P-silyl carbocations

Competition studies reported by Kuwajima, ° which also complement the results of Nakai, illustrate the limitations of the -effect as a tool for predicting the outcome of vinylsilane-terminated cyclizations (Scheme 4). Acylium ion initiated cyclizations of (7a) and (7b) gave the expected cyclopentenones (8a) and (8b). However, compound (7c), upon treatment with titanium tetrachloride, gave exclusively the cyclopentenone product (8c) arising from the chemoselective addition on the 1,1-disubstituted alkene followed by protodesilylation of the vinylsilane. The reversal observed in the mode of addition may be a reflection of the relative stabilities of the carbocation intermediates. The internal competition experiments of Kuwajima indicate that secondary B-silyl cations are generated in preference to secondary carbocations (compare Schemes 3 and 4), while tertiary caibocations appear to be more stable than secondary P-silyl carbocations, as judged by the formation of compound 9c). 

The vinylsilane-terminated cyclization strategy has been extended to the preparation of eight-membered cyclic ethers. Oxocenes (Scheme 17) with A -unsaturation (3,6,7,8-tetrahydro-2//-oxocins) were prepared efficiently by the SnCU-catalyzed cyclization of the mixed acetals (39) with complete re-giochemical control. Electrophilic addition on the 2-(trimethylsilyl)-l-alkene occurs predominantly at the terminal position of the alkene to form a tertiary a-silyl, rather than a primary P-silyl, carbocation. The... 

Propargylsilanes undergo electroftiiilic addition reactions to generate p-silyl carbocations (109) that can, in principle, react further to give eidicr addition (110) or sutatitution (111) products, as illustrated in Sdieme 51. As in the case of allylsilanes, however, substitution predominates. ... 

Akiyama and coworkers noticed that treatment of diallylisopropylsilane (108) and a,P-unsaturated ketones with Bp3 OEt2 results in a Michael addition of one of the allyl moieties, followed by an intramolecular allylation onto the P-silyl carbocation intermediate to furnish a domino allylation product, (109) (Equation 65) [68]. 

Lambert JB, Liu C, Kouliev T (2002) A stable p-silyl carbocation with allyl conjugation. J Phys Org Chem 15 667... 

Early attempts to generate ot-aryl-(3-silyl substituted carbocations by ionization of 1,1-diphenyl-2-(trimethylsilyl)ethanol 10 usipg FS03H in S02C1F even at very low temperature of -140 °C were unsuccessful (77). Only 1,1-diphenylethyl cation 11 and trimethylsilyl fluorosulfate, fhe products of P-silyl cleavage were observed. 

The ferrocenyl group is a very good electron donor. The a-ferrocenyl P-silyl substituted carbocation 20 is accessible by protonation of ( )-1 -ferrocenyl-2-(triisopropylsilyl)alkene 21 with trifluoroacetic acid in SO2CIF at - 95 °C (13, 22). 

The generation of a-ferrocenyl-P-silyl substituted vinyl cations of type 28 does not require superacidic conditions, they can be generated by protonation of l-ferrocenyl-2-trialkylsilyl alkynes with trifluoroacetic acid at room temperature. The SiR3-groups with larger alkyl substituents increase the lifetime of this type of carbocations. 

In addition to its interesting structure, the triethylsilylium-aromatic complex has proved useful in preparing other cations. Reaction with 1,1-diphenylethylene, for example, provided the cation 95, the first example of a persistent p-silyl substituted carbocation (i.e., where decomposition by loss of the silyl group did not occur). 

The major reaction product formed from the solvolysis of the trans y-silyl ester was cyclohexene formed from the carbocation 138 (Scheme 18) by 1,2-hydrogen migration to give the p-silyl cation 139 followed by loss of the silicon substituent. In contrast to the behavior shown by 136, the cis y-silyl ester 134 exhibited a small inverse p-d4 isotope effect (kH/kn = 0.97) attributed to the inductive effect of the P-deuteriums and implies that there is very little hyperconjugative stabilization of... 

The familiar hierarchy of carbocation stability—tertiary > secondary > primary—is due to the stabilization of the positive charge by donation of electron density from adjacent C-H or C—C bonds (their filled 0 orbitals to be precise) that are aligned correctly with the vacant orbital (Chapter 17). The electropositive nature of silicon makes C-Si bonds even more effective donors so that a P-silyl... 

---