Hydrogen Donor Abilities of the Group 14 Hydrides

Regioselective hydrosilylations of propiolate esters with tris(trimethylsilyl)silane

Lewis acid and substituent dependency on the regioselectivity of hydrosilylation of propiolate esters 1a-c with tris(trimethylsilyl)silane (2a) was found. The reaction of methyl and ethyl propiolate esters and 2a without Lewis acid and in the presence of EtAlCl2 and Et2AlCl gave beta-silicon-substituted Z-alkenes 3 selectively. On the other hand, reaction in the presence of AlCl3 in dichloromethane gave alpha-silicon-substituted alkenes 4. In the case of trifluoroethyl propiolate ester 1c, reaction with aluminum chloride-based Lewis acids gave alpha-silicon-substituted alkenes 4 exclusively. Two competitive mechanisms, free-radical and ionic, are proposed as the source of the complementary regioselectivity displayed in these reactions. A transition state of the radical-forming step was obtained computationally. The reaction of various reactive acetylene substrates and 2a without Lewis acid and without solvent at room temperature gave beta-silicon-substituted Z-alkenes 3 selectively.

Novel Methodology for the Synthesis of N-Alkoxyamines

[reaction: see text] We report a new methodology for the synthesis of the N-alkoxyamines, which can be used as initiators in "living" free radical polymerization. Silyl radical abstraction from alkyl halides allows the synthesis of N-alkoxyamines inaccessible by other methods.

Determination of the rate constant for ring opening of an α-cyclopropylvinyl radical

The rate constant for ring opening of the 1-(trans-2-phenylcyclopropyl)ethen-1-yl radical, 4, generated by photolysis of the corresponding vinyl iodide 2, is reported. The value of the rate constant was determined by the tin hydride method and was found to be (1.6+/-0.2)x10(10) s-1, one order of magnitude smaller than the rate constant for rearrangement of the trans-2-phenylcyclopropylcarbinyl radical.

A new class of chiral organogermanes derived from C2-symmetric dithiols: synthesis, characterization and stereoselective free radical reactions

A new class of dithiostannanes and dithiogermanes have been prepared from 1,1'-binaphthyl-2,2'-dithiol and 3,3'-bis(trimethylsilyl)-1,1'-binaphtho-2,2'-dithiol. While reduction of 4-butyl-4-chloro-3,5-dithia-4-stanna-cyclohepta[2,1-a;3,4-a']dinaphthalene to the corresponding tin hydride was unsuccessful, 4-tert-butyl-3,5-dithia-4-germa-cyclohepta[2,1-a;3,4-a']dinaphthalene and 4-tert-butyl-2,6-bis(trimethylsilyl)-3,5-dithia-4-germa-cyclohepta[2,1-a;3,4-a']dinaphthalene were obtained by reduction of the parent germanium chlorides with NaBH(4) and LiBH(4), respectively. Kinetic constants for hydrogen transfer to a primary alkyl radical were measured for both germanium hydrides. Reduction of alpha-halo carbonyl compounds by these germanium hydrides occurs with moderate ee values (up to 42%), while hydrogermylation of methyl methacrylate occurs with low selectivity (<3/1) for the former hydride but high selectivity (>10/1) for the latter.

Relative rates and approximate rate constants for inter- and intramolecular hydrogen transfer reactions of polymer-bound radicals

Measurements of relative rates and rate constants for inter- and intramolecular hydrogen transfer reactions of polymer-bound radicals are reported. The relative rate of reaction of resin-bound primary alkyl radical with tributyltin hydride is about 2 times slower than that of the benchmark reaction in solution. The data do not reveal whether this is due to a reduced rate constant or a lower concentration of tin hydride in the resin phase. Yet the difference between solid and solution reactions is small enough to be neglected, and it appears that rate constants measured in solution can be applied directly to resin-bound radicals. A resin-bound aryl radical abstracts a hydrogen atom rapidly (k = 3 x 10(6) s(-1)) from its own polymer backbone and linker, and a simplified view of the resin as a "solvent" is suggested for predicting such effects with other polymers and linkers. Rapid cyclizations of resin-bound aryl radicals will be possible, but slower cyclizations and most bimolecular reactions will be difficult due to the competing polymer/linker hydrogen transfer.

Remote substituent effects on bond dissociation energies of para-substituted aromatic silanes

UB3LYP/6-31G(d) and ROMP2/6-311++G(d,2p) methods were used to calculate the Si-X bond dissociation energies (BDEs) of a number of para-substituted aromatic silanes (4-Y-C(6)H(4)-SiH(2)X, where X = H, F, Cl, or Li). It was found that the substituent effect on the Si-H BDE of 4-Y-C(6)H(4)-SiH(3) was small, as the slope (rho(+)()) of the BDE- regression was only 0.09 kJ/mol. In comparison, the substituent effect on the Si-F BDE of 4-Y-C(6)H(4)-SiH(2)F was much stronger, whose rho(+ )()value was -2.34 kJ/mol. The substituent effect on the Si-Cl BDE of 4-Y-C(6)H(4)-SiH(2)Cl was also found to be strong with a rho(+)() value of -1.70 kJ/mol. However, the substituent effect on the Si-Li BDE of 4-Y-C(6)H(4)-SiH(2)Li was found to have a large and positive slope (+9.12 kJ/mol) against. The origin of the above remarkably different substituent effects on the Si-X BDEs was found to be associated with the ability of the substituent to stabilize or destabilize the starting material (4-Y-C(6)H(4)-SiH(2)X) as well as the product (4-Y-C(6)H(4)-SiH(2)* radical) of the homolysis. Therefore, the direction and magnitude of the effects of Y-substituents on the Z-X BDEs in compounds such as 4-YC(6)H(4)Z-X should have some important dependence on the polarity of the Z-X bond undergoing homolysis. This conclusion was in agreement with that from earlier studies (for example, J. Am. Chem. Soc. 1991, 113, 9363). However, it indicated that the proposal from a recent work (J. Am. Chem. Soc. 2001, 123, 5518) was unfortunately not justified.

Studies on the mechanism of B(C(6)F(5))(3)-catalyzed hydrosilation of carbonyl functions

The strong organoborane Lewis acid B(C(6)F(5))(3) catalyzes the hydrosilation (using R(3)SiH) of aromatic and aliphatic carbonyl functions at convenient rates with loadings of 1-4%. For aldehydes and ketones, the product silyl ethers are isolated in 75-96% yield; for esters, the aldehydes produced upon workup of the silyl acetal products can be obtained in 45-70% yield. Extensive mechanistic studies point to an unusual silane activation mechanism rather than one involving borane activation of the carbonyl function. Quantitative kinetic studies show that the least basic substrates are hydrosilated at the fastest rates; furthermore, increased concentrations of substrate have an inhibitory effect on the observed reaction rate. Paradoxically, the most basic substrates are reduced selectively, albeit at a slower rate, in competition experiments. The borane thus must dissociate from the carbonyl to activate the silane via hydride abstraction; the incipient silylium species then coordinates the most basic function, which is selectively reduced by [HB(C(6)F(5))(3)](-). In addition to the kinetic data, this mechanistic proposal is supported by a kinetic isotope effect of 1.4(5) for the hydrosilation of acetophenone, the observation that B(C(6)F(5))(3) catalyzes H/D and H/H scrambling in silanes in the absence of substrate, computational investigations, the synthesis of models for proposed intermediates, and other isotope labeling and crossover experiments.