Platinum-catalyzed acrylonitrile hydrophosphination via olefin insertion into a Pt-P bond [3]

Full text of DOI:10.1021/ja970355r

J. Am. Chem. Soc. 1997, 119, 5039-5040
5039
Scheme 1
Platinum-Catalyzed Acrylonitrile
Hydrophosphination via Olefin Insertion into a
Pt-P Bond
Denyce K. Wicht, Igor V. Kourkine, Belinda M. Lew,
J. Mulei Nthenge, and David S. Glueck*
6128 Burke Laboratory, Department of Chemistry
Dartmouth College, HanoVer, New Hampshire 03755
ReceiVed February 3, 1997
Catalytic addition of P-H bonds to unsaturated substrates
mediated by platinum group metal complexes offers a regio-
controlled way to prepare biologically active substances and
useful ligands for homogeneous catalysis.1 Further rational
development of these reactions requires mechanistic understand-
ing, which so far is limited; Scheme 1 shows possible mecha-
nisms for an olefin substrate as an example. After oxidative
addition of the P-H bond,2 it is not known whether catalytic
P-C bond formation occurs by reductive elimination3 (path A)
after insertion of the unsaturated substrate into the M-H bond
or by insertion into the M-P bond4 (path B), followed by C-H
reductive elimination. We report evidence for the latter pathway
in Pt-catalyzed hydrophosphination of acrylonitrile and direct
observation of both proposed P-C bond-forming steps in model
systems.
The complex Pt(dppe)(CH2CHCN) (1) catalyzes hydrophos-
phination of acrylonitrile with PH2Mes*, which yields PHMes*-
(CH2CH2CN) (2) (dppe ) Ph2PCH2CH2PPh2, Mes* ) 2,4,6-
(t-Bu)3C6H2, Scheme 2). The reaction proceeds slowly (10 mol
% 1, THF, 55 °C, one turnover per 24 h), and no intermediates
are observed by 31P NMR during catalysis. However, oxidative
addition of P-H bonds to the catalyst precursor Pt(dppe)(trans-
stilbene) (3) generates the phosphido hydride complexes Pt-
(dppe)(PR1R2)H [R1 ) R2 ) Mes (4); R1 ) H, R2 ) Mes* (5),
Mes ) 2,4,6-Me3C6H2]. Treatment of these hydrides with 2
equiv of acrylonitrile affords the phosphines PMes2CH2CH2-
CN (6) and 2, respectively, and Pt complex 1.
Scheme 2a Platinum Phosphido Hydrides and Alkyls in
Catalytic Acrylonitrile Hydrophosphination
a Pt ) Pt(dppe), R1 ) R2 ) Mes (4, 7, 12); R1 ) H, R2 ) Mes* (5,
8, 13). Reagents: (i) CH2CHCN, PH2Mes*, catalytic in Pt; (ii) PHR1R2,
-stilbene; (iii) 2CH2CHCN, -PR1R2CH2CH2CN (2, 6); (iv) CH2CHCN;
(v) LiN(SiMe3)2 or NaN(SiMe3)2.
Scheme 3a Synthesis and P-C Bond-Forming Reactions of
Platinum and Palladium Phosphido Alkyl Complexes
The putative intermediates in this transformation, Pt(dppe)-
(PR1R2)CH2CH2CN [R1 ) R2 ) Mes (7); R1 ) H, R2 ) Mes*
(8)] were prepared by deprotonation of the cationic phosphine
complex precursors 12 and 13,5 while the analogous methyl
compounds M(dppe)(PR1R2)Me [R1 ) R2 ) Mes, M ) Pt (9);
R1 ) H, R2 ) Mes*, M ) Pt (10), Pd (11)] could be synthesized
a M ) Pt(dppe), except M ) Pd(dppe) for 11, 15, and 18, X )
halide. R1 ) R2 ) Mes: R ) Me (9, 16), R ) CH2CH2CN (7, 12). R1
) H, R2 ) Mes*: R ) Me (10, 11, 14, 15, 17, 18), R ) CH2CH2CN
(8, 13). Reagents: (i) PHR1R2, -MeOH; (ii) AgBF4, PHR1R2; (iii)
LiN(SiMe3)2 or NaN(SiMe3)2; (iv) dppe; (v) CH2CHCN.
(1) (a) Pringle, P. G.; Smith, M. B. J. Chem. Soc., Chem. Commun. 1990,
1701-1702. (b) Hoye, P. A. T.; Pringle, P. G.; Smith, M. B.; Worboys, K.
J. Chem. Soc., Dalton Trans. 1993, 74, 269-74. (c) Pringle, P. G.; Brewin,
D.; Smith, M. B.; Worboys, K. In Aqueous Organometallic Chemistry and
Catalysis; Horvath, I. T., Joo, F., Eds.; Kluwer: Dordrecht, 1995; Vol. 5,
pp 111-122. (d) Han, L.-B.; Choi, N.; Tanaka, M. Organometallics 1996,
15, 3259-3261. (e) Han, L.-B.; Tanaka, M. J. Am. Chem. Soc. 1996, 118,
1571-1572. (f) Nagel, U.; Rieger, B.; Bublewitz, A. J. Organomet. Chem.
1989, 370, 223-239. See also: (g) Han, L.-B.; Choi, N.; Tanaka, M. J.
Am. Chem. Soc. 1996, 118, 7000-7001. For lanthanide-catalyzed reactions,
see: Giardello, M. A.; King, W. A.; Nolan, S. P.; Porchia, M.; Sishta, C.;
Marks, T. J. In Energetics of Organometallic Species; Martinho Simoes, J.
A., Ed.; Kluwer: Dordrecht, 1992; pp 35-51.
by this method from 14 and 15 or by direct proton transfer to
a Pt-OMe group (Scheme 3).6 The Pt phosphido alkyls 7-10
are readily isolated and stable to reductive elimination in
solution, even on heating; as expected from these observations,
complexes 7 and 8 are inactive in catalytic hydrophosphination.
In contrast, Pd complex 11 must be generated at -78 °C, since
it decomposes at room temperature to yield PH(Me)Mes*.7 In
the absence of trapping reagents, the Pd products are Pd(dppe)2
and Pd(0); adding dppe avoids this disproportionation and gives
only Pd(dppe)2 (Scheme 3).8
(2) For examples, see: (a) Ebsworth, E. A. V.; Gould, R. O.; Mayo, R.
A.; Walkinshaw, M. J. Chem. Soc., Dalton Trans. 1987, 2831-2838. (b)
Powell, J.; Fuchs, E.; Gregg, M. R.; Phillips, J.; Stainer, M. V. R.
Organometallics 1990, 9, 387-393. (c) Reference 1e.
These results suggest that catalysis operates by insertion of
acrylonitrile into the Pt-P bond of phosphido hydride 5,
followed by C-H reductive elimination from an unobserved
(3) See refs 1a-c. For previous examples of P-C reductive elimination,
see: (a) Geoffroy, G. L.; Rosenberg, S.; Shulman, P. M.; Whittle, R. R. J.
Am. Chem. Soc. 1984, 106, 1519-1521. (b) Fryzuk, M. D.; Joshi, K.;
Chadha, R. K.; Rettig, S. J. J. Am. Chem. Soc. 1991, 113, 8724-8736.
(4) For insertion of alkynes,4a,b CO,4c CO2,4d diazoalkanes,4e isonitriles,4f
and carbodiimides4f into M-P bonds, see: (a) Barnett, B. L.; Krueger, C.
Cryst. Struct. Commun. 1973, 2, 347-354. (b) Hey-Hawkins, E.; Linden-
berg, F. Chem. Ber. 1992, 125, 1815-1819. (c) Roddick, D. M.; Santarsiero,
B. D.; Bercaw, J. E. J. Am. Chem. Soc. 1985, 107, 4670-4678. (d) Buhro,
W. E.; Chisholm, M. H.; Folting, K.; Huffman, J. C. Inorg. Chem. 1987,
26, 3087-3088. (e) Hey, E.; Muller, U. Z. Naturforsch. B 1989, 44, 1538-
1544. (f) Lindenberg, F.; Sieler, J.; Hey-Hawkins, E. Polyhedron 1996,
15, 1459-1464.
(6) The thermodynamics of the reactions with Pt(dppe)Me(OMe) (see:
Bryndza, H. E.; Domaille, P. J.; Tam, W.; Fong, L. K.; Paciello, R. A.;
Bercaw, J. E. Polyhedron 1988, 7, 1441-52) are under investigation (Wicht,
D. K.; Paisner, S. N.; Glueck, D. S.; Li, C.; Nolan, S. P. Unpublished
results).
(7) See: Brauer, D. J.; Bitterer, F.; Dorrenbach, F.; Hessler, G.; Stelzer,
O.; Kruger, C.; Lutz, F. Z. Naturforsch. B 1996, 51, 1183-1196 and ref 5b
therein.
(8) Such apparent reductive eliminations from Pd(II) phosphido alkyl
complexes, which have been previously proposed to occur in Pd-catalyzed
P-C coupling reactions (see for example: Tunney, S. E.; Stille, J. K. J.
Org. Chem. 1987, 52, 748-753), appear to be general. Details and related
mechanistic studies will be reported separately.
(5) Prepared (Scheme 3) from the precursor Pt(dppe)(CH2CH2CN)Br,
which was synthesized from 3 and BrCH2CH2CN.
S0002-7863(97)00355-7 CCC: $14.00 © 1997 American Chemical Society

5040 J. Am. Chem. Soc., Vol. 119, No. 21, 1997
Communications to the Editor
alkyl hydride intermediate (path B above). In support of this
hypothesis, the phosphido alkyls 9-11 undergo regiospecific
insertion of acrylonitrile into the M-P bonds to give M(dppe)-
[CH(CN)CH2PR1R2]Me [R1 ) R2 ) Mes, M ) Pt (16); R1 )
H, R2 ) Mes*, M ) Pt (17), Pd (18)] (Scheme 3). Platinum
complexes 16 and 17 were isolated as stable pale yellow solids,
while Pd complex 18 decomposes on attempted workup. For
10 and 11, which contain a chiral phosphido group, the insertion
is diastereoselective; 17 and 18 exist as ∼2:1 mixtures of
diastereomers. This is probably a thermodynamic ratio, since
insertion is reversible. Isolated, acrylonitrile-free 16 and 17 de-
insert acrylonitrile slowly in THF solution to reach an equilib-
rium with 9 and 10, while 18, generated in solution, decomposes
to PH(Me)Mes*, via phosphido complex 11.9
However, the observed P-C bond formation from a palladium
phosphido alkyl suggests that path A may also be important in
Pd-catalyzed additions of P-H bonds to unsaturated substrates.
The effects of metal, ancillary ligands, and substrates on these
and related catalyses are currently under investigation.
Acknowledgment. We thank the donors of the Petroleum Research
Fund, administered by the American Chemical Society, the Exxon
Education Foundation, and DuPont for partial support. We also thank
Dartmouth College for support, including an E. E. Just fellowship for
J.M.N., the NSF for a REU fellowship for B.M.L., and Johnson-
Matthey/Alfa/Aesar for loans of Pd and Pt salts.
Supporting Information Available: Analytical and spectroscopic
data for complexes 1-18 (9 pages). See any current masthead page
for ordering and Internet access instructions.
In conclusion, our results suggest that, in this system, Pt-
catalyzed acrylonitrile hydrophosphination proceeds by selective
insertion into the M-P bond in preference to the M-H bond.10
JA970355R
(10) Related pathways are possible in metal-catalyzed hydroamination
and hydroxylation of unsaturated substrates. (a) Cowan, R. L.; Trogler, W.
C. J. Am. Chem. Soc. 1989, 111, 4750-4761. (b) Bennett, M. A.; Jin, H.;
Li, S.; Rendina, L. M.; Willis, A. C. J. Am. Chem. Soc. 1995, 117, 8335-
8340.
(9) These equilibria lie to the right, favoring insertion. Decomposition
during the long reaction times required to reach equilibrium prevented
precise measurement of Keq.