J. Am. Chem. Soc. 1998, 120, 2919-2924
2919
R-Stabilization by Silyl and Phosphino Substitution
Bettina Ro1mer, Geo G. Gatev, Meili Zhong, and John I. Brauman*
Contribution from the Department of Chemistry, Stanford UniVersity, Stanford, California 94305-5080
ReceiVed January 27, 1997
Abstract: The electron affinity of the bis(dimethylphosphino)methyl radical was measured to be 35.3 ( 0.2
kcal/mol, using electron photodetachment spectroscopy in an ion cyclotron resonance spectrometer. Using
equilibrium measurements, ∆H° of bis(dimethylphosphino)methane and bis(trimethylsilyl)methane was
acid
determined to be 370 ( 3 and 373 ( 3 kcal/mol, respectively. From measured and known electron affinities
and gas-phase acidities, we derive C-H bond dissociation energies: bis(dimethylphosphino)methane, 92 ( 3
kcal/mol, and bis(trimethylsilyl)methane, 95 ( 3 kcal/mol. ∆H° of trimethylphosphine was bracketed at
acid
383-387 kcal/mol. The R-stabilization effect of silyl and phosphino substitution is large and comparable in
size to stabilization by thio and chloro substitution. Possible mechanisms of stabilization are discussed.
Introduction
substituted carbanions is increased by an additional 16 kcal/
mol relative to the singly substituted compound.14
The properties of a molecule, such as its acidity or equiva-
lently the stability of its anion, can be changed dramatically by
adding a substituent. Organic chemists have made use of the
enhanced stability of silyl-substituted1,2 and phosphino-substi-
tuted3 anions in the synthesis of new carbon-carbon bonds,
providing greater possibilities in such syntheses. Measuring the
properties of substituted anions in the gas phase allows us to
evaluate the stabilization provided by the substituents without
solvation or counterion effects.
In this work, we examine further the effects of substitution
on carbanions: we have determined the electron affinity of the
bis(dimethylphosphino)methyl radical and the gas-phase acidi-
ties of trimethylphosphine, bis(dimethylphosphino)methane, and
bis(trimethylsilyl)methane. Using known electron affinities and
proton affinities,15 we have also derived the corresponding bond
dissociation energies. Comparing the thermochemical quantities
for the different compounds, we evaluate the magnitude of
stabilization for single and double R-substitution.
Substituent effects have been the subject of many gas-phase
studies,4-11 which have shown that R-substitution with third-
row elements stabilizes carbanions. From electron affinity and
gas-phase acidity measurements, it is known that carbanions
substituted with a silyl group are stabilized by about 20 kcal/
mol6,12 with respect to simple carbanions in which the electrons
are typically weakly bound or unbound.13 Gas-phase acidity
measurements indicate similar stabilization for singly phosphino-
substituted carbanions.5-7 The electron affinity of doubly silyl-
Several factors can determine the stabilization upon substitu-
tion with third-row elements: the electronegativity of the
substituent, its polarizability, the presence of d orbitals, hyper-
conjugation, and steric considerations.16-25 By comparing the
measured stabilization for single and double substitution with
silyl and phosphino groups to known stabilization for thio and
chloro groups, we can learn about the relative importance of
the stabilizing and destabilizing factors. We contrast the effects
of third-row substituents with those of second-row substituents.
Experimental Section
(1) Colvin, E. W. In ComprehensiVe Organometallic Chemistry; Abel,
E. W., Stone, F. G. A., Wilkinson, G., Eds.; Elsevier Science Ltd.: Oxford,
1995; Vol. 11, pp 131-354.
Photodetachment Experiments. Photodetachment experiments
were performed by using an ion cyclotron spectrometer (ICR) operating
in the CW mode, which allows continuous generation and detection of
(2) Panek, J. S. In ComprehensiVe Organic Synthesis; Trost, B. M.,
Fleming, I., Eds.; Pergamon: New York, 1991; Vol 1, pp 579-627.
(3) Edwards, G. L. In ComprehensiVe Organic Functional Group
Transformations; Katritzky, A. R., Meth-Cohn, O., Rees, C. W., Eds.;
Elsevier: Oxford, 1995; pp 105-171.
(14) Brinkman, E. A.; Berger, S.; Brauman, J. I. J. Am. Chem. Soc. 1994,
116, 8304-8310.
(15) Bartmess, J. E. NIST NegatiVe Ion Energetics Database, Version
3.0; Standard Reference Database 19B, National Institute of Standard
Technology, 1993.
(4) Bartmess, J. E.; Hays, R. L.; Khatri, H. N.; Misra, R. N.; Wilson, S.
R. J. Am. Chem. Soc. 1981, 103, 4746-4751.
(5) Ingemann, S.; Nibbering, N. M. M. J. Chem. Soc., Perkin Trans. 2
1985, 837-840.
(16) Bernardi, F.; Bottoni, A.; Tonachini, G. Theor. Chim. Acta (Berlin)
1979, 52, 37-43.
(6) Damrauer, R.; Kass, S. R.; DePuy, C. H. Organometallics 1988, 7,
637-640.
(17) Bernardi, F.; Mangini, A.; Tonachini, G.; Vivarelli, P. J. Chem.
Soc., Perkin Trans. 2 1985, 111-114.
(7) Grabowski, J. J.; Roy, P. D.; Leone, R. J. Chem. Soc., Perkin Trans.
2 1988, 1627-1632.
(18) Bernardi, F.; Bottoni, A.; Venturini, A.; Mangini, A. J. Am. Chem.
Soc. 1986, 108, 8171-8175.
(8) Moran, S.; Ellison, G. B. J. Phys. Chem. 1988, 92, 1794-1803.
(9) Kass, S. R.; Guo, H.; Gahlke, G. D. J. Am. Soc. Mass Spectrom.
1990, 1, 366-371.
(19) Bernardi, F.; Bottoni, A.; Valli, G. S.; Venturini, A. Gazz. Chim.
Ital. 1990, 120, 301-307.
(20) Larson, J. R.; Epiotis, N. D. J. Am. Chem. Soc. 1981, 103, 410-
(10) Paulino, J. A.; Squires, R. R. J. Am. Chem. Soc. 1991, 113, 5573-
416.
5580.
(21) Magnusson, E. Aust. J. Chem. 1985, 38, 23-46.
(22) Magnusson, E. Tetrahedron 1985, 41, 2945-2948.
(23) Rodriquez, C. F.; Sirois, S.; Hopkinson, A. C. J. Org. Chem. 1992,
57, 4869-4876.
(24) Schleyer, P. v. R.; et al. J. Am. Chem. Soc. 1984, 106, 6467-6475.
(25) Wiberg, K. B.; Castejon, H. J. Am. Chem. Soc. 1994, 116, 10489-
10497.
(11) Hierl, P. M.; Henchman, M. J.; Paulson, J. F. Int. J. Mass Spectrom.
Ion. Proc. 1992, 117, 475-485.
(12) Wetzel, D. M.; Brauman, J. I. J. Am. Chem. Soc. 1988, 110, 8333-
8336.
(13) Ellison, G. B.; Engelking, P. C.; Lineberger, W. C. J. Am. Chem.
Soc. 1978, 100, 2556-2558.
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Published on Web 03/13/1998