Potential carbon-fluorine reductive elimination from pincer-supported Rh(III) and dominating side reactions: Theoretical and experimental examination

Article abstract of DOI:10.1021/om5008902

This paper explores the potential for C-F reductive elimination from Rh(III) pincer complexes. A DFT computational study indicated that concerted C-F reductive elimination from (POCOP)Rh(CHCH₂)(F) (3) (where POCOP is κ³P,C,P-2,6(

Full text of DOI:10.1021/om5008902

Article
pubs.acs.org/Organometallics
Potential Carbon−Fluorine Reductive Elimination from Pincer-
Supported Rh(III) and Dominating Side Reactions: Theoretical and
Experimental Examination
Samuel D. Timpa, Jia Zhou, Nattamai Bhuvanesh, and Oleg V. Ozerov*
Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77842, United States
S
* Supporting Information
ABSTRACT: This paper explores the potential for C−F reductive elimination
from Rh(III) pincer complexes. A DFT computational study indicated that
concerted C−F reductive elimination from (POCOP)Rh(CHCH2)(F) (3) (where
POCOP is κ3P,C,P-2,6(iPr2PO)2C6H3, and aryl/bis(phosphinite) pincer ligand)
possesses an experimentally plausible activation barrier of ΔG⧧ = 28.3 kcal/mol.
This barrier is considerably lower than that calculated (35.7 kcal/mol) for the analogous C−F reductive elimination from
(POCOP)Rh(Ph)(F) (1). The difference is ascribed to the need for a partial rotation of a phenyl or vinyl group in the transition
state, with the phenyl being more encumbered by the steric bulk of the supporting pincer ligand. DFT calculations did not
analyze the full range of possible side reactions, which have proven to be dominant. The attempted synthesis of 1 was
unsuccessful because of competing C−C reductive elimination at the stage of the preparation of the (POCOP)Rh(CHCH2)(I)
precursor. DFT calculations predicted C−C reductive elimination to be facile but markedly unfavorable in the monomeric unit
because of inherent strain in the product. That strain is apparently relieved in dimerization that takes place with opening up of
the pincer to become bridging between two Rh centers. The opening up of the pincer, and thus dimerization and C−C reductive
elimination, was prevented by the use of the tBuPOCOP ligand (κ3P,C,P-2,6(iPr2PO)2-3,5-But2C6H3), which allowed isolation of
(
tBuPOCOP)Rh(CHCH2)(I) (11). Compound 11 was converted to (tBuPOCOP)Rh(CHCH2)(OTf) (12) via reaction with
AgOTf. However, treatment of 11 with AgF or of 12 with CsF failed to produce (tBuPOCOP)Rh(CHCH2)(F), resulting instead
in multiple products containing P−F bonds. On the other hand, 12 was cleanly converted to (tBuPOCOP)Rh(CHCH2)(OBut)
(13) via reaction with NaOBut and then to (tBuPOCOP)Rh(CHCH2)(OC6H4F-p) (14) by treatment of 13 with p-fluorophenol.
Neither 13 nor 14 gave any evidence of C−O reductive elimination. Instead, thermolysis of either 13 or 14 resulted in
dehydroalkoxylation to the bimetallic divinylacetylene complex (tBuPOCOP)Rh(CH2CHCCCHCH2)Rh(tBuPOCOP)
(15) as the major product.
becomes more thermodynamically favorable and kinetically
accessible for metal centers in higher oxidation states. C−F RE
has been implicated for a number of complexes of group 10
metals (Ni, Pd, Pt) in the +4 oxidation state,13−16 as well as
proposed for Ag(II)/Ag(III)17 and Cu(III)18 complexes in Ag-
and Cu-based catalysis. However, these high-oxidation-state
complexes can only be reached with the use of strong oxidants
and typically cannot be accessed via oxidative addition of C−X
bonds under mild conditions, except perhaps with the Cu(I)/
Cu(III) couple.19,20 This has been exploited in electrophilic
fluorination catalysis,21 where the high-valent metal fluoride is
obtained by oxidation with an F2-derived “F+” reagent, such as
XeF2 or one of the N−F22 reagents. The aryl group for C−F
coupling is delivered in the “nucleophilic” capacity, either by
directed C−H activation or via an arylboron or aryltin reagent.
In a hybrid version of electrophilic fluorination, high-valent
metal fluorides have been accessed via a combination of a
fluoride-free strong oxidant and nucleophilic fluoride.23
Electrophilic fluorination appears especially viable for the
synthesis of high-value C−F-containing products, including
INTRODUCTION
■
The C−F functional group is prevalent in a number of
pharmaceuticals,1 agrochemicals,2 imaging agents,3 and new
high-performance materials.4 However, well-evolved methods
for C−F bond formation are still uncommon. Many of the
more traditional methods for forming C−F bonds are limited in
scope, especially for aromatic and C(sp2)−F bonds in general.
Direct oxidative fluorination with F2 or CuF2 involves rather
harsh conditions. Nucleophilic aromatic substitution with
fluoride is applicable only to rather specific substrates with
strongly electron withdrawing groups.7 The Balz−Schiemann
fluorodediazonization8 and fluoridation of diaryliodonium salts9
require introduction of a nontrivial functionality early in the
synthesis, which reduces the scope of these processes.
Transition-metal catalysis offers the promise of C−F bond
formation under milder conditions and with a broader scope of
substrates.10−12 Concerted C−F reductive elimination (RE) is
the common C−F bond-making step in most of the various
transition-metal-based approaches to the synthesis of C(sp2)−F
bonds. It has been pointed out that C−F RE is intrinsically the
most difficult concerted C−X RE process, because of the low
polarizability of fluorine and also because of the relative facility
of side reactions for fluoride complexes.10−12 RE generally
5
6
Received: August 29, 2014
Published: October 2, 2014
© 2014 American Chemical Society
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late-stage synthesis modification24 and 18F-labeled radiophar-
maceuticals.25−27
it involves different d electron configurations (d6/d8 for Rh vs
d8/d10 for Pd) at the metal and different coordination
geometries. OA of aryl halides requires a three-coordinate
(pincer)RhI intermediate and forms a five-coordinate (pincer)-
RhIII product. RE of C−F would then have to take place from
the corresponding five-coordinate Rh(III) aryl/fluoride com-
plex. Five-coordinate (pincer)RhIII complexes are readily
isolable and do not show propensity for dimerization,38−41 in
contrast to the difficulty in accessing truly three-coordinate
Pd(II) complexes.42 In addition, OA/RE processes at Rh do
not require changes in the disposition of the donor atoms of
the pincer. These considerations held some promise for Rh-
based C−F coupling catalysis and encouraged us to explore
whether C−F RE may be accessible with Rh. Presently, we
report the results of our initial foray into this problem, which
uncovered significant competing reactions.
Catalytic nucleophilic fluorination10 is conceived to mimic
the well-established Pd(0)/Pd(II) chemistry of carbon−carbon
and carbon−heteroatom coupling (Scheme 1).28,29 This
Scheme 1. Mechanism for Nucleophilic C−F Bond
Formation via the Pd(0)/Pd(II) Cycle and the Proposed
Analogous Rh(I)/Rh(III) Cycle
RESULTS AND DISCUSSION
■
Theoretical Analysis. We selected (POCOP)Rh(Ph)(F)
(1) and (POCOP)Rh(CHCH2)(F) (3) for our theoretical
analysis of the kinetic and thermodynamic accessibility of C−F
reductive coupling, as shown in Scheme 2 and Figure 1. The
Scheme 2. Calculated ΔG and ΔG⧧ Values for C−F
Reductive Coupling from (POCOP)Rh(Ph)(F) (1) and
(POCOP)Rh(CHCH2)(F) (3) at the M06/SDD/6-
311G(d,p) Level of Theory
approach utilizes simple fluoride salts (e.g., CsF, AgF) as
sources of nucleophilic fluoride and aryl halides or sulfonates as
formal carbon electrophiles. Aryl halide oxidative addition
(OA) to Pd(0) forms a PdII−C bond, and transmetalation with
a fluoride source establishes a Pd(II) aryl/fluoride complex for
C−F reductive elimination.
Execution of C(sp2)-F coupling via a nucleophilic pathway
with Pd has encountered many obstacles. Pioneering work by
Grushin30 and co-workers produced a series of PR3-substituted
LnPd(Ar)(F) compounds (L = PR3); however, none of these
complexes underwent successful Ar-F RE. Theoretical analysis
by Yandulov31 and more recently by Saeys32 identified two
general issues with LnPd(Ar)(F): (1) the necessity of access to
three-coordinate LPd(Ar)(F) for reasonable C−F RE activation
barriers and (2) the competitiveness of side reactions such as
C−C and P−F coupling, because of the relatively high barrier
for C−F RE.
More recently, Buchwald and co-workers have demonstrated
the utility of their bulky phosphinobiaryl ligands, such as
BrettPhos and RockPhos, with Pd to form Ar-F bonds both
stoichiometrically and catalytically.33 The success of these
ligands for challenging RE processes, including C−N and C−O
coupling, is attributed to their large and enveloping steric
profile, which facilitates pseudo-monocoordinate Pd(0) as well
as pseudo-three-coordinate Pd(II).34 These ligands can also be
broadly tuned by altering the phosphine substituents to achieve
the desired steric and electronic environment at the metal
center.35,36 However, the significant cost of the phosphinobiaryl
ligands that support C−F coupling and the relatively low
turnover numbers reduce the practicality of this method at
present.
Our group has been pursuing chemistry relevant to catalysis
of aryl halide coupling reactions with Rh complexes of pincer-
type37 ligands. In addition to demonstrating the viability of the
requisite C(sp2)−X OA/RE elementary reactions,38 we have
recently reported that C−C,39 C−N,40 and C−S41 coupling of
aryl halides can be carried out catalytically using simple aryl/
bis(phosphinite) POCOP pincer ligands with Rh. The putative
Rh(I)/Rh(III) catalytic cycle (Scheme 1) for C−F coupling is
functionally analogous to the Pd(0)/Pd(II) cycle, even though
products were optimized as the F-bound adducts of
fluorobenzene and fluoroethylene with (POCOP)Rh (2 and
4a, respectively) in order to avoid the steric and electronic
differences between fluorobenzene and fluoroethylene when
binding via their π systems. Binding to the π system of
fluoroethylene (4b) was calculated to be 18.3 kcal/mol more
favorable than binding via the fluorine (Scheme 2), but as it
occurs following the transition state, it should only affect the
thermodynamics and not the barrier for C−F reductive
coupling. In a putative catalytic reaction, any C−F coupled
product would most likely dissociate and be replaced by OA of
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Figure 1. Potential energy surface of C(vinyl)−F reductive elimination and C(vinyl)−C(aryl) reductive elimination with (POCOP)Rh at the M06/
SDD/6-311G(d,p) level of theory.
the substrate organohalide (halide = Br, I, OTf) to the
unsaturated (POCOP)RhI center.43
L2Pd(Ar)(X) or LPd(Ar)(X), where the bulky L ligands
encourage the “face-on” arrangement expected for the
transition state.
The thermodynamics of the formation of 2 and 4a by C−F
reductive coupling were calculated to be nearly the same (ΔGrxn
values of 4.3 and 3.6 kcal/mol, respectively), suggestive of
similar electronic factors in vinyl− and phenyl−fluoride bond
formation. However, the reaction barrier was considerably
lower for the vinyl−fluoride coupling (ΔG⧧ = 28.3 kcal/mol)
than for the phenyl−fluoride coupling (ΔG⧧ = 35.7 kcal/mol).
The disparity in reaction barriers for concerted reductive
coupling reactions of phenyl vs vinyl has been previously
studied in the (POCOP)Ir system, where it was attributed to
the smaller steric profile of the vinyl group.44 For concerted
reductive coupling, the aryl (or vinyl) group has to “face” its
coupling partner (in our work fluoride) in the transition state.
However, in pincer systems similar to (POCOP)Rh, the bulky
phosphines force a perpendicular, “side-on” orientation of the
aryl (or vinyl) group (Scheme 3). Therefore, the aryl or vinyl
group must rotate about the metal−carbon bond to attain the
“face-on” arrangement in the transition state. This rotation
should be more facile for the smaller vinyl group. Notably, this
is not a concern in the concerted Ar-X bond formation from
We have also examined the potential for C−C reductive
coupling involving the Rh-bound carbon of the POCOP ligand.
Not surprisingly, calculations revealed the barrier for aryl−vinyl
C−C coupling from 3 is much lower than that for C−F
coupling (TS3-5, Figure 1). However, the product formed via
C−C reductive coupling (5)45 is 6.1 kcal/mol less favorable
than 4a, is 23 kcal/mol less favorable than 4b, and is
considerably unfavorable (by 9.7 kcal/mol) with respect to 3.
C−C reductive elimination from (pincer)RhIII species is
typically quite favorable, but in this case, the constraint of the
chelate ligand makes for a product (5) with a high degree of
congestion and strain. Bearing this in mind, we decided to
explore the experimental potential of C−F reductive coupling
from (POCOP)Rh(CHCH2)(F).
Attempted Synthesis of (POCOP)Rh(CHCH2)(X) Com-
pounds. We previously described oxidative addition of aryl
halides to (PNP)Rh(SiPr2)38,43 and believed an analogous
reaction between CH2CHI and (POCOP)Rh(SiPr2) could
provide a synthetically useful precursor toward the synthesis of
3. The reaction of (POCOP)Rh(SiPr2) (6) with vinyl iodide
resulted in a reaction upon mixing at room temperature. By
analogy with (POCOP)Rh(Ar)(I),39 (POCOP)Rh(CHCH2)-
(I) would be expected to display a doublet resonance in the
31P{1H} NMR spectrum with JRh−P ≈ 120 Hz; however, such a
resonance was not in evidence. The reaction instead resulted in
a mixture of compounds with one dominant product (ca. 60%).
This compound was isolated in a pure form in 52% yield from
the reaction mixture as an orange crystalline solid and identified
as 7 on the basis of the solution NMR data and an X-ray
structural study on a single crystal (Scheme 4). 7 displayed two
sets of doublets of doublets in the 31P{1H} NMR spectrum at
200.5 ppm (JRhP = 140 Hz, JPP = 473 Hz) and 163.7 ppm (JRhP
= 128 Hz, JPP = 473 Hz). The solid-state structure showed a
bimetallic complex containing two square-planar Rh(I) centers,
ostensibly arising from 2 by way of C(aryl)−C(vinyl) reductive
elimination to form 5 and subsequent dimerization of the latter.
The C−C bond lengths (1.420(6) and 1.412(6) Å) of the
bound olefins are typical for an η2-olefin complex of Rh(I).46
The key difference between the computationally evaluated 5
and its dimer 7 is that the two phosphinite donors of the same
Scheme 3. Illustration of the Rotation about the Rh−C(sp2)
Bond Necessary in the Transition State vs the Ground State
a
of 1 or 3
a
Overlayed stick representations of the calculated transition states for
the Ph−F (TS1-2) and vinyl−F (TS3-4) coupling are shown at the
bottom.
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straightforward than that of the corresponding (POCOP)Rh-
(H)(Cl).39 9 formed in nearly quantitative yield by the reaction
of (tBuPOCOP)H (8) with [(cod)RhCl]2 after 2 h at 100 °C
and displayed a characteristic hydride resonance appearing as a
doublet of triplets (JRhH = 44 Hz, JPH = 13 Hz) at −25.19 ppm
Scheme 4. Reactivity of Vinyl Iodide with
(POCOP)Rh(SiPr2) (5) and ORTEP Drawing (50%
Probability Ellipsoids) of 7 Showing Selected Atom
a
Labeling
1
in the H NMR spectrum. There was no indication of the
POCOP-bridged side product that plagued the reaction of
(POCOP)H with [(cod)RhCl]2.39 Notably, that side product
must require the POCOP ligand to contort in a fashion similar
to that observed in 7.
(
tBuPOCOP)Rh(SiPr2) (10) was formed by reaction of 9
with NaOtBu in the presence of SiPr2. Treatment of 10 with
vinyl iodide at room temperature resulted in an immediate
color change to red and 95% conversion to (tBuPOCOP)Rh-
(CHCH2)(I) (11)a product analogous to P2. 11 was
isolated as a red solid and displayed a doublet at 177.4 ppm
with JRhP = 122 Hz in the 31P{1H} NMR spectrum. The
reaction consistently produced ca. 5% of a side product at 204
ppm (JRhP = 155 Hz) in the 31P{1H} NMR spectrum, which
would be consistent with formation of an Rh(I) olefin adduct.
For example, the 1-hexene adduct of (tBuPOCOP)Rh displays a
doublet at 197.3 ppm with JRhP = 160 Hz. We initially thought
the impurity could be an equilibrium between the isomeric η2-
iodoethylene adduct and 11. However, the reaction of
a
Hydrogen atoms are omitted for clarity, with the exception of the
vinylic protons on C1, C2, C27, and C28. Selected bond distances (Å)
and angles (deg) for 7: P1−Rh1, 2.359(1); P2−Rh1, 2.259(1); Rh1−
C1, 2.140(4); Rh1−C2, 2.123(4); C1−C2, 1.420(6); P1−Rh1−P2,
168.47(4); I1−Rh1−C1, 171.2(1); P2−O2−C4, 119.0(3); C8−O3−
P3, 132.1(2), C1−C2−C3, 123.4(3).
(
tBuPOCOP)Rh(CHCH2)(OTf) (12), which is formed by
treatment of 11 with AgOTf, with Me3SiI resulted in
conversion to 11 with no observation of the 5% impurity.
The vinyl iodide was used as received from the distributor with
reported 95% purity, with no additional purification. Examina-
POCOP ligand are bound to two different Rh atoms. This
rearrangement relieves the strain inherent in 5 and leads to
geometrically “normal”, square-planar Rh(I) product. From the
X-ray structure, it is clear that, in order for 7 to form, the
phosphinites must be able to rotate about the C−O bond. We
surmised that, if this movement could be restricted, it should
prevent formation of 7 and preclude formation of a stable
Rh(I) product after C(aryl)−C(vinyl) reductive elimination.
Synthesis of (tBuPOCOP)Rh Complexes. We envisioned
that the analogous tert-butyl-substituted tBuPOCOP ligand
would provide just such a restriction and investigated the
synthesis of (tBuPOCOP)Rh(CHCH2)(I) (11, Scheme 5).
Synthesis of (tBuPOCOP)Rh(H)(Cl) (9) proved much more
1
tion of the H NMR spectrum vinyl iodide showed a trace of
divinyl ether, which could be the olefin source for the 5%
impurity.
The reaction of 12 with CsF in C6D6 after 24 h at room
temperature resulted in complete conversion of 12 to a mixture
of products, with no indication of the desired (tBuPOCOP)-
Rh(CHCH2)(F) product. Analysis of the reaction mixture by
19F NMR spectroscopy indicated P−F bond formation, as
evidenced by the presence of 19F NMR resonances displaying
very strong coupling to 31P (JPF = 480−640 Hz), but no signal
attributable to a Rh−F bond.47 Similarly, treatment of 11 with
AgF produced no evidence of Rh−F and NMR resonances with
Scheme 5. Reactivity of (tBuPOCOP)Rh in the Direction of a Rh Vinyl Fluoride Complex
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large JPF coupling constants. P−F bond formation has been
observed previously with phosphine-substituted rhodium
fluoride complexes, including the fluoride analogue of
Wilkinson’s catalyst.48
The major product was isolated from the reaction mixture from
the thermolysis of 14 and analyzed by 1H NMR and 31P NMR
1
spectroscopy. The H NMR spectrum displayed no downfield
vinylic proton resonances, indicating the absence of an η1-vinyl
substituent. The spectrum also showed one aromatic resonance,
corresponding to the C−H on the POCOP backbone, which
indicated that the OC6H4F group was not present in the major
product from the thermolysis of 14. The product displayed two
doublets in the 31P{1H} NMR spectrum (197.6 ppm, JRhP = 158
Hz; 197.5 ppm, JRhP = 158 Hz), which we interpreted as an
ABX pattern with outer bands too small to observe due to the
close proximity of the chemical shifts of the inequivalent 31P
nuclei. The identity of the major product was determined to be
the bridging divinylacetylene complex 15 by X-ray analysis of a
single crystal grown from a saturated toluene solution at room
temperature (Figure 2). The exact mechanism of how 15 forms
is unclear. However, the composition of 15 suggests a
stoichiometry whereby it arose from two (tBuPOCOP)Rh
fragments combined with three C2 units, with loss of three
ROH molecules and one (tBuPOCOP)Rh fragment. This is
consistent with the ca. two-thirds yield of 15. In addition, when
thermolysis of 14 was carried out in the presence of SiPr2, a ca.
60:40 mixture (by 31P NMR intensity) of 15 and 10 was
observed, and p-FC6H4OH was the only product detectable by
19F NMR spectroscopy.
In order to probe for potential side reaction pathways in a
related system, we turned to the analogous vinyl/alkoxide
complexes. C−O reductive elimination is also regarded as quite
challenging, largely for the same reasons as for C−F, if to a
lesser degree.49 In this vein, we prepared two (tBuPOCOP)Rh-
t
(CHCH2)(OR) (R = Bu, p-C6H4F) complexes (Scheme 6).
Scheme 6. Reactivity of (tBuPOCOP)Rh(CHCH2)(OR)
Complexes
CONCLUSION
■
The (POCOP)Rh and (tBuPOCOP)Rh systems were un-
successful for C−F bond formation. Both reactions suffered
from dominating lower energy side processes, a problem that
has plagued many metal systems. The (POCOP)Rh system
underwent C−C RE with the aryl backbone of the pincer
ligand. Installation of tert-butyl groups on the central aryl ring
of the POCOP system obviated C−C coupling as the side
reaction, but the (tBuPOCOP)Rh system underwent P−F bond
formation upon attempts to make (tBuPOCOP)Rh(CHCH2)-
(F). (tBuPOCOP)Rh(CHCH2)(OR) complexes were also
examined. C−O reductive elimination was not observed, and
instead a divinylacetylene complex was formed.
(
tBuPOCOP)Rh(CHCH2)(OtBu) (13) was synthesized by
reacting 12 with 1.2 equiv of NaOtBu in toluene at room
temperature and isolated in 49% yield. (tBuPOCOP)Rh-
(CHCH2)(OC6H4F) (14) was isolated in 68% yield from the
reaction of 13 with 1 equiv of p-FC6H4OH. 13 and 14
displayed doublets at 165.9 ppm (JRhP = 130 Hz) and 169.2
ppm in 31P NMR spectroscopy, respectively.
Independent thermolyses of 13 and 14 at 110 °C in toluene
resulted in mixtures of products; however, both reactions
formed the same major product as ca. 70% of the 31P content of
the mixture.50 Neither reaction displayed resonances consistent
with formation of the free or coordinated C−O reductive
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise specified, all manip-
elimination product by H NMR or 31P NMR spectroscopy.
ulations were performed under an argon atmosphere using standard
1
Figure 2. ORTEP drawings (50% probability ellipsoids) of 13 (left) and 15 (right) showing selected atom labeling. Hydrogen atoms are omitted for
clarity. Selected bond distances (Å) and angles (deg) for 13: Rh1−P1, 2.286(1); Rh1−P2, 2.308(1); Rh1−C1, 1.993(3); Rh1−O3, 2.053(2); C1−
C2, 1.321(4); O3−C3, 1.419(3); P1−Rh1−P2, 151.19(3); C7−Rh1−O3, 159.69(9); Rh1−C1−C2, 135.6(2); Rh1−O3−C3, 134.7(2). Selected
bond distances (Å) and angles for 15: Rh1−P1, 2.261(2); Rh1−P2, 2.242(2); Rh1−C1, 2.175(7); Rh1−C2, 2.206(8), C1−C2, 1.39(1); C2−C3,
1.42(1); C3−C3′, 1.23(1); P1−Rh1−P2, 156.72(7), C16−Rh1−C1, 159.1(3); C16−Rh1−C2, 163.9(3); C1−C2−C3, 123.4(8); C2−C3−C3′,
176.7(9).
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Schlenk line or glovebox techniques. Toluene, THF, pentane, and
isooctane were dried and deoxygenated (by purging) using a solvent
purification system and stored over molecular sieves in an Ar-filled
glovebox. C6D6 and hexanes were dried over and distilled from NaK/
Ph2CO/18-crown-6 and stored over molecular sieves in an Ar-filled
glovebox. Fluorobenzene was dried with and then distilled or vacuum-
transferred from CaH2. All other chemicals were used as received from
commercial vendors. NMR spectra were recorded on a Varian NMRS
500 (1H NMR, 499.686 MHz; 13C NMR, 125.659 MHz, ;31P NMR,
solution of toluene layered with pentane at −35 °C. 13P{1H} NMR
1
(C6D6): δ 184.4 (d, 122 Hz). H NMR (C6D6, Figure 5-5 in the
Supporting Information): δ 7.22 (s, 1H, POCOP), 2.50 (m, 2H,
t
CHMe2), 2.10 (m, 2H, CHMe2), 1.47 (s, 18H, Bu), 1.29 (q, 9.0 Hz,
6H, CHMe2), 1.16 (q, 9.0 Hz, 6H, CHMe2), 1.07 (m, 12H, CHMe2),
−25.19 (dt, JRhH = 44 Hz, JPH = 13 Hz, 1H, Rh-H). 13C{1H} NMR
(C6D6): 162.3 (t, JPC = 6.4 Hz), 130.5 (dt, JRhC = 30 Hz, JPC = 5.0 Hz),
127.6 (t, JPC = 5.1 Hz), 122.0 (s), 34.7 (s, CMe3), 30.4 (s, CMe3), 30.0
(t, JPC = 11 Hz, CHMe2), 28.5 (td, JPC = 14 Hz, JRhC = 2.0 Hz,
CHMe2), 17.6 (s, CHMe2), 17.4 (t, JPC = 2.4 Hz, CHMe2), 17.1 (t, JPC
1
202.298 MHz; 19F NMR, 470.111 MHz) spectrometer. For H and
13C NMR spectra, the residual solvent peak was used as an internal
reference. 31P NMR spectra were referenced externally using 85%
H3PO4 at δ 0 ppm. 19F NMR spectra were referenced externally using
1.0 M CF3CO2H in CDCl3 at −78.5 ppm. Elemental analyses were
performed by CALI Laboratories, Inc. (Parsippany, NJ).
Computational Details. All computations were carried out with
the Gaussian09 program.51 All of the geometries were fully optimized
in toluene solvent via the PCM model52 by the M0653 functional. The
Stuttgart basis set (SDD) and the associated effective core potential
(ECP) was used for Rh atoms, and an all-electron 6-311G(d,p) basis
set was used for the other atoms. The harmonic vibrational frequency
calculations were performed to ensure that either a minimum or first-
order saddle point was obtained. The energies reported here are Gibbs
free energies with solvent effect corrections at 298.15 K and 1 atm
unless noted otherwise.
Reaction of 6 with CH2CHI. 6 (110 mg, 0.20 mmol) was placed in
a J. Young tube and dissolved in C6D6. Vinyl iodide (15 μL, 0.21
mmol) was added to the solution, resulting in the immediate
precipitation of a large amount of orange solid. Analysis of the
solution after 24 h at room temperature by 31P{1H} NMR
spectroscopy indicates no remaining (POCOP)Rh(SiPr2) and several
products. The major product (60%) appears as two sets of doublets of
doublets (200.5 ppm, dd, JRhP = 140 Hz, JPP = 473 Hz; 163.7 ppm, dd,
JRhP = 128 Hz, JPP = 473 Hz). There is no indication of the desired
product, (POCOP)Rh(CHCH2)(I), by 31P{1H} NMR, which would
be expected to display a doublet with Rh−P coupling around 120 Hz.
The sample was transferred to a Schlenk flask, and the solvent was
decanted off from the orange solid. The solid was washed with pentane
and dried under vacuum. Analysis of the solid by 31P{1H} NMR
identified the precipitate as the major product 7. X-ray-quality crystals
of the solid were grown from a concentrated solution of the solid in
toluene at room temperature. 31P{1H} NMR (CD2Cl2): δ 200.2 (dd,
JRhP = 140 Hz, JPP = 468 Hz), 163.0 (dd, JRhP = 128 Hz, JPP = 468 Hz.
1H NMR (CD2Cl2, Figure 5-3 in the Supporting Information): δ 6.99
(t, 7.5 Hz, 2H, POCOP), 6.77 (d, 9.0 Hz, 2H, POCOP), 6.63 (d, 9.0
Hz, 2H, POCOP), 5.41 (m, 2H, CHCH2), 3.33 (m, 2H, CHCH2),
3.22 (m, 2H, CHMe2), 3.04 (m, 2H, CHMe2), 2.85 (m, 2H, CHCH2),
2.39 (m, 2H, CHMe2), 1.68 (dd, 15 Hz, 7.5 Hz, 6H, CHMe2), 1.59 (m,
12H, CHMe2), 1.45 (m, 12H, CHMe2), 1.18 (dd, 10 Hz, 7.0 Hz, 6H,
CHMe2), 1.08 (dd, 11 Hz, 7.5 Hz, 6H, CHMe2), 1.00 (dd, 16 Hz, 7.0
Hz, CHMe2). Anal. Calcd for C40H68I2O4P4Rh2: C, 40.15; H, 5.73.
Found: C, 39.99; H, 5.59.
=
4.4 Hz, CHMe2), 16.8 (s, CHMe2). Anal. Calcd for
C26H48ClO2P2Rh: C, 52.66; H, 8.16. Found: C, 52.64; H, 8.20.
Synthesis of (tBuPOCOP)Rh(SiPr2) (10). 9 (1.0 g, 1.7 mmol) was
placed in a Schlenk flask and dissolved in toluene. NaOtBu (300 mg,
3.4 mmol) and SiPr2 (500 μL, 3.4 mmol) were added to the solution,
and the reaction mixture was stirred for 1 h at room temperature. This
resulted in an immediate color change to red-brown. The volatiles
were removed by vacuum. The product was extracted with pentane
and passed through a pad of Celite. The volatiles were removed by
vacuum to give clean 10 as a brown-orange solid (1.1 g, 91%). 31P{1H}
NMR (C6D6): δ 183.1 (d, 173 Hz). 1H NMR (C6D6, Figure 5-6 in the
Supporting Information): δ 7.24 (s, 1H, POCOP), 2.72 (m, 2H,
t
CHMe2), 2.20 (m, 2H, CHMe2), 1.65 (s, 18H, Bu), 1.27 (m, 36H,
CHMe2 and S(iPr)2). 13C{1H} NMR (C6D6): δ 162.8 (t, JPC = 8.5 Hz),
144.2 (dt, JRhC = 33 Hz, JPC = 2.5 Hz), 124.4 (t, JPC = 6.0 Hz), 119.9
(s), 41.0 (t, JPC = 3.0 Hz, SCHMe2), 34.7 (s, CMe3), 30.8 (s, CMe3),
30.7 (td, JPC = 10 Hz, JRhC = 3.0 Hz, CHMe2), 24.2 (s, SCHMe2), 18.7
(t, JPC = 3.5 Hz, CHMe2), 17.8 (s, CHMe2). Anal. Calcd for
C32H61O2P2RhS: C, 56.96; H, 9.11. Found: C, 56.94; H, 9.08.
Synthesis of (tBuPOCOP)Rh(CHCH2)(I) (11). 10 (500 mg, 0.74
mmol) was placed in a Schlenk flask and dissolved in toluene. Vinyl
iodide (165 μL, 2.2 mmol) was added to the solution, resulting in an
immediate color change to red. The reaction mixture was stirred for 20
min at room temperature. The solution was passed through a pad of
Celite, and the volatiles were removed by vacuum to give a red solid
(489 mg, 93%). The red solid was 95% clean by 31P{1H} NMR
spectroscopy, with a minor impurity at 204 ppm (d, 155 Hz). The
identity of the impurity is still unknown. 31P{1H} NMR (C6D6): δ
1
177.4 (d, 122 Hz); H NMR (C6D6, Figure 5-7 in the Supporting
Information): δ 7.30 (s, 1H, POCOP), 6.78 (d, 11 Hz, 1H, CHCH2),
3.88 (s, 1H, CHCH2), 3.79 (d, 14 Hz, 1H, CHCH2), 2.65 (m, 4H,
t
CHMe2), 1.51 (s, 18H, Bu), 1.41 (q, 9.0 Hz, 6H, CHMe2), 1.31 (q,
9.0 Hz, 6H, CHMe2), 0.96 (q, 9.0 Hz, 6H, CHMe2). 13C{1H} NMR: δ
160.6 (t, JPC = 5.9 Hz), 145.6 (d, JRhC = 35 Hz, CHCH2), 142.4 (dt,
JRhC = 34 Hz, JPC = 10 Hz), 128.4 (s), 122.1 (d, JRhC = 6.0 Hz,
CHCH2), 119.8 (t, JPC = 4.4 Hz), 34.9 (s, CMe3), 31.6 (t, JPC = 11 Hz,
CHMe2), 30.4 (s, CMe3), 28.6 (td, JPC = 13 Hz, JRhC = 2.0 Hz,
CHMe2), 18.3 (s, CHMe2), 18.1 (s, CHMe2), 17.8 (s, CHMe2), 16.8 (s,
CHMe2).
Synthesis of (tBuPOCOP)Rh(CHCH2)(OTf) (12). 11 (101 mg, 0.15
mmol) was placed in a Schlenk flask and dissolved in toluene. AgOTf
(46 mg, 0.18 mmol) was added to the sample, resulting in immediate
precipitation and a color change to brown. The sample was stirred at
room temperature for 60 min. The volatiles were removed by vacuum.
The product was extracted with pentane and passed through a pad of
Celite. The volatiles were removed to give 11 as a brown-orange solid.
Synthesis of (tBuPOCOP)H (8). 4,6-Di-tert-butylresocinol (1.07 g,
4.8 mmol) was placed in a reaction vial and dissolved in toluene. Et3N
(2.0 mL, 14.5 mmol) and ClPiPr2 (1.51 mL, 10.1 mmol) were added
to the solution, and the reaction mixture was heated at 110 °C for 24
h. The mixture was passed through a pad of Celite, and the volatiles
were removed by vacuum, leaving an oily white solid. The solid was
dissolved in a minimum of pentane and left at −35 °C. The product
crashed out of solution as a crystalline white solid (1.0 g, 46%).
31P{1H} NMR (C6D6): δ 139.0 (s). 1H NMR (C6D6, Figure 5-4 in the
Supporting Information): 8.52 (t, 7.0 Hz, 1H, Ar-H), 7.46 (s, 1H, Ar-
1
31P{1H} NMR (C6D6): δ 173.0 (d, 126 Hz); H NMR (C6D6, Figure
5-8 in the Supporting Information): δ 7.25 (s, 1H, POCOP), 6.80 (d,
13 Hz, 1H, CHCH2), 4.13 (m, 1H, CHCH2), 3.94 (d, 13 Hz, 1H,
CHCH2), 2.66 (m, 2H, CHMe2), 2.56 (m, 2H, CHMe2), 1.43 (s, 18H,
tBu), 1.21 (m, 12H, CHMe2), 1.12 (q, 9.0 Hz, 6H, CHMe2), 0.92 (q,
9.0 Hz, 6H, CHMe2); 19F NMR (C6D6): δ-77.7 (s). 13C{1H} NMR
(C6D6): δ 162.0 (t, JPC = 5.4 Hz), 140.2 (dt, JRhC = 37 Hz, JPC = 9.6
Hz, CHCH2), 133.3 (dt, JRhC = 40 Hz, JPC = 5.1 Hz), 123.4 (s), 122.1
(s, CHCH2), 118.1 (t, JPC = 4.0 Hz), 34.8 (s, CMe3), 30.9 (t, JPC = 10
Hz, CHMe2), 30.3 (s, CMe3), 28.3 (td, JPC = 13 Hz, JRhC = 2.4 Hz,
CHMe2), 18.5 (t, JPC = 3.0 Hz, CHMe2), 18.1 (s, CHMe2), 16.7 (t, JPC
t
H), 1.83 (m, 4H, CHMe2), 1.54 (s, 18H, Bu), 1.14 (dd, 7.5 Hz, 3.5
Hz, 12H, CHMe2), 1.02 (dd, 7.5 Hz, 7.5 Hz, 12H, CHMe2).
Synthesis of (tBuPOCOP)Rh(H)(Cl) (9). 8 (1.5 g, 3.3 mmol) was
placed in a reaction vial and dissolved in toluene. [(cod)RhCl]2 (0.81
g, 1.6 mmol) was added to the solution, and the mixture was stirred at
100 °C for 2 h. The solution was passed through a pad of Celite. The
volatiles were removed by vacuum to give clean 9 as an orange solid
(1.9 g, 95%). The product can be recrystallized from a saturated
=
2.2 Hz, CHMe2), 16.1 (s, CHMe2). Anal. Calcd for
C29H50F3O5P2RhS: C, 47.54; H, 6.88. Found: C, 47.60; H, 6.77.
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Treatment of 11 with AgF. (tBuPOCOP)Rh(CHCH2)(I) (11; 27
mg, 0.04 mmol) was combined in a Schlenk flask with AgF (25 mg,
0.20 mmol) and dissolved in THF. The reaction mixture was stirred at
room temperature overnight, resulting in a brown suspension. Analysis
of the reaction mixture by 31P NMR spectroscopy showed a complex
mixture of products. The 19F NMR spectrum showed several signals
with large coupling (J = 640 Hz, J = 853 Hz, J = 982 Hz), consistent
with P−F bond formation.
Treatment of 12 with CsF. 12 (33 mg, 0.045 mmol) was placed
in a Schlenk flask and dissolved in C6D6. CsF (14 mg, 0.090 mmol)
was added to the flask, and the reaction mixture was stirred at room
temperature. After 24 h at room temperature, the mixture showed
complete conversion to a mixture of new products by 31P{1H} NMR.
Analysis of the 19F NMR spectrum indicated the presence of P−F
bond formation (J = 639 Hz and J = 480 Hz), but there were no
signals indicating the formation of a Rh−F bond.
115.5 (s), 115.4 (s), 34.8 (s, CMe3), 30.8 (t, JPC = 9.5 Hz, CHMe2),
30.4 (s, CMe3) 28.4 (td, JPc = 15 Hz, JRhC = 2.5 Hz, CHMe2), 18.6 (s,
CHMe2), 17.1 (t, JPC = 3.8 Hz, CHMe2), 16.7 (t, JPC = 2.0 Hz,
CHMe2), 16.2 (s). Anal. Calcd for C34H54FO3P2Rh: C, 58.79; H, 7.84.
Found: C, 58.76; H, 7.79.
Thermolysis of 14. 14 (14 mg, 0.020 mmol) was placed in a J.
Young tube and dissolved in C6D6. The sample was heated in a 110 °C
oil bath and monitored by 31P{1H} NMR and 1H NMR spectroscopy.
Analysis of the 31P{1H} NMR spectrum after 60 min indicated 30%
conversion to two major products (197.5 ppm, d, J = 157.7 Hz, 17%;
183.9 ppm, d, J = 205.4 Hz, 15%). After 24 h at 110 °C, the reaction
mixture displayed complete conversion from the starting material and
the presence of four products by 31P{1H} NMR spectroscopy. The
major product (197.5 ppm, d, J = 157.7 Hz, 70%) is the same major
product from the thermolysis of 13, identified as 15. The identity of
the side products could not be determined. The product mixture was
washed with pentane, resulting in the isolation of a small amount of
pure 15 as an orange solid. X-ray-quality crystals of 15 were grown
from a saturated toluene solution of pure 15 at room temperature. 1H
and 31P{1H} NMR data for 15 are as follows. 31P{1H} NMR (C6D6): δ
Synthesis of (tBuPOCOP)Rh(CHCH2)(OtBu) (13). 12 (240 mg,
0.327 mmmol) was combined with NaOtBu (38 mg, 0.39 mmol) in a
flask and dissolved in toluene. The reaction mixture was stirred at
room temperature for 60 min with no noticeable color change. The
volatiles were removed by vacuum. The orange solid was extracted
with pentane and passed through a pad of Celite. The volatiles were
removed to yield an orange solid (154 mg, 72%). The solid was
recrystallized from pentane at −35 °C to yield a crystalline orange
solid (105 mg, 49%) and X-ray-quality crystals. Complex 13 displays
limited stability at ambient temperature, decomposing over a few
hours in C6D6 solution. This prevented us from collecting a reliable
13C NMR spectrum and elemental analysis data. 31P{1H} NMR
(C6D6): 165.9 (d, JRh−P = 130 Hz). 1H NMR (C6D6, Figure 5-9 in the
Supporting Information): 7.49 (m, 1H, CHCH2), 7.22(s, 1H,
POCOP), 4.54 (t, 5.5 Hz, 1H, CHCH2), 3.93 (d, 14 Hz, 1H,
CHCH2), 2.58 (m, 2H, CHMe2), 2.36 (m, 2H, CHMe2), 1.57 (s, 27H,
tBu and OtBu), 1.32 (q, 8.0 Hz, 6H, CHMe2), 1.25 (m, 12H, CHMe2),
1.09 (q, 8.0 Hz, 6H, CHMe2).
1
197.5 (d, 158 Hz), 197.6 ppm (d, 158 Hz). H NMR (C6D6): δ 7.31
(s, 2H, POCOP), 4.39 (m, 2H, vinyl), 3.52 (m, 2H, vinyl), 2.59 (t, 5
Hz, 2H, vinyl), 2.38 (m, 4H, CHMe2), 2.23 (m, 4H, CHMe2), 1.62 (s,
36H, tBu), 1.37 (m, 12H, CHMe2), 1.30 (m, 12H, CHMe2), 1.14 (m,
12H, CHMe2), 1.04 (m, 12H, CHMe2).
Thermolysis of 14 in the Presence of SiPr2. 14 (25 mg, 0.039
mmol) and SiPr2 (6 μL, 0.039 mmol) were combined in a J. Young
tube and dissolved in C6D6. A sealed capillary containing a solution of
PPh3 in C6D6 was also added to the J. Young tube to act as an
integration standard. Analysis of the sample by 31P{1H} NMR
spectroscopy at room temperature showed only the presence of 14
and the signal for PPh3. After the sample was heated overnight in a 110
°C oil bath, the 31P{1H} NMR spectrum showed complete conversion
of 14 to a mixture of 15 (60%) and 10 (40%). The 19F NMR spectrum
showed only the presence of p-FC6H4OH.
Thermolysis of 13. 13 (23 mg, 0.035 mmol) was placed in a J.
Young tube and dissolved in C6D6. The sample was heated in a 110 °C
oil bath and monitored by 31P{1H} NMR and 1H NMR spectroscopy.
After 30 min at 110 °C, the reaction mixture showed complete
conversion from the starting material to three new compounds, with a
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, and CIF and mol files giving select details of
experimental procedures, graphical NMR spectra, details of X-
ray structural determinations, and a calculated structure. This
material is available free of charge via the Internet at http://
pubs.acs.org.
1
major compound making up 70% of the reaction mixture. The H
NMR spectrum no longer displayed any downfield vinyl resonances,
indicating there were no longer any η1 vinyl complexes present in the
mixture. Additional heating at 110 °C for 24 h resulted in the presence
of a new compound at 204.4 ppm (d, J = 154.3 Hz, 5%) in the
31P{1H} NMR spectrum; however, the fraction of the major product
was unchanged (70%). The reaction was passed through a pad of
Celite, and the volatiles were removed by vacuum. The brown-orange
solid was washed with pentane, leaving a bright orange solid. Analysis
of the solid by 31P{1H} NMR and 1H NMR showed only the presence
of the major product, identified as 15. The 31P{1H} NMR spectrum
displays the complex as two doublets with the same Rh−P coupling
constant (J = 157.7 Hz). The identity of the side products could not
be determined.
AUTHOR INFORMATION
Corresponding Author
*E-mail for O.V.O.: ozerov@chem.tamu.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Synthesis of (tBuPOCOP)Rh(CHCH2)(OC6H4F) (14). 13 (43 mg,
0.065 mmol) was placed in a Schlenk flask and dissolved in toluene. p-
FC6H4OH (7.4 mg, 0.065 mmol) was added to the sample, and the
reaction mixture was stirred for 30 min at room temperature. The
reaction mixture was passed through a pad of Celite, and the volatiles
were removed by vacuum to give a red solid. The solid was
recrystallized from pentane at −35 °C to give a red crystalline product
■
We are thankful for the support of this work by the U.S.
National Science Foundation (Grants CHE-0944634 and CHE-
1300299 to O.V.O.) and by the Welch Foundation (Grant A-
1717 to O.V.O.).
REFERENCES
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(31 mg, 68%). 31P{1H} NMR (C6D6): δ 169.2 (d, J = 128.2 Hz); H
■
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