A-Silyl carbocations

However, when R is phenyl, the bromonium ion 39 is found to open to give a /)-silyl carbocation 40. Overall syn addition can then occur, followed by anti elimination to give the trans alkene 41, as seen in Scheme 2. 

These results suggest that the tertiary a-silyl carbocation 105 has a higher stability than the primary /J silyl cation 107, formed by a-cyclization. This shows that the /1-effect does not always dominate the regioselectivity of reactions of vinylsilanes. 

Alkynylsilanes can function as carbon nucleophiles in addition reactions to electrophilic ir-systems. In principle electrophilic addition reactions to alkynylsilanes can occur to produce a- or P-silyl-substituted vinyl cations, as illustrated in Scheme 37. The a-silyl carbocation is not the most stabilized cation, the reason being that the caibon-silicon bond can achieve coplanarity with the vacant orbital on the -carbo-nium ion, making possible 3-stabilization through hyperconjugation. Depending on the configuration of the carbocation, the developing vacant orbital can exist as a p-orbital, as in structure (75a), or an rp -hy-brid, as in structure (75b). 

In general, carbonyl groups do not function as good initiators in vinylsilane-mediated cyclization reactions and relatively few examples exist. The cyclization of vinylsilanes with ketones or aldehydes as initiators is a highly underdeveloped reaction that holds considerable potential. As reported by Tius and coworkers, treatment of aldehyde (13) with a catalytic amount of p-toluenesulfonic acid gave a mixture of the tetralins (14) and (15) in a combined yield of 53%, with lesser amounts of the enone (16 Scheme 8). The enone system presumably arises from the intramolecular 1,3-hydride transfer of the intermediate a-silyl carbocation (17). Similarly, the vinylsilane (18) undergoes cyclization to produce the disub-stituted benzene derivative (19), although yields are low. 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

It should be recognized that the stability of cation radicals generated by anodic oxidation is also affected by jS-silyl substitution. Stabilization of car-bocations by a silyl group situated at the -position is well known as the / effect . The interaction of the C Si a orbital with the empty p orbital of the carbon stabilizes the carbocation. Therefore, we can expect similar effects of silicon for cation radical species. The interaction of the filled C-Si a orbital with the half-filled orbital of the carbon may stabilize the cation radical. 

Pyrrolo[l,2-c][l,3]oxazin-l-one 248 has been obtained by reaction of allylsilanes with a pyrrolidine-lV-acyliminium ion 247 (Scheme 32), formed by addition of a Lewis acid on compound 244. The /3-silyl carbocation formed by the reaction with allylsilane 246 reacted with the oxygen of the N-15OC group followed by the loss of 2-methylpropene. The reaction was not very stereoselective when trimethylsilane was used, whereas with larger group on the silicon the selectivity was increased <1997J(P1)2163>. 

The destabilizing effect of a silyl group compared with an alkyl group in trivalent carbocations was explained by the weaker hyperconjugation of the Si-R a-bond (R = alkyl) relative to a C-R cr-bond (R = H or alkyl) and by electrostatic repulsion between the adjacent positively charged cationic carbon and the electropositive silicon (10). 

Removable cation-stabilizing auxiliaries have been investigated for polyene cyclizations. For example, a silyl-assisted carbocation cyclization has been used in an efficient total synthesis of lanosterol. Other conditions for the cyclization of polyenes and of ene-ynes to steroids have been investigated. Oxidative free-radical cyclizations of polyenes produce steroid nuclei with exquisite stereocontrol Besides the aforementioned A-ring aromatic steroids and contraceptive agents, partial synthesis from steroid raw materials has also accounted for the vast majority of industrial-scale steroid synthesis. 

The first X-ray structure of an a-silyl-substituted carbocation (33) is reported its pXR+ value is predicted to be 4.73 The trimesitylsilylium cation is proposed to be a nearly free, tricoordinate species.74 The dimethylsilylium cation undergoes isomer interconversion via (34), according to high-level calculations the most stable structure is... 

Kresge and Tobin48 also studied the hydrolysis of vinyl ethers and found a rate ratio of 130 between methyl vinyl ether and ethyl cA-trimethylsilylvinyl ether, corresponding to a stabilization of the /J-silyl carbocation of 2.9 kcal mol-1. In this case the small rate acceleration (compared to the cyclohexyl systems studied by Lambert) can be attributed to the unfavourable dihedral angle. The dihedral angle in the vinyl ether is 90° (24), and on protonation it drops to 60° (25), whereas maximum hyperconjugative interaction requires a dihedral angle of 0°. 

Shimizu and coworkers have extensively studied the solvolyses of /1-silyl benzyl systems. They observed a rate acceleration of 3 x 105 for solvolysis of l-phenyl-2-(trimethylsilyl) ethyl trifluoroacetate 26 compared to the corresponding /1-r-butyl system 27. This indicates that the solvolytic generation of the /3-silyl carbocation is about 7.5 kcal mol 1 more favourable52. 

This product must arise from /1-cyclization followed by migration of the methyl group to give a /1-silyl carbocation intermediate 106, as shown in equation 55. 

This can be explained if the initial attack on 113 forms a carbocation a to the phenyl group (116) (Markovnikov attack), followed by a 1,2 shift of hydride or deuteride to give the yS-silyl carbocations 117 and 118 respectively, and hence the alkenes 114 and 115 (Scheme 6). 

Attack of a nucleophile on the /1-silyl carbocation 127 or the cyclic siliconium ion 128 leads to desilylation and formation of the Sakurai product. When nucleophilic attack is disfavoured by steric hindrance at the silicon, competing intramolecular attack by the enolate becomes important. This 5-exo-tet cyclization gives the trimethylsilylcyclopentane product with high stereospecificity, the trimethylsilyl group having undergone a 1,2 shift. 

The ring contraction shown in equation 111 is also promoted by formation of a /J-silyl carbocation in the intermediate 146179. 

It might be predicted that electrophilic attack would take place by a S l - type process, to give a /J-silyl carbocation on cleavage of the fi C—O bond. However, the relative orientations of the C—Si bond and the developing positive charge are such that hyperconjugative overlap is minimal (169). 

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