Bidirectional transfer of phenyl and methyl groups between PtIV and boron in platinum dipyridylborato complexes

Article abstract of DOI:10.1021/ja804222c

In the presence of weakly Lewis basic DMSO methyl group migration between B and Pt^IV in dipyridylborato trihydrocarbyl Pt^IV complexes is reversible; the preferred direction of Ph group migration in THF solution is from Pt^IV

Full text of DOI:10.1021/ja804222c

Published on Web 07/12/2008
Bidirectional Transfer of Phenyl and Methyl Groups between PtIV and Boron
in Platinum Dipyridylborato Complexes
Eugene Khaskin, Peter Y. Zavalij, and Andrei N. Vedernikov*
Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742
Received June 4, 2008; E-mail: avederni@umd.edu
Scheme 1
Organoboron compounds are widely used in organic synthesis
as a source of nucleophilic hydrocarbyl groups for transition metal
catalyzed construction of C-C bonds.1 Despite the widespread use
of Suzuki-Miyaura coupling reaction, the mechanism of one of
its key steps (b, Scheme 1) involving hydrocarbyl transfer between
boron and a transition metal atom catalyst is poorly studied.2–4
Previously we reported facile boron-to-PtIV methyl group transfer
observed upon oxidation of PtII with O2 in dimethyldipyridylborato
complexes Na[LPtIIR2] (Scheme 2; R ) Me, 1; Ph, 4) and in
LPtIVR2Me complexes (R ) Me, 2; Ph, 5) in hydroxylic solvents
R′OH, both leading to products of the structural type 3.4 The
hydrocarbyl migration was suggested to occur as an electrophilic
substitution at one of the carbon atoms of the BMe2 fragment with
five-coordinate electrophilic PtIV assisted by nucleophilic attack of
the solvent at the boron atom. In this work we disclose that methyl
migration between B and PtIV can be reVersible and that phenyl
group transfer between B and PtIV centers can occur in both
directions, from B to PtIV and from PtIV to B. These observations
imply, in particular, that under certain basic conditions there might
be an additional pathway leading to products of homocoupling of
organoboron compounds5 occurring along with the desired cross-
coupling between a boron reagent and an electrophile.
Scheme 2
Scheme 3
Considering formation of a strong B-O bond in 3 as the driving
force of B-to-PtIV hydrocarbyl migration in hydroxylic solvents4
we hypothesized that in the presence of a poor nonhydroxylic Lewis
base hydrocarbyl transfer between B and PtIV may become
reversible. To probe reversibility of methyl transfer in 2 a Pt-13C-
labeled complex [Me2B(py)2]PtIVMe2*Me, 2-Pt-13C, was prepared
from 1 and 1 equiv of *MeI (*Me ) 13CH3). Five minutes after
mixing the reagents, a statistical distribution of the label among
the three Pt-bound methyl ligands was reached resulting in their
effective 33% 13C-enrichment (compare to fluxional behavior of
54). The labeled complex 2-Pt-13C is stable in benzene-d6 or THF-
d8 in the temperature range of 20°-60 °C for at least 2 weeks and
shows no sign of 13C-label incorporation in the BMe2 moiety.7 This
situation changed when we used DMSO-d6, a stronger nonhydroxy-
lic Lewis base compared to THF, as the solvent. Slow formation
of 2-B-13C isomers with the 13C-label evenly distributed between
exo- (JCH ) 115 Hz) and endo- (JCH ) 109.3 Hz) methyl groups
of the BMe2 fragment was seen at 60 °C with the observed pseudo-
first order rate constant of (5.1 ( 0.2) ×10-5 min-1 (Scheme 3).
The 13C-enrichment for each of the boron-bound methyl groups
was 15% versus the expected statistical value of 20% after 19 days
(Figure 1).
transfer from PtIV to B leads to an endo-B-13C-labeled species,
2-B-13C. Finally, the interconversion of the endo- to the exo-B-13C
labeled 2-B-13C might imply platinum chelate ring inversion
combined with a Berry pseudorotation at the 5-coordinate PtIV
center.8
Similar experiments were performed with the diphenyl analogue
of 2, LPtIVPh2Me, 5. No reaction was seen in benzene solutions of
5 at 60 °C after 3 days. By contrast, isomerization of 5 to
Cs-symmetrical phenylborato complex [MeBPh(py)2]PtIVPhMe2 7,
involving PtIV-to-B phenyl migration, was complete at 60 °C after
24 h in THF solution. As in the case of trimethyl PtIV complex 2,
the use of more Lewis basic solvent DMSO allowed for faster
transformation; isomerization of 5 to 7 was possible at 22 °C. These
observations also suggest that 7 is more thermodynamically stable
than 5. According to our DFT estimates, the standard Gibbs energy
of isomerization of 5 to 7 is -3.1 kcal/mol. The effect of the boron-
bound Ph group on the position of the hydrocarbyl transfer
equilibrium shown in Scheme 3 may be due to a better ability of
the B-Ph fragment to donate to the PtIV center compared to the
B-Me group in 5. No second PtIV-to-B phenyl migration that would
lead to diphenyldipyridylborato complex L′PtIVMe3, 8 (vide infra)
was observed in this system.
To account for these observations we suggest the following
mechanism (Scheme 3). The first step involves B-to-PtIV migration
leading to a tetrahydrocarbyl PtIV species 6 that is facilitated by
concurrent nucleophilic attack of solvent at the boron atom.
Dissociation of one of the pyridine nitrogens from PtIVMe(R)2R′
in 6 with subsequent scrambling of hydrocarbyl groups in the
resulting five coordinate metal intermediate, re- coordination of the
pyridyl group leading to transient 6′ and, ultimately, 13CH3 group
9
10088 J. AM. CHEM. SOC. 2008, 130, 10088–10089
10.1021/ja804222c CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
isomerization of 6 and 6′. The presence of a bulky Ph group in the
equatorial plane of 6 would diminish the energy required for
dissociation of an adjacent pyridyl.
To probe the option of B-to-PtIV Ph group transfer we attempted
transformation of 8 to its isomer 7. Complex 88 was characterized
by 1H and 13C NMR spectroscopy and by X-ray diffraction (Figure
2b). Similar to 7, one of the boron-bound phenyl groups in 8 is
involved in stabilization of the PtIV center with a B1A-C31A
distance of 1.643(7) Å and Pt1A-C31A distance of 2.624(4) Å.
According to our DFT calculations, the Gibbs energy of isomer-
1
ization of 8 to 7 is -0.4 kcal/mol. No changes were seen in H
NMR spectra of 8 in both THF-d8 and DMSO-d6 after 5 days at
60 °C. We presume that the stability of phenylborato PtIV species
8 as well as 7 might be of kinetic origin, and solvent nucleophilicity
is not sufficient to decrease the reaction barrier noticeably.
TheviabilityofB-to-PtIV phenylgrouptransferinPt-dipyridylborate
systems was demonstrated in a reaction of Na[L’PtIIMe2] with O2
in i-PrOH solution leading to isopropoxo-bridged PtIVMe2Ph
complex 9 (Figure 2c) (eq 1).
Na[Ph2B(py)2]PtMe2 + 0.5O2 + 2i-PrOH f
PhB(py)2(µ-O-i-Pr)PtMe2Ph + NaO-i-Pr + H2O (1)
Strongly nucleophilic i-PrO- present in the reaction mixture might
contribute substantially in diminishing the activation barrier of the
overall B-to-PtIV phenyl group transfer.
In summary, we showed that phenyl group migration between
PtIV and B centers can occur in both directions and is accelerated
by Lewis bases. The prevailing product is determined by a stronger
donor group bridging Lewis acidic B and PtIV centers. In the case
of dimethylborato trimethyl PtIV complex 2 and the use of aprotic
weakly nucleophilic DMSO, methyl migration between PtIV and
B centers is reversible.
Figure 1. Transformation of 2-Pt-13C to 2-B-13C in DMSO-d6 solution at
60 °C: (a) plot of the 13C-enrichment of a boron-bound methyl group vs
time; (b) high-field region of 1H NMR spectrum of a mixture of 25% 2-Pt-
13C and 75% 2-B-13C with 195Pt and 13C satellites after 19 days. The
horizontal axis labels are in ppm; the peak labels are in Hertz.
Acknowledgment. We thank the University of Maryland and
the NSF (Grant CHE-0614798) for the financial support of this
work.
Supporting Information Available: Experimental details, CIF files
for 7, 8, and 9. This material is available free of charge via the Internet
at http://pubs.acs.org.
References
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Figure 2. ORTEP drawings (50% probability ellipsoids) of complexes 7
(a), 8 (b), and 9 (c). Hydrogen atoms are omitted for clarity.
Complex 7 could be isolated from the solution in THF in pure
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group, BMeendo, JPtH ) 58.1 Hz).
(8) See Supporting Information for details.
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