Electron-rich trialkyl-type dihydro-KITPHOS monophosphines: Efficient ligands for palladium-catalyzed suzuki-miyaura cross-coupling. Comparison with their biaryl-like KITPHOS monophosphine counterparts

Article

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Electron-Rich Trialkyl-Type Dihydro-KITPHOS Monophosphines:

Eﬃcient Ligands for Palladium-Catalyzed Suzuki−Miyaura Cross-

Coupling. Comparison with Their Biaryl-Like KITPHOS

Monophosphine Counterparts

Simon Doherty,* Julian G. Knight, Nicholas A. B. Ward, Dror M. Bittner, Corinne Wills,

William McFarlane, William Clegg, and Ross W. Harrington

NUCat, School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne NE1 7RU, U.K.

S

* Supporting Information

ABSTRACT: The Diels−Alder cycloaddition between dicy-

clohexylvinylphosphine oxide and anthracene or 9-methylan-

thracene aﬀords the bulky electron-rich trialkyl-type dihydro-

KITPHOS monophosphines 11-(dicyclohexylphosphinoyl)-

12-phenyl-9,10-dihydro-9,10-ethenoanthracene and 11-(dicy-

clohexylphosphinoyl)-9-methyl-12-phenyl-9,10-dihydro-9,10-

ethenoanthracene, respectively, after reduction of the corre-

sponding oxide. Both phosphines are highly air-sensitive and

rapidly oxidize on silica gel during puriﬁcation but have been

isolated as air-stable cyclometalated palladium precatalysts of the type [Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)C₆H₄}Cl(L)]. Both

palladium precatalysts form highly active systems for the Suzuki−Miyaura cross-coupling of a range of aryl chlorides with aryl

boronic acids, giving the desired products in good to excellent yield under mild conditions and a catalyst loading of 0.25 mol %. A

comparison of the performance of catalysts based on dihydro-KITPHOS monophosphines against their ﬁrst-generation biaryl-

like KITPHOS counterparts revealed that the latter are consistently more eﬃcient for the vast majority of substrate combinations

examined, albeit by only a relatively small margin in some cases. This, together with the greater air stability and ease of handling

of biaryl-like KITPHOS monophosphines, renders them more practical ligands for palladium-based cross-coupling. The steric

parameters of both classes of KITPHOS monophosphine and a selection of electron-rich biaryl monophosphines have been

quantiﬁed using a combination of Solid-G to determine the percentage of the metal coordination sphere shielded by the

phosphine (the G parameter), and Salerno molecular buried volume calculations (SambVca) to determine the percent buried

volume (%V_bur); the corresponding Tolman cone angles have also been determined from correlations and the relative merits of

the two approaches discussed. The electronic properties of these phosphines have also been investigated using DFT to calculate

the A₁ν(CO) frequency in LNi(CO)₃(B3LYP/6-31G(2d,p)[LanL2DZ on Ni]), and the resulting computed electronic

parameters (CEP) were used to estimate the corresponding experimental Tolman electronic parameters (TEP).

phosphine are matched, as too much bulk has been shown to

be detrimental for selected cross-coupling reactions.¹⁸The

second class are biaryl-based monophosphines such as 5−7,

ﬁrst introduced by Buchwald in 1998, characterized by their

steric bulk, high basicity, and a proximal non-phosphine-

containing aryl ring and assembled via an elegant modular and

versatile one-pot procedure.¹⁹The eﬃcacy of these phosphines

has been attributed to a number of factors: (i) their electron-

rich character which facilitates oxidative addition of unreactive

aryl chlorides,²⁰(ii) their bulk which promotes formation of the

active monophosphine complex L₁Pd⁰,¹⁷(iii) increased catalyst

lifetime due to stabilization of intermediate complexes through

either a Pd−arene interaction with the ipso carbon atom or a

Pd−O interaction with an oxygen atom of the methoxy group

on the non-phosphine-containing aromatic ring,^2f,21and (iv)

INTRODUCTION

■

Since the late 1990s, bulky electron-rich monodentate

phosphines¹have evolved into a highly powerful class of ligand

for a host of palladium-catalyzed transformations; examples

include Suzuki−Miyaura,²Negishi,³Kumada,⁴Stille.⁵and

Hiyama⁶cross-couplings, Buchwald−Hartwig amination,⁷

borylation,⁸silylation,⁹etheriﬁcation,¹⁰α-arylation,¹¹direct

arylation,¹²and carbonylation.¹³Two distinct classes of

monophosphines can be identiﬁed: the ﬁrst of these are the

trialkyl-based phosphines such as P(t-Bu)₃,¹⁴PCy₃,¹⁴and

PAd₂R (Ad = 1-adamantyl, R = CH₂Ph, n-Bu, t-Bu)¹⁵and most

recently 9-ﬂuorenylphosphines¹⁶(Figure 1, 1−4); the last

species were introduced as architecturally modular trialkylphos-

phines to overcome the limited structural versatility associated

with this class of ligand. The eﬃciency of trialkylphosphines

appears to be due to a combination of their steric bulk and high

basicity, favoring formation of an active low-coordinate

complex,¹⁷provided the steric bulk of the substrate and

Received: December 11, 2012

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Figure 1. Bulky electron-rich monodentate phosphines 1−8.

the substitution pattern of both the non-phosphine-containing

aryl ring and the upper arene;²²in particular, bulky biaryl

monophosphines 3,6-substituted on the upper arene are

uniquely eﬀective ligands for certain classes of challenging

palladium-catalyzed carbon−heteroatom coupling reactions.²³

As the biaryl framework is integral to the success of these

ligands, it has been used as the lead motif for developing

alternative systems based on biaryl-like units such as N-

phenylpyrrole,²⁴2-phenylindole,²⁵arylpyrazole, and bipyra-

zole,²⁶as well as the 2,3-dihydrobenzo[d][1,3]oxaphosphole

framework.²⁷We have also embraced this design concept and

recently introduced a new class of biaryl-like monophosphines

8,²⁸KITPHOS, the basic architecture of which closely

resembles Buchwald’s biaryl monophosphines in that a bulky

electron-rich PR₂group is connected to a carbon−carbon

double bond, albeit as part of an anthracene-derived bicyclic

framework, with a proximal non-phosphine-containing aryl ring

that can be systematically modiﬁed. In addition to their being

architectural analogues, we have been exploring whether

electron-rich KITPHOS monophosphines could be surrogates

for their biaryl-based counterparts. Encouragingly, preliminary

studies have revealed that these ligands either rival or

outperform their biaryl-based counterparts across a wide

range of palladium-catalyzed Suzuki−Miyaura and Buchwald−

Hartwig cross-couplings^29a−cas well as gold(I)-catalyzed

intramolecular cycloisomerizations^29d,eand most recently the

ruthenium-catalyzed direct ortho arylation of 2-phenylpyridine

and N-phenylpyrazole.³⁰Key features of KITPHOS mono-

phosphines include their operationally straightforward synthesis

via [4 + 2] cycloaddition between a 1-alkynylphosphine oxide

and anthracene in moderate to reasonable yield from relatively

inexpensive starting materials, as well as their highly modular

construction, which allows the substitution pattern of the 1-

alkynylphosphine oxide aryl group to be systematically varied,

the steric bulk and the basicity of the phosphino group to be

ﬁne-tuned, and the biaryl-like fragment to be modiﬁed by

performing the cycloaddition with a substituted anthracene or

its equivalent. For comparison, electron-rich biaryl mono-

phosphines are typically prepared in an elegant, modular one-

pot protocol that involves addition of an aryl Grignard to an in

situ generated benzyne followed by trapping of the resulting

biaryl−metal intermediate with an appropriate chlorophos-

phine.³¹

the Diels−Alder cycloaddition between a vinylphosphine oxide

and anthracene to aﬀord the dihydro-KITPHOS class of

monophosphine (Figure 2) and exploring the extent to which

Figure 2. Retrosynthesis of dihydro-KITPHOS monophosphines.

these phosphines resemble or mimic bulky electron-rich

trialkyl-type systems such as 1−4. Moreover, this approach

would not be constrained by the limited structural ﬂexibility/

versatility associated with conventional trialkylphosphines, since

the modular construction of dihydro-KITPHOS monophos-

phines will enable their steric bulk and basicity to be modiﬁed

through the phosphino group as well as the substituents

attached to the vinyl-derived two-carbon bridge and the

anthracene unit. Herein, we report the synthesis of two

trialkyl-type dihydro-KITPHOS monophosphines, isolation of

their borane adducts and phosphonium salts, the synthesis of

palladium precatalysts, an evaluation of their eﬃciency as

ligands in the palladium-catalyzed Suzuki−Miyaura cross-

coupling of aryl chlorides, a comparison of their performance

against the corresponding biaryl-like KITPHOS monophos-

phine-based systems, and an initial quantiﬁcation of their steric

and electronic parameters.

RESULTS AND DISCUSSION

■

Synthesis of KITPHOS-Based Phosphines, Their Bor-

ane Adducts, and Phosphonium Salts. Our interest in

dihydro-KITPHOS monophosphines 9a,b began after recog-

nizing their trialkyl-type character and the close similarity of

their steric and electronic properties to those of tricyclohex-

ylphosphine and tri-tert-butylphosphine. Monophosphines 9a,b

were prepared in good yield following the procedure outlined

in Scheme 1, which involves the Diels−Alder cycloaddition

between dicyclohexylvinylphosphine oxide (10) and either

anthracene or 9-methylanthracene followed by reduction of the

resulting monoxides 11a,b by heating the oxide in a mixture of

chlorosilane, toluene, and triethylamine at 110 °C for 48 h in a

sealed vessel. The dicyclohexylvinylphosphine oxide required

Having demonstrated the validity of the biaryl-like analogy,

we became interested in extending the same methodology to

B

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Scheme 1

for the Diels−Alder cycloaddition was prepared in near-

quantitative yield by reaction of vinylmagnesium chloride with

chlorodicyclohexylphosphine,³²followed by oxidation immedi-

ately prior to workup; the product can typically be used for the

subsequent step without further puriﬁcation. The Diels−Alder

reactions were most conveniently carried out by heating the

reactants in sealed 20 mL Wheaton V-20 reaction vials using an

Asynt DrySyn multiposition heating block and the progress

monitored using ³¹P NMR spectroscopy to analyze samples of

the reaction mixture at regular time intervals.

possible steric interactions between the 9-methyl substituent

and the dicyclohexylphosphinoyl group in the transition state

for cycloaddition. The rapid oxidation of 9a,b is in stark

contrast to their biaryl-like counterparts 8, which can be

puriﬁed by column chromatography and are air-stable solids

that can be stored for several months with no evidence of

oxidation.^28a

As 9a,b are highly air-sensitive, they were immediately

transformed into their borane adducts by reaction with

BH₃·SMe₂in THF; workup and puriﬁcation by column

chromatography gave 12a,b, both as air-stable white solids, in

near-quantitative yield. The ³¹P NMR spectrum of 12a contains

a doublet at δ 33.7 (J = 69.2 Hz), and the corresponding signal

for 12b appears at δ 32.8 (J = 65.3 Hz); as expected, both are

shifted downﬁeld relative to the corresponding phosphine. The

¹¹B NMR spectra of 12a,b contain characteristically broad

Unfortunately, attempts to purify 9a,b by ﬂash chromatog-

raphy resulted in rapid oxidation, and the corresponding oxide

was the sole product to elute from the column, which is

perhaps not surprising considering their trialkyl-type nature.

However, spectroscopically pure samples were isolated by

conducting the base hydrolysis and aqueous workup of the

reduction mixture under a nitrogen atmosphere and analytically

pure samples were obtained by careful crystallization. The

Diels−Alder reaction between 9-methylanthracene and 10

occurs with high regioselectivity to aﬀord a single adduct,

presumably that with the bulky methyl-substituted bridgehead

atom attached to the least substituted carbon atom of the

vinylphosphine oxide derived two-carbon bridge, in much the

same manner as previously reported for the corresponding

double Diels−Alder cycloaddition with 1,4-bis-

(diphenylphosphinoyl)buta-1,3-diyne.³³The regioselectivity of

addition is supported by a comparison of the ¹H and ¹³C NMR

spectra of 9a,b; speciﬁcally the bridgehead hydrogen atoms of

9a appear as a doublet and broad singlet at δ 4.37 and 4.29,

respectively. The coupling constant of 5.9 Hz in the former is

consistent with a three-bond phosphorus−hydrogen coupling

and is similar to that of 4.9 Hz for the doublet at δ 4.31

associated with the sole bridgehead hydrogen atom of 9b. The

bridgehead carbon atoms of 9a,b show a similar distinctive

pattern of P−C coupling; in both cases one bridgehead carbon

atom appears as a doublet (J_PC= ca. 16 Hz) and the other as a

singlet; the doublet associated with 9b has been unequivocally

assigned to the bridgehead CH using the DEPT NMR

sequence: the magnitude of this coupling is entirely consistent

with a two-bond coupling in the proposed regioisomer. The

high regioselectivity can be accounted for by considering the

1

resonances at δ −43.4 and −43.5, respectively. The H NMR

spectra of 12a,b both contain a high-ﬁeld broad multiplet

associated with the BH₃group, while the protons attached to

the bridgehead carbon atoms of 12a appear as a doublet and a

broad triplet at δ 4.74 (J_PC= 5.4 Hz) and 4.36 (J_PC= 2.1 Hz),

respectively; the sole bridgehead proton of 12b appears as a

doublet at δ 4.72 (J_PC= 4.6 Hz). As 12a,b are the ﬁrst examples

of dihydro-KITPHOS monophosphines, a single-crystal X-ray

study of 12a was undertaken to compare the key structural

features with those of other trialkyl monophosphine−borane

adducts; a perspective view of the molecular structure is shown

in Figure S2 of the Supporting Information together with a

brief discussion. Deprotection of 12a,b using the catalyst-free

alcoholysis procedure recently developed by Van der Eycken

was attempted, as this generates volatile byproducts that can be

easily removed and avoids the need for further puriﬁcation.³⁴

Unfortunately, there was no evidence for liberation of the

phosphine even after heating ethanol solutions of 12a,b at

reﬂux for 48 h and near-quantitative amounts of the borane

adducts were recovered.

Taking a lead from the pioneering work of Fu,³⁵9a,b were

expected to be stable for long periods of time as their

phosphonium salts, with the advantage that they could

ultimately be used as replacements for the corresponding

phosphine, since deprotonation by a Brønsted base under the

C

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Scheme 2

Scheme 3

reaction conditions typically used for palladium-catalyzed cross-

coupling would liberate the trialkylphosphine. Following a

procedure developed by Denmark to transform air-stable

phosphine−borane adducts directly into phosphonium tetra-

ﬂuoroborate salts,^36atreatment of 12a,b with HBF₄·OEt₂in

dichloromethane at 0 °C followed by an aqueous ﬂuoroboric

acid wash gave 13a,b as spectroscopically and analytically pure

white crystalline solids in high yield.³⁶The proton-coupled ³¹P

NMR spectra of 13a,b contain doublets at δ 33.7 and 31.9,

respectively, with characteristically large one-bond phospho-

rus−hydrogen coupling constants of 477 and 475 Hz,

Synthesis of Palladium Precatalysts. With the intention

of comparing the performance of dihydro-KITPHOS mono-

phosphines and that of their associated biaryl-like counterparts

as ligands for palladium-catalyzed Suzuki−Miyaura cross-

coupling, precatalysts 16a,b and 17a,b were prepared by

reaction of the corresponding phosphine with the chloride-

bridged dimer [Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)C₆H₄}(μ-

Cl)]₂,⁴⁹in a 1/1 mixture of dichloromethane and acetone at

room temperature (Scheme 3). Precatalysts of this type have

recently been introduced by Buchwald and co-workers as air-

and moisture-stable complexes that can be “activated” under

standard conditions by elimination of HCl and 9H-carbazole to

aﬀord the active monocoordinated species L₁Pd⁰.⁴⁰The use of

this new class of one-component precatalysts is markedly more

eﬃcient than generating the catalyst from a source of Pd(0)

such as Pd₂(dba)₃, as competitive coordination of dba retards

formation of the catalyst and/or diminishes reactivity, or from

Pd(OAc)₂, which must be reduced in situ and generally

requires more than 2 equiv of phosphine. Precatalysts 16a,b

and 17a,b were all puriﬁed by column chromatography, isolated

as air-stable oﬀ-white solids, and crystallized by slow diﬀusion

techniques.

1

respectively; similar values of J_PHhave been reported for

[Cy₂(t-Bu)PH][BF₄],³⁶[Cy₂EtPH][BF₄],³⁷and [Cy₃PH]-

[BF₄].³⁸The corresponding phosphonium protons appear as

1

doublets at δ 5.91 and 5.90 in the H NMR spectra of 13a,b,

respectively; a doublet at δ 4.95 (J_PC= 6.1 Hz) and a singlet at

δ 4.50 belong to the protons attached to the two bridgehead

carbon atoms of 13a, and a doublet at δ 4.93 (J = 6.4 Hz)

corresponds to the sole bridgehead proton of 13b.

With the intention of undertaking a systematic comparison

between the performance of dihydro-KITPHOS monophos-

phines and that of their corresponding biaryl-like counterparts

as ligands for palladium-catalyzed Suzuki−Miyaura cross-

coupling, it was also necessary to prepare 15b; this was

achieved in good yield by following the procedure reported for

15a,^28awhich involved a Diels−Alder reaction between

(dicyclohexylphosphinoylethynyl)benzene and 9-methylanthra-

cene followed by reduction of the resulting KITPHOS

monoxide 14a,b with chlorosilane/triethylamine in toluene at

110 °C (Scheme 2). KITPHOS monophosphine 15b was

isolated as an air-stable oﬀ-white crystalline solid after

puriﬁcation by column chromatography; the identity and purity

were unequivocally established using conventional spectro-

scopic and analytical techniques.

Interestingly, the room-temperature ³¹P NMR spectra of

16a,b each contain two broad signals, while those of 17a,b

contain only one broad resonance. Buchwald⁴⁰and Krska⁴¹

have both reported that the ³¹P NMR spectra of the

corresponding precatalysts based on SPHOS and a trioxa-6-

phosphaadamantane ligand, respectively, also contain two

broad signals; in both cases the line broadening was

unspeciﬁcally attributed to the presence of rotamers in

exchange. In the present work, the origin of this line

broadening was investigated further by conducting a variable-

temperature ³¹P NMR study on 17a in d₂-DCM, the result of

which is shown in Figure 3. At room temperature the spectrum

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Table 1. Approximate Relative Energies of Conformers A−C

of 17a and the Barriers to Conversion

conformer or barrier

rel energy/kJ mol⁻¹

A

0

A/B

B

25.1

4.2

B/C

C

41.2

2.5

C/A

31.1

Figure 3. Selected variable-temperature ³¹P{¹H} NMR spectra of

[Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)C₆H₄}Cl{11-(dicyclohexylphosphino)-

9-methyl-12-phenyl-9,10-dihydro-9,10-ethenoanthracene}] (17a):

●

△

( ) major rotamer; ( ) minor rotamer.

Figure 4. Approximate energies involved in the interconversion among

conformers A−C of 17a.

consists of a single peak at 47.1 ppm with a half-height line

width of 156 Hz. As the temperature is lowered, the peak

broadens and at 243 K has a half-height line width of 700 Hz.

Below this temperature two distinct sets of signals begin to

evolve; one of these appears as two resonances at 55.5 and 31.2

ppm (△) that sharpen rapidly as the temperature is lowered.

However, these comprise only a very minor component of the

exchanging system and will be disregarded in the following

discussion. The major components (●) appear as a set of three

signals at 51.7 (A), 43.8 (B), and 23.4 ppm (C) which sharpen

more slowly over a wider reduced temperature range and have

relative proportions of 79.8, 5.5, and 14.7%, respectively, at 183

K.

sulﬁde and in neither case were any signiﬁcant changes in the

spectra observed at temperatures down to 183 K, thus

suggesting that rotation about the P −anthracyl bond in 17a

would also be relatively free. On the basis that hindrance to

rotation about the Pd−P bond is responsible for the NMR line

broadening, the three idealized conformers 17a_i, 17a_ii, and

17a_iiimay be considered; these are shown in Figure 5 as

More detailed information concerning the exchange process

was obtained from a magnetization transfer experiment

(EXSY). The low-temperature ³¹P EXSY spectrum of 17a

(see Figure S3 in the Supporting Information) clearly shows

that the signals at 51.7, 43.8, and 23.4 ppm are in mutual

chemical exchange, whereas the minor signals at 55.5 and 31.2

ppm are involved in a separate process where the exchange is

much slower. These observations are consistent with the major

components comprising a system of three interchanging

conformers, and line-shape analysis using the program gNMR

was undertaken of the ³¹P NMR spectra across the overlapping

temperature ranges 183−313 K (dichloromethane-d₂) and

295−373 K (toluene-d₈). Apparently satisfactory ﬁts using a

single exchange rate were obtained between 223 and 373 K, but

this is clearly an oversimpliﬁcation, and from 183 to 213 K it

was necessary to use three separate rates to achieve an

acceptable simulation. The application of the Boltzmann and

Eyring equations to this rather limited data set was used to

determine the relative ground-state energies and barrier heights

shown in Table 1; the results are also presented schematically

in Figure 4 for the three-conformer system. It is noteworthy

that the most stable conformer (A) is associated with the

lowest barriers to interconversion, while the least stable

conformer (B) is associated with lower barriers than

intermediate C. The foregoing behavior can be attributed

most reasonably to restricted rotation about either the Pd−P

bond or the P−“anthracyl” bond of 17a (but probably not both,

since this would lead to more complex spectra). To test this,

low-temperature ³¹P NMR spectra were obtained for 9a and its

Figure 5. Newman projections of the three possible solution-state

conformations of 17a.

Newman projections down the P−Pd bond. Of the three

possible solution-state conformations 17a_iicorresponds most

closely to the situation in the crystal structure and is therefore

probably the most stable (A).

Even though the spectroscopic properties and analytic data

for 16a,b and 17a,b are all fully consistent with the proposed

1

formulation, the exchange broadening in the ³¹P and H NMR

spectra prompted us to undertake single-crystal X-ray analyses

of 16a and 17a,b to compare their key structural features with

those for closely related complexes of electron-rich biaryl

monophosphines. Perspective views of the molecular structures

of 16a and 17a,b are given in Figures 6−8, respectively. In each

precatalyst, the phosphine occupies the coordination site trans

to the amin,e since it has a much lower trans inﬂuence than the

strongly σ-donating metalated aromatic ring. The Pd−P(1)

distances of 2.2737(11) Å (16a), 2.2699(7) Å (17a), and

2.2641(13) Å (17b) are similar to those of 2.270(1) and

2.2786(9) Å in [Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)C₆H₄}Cl-

(XPHOS)]⁴²and [Pd(κ²C,N-C₆H₄CH₂CH₂NH₂-2)Cl-

(SPHOS)],^40arespectively, as are the Pd−C(1) bond lengths

of 2.029(5) Å (16a), 2.005(3) Å (17a), and 2.005(5) Å (17b);

the corresponding distances in [Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)-

E

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C₆H₄}Cl(XPHOS)]⁴²and [Pd(κ²-C,N-C₆H₄CH₂CH₂NH₂-2)-

Cl(SPHOS)]^40aare 2.022(4) and 2.004(4) Å, respectively. The

P(1)−Pd−Cl(1) angles of 97.08(4)° (16a), 96.99(2)° (17a),

and 97.95(4)° (17b) lie within a fairly narrow range, as do the

P(1)−Pd−C(1) angles of 93.52(12)° (16a), 91.83(8)° (17a),

and 91.67(14)° (17b); all are greater than the ideal value of 90°

and close to the corresponding angles in [Pd{κ²N2′,C1-2-(2′-

NH₂C₆H₄)C₆H₄}Cl(SPHOS)]⁴⁵and [Pd(κ²-C,N-

C₆H₄CH₂CH₂NH₂-2)Cl(SPHOS)].^40a

Steric and Electronic Properties of KITPHOS-Based

Monophosphines: Comparison with Biaryl Monophos-

phines. The steric properties of electron-rich trialkyl- and

biaryl-based monophosphines are integral to their eﬃcacy in

palladium-catalyzed C−C and C−N bond formation. Exper-

imental and computational studies have suggested this to be

associated with promoting the formation and stabilization of a

monocoordinate complex and, in the case of biaryl mono-

phosphines, to the longevity of the catalyst due to additional

stabilization of intermediates by interaction of the non-

phosphine-containing aromatic π system with the palladium

center. Surprisingly, until recently, there have been no attempts

to quantify the steric properties of this highly versatile class of

ligand. As calculations using the Tolman model⁴³have often

proven diﬃcult for elaborate phosphines and N-heterocyclic

carbenes (NHC), Nolan and Cavello recently proposed an

alternative model to measure steric bulk,⁴⁴the percent buried

volume (%V_bur), deﬁned as the percent of the total volume of a

sphere occupied by a ligand; the volume of this sphere

represents the potential coordination sphere space around the

metal occupied by a ligand and is calculated using the SambVca

software.⁴⁵Gold(I) complexes were shown to be a suitable

model for quantifying the steric parameters of phosphines, as

there was an excellent correlation between the %V_burvalue in

[Au(PR₃)Cl] and the Tolman cone angle (θ). Since crystallo-

graphic data are available for several gold(I) complexes of

KITPHOS and dihydro-KITPHOS monophosphines, their

percent buried volumes have been determined and compared

with the corresponding values for selected gold(I) complexes of

biaryl monophosphines, full details of which are presented in

Table 2. The %V_burvalues of 43.8 and 45.0 for KITPHOS

monophosphine 15a coordinated to AuNTf₂and Au(tht),

respectively, are similar, which suggests that buried volume

calculations carried out with Au(I) complexes provide a reliable

and accurate measure of the steric properties of the

monophosphine (entries 1 and 2); this is reinforced by values

of 45.5 and 46.0 determined for the corresponding gold(I)

complexes of 2-MeOKITPHOS (entries 5 and 6).

Figure 6. Molecular structure of [Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)-

C₆H₄}Cl{11-(dicyclohexylphosphino)-9,10-dihydro-9,10-ethanoan-

thracene}] (16a). Hydrogen atoms have been omitted for clarity.

Ellipsoids are at the 40% probability level.

Figure 7. Molecular structure of [Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)-

C₆H₄}Cl{11-(dicyclohexylphosphino)-12-phenyl-9,10-dihydro-9,10-

ethenoanthracene}] (17a). Hydrogen atoms and the dichloromethane

and hexane molecules of crystallization have been omitted for clarity.

Ellipsoids are at the 40% probability level.

Surprisingly, the %V_burvalue of 44.4 calculated for dihydro-

KITPHOS monophosphine 9a in [LAuCl] is similar to those of

43.8 and 45.8 for biaryl-like KITPHOS monophosphines 15a,b,

respectively, in the same complexes and close to that of 45.0 in

the corresponding tetrahydrothiophene adduct (entries 1−4).

As 9a lacks a proximal phenyl ring, its larger than expected %

V_burmay suggest that this fragment does not contribute

signiﬁcantly to the steric proﬁle, although the diﬀerent

geometrical constraints imposed by the sp³nature of the two-

carbon bridge of the bicyclic framework in 9a in comparison

with the planar double bond in 15a,b might be responsible for

the unexpected similarity in their steric parameters. It will be

necessary to analyze a much larger cohort of “substituted”

KITPHOS-based monophosphines in order to understand what

factors control the steric proﬁle of these ligands. This is in stark

contrast to the corresponding comparison between PCy₂Ph and

Figure 8. Molecular structure of [Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)-

C₆H₄}Cl{11-(dicyclohexylphosphino)-9-methyl-12-phenyl-9,10-dihy-

dro-9,10-ethenoanthracene}] (17b). Hydrogen atoms and the

dichloromethane molecule of crystallization have been omitted for

clarity. Ellipsoids are at the 40% probability level.

F

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catalyzed transformations, the additional steric bulk associated

with dihydro-KITPHOS monophosphines may well manifest

itself in catalyst performance. As %V_burcalculations have shown

that the steric bulk of P^tBu₃depends on the number and size of

additional ligands coordinated directly to the metal center, with

an interest in further developing the use of KITPHOS-based

monophosphines in palladium catalysis, the buried volumes for

9a and 15a,b in their precatalysts [Pd{κ²N2′,C1-2-(2′-

NH₂C₆H₄)C₆H₄}(Cl)L] (16a, 17a,b) have also been calcu-

lated, the results of which are given in Table 2 (entries 15−17).

For each KITPHOS monophosphine, the %V_burvalue is

markedly smaller than the corresponding value for its LAuX

complex; the average value of 37.4 represents a 16% decrease in

%V_bur. Considering PCy₂Ph as the reference for biaryl

monophosphines, precatalyst [Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)-

C₆H₄}Cl(PCy₂Ph)] was also prepared and its crystal structure

determined; a perspective view of the molecular structure is

shown in Figure S2 of the Supporting Information. As for

KITPHOS monophosphines, the buried volume of 30.5% for

PCy₂Ph in this precatalyst is also smaller than the

corresponding value of 32.7 in (PCy₂Ph)AuCl, though this

reduction is much less pronounced.

Interestingly, dihydro-KITPHOS monophosphine 9a has the

largest steric bulk in its precatalyst with a %V_burof 39.5 and a

cone angle of 189°, while 15a,b are decidedly smaller with %

V_burvalues of 36.5 and 35.7, respectively. On the basis of this

limited comparison, KITPHOS monophosphines appear to be

sterically malleable and capable of adjusting their bulk

according to the immediate surroundings/environment: i.e.,

the space available around the metal center. Thus, there appear

to be subtle diﬀerences in the factors that inﬂuence the steric

bulk of KITPHOS monophosphines in comparison with their

biaryl counterparts, as gauged by the trends and absolute values

of %V_burin Table 2; however, more data will clearly be required

to lend credibility and validity to this interpretation.

Table 2. %V_burValues Calculated for KITPHOS

Monophosphines and Biaryl Monophosphines in [(L)Au(I)]

and [Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)C₆H₄}Cl(L)] Complexes

a

b

entry

complex

29d

%V_bur

cone angle θ/deg

1

15a·AuNTf₂

43.8

45.0

44.2

45.8

45.5

46.0

45.7

46.0

32.7

46.7

49.3

49.7

53.1

38.1

39.9

36.5

35.7

30.5

206

209

206

212

211

213

212

213

159

226

238

240

256

182

189

176

173

152

2

15a·Au(tht)^29c

3

9a·AuCl

15b·AuCl

4

5

2-MeOKITPHOS·AuCl^29d

2-MeOKITPHOS·AuNTf₂

29d

29e

6

7

2,6-MeKITPHOS·AuNTf₂

29e

8

2,6-MeOKITPHOS·AuNTf₂

PCy₂Ph·AuCl⁴⁶

9

10

11

12

13

14

15

16

17

18

Cy-JohnPhos·AuCl⁴⁷

MePHOS·AuCl⁴⁸

SPHOS·AuCl⁴⁷

XPHOS·AuCl⁴⁸

(t-Bu)₃P·AuCl⁴⁹

precatalyst 16a

precatalyst 17a

The steric parameters of selected KITPHOS-based and biaryl

monophosphines have also been determined using Solid-G

calculations (Table 3), as this method provides a measure of

steric congestion in terms of percentage of the metal

coordination sphere shielded by the ligand; the G parameter

essentially measures the probability of an incoming reagent not

precatalyst 17b

precatalyst 18

a

b

%V_burfor Au−P bond length at 2.28 A. Calculated by linear

regression.

Cy-JohnPhos in [LAuCl], which has been used to evaluate the

steric inﬂuence of the proximal phenyl ring; the %V_burvalues of

32.7 and 46.7, respectively, clearly show that the biaryl phenyl

ring has a marked inﬂuence and increases the buried volume by

14% (entries 9 and 10). Moreover, the buried volume of biaryl

monophosphines increases quite dramatically as a function of

the biaryl substitution, reaching a value of 53.1 for XPHOS,

which corresponds to a Δ%V_burvalue of 6.4 from Cy-JohnPhos

(entries 11−13). In contrast, substitution of the aryl ring in

biaryl-like KITPHOS monophosphines does not appear to

aﬀect steric bulk either to the same extent or in the same

manner, as the buried volumes for 15a and its 2-OMe-, 2,6-

OMe₂-, and 2,6-Me₂-substituted counterparts in LAuX all lie

within in a narrow range between 43.8 and 46.0, corresponding

to a Δ%V_burvalue of 2.2% (entries 5−8).

Table 3. G Parameters for KITPHOS and Biaryl

Monophosphines Calculated from the Solid Angle Data

a

b

entry

phosphine

G param

cone angle θ/deg

1

9a

49.7

50.57

53.5

52.5

55.0

56.9

58.0

33.9

52.9

54.5

61.0

73.1

37.6

179

181

188

186

192

196

198

142

187

190

205

234

151

2

9b

3

15a

15b

4

5

2-MeOKITPHOS

2,6-MeKITPHOS

2,6-MeOKITPHOS

PCy₂Ph

6

7

8

9

Cy-JohnPhos

MePHOS

10

11

12

13

SPHOS

The gold(I) chloride complexes of 9a and P^tBu₃have been

used to evaluate and compare their steric properties, and the %

V_burvalues of 44.2 and 38.1, respectively, clearly lend some

validity to our description of dihydro-KITPHOS mono-

phosphines as bulky trialkyl-type monophosphines. Moreover,

since P^tBu₃is an eﬀective ligand for a range of palladium-

XPHOS

(t-Bu)₃P

a

b

Calculated using Solid-G.⁵³Equivalent cone angle calculated from

the on-linear relationship Ω = 100[1 − cos(θ/2)], where G = 100(Ω/

(4π)).

G

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accessing the metal center.⁵⁰The G parameter is calculated

from the solid angle, Ω, which corresponds, at least visually, to

the area of the ligand projection shadow (A) formed by a light

source originating from the central metal of the organometallic

complex and located at the center of a sphere of radius 13 Å.

Qualitatively, the G parameters calculated for KITPHOS

monophosphines show a trend similar to that for the %V_bur

values in that 9a and 15a,b are all only marginally smaller than

their aryl ring substituted counterparts and, regardless of the

extent of substitution, all the G parameters lie within a narrow

range between 49.7 and 58.0 corresponding to cone angles of

179 and 198°, respectively (entries 1−7). For comparison, the

G parameters for the biaryl monophosphines show that steric

congestion at the metal center increases rapidly with increasing

substitution of the lower aryl ring (entries 8−12); however, the

G values of 52.9 and 54.5 for JohnPhos and MePHOS,

respectively, are slightly smaller than anticipated on the basis of

the trend in buried volumes and present steric congestion

similar to that of unsubstituted or 2-OMe-substituted

KITPHOS monophosphines, which have G parameters of

53.5 and 55.0, respectively.

Table 4. CEP (cm⁻¹) and TEP (cm⁻¹) for Selected

Monophosphines

ligand

P(t-Bu)₃

TEP

CEP

2056.1

2059.2

2066.6

2067.0

2072.0

2076.3

2079.5

2057.9

2059.4

2058.7

2052.6

2056.0

2057.0

2056.6

2058.1

2057.6

2050.9

2132.5

2138.0

2142.6

2144.7

2148.4

2155.1

2157.6

2134.8

2136.3

2135.6

2129.1

2132.7

2133.8

2133.4

2135.0

2134.4

2127.4

P(i-Pr)₃

P(o-tolyl)₃

PPh₂Me

PPh₂OMe

P(OEt)₃

P(OMe)₃

i

PCy₂Pr

Cy-JohnPhos

MePHOS

SPHOS

XPHOS

9a

9b

15a

15b

2,6-MeOKITPHOS

As the donor properties of monophosphines also play an

integral role in achieving eﬃcient catalysis by inﬂuencing either

the oxidative addition and/or transmetalation step, it is

surprising that there are so few reports that provide details

on the electronic properties of commonly used bulky electron-

rich monodentate phosphines. Thus, we have undertaken a

comparative study and computed the ligand electronic

parameters for selected KITPHOS-based monophosphines

and their corresponding biaryl-based counterparts. Initially a

correlation between the computed electronic parameter (CEP)

and the experimentally measured Tolman electronic parameter

(TEP) was determined for a range of phosphines, as previously

described by Clot and co-workers.⁵¹DFT (B3LYP) calculations

were conducted on LNi(CO)₃, and a frequency calculation on

the fully optimized structure provided the A₁ν(CO) vibrational

frequency and comparison with the corresponding experimen-

tal IR stretching frequencies gave a correlation coeﬃcient of

0.984. Regression analysis gave the following equation relating

the computed electronic parameter (cm⁻¹) to the experimen-

tally measured TEP (cm⁻¹):

highly eﬃcient catalysts for the Suzuki−Miyaura coupling^29a,b

and because bulky electron-rich trialkylphosphines also

generally form eﬀective catalysts for this reaction, we chose

to use this transformation to investigate the eﬃcacy of trialkyl-

type dihydro-KITPHOS monophosphines against their biaryl-

like counterparts and to explore the eﬀect on catalyst

performance of removing the proximal aryl ring and the double

bond in the anthracene-derived bicyclic framework. Precursors

16a,b and 17a,b were used for the catalyst evaluation, as they

are easy to prepare, air-stable, a convenient source of the active

monocoordinated complex L₁Pd⁰, and cost-eﬀective, since only

1 equiv of phosphine is required per palladium atom; compare

this with catalysts generated from palladium acetate, which

typically require 2.5 equiv of the phosphine to achieve optimum

performance. Our preliminary comparison and optimization

focused on the benchmark coupling between 4-chloroaceto-

phenone and benzeneboronic acid using 0.25 mol % precatalyst

in toluene at 50 °C with K₃PO₄as base; for brevity and clarity

selected results are presented in Table 5 and full details of the

optimization study are provided in Table S1 of the Supporting

Information. Under these conditions precatalysts 17a,b were

marginally more eﬃcient than 16a,b; the former gave

conversions of 91% (17a) and 87% (17b) after 30 min in

comparison with 81% (16a) and 79% (16b) for the latter, in

the same time (entries 1−2). Excellent conversions were also

obtained in THF, under otherwise identical conditions;

however, the presence of added water resulted in slightly

lower conversions.⁵³As previously reported, the correct

combination of base and solvent is crucial to achieving good

conversions; K₃PO₄was most eﬀective for reactions conducted

in toluene, while KF and K₃PO₄were both highly eﬀective for

reactions in THF, the combination of toluene and KF was

slightly less eﬃcient, K₂CO₃was more eﬀective in toluene than

THF, and other bases, such as alkali-metal acetates and sodium

tert-butoxide, were markedly less eﬀective in both solvents.

Although 9a,b are highly air-sensitive, following a protocol

popularized by Fu and co-workers,⁵⁴catalyst mixtures

generated from Pd(OAc)₂(0.5 mol %), 2.5 equiv of the

corresponding phosphonium salt, 13a or 13b, and K₃PO₄in

THF gave conversion of 73% and 84%, respectively, for the

cross-coupling between 4-chloroacetophenone and benzene-

TEP = 0.9473(CEP) + 35.638

This function has been used to estimate the TEPs for

KITPHOS monophosphines 9a,b, 15a,b, and 2,6-MeOKIT-

PHOS together with a range of related biaryl monophosphines;

values are given in Table 4. The estimated TEPs provide a

measure of the net donor power of KITPHOS mono-

phosphines, which decreases in the order 2,6-MeOKITPHOS

> 9b > 9a > 15b > 15a; this series is supported by the J_PSe

values, which increase in the order 2,6-MeOKITPHOS (684

Hz), 9b (694 Hz), 9a (696 Hz), 15b (702 Hz), and 15a (702

Hz). The magnitude of this parameter is inversely correlated

with the σ-donor strength of PR₃.⁵²The estimated TEPs for the

biaryl monophosphines show a similar trend, with the donor

strength decreasing in the order SPHOS > XPHOS >

MePHOS > Cy-JohnPhos. A comparison of the estimated

TEPs shows the net donor power of 9a,b and 15a,b to be

similar to that of MePHOS and Cy-JohnPhos, while 2,6-

OMeKITPHOS more closely resembles SPHOS.

Palladium-Catalyzed Suzuki−Miyaura Coupling of

Aryl Chlorides. Having previously shown that biaryl-like

KITPHOS monophosphines combine with Pd(OAc)₂to form

H

dx.doi.org/10.1021/om3011992 | Organometallics XXXX, XXX, XXX−XXX

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chlorobenzonitrile and benzeneboronic acid; 0.25 mol % of

17a,b gave conversions of 86% and 88%, respectively, after 0.5

h while 16a,b only reached 59% and 57% conversion,

respectively, in the same time, whereas, comparable conversions

of 88/86% and 89/92% were obtained with 16a,b and 17a,b,

respectively, for the reaction between 2-chloroacetophenone

and benzeneboronic acid. Precatalysts 16a,b and 17a,b also

eﬃciently promoted the cross-coupling of 2- and 3-chloropyr-

idine with benzeneboronic acid, giving good conversions to the

corresponding heterobiaryl under mild conditions in short

reaction times. However, for these substrate combinations the

diﬀerence in catalyst performance between 16a,b and 17a,b was

less pronounced than for the aryl chlorides.

Table 5. Palladium-Catalyzed Suzuki−Miyaura Coupling of

Aryl and Heteroaryl Chlorides Using Precatalysts 16a,b and

a

17a,b

temp

time

(h)

conversn

b

entry

Ar

C₆H₅

R

precatalyst

(°C)

(%)

1

H

16a,b

17a,b

16a,b

17a,b

16a,b

17a,b

16a,b

17a,b

16a,b

17a,b

16a,b

17a,b

16a,b

17a,b

16a,b

17a,b

16a,b

17a,b

16a,b

17a,b

16a,b

17a,b

16a,b

17a,b

16a,b

17a,b

16a,b

50

60

50

60

70

50

60

70

80

70

0.5

1

81, 79

91, 87

59, 57

86, 88

81, 83

97, 88

51, 56

72, 80

55, 59

84, 90

88, 86

89, 92

84, 82

95, 85

70, 71

88, 78

38, 44

71, 69

74, 69

87, 75

83, 79

92, 93

81, 76

96, 89

95, 89

41, 46

76, 74

2

C₆H₅

H

3

4-CN

H

4

4-CN

H

5

2-CN

H

Electron-rich and sterically hindered coupling partners

typically required longer reaction times and/or higher temper-

atures to reach good conversions. For example, 17a,b formed

highly eﬃcient catalysts for the reaction between 4-

chloroanisole and benzeneboronic acid; the conversions of

71% and 69%, respectively, after 3 h at 70 °C were markedly

higher than those of 38% and 44% obtained with catalysts

generated from 16a,b, respectively. Similarly, 17a,b both

outperformed their dihydro-KITPHOS-based counterparts for

the coupling of 4-chlorotoluene and benzeneboronic acid,

although the diﬀerence in performance was less marked than

for 4-chloroanisole. Interestingly, precatalysts 16a,b gave

conversions of 95% and 89%, respectively, for the sterically

challenging substrate combination of 1-chloro-2,6-dimethyl-

benzene and 2-methylbenzeneboronic acid after 7 h at 70 °C,

which is a marked improvement on the conversions of 41% and

46% obtained with 17a,b, respectively, after the same time;

however, conversions of 76% and 74%, respectively, were

obtained after a reaction time of 14 h. Similarly, 16a,b and

17a,b also form eﬃcient catalysts for the Suzuki−Miyaura

coupling between aryl chlorides and 2,6-dimethoxybenzenebor-

onic acid, with 17a,b giving slightly better conversions than

16a,b across the limited range of substrates examined. Even

though catalysts based on dihydro-KITPHOS monophosphines

appear to be less eﬃcient than their biaryl-like counterparts,

good conversions have been obtained at relatively low catalyst

loadings. However, more exhaustive catalyst testing will be

required in order to establish the relative merits of these

phosphines and to begin developing a structure−eﬃciency

relationship. In this regard, we also note that while dihydro-

KITPHOS monophosphines lack the proximal phenyl ring

characteristic of biaryl-like KITPHOS monophosphines and

Buchwald biaryl monophosphines, the aromatic rings of the

anthracene-derived bicyclic architecture may be in an

appropriate spatial arrangement with respect to the phosphorus

donor to form a weak interaction to the palladium center and

thereby stabilize Pd(0) intermediates; i.e. dihydro-KITPHOS

monophosphines may combine features of electron-rich trialkyl

and biaryl monophosphines. In this regard, a hybrid phosphine

based on three tertiary alkyl groups, one of which contains a

phenyl ring which adopts a spatial arrangement with respect to

phosphorus that resembles a biaryl monophosphine, has

recently been designed and shown to be an eﬀective ligand

for the palladium-catalyzed amination of aryl chlorides.⁵⁵

6

2-CN

H

1

7

4-CHO

2-CHO

2-C(O)Me

3-py

H

1

8

H

1

9

H

1

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

H

1

H

4

H

4

H

1

3-py

H

1

2-py

H

1

2-py

H

1

4-OMe

4-Me

H

3

H

3

H

3

4-Me

H

3

4-NH₂

2,6-Me₂

H

6

H

6

H

6

H

6

2-Me

7

14

a

Reaction conditions: 1 mmol of Ar-Cl, 1.5 mmol of ArB(OH)₂, 2.0

mmol of K₃PO₄, 2.5 μmol (0.25 mol %) of 16a,b and 17a,b, 2 mL of

b

toluene. Conversions determined by GC analysis of the reaction

mixture using decane as internal standard and based on aryl chloride.

Average of three runs.

boronic acid. While these conversions were comparable to

those of 84% and 92% obtained with 16a,b, respectively, the

need for a higher catalyst loading together with 2.5 equiv of the

phosphonium salt further underpins the cost eﬀectiveness and

eﬃcacy of the palladium precatalysts. Good conversions could

also be obtained in toluene with a catalyst loading as low as

0.02 mol %, although signiﬁcantly longer reaction times were

required to reach acceptable conversions.

Having identiﬁed a set of optimum conditions and obtained

encouraging conversions, catalyst testing was extended to the

coupling of a range of aryl and heteroaryl chlorides with

arylboronic acids. Gratifyingly, 16a,b and 17a,b formed

eﬀective catalysts for the Suzuki−Miyaura coupling of a range

of activated aryl and heteroaryl chlorides with arylboronic acids

as well as unactivated and sterically challenging substrate

combinations; full details are presented in Tables 5. Each of the

precatalysts 16a,b and 17a,b gave good conversion for activated

aryl chlorides under mild conditions; however, catalysts based

on biaryl-like KITPHOS monophosphines 17a,b consistently

outperformed their dihydro-KITPHOS counterparts, albeit by

only a small margin in some cases. For example, disparate

conversions were obtained for the cross coupling between 4-

CONCLUSIONS

■

Bulky electron-rich trialkyl-type dihydro-KITPHOS mono-

phosphines have been prepared via Diels−Alder cycloaddition

between an anthracene and dicyclohexylvinylphosphine oxide;

these phosphines lack the “proximal” phenyl ring and adjacent

I

dx.doi.org/10.1021/om3011992 | Organometallics XXXX, XXX, XXX−XXX

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60F 254, and column chromatography was performed using Merck

Kieselgel 60. Gas chromatography−mass spectrometry was performed

on a Saturn 2220 GC-MS system using a factorFour VF-5 ms capillary

column (30 m, 0.25 mm, 0.25 μm), and high-resolution mass

spectrometry was conducted on a Waters Micromass LCT Premier

mass spectrometer.

alkyne-derived double bond of their biaryl-like KITPHOS

counterparts. They are highly air-sensitive and are rapidly

oxidized on silica gel during puriﬁcation, which is entirely

consistent with their trialkyl-type character. However, air-stable

phosphonium salts and borane adducts have been prepared as

well as cyclometalated palladium precatalysts of the type

[Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)C₆H₄}Cl(L)]. Most impor-

tantly, this study has revealed that biaryl-like KITPHOS

monophosphines are more practical ligands for palladium-

catalyzed Suzuki−Miyaura cross-coupling than their trialkyl-

type counterparts, since they gave higher conversions across the

majority of substrate combinations tested, albeit in some cases

by a relatively small margin, and are also air-stable, easy to

handle, available in higher overall yield, and markedly more cost

eﬀective. This study has also reinforced recent reports that the

use of precatalysts is a more practical, eﬃcient, and cost-

eﬀective approach than generating catalysts in situ from either

the phosphine or phosphonium salt and a source of Pd(II) or

Pd(0), since catalyst loadings can be reduced quite signiﬁcantly,

to achieve comparable conversions under similar conditions,

and only 1 equiv of phosphine is required; both factors could

translate into a signiﬁcant cost savings. While dihydro-

KITPHOS monophosphines lack the proximal biaryl-like

phenyl ring of KITPHOS monophosphines, the aromatic

rings of the anthracene-derived bicyclic architecture may be

able to form a weak interaction to the palladium center and

thereby stabilize Pd(0) intermediates; i.e. dihydro-KITPHOS

monophosphines may combine features of electron-rich trialkyl

and biaryl monophosphines. The steric parameters of both

classes of KITPHOS monophosphine and a selection of biaryl

monophosphines have been quantiﬁed using a combination of

Solid-G and Salerno molecular buried volume calculations

(SambVca) and the corresponding Tolman cone angles

determined from correlations. Interestingly both methods

show that the steric properties of KITPHOS monophosphines

lie within a relatively narrow range, regardless of the

substitution pattern of the aryl ring, whereas the buried

volumes of their biaryl counterparts increase quite dramatically

as a function of the biaryl substitution. Synthetic, spectroscopic,

and computational studies are now underway to (i) further

modify the KITPHOS framework in order to determine what

features are key to improving catalyst performance, with an

emphasis on understanding steric eﬀects, (ii) explore whether

the anthracene-based π system can interact with the palladium

center and stabilize the active species, and (iii) identify the

precise form of the rotamers responsible for the line broadening

in the ³¹P NMR spectra of the precatalysts.

Synthesis of Dicyclohexylvinylphosphine Oxide (10). An

oven-dried Schlenk ﬂask was cooled to room temperature under

vacuum, back-ﬁlled with nitrogen, charged with THF (20 mL) and

vinylmagnesium bromide (1.0 M in THF, 6.45 mL, 6.45 mmol), and

then cooled to −78 °C in a dry ice/acetone bath. Another ﬂame-dried

Schlenk ﬂask was charged with THF (5 mL) and chlorodicyclohex-

ylphosphine (1.45 g, 6.25 mmol), and the resulting solution was added

dropwise to the solution of vinylmagnesium bromide via cannula

transfer. The reaction mixture was warmed to room temperature and

stirred for a further 2 h. The clear colorless solution was then cooled to

0 °C and H₂O₂(35% aqueous solution, 1.76 mL, 20.0 mmol) added

dropwise. After it was stirred for 30 min, the solution was diluted with

diethyl ether (50 mL) and washed with water (50 mL) and the

aqueous layer extracted with diethyl ether (2 × 50 mL). The organic

phases were combined, dried over magnesium sulfate, and ﬁltered, and

the solvent was removed under vacuum to give the desired product as

a highly viscous colorless liquid in 83% yield (1.24 g). ³¹P{¹H} NMR

1

(161.83 MHz, CDCl₃, δ): 46.1. H NMR (500.16 MHz, CDCl_3,δ):

6.31 (m, 2H, CHCH_aH_b), 6.01 (dddd, J = 23.5, 15.8, 11.9, 4.1 Hz,

1H, CHCH_aH_b), 1.9 (m, 2H, Cy-H) 1.84−1.55 (m, 8H, Cy-H),

1.39−1.12 (m, 12H, Cy-H). ¹³C{¹H} NMR (125.76 MHz, CDCl₃, δ):

136.5 (CHCH₂), 127.0 (d, ¹J = 83.0 Hz, PCHCH₂), 34.7 (d, J =

69.2 Hz, Cy), 26.5 (d, J = 12.5 Hz, Cy), 26.3 (d, J = 12.4 Hz, Cy), 25.9

(Cy), 25.6 (Cy), 24.4 (d, J = 3.9 Hz, Cy). LRMS (ESI⁺): m/z 241 [M

+ H]⁺. HRMS (ESI⁺): exact mass calcd for C₁₄H₂₅PO [M + H]⁺m/z

241.1721, found m/z 241.1725.

Synthesis of 11-(Dicyclohexylphosphinoyl)-9,10-dihydro-

9,10-ethanoanthracene (11a). Four 25 mL Wheaton V vials were

each charged with dicyclohexylvinylphosphine oxide (0.400 g, 1.67

mmol) and anthracene (0.891 g, 5.01 mmol). The vials were sealed

and gradually heated to 220 °C. After 20 min the temperature was

lowered to 210 °C and the mixture heated for a further 12−16 h. The

resulting dark brown residue was dissolved in the minimum amount of

dichloromethane, placed on top of a silica gel column, and eluted with

dichloromethane to remove unreacted anthracene before eluting with

CH₂Cl₂/MeOH (95/5) to aﬀord 11a as a beige solid in 76% yield

(2.12 g). Mp: 177−181 °C. ³¹P{¹H} NMR (161.83 MHz, CDCl₃, δ):

1

52.8. H NMR (399.78 MHz, CDCl₃, δ): 7.40 (m, 1H, C₆H₄) 7.31−

7.24 (m, 3H, C₆H₄), 7.12−7.06 (m, 4H, C₆H₄), 4.71 (dd, J = 4.7, 1.4

Hz, 1H, bridgehead CH), 4.36 (br, 1H, bridgehead CH), 2.30 (m, 1H,

Cy-H), 2.0−0.85 (m, 24H, Cy-H + PCHCH₂). ¹³C{¹H} NMR (100.52

MHz, CDCl₃, δ): 144.9 (C₆H₄Q), 144.3 (C₆H₄Q), 142.9 (C₆H₄Q),

140.7 (C₆H₄Q), 126.3 (C₆H₄), 126.2 (C₆H₄), 125.9 (C₆H₄), 125.7

(C₆H₄), 123.9 (C₆H₄), 123.2 (C₆H₄), 122.9 (C₆H₄), 120.4 (C₆H₄),

44.1 (d, J = 3.7 Hz, bridgehead CH), 43.4 (bridgehead CH), 36.3 (d, J

= 57.0 Hz, CH), 36.2 (d, J = 60.1 Hz, CH), 35.7 (d, J = 59.1 Hz, CH),

28.8 (CH₂), 27.6 (d, J = 1.9 Hz, CH₂), 27.9 (d, J = 11.2 Hz, CH₂), 27.8

(d, J = 9.5 Hz, CH₂), 27.6 (d, J = 5.2 Hz, CH₂), 26.7 (d, J = 7.9 Hz,

CH₂), 26.2 (d, J = 3.2 Hz, CH₂), 26.1 (d, J = 3.1 Hz, CH₂), 26.0 (2 ×

CH₂), 25.8 (d, J = 2.7 Hz, CH₂). LRMS (ESI⁺): m/z 419 [M + H]⁺.

HRMS (ESI⁺): exact mass calcd for C₂₈H₃₆OP [M + H]⁺m/z

419.2504, found m/z 419.2505. Anal. Calcd for C₂₈H₃₅OP: C, 80.35;

H, 8.43. Found: C, 80.66; H, 8.79.

EXPERIMENTAL SECTION

■

General Comments. All manipulations involving air-sensitive

materials were carried out using standard Schlenk line techniques

under an atmosphere of nitrogen or argon in oven-dried glassware.

Dichloromethane and chloroform were distilled from calcium hydride,

THF and toluene from sodium, and hexane and diethyl ether from

Na/K alloy, under an atmosphere of nitrogen. Vinylmagnesium

chloride, chlorodicyclohexylphosphine, dicyclohexylphenylphosphine,

hydrogen peroxide, trichlorosilane, anthracene, and 9-methylanthra-

cene were purchased from commercial suppliers and used without

further puriﬁcation. KITPHOS monophosphine 15a,^28a

dicyclohexylphosphinoylethynyl)benzene,³²and [Pd{κ²N2′,C1-2-(2′-

Synthesis of 11-(Dicyclohexylphosphinoyl)-9-methyl-9,10-

dihydro-9,10-ethanoanthracene (11b). Compound 11b was

prepared according to the procedure described above for 11a on the

same scale (4 × 1.67 mmol) and isolated as an analytically pure pale

brown solid in 79% yield (2.29 g, 5.27 mmol) after puriﬁcation by

column chromatography. Mp: 173−175 °C. ³¹P{¹H} NMR (161.83

MHz, CDCl₃, δ): 52.1. ¹H NMR (399.78 MHz, CDCl₃, δ): 7.40 (d, J

= 8.1 Hz, 1H, C₆H₄), 7.27 (d, J = 7.9 Hz, 1H, C₆H₄), 7.23 (d, J = 8.1

Hz, 1H, C₆H₄), 7.11 (m, 4H, C₆H₄), 4.70 (d, J = 3.9 Hz, 1H,

bridgehead CH), 2.35 (m, 1H, Cy-H) 1.95 (s, 3H, CH₃) 1.90−0.82

(m, 24H, Cy-H + PCHCH₂). ¹³C{¹H} NMR (100.52 MHz, CDCl₃,

39

NH₂C₆H₄)C₆H₄}(μ-Cl)]₂were prepared as previously described.

¹H, ¹³C{¹H}, and ³¹P{¹H} NMR spectra were recorded on JEOL

LAMBDA-500 and ECS-400 instruments. Thin-layer chromatography

(TLC) was carried out on aluminum sheets precoated with silica gel

J

dx.doi.org/10.1021/om3011992 | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

δ); 146.4 (C₆H₄Q), 145.3 (C₆H₄Q), 145.2 (d, J = 11.4 Hz, C₆H₄Q),

141.1 (C₆H₄Q), 126.2 (C₆H₄), 126.0 (C₆H₄), 125.9 (C₆H₄), 125.8

(C₆H₄), 125.4 (C₆H₄), 122.6 (C₆H₄), 121.3 (C₆H₄), 120.4 (C₆H₄),

43.7 (bridgehead CH), 42.4 (d, J = 5.0 Hz, bridgehead CMe), 37.7 (d,

J = 57.2 Hz, CH), 36.3 (d, J = 60.1 Hz, CH), 36.1 (CH₂), 35.7 (d, J =

60.1 Hz, CH), 27.6 (d, J = 1.9 Hz, CH₂), 27.0 (d, J = 12.4 Hz, CH₂),

26.9 (d, J = 9.4 Hz, CH₂), 26.5 (CH₂), 26.6 (d, J = 4.5 Hz, CH₂), 26.3

(d, J = 3.1 Hz, CH₂), 26.1 (d, J = 3.1 Hz, CH₂), 26.0 (s, 2 × CH₂), 25.7

(d, J = 3.0 Hz, CH₂), 17.9 (CH₃). LRMS (ESI⁺): m/z 433 [M + H]⁺.

HRMS (ESI⁺): exact mass calcd for C₂₉H₃₈OP [M+H]⁺m/z 433.2660,

found m/z 433.2664. Anal. Calcd for C₂₉H₃₇OP: C, 80.52; H, 8.62.

Found: C, 80.64; H, 8.77.

NH₄Cl (5 mL) was slowly added. The mixture was stirred for 1 h and

then poured into water (50 mL) and extracted with dichloromethane

(3 × 50 mL). The organic phases were combined, washed with

saturated NaHCO₃(50 mL), dried over MgSO₄, and ﬁltered, and the

solvent was removed under vacuum. The product was puriﬁed by

column chromatography, with hexane/ethyl acetate (98/2) as eluent,

to aﬀord 12a as a spectroscopically pure white solid in 93% yield (0.29

g). Crystals suitable for X-ray structure determination were grown by

slow diﬀusion of hexane into a dichloromethane solution at room

temperature. Mp: 144−146 °C dec. ³¹P{¹H} NMR (161.83 MHz,

CDCl₃, δ): 33.7 (q, J = 39.2 Hz). ¹¹B{¹H} NMR (128.27 MHz,

1

CDCl₃, δ): −43.4 (d, J = 39.2 Hz). H NMR (399.78 MHz, CDCl₃,

δ): 7.40 (m, 1H, C₆H₄), 7.31−7.24 (m, 3H, C₆H₄), 7.15−7.07 (m, 4H,

C₆H₄), 4.74 (d, J = 5.4 Hz, 1H, bridgehead CH), 4.36 (br t, J = 2.1 Hz,

1H, bridgehead CH), 2.18 (m, 1H, Cy-H), 2.05 (m, 1H, Cy-H), 1.95−

0.85 (m, 23H, Cy-H + PCHCH₂), 0.4 (d, J = 98 Hz, 3H, BH₃).

¹³C{¹H} NMR (100.52 MHz, CDCl₃, δ): 145.0 (d, J = 11.4 Hz,

Reduction of 11-(Dicyclohexylphosphinoyl)-9,10-dihydro-

9,10-ethanoanthracene (11a). A ﬂame-dried Schlenk ﬂask was

charged with 11a (2.128 g, 5.09 mmol), toluene (75 mL), and

triethylamine (34.1 mL, 245 mmol). Trichlorosilane (7.5 mL 74.5

mmol) was added slowly, the ﬂask was sealed, and the mixture was

heated at 110 °C for 72 h. The reaction mixture was diluted with

diethyl ether (40 mL) and added slowly to a mixture of ice (10 g) and

20% aqueous sodium hydroxide (20 mL). After the mixture was stirred

vigorously for 30 min, the organic layer was removed and the aqueous

phase extracted with diethyl ether (3 × 30 mL). The organic fractions

were combined, washed with NaHCO₃(2 × 30 mL), water (2 × 20

mL), and brine (2 × 20 mL), dried over MgSO₄, and ﬁltered, and the

solvent was removed under vacuum to aﬀord 9a as a spectroscopically

pure beige solid in 83% yield (1.69 g). Mp: 116−118 °C dec. ³¹P{¹H}

NMR (161.83 MHz, CDCl₃, δ): 11.4. ¹H NMR (500.16 MHz, CDCl₃,

δ): 7.31−7.24 (m, 4H, C₆H₄), 7.13−7.10 (m, 4H, C₆H₄), 4.37 (br, 1H,

bridgehead CH), 4.29 (d, J = 5.9 Hz, bridgehead CH), 2.19 (m, 1H,

Cy-H), 2.00−1.01 (m, 24H, Cy-H + PCHCH₂). ¹³C{¹H} NMR

(125.76 MHz, CDCl₃, δ): 145.2 (d, J = 6.7 Hz, C₆H₄Q), 144.1

(C₆H₄Q), 143.2 (C₆H₄Q), 140.8 (C₆H₄Q), 126.1 (C₆H₄), 125.7 (2 ×

C₆H₄), 125.6 (C₆H₄), 125.1 (C₆H₄), 123.7 (C₆H₄), 122.9 (C₆H₄),

122.7 (C₆H₄), 47.6 (d, J = 16.1 Hz, bridgehead CH), 44.5 (bridgehead

CH), 33.5 (d, J = 18.1 Hz, CH), 32.9 (d, J = 18.1 Hz, CH), 32.8 (d, J =

15.3 Hz, CH₂), 31.8 (d, J = 20.0 Hz, CH), 31.6 (d, J = 4.7 Hz, CH₂),

31.5 (d, J = 7.6 Hz, CH₂), 31.4 (d, J = 6.4 Hz, CH₂), 30.9 (d, J = 7.6

Hz, CH₂), 28.1 (d, J = 9.5 Hz, CH₂), 28.0 (d, J = 7.3 Hz, CH₂), 27.8

(d, J = 9.7 Hz, CH₂), 27.8 (d, J = 8.3 Hz, CH₂), 26.7 (CH₂), 26.6

(CH₂). LRMS (ESI⁺): m/z 403 [M + H]⁺. HRMS (ESI⁺): exact mass

calcd for C₂₈H₃₆P [M + H]⁺m/z 403.2555, found m/z 403.2564. Anal.

Calcd for C₂₈H₃₅P: C, 83.54; H, 8.76. Found: C, 83.77; H, 9.03.

Reduction of 11-(Dicyclohexylphosphinoyl)-9-methyl-9,10-

dihydro-9,10-ethanoanthracene (11b). Compound 11b was

reduced according to the procedure described above for 11a on the

same scale to give 9b as a spectroscopically pure oﬀ-white solid in 79%

yield (1.63 g). An analytically and spectroscopically pure sample was

obtained by slow diﬀusion of a chloroform solution layered with

methanol at room temperature. Mp: 119−123 °C dec. ³¹P{¹H} NMR

C₆H₄Q), 144.4 (C₆H₄Q), 142.6 (C₆H₄Q), 140.4 (C₆H₄Q), 126.2

(C₆H₄), 125.4 (4 × C₆H₄), 123.7 (C₆H₄), 123.1 (C₆H₄), 122.6

(C₆H₄), 44.7 (bridgehead CH), 44.0 (d, J = 2.9 bridgehead CH), 32.3

(d, J = 24.6 Hz, CH), 31.0 (d, J = 30.6 Hz, CH), 30.7 (d, J = 28.1 Hz,

CH), 29.8 (CH₂), 27.9 (d, J = 3.0 Hz, CH₂), 27.9 (CH₂), 27.6 (CH₂),

27.2 (d, J = 11.5 Hz, CH₂), 27.1 (CH₂), 26.9 (CH₂), 26.8 (CH₂), 26.7

(d, J = 11.4 Hz, CH₂), 25.8 (CH₂), 25.7 (CH₂); Anal. Calcd for

C₂₈H₃₈BP: C, 80.77; H, 9.20. Found: C, 81.08; H, 9.39.

Synthesis of 11-(Dicyclohexylphosphino)-9-methyl-9,10-di-

hydro-9,10-ethanoanthracene−Trihydroborane (12b). Com-

pound 12b was prepared according to the procedure described

above for 12a on the same scale and isolated as a spectroscopically and

analytically pure white solid in 89% yield (0.28 g), after puriﬁcation by

column chromatography, with hexane/ethyl acetate (98/2) as eluent.

Mp: 149−153 °C dec. ³¹P{¹H} NMR (161.83 MHz, CDCl₃, δ): 32.8

(q, J = 36.8 Hz). ¹¹B{¹H} NMR (128.66 MHz, CDCl₃, δ): −43.5 (d, J

= 36.8 Hz). ¹H NMR (399.78 MHz, CDCl₃, δ): 7.40 (dd, J = 6.9, 1.5

Hz, 1H, C₆H₄), 7.32−7.24 (m, 3H, C₆H₄), 7.18−7.10 (m, 4H, C₆H₄),

4.72 (d, 1H, J = 4.6 Hz, bridgehead CH), 2.27 (m, 1H, Cy-H), 1.96 (s,

3H, CH₃), 1.85−0.85 (m, 24H, Cy-H + PCHCH₂), 0.4 (br d, J = 111

Hz, 3H, BH₃). ¹³C{¹H} NMR (100.52 MHz, CDCl_3,δ); 146.6

(C₆H₄Q), 145.5 (C₆H₄Q), 144.9 (d, J = 11.1 Hz, C₆H₄Q), 141.0

(C₆H₄Q), 126.3 (C₆H₄) 126.0 (C₆H₄), 125.9 (C₆H₄), 125.8 (C₆H₄),

125.8 (C₆H₄), 122.6 (C₆H₄), 121.3 (C₆H₄), 120.5 (C₆H₄), 45.2

(bridgehead CH), 42.5 (d, J = 2.8 Hz, bridgehead CMe), 37.3 (CH₂),

33.9 (d, J = 25.8 Hz, CH), 31.2 (d, J = 29.6 Hz, CH), 30.8 (d, J = 28.6

Hz, CH), 28.2 (2 × CH₂), 27.8 (CH₂), 27.4 (d, J = 11.4 Hz, CH₂),

27.3 (CH₂), 27.1 (CH₂), 27.0 (CH₂), 26.8 (d, J = 11.5 Hz, CH₂), 26.1

(CH₂), 26.0 (CH₂), 17.9 (CH₃). Anal. Calcd for C₂₉H₄₀BP: C, 80.92;

H, 9.37. Found: C, 81.11; H, 9.52.

Synthesis of 9,10-Dihydro-9,10-ethanoanthracenyl-11-(di-

cyclohexylphosphonium) Tetraﬂuoroborate (13a). A ﬂame-

dried Schlenk ﬂask was cooled under vacuum, back-ﬁlled with

nitrogen, charged with phosphine−borane adduct 12a (0.300 g,

0.721 mmol), and evacuated and ﬁlled with nitrogen. Dichloro-

methane (15 mL) was added, the resulting solution was cooled to 0

°C, and HBF₄·OEt₂(54 wt %, 1.46 mL, 10.8 mmol) was added

dropwise by syringe. The mixture was stirred for 30 min at 0 °C and

then warmed to room temperature (ca. 22 °C) and stirred for a further

30 min before HBF₄(aq) (48 wt %, 10 mL, 76 mmol) was added. The

resulting biphasic mixture was stirred vigorously for 20 min and

diluted with CH₂Cl₂(15 mL), and the aqueous phase was separated

and extracted with CH₂Cl₂(2 × 10 mL). The organic phases were

combined, dried, ﬁltered, and concentrated to a thick syrup, which was

added dropwise to a conical ﬂask containing rapidly stirred diethyl

ether (60 mL). The resulting solid was ﬁltered, washed with diethyl

ether, and dried under vacuum to aﬀord 13a as a spectroscopically

pure white solid in 68% yield (0.24 g). Mp: 178−183 °C. ³¹P{¹H}

NMR (161.83 MHz, CDCl₃, δ): 33.7. ¹H NMR (399.78 MHz, CDCl₃,

δ): 7.55 (d, J = 7.1 Hz, 1H, C₆H₄), 7.46 (dd, J = 5.2, 3.4 Hz, 1H,

C₆H₄), 7.36 (d, J = 7.2 Hz, 1H, C₆H₄), 7.29 (dd, J = 5.3, 3.4 Hz, 1H,

C₆H₄), 7.24−7.12 (m, 4H, C₆H₄), 5.91 (d, J = 477 Hz, 1H, PH), 4.95

(d, J = 6.1 Hz, 1H, bridgehead CH), 4.50 (s, 1H, bridgehead CH),

1

(202.46 MHz, CDCl₃, δ): 11.7. H NMR (500.16 MHz, CDCl₃, δ):

7.31−7.24 (m, 4H, C₆H₄), 7.16−7.13 (m, 4H, C₆H₄), 4.29 (d, J = 4.9

Hz, 1H, bridgehead CH), 2.30 (m, 1H, Cy-H), 1.98 (s, 3H, CH₃),

1.93−1.10 (m, 24H, Cy-H + PCHCH₂). ¹³C{¹H} NMR (125.76 MHz,

CDCl₃, δ): 146.1 (C₆H₄Q), 145.6 (d, J = 7.6 Hz, C₆H₄Q), 145.1

(C₆H₄Q), 141.2 (d, J = 1.9 Hz, C₆H₄Q), 125.9 (C₆H₄), 125.6 (C₆H₄),

125.5 (C₆H₄), 125.4 (C₆H₄), 125.1 (C₆H₄), 122.6 (C₆H₄), 121.1

(C₆H₄), 120.2 (C₆H₄), 47.9 (d, J = 15.9, bridgehead CH), 42.6

(bridgehead CMe), 40.3 (d, J = 13.4 Hz, CH₂), 33.5 (d, J = 18.1 Hz,

CH), 33.2 (d, J = 19.0 Hz, CH), 32.9 (d, J = 18.1 Hz, CH), 31.6

(CH₂), 31.5 (CH₂), 31.5 (d, J = 10.4 Hz, CH₂), 30.9 (d, J = 8.6 Hz,

CH₂), 28.2 (d, J = 9.5 Hz, CH₂), 28.1 (d, J = 6.1 Hz, CH₂), 27.9 (d, J =

9.5 Hz, CH₂), 27.7 (d, J = 6.7 Hz, CH₂), 26.8 (CH₂), 26.6 (CH₂), 18.0

(CH₃). LRMS (ESI⁺): m/z 417 [M + H]⁺. HRMS (ESI⁺): exact mass

calcd for C₂₉H₃₉P [M + H]⁺m/z 417.2711, found m/z 417.2705. Anal.

Calcd for C₂₉H₃₈P: C, 83.61; H, 8.95. Found: C, 83.82; H, 9.17.

Synthesis of 11-(Dicyclohexylphosphino)-9,10-dihydro-

9,10-ethanoanthracene−Trihydroborane (12a). A Schlenk ﬂask

containing 9a (0.300 g, 0.744 mmol) in THF (20 mL) was treated

dropwise with BH₃·SMe₂(1.0 M, 2.32 mL, 2.23 mmol) and the

resulting solution stirred for 2−3 h, after which time saturated aqueous

K

dx.doi.org/10.1021/om3011992 | Organometallics XXXX, XXX, XXX−XXX

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2.78 (br m, 1H, Cy-H), 2.45 (br m, 1H, Cy-H), 2.25 (br m, 1H, Cy-

H), 1.91−1.09 (m, 22H, Cy-H + PCHCH₂). ¹³C{¹H} NMR (100.52

MHz, CDCl_3,δ); 143.4 (C₆H₄Q), 143.7 (d, J = 14.1 Hz, C₆H₄Q),

141.5 (C₆H₄Q), 138.7 (C₆H₄Q), 127.7 (C₆H₄), 127.0 (C₆H₄), 126.9

(C₆H₄), 126.8 (C₆H₄), 125.2 (C₆H₄), 124.2 (C₆H₄), 124.0 (C₆H₄),

123.8 (C₆H₄), 45.1 (bridgehead CH), 42.3 (d, J = 2.9 Hz, bridgehead

CH), 29.6 (d, J = 12.4 Hz, CH₂), 29.4 (d, J = 15.3 Hz, CH₂), 29.3 (d, J

= 38.1 Hz, CH), 28.7 (d, J = 37.2 Hz, CH), 28.3 (d, J = 39.1 Hz, CH),

28.0 (d, J = 2.9 Hz, CH₂), 27.7 (d, J = 3.8 Hz, CH₂), 27.0 (d, J = 3.9

Hz, CH₂), 26.3 (d, J = 13.3 Hz, CH₂), 26.1 (CH₂), 25.9 (d, J = 13.4

Hz, CH₂), 25.9 (CH₂), 24.8 (CH₂), 24.7 (CH₂). LRMS (ESI⁺): m/z

403 [M]⁺. HRMS (ESI⁺): exact mass calcd for C₂₈H₃₆P [M]⁺m/z

403.2555, found m/z 403.2546. Anal. Calcd for C₂₈H₃₆BF₄P: C, 68.58;

H, 7.40. Found: C, 68.91; H, 7.68.

with diethyl ether (20 mL) and added slowly to a mixture of ice (10 g)

and 20% aqueous NaOH (20 mL). After the mixture was stirred

vigorously at room temperature for 30 min, the organic layer was

removed and the aqueous phase extracted with diethyl ether (3 × 30

mL). The organic fractions were combined, washed with saturated

NaHCO₃(2 × 20 mL), water (2 × 20 mL), and brine (2 × 20 mL),

dried over MgSO₄, and ﬁltered, and the solvent was removed in vacuo.

The product was puriﬁed by column chromatography, with hexane/

ethyl acetate (9/1) as eluent, to aﬀord 15b as a spectroscopically pure

white solid in 76% yield (0.51 g). An analytically pure sample was

obtained by slow diﬀusion of a chloroform solution layered with

methanol at room temperature. Mp: 148−151 °C. ³¹P{¹H} NMR

(202.46 MHz, CDCl₃, δ): −12.1. ¹H NMR (399.78 MHz, CDCl₃, δ):

7.47−7.34 (m, 2H, Ar-H), 7.28−7.24 (m, 5H, Ar-H), 7.05−6.99 (m,

4H, Ar-H), 6.72−6.68 (m, 2H, Ar-H), 5.40 (br, 1H, bridgehead CH),

2.01 (br m, 2H, Cy-H), 1.75−1.55 (m, 48, Cy-H), 1.40−0.90 (m, 10H,

Cy-H). ¹³C{¹H} NMR (100.52 MHz, CDCl₃, δ): 165.9 (d, J = 22.9

Hz, CCP), 147.7 (C₆H₄Q), 147.2 (C₆H₄Q), 144.6 (d, J = 21.4 Hz,

CCP), 138.7 (d, J = 6.1 Hz, C₆H₅Q), 128.9 (C₆H₅o-C), 127.6

(C₆H₅m-C), 126.9 (C₆H₅p-C), 124.4 (C₆H₄), 124.3 (C₆H₄), 122.9

(C₆H₄), 120.3 (C₆H₄), 53.8 (d, J = 5.3 Hz, bridgehead CMe), 53.7 (d,

J = 4.5 Hz, bridgehead CH), 34.2 (d, J = 9.9 Hz, Cy), 30.6 (d, J = 12.2

Hz, Cy), 30.3 (d, J = 7.6 Hz, Cy), 27.4 (d, J = 6.9 Hz, Cy), 26.5 (Cy),

26.3 (Cy), 15.2 (CH₃). LRMS (ESI⁺): m/z 491 [M + H]⁺. HRMS

(ESI⁺): exact mass calcd for C₃₅H₄₀P [M + H]⁺m/z 491.2868, found

m/z 491.2870. Anal. Calcd for C₃₅H₃₉P: C, 85.68; H, 8.01. Found: C,

85.87; H, 8.21.

Synthesis of 9-Methyl-9,10-dihydro-9,10-ethanoanthracen-

yl-11-(dicyclohexylphosphonium) Tetraﬂuoroborate (13b).

Compound 13b was prepared according to the procedure described

above for 13a, on the same scale, and isolated as a spectroscopically

and analytically pure white solid in 72% yield (0.26 g). Mp: 174−178

1

°C. ³¹P{¹H} NMR (202.46 MHz, CDCl₃, δ): 31.9. H NMR (500.16

MHz, CDCl₃, δ): 7.55 (d, J = 7.4, Hz, 1H, C₆H₄), 7.48 (d, J = 7.4 Hz,

1H, C₆H₄), 7.35 (d, J = 7.4 Hz, 1H, C₆H₄), 7.30−7.26 (m, 2H, C₆H₄),

7.22−7.14 (m, 3H, C₆H₄), 6.39 (d, J = 475 Hz, 1H, P-H), 5.90 (d, J =

475 Hz, 1H, P-H), 4.93 (d, J = 6.4 Hz, 1H, bridgehead CH), 2.87 (m,

1H, Cy-H), 2.42 (m, 1H, Cy-H), 2.01 (s, 3H, CH₃), 1.91−0.92 (m,

3H, Cy-H + PCHCH₂). ¹³C{¹H} NMR (125.76 MHz, CDCl_3,δ):

145.7 (C₆H₄Q), 143.8 (C₆H₄Q), 142.0 (d, J = 13.4 Hz, C₆H₄Q), 139.0

(C₆H₄Q), 127.7 (C₆H₄), 127.0 (C₆H₄), 126.9 (C₆H₄), 126.7 (C₆H₄),

125.2 (C₆H₄), 124.2 (C₆H₄), 121.4 (C₆H₄), 121.3 (C₆H₄), 45.3

(bridgehead CH), 42.3 (d, J = 2.9 Hz, bridgehead CMe), 36.7 (CH₂),

29.6 (d, J = 41.0 Hz, CH), 29.4 (d, J = 7.6 Hz, CH₂), 29.2 (d, J = 38.3

Hz, CH), 28.8 (d, J = 38.1 Hz, CH), 28.1 (d, J = 2.9 Hz, CH₂), 27.9 (d,

J = 3.8 Hz, CH₂), 27.8 (d, J = 3.2 Hz, CH₂), 26.4 (d, J = 13.4 Hz,

CH₂), 26.2 (CH₂), 26.1 (d, J = 12.2 Hz, CH₂), 26.0 (CH₂), 25.0

(CH₂), 24.9 (CH₂), 17.6 (CH₃). LRMS (ESI⁺): m/z 417 [M]⁺. HRMS

(ESI⁺): exact mass calcd for C₂₉H₃₈P [M]⁺m/z 417.2711, found m/z

417.2705. Anal. Calcd for C₂₉H₃₈BF₄P: C, 69.06; H, 7.59. Found: C,

69.29; H, 7.79.

[Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)C₆H₄}Cl{11-(dicyclohexylphos-

phino)-9,10-dihydro-9,10-ethanoanthracene}] (16a). To a sol-

ution of [Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)C₆H₄}(μ-Cl)]₂(0.25 g, 0.405

mmol) in acetone (8−10 mL) was added a solution of 9a (0.326 g,

0.81 mmol) in dichloromethane (5−7 mL). After the mixture was

stirred for 2 h, the solvent was removed under reduced pressure and

the resulting brown oily residue puriﬁed by column chromatography,

with CH₂Cl₂/methanol (97/3) as eluent, to give 16a as a pale yellow

spectroscopically pure solid in 72% yield (0.414 g). An analytically

pure sample that was also suitable for X-ray structure determination

was grown by slow diﬀusion of a dichloromethane solution layered

with hexane at room temperature. Mp: 186−188 °C dec. ³¹P{¹H}

NMR (161.83 MHz, CDCl₃, δ): 43.9 (br, minor isomer), 43.2 (br,

Synthesis of 11-(Dicyclohexylphosphinoyl)-9-methyl-12-

phenyl-9,10-dihydro-9,10-ethenoanthracene (14b). Four 25

1

major isomer). H NMR (399.78 MHz, CDCl₃, δ): minor + major

m L W h e a t o n

V

v i a l s w e r e e a c h c h a r g e d w i t h

isomers, 7.97 (m, 1H, Ar-H), 7.54 (m, 1H, Ar-H), 7.47 (m, 1H, Ar-H),

7.44 (d, J = 7.3 Hz, 1H, Ar-H), 7.31−7.17 (m, Ar-H), 7.08−7.17 (m,

Ar-H), 6.80 (td, J = 7.4, 1.4 Hz, 1H, Ar-H), 6.32 (br d, J = 4.6 Hz, 1H,

Ar-H), 4.84 (br d, J = 6.0 Hz, 1H, NH, major isomer), 4.74 (d, J = 6.4

Hz, 1H, bridgehead CH, minor isomer), 4.66 (br, 1H, NH, major

isomer), 4.59 (br, 1H, NH, major + minor isomer), 4.55 (d, J = 6.9 Hz,

1H, bridgehead CH, major isomer), 4.36 (br, 1H, bridgehead CH,

major isomer), 4.30 (br, 1H, bridgehead CH, minor isomer), 2.57 (q, J

= 6.3 Hz, 1H, Cy-H, minor isomer) 2.45 (q, J = 8.3 Hz, 1H, Cy-H,

major isomer), 2.19−1.9 (m, 3H, Cy-H + PCHCH₂), 1.78−0.85 (m,

19H, Cy-H + PCHCH₂), 0.62 m, 1H, Cy-H), 0.27 (m, 1H, Cy-H).

¹³C{¹H} NMR (100.52 MHz, CDCl₃, δ): minor + major isomers,

152.9 (C₆H₄Q), 152.0 (C₆H₄Q), 146.2 (d, J = 12.4 Hz, C₆H₄Q), 145.8

(d, J = 12.4 Hz, C₆H₄Q), 145.6 (C₆H₄Q), 145.4 (C₆H₄Q), 142.8

(C₆H₄Q), 142.6 (C₆H₄Q), 140.9 (C₆H₄Q), 140.5 (C₆H₄Q), 140.4 (2

× C₆H₄Q), 139.2 (2 × C₆H₄Q), 137.9 (d, J = 7.6 Hz, Ar-CH), 137.6

(d, J = 6.7 Hz, Ar-CH), 136.1 (C₆H₄Q), 136.0 (C₆H₄Q), 128.3 (Ar-

CH), 128.0 (Ar-CH), 127.9 (Ar-CH), 127.5 (Ar-CH), 127.4 (Ar-CH),

127.3 (Ar-CH), 127.2 (Ar-CH), 126.6 (Ar-CH), 126.1 (Ar-CH), 125.9

(Ar-CH), 125.7 (Ar-CH), 125.6 (3 × Ar-CH), 125.5 (Ar-CH), 125.4

(Ar-CH), 125.2 (2 × Ar-CH), 125.0 (2 × Ar-CH), 124.9 (Ar-CH),

124.8 (Ar-CH), 123.9 (Ar-CH), 123.8 (Ar-CH), 123.0 (Ar-CH), 122.7

(2 × Ar-CH), 122.6 (Ar-CH), 119.9 (Ar-CH), 46.8 (d, J = 4.8 Hz,

bridgehead CH, minor isomer), 45.7 (d, J = 5.2 Hz, bridgehead CH,

major isomer), 44.6 (d, J = 4.8 Hz, bridgehead CH, major isomer),

44.4 (d, J = 4.7 Hz, bridgehead CH, minor isomer), 36.8 (d, J = 22.9

Hz, CH), 36.0 (d, J = 27.6 Hz, CH), 35.8 (d, J = 21.0 Hz, CH), 35.3 (2

× d, J = 16.2 Hz, CH), 34.8 (d, J = 21.9 Hz, CH), 30.9 (Cy-CH₂), 30.7

(Cy-CH₂), 30.4 (Cy-CH₂), 30.3 (Cy-CH₂), 30.0 (2 × Cy-CH₂), 29.7

(dicyclohexylphosphinoylethynyl)benzene (0.500 g, 1.60 mmol) and

9-methylanthracene (0.917 g, 4.78 mmol). The vials were sealed and

gradually heated to 220 °C, and then after 20 min the temperature was

lowered to 210 °C and the mixture was heated for a further 16 h. The

resulting dark solid residue was puriﬁed by column chromatography,

with CH₂Cl₂as eluent to remove the unreacted 9-methylanthracene

and then CH₂Cl₂/ethyl acetate (3/2), to aﬀord 14b as an oﬀ-white

solid in 77% yield (2.49 g). ³¹P{¹H} NMR (161.83 MHz, CDCl₃, δ):

1

49.0. H NMR (500.0 MHz, CDCl₃, δ): 7.41 (d, J = 6.1 Hz, 2H, Ar-

H), 7.30−7.23 (m, 5H, Ar-H, 7.05−7.01 (m, 4H, Ar-H), 6.71 (dd, J =

9.6, 3.8 Hz, Ar-H), 5.6 (d, J = 7.01 Hz, 1H, bridgehead CH), 5.23 (d, J

= 2.3 Hz, 1H, bridgehead CH), 1.72 (s, 3H, CH₃), 1.70−0.90 (m,

22H, Cy-H). ¹³C{¹H} NMR (100.52 MHz, CDCl₃, δ): 164.6 (d, J =

5.1 Hz, CCP), 146.8 (C₆H₄Q), 146.0 (C₆H₄Q), 139.8 (d, J = 79.7

Hz, CCP), 137.1 (d, J = 3.5 Hz, C₆H₅Q), 127.6 (C₆H₅p-C), 127.4

(C₆H₅o-C), 127.3 (C₆H₅m-C), 124.7 (C₆H₄), 124.6 (C₆H₄), 123.4

(C₆H₄), 120.6 (C₆H₄), 54.6 (d, J = 8.6 Hz, bridgehead CMe), 51.5 (d,

J = 7.6 Hz, bridgehead CH), 37.2 (d, J = 67.1 Hz, Cy), 26.4 (d, J = 2.9

Hz, Cy), 26.3 (d, J = 3.8 Hz, Cy), 25.7 (d, J = 2.9 Hz, Cy), 25.6 (Cy),

25.3 (d, J = 2.9 Hz, Cy). LRMS (ESI⁺): m/z 507 [M + H]⁺. HRMS

(ESI⁺): exact mass calcd for C₃₅H₄₀OP [M + H]⁺m/z 507.2817,

found m/z 507.2811. Anal. Calcd for C₃₅H₃₉OP: C, 82.97; H, 7.76.

Found: C, 83.19; H, 8.03.

Reduction of 11-(Dicyclohexylphosphinoyl)-9-methyl-12-

phenyl-9,10-dihydro-9,10-ethenoanthracene (14b). A ﬂame-

dried Schlenk ﬂask was charged with 14b (0.70 g, 1.38 mmol),

toluene (25 mL), and triethylamine (7.5 mL, 53.6 mmol).

Trichlorosilane (1.35 mL, 13.4 mmol) was added slowly and the

mixture heated at 110 °C for 72 h. The reaction mixture was diluted

L

dx.doi.org/10.1021/om3011992 | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

(Cy-CH₂), 29.8 (Cy-CH₂), 29.6 (Cy-CH₂), 29.4 (Cy-CH₂), 29.2 (Cy-

CH₂), 27.9 (Cy-CH₂), 27.8 (Cy-CH₂), 27.7 (Cy-CH₂), 27.6 (2 × Cy-

CH₂), 27.5 (Cy-CH₂), 27.4 (Cy-CH₂), 27.3 (2 × Cy-CH₂), 26.2 (Cy-

CH₂), 26.0 (Cy-CH₂). LRMS (ESI⁺): m/z 676 [M − Cl]⁺. HRMS

(ESI⁺): exact mass calcd for C₄₀H₄₅NPPd [M − Cl]⁺m/z 675.2332,

found m/z 675.2339. Anal. Calcd for C₄₀H₄₅ClNPPd: C, 67.42; H,

6.36; N, 1.97. Found: C, 67.79; H, 6.45; N, 2.03.

0.23 (br, 1H, Cy-H), 0.09 (br, 1H, Cy-H). ¹³C{¹H} NMR (125.76

MHz, CDCl₃, δ): 159.4 (d, J = 22.9 Hz, CCP), 148.2 (C₆H₄Q),

145.0 (C₆H₄Q), 144.7 (C₆H₄Q), 144.4 (C₆H₄Q), 143.8 (C₆H₄Q),

140.4 (d, J = 2.0, C₆H₄Q), 140.1 (C₆H₄Q), 139.3 (C₆H₄Q), 135.9 (d, J

= 8.6 Hz, Ar), 135.2 (d, J = 2.1 Hz, C₆H₅Q), 134.9 (d, J = 29.6 Hz,

CCP), 128.7 (Ar-CH), 128.1 (Ar-CH), 127.7 (Ar-CH), 127.6 (Ar-

CH), 126.9 (Ar-CH), 126.8 (Ar-CH), 126.3 (Ar-CH), 125.2 (Ar-CH),

125.1 (Ar-CH), 125.0 (br, ArCH), 124.7 (br, Ar-CH), 123.8 (br, Ar-

CH), 122.9 (br, Ar-CH), 122.6 (br, Ar-CH), 120.5 (Ar-CH), 62.4 (d, J

= 5.3 Hz, bridgehead CH), 56.6 (d, J = 12.2 Hz, bridgehead CH), 37.4

(d, J = 21.9 Hz, Cy-CH), 36.2 (d, J = 25.3 Hz, Cy-CH), 32.4 (Cy-

CH₂), 30.8 (Cy-CH₂), 30.2 (Cy-CH₂), 29.8 (Cy-CH₂), 28.0 (Cy-

CH₂), 27.6 (Cy-CH₂), 27.5 (Cy-CH₂), 27.3 (Cy-CH₂), 26.3 (Cy-

CH₂), 25.9 (Cy-CH₂). LRMS (EI⁺): m/z 787.2 [M]⁺. Anal. Calcd for

C₄₆H₄₇ClNPPd: C, 70.23; H, 6.02; N, 1.78. Found: C, 70.67; H, 6.44;

N, 1.91.

[Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)C₆H₄}Cl{11-(dicyclohexylphos-

phino)-9-methyl-9,10-dihydro-9,10-ethanoanthracene}] (16b).

Compound 16b was prepared and puriﬁed according to the procedure

described above for 16a and isolated as a spectroscopically pure beige

solid in 77% yield (0.45 g) by slow diﬀusion of hexane into a

dichloromethane solution at room temperature. Mp: 197−199 °C dec.

³¹P{¹H} NMR (161.83 MHz, CDCl₃, δ): 45.1 (br, minor), 42.4 (br,

major) mixture of rotamers. ¹H NMR (399.78 MHz, CDCl₃, δ): minor

+ major isomers 7.99 (m, 1H, Ar-H), 7.47 (m, 1H, Ar-H), 7.33−7.18

(m, 7H, C₆H₄), 7.12−6.98 (m, 6H, C₆H₄), 6.78 (br t, J = 7.3 Hz, 1H,

Ar-H), 6.22 (br, 1H, Ar-H), 4.82 (d, J = 6.9 Hz, bridgehead CH, minor

isomer), 4.74 (br, 1H, NH), 4.62 (br, 1H, NH), 4.59 (br, 1H, NH),

4.50 (d, J = 5.5 Hz, bridgehead CH major isomer + NH), 2.68 (q, J =

9.16 Hz, 1H, Cy-H, minor isomer), 2.54 (q, J = 10.1 Hz, 1H, Cy-H

major isomer), 1.96 (s, 3H, CH₃major isomer), 1.90 (s, 3H, CH₃

minor isomer), 1.86−0.31 (m, 24H, Cy-H + PCHCH₂major + minor

isomer). ¹³C{¹H} NMR (100.52 MHz, CDCl₃, δ): 152.3 (C₆H₄Q,

minor isomer), 151.4 (C₆H₄Q, major isomer), 147.2 (C₆H₄Q, major

isomer), 146.9 (C₆H₄Q, minor isomer),146.2 (d, J = 11.4 Hz, C₆H₄Q,

minor isomer), 145.8 (d, J = 12.4 Hz, C₆H₄Q, major isomer), 144.6

(C₆H₄Q, minor isomer), 144.4 (C₆H₄Q, major isomer), 140.9

(C₆H₄Q, major isomer), 140.6 (C₆H₄Q, minior isomer), 140.4

(C₆H₄Q, minor isomer), 140.1 (2 × C₆H₄Q, major + minor isomers),

139.5 (C₆H₄Q, major isomer), 137.5 (d, J = 6.9 Hz, Ar-CH, minor

isomer), 137.3 (d, J = 6.7 Hz, Ar-CH, major isomer), 135.6 (C₆H₄Q,

major isomer), 135.5 (C₆H₄Q, minor isomer), 128.2 (2 × Ar-CH),

128.1 (Ar-CH), 127.9 (Ar-CH), 127.4 (Ar-CH), 127.3 (2 × Ar-CH),

127.1 (Ar-CH), 127.0 (2 × Ar-CH), 126.1 (Ar-CH), 125.8 (Ar-CH),

125.7 (Ar-CH), 125.6 (2 × Ar-CH), 125.5 (2 × Ar-CH), 125.3 (Ar-

CH), 125.1 (2 × Ar-CH), 125.0 (Ar-CH), 124.9 (Ar-CH), 122.6 (2 ×

Ar-CH), 121.1 (Ar-CH), 121.0 (Ar-CH), 120.1 (Ar-CH), 119.9 (2 ×

Ar-CH), 119.7 (Ar-CH), 47.4 (d, J = 3.0 Hz, bridgehead CH, minor

isomer), 45.8 (d, J = 3.8 Hz, bridgehead CH, major isomer), 45.2 (d, J

= 2.9 Hz, bridgehead CMe, minor isomer), 45.1 (d, J = 2.9 Hz,

bridgehead CH, major isomer), 37.8 (CH₂, major isomer), 37.1 (d, J =

17.1 Hz, CH, minor isomer), 37.0 (d, J = 21.9 Hz, CH, major isomer),

36.6 (CH₂, minor isomer), 36.5 (d, J = 16.2 Hz, CH, major isomer),

35.9 (d, J = 21.9 Hz, CH, major isomer), 35.8 (d, J = 21.7 Hz, CH,

minor isomer), 35.1 (d, J = 21.9 Hz, CH, minor isomer), 30.6 (Cy-

CH₂), 30.4 (Cy-CH₂), 30.2 (Cy-CH₂), 30.0 (2 × Cy-CH₂), 29.7 (Cy-

CH₂), 29.6 (Cy-CH₂), 29.2 (Cy-CH₂), 27.8 (Cy-CH₂), 27.7 (2 × Cy-

CH₂), 27.6 (Cy-CH₂), 27.5 (Cy-CH₂), 27.4 (Cy-CH₂), 27.3 (Cy-

CH₂), 27.2 (Cy-CH₂), 26.1 (2 × Cy-CH₂), 26.0 (Cy-CH₂), 25.9 (Cy-

CH₂), 18.0 (CH₃, major isomer), 17.7 (CH₃, minor isomer). LRMS

(ESI⁺): m/z 686 [M − Cl]⁺. HRMS (ESI⁺): exact mass calcd for

C₄₁H₄₇NPPd [M − Cl]⁺m/z 686.2497, found m/z 686.2480. Anal.

Calcd for C₄₁H₄₇ClNPPd: C, 67.77; H, 6.52; N, 1.93. Found: C, 67.93;

H, 6.66; N, 2.11.

[Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)C₆H₄}Cl{11-(dicyclohexylphos-

phino)-9-methyl-12-phenyl-9,10-dihydro-9,10-ethenoanthra-

cene}] (17b). Compound 17b was prepared and puriﬁed according to

the procedure described above for 16a and isolated as a spectroscopi-

cally pure light yellow solid in 83% yield. Crystals suitable for X-ray

structure determination were obtained by slow diﬀusion of hexane into

a dichloromethane solution at room temperature. Mp: 190 °C dec.

³¹P{¹H} NMR (202.46 MHz, CDCl₃, δ): 51.3. ¹H NMR (399.98

MHz, CDCl₃, δ): 7.91 (br, 1H, Ar-H), 7.51 (br, 1H, Ar-H), 7.32−7.01

(m, 15H, Ar-H), 6.98 (t, J = 7.4 Hz, 1H, Ar-H), 6.83 (br, 1H, Ar-H),

6.59 (t, J = 7.3 Hz, 1H, Ar-H), 6.20 (br, 1H, bridgehead CH), 6.00 (br,

1H, Ar-H), 4.90 (br, 1H, N-H), 4.56 (br, 1H, N-H), 2.90 (br, 1H, Cy-

H), 2.75 (br, 1H, Cy-H), 1.70 (s, 3H, CH₃), 1.66 (br m, 3H, Cy-H),

1.16−0.69 (br, 14H, Cy-H), 0.08 (br, 1H, Cy-H), −0.14 (br, 1H, Cy-

H). ¹³C{¹H} NMR (100.52 MHz, CDCl₃, δ): 160.5 (CCP), 148.7

(C₆H₄Q), 147.1 (C₆H₄Q), 146.4 (C₆H₄Q), 145.8 (C₆H₄Q), 140.1

(C₆H₄Q), 139.2 (C₆H₄Q), 137.5 (d, J = 2.9 Hz, C₆H₄Q), 137.4 (d, J =

29.6 Hz, CCP), 135.5 (C₆H₅Q), 135.3 (d, J = 5.7 Hz, C₆H₄Q),

128.3 (Ar-CH), 128.2 (Ar-CH), 129.7 (Ar-CH), 129.8 (Ar-CH) 129.7

(Ar-CH) 125.6 (Ar-CH), 125.4 (Ar-CH), 126.7 (d, J = 3.8 Hz, Ar-

CH), 126.0 (Ar-CH), 125.1 (Ar-CH), 124.9 (Ar-CH), 124.7 (Ar-CH),

124.6 (2 × Ar-CH), 124.5 (Ar-CH), 124.3 (Ar-CH), 123.4 (Ar-CH),

120.3 (Ar-CH), 120.2 (2 × Ar-CH), 120.1 (Ar-CH), 56.1 (d, J = 17.1

Hz, bridgehead CH), 55.3 (d, J = 7.0 Hz, bridgehead CH), 37.6 (d, J =

7.0 Hz, bridgehead CMe), 37.2 (d, J = 25.7 Hz, Cy-CH), 36.6 (d, J =

21.1 Hz, Cy-CH), 33.1 (Cy-CH₂), 31.7 (Cy-CH₂), 31.1 (Cy-CH₂),

30.1 (Cy-CH₂), 28.1 (d, J = 17.2 Hz, Cy-CH₂), 27.7 (Cy-CH₂), 27.6

(Cy-CH₂), 27.4 (d, J = 17.1 Hz, Cy-CH₂), 26.3 (Cy-CH₂), 25.8 (Cy-

CH₂). 15.2 (CH₃). LRMS (EI⁺): m/z 797 [M]⁺. HRMS (ESI⁺): exact

mass calcd for C₄₁H₄₇NPPd [M]⁺m/z 797.2326, found m/z 797.2336.

Anal. Calcd for C₄₇H₄₉ClNPPd: C, 70.50; H, 6.17; N, 1.75. Found: C,

70.77; H, 6.43; N, 1.88.

[Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)C₆H₄}Cl(PCy₂Ph)] (18). Compound

18 was prepared and puriﬁed according to the procedure described

above for 16a and isolated as a spectroscopically pure beige solid in

68% yield (0.160 g) by slow diﬀusion of hexane into a dichloro-

methane solution at room temperature. Mp: 168−172 °C dec.

³¹P{¹H} NMR (161.83 MHz, CDCl₃, δ): 41.5. ¹H NMR (500.16

MHz, CDCl₃, δ): 7.49 (br dd, J = 3.2, 2.9 Hz, 1H, Ar-H), 7.29−7.17

(m, 8H, Ar-H), 6.92 (t, J = 7.3 Hz, 1H, Ar-H), 6.78 (dd, J = 7.8, 4.7

Hz, 1H, Ar-H), 6.61 (t, J = 7.4 Hz, 1H, Ar-H), 4.78 (br, 2H,. NH₂),

2.37 (br, 2H, Cy-H), 2.02 (br 2H, Cy-H), 1.9−1.0 (br, 18H, Cy-H).

¹³C{¹H} NMR (125.76 MHz, CDCl₃, δ): 151.7 (Ar-CH), 140.3 (Ar-

CH), 128.3 (d, J = 9.1 Hz, Ar-CH), 135.8 (d, J = 3.1 Hz, Ar-CH),

134.3 (d, J = 9.1 Hz, Ar-CH), 129.7 (d, J = 2.4 Hz, Ar-CH), 127.9 (Ar-

CH), 127.6 (Ar-CH), 127.3 (Ar-CH), 127.2 (Ar-CH), 127.0 Ar-CH),

126.9 (Ar-CH), 126.7 (Ar-CH), 126.4 (Ar-CH), 125.9 (Ar-CH), 125.2

(Ar-CH), 124.8 (Ar-CH), 1201. (Ar-CH), 33.2 (br d, J = 25.1 Hz, Cy-

CH), 32.9 (br d, J = 23.8 Hz, Cy-CH), 29.4 (br, Cy-CH₂), 29.1 (br,

Cy-CH₂), 28.3 (br, Cy-CH₂), 28.3 (br, Cy-CH₂), 27.0 (br overlapping,

Cy-CH₂), 26.2 (Cy-CH₂). Anal. Calcd for C₃₀H₃₇ClNPPd: C, 61.65;

H, 6.38; N, 2.40. Found: C, 61.79; H, 6.67; N, 2.78.

[Pd{κ²N2′,C1-2-(2′-NH₂C₆H₄)C₆H₄}Cl{11-(dicyclohexylphos-

phino)-12-phenyl-9,10-dihydro-9,10-ethenoanthracene}]

(17a). Compound 17a was prepared and puriﬁed according to the

procedure described above for 16a and isolated as a spectroscopically

pure pale beige powder in 81% yield. Crystals suitable for X-ray

structure determination were obtained by slow diﬀusion of hexane into

a dichloromethane solution at room temperature. Mp: 188−188 °C

dec. ³¹P{¹H} NMR (202.46 MHz, CDCl₃, δ): 49.5. ¹H NMR (500.16

MHz, CDCl₃, δ): 7.82 (br, 1H, Ar-H), 7.46 (br, 1H, Ar-H), 7.34 (br,

1H, Ar-H), 7.32−7.14 (m, 11H, Ar-H), 7.02 (br, 4H, Ar-H), 6.96 (t, J

= 7.3 Hz, 1H, Ar-H), 6.58 (t, J = 7.3 Hz, 1H, Ar-H), 6.15 (t, J = 7.3 Hz,

1H, Ar-H), 6.10 (t, J = 5.5 Hz, 1H, bridgehead CH), 5.72 (t, J = 2.3

Hz, 1H, bridgehead CH), 4.86 (br, 1H, N-H), 4.53 (br, 1H, N-H),

2.84 (br, 1H, Cy-H), 2.21 (br, 1H, Cy-H), 1.64 (br, 3H, Cy-H), 1.43−

1.14 (br, 5H, Cy-H), 1.03−0.87 (br, 8H, Cy-H), 0.74 (br, 2H, Cy-H),

Preparation of Selenium Adducts. A round-bottomed ﬂask was

charged with phosphine (0.125 mmol) and methanol (0.5 mL)

M

dx.doi.org/10.1021/om3011992 | Organometallics XXXX, XXX, XXX−XXX

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Article

followed by a solution of KSeCN (0.035 g, 0.025 mmol) in methanol

(0.7 mL) and placed under a nitrogen atmosphere, and the mixture

was stirred. The reactions were typically complete within 15 min, after

which time the solvent was removed under reduced pressure, the solid

residue was extracted into chloroform (2 × 2 mL), and the extract was

ﬁltered. The products were obtained as either white solids or oils after

removing the solvent under reduced pressure and characterized by ³¹P

NMR spectroscopy only. ³¹P{¹H} NMR (202.46 MHz, CDCl₃, δ): 9a,

60.6 ppm (J_PSe= 694 Hz); 9b, 61.8 ppm (J_PSe= 696 Hz); 15a, 60.5

ppm (J_PSe= 702 Hz); 15b, 60.1 ppm (J_PSe= 702 Hz); 2,6-

OMeKITPHOS, 59.2 ppm (J_PSe= 684 Hz).

General Procedure for the Suzuki−Miyaura Coupling of Aryl

Chlorides. A ﬂame-dried Schlenk ﬂask was cooled to room

temperature under vacuum, back-ﬁlled with nitrogen, and charged

with potassium phosphate (0.424 g, 2.00 mmol), boronic acid (1.50

mmol), precatalyst (2.5 μmol, 0.25 mol %), and toluene (2.0 mL).

Aryl chloride (1.00 mmol) was added and the resulting mixture heated

at the required temperature for the allocated time. After the reaction

mixture was cooled to room temperature, decane (0.194 mL, 1.00

mmol), diethyl ether (5 mL), and water (5 mL) were added, the

mixture wasshaken vigorously, the organic layer was passed through a

short silica plug, and the solution was analyzed by gas chromatog-

raphy−mass spectrometry to determine the conversion. The solvent

was then removed under reduced pressure and the product puriﬁed by

column chromatography, with hexane/ethyl acetate (typically 5/1 v/v)

as eluent. Known products were characterized by NMR spectroscopy

and mass spectrometry and unknown products by NMR spectroscopy,

mass spectrometry, and high-resolution mass spectrometry (HRMS).

Computational Details. DFT calculations were performed on the

gas-phase molecules with the Gaussian 09 (revision B.01) suite of

programs.⁵⁶Geometries were optimized using the hybrid B3LYP

functional,⁵⁷with a LanL2DZ eﬀective core potential basis set⁵⁸for Ni

and a 6-31G(2d,p)⁵⁹basis set for all other atoms. The location of

minima was conﬁrmed by the absence of imaginary vibrational

frequencies in each case. The highest ν(CO) vibrational frequency

without any scaling factor deﬁnes the computed electronic parameter

(CEP).

X-ray Crystal Structure Determinations of 12a, 16a, 17a,b,

and 18. All data were measured on an Oxford Diﬀraction (now

Agilent Technologies) Gemini A Ultra diﬀractometer at 150 K, using

Mo Kα radiation (λ = 0.71073 Å) for compounds 12a, 17b, and 18 or

Cu Kα radiation (λ = 1.54178 Å) for compound 17a, except for

compound 16a, for which data were measured at 120 K using

synchrotron radiation (λ = 0.6889 Å) at beamline I19, Diamond Light

Source. Semiempirical absorption corrections were applied on the

basis of symmetry-equivalent and repeated reﬂections. Structures were

solved by direct methods and reﬁned on all unique F²values, with

anisotropic non-H atoms and constrained riding isotropic H atoms,

except in the case of H atoms bonded to nitrogen and boron, which

were located in a diﬀerence map and either reﬁned freely or reﬁned

with distance restraints. Programs used were Oxford Diﬀraction

CrysAlisPro^60a(12a, 17a,b, and 18) or Rigaku CrystalClear^60band

Bruker APEX2^60c(16a) for data collection, integration, and absorption

corrections, SHELXTL⁶¹for structure solution and reﬁnement, and

Olex2⁶²for graphics. Where applicable, the absolute conﬁguration was

determined using anomalous dispersion eﬀects. The structure of

compound 16a contains two independent molecules in the asymmetric

unit and was integrated and reﬁned as a pseudo-nonmerohedral twin

(for which symmetry-equivalent reﬂections cannot be merged before

reﬁnement). The structure of compound 17a contains two

independent molecules, one dichloromethane solvent molecule, and

half a hexane molecule in the asymmetric unit, the remainder being

generated by inversion symmetry. The structure of compound 17b

contains four independent molecules and three dichloromethane

solvent molecules in the asymmetric unit. An additional solvent

molecule was very disordered and was treated using the PLATON/

SQUEEZE⁶³algorithm, as it could not be successfully modeled. The

structure of compound 18 contains two independent molecules and

two toluene solvent molecules, related by a pseudo inversion center.

Details are provided in the Supporting Information.

ASSOCIATED CONTENT

■

S

* Supporting Information

Figures, tables, text, and CIF ﬁles showing the low-temperature

³¹P NMR EXSY spectrum for 16a and perspective views of the

molecular structures of 12a and 18, full details of catalyst

optimization studies, details of crystal data, structure solution

and reﬁnement, atomic coordinates, bond distances, bond

angles, anisotropic displacement parameters for compounds

12a, 16a, 17a,b, and 18, and details of atomic coordinates and

ﬁnal energies for all calculated geometries. This material is

available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

■

Corresponding Author

*E-mail for S.D.: simon.doherty@newcastle.ac.uk.

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

We gratefully acknowledge the EPSRC for a studentship

(N.A.B.W.) and an equipment grant (W.C.), Diamond Light

Source for access to synchrotron facilities, and Johnson

Matthey for generous loans of palladium salts. We acknowledge

the use of the EPSRC UK National Service for Computational

Chemistry Software (NSCCS) at Imperial College London in

carrying out this work as well as the National Mass

Spectrometry Service Centre in Swansea.

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dx.doi.org/10.1021/om3011992 | Organometallics XXXX, XXX, XXX−XXX

Article Doi

Electron-rich trialkyl-type dihydro-KITPHOS monophosphines: Efficient ligands for palladium-catalyzed suzuki-miyaura cross-coupling. Comparison with their biaryl-like KITPHOS monophosphine counterparts

DOI: 10.1021/om3011992

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