Allyl Silanes and Stannanes

Allylic silanes and stannanes react with various electrophiles with demetallation. These reactions can occur via several related mechanisms. Both types of reactants can deliver alkylic groups to electrophilic centers such as carbonyl and iminium. 

Reactions proceeding through open TS In this group, exemplified by BF3-catalyzed additions of allylic silanes and stannanes, the degree of stereochemical control is variable and often moderate. The stereoselectivity depends on steric factors in the open TS and can differ significantly for the E- and Z-isomers of the allylic reactant. 

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. 

Suitably protected glycosyl halides or acetates, upon Lewis-acid promoted SN1 heterolysis, generate glycosyl cation intermediates that can react with electron-rich arenes, heteroarenes, Me3SiCN, enoxysilanes, enamines, allyl silanes and stannanes, acetylenyl silanes and stannanes affording C-glycosyl compounds. 

In the presence of Lewis acids allyl silanes and stannanes react with epoxides generally at the sterically less demanding carbon atom. Other electron-rich alkenes, such as ketene acetals, can also be used as nucleophiles. The strong Lewis acids required might, however, also lead to rearrangement of the epoxide before addition of the nucleophile can occur (last reaction, Scheme 4.72). 

Imai, T. Nishida, S. Lewis acid promoted ringopening allylation of epichlorohydrin with allylic silanes and stannanes to afford... 

Allyl silanes and stannanes are established as useful reagents in organic synthesis (107-110). Electrophilic attack occurs at the terminal carbon atom of the allyl system, leading to allylic transposition in the product [Eq. (7)]. 

Cozzi, P. G. Tagliavini, E. Umani-Ronchi, A. Enantioselective Addition of Allylic Silanes and Stannanes to Aldehydes Mediated by Chiral Lewis Acids, Gazz. Chim. Ital. 1997,127, 247-254. 

The chiral titanium complex BINOL-T1CI2 also catalyzes the asymmetric carbonyl addition reaction of allylic silanes and stannanes. Thus the addition reaction of glyoxylate with (E)-2-butenylsilane and -stannane proceeds smoothly to give the syn product in high enantiomeric excess (eq 12). The syn product thus obtained can be readily converted to the lactone portion of verrucaline A. The reaction of aliphatic and aromatic aldehydes with allylstannane is also catalyzed by BINOL-TiC to give... 

Aoki, S., Mikami, K., Terada, M., Nakai, T. Enantio- and diastereoselective catalysis of addition reaction of allylic silanes and stannanes to glyoxylates by binaphthol-derived titanium complex. Tetrahedron 1993, 49,1783-1792. 

During the course of our research project on asymmetric catalysis of the carbo-nyl-ene reaction, we have found that the BINOL-Ti complexes (1) catalyze rather than promote stoichiometrically the carbonyl addition reaction of allylic silanes and stannanes [11,71]. The addition reactions to glyoxylate of ( )-2-butenylsi-lane and -staimane proceed smoothly to afford the sj -products in high enantiomeric excess (Scheme 27) [71]. The sjn-product thus obtained could readily be converted to the lactone portion of verrucaline A [72]. 

Special substituents have been intrcjduced onto the alkene in order to direct the cyclization reaction to five-meml red formation. Examples of electronically biased alkenes for this purpose are vinyl chlorides, allylsilanes, ° allylstannanes and ketene thioacetals. The cyclization of allyl-silanes and -stannanes to the pyrrolizidine system is highly stereoselective. " The chair geometry of Tr-complex (102) explains this selectivity (equation 67). 

Phosphonium salts can also be prepared by alkylating existing phosphonium salts. This method has been used to incorporate interesting groups, such as silyl and stannyl functionalities [17, 18] (Scheme 1.5), into phosphonium salts. Such salts, upon reaction with base and aldehyde, produce synthetically usefiil allyl-silanes and -stannanes. 

Fukuzumi, S., Fujita, M., and Otera, J., Regio-reversal in the thermal and photochemical reduction of 10-methylacridinium ion by allylic silanes and stannanes,/. Chem. Soc., Chem. Commun., 1536, 1993. 

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