Alkenes allylic stannanes

Although the Grubbs ruthenium benzylidene 17 has a significant advantage over the Schrock catalyst 3 in terms of its ease of use, the molybdenum alkylidene is still far superior for the cross-metathesis of certain substrates. Acrylonitrile is one example [28] and allyl stannanes were recently reported to be another. In the presence of the ruthenium catalyst, allyl stannanes were found to be unreactive. They were successfully cross-metathesised with a variety of alkenes, however, using the molybdenum catalyst [39] (for example Eq. 20). 

A subsequent publication by Blechert and co-workers demonstrated that the molybdenum alkylidene 3 and the ruthenium benzylidene 17 were also active catalysts for ring-opening cross-metathesis reactions [50]. Norbornene and 7-oxanorbornene derivatives underwent selective ring-opening cross-metathesis with a variety of terminal acyclic alkenes including acrylonitrile, an allylsilane, an allyl stannane and allyl cyanide (for example Eq. 34). 

Three-component coupling. Addition of AIBN to a benzene solution of an allylic stannane, an alkyl iodide, and an electron-deficient alkene initiates a radical process that results in 1,2-addition of the alkyl and allyl groups to the alkene.4 Example ... 

C-Glycoside synthesis may be achieved in twro ways. Intermolecular radical addition reactions are observed with (i) polarized, electron-deficient alkenes, (ii) alkenes that provide a high level of stabilization to the initial radical adduct and (in) substrates that undergo a facile fragmentation (e.g. allyl stannanes). Additions to less reactive substrates, though not favored for intermolecular processes, are observed if the two components are tethered in an intramolecular array. 

Deoxygenation of allyttc alcohols.3 A method for conversion of allylic alcohols to l-alkenes is outlined in equation (I). The first step is an allylic rearrangement of an Oallyl xanthate to 2. The second step is an allyl transfer from sulfur to tin with tri-n-butyltin hydride to give the allylic stannane (3). The last step, destannylation, is a well-known mule to terminal alkcnes.4... 

A number of methods for the preparation of vinyl and allyl sulfones are available [1U9, 110], and the syntheses of vinyl sulfones from alkenes has been reviewed [116]. A simple one-step procedure of wide applicability makes use of a palladium-catalysed cross-coupling reaction between aryl and alkyl sulfonyl chlorides and substituted vinyl and allyl stannanes... 

The origin of this activating effect is not clear. An allylic C—Sn bond should not lower the LUMO of an alkene. Crystal structures of several allylic stannanes have been determined and several appear to show unusually short C=C bonds. See W. Kitching, K. G. Penman, G. Valle, G. Tagliavini and P. Ganis, Organometallics, 1989, 8, 785. 

Williams and coworkers have expanded the utility of this methodology via the application to highly functionalized substrates. transmetalation with R.R)-ot (S,S)-228 is quantitative and dependable, with a variety of allylic stannanes with C-2 substitution. Carbon chains at C-2 of the allyl component may contain additional functionality, including benzyl and allyl ethers, silyl ethers, esters, alkenes, halogens, or thioacetals, as well as stereogeificity. Asymmetric induction in the condensation with aldehydes is dominated by the chiral auxiliary. In this fashion, the allylation reaction may be designed as a convergent... 

Closely related to the previous section is the asymmetric addition of allyl stannanes to aldehydes. The catalyst is the same and the main difference is that an R3Sn group is lost instead of a proton. This has the advantage that there is no ambiguity about the position of the alkene in the product. An example is the addition of an allyl group to the functionalised aldehyde 199. Complexation of either or both oxygen atoms to the Ti is indicated.45... 

A useful intermolecular radical reaction that avoids tributyltin hydride and excess alkene makes use of the ready p-ehmination of fin or sulfur radicals. Addition of the carbon-centred radical to the y-position of an unhindered allyl stannane gives an intermediate radical that eliminates a tin radical (4.38). The product is therefore the result of overall allyl addition and the released fin radical reacts... 

Addition reactions to alkenes are by far the most common type of radical carbon-carbon bond forming reaction. However, there are also examples of the addition of radicals to carbon monoxide or isonitriles. Under appropriate conditions, normally high pressure, an alkyl radical will add to carbon monoxide faster than to an alkene (or hydrogen atom abstraction). The resulting acyl radical can then be reduced or add to an alkene. For example, the radical generated from 1-iodo-octane adds to carbon monoxide followed by the allyl stannane 52 in a three-component coupling process (4.48). 

Allylation of perfluoroalkyl halides with allylsilanes is catalyzed by iron or ruthenium carbonyl complexes [77S] (equation 119) Alkenyl-, allyl-, and alkynyl-stannanes react with perfluoroalkyl iodides 111 the presence ot a palladium complex to give alkenes and alkynes bearing perfluoroalkyl groups [139] (equation 120)... 

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. 

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). 

In the case of tertiary stannanes, oxidation leads to mixtures of alcohols and alkenes and some allylic oxidation, although 1-adamantyltrimethylstannane, incapable of elimination, provided the tertiary alcohol in good yield (Scheme 2). 

The radical anions of carbonyl groups can also be generated via PET from activated alkenes, e.g. allylic silanes or stannanes. Triplet excited aromatic ketones, a-dicarbonyls and Michael systems are suitable substrates for oxidizing allylic Group 14 organometallic compounds with subsequent formation of homoallylic alcohols or S-allylated ketones (Scheme 32) [120-122]. 

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