1 - - 3-stannyl-1 -alkene

Alkenyl(phenyl)iodonium salts have attracted a significant interest recently as stable and readily available powerful alkenylating reagents. Several convenient, general procedures for the stereoselective synthesis of alkenyliodonium salts from silylated or stannylated alkenes and the appropriate hypervalent iodine reagents are known [5]. The chemistry of alkenyliodonium salts has been extensively covered in several recent reviews [42 - 45]. 

When the hydrostannation of l- -dodecyne is performed with Bu2SnIH systems, the sole use of Bu2SnIH gives a mixture of E/Z isomers. The tin hydride ate complex, Bu2SnIH-MgBr2-OEt2 shows the highest effect where an cr-stannylated alkene is obtained (Equation (36)).106 The employment of EtOAc as a solvent sharply increases the yield of the cr-stannylated adduct. 

A very general and mild procedure for the stereospecific synthesis of aIkenyl(aryl)iodonium triflates 309 involves aryl(cyano)iodonium triflates 308 as iodonium transfer reagents in reactions with stannylated alkenes 307 (Scheme 2.89) [367,443,444], This method was also applied to the preparation of the parent vinyliodonium triflate from tributyl(vinyl)tin [445],... 

There has been a full account of the synthesis of the 2 -stannylated alkene 135 (X = SnBua) by base-induced stannyl migration from C-6 (see Vol. 32, p. 275), and the application of this compound to the preparation of the alkenyl halides 135 (X = Cl, Br, I), and products with carbon substituents at C-2 through Stille couplings. Reaction of di-O-acetyl-L-rhamnal with silylated thjmiine gave the 2 -enopyranosyl nucleoside by allylic rearrangement, as a mixture of anomers. A paper discussing a glycal substituted at C-3 with a nucleobase is mentioned in Chapter 10, and a 3 -ene derived from thymidine is mentioned in Section 17. 

Proximal heteroatoms, such as Si, N, O, S, and halogens, can exert significant effects on some critical aspects of the reaction such as the regio- and stereochemistries. With some groups that contain Si, S, and Br as well as alkoxycarbonyl, exclusive formation of a-stannylated alkenes may be achieved. 

Complexes of internal alkynes of general formula Pd(7] -alkyne)(PR3)2 or Pd( 7 -alkyne)(diphos) have been reported, often prepared in the course of palladium-catalyzed reactions and other processes. Thus, most of them have been synthesized by decomposition of Pd(ii) complexes in the presence of the alkyne as shown in Equations (20) and (21). Insertion into a Pd-E bond and reductive elimination generates the silylated or stannylated alkene and Pd(0), which is trapped by the alkyne in excess. 

Tin Reagents. The presence of a 3-methyl group in the 2-chloroquinoline 202 has a beneficial effect on reaction rates and efficiency in Stille-type alkenylations with terminal stannyl alkenes (Scheme 77). This was attributed to steric acceleration in the reductive elimination of Pd(0) from a Pd(II) complex. The purine 6-position is highly electrophilic. A chloro substituent is readily replaced under Stille conditions using tri(n-butyl)vinylstannane. The 6-vinylpurine product 203 from the coupling is reacted further in situ in Heck couphngs. These reactions proceed readily because of the electron-withdrawing effect from the ir-deficient pyrimidine moiety of the heterocycle.t ... 

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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