Stannane radicals

Stannane Radical Solvent, method Rate constant (M-1 s-1) Rate expression (0 = 2.3RT kcal/mol) Ref. 

SET photochemistry is involved in the reaction between the enones (371) and the a-stannyl ethers (372) in methanol. The products are the 3-sub-stituted cycloalkanes (373) which arise from addition of aryloxymethyl radicals to the enones. Irradiation (X > 400 nm) of the stannanes (374) in the presence of the ketones and aldehydes (375) affords two products identified as (376) and (377). The former of these is dominant and the reaction arises by an electron transfer from the stannane to the ketone. The resultant stannane radical-cation undergoes fission to yield an alkoxy allyl radical and the tin cation. The alkoxyalkyl radical adds to the carbonyl radical-anion with a preference for... 

From Marco Polo to chiral stannanes - radical chemistry for the new millennium... 

Stannanes also add across double bonds offiuonnated olefins in a free radical reaction Trimethylstannane undergoes stereospecific addition to hexafluorocyclo-butene to afford trans 1,2,3,3,4,4 hexafluoro 1 (trimethylstannyl)cyclobutane [5] (equation 1)... 

Stannylation of lithiated allyl ethers gives (Z)-3-alkoxyallylstannanes (1)115,116, whereas mixtures of (Z)- and ( )-tributyl(3-methoxy-2-propenyl)stannanes (2) were obtained from free-radical addition of tributyltin hydride to l-methoxy-l,2-propadienel16. 

In addition to the radical ipso-substitution of indolyl sulfones producing stannanes described earlier <96T11329>, Caddick has also reported an approach to fused [l,2-a]indoles based on the intramolecular cyclization of alkyl radicals. Thus, treatment of 112 with BuaSnH leads to the fused ring derivatives 113 (n = 1-4) <96JCS(P1)675>. 

The readily available organotin compounds include tin hydrides (stannanes) and the corresponding chlorides, with the tri-n-butyl compounds being the most common. Trialkylstannanes can be added to carbon-carbon double and triple bonds. The reaction is usually carried out by a radical chain process,137 and the addition is facilitated by the presence of radical-stabilizing substituents. 

There are several procedures for synthesis of terminal alkenyl stannanes that involve addition to aldehydes. A well-established three-step sequence culminates in a radical addition to a terminal alkyne.150... 

There is a discussion of some of the sources of radicals for mechanistic studies in Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as initiators, are also important in synthetic chemistry. One of the most useful sources of free radicals in preparative chemistry is the reaction of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and the stannyl radical can then abstract halogen from the alkyl group. For example, net addition of an alkyl group to a reactive double bond can follow halogen abstraction by a stannyl radical. 

This generalized reaction sequence consumes the halide, the stannane, and the reactant X=Y, and effects addition to the organic radical and a hydrogen atom to the X=Y bond. The order of reactivity of organic halides toward stannyl radicals is iodides > bromides > chlorides. 

These reactions result in iodine atom transfer and introduce a potential functional group into the product. The trialkylborane method of radical generation can also be used in conjunction with either tri-n-butyl stannane or fnT-(trimethylsilyl)silane, in which case the product is formed by hydrogen atom transfer. 

Allylic stannanes are an important class of compounds that undergo substitution reactions with alkyl radicals. The chain is propagated by elimination of the trialkyl -stannyl radical.315 The radical source must have some functional group that can be abstracted by trialkylstannyl radicals. In addition to halides, both thiono esters316 and selenides317 are reactive. 

A. With radical generation using trisubstituted stannanes... 

Scheme 10.17 illustrates allylation by reaction of radical intermediates with allyl stannanes. The first entry uses a carbohydrate-derived xanthate as the radical source. The addition in this case is highly stereoselective because the shape of the bicyclic ring system provides a steric bias. In Entry 2, a primary phenylthiocar-bonate ester is used as the radical source. In Entry 3, the allyl group is introduced at a rather congested carbon. The reaction is completely stereoselective, presumably because of steric features of the tricyclic system. In Entry 4, a primary selenide serves as the radical source. Entry 5 involves a tandem alkylation-allylation with triethylboron generating the ethyl radical that initiates the reaction. This reaction was done in the presence of a Lewis acid, but lanthanide salts also give good results. 

The success of such reactions depends on the intramolecular hydrogen transfer being faster than hydrogen atom abstraction from the stannane reagent. In the example shown, hydrogen transfer is favored by the thermodynamic driving force of radical stabilization, by the intramolecular nature of the hydrogen transfer, and by the steric effects of the central quaternary carbon. This substitution pattern often favors intramolecular reactions as a result of conformational effects. 

Radical reactions used in synthesis include additions to double bonds, ring closure, and atom transfer reactions. Several sequences of tandem reactions have been developed that can close a series of rings, followed by introduction of a substituent. Allylic stannanes are prominent in reactions of this type. 

Some mechanisms of radical-initiated migration copolymerization of di-alkyl(diphenyl)stannanes with non-conjugated epoxyalkadienes such as 4,4-epoxy-l,7-heptadiene (I) and 3-glycidyl-oxy-l,6-hexadiene (II) have been discussed 98). 

These studies were subsequently extended to the analogous stannanes and the 3-methyl and 2-methylcyclohexyl bromides (Table 9)47. The MesSnLi reactions, as expected, afforded similar ratios of isomeric product from a given set of stereoisomeric bromides. However, Pt SnLi led to clean inversion with 4-methyl and trans-3-methylcyclohexyl bromides. Evidently, the Ph3Sn radical is not easily formed under these conditions. 

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