First optically active PC-palladacycle bearing a phosphorus atom in an axially chiral environment

Russian Chemical Bulletin, International Edition, Vol. 58, No. 11, pp. 2318—2335, November, 2009

2318

First optically active PCꢀpalladacycle bearing a phosphorus atom

in an axially chiral environment

P. A. Zykov,^a,bV. V. Dunina,^aM. V. Livantsov,^aI. V. Glukhov,^b

K. A. Kochetkov,^bI. P. Gloriozov,^aand Yu. K. Grishin^a

^aDepartment of Chemistry, M. V. Lomonosov Moscow State University,

3 Build., 1 Leninskie Gory, 119992 Moscow, Russian Federation.

Fax: + 7 (495) 932 8846. Eꢀmail: dunina@org.chem.msu.ru

^bA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,

28 ul. Vavilova, 119991 Moscow, Russian Federation

Direct aromatic C—H bond activation in the (S_a)ꢀBINOLꢀderived phosphite (S_a)ꢀHL

2

afforded the dimeric cyclopalladated complex (S_a,S_a)ꢀ{Pd(η ꢀL)(μꢀCl)}₂(2) which is the first

optically active PCꢀpalladacycle bearing a phosphorus atom in an axially chiral environment.

orthoꢀPalladated structure of dimer 2 was confirmed by spectral (¹H and ³¹P NMR) examinaꢀ

tion of its mononuclear derivatives and by Xꢀray diffraction analysis of the phosphine adduct

2

(η ꢀL)PdCl(PPh₃) (4). The enantiomeric purity of the starting ligand remained unchanged in

the PCꢀpalladacycle under the thermal conditions used for the cyclopalladation (∼110 °C); this

fact was confirmed by the ³¹P NMR spectroscopy after chiral derivatization in situ of dimer

(S_a,S_a)ꢀ2 with the (R_C)ꢀvalinate chiral auxiliary (Val). trans(N,C)ꢀConfiguration of the valinate

2

1

complex (η ꢀL)Pd(Val) (5) was established by H NMR and supported by DFT calculations.

The chirality transfer in the new PCꢀpalladacycle was discussed on the basis of Xꢀray diffracꢀ

tion data for the phosphine adduct racꢀ4 and DFT calculations performed for both phosphine

and valinate mononuclear derivatives.

Key words: C—H bond activation, cyclopalladated phosphites, optically active PCꢀpalladaꢀ

cycles, enantiomeric purity, geometric isomerism, axial chirality, chirality transfer, DFT.

Transition metal coordination complexes with monoꢀ¹

Scheme 1

and bidentate²Pꢀdonor ligands, atropoisomeric or bearꢀ

ing axially chiral moieties, are widely known as highly

effective enantioselective catalysts. In particular, phosꢀ

phites,³phosphoramidites^3e,4and other Pꢀdonor ligands⁵

derived from commercially available enantiomerically

pure diol BINOL⁶or its analogs provide up to 99.9% ee

selectivity in the asymmetric catalysis of various transforꢀ

mations including palladium catalyzed reactions.^3h,4d,5c,7

Monodentate ligands of the above type often prove more

effective than bidentate ones, affording optical yields

exceeding 99% ee.^{3a,b,d,e,k—m,4a,b,5b,7e}A number of processes

catalyzed by coordination compounds of iridium and

palladium were found^8a,bor supposed^8сto be mediated

by the appropriate phosphametallacycles. In particular,

for the allylic amination involving phosphoramidite HL´,

the cyclometalated derivative of the latter, I formed in situ

under catalysis conditions was shown^8a,bto be a true

catalyst (Scheme 1). These results may contribute to the

explanation of the high enantioselectivity exhibited by

monodentate Pꢀdonor ligands of this type.

B is base, cod is cyclooctaꢀ1,5ꢀdiene

metric catalysis with the BINOLꢀderived transition metal

complexes with monodentate phosphites,³cyclopalladated

derivaives of such chiral ligands remained unknown until

this time. Only recently there have emerged the reports¹⁰

on synthesis and application in catalysis of several РСPꢀ

Despite the high catalytic activity of achiral phosphite

palladacycles⁹and the excellent results achieved in asymꢀ

Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2247—2264, November, 2009.

Axial chirality in phosphite phosphapalladacycles

Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009 2319

pincer cyclopalladated complexes (CPCs) based on bisꢀ

phosphites with 1,3ꢀphenylene backbone derived from

binaphthol (II, R = H, Me, Cl, SMe, SEt, SPh; R´ = H,

Bu^t) or biphenanthrol (III).

maximum efficiency.^12a,cInvolvement of the related

РСꢀpalladacycles in catalysis is strongly implicated by the

structure of the ligands optimal for these systems (P(OAr)₃,

Ar = Ph, 2,4ꢀBu^t₂C₆H₃) and the reaction conditions.

We found attractive an idea of combining high enantioꢀ

selection potential of BINOL derivatives with high

catalytic activity of phosphite PCꢀpalladacycles. Our speꢀ

cific target was to develop synthetic routes employing

direct phosphite cyclopalladation to yield enantiomerically

pure phosphite РСꢀpalladacycle bearing a phosphorus

atom in an axially chiral environment, as well as to estiꢀ

mate the effectiveness of chirality transfer from its origiꢀ

nal source to the reaction center. Preliminary results of

this work were reported previously.¹³

(S_a,S_a)ꢀII

Results and Discussion

Synthesis and cyclopalladation of phosphite HL. Steric

promotion of intramolecular C–H bond activation is

widely practiced in cyclopalladation chemistry.¹⁴Thereꢀ

fore we have chosen for cyclopalladation a rather bulky

phosphite ligand, (S_a)ꢀHL. The racemic and enantioꢀ

merically pure phosphites HL were synthesized by an

adaptation of the known procedure^10e,14c,15based on

reacting phosphachloridite 1 formed in situ from BINOL

(see Ref. 4q) with 2,4ꢀdiꢀtertꢀbutylphenol in the presence

of a base (Scheme 2).

(S_a,S_a)ꢀIII

Phosphite HL is relatively stable to oxidation and

hydrolysis: after 1ꢀweek storage in air of its chloroform

solution, the content of decomposition products did not

Except for these РСРꢀcomplexes, known are only two

optically active phosphite PCꢀpalladacycles with the

distant stereocenters (IV and V).¹¹

Scheme 2

(S_C)ꢀIV

(S_a)ꢀ1

(8S_C,9S_C,13S_C,14S_C)ꢀV

Achiral monophosphites are extremely rarely used in

Pdꢀcatalyzed reactions generating С—С and С—heteroꢀ

atom bonds.¹²It was a fairly new finding that the replaceꢀ

ment of σꢀdonating phosphines by πꢀacceptor phosphites

in such reactions provides high regioselectivity^12beither

(S_C)ꢀHL

i. PCl₃, Et₃N, toluene (35 mL), from –70 to 25 °С, 2 h; ii. Et₃N,

2,4ꢀBu^t₂C₆H₃OH, toluene (5 mL), from –70 to 25 °C, 13 h.

2320

Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009

Zykov et al.

exceed 10% (by ³¹Р NMR). Using of HL ligand in catalyꢀ

sis was reported before,^8chowever, since no comment was

made on its synthesis and no spectral or other characterꢀ

ization data were provided, HL can be regarded as a new

member of an extensive family of ArOꢀsubstituted phosꢀ

phites based on BINOL with Ar = Ph,^3m2ꢀNp,^3c

2ꢀBrC₆H₄,^3m4ꢀMeOC₆H₄,¹⁶4ꢀCF₃C₆H₄,¹⁷2,6ꢀMe₂C₆H₃,^3m

2,6ꢀPh₂C₆H₃,^3mand 2,6ꢀBu^t₂ꢀ4ꢀMeC₆H₂.¹⁸

Scheme 3

There are two methods for cyclopalladation of achiral

phosphites (HL´). For a long time it was effected by a

twoꢀstage procedure involving isolation of the interꢀ

mediate coordination complex Pd(HL´)₂Cl₂with its

following thermolysis in the presence of additional molar

equivalent of PdCl₂.^14bThe yields of thus obtained

PCꢀpalladacycles vary from moderate¹⁹to rather high.^11a,20

The more recent oneꢀstep route is based on reacting

palladiumꢀcontaining reagents (usually PdCl₂) with phosꢀ

phites HL´ (1 : 1) in 2ꢀmethoxyethanol^14cor toluene^14c,15

under reflux. This method allows obtaining cyclopalꢀ

ladated derivatives of simple triarylphosphites in high

yields (up to 98%)^14c,21yet affording only moderate yields

(43—53%) in the case of bulky ligands incorporating

sevenꢀ or eightꢀmembered heterocycles, [1,3,2]ꢀdioxaꢀ

phosphepine^10eor [1,3,2]ꢀdioxaphosphocene.¹⁵

With the foregoing in view, we expected that cycloꢀ

palladation of phosphite HL could be difficult. Indeed,

the twoꢀstep procedure turned out to be nonproductive as

it was accompanied by palladium black precipitation and

yielded a multicomponent product mixture even at the

stage leading to coordination complex (Pd : HL = 1 : 1,

CH₂Cl₂, 25 °C). The similar problems were encountered

when exploiting the oneꢀstep procedure with PdCl₂as a

palladating agent (Pd : HL = 1 : 1): the synthesis in the

refluxing 2ꢀmethoxyethanol proceeded with intense forꢀ

mation of Pd⁰, yielding the target CPC in only trace

amounts.²²Also unsuccessful was an attempt of using more

electrophilic Pd(OAc)₂— reactions in MeCN were

dominated by intense palladium black precipitation which

occurred already at room temperature.

(S_a,S_a)ꢀ2

purification considerably. The problem has been solved

by in situ converting insoluble dimer racꢀ2 into its well

soluble mononuclear pyridine derivative, PdCl(η²ꢀL)(Py)

(3), which was further purified chromatographically. Preꢀ

viously²³we have already demonstrated the possibility

of regenerating dimeric CPCs from their mononuclear

derivatives on silica. In the present case, strong trans

influence of the carbanion and electronꢀdonating phosꢀ

porus atom of PCꢀpalladacycle facilitated the elimination

of the auxiliary pyridine ligand from adduct racꢀ3 appreꢀ

ciably, which allowed isolation of the purified dimer

racꢀ2 under conditions of column chromatography

(Scheme 4).

Scheme 4

We could synthesize CPC in a moderate yield only

by reaction of HL with PdCl₂(NCPh)₂in the refluxing

toluene (Scheme 3). The racemic and optically active

dimers, racꢀ2 and (S_a,S_a)ꢀ2 were obtained in 53 and 40%

isolated yields, respectively (after chromatograpihc puriꢀ

fication). In the similar reaction conducted in 1,2ꢀdiꢀ

chloroethane, the yield of dimer racꢀ2 has dropped to

37%, while the attempts to induce C—H bond activation

by introduction of Et₃N as a base^10еfailed because of the

rapid palladium(^II) reduction under these conditions.

A slightly higher yield of a racemic dimer racꢀ2 (53%)

compared with enantiomerically pure dimer (S_a,S_a)ꢀ2

(40%) is probably due to the extremely low solubility of

the former, owing to which the equilibrium is shifted

towards palladacycle formation. This feature of racꢀ2

useful as it was at the synthesis stage, complicated its

Spectral study of dimeric CPC cannot provide the full

information, being restricted by dynamic mobility of these

species²⁴and their occurrence as a mixture of syn/anti

and meso/dl isomers (in case of racemic compounds).

Thus dimer racꢀ2 was converted to its mononuclear phosꢀ

phine derivative racꢀ4 via the chloride bridge splitting

reaction (Scheme 5).

Another problem was associated with the possible

1,1´ꢀbinaphthyl moiety racemization upon thermal actiꢀ

vation of the C—H bond in HL ligand. The BINOLꢀ

derived bisꢀphosphites were demonstrated^10bto racemize

partially at temperatures exceeding 60 °C. Therefore, most

pincer PCPꢀcomplexes of II and III types were syntheꢀ

Axial chirality in phosphite phosphapalladacycles

Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009 2321

Scheme 5

such processes are obviously unfeasible for the HL related

BINOL derivatives with the sevenꢀmembered dioxaphosꢀ

phepine ring.

Anyhow it was necessary to check if enantiomeric

purity of (S_a)ꢀBINOL is retained in the palladacycle

(S_a)ꢀ2 formed under thermal conditions applied for C—H

bond activation in HL ligand (toluene, 110 °C). It should

be noted that much more severe conditions were used

for direct C—H bond activation in the synthesis of one of

the pincer complexes of type II (150 °C, microwave

activation).^10eHowever, the cited communication gives

no data on the enantiomeric composition of reaction prodꢀ

ucts, hence, the question about applicability of direct

C—H bond activation for producing optically active PCPꢀ

complexes of that type remained unanswered.

Enantiomeric purity of dimer (S_a,S_a)ꢀ2 was determined

by ³¹P NMR after its chiral derivatization in situ with the

(R_C)ꢀvalinate auxiliary ligand (Scheme 6).

Scheme 6

Reaction conditions: PPh₃, PhCH₃, 25 °C, 1 h.

sized using a rather long and tedious route, i.e., through

the functionalization of the ligands followed by their

oxidative addition to zerovalent palladium.^10a—d

A priori the possibility of 1,1´ꢀbinaphthyl moiety raceꢀ

mization under so mild conditions can be excluded on the

basis of the following arguments. First, this is inconsistent

with the high optical yields (up to 99.8% ee) observed in

enantioselective reactions catalyzed by transition metal

complexes with BINOLꢀderived phosphites^3fand phosꢀ

phoramidites^4c,f,g,7d,25under the same or more severe temꢀ

perature conditions (up to 120 °C). Second, it is known

that a partial racemization of parent BINOL through

rotation about C(1)—C(1´) bond requires more severe

(S_a,S_a)ꢀ2

conditions (τ^1/2

= 60 min at 220 °C),²⁶whereas its

rac

monoacylated derivatives racemize only at 275—280 °C.²⁷

Third, the quantum chemical estimation yielded rather

high racemization barriers for diol BINOL²⁶and its

monoacylated derivatives²⁷(ΔG^#

= 158 kJ mol^–1and

rac

(S_a,R_C)ꢀ5a

E_a= 159—165 kJ mol^–1, respectively). Lastly, a mechaꢀ

nistically alternative route to binaphthyl moiety racemꢀ

ization that was discussed in the literature is the interaction

between the 2,2´ꢀderivatives resulting in a fiveꢀmembered

ring formation. For example, this type of racemization

proceeding via the fiveꢀmembered ССꢀpallada(^IV)cycle

was suggested as the mechanism accounting for the loss

of enantiomeric purity by 2,2´ꢀdiiodoꢀ1,1´ꢀbinaphthyl

under conditions of Suzuki reaction.²⁸The similar raceꢀ

mization mechanism can be supposed for monoacylated

BINOL derivatives undergoing Newman—Quote rearꢀ

rangement,²⁷in which case quite probable is the formation

of an intermediate bearing dinaphthofuran or dinaphthoꢀ

thiophene fiveꢀmembered cycle closed through C(2) and

C(2´) atoms. However, under relatively mild conditions

Reaction onditions: (R)ꢀValNa, CHCl₃, 25 °C, 4 h.

The similar reaction with the racemic dimer 2 was

used to yield the mixture of two diastereomers of the

mononuclear (R_C)ꢀvalinate derivative, (S_a,R_C)ꢀ5a and

(R_a,R_C)ꢀ5b. The ³¹P NMR spectra of an individual

(R_C)ꢀvalinate derivative, (S_a,R_C)ꢀ5a, as well as the mixꢀ

ture of diastereomers (S_a,R_C)ꢀ5a/(R_a,R_C)ꢀ5b prepared from

dimers (S_a,S_a)ꢀ2 and racꢀ2, respectively were measured

in situ, prior to any purification procedures. The spectrum

of (S_a,R_C)ꢀ5a shows only one singlet at δ 147.96. In the

spectrum of the mixture of diastereomers (S_a,R_C/R_a,R_C)ꢀ5a,b

there are two closely located singlets at δ 147.73 and 148.05

(Δδ 0.32). However, such resolution is quite sufficient to

2322

Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009

Zykov et al.

Table 2. The DFT calculated and experimental values of

confirm completely retained enantiomeric composition

of the starting (S_a)ꢀBINOL in dimer (S_a,S_a)ꢀ2. Thus we

have proved the possibility of direct cyclopalladation of

chiral ligands based on BINOL without loss of enantioꢀ

meric purity.

chemical shifts (δ) in the ¹H NMR spectrum of valinate

derivative (S_a,R_C)ꢀ5a

Proton

PBE/TZ2P

Experiment

Spectral studies of cyclopalladated complexes. Signal

assignments in the H NMR spectra of HL ligand and

NH^ax

1.57

1.27

0.53

0.78

8.73

7.26

7.81

8.24

7.65

7.56

8.22

7.75

7.83

8.23

8.22

7.77

7.60

7.77

1.32

1.37

2.72

3.22

2.10

1.69

0.94

1.16

7.83

7.23

7.48

8.09

7.38—

—7.43

8.04

7.57

7.72

8.17

8.06

7.63

7.46

7.53

1.27

1.33

2.47

3.40

1

NH^eq

CPCs 2—5 were based on homoꢀ and heteronuclear

decoupling experiments, COSY and NOE techniques, and

also quantum chemical calculations of the structure

and ¹H NMR spectra for complexes racꢀ4 (Table 1),

(S_a,R_C)ꢀ5a (Table 2), and (R_a,R_C)ꢀ5b.

The chemical shift of HL ligand in ³¹P{¹H} NMR

spectrum (δ 145.1) lies in the range δ 143.5—148.4 typical

of the known monoꢀ^16,18and bisꢀphosphites with

1,1´ꢀbinaphthyl fragment.^10d,e,16,29The relatively large

δ_Pvalues produced by these ligands are most probably

attributable to their cyclic dioxaphosphepine structure.

For comparison, this shift value increases from δ ∼127

for the simplest phosphite P(OPh)₃(see Ref. 30) to

δ 132.9—134.0 when we move to the analogs containꢀ

ing eightꢀmembered dioxaphosphocane ring.^14c,15The

¹Н NMR spectrum of HL ligand is in full agreement with

its presumed structure.

The ³¹P{¹H} NMR spectrum of a racemic dimer 2

contains four broadened singlets in the narrow range at

δ 138.45 and 138.54 (86%), 140.75 and 140.81 (14%).

The two of them, found in the spectrum of optically

active dimer (S_a,S_a)ꢀ2 (δ 138.43 and 140.68) must be

assigned to its anti and sin isomers, respectively. The

chemical shift for dimer 2 in the ³¹P{¹H} spectrum falls

Me

H(6″)

H(4″)

H(3/3´)

H(4/4´)

H(8/8´)

H(7/7´)

H(5/5´)

H(6/6´)

H(3´/3)

H(4´/4)

H(5´/5)

H(6´/6)

H(7´/7)

H(8´/8)

Bu^t

CHMe₂

αꢀCH

out from the δ range 115—125 characteristic for its anaꢀ

logs with an acyclic moiety P(OAr)₂^14c,20,21bor with a less

sterically strained eightꢀmembered dioxaphosphocane

Рꢀdonor.^14c,15It is possible that the significantly larger

chemical shift in the ³¹P{¹H} NMR spectrum of dimer 2

results from strongly distorted bond angles in the molꢀ

ecule. As the chemical shifts δ_Pfor monodentate phosꢀ

phites P(OAr)₃in the spectra of their coordination comꢀ

plexes of Pd(HL´)₂Cl₂type are typically found at the much

lower field (δ 81—87)^14c,20,31as compared with the signals

from free ligands, the shift produced by dimer 2 can be

indicative of its cyclopalladated structure.

As the ¹Н NMR spectrum of dimer 2 is unsuitable for

structural characterization (see above), to identify the

structures of the new complexes, we examined spectrally

mononuclear derivatives of 2.

The ³¹P{¹H} NMR spectrum of a mononuclear adduct

racꢀ4 displays two doublets at δ 149.9 and 17.66, correꢀ

sponding to the phosphorus atoms of PCꢀpalladacycle

(P(1)) and of an auxiliary phosphine ligand, PPh₃(P(2)).

Table 1. The DFT calculated and experimental values

of chemical shifts (δ) in the ¹H NMR spectrum of phosꢀ

phine adduct racꢀ4

Proton

PBE/TZ2P

Experiment

orthoꢀH (PPh₃)

metaꢀH (PPh₃)

paraꢀH (PPh₃)

H(6″)

7.68

7.17

7.14

9.27

7.37

7.80

8.16

8.13

7.65

7.40

7.41

7.81

7.92

7.97

7.61

7.38

7.31

1.18

1.35

7.35—7.41

7.01—7.08

7.10—7.15

8.47

H(4″)

7.17

7.38

7.58

7.84

H(3/3´)

H(4/4´)

H(5/5´)

H(6/6´)

H(7/7´)

H(8/8‘)

H(3´/3)

H(4´/4)

H(5´/5)

H(6´/6)

H(7´/7)

H(8´/8)

Bu^t

7.48—7.52

7.22—7.28

7.21

2

The coupling constant J_P,P= 45.2 Hz unequivocally

7.46

8.03

7.98

indicates the cisꢀ(P,P) geometry of the complex.³²

cisꢀ(P,P)ꢀCoordination of Pꢀdonor ligands is typical of

the most phosphite palladacycles;^{14c,19b,20,21b,33}examples

of cis/trans isomer mixture formation are extremely

rare.^9c,14c

7.50—7.54

7.33

7.22—7.28

1.14

1

The H NMR spectra of phosphine adduct racꢀ4 are

Bu^t

1.40

consistent with its orthoꢀpalladated structure completely.

Axial chirality in phosphite phosphapalladacycles

Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009 2323

The metalation at the orthoꢀC—H bond of 2,4ꢀdiꢀtertꢀ

butylꢀsubstituted phenyl ring of HL ligand follows from

the presence of only two 1H signals at δ 8.57 (ddd) and

obtained by the quantum chemical simulation of the

¹H NMR spectrum of the transꢀ(N,C)ꢀisomer of adduct

(S_a,R_C)ꢀ5а: δ_H1.27 and 1.57 for NH^eqand NH^axprotons,

respectively (see Table 2).

3

7.18 (dd) without J_H,Hcoupling. The correspondence of

these signals to H(6″) and H(4″) protons, respectively,

was concluded on the following grounds. First, the lowꢀ

field shift of the first signal (see Table 1) is indicative of

the chloride anisotropy influence: the DFT and Xꢀray

Identification of the geometric configuration of a

valinate complex (S_a,R_C)ꢀ5a presented a difficult probꢀ

lem. The spectral data only provide an indirect indication

in favor of the transꢀ(N,C) configuration. First, the lowꢀ

field shift of the H(6″) proton signal compared to that of

the H(4″) proton (δ 7.87 and 7.23, respectively) points

toward the influence of the carboxylate oxygen anisotropy

...

diffraction yielded the H(6″) Cl separation of 2.49 and

2.66 Å, respectively. Second, a useful information is proꢀ

vided by the difference in the spinꢀspin coupling conꢀ

stants of these protons with two phosphorus atoms: both

protons show coupling with the PCꢀpalladacycle phosꢀ

...

(Fig. 1). The DFT calculated H(6″) O(CO) distance is

2.31 Å (the sum of van de Waals radii of these atoms being

5

1

phorus P(1) with J_H,P(1)of 5.6 and 2.6 Hz, respectively,

∼2.6 Å).³⁴The Н NMR spectral parameters calculated

but only the H(6″) proton has a rather strong coupling

with the P(2) atom of auxiliary ligand PPh₃(⁴J_H,P(2)= 8 Hz).

The J_H,Pconstants were assigned by the ¹Н {³¹P} selective

double resonance technique.

for transꢀ(N,C)ꢀ(S_a,R_C)ꢀ5a isomer predict even more proꢀ

nounced carboxylate group anisotropy influence on the

H(6″) proton and a larger lowꢀfield shift of its signal from

the signal of H(4″) proton (δ_H(DFT) 8.73 and 7.26, respecꢀ

...

The assignment of signals from 1,1´ꢀbinaphthyl moiꢀ

ety was based on H(3)/H(3´) resonances (δ 7.38 and 7.46)

tively). Second, since the DFT calculated H(6″) NH^eq

distance for alternative cisꢀ(N,C) configuration is only

2.22 Å, it was reasonable to expect significant dipoleꢀ

dipole interaction between the H(6″) proton of the

palladated phenyl ring and the equatorial valinate NH

proton (see Fig. 1). However, the NOE experiments have

not shown any interaction between NH^eqand H(6″) proꢀ

tons. It is noteworthy that in the case of transꢀ(N,C)ꢀ

isomer, the shortest interꢀligand contact between the

4

with the coupling constants J_H,P(1)0.9 and 1.2 Hz, reꢀ

spectively. To assign H(5)/H(5´) (δ 7.84) and H(4)/H(4´)

(δ 7.59) proton signals we have conducted the NOE

experiments where irradiation at H(5)/H(5´) resonance

frequency resulted in the responses from H(4)/H(4´) proꢀ

tons and vice versa, with 4.8 и 3.9% efficiency, respecꢀ

...

tively. Note that the DFT estimate of the H(5) H(4)

...

distance was 2.44 Å.

amino group and 1,1´ꢀbinaphthyl moiety is H(3´) NH^ax

The similar procedure was used for assigning the

signals in the Н NMR spectrum of valinate derivative

with a separation of 3.25 Å (DFT) which is too large for

observing dipoleꢀdipole interaction.

1

(S_a,R_C)ꢀ5a. To distinguish between two diastereotopic proꢀ

The transꢀ(N,C) configuration of adduct (S_a,R_C)ꢀ5a is

preferred also due to the “transphobia” effect.^33a,35Strucꢀ

tural trans influence (STI) of the primary amino group is

somewhat stronger than that of the carboxylate oxygen,^33a

while STI of the phosphite Рꢀdonor atom is comparable

or even surpassing STI of the aromatic carbanion center.^33a

As a result, the trans position of the two softer donor

atoms (N and P) must destabilize the molecule to some

extent. Besides that, transꢀ(N,C) coordination of the

aminoacidate ligands is consistent with the abnormal

tons of the valinate NH₂group we correlated the coupling

3

constants J_{HNC H}(4.4 and 7.2 Hz, respectively) to

α

dihedral angles H—N—C_α—H (+44.67 and +161.84°,

respectively) for NH^eqand NH^axprotons. The dihedral

angles were taken from the DFT calculated structural

data for the transꢀ(N,C)ꢀisomer of adduct (S_a,R_C)ꢀ5а. The

additional evidence for the signals at δ 1.69 and 2.10

generated by NH^eqand NH^axprotons, respectively, comes

from the similarity of their spectral pattern with that

cisꢀ(N,C)ꢀ(S_a,R_C)ꢀ5a

transꢀ(N,C)ꢀ(S_a,R_C)ꢀ5a

Fig. 1. Mutual disposition of valinate NH protons and aromatic protons of РСꢀpalladacycle in the two geometric isomers of valinate

derivative (S_a,R_C)ꢀ5a.

2324

Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009

a

Zykov et al.

b

transꢀ(S_a,R_C)ꢀ5a

cisꢀ(S_a,R_C)ꢀ5a

c

d

transꢀ(R_a,R_C)ꢀ5b

cisꢀ(R_a,R_C)ꢀ5b

Fig. 2. The PBE/TZ2P optimized structures of most stable conformers of transꢀ(N,C)ꢀ (a, c) and cisꢀ(N,C)ꢀisomers (b, d) for two

diastereomers of valinate derivative 5, (S_a,R_C)ꢀ5a and (R_a,R_C)ꢀ5b.

(i) For both diastereomers, (S_a,R_C)ꢀ5a and (R_a,R_C)ꢀ5b,

the transꢀ(N,C) configuration is more energetically

favorable than cisꢀ(N,C) configuration, ΔE° = 2.43 and

2.57 kcal mol^–1, respectively.

(ii) The valinate chelate ring in both (S_a,R_C)ꢀ5a and

(R_a,R_C)ꢀ5b diastereomers shows preference for λ(R_C)

conformation with equatorial orientation of the isopropyl

group at carbon stereocenter.³⁶

(iii) Nonplanarity of PCꢀpalladacycle is strongly

dependent on the metal coordination sphere geometry:

in (S_a,R_C)ꢀ5a and (R_a,R_C)ꢀ5b complexes with cisꢀ(N,C)

configuration, PCꢀpalladacycle is markedly distorted

(ϖ_av= 11.93 and 9.52°, respectively), whereas in transꢀ

(N,C) isomers it retains an essentially planar structure

(ϖ_av= 0.71 and 2.25°, respectively).

(compared to CNꢀsystems) transꢀ(P(2),C) regiochemistry

of the coordination of auxiliary phosphine ligands with

the phosphite PCꢀpalladacycles.^33a

Quantum chemical calculation of the structure of

valinate adducts 5a,b. To verify the conclusions based

on the spectral data, we performed quantum chemical

calculations of the structures of transꢀ(N,C) and cisꢀ(N,C)

isomers for each of valinate adduct diastereomers,

(S_a,R_C)ꢀ5a and (R_a,R_C)ꢀ5b, for the gas phase conditions

and without account for the solvation effect. Fully optiꢀ

mized geometries of these structures are presented in

Fig. 2 and their selected parameters* are collected in

Table 3.

The comparison of the parameters for the four isomers

has led to the following conclusions.

(iv) Only cisꢀ(N,C) isomers of diastereomers (S_a,R_C)ꢀ5a

and (R_a,R_C)ꢀ5b show considerable distortions of PCꢀ

palladacycles. The latter adopt the conformation which

can be described as an envelope with the apical phosphoꢀ

rus atom. The envelopes are slightly twisted with regard to

* Definitions for the parameters used in the discussion of conꢀ

formation of N,Oꢀchelate valinate cycle and PCꢀpalladacycle

are given below in the section describing Xꢀray characterization

of racꢀ4.

Axial chirality in phosphite phosphapalladacycles

Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009 2325

Table 3. Selected parameters of the geometric isomers of two diastereoisomers of valinate derivative 5, (S_a,R_C)ꢀ5a and (R_a,R_C)ꢀ5b (DFT)

Isomer

ΔE°

N,OꢀChelate ring^a

PCꢀPalladacycle

/kcal mol^–1

b

ϖ

O—N—C_α—C(O) angle/deg

ϖ

C(1)—P—O—C(2) angle/deg

Conformation

av

Diastereomer (S_a,R_C)ꢀ5a

c

transꢀ5a

cisꢀ5a

0.10

2.43

18.55

17.62

+10.97

+10.61

0.71

11.93

+0.60

+4.64

—

λ

Diastereomer (R_a,R_C)ꢀ5b

c

transꢀ5b

cisꢀ5b

0.00

2.57

16.87

17.30

+10.03

+12.83

2.25

9.52

–1.43

–3.62

—

δ

^aN,OꢀChelate ring in all isomers exists in the λꢀconfiguration with Prⁱgroup positioned equatorially.

^bNonplanarity of N,Oꢀchelate ring and PCꢀpalladacycle is characterized by the average of the absolute value of the intraꢀchelate

torsion angles (ϖ_av/deg).

^cNonꢀtwisted PCꢀpalladacycle conformation is assumed.

basal planes with C(1)—P—O—C(2) torsion angles of

+4.64 and –3.62° for (S_a,R_C)ꢀ5a and (R_a,R_C)ꢀ5b, respecꢀ

tively. The opposite signs of the angles can be interpreted

to mean that stereochemistry of PCꢀpalladacycles depends

on dioxaphosphepine fragment configuration: diastereꢀ

omer (S_a,R_C)ꢀ5a bears PCꢀpalladacycle in the λ conforꢀ

mation, and (R_a,R_C)ꢀ5b — in δ conformation.

by the nearly zero torsion angles formed by the four donor

...

... ...

atoms (P C(1) O N angle is +0.97 and –0.03°, respecꢀ

tively). For cisꢀ(N,C) isomers (S_a,R_C)ꢀ5a and (R_a,R_C)ꢀ5b

it is only slightly larger (+1.21 and +2.57°, respectively).

Thus, the DFT calculations confirm the transꢀ(N,C)

configuration of diastereomer (S_a,R_C)ꢀ5a suspected on

1

the basis of Н NMR data. The lower thermodynamic

(v) Coordination environment of palladium atom is

also dependent on the geometry of the complexes. The

both transꢀ(N,C) isomers of diastereomers (S_a,R_C)ꢀ5a and

(R_a,R_C)ꢀ5b can be described as squareꢀplanar as indicated

stability of cisꢀ(N,C) isomers compared to transꢀ(N,C)

structures for both diastereomers (S_a,R_C)ꢀ5a and (R_a,R_C)ꢀ5b

can be ascribed to the stronger distortion of the palladium

coordination sphere from squareꢀplanar geometry most

C(10)

C(13)

C(7)

C(4)

C(12)

C(9)

C(8)

C(11)

C(3)

C(31)

C(30)

C(29)

C(5)

C(6)

C(14)

C(28)

O(1)

O(2)

C(2)

C(27)

C(32)

C(1)

C(26)

P(1)

C(33)

C(34)

C(16)

C(25)

C(15)

Pd(1)

O(3)

C(17)

Cl(1)

C(24)

C(48)

C(18)

C(40)

C(23)

C(22)

C(49)

C(50)

P(2)

C(47)

C(19)

C(39)

C(35)

C(42)

C(43)

C(52)

C(38)

C(20)

C(36)

C(21)

C(51)

C(46)

C(37)

C(44)

C(45)

Fig. 3. Molecular structure of (S_a)ꢀenantiomer of phosphine adduct racꢀ4. 1,2ꢀDichloroethane solvate molecule is omitted, thermal

ellipsoids are scaled to the 50% probability level.

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Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009

Zykov et al.

typical of palladium(^II), as well as to conformation of

PCꢀpalladacycle distorted from planar, which is usually

adopted by sterically nonꢀhindered palladacycles (see

below). The lack of these two important elements of

chirality in transꢀ(N,C) valinate adduct isomers may

reduce the chiral information transfer efficiency for

such systems considerably; however, this is amendable by

introducing more sterically demanding auxiliary ligands.

Xꢀray diffraction analysis of phosphine adduct racꢀ4.

The cyclopalladated structure and orthoꢀmetallation site

in dimer racꢀ2 are established unambiguously by Xꢀray

examination of its mononuclear phosphine derivative

racꢀ4; the Xꢀray data have also confirmed the cisꢀ(P,P)

two enantiomeric molecules in the unit cell. The molecuꢀ

lar structure of the complex is shown in Fig. 3. Selected

bond lengths and angles are summarized in Table 4. Strucꢀ

tural and stereochemical features of complex 4 are disꢀ

cussed for the (S_a)ꢀenantiomer.

To estimate the efficiency of chirality transfer in

CPCs of this type (in particular, in the presence of a bulky

1,3,2ꢀdioxaphosphepine ring) we compared the structures

of adduct (S_a)*ꢀ4, the analogous PCPꢀpincer complex of

type II (R = R´ = H, X = I, IIa)^8cand a series of achiral

phosphite CPCs (VI—XII) characterized previously by

configuration of the metal coordination sphere. Racemic

–

complex 4 crystallizes in the triclinic space group P1 with

Table 4. Comparison of experimental (Xꢀray) and DFT calculꢀ

ated values of selected bond lengths (d) and angles (ϕ) for phosꢀ

phine adduct racꢀ4

VI

Ar

Ph

R

H

a^21b

b^21b

c⁹ⁱ

2ꢀBu^tC₆H₄

2ꢀBu^t

2,4ꢀBu^t₂

2,4ꢀMe₂

Parameter

Xꢀray

DFT

d/Å

Δ

2,4ꢀBu^t₂C₆H₃

(2ꢀBu^tꢀ4ꢀMeC₆H₂ꢀ6)₂CH₂

d^14c

Bond length

Pd(1)—C(1)

Pd(1)—P(1)

Pd(1)—P(2)

Pd(1)—Cl(1)

P(1)—O(3)

P(1)—O(1)

P(1)—O(2)

O(1)—C(2)

O(2)—C(16)

O(3)—C(26)

C(1)—C(2)

C(1)—C(6)

C(2)—C(3)

C(3)—C(4)

C(4)—C(5)

C(5)—C(6)

2.072(3)

2.1517(9)

2.3567(9)

2.3813(8)

1.588(2)

1.609(2)

1.421(3)

1.406(4)

1.414(3)

1.387(4)

1.395(4)

1.398(4)

1.392(4)

1.391(4)

1.384(4)

2.097

2.180

2.409

2.360

1.635

1.654

1.628

1.416

1.391

1.402

1.399

1.398

1.407

1.401

1.402

1.400

0.025

0.028

0.052

–0.022

0.047

0.066

0.019

–0.005

–0.015

0.012

0.022

0.003

0.009

0.011

0.016

VII

Ar

R

E^∩E´

a^14c2,4ꢀBu^t₂C₆H₃2,4ꢀBu^t₂

Et₂NCSS^–

b³⁹

c²⁰

2,4ꢀBu^t₂C₆H₃2,4ꢀBu^t₂Ph₂P(O)CH₂C(Me)(CH₂PPh₂)₂

5

Ph

H

η ꢀCp^–

Angle

ϕ/deg

C(1)—Pd(1)—P(1)

C(1)—Pd(1)—P(2)

P(1)—Pd(1)—P(2)

C(1)—Pd(1)—Cl(1)

P(1)—Pd(1)—Cl(1)

P(2)—Pd(1)—Cl(1)

O(3)—P(1)—O(1)

O(3)—P(1)—O(2)

O(1)—P(1)—O(2)

O(3)—P(1)—Pd(1)

O(1)—P(1)—Pd(1)

O(2)—P(1)—Pd(1)

C(2)—O(1)—P(1)

C(16)—O(2)—P(1)

C(26)—O(3)—P(1)

C(2)—C(1)—Pd(1)

C(6)—C(1)—Pd(1)

76.28(9)

169.84(9)

104.31(3)

95.22(9)

167.02(3)

85.81(3)

103.47(12)

104.03(11)

100.20(12)

120.97(9)

111.13(9)

114.56(9)

110.33(18)

115.84(19)

120.15(19)

119.6(2)

77.40

173.25

105.94

92.94

169.12

84.20

103.00

101.56

98.07

121.93

109.67

119.06

110.95

119.32

119.68

118.99

122.96

1.12

3.41

1.63

–2.28

2.10

–1.61

–0.47

–2.47

–2.13

0.96

–1.46

4.50

0.62

Ar = 2,4ꢀBu^t₂C₆H₃, R = 2,4ꢀBu^t₂(see Ref. 37)

3.48

IX

a

Ar

Ph

R

H

R´

–0.47

–0.61

–0.14

2,6ꢀPrⁱ₂C₆H₃

Mes

b

c^9b

2,4ꢀBu^t₂C₆H₃

2,4ꢀBu^t₂

123.1(2)

Axial chirality in phosphite phosphapalladacycles

Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009 2327

(2.340—2.405 Å). The C—Pd—P(1) angle in the palladaꢀ

cycle of 4 (76.27°) is smaller compared with the values

typical of all phosphite PCꢀpalladacycles (77.83—81.47°

for V—XI). It is most likely due to steric strain in adduct

4, as the minimal angles were found in the structures

bearing bulky substituents (78.08 and 77.83° for VId and

VIII, respectively).

The formally squareꢀplanar metal coordination enviꢀ

ronment in complex (S_a)*ꢀ4 displays a significant tetraꢀ

hedral distortion with a {C(1)Pd(1)P(1)}/{Cl(1)Pd(1)P(2)}

dihedral angle of 14.83° that is larger than that observed

in all known phosphite PCꢀpalladacycles (0.81—13.45°).

Most phosphite CPCs show characteristic tendency for

the squareꢀplanar coordination (0.81—5.76°), which

acquires tetrahedral distortion only on the introduction of

bulky substituents (8.36—8.97° for VIb,d, X), maximum

distortion (13.45°) being found for the sterically strained

adduct IXb. To describe stereochemistry of palladium

pseudotetrahedral coordination environment and palladaꢀ

cycle conformation in complex (S_a)*ꢀ4 we applied the

skew line convention.⁴¹The pseudotetrahedral configuraꢀ

tion is controlled by a sign of the torsion angle formed by

four metalꢀbound donor atoms, preferably starting from

the PCꢀpalladacycle P(1) and C atoms (Fig. 4). In adduct

(see Ref. 38)

(see Ref. 15)

... ... ...

(S_a)*ꢀ4, the P(1) C Cl P(2) torsion angle has a posiꢀ

tive sign (+15.48°), accordingly, pseudotetrahedral conꢀ

figuration is Λ. It should be noted that in the formally

achiral dimeric CPCs VIb,d with the pronounced tetraꢀ

hedral distortion, pseudotetrahedral configurations in their

two halves are opposite (Δ and Λ), thus, these molecules

can be regarded as configurational racemates.⁴²

The fiveꢀmembered PCꢀpalladacycle in complex (S_a)ꢀ4

has been found in envelope conformation with a base

P(1)O(1)C(2)C(1) (average displacement 0.0344 Å)

with palladium atom shifted 0.587 Å out of the basal

Ar = 4ꢀMeOC₆H₄(see Ref. 40)

Xꢀray diffraction. The latter included several dimeric

PCꢀcomplexes (VI,^9i,14c,21bXI (see Ref. 15), and their

...

plane and a twist with regard to C(1) P(1) line of 20.62°.

mononuclear derivatives with monoꢀ (VIII,³⁷IX,^9b

(see Ref. 38)) and bidentate auxiliary ligands (VII),^14c,20,39

and PCPꢀpincer complex XII.⁴⁰

X

An alternative description for this conformation is the

envelope with a base Pd(1)C(1)C(2)O(1) (average disꢀ

placement from the basal plane 0.039 Å), P(1) atom

The Pd—C bond length (2.072 Å) in complex 4 is

close to those of transꢀ(P(2),C)ꢀconfigured CPCs VIII

and X (2.084 and 2.064 Å, respectively), but less than in a

cationic diphosphine derivative VIIb (2.118 Å). The

endocyclic Pd—P(1) bond length in adduct 4 (2.152 Å)

falls in the range 2.142—2.177 Å characteristic of triarylꢀ

phosphite CPCs containing bridging (VI) or terminal

(VIII, X) chloride on the P(1)—Pd—Cl diagonal. The

Pd—Cl bond in adduct 4 (2.381 Å) is slightly lengthened

compared to that in mononuclear analogs VIII and X

(2.349 and 2.319 Å, respectively), but is expectedly

stronger than the (μ)Cl—Pd bonds positioned trans to

phosphite Pꢀdonor atom in dimers VI (2.390—2.419 Å).

The exocyclic Pd—P(2) bond length in adduct 4

(2.357 Å) is within the range established for phosphine

derivatives VIIb, VIII, and X with P(2)—Pd—C diagonal

¹P

²P

¹P

²P

Cl

C

Δ (δ)

Λ (λ)

¹P

²P

C

(–)

(+)

Fig. 4. Correlations between the skewꢀline projections and the

sign of torsion angle for phosphine adduct 4 coordination sphere.

Hereafter, the choice of four envelope basal atoms was based on

the minimum values of appropriate intraꢀchelate torsion angles.

2328

Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009

Zykov et al.

...

0.464 Å outꢀofꢀplane, a twist with regard to O(1) Pd(1)

line 26.23°. This duality results from the presence of two

nearly equal intraꢀchelate torsion angles (8.97 and 10.85°

compared to 19.10—23.32° for the three other angles).

Nonplanarity of PCꢀpalladacycle (S_a)ꢀ4 is substantial:

the average of the absolute value of the torsion angles

(ϖ_av) is 16.80°. Among the known analogs only three

complexes were found to contain palladacycles with a

comparable (VIII) or even stronger (VId, IXc) distortion

(ϖ_av15.19, 21.08, and 17.81°, respectively). Not surprisꢀ

ingly, these falling out structures are those in which steric

strain is induced either by the bulky Рꢀsubstituents (VId)

or auxiliary carbene ligand (IXc), or by the cisꢀcoorꢀ

dinated and rather bulky phosphine PCy₃(the Tolman

cone angle θ is 179°)⁴³adjacent to a cumbersome P(OAr)₂

group (VIII). By contrast, CPCs VIa, VIIc, IXa based on

nonsubstituted phosphites P(OPh)₃have the essentially

planar palladacycle as evidenced by minimal ϖ_avvarying

in the range 1.16—4.20°. In most of other complexes

with relatively low steric demands, palladacycle adopts

the flattened envelope conformation (ϖ_av5.89—10.29°).

Thus it is safe to state that in the absence of considerable

steric hindrance, phosphite palladacycles show clear

tendency to adopting a virtually planar conformation.

The conformation of the palladacycle in adduct

(S_a)*ꢀ4 can be described as twistedꢀenvelope. The average

displacement of the basal atoms from the basal leastꢀ

squares plane is 0.039 Å that is comparable with distortions

exhibited by sterically strained analogs (0.020—0.036 Å

for VId, VIIb, VIII) but exceeds those observed for other

complexes (0.001—0.010 Å). In accordance with the skewꢀ

line convention,⁴¹configuration of this new element of

chirality in the structure of adduct (S_a)*ꢀ4 must be defined

as δꢀconformation as being characterized by negative

exploring the stereochemistry of PPh₃chiral auxiliary in

adduct (S_a)*ꢀ4, we have evaluated the orientation (ω_i) of

three Pꢀphenyl rings (А—С) on the basis of three pairs

of C_o—C_i—P—Pd torsion angles (ω_iand ω_i) using

1

2

the known formula ω_i= 0.5(ω_i+ ω_i+ 180°).^44gThe ω_i

1

values obtained for PPh rings demonst²rate the pronounced

dependence upon the ring arrangement with regard to

dioxaphosphepine heterocycle containing the fragment

of BINOL: angles ω_A, ω_B, and ω_Care 66.8, 33.7, and

15.4°, respectively. The average value of the torsion angle

(ω_av= (1/3)(ω_A+ ω_B+ ω_C) = 38.6°) corresponds to the

Рꢀconfiguration of phosphine propeller in (S_a)*ꢀ4.

From comparison of the Xꢀray diffraction data for

(S_a)*ꢀ4 and its analogs it is seen that the introduction of

bulky substituents (including dioxaphosphepine ring

formed by BINOL) into phosphite CPC structure inꢀ

creases the tetrahedral distortion of coordination sphere

and the twist of palladacycle, also resulting in Pd—Cl

bond weakening and in the reduction of intraꢀchelate

C—Pd—P(1) angle.

Quantum chemical study of phosphine adduct (S_a)ꢀ4.

In order to estimate the effect of crystal packing on

the structure and stereochemistry of complex 4, we have

carried out quantum chemical modeling of its (S_a)ꢀenanꢀ

tiomer (Fig. 5). The structural parameters of (S_a)ꢀ4 calꢀ

culated and optimized for the gas phase appeared to be

rather close to those obtained crystallographically with

the discrepancy between the calculated and Xꢀray deterꢀ

mined bond lengths and angles not exceeding 2% (see

Table 4). More important, however, is the fact that stereoꢀ

chemical parameters of the complex in the gas and crysꢀ

talline phases also differ insignificantly.

In particular, DFT theory also predicts the tetrahedral

distortion of the metal coordination sphere, although

...

С(1) P(1)—O(1)—C(2) torsion angle (–5.48°). Hence

configuration of palladacycle in (S_a)*ꢀ4 (δ) is opposite to

that of the coordination polyhedron (Λ). The same relaꢀ

tion between palladacycle and coordination polyhedron

configurations is found for the formally achiral sterically

strained dimer VId whose two halves have enantiomeric

configurations Λδ and Δλ.

The primary source of stereochemical information in

complex (S_a)*ꢀ4 is axial chirality of (S_a)ꢀ1,1´ꢀbinaphthyl

fragment in 1,3,2ꢀdioxaphosphepine heterocycle. This

ring is strongly twisted (ϖ_av42.35°), and applying the

skewꢀline convention to sevenꢀmembered rings, its conꢀ

...

formation will be δ(S_a) based on the negative O(2) O(3)—

...

C(26) C(16) torsion angle (–28.59°). The both results

compare well with the conformations of two dioxaphosꢀ

phepine rings of the PCPꢀanalog, (R_a)ꢀIIa characterized

by a slightly higher distortion (ϖ_av44.12 and 46.03°) at

identical λ(R_a)ꢀrelations between the axial and conforꢀ

mational chirality.

The metalꢀcoordinated triarylphosphines are known⁴⁴

to exist in chiral propellerꢀtype rotamer forms. While

Fig. 5. The PBE/TZ2P optimized structure of phosphine adduct

(S_a)ꢀ4 (gas phase).

Axial chirality in phosphite phosphapalladacycles

Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009 2329

From comparison of the calculated parameters for

two phosphite PCꢀpalladacycle derivatives, the phosphine

adduct (S_a)ꢀ4 and valinate complex (S_a,R_C)ꢀ5a it is

obvious that steric demands of auxiliary ligands affect the

complex stereochemistry considerably. In particular, in a

phosphine adduct (S_a)ꢀ4, the squareꢀplanar palladium

environment is distorted stronger than in a less strained

valinate derivative (S_a,R_C)ꢀ5а, P(1) C(1) Cl P(2) angles

being +8.68 and +0.97°, respectively. This also holds

for РСꢀpalladacycle conformation which is much more

twisted in (S_a)ꢀ4 compared to (S_a,R_C)ꢀ5a, ϖ_av= 15.41 and

0.60°, respectively. It is most likely that chirality transfer

in the phosphite CPCs of this type will be more effective

in the presence of sterically demanding auxiliary ligands

due to involvement of two additional elements of chirality,

the pseudotetrahedral coordination sphere and twisted

palladacycle conformation.

a

b

Pd

P

O

Pd

Cl

d

...

... ...

c

O

P

Pd

P

O

Fig. 6. Elements of chirality in phosphine adduct (S_a)ꢀ4.

The calculation of conformational equilibria in adduct

(S_a)ꢀ4. An important factor of effective chirality transfer

is the conformational stability of the complex. To estimate

the conformational rigidity of adduct (S_a)ꢀ4 and reveal

possible conformational equilibria, we have examined the

complex structure with the aid of additional DFT calculꢀ

ations with the modified configurational or conforꢀ

mational parameters. The previously optimized structure

of complex (S_a)ꢀ4 was taken as a starting point. The calꢀ

culation was run in three series, each characterized by a

preset iteration step of variation of the torsion angles

controlling i) palladacycle conformation, ii) pseudoꢀ

tetrahedral configuration or iii) PPh₃propeller configuraꢀ

tion. At each step one of parameters was kept fixed to fully

optimize the CPC geometry with respect to all other

parameters, then the CPC energy was determined.

In the first series, РС palladacycle conformation was

... ... ...

weaker than that found for the crystal: P(1) C Cl P(2)

angles for the two phases are +8.68 and +15.48°, respecꢀ

tively. The positive sign of this angle is indicative of the

retained pseudotetrahedral Λꢀconfiguration in the gas

phase (Fig. 6, a).

The calculated degree of nonplanarity of РСꢀpalladaꢀ

cycle is close to the Xꢀray result for the crystal (ϖ_av= 15.41

and 16.80°, respectively). The envelope geometry of

palladacycle with apical phosphorus is more pronounced

in the gas phase: only one of the five intraꢀchelate angles,

O—C(2)—C(1)—Pd is characterized by the considerably

smaller value (+6.29°) compared to others (11.84—23.22°).

Phosphapalladacycle in the gas phase retains its δꢀconforꢀ

mation identified in the crystal as follows from the negaꢀ

...

tive sign of both calculated and experimental С(1) P(1)—

O(1)—C(2) torsion angle values, –6.15 and –5.48°, reꢀ

spectively (Fig. 6, b).

...

changed by varying С(1) P(1)—O(1)—C(2) angle value.

All calculated structures had the higher energy than the

starting one (Fig. 7). The energy curve showed no addiꢀ

The calculated parameters of 1,3,2ꢀdioxaphosphepine

ring bearing (S_a)ꢀbinaphthyl fragment agreed with the

experimental geometry derived from the Xꢀray data. The

twist of the sevenꢀmembered ring is nearly the same in

both cases, with angles ϖ_av= 41.58 and 42.35°, respecꢀ

tively. The conformation of the sevenꢀmembered ring was

also found to be the same (δ(S_a)) for the crystal and

gas phase as determined from the sign and value of

ΔE/kcal mol^–1

6

5

4

3

2

1

0

...

O(2) O(3)—C(26) C(16) torsion angle: –27.89 and

–28.59°, respectively (Fig. 6, с).

Finally, the rotameric state of PPh₃ligand coorꢀ

dinated to palladium in a phosphine adduct (S_a)ꢀ4 was

estimated almost identically by Xꢀray and gasꢀphase model

DFT calculation, with ω_A, ω_B, and ω_Cangles of 68.74,

35.14, and 19.73°, respectively for the gas phase and

66.8, 33.7, and 15.4° for crystal. The average torsion angles

(ω_av) for the gas phase and crystal are 41.20 and 38.6°,

indicating Pꢀconfigured PPh₃propeller in both cases

(Fig. 6, d).

–5

0

...

C(1) P(1)—O(1)—C(2) angle/deg

Fig. 7. Dependence of the energy of complex (S_a)ꢀ4 on

РСꢀpalladacycle conformation.

2330

Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009

Zykov et al.

ΔE/kcal mol^–1

P₂

M₂

M₃

6

5

4

3

2

1

0

2.5

2.0

1.5

1.0

0.5

0

P₃

... ... ...

P(1) C Cl P(2) angle/deg

–10

0

–100

–50

0

Pd—P—C_i—C₀angle/deg

Fig. 8. Dependence of the energy of complex (S_a)ꢀ4 on configuꢀ

ration of pseudotetrahedral metal environment.

Fig. 9. Dependence of the energy of complex (S_a)ꢀ4 on the

rotameric state of PPh₃propeller as determined by A ring

(Рꢀphenyl ring oriented toward binaphthyl moiety) rotation.

M₂and P₂are twoꢀbladed propellers, M₃and P₃are threeꢀ

bladed propellers.

tional minima corresponding to the different, more stable

conformation. So it can be concluded that 1,1´ꢀbinaphthyl

fragment hinders any conformational alterations of PCꢀ

palladacycle in complex (S_a)ꢀ4, hence strictly dictating its

δꢀstereochemistry.

rings is typical of the transition metal complexes with

triphenylphosphine.^44a,e,f

The second series of calculations was performed to

check for palladium in complex (S_a)ꢀ4 the possibility of

adopting an alternative pseudotetrahedral coordination

environment. For this purpose we calculated a set of strucꢀ

Thus, the quantum chemical calculations of the conꢀ

formational equilibria in complex (S_a)ꢀ4 demonstrated

that axially chiral 1,1´ꢀbinaphthyl moiety strictly dictates

the δꢀconformation for the PCꢀpalladacycle and Λꢀconꢀ

figuration for the metal pseudotetrahedral environment.

The alternative λΔ(S_a)ꢀstereochemistry of these structural

units cannot be realized. The stereochemical information

transfer from axially chiral (S_a)ꢀ1,1´ꢀbinaphthyl moiety to

the reaction center in complex (S_a)ꢀ4 is anticipated to be

rather effective due to involvement of additional induced

elements of chirality in the process.

As shown by the gas phase calculations, the state of

the PPh₃propeller which was determined for the (S_a)*ꢀ4

crystal is not the only possibility. There are no significant

energetic barriers for the propeller phenyl ring rotation

at room temperature.

In summary, in this work we described the synthesis

of the first optically active PCꢀpalladacycle with the phosꢀ

phorus atom in axially chiral environment. The feasibility

of direct thermal (at ∼110 °C) C—H bond activation in

(S_a)ꢀBINOLꢀderived phosphites without loss of enantioꢀ

meric purity was proved spectrally (³¹P NMR) with using

(R_C)ꢀvalinate auxiliary in the step of chiral derivatization

of dimer (S_a,S_a)ꢀ2а. The orthoꢀpalladated structure of a

new palladacycle was confirmed by the singleꢀcrystal

Xꢀray diffraction analysis of the mononuclear phosphine

derivative of dimer racꢀ2. Chirality transfer in the novel

CPCs is discussed on the basis of comparing the results

from DFT studies and Xꢀray data for (R_C)ꢀvalinate deꢀ

rivative (S_a)ꢀ5 and phosphine adduct (S_a)ꢀ4. It is shown

that axial chirality of (S_a)ꢀ1,1´ꢀbinaphthyl moiety

determines the δꢀconformation of the sevenꢀmembered

1,3,2ꢀdioxaphosphepine ring and fiveꢀmembered PCꢀpalꢀ

... ... ...

tures having different P(1) C Cl P(2) torsion angles.

As in the first series, the energy curve showed no addiꢀ

tional minima corresponding to configurations different

from the starting point; all modified structures are charꢀ

acterized by a higher energy (Fig. 8). These results

conclusively indicate that in adduct (S_a)ꢀ4, the BINOL

fragment determines the Λꢀconfiguration of palladium

coordination environment; (S_a)ꢀaxially chiral structure

does not admit the possibility of the opposite pseudoꢀ

tetrahedral Δꢀconfiguration

The last calculation series was aimed at energy estimaꢀ

tion for various rotameric states of PPh₃propeller. Since

the energy calculation for all probable structures would

require enormous computation time, we have restricted

their number to those with only one Рꢀphenyl ring posiꢀ

tion varied, i.e., the one proximate to binaphthyl moiety

and thus most sensitive to the primary chirality inducer

(hereafter, A ring). Despite the identical positions of A

ring in the structures of the starting and final (yielded by a

180° rotation) points in the energy curve (Fig. 9), these

states differ in energy because of the different orientations

of the two other Pꢀphenyl rings. The calculation results

demonstrate the absence of any hindrance for the conꢀ

certed rotation of PPh rings about the P—C_ipsoaxes at

room temperature: the obtained barrier for two propeller

configurations, Р and M, is not higher than 2.5 kcal mol^–1

that is only slightly exceeding the rotation barrier for free

PPh₃group (∼1.5 kcal mol^–1).^44f,hOur estimate for the

barrier to concerted rotation of the propeller Рꢀphenyl

Axial chirality in phosphite phosphapalladacycles

Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009 2331

The residue was purified by dry column chromatography⁴⁵on

deaerated Silpearl silica (h = 6.5 cm, d = 2.5 cm, eluting with

toluene—hexane mixtures (50 : 50—100 : 0)) under argon to

yield 1.17 g (78%) of phosphite racꢀHL as colorless amorphous

powder. M.p. (decomp.) 182 °C, R_f0.40 (toluene—hexane (1 : 5)).

Found (%): C, 78.25; H, 6.58. C₃₄H₃₃O₃P. Calculated (%):

C, 78.43; H, 6.40. ³¹P{¹H} NMR, δ: 145.11 (s). ¹H NMR, δ:

1.34 (s, 9 H, Bu^t); 1.39 (s, 9 H, Bu^t); 7.20 (dd, 1 H, H(5″),

ladacycle, as well as the Λꢀconfiguration of pseudoꢀ

tetrahedral palladium coordination environment that is

confirmed by DFT calculations of the conformational

transition energy in the complex (S_a)ꢀ4.

Experimental

4

³J_H,H= 8.3 Hz, J_H,H= 2.5 Hz); 7.24 (d, 1 H, H(6″),

¹Н and ³¹Р{¹H} NMR spectra were recorded on Varian

VXRꢀ400 and Bruker DPXꢀ400 spectrometers with operating

frequency of 400 and 169 MHz, respectively, in CDCl₃at room

temperature (unless otherwise stated). Chemical shifts in ¹Н

and ³¹Р NMR spectra were referenced internally to Me₄Si

and externally to Н₃РО₄, respectively. Signal assignments

were based on homoꢀ and heteronuclear decouplings, and also

on COSY and NOE techniques. Optical rotation was measured

using a Perkin—Elmer 341 polarimeter in 0.5ꢀdm cuvettes

at 25 °C. Melting points were measured in vacuumꢀsealed

capillary tube with an Electrothermal IA 9000 digital melting

point apparatus. TLC (on Silufol UVꢀ254 plates) was used

to follow the course of the reaction and control product purity.

All manipulations with the free phosphite HL were perꢀ

formed under purified Ar in absolute deaerated solvents using

Schlenk techniques. The products were isolated by preparative

flashꢀchromatography or dry column chromatography⁴⁵on

Fluka 60 silica.

The solvents were purified by standard procedures: benzene

and toluene were dried over CaCl₂, refluxed over Na and then

distilled under argon; anhydrous MeOH was obtained by

distillation over MeONa; chloroform and methylene chloride

were passed through the column of neutral Al₂O₃(Brockmann

activity grade II) (L 40/250) and distilled under argon; 1,2ꢀdiꢀ

chloroethane was passed through the column of neutral Al₂O₃

(Brockmann activity II) (L 40/250) and distilled over P₂O₅;

chloroformꢀd₁and methylene chlorideꢀd₂(Aldrich) were disꢀ

tilled over CaH₂under argon directly before use; hexane and

light petroleum were distilled over Na under argon.

3

³J_H,H= 8.3 Hz); 7.27 (ddd, 1 H, H(7)/H(7´), J_H,H= 8.3 Hz,

4

³J_H,H= 7.0 Hz, J_H,H= 1.3 Hz); 7.29 (ddd, 1 H, H(7´)/H(7),

4

³J_H,H= 8.2 Hz, ³J_H,H= 6.9 Hz, J_H,H= 1.5 Hz); 7.39 (d, 1 H,

H(3″), ⁴J_H,H= 2.5 Hz); 7.40 (d, 1 H, H(8)/H(8´), ³J_H,H= 8.2 Hz);

3

7.42 (d, 1 H, H(8´)/H(8), J_H,H= 8.2 Hz); 7.43 (ddd, 1 H,

3

4

H(6)/H(6´), J_H,H= 8.3 Hz, J_H,H= 7.0 Hz, J_H,H= 1.3 Hz);

3

7.46 (ddd, 1 H, H(6´)/H(6), J_H,H= 8.2 Hz, J_H,H= 6.9 Hz,

⁴J_H,H= 1.5 Hz); 7.49 (dd, 1 H, H(3)/H(3´), J_H,H= 8.8 Hz,

3

⁴J_H,P= 0.8 Hz); 7.59 (d, 1 H, H(3´)/H(3), ³J_H,H= 8.8 Hz); 7.90

(d, 1 H, H(4)/H(4´), ³J_H,H= 8.8 Hz); 7.91 (d, 1 H, H(5)/H(5´),

³J_H,H= 8.2 Hz); 7.95 (d, 1 H, H(5´)/H(5), J_H,H= 8.2 Hz);

3

8.01 (d, 1 H, H(4´)/H(4), ³J_H,H= 8.3 Hz).

(S_a)ꢀ(1,1´ꢀBinaphthylꢀ2,2´ꢀdiyl)(2,4ꢀdiꢀtertꢀbutylphenyl)ꢀ

phosphite ((S_a)ꢀHL) was synthesized and purified as described in

the previous step, from (S_a)ꢀBINOL (1040 mg, 3.63 mmol),

PCl₃(0.32 mL, 500 mg, 3.63 mmol), 2,4ꢀdiꢀtertꢀbutylphenol

(749 mg, 3.63 mmol), and Et₃N (1.50 mL, 1100 mg, 10.89 mmol)

in anhydrous toluene (40 mL) in a yield of 1.34 g (71%) as

colorless amorphous powder. M.p. (decomp.) 187 °C, R_f0.40

25

(toluene—hexane (1 : 5)), [α]_D+103.5 (c 1.31, CH₂Cl₂).

Found (%): C, 78.60; H, 6.48. C₃₄H₃₃O₃P. Calculated (%):

C, 78.43; H, 6.40. ³¹P{¹H} NMR, δ: 145.07 (s). ¹H NMR, δ:

1.32 (s, 9 H, Bu^t); 1.37 (s, 9 H, Bu^t); 7.19 (dd, 1 H, H(5″),

4

³J_H,H= 8.3 Hz, J_H,H= 2.5 Hz); 7.24 (d, 1 H, H(6″),

3

³J_H,H= 8.3 Hz); 7.27 (ddd, 1 H, H(7)/H(7´), J_H,H= 8.3 Hz,

4

³J_H,H= 7.0 Hz, J_H,H= 1.3 Hz); 7.29 (ddd, 1 H, H(7´)/H(7),

4

³J_H,H= 8.3 Hz, ³J_H,H= 6.9 Hz, J_H,H= 1.5 Hz); 7.39 (d, 1 H,

H(3″), ⁴J_H,H= 2.5 Hz); 7.40 (d, 1 H, H(8)/H(8´), ³J_H,H= 8.3 Hz);

3

7.42 (d, 1 H, H(8´)/H(8), J_H,H= 8.3 Hz); 7.43 (ddd, 1 H,

The reagents, PdCl₂, Pd(OAc)₂, (R)ꢀvaline (Aldrich),

Pd(PhCN)₂Cl₂(Merck), and (S_a)ꢀ1,1´ꢀbinaphthylꢀ2,2´ꢀdiol

(BINOL) (Fluka) were used without additional purification.

Racemic BINOL (Aldrich) and PPh₃were purified by twofold

recrystallizations from benzene—hexane mixture and acetone,

respectively; 2,4ꢀdiꢀtertꢀbutylphenol (Aldrich) was recrystallized

twice from light petroleum and dried over P₂O₅and paraffin

in vacuo.

3

4

H(6)/H(6´), J_H,H= 8.3, J_H,H= 7.0 Hz, J_H,H= 1.3 Hz);

3

7.46 (ddd, 1 H, H(6´)/H(6), J_H,H= 8.3 Hz, J_H,H= 6.9 Hz,

⁴J_H,H= 1.3 Hz); 7.48 (dd, 1 H, H(3)/H(3´), J_H,H= 8.8 Hz,

3

⁴J_H,P= 0.8 Hz); 7.58 (d, 1 H, H(3´)/H(3), ³J_H,H= 8.8 Hz); 7.90

(d, 1 H, H(4)/H(4´), ³J_H,H= 8.8 Hz); 7.91 (d, 1 H, H(5)/H(5´),

³J_H,H= 8.3 Hz); 7.95 (d, 1 H, H(5´)/H(5), J_H,H= 8.3 Hz);

3

8.01 (d, 1 H, H(4´)/H(4), ³J_H,H= 8.8 Hz).

Racemic diꢀμꢀchloroꢀbis[(1,1´ꢀbinaphthylꢀ2,2´ꢀdiyl)(2,4ꢀdiꢀ

tertꢀbutylphenyl)phosphiteꢀC,P]dipalladium(^II) (racꢀ2). To the

solution of Pd(PhCN)₂Cl₂(71.5 mg, 0.186 mmol) in 1,2ꢀdiꢀ

chloroethane (3 mL) was added phosphite HL (97.0 mg,

0.186 mmol), the homogeneous reaction mixture was heated to

reflux with stirring for 9 h. The precipitate formed was filtered

off and dried in vacuo to yield 0.0707 g (57%, 0.0534 mmol) of

impure dimer 2 as absolutely insoluble powder. The suspension

of dimer racꢀ2 in methylene chloride (3 mL) was further treated

with pyridine (8.5 mg, 0.107 mmol), this followed by stirring of

the reaction mixture for 15 min. The homogeneous solution

of pyridine adduct 3 (R_f0.30, toluene—acetone (1 : 1))* was

evaporated and passed through the flashꢀcolumn (h = 13 cm,

Racemic (1,1´ꢀbinaphthylꢀ2,2´ꢀdiyl)(2,4ꢀdiꢀtertꢀbutylphenyl)ꢀ

phosphite (racꢀHL). The solution of racemic BINOL (860 mg,

3.0 mmol) in anhydrous toluene (25 mL) heated to 60 °C was

added dropwise at vigorous stirring over 10 min to the solution

of PCl₃(0.26 mL, 410 mg, 3.0 mmol) and Et₃N (0.83 mL,

606 mg, 6.0 mmol) in the same solvent (10 mL) cooled to

–70 °C. After stirring at –70 °C (2 h) and more stirring at ∼20 °C

(1 h), formation of 4ꢀchlorodinaphtho[2,1ꢀd:1´,2´ꢀf][1,3,2]ꢀ

dioxaphosphepine 1 was confirmed by ³¹P NMR examination

of an aliquote of the reaction mixture: δ 179.51 (cf. Ref. 29:

Р

δ

179.0). Immediately after that, the reaction mixture was

Р

treated with additional Et₃N (302 mg, 3.0 mmol) under vigorous

stirring and then dropwise by the solution of 2,4ꢀdiꢀtertꢀbutylꢀ

phenol (600 mg, 2.9 mmol) in anhydrous toluene (5 mL). The

reaction mixture was stirred for 12 h at ∼20 °C, the precipitate

formed was filtered off, mother liquor was evaporated in vacuo.

* Pyridine adduct 3 is completely decomposed on silica with

formation of dimer 2 (R_f0.98).

2332

Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009

Zykov et al.

d = 2 cm) eluting with toluene—hexane (2 : 1) → toluene →

toluene—acetone (5 : 1) to isolate pure dimer racꢀ2 (a product

of complete decomposition of adduct 3 in the column)

as a colorless amorphous powder (yield 53.3 mg, 43%).

M.p. (decomp.) 247 °C, R_f0.5 (toluene—hexane (1 : 1)).

Found (%): C, 61.44; H, 5.05. C₆₈H₆₄Cl₂O₆P₂Pd₂. Calculꢀ

ated (%): C, 61.73; H, 4.89. ³¹P{¹H} NMR, δ: 138.45 (s), 138.54

(s) (86%); 140.75 (s), 140.81 (s) (14%).

two equal intensity signals at δ 147.73 (s) and 148.07 (s). The

residue was recrystallized from methylene chloride—hexane

mixture at low temperature to yield 14.1 mg (84%) of the mixture

of diastereomers 5a,b as colorless fine crystals. M.p. (decomp.)

25

235—237 °C, R_f0.53 (toluene—acetone (1 : 1))*, [α]_D∼0.00,

25

[α]₄₃₆+17.0 (c 0.19, CHCl₃). Found (%): C, 63.07; H, 5.74;

N, 1.93. C₃₉H₄₂NO₅PPd. Calculated (%): C, 63.11; H, 5.72;

N, 1.89. ³¹P{¹H} NMR, δ: 147.73 (s); 148.07 (s).

(S_a,S_a)ꢀDiꢀμꢀchloroꢀbis[(1,1´ꢀbinaphthylꢀ2,2´ꢀdiyl)(2,4ꢀdiꢀ

tertꢀbutylphenyl)phosphiteꢀC,P]dipalladium(^II) ((S_a,S_a)ꢀ2). The

solution of Pd(PhCN)₂Cl₂(46.5 mg, 0.1212 mmol) and phosꢀ

phite (S_a)ꢀHL (63.1 mg, 0.1212 mmol) in toluene (2 mL) was

refluxed for 7 h with stirring. Palladium black was filtered off,

the mother liquor was evaporated. The residue was purified by

flashꢀchromatography (h = 13 cm, d = 2 cm; elution with

toluene—hexane (1 : 1) → toluene → toluene—acetone (10 : 1))

to yield 32.0 mg (40%) of dimer (S_a,S_a)ꢀ2 as colorless amorphous

powder. M.p. (decomp.) 230 °C, R_f0.5 (toluene—hexane (1 : 1)).

Found (%): C, 61.49; H, 4.88. C₆₈H₆₄Cl₂O₆P₂Pd₂. Calculꢀ

ated (%): C, 61.73; H, 4.89. ³¹P{¹H} NMR, δ: 140.68 (s, 16%);

138.43 (s, 84%). ¹H NMR, δ: 1.12 (s, Bu^t); 1.20 (s, Bu^t);

7.10 (br.m); 7.14—7.19 (m); 7.24—7.26 (m); 7.28—7.29 (m);

7.31—7.35 (m); 7.37—7.42 (m); 7.49—7.53 (m); 7.81 (br.d);

7.97 (br.d); 8.06 (br.d).

[{(S_a)ꢀ1,1´ꢀBinaphthylꢀ2,2´ꢀdiyl}(2,4ꢀdiꢀtertꢀbiphenyl)phosꢀ

phiteꢀC,P][(R_C)ꢀvalinatoꢀN,O]palladium(II) ((S_a,R_C)ꢀ5a). Dimer

(S_a,S_a)ꢀ2 (8.2 mg, 0.0062 mmol) was introduced into the soluꢀ

tion of sodium (R)ꢀvalinate excess (2.8 mg, 0.0248 mmol) in

chloroform (2 mL). The reaction mixture was stirred for 4 h,

then filtered and evaporated. Recrystallization from chloroꢀ

form—hexane mixture at low temperature gave 8.3 mg (93%

yield) of diastereomer (S_a,R_C)ꢀ5a as fine colorless crystals.

M.p. (decomp.) 228—230 °C, R_f0.48 (toluene—acetone

25

(1 : 1))*, [α]_D+155.6 (c 0.25, CHCl₃). Found (%): C, 63.09;

H, 5.70; N, 1.94. C₃₉H₄₂NO₅PPd. Calculated (%): C, 63.11;

H, 5.72; N, 1.89. ³¹P{¹H} NMR, δ: 148.12 (s). ¹H NMR, δ: 0.94

(d, 3 H, Me, ³J_H,H= 7.1 Hz); 1.16 (d, 3 H, Me, ³J_H,H= 7.0 Hz);

1.27 (s, 9 H, Bu^t); 1.33 (s, 9 H, Bu^t); 1.69 (br.dd, 1 H, NH^eq

,

3

²J_H,H= 11.0 Hz, J_{HNC H}= 4.4 Hz); 2.10 (br.dd, 1 H, NH^ax

α

3

²J_H,H= 11.0 Hz, J_HNCαH= 7.2 Hz); 2.47 (m, 1 H, CHMe₂);

3

Racemic chloro[(1,1´ꢀbinaphthylꢀ2,2´ꢀdiyl)(2,4ꢀdiꢀtertꢀbutylꢀ

phenyl)phosphiteꢀC,P](triphenylphosphineꢀР)palladium(^II)

(racꢀ4). To the suspension of racemic dimer 2 (18.2 mg,

0.0138 mmol) in toluene (1.5 mL) was added triphenylphosphine

(7.2 mg, 0.0275 mmol). The reaction mixture was stirred for

30 min at ∼20 °C and evaporated in vacuo. The residue was

purified by dry column chromatography⁴⁵(h = 5 cm, d = 4 cm;

eluting with toluene → toluene—acetone (5 : 1)). Two crystalꢀ

lizations from methylene chloride—hexane mixture afforded

20.0 mg (60% yield) of adduct racꢀ4 as lightꢀorange crystals.

M.p. (decomp.) 225 °C, R_f0.73 (toluene—acetone (10 : 1)).

Found (%): C, 67.43; H, 5.25. C₅₂H₄₇ClO₃P₂Pd. Calculꢀ

ated (%): C, 67.60; H, 5.14. ³¹P{¹H} NMR (CD₂Cl₂), δ: 17.66

3.40 (ddd, 1 H, αꢀCH, J_HCNH= 7.2 Hz, J_HCNH= 4.4 Hz,

³J_HCCH= 6.9 Hz); 7.23 (dd, 1 H, H(4″), J_H,P= 3.5 Hz,

5

⁴J_H,H= 2.1 Hz); 7.38—7.43 (m, 2 H, H(7)/H(7´), H(8)/H(8´));

3

7.46 (ddd, 1 H, H(7´)/H(7), J_H,H= 8.7 Hz, J_H,H= 6.9 Hz,

⁴J_H,H= 1.3 Hz); 7.48 (br.d, 1 H, H(3)/H(3´), J_H,H= 8.8 Hz);

3

7.53 (d, 1 H, H(8´)/H(8), J_H,H= 8.7 Hz); 7.57 (m, 1 H,

3

H(6)/H(6´)); 7.63 (ddd, 1 H, H(6´)/H(6), J_H,H= 8.3 Hz,

³J_H,H= 6.9 Hz, J_H,H= 1.3 Hz); 7.72 (br.d, 1 H, H(3´)/H(3),

4

³J_H,H= 8.8 Hz, ⁴J_H,P= 0.4 Hz); 7.87 (dd, 1 H, H(6´), H(5)/H(5´),

⁴J_H,P= 7.1 Hz, ⁴J_H,H= 2.1 Hz); 8.04 (d, 1 H, ³J_H,H= 8.3 Hz);

3

8.06 (d, 1 H, H(5´)/H(5), J_H,H= 8.3 Hz); 8.09 (d, 1 H,

H(4)/H(4´), J_H,H= 8.8 Hz); 8.17 (d, 1 H, H(4´)/H(4),

3

³J_H,H= 8.8 Hz).

2

(d, J_P,P= 45.2 Hz); 149.91 (d, J_P,P= 45.2 Hz). ¹H NMR

(CD₂Cl₂), δ: 1.14 (s, 9 H, Bu^t); 1.40 (s, 9 H, Bu^t); 7.01—7.08

(m, 6 H, mꢀPPh, ⁴J_H,P(2)= 2.0 Hz); 7.10—7.15 (m, 3 H, pꢀPPh,

Quantum chemical calculations. All DFT calculations were

performed for the gas phase using the nonempirical PBE

functional within the Generalized gradient approximation⁴⁶

(PRIRODAꢀ6 program package⁴⁷). For expansion of oneꢀ

electron wave functions we used more extensive TZ2P basis set

constructed from Gaussian functions: {311/1} for H atoms,

{51111/51111/5111} for Pd atoms, and {6,11111/411/11} for

other (C, N, O, and P) atoms. Relativistic SBK pseudopotential

was used as implemented in Ref. 48. The energy E⁰was calculated

taking into account the zeroꢀpoint vibration in the harmonic

approximation. All structures were fully optimized. Zeroꢀpoint

energy corrections were included within the harmonic approxiꢀ

mation. ¹H NMR spectra were simulated by GIAO (gaugeꢀ

includingꢀatomicꢀorbital)⁴⁹technique using РВЕ functional with

TZ2P allꢀelectron basis set. Calculated chemical shifts are

presented as the difference in shielding between the standard

(Me₄Si) and the protons in the complex in question. All

computation was run on a MVS 15000BM cluster at the Joint

Supercomputer Center (JSCC, Moscow).

⁵J_H,P(2)= 2.0 Hz); 7.17 (dd, 1 H, H(4″), J_H,P(1)= 2.6 Hz,

5

⁴J_H,H= 2.1 Hz); 7.21 (d, 1 H, H(8)/H(8´), J_H,H= 8.6 Hz);

3

7.22—7.28 (m, 2 H, H(7)/H(7´), H(8´)/H(8)); 7.33 (m, 1 H,

3

H(7´)/H(7))*; 7.35—7.41 (m, 6 H, oꢀPPh, J_H,P(2)= 10.9 Hz);

3

4

7.38 (dd, 1 H, H(3)/H(3´), J_H,H= 8.8 Hz, J_H,P(1)= 0.9 Hz);

3

4

7.46 (dd, 1 H, H(3´)/H(3), J_H,H= 8.9 Hz, J_H,P(1)= 1.2 Hz);

7.48—7.52 (m, H, H(6)/H(6´)); 7.50—7.54 (m, 1 H,

1

3

H(6´)/H(6)); 7.58 (d, 1 H, H(4)/H(4´), J_H,H= 8.8 Hz); 7.84

(d, 1 H, H(5)/H(5´), ³J_H,H= 8.2 Hz); 7.98 (d, 1 H, H(5´)/H(5),

³J_H,H= 8.2 Hz); 8.03 (d, 1 H, H(4´)/H(4), J_H,H= 8.9 Hz);

3

4

8.47 (ddd, 1 H, H(6″), J_HP(1)= 5.6 Hz, J_HP(2)= 8.0 Hz,

⁴J_H,H= 2.2 Hz).

The mixture of diastereomers [(1,1´ꢀbinaphthylꢀ2,2´ꢀdiyl)ꢀ

(2,4ꢀdiꢀtertꢀbutylphenyl)phosphiteꢀC,P][(R_C)ꢀvalinatoꢀN,O]ꢀ

palladium(^II) ((S_a,R_C)ꢀ5a and (R_a,R_C)ꢀ5b). Dimer racꢀ2

(15.7 mg, 0.0118 mol) was added to the solution of sodium

(R)ꢀvalinate (2.9 mg, 0.0237 mmol) in chloroform (2 mL). The

reaction mixture was stirred for 3 h at ∼20 °C, filtered and

evaporated. At this step, the ³¹P{¹H} NMR spectrum shows

Xꢀray diffraction analysis of phosphine adduct racꢀ4. Crystals

of racꢀ4 suitable for Xꢀray examination were grown by the slow

* Valinate complexes 5а,b are partially decomposed on silica

with formation of dimer 2 (R_f0.90).

* The signal overlaps with signals of PPh₃orthoꢀprotons.

Axial chirality in phosphite phosphapalladacycles

Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009 2333

diffusion of pentane vapor into a dichloroethane solution.

Crystals of racꢀ4 (C₅₆H₅₅Cl₅O₃P₂Pd, M = 1121.59) are triclinic,

space group Р^–1, at 100 K a = 12.9201(7) Å, b = 14.3239(8) Å,

s = 16.3593(14) Å, α = 111.534(2)°, β = 106.222(2)°,

γ = 98.2940(10)°, V = 2598.8(3) Å³, Z = 2, F(000) = 1152,

d_calc= 1.433 g cm^–3, μ = 0.719 mm^–1. The lattice parameters

and intensities of 32453 reflections were measured on a

Bruker SMART APEX2 CCD Area Detector diffractometer

(λ(MoꢀKα) = 0.71072 Å, graphite monochromator, 100 K,

J.ꢀA. Ma, R. Goddard, Angew. Chem., Int. Ed. Engl., 2005,

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Tetrahedron, 2005, 61, 10514; (i) I. G. Gergely, C. Hegedüs,

H. Gulyás, A. Scöllósy, A. Monsees, Th. Riermeier, J. Bakos,

Tetrahedron: Asymmetry, 2003, 14, 1087; (j) Sh. J. Sturla,

St. L. Buchwald, J. Org. Chem., 2002, 67, 3398; (k) M. T.

Reetz, G. Mehler, A. Meiswinkel, Th. Sell, Tetrahedron

Lett., 2002, 43, 7941; (l) W. Chen, J. Xiao, Tetrahedron Lett.,

2001, 42, 2897; (m) M. T. Reetz, G. Mehler, Angew.

Chem., Int. Ed., 2000, 39, 3889; (n) M. Yan, K.ꢀY. Yang,

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ωꢀscans, θ

= 29°). The reflection intensities were integrated

max

using SAINT program,⁵⁰SADABS v2.03 program⁵¹was used to

apply semiempirical absorption correction. The structure was

solved by the direct method and refined using the fullꢀmatrix

leastꢀsquares on F²in anisotropic approximation for nonꢀ

hydrogen atoms. The hydrogen atoms were placed in the

geometrically calculated positions and then refined isotropically,

“riding” on their parent atoms. A crystal of racꢀ4 contains two

1,2ꢀdichloroethane solvate molecules, with one having carbon

atoms disordered about two positions with a 3 : 7 population

ratio. All calculations were performed using SHELXTL program

package.⁵²Final Rꢀfactors: R₁= 0.0517 for 9366 reflections

with I > 2σ(I) and wR₂= 0.0749 for all 14061 independent

reflections. Crystallographic data for the structure of complex

racꢀ4 have been deposited with the Cambridge Crystallographic

Data Center (CCDCꢀ677989); Xꢀray data are available at

www.ccdc.cam.ac.uk/data_request/cif.

4. (a) D.ꢀY. Zou, Z.ꢀC. Duan, X.ꢀP. Hu, Z. Zheng, Tetraheꢀ

dron: Asymmetry, 2009, 20, 235; (b) Z.ꢀC. Duan, X.ꢀP. Hu,

D.ꢀY. Wang, J.ꢀD. Huang, S.ꢀB. Yu, J. Deng, Z. Zheng,

∨

Adv. Synth. Cat., 2008, 350, 1979; (c) N. Mrsic´, L. Lefort,

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Chem., 2008, 693, 3519; (f) A. J. Minnaard, B. L. Feringa,

L. Lefort, J. G. de Vries, Acc. Chem. Res., 2007, 40,

1267; (g) K. Kurihara, N. Sugishita, K. Oshita, D. Piao,

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(j) K. A. Vallianatou, I. D. Kostas, J. Holz, A. Börner,

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Moreno, St. Streiff, G. Helmchen, Org. Biomol. Chem., 2005,

3, 3266; (l) M. van den Berg, A. J. Minnaard, R. M. Haak,

M. Leeman, E. P. Schudde, A. Meetsma, B. L. Feringa,

A. H. M. de Vries, C. E. P. Maljaars, C. E. Willans, D. Hyett,

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This work was financially supported by the Division of

Chemistry and Materials Science of the Russian Academy

of Sciences (Grant DCMSꢀ01).

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Received May 26, 2009;

in revised form October 1, 2009

Article Doi

First optically active PC-palladacycle bearing a phosphorus atom in an axially chiral environment

DOI: 10.1007/s11172-009-0324-8

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