Electrophilic reactions allylstannane

The palladium-catalyzed multicomponent coupling reactions have attracted considerable interest.12,12a 12e A reaction using allylstannane 39 and allyl chloride 40 was applied to the three-component diallylation of benzylidenema-lonitrile and its congeners by Yamamoto et al 2 Analogous diallylation of isocyanate 41 was studied by Szabo et al. (Scheme 7).12a The reaction mechanism can be explained by formation of an amphoteric bis-allylpalladium intermediate 43 which undergoes an initial electrophilic attack on one of the allyl moieties followed by a nucleophilic attack on the other. 

Regio- and stereo-selective allylation of sulfonylimines has been carried out with trifluoro(allyl)borates and allylstannanes, using palladium-pincer complexes as catalysts.47 Syn products predominate, in contrast to the corresponding reaction of aldehyde electrophiles DFT calculations have been employed to probe the mechanistic differences. 

Allylstannane 176 is formed by the reaction of allyl acetate with distannanes [97,98], In this reaction, umpolung of the electrophilic 7r-ally] palladium to the nucleophilic allylstannane occurs. Allylation of bromoindole 198 to give allylindole 199 involves the oxidative addition of 198 to Pd, transmetallation with the allylstannane 176, and final reductive elimination [99],... 

The intramolecular carbostannylation reported in 2007 by Echavarren et al. and related reactions are also initiated by the n-Lewis acidity of silver ions.108 Silver salts catalyzed the cyclization of alkynylated allylstannanes to the corresponding 1-stannylmethylene-2-vinyl hve-membered rings (Scheme 10.69). Interestingly, the more electrophilic the salt is, the more rapid the reaction is the hexafluroroantimonate... 

Finally, ruthenium-catalyzed carbocyclization by intramolecular reaction of allylsilanes and allylstannanes with alkynes also led to the formation of vinyl-alkylidenecyclopentanes [81] (Eq. 60). This reaction is catalyzed by RuC13 or CpRuCl(PPh3)2/NH4PF6 in methanol. The postulated mechanism involves the coordination of the alkyne on the ruthenium center to form an electrophilic /f-alkyne complex. This complex can thus promote the nucleophilic addition of the allylsilane or stannane double bond. 

A complementary sequence uses an alkyl halide 7.46 with a Z-substituent to create an electrophilic radical 7.47 in the presence of a nucleophilic alkene 7.48. In this case, the radical 7.49 expels the low-energy tributyltin radical to regenerate the tin radical achieving overall the allylation of the ester, catalytic in both the AIBN and the tin hydride. The Z-substituent in the radical 7.47 is necessary for an efficient reaction—in its absence the allylstannane has to be used in large excess. 

The transmetallation or the metal-metal exchange reaction of an allylic tin species with an electrophile was first observed in 1970 [77]. The possibility that transmetallation may play a role in the Lewis acid-promoted reaction of allylstannanes with aldehydes was initially discussed by Tagliavini [78], Keck [79], Yamamoto [71b, 80], and Maruyama [81]. It is believed that upon transmetallation with either SnCU or TiCl4, the addition of an allylstannane and aldehyde will occur via a cyclic, six-membered transition structure. The reaction occurs by coordination of the aldehyde carbonyl with the Lewis acidic trichlorotin or trichlorotitanium reagent, thus affording the anti homoallylic alcohol (Scheme 10-42). 

The P-effect similarly enhances the reactivity of alkynyl- (Section 8.2.2), alkenyl-(Section 8.1.2), allyl- (Section 9.1.3.2), and aryl- (Section 7.1) stannanes in their reactions with electrophiles (equations 3-33-3-36), and its effect can be recognised in other contexts such as the ene reactions of allylstannanes (Section 9.1.3.4), and the charge-transfer reactions and ring-substitution reactions of benzylstannanes.22... 

Seyferth showed in 1959 that allyllithium, which is not available from allyl halides because of competing substitution reactions, can be prepared from the reaction of allylstannanes with organolithium reagents (XM = R Li Section 22.1). These reactions involve nucleophilic attack of R upon tin to form a pentacoordinate intermediate. Lewis acids, MX, on the other hand, react by electrophilic attack at the y-position. 

Allylstannanes rapidly undergo the Kocheshkov comproportionation reaction with SnCU, even at -50 °C. The initial product is apparently formed with allylic inversion, but further transmetallation may give other allylic isomers.35 The SnCl3 group which is formed enhances the electrophilic power of the allyl group. This has been exploited in aromatic allylation (equation 9-13),36 and transmetallations of this type provide the basis for some, but not all, of the examples of the catalysis of the allylation of carbonyl compounds by allylstannanes in the presence of Lewis acids37,38 (Section 9.1.3.4). 

A-Acyliminium ions are suitable partners for electrophilic addition reactions of allylstannanes. Although the corresponding silanes have been more widely studied in these cases/ allylic stannanes are fully competent reaction participants, as illustrated by formation of the l-azabicyclo[3.1.0]pentane 159 via intra-molecular cyclization of the fi-allylic stannane 157 (Scheme 5.2.33). The reaction produces the vinyl cyclopropane under protic acid conditions with complete stereocontrol. s... 

In the Lewis acid-promoted allylation with allylsilanes, the promoter is usually regarded as activating electrophiles. Thomas [374] and Dias [375] have, however, described SnCLj-promoted allylation reactions proceeding via a transmetalation mechanism (Scheme 10.130). In the latter equation of Scheme 10.130, the formation of the allylstannane species has been observed by NMR spectroscopy [375 b]. 

These reactions are typical of allylsilanes and proceed under mechanism-con-trolled anti-addition of electrophiles such as aldehydes and pseudo-halides [reactions (13a,b,c). Scheme 5] [16] and reaction (14) (Scheme 5)[17, 18]). In reaction (13c) a closed transition state 5-1 has been assumed. A very interesting, highly stereo controlled example is given in reaction (14). An open transition state 5-2 has been postulated with an anti-Sg2 attack at the chiral allylstannane... 

There is an extensive review of the electrophilic substitution reactions of allylsilanes and vinylsilanes in Organic Reactions and the reactions of allylstannanes have been reviewed briefly. The major books on synthetic organosilicon and organotin chemistry also cover the types of reaction discussed here. 

In the absence of Lewis acids, stereoselectivity is strongly dependent upon the geometry of the double bond in the allylstannane, both in simple thermal reactions (Scheme 28) and in pressure-induced reactions, as expected for a cyclic transition state. As well as being selective for the formation of the anti arrangement of substituents on the carbon chain, the overall electrophilic substitution in the thermal reaction of an ( )-allylstannane, is stereospecifically syn (Scheme 29), with a cyclic transition state (51),... 

Oxidative addition of certain stannanes to Pd(0) complejKS is also possible. Thus, aUcynylstannanes have been shown to react with Pd(0) complexes [207, 208). In addition, the Pd(0)-catalyzed reaction of allylstannanes with alkynes has been found to afford aUylstannylation products 31 (Scheme 1.23) [209]. A likely mechanism involves oxidative addition of the allylstannanes to Pd(0) to give (ri -allyl) palladium complexes 32 (L = alkyne) (Scheme 1.23). In this transformation, the usually nucleophilic allylstannanes behave as electrophiles. Complexes of type 32 are probably formed by transmetallation of (T) -aUyl)palladium complexes with hexamethylditin [210]. An oxidative addition to form complexes 32 has been proposed in the Pd(0)-catalyzed carboxylation of allylstannanes with COj [211]. Although compleres 32 have not been isolated as stable species, work on the intramolecular reachon of allylstannanes with alkynes and theorehcal calculations give support to the formation of these complexes by the oxidahve addition of allylstarmanes to Pd(0) [212]. 

Allylation via Bisallylpalladium Intermediates As mentioned in Scheme 6.4, Yamamoto et al. [9] have shown that successive addition of an aUyl chloride and an allylstannane to a palladium(O) precursor generates a bisallylpalladium complex. This reaction can be employed under catalytic conditions and has led to many useful apphcations of allylstannanes in organic synthesis [80-82]. The aUyl moieties of bisallylpalladium complexes may undergo an allyl-aUyl couphng reaction, and thus, the process can be employed for syntheses of 1,5-dienes [82a]. Another interesting area involves reactions of electrophiles, such as aldehydes, imines (Scheme 6.19), and the Michael acceptors (Scheme 6.20) with bisallylpalladium intermediates [80-82b]. 

The issue of transmetalation reactions, or metathesis or redistribution reactions (all terms have been used to describe this) is an important one in the chemistry of organostannanes The typical electrophilic chemistry of allylstannanes involves reaction with an electrophile in an Se2 process. Since all of the Lewis acids we are using to mediate this process are electrophilic, one can easily envision the sequences outlined in Fig.2. Thus a... 

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