2- -4-alkenal 3-stannyl-1 -alkene

Stannylation of lithiated alkenes also provides allylstannanes11 13. 

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. 

Radicals for addition reactions can be generated by halogen atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism addition, hydrogen atom abstraction, and halogen atom transfer. 

Oxidation of (3-silyl and (3-stannyl acids leads to loss of the substituent and alkene formation.283... 

A new entry to exocyclic dienes was reported by Sha who uncovered that a radical cyclization of the vinyl iodide 100 can lead to the formation of an exocyclic dienes fused with a tetrahydrofuran ring. The cyclization is proposed to proceed in a 5-(n-exo)-exo-dig fashion <00OL2011>. 3,4-Disubstituted tetrahydrofurans can also be constructed via the cyclization of O-stannyl ketyls and allylic 0-stannyI ketyls onto electron-rich or electron-poor alkenes <00TL8941>. 

The same stannyl phosphine will also add to both terminal and non-terminal alkynes32, giving stannyl-substituted alkenes. In the case of terminal alkynes E- and Z-isomers are formed, with a mixture of the two possible regioisomers. Total yields are 60-80%, with 60-90% preference for the E-isomer, depending on the substituents on the alkyne, although exact experimental details are not given (reaction 24). 

Mechanistically, a (silyl)(stannyl)palladium initially formed undergoes regioselective silylpalladation to the alkyne moiety of the enyne (Scheme 66). Then, two possible pathways are conceivable for addition to the alkene moiety, that is, stannylpalladation and carbopalladation. It has not been established that which pathway operates. 

Similarly, trityl cation in aromatic hydrocarbons initiates the fragmentation of simple tetraalkyl plumbanes and stannanes yielding the plumbyl or stannyl cationic species, e.g. 11, and alkenes. The reaction is thought to proceed via plumbyl-or stannyl-substituted carbocations 12, which in a second step eliminate the al-kene. This approach was used in the synthesis of norbornyl cations of the elements tin and lead, e.g. 13, (Scheme 5). ... 

The increased reactivity of the silyl- and stannyl-substituted alkenes enhances the synthetic utility of carbocation-alkene reactions. 

Methylenation of sulfones.1 This reagent reacts rapidly with the anion (LDA) of even moderately hindered sulfones to form (3-stannyl sulfones, which undergo desulfonylstannylation on treatment with Bu4NF to form alkenes. 

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