Stannanes formation

Lithiation at C2 can also be the starting point for 2-arylatioii or vinylation. The lithiated indoles can be converted to stannanes or zinc reagents which can undergo Pd-catalysed coupling with aryl, vinyl, benzyl and allyl halides or sulfonates. The mechanism of the coupling reaction involves formation of a disubstituted palladium intermediate by a combination of ligand exchange and oxidative addition. Phosphine catalysts and salts are often important reaction components. 

In contrast to the intermediate hydroxystannanes, O-protected stannanes 7 are stable compounds which can be distilled or chromatographed and stored under nitrogen for months. Treatment of 7 with butyllithium in tetrahydrofuran at — 78,JC results in rapid tin/lithium exchange (< 1 min). No products resulting from Wittig rearrangement or formation of an ate complex 8 could be detected9. 

Transmetalation to give l-methyl-2-propenylaluminum followed by isomerization to 2-butenyl isomers may be involved in reactions between aldehydes and 2-butenyl(tributyl)-stannane induced by aluminum(III) chloride in the presence of one mole equivalent of 2-propanol. Benzaldehyde and reactive, unhindered, aliphatic aldehydes give rise to the formation of linear homoallyl alcohols, whereas branched products are obtained with less reactive, more hindered, aldehydes66,79. 

Allyl carbamates also can serve as amino-protecting groups. The allyloxy group is removed by Pd-catalyzed reduction or nucleophilic substitution. These reactions involve formation of the carbamic acid by oxidative addition to the palladium. The allyl-palladium species is reductively cleaved by stannanes,221 phenylsilane,222 formic acid,223 and NaBH4,224 which convert the allyl group to propene. Reagents... 

The allenic stannanes can be transmetallated by treatment with SnCl4, a reaction that results in the formation of the a propargyl stannane. If the transmetallation reaction is allowed to equilibrate at 0°C, an allenic structure is formed. These reagents add stereospecifically to the aldehyde through cyclic TSs.194... 

Indeed, a Stille reaction of the TES-protected ( )-bromoxone (6/1-175) with the stannane 6/1-173 led to 6/1-176 upon standing overnight, via the diene 6/1-172 (Scheme 6/1.46). This synthesis is a classic example of a transition metal-catalyzed formation of a 1,3-diene, followed by a cycloaddition. 

The analogous formation of aryl stannanes and aryl germanes from aryl halides occurs in the presence of catalytic amounts of palladium complexes of triphenylphosphine. Hexabutyl-... 

Racemization of stannane 37 was also observed in donor solvents as shown in Table 2. Although stable for short periods in phenethylamine and acetonitrile, stannane 37 is appreciably racemized in DMSO, HMPA and especially MeOH. Pentacoordinated tin species are likely intermediates. Interestingly, exposure to HMPA leads to significant formation... 

A number of related stannane compounds have now been characterized, although to date no plumbanes. In addition, there is no definite evidence for the formation of M—Ge—H interactions, although Carre and coworkers90 have suggested that one may exist in MeCpMn(CO)2(H)GePh3. For a full coverage of this subject see the review by Schubert92. 

The palladium-catalyzed hydrostannylative cyclization of enynes is dealt with first, since mechanistically it is closely related to hydrometallation. Lautens262 reported the formation of homoallyl stannanes through the reaction of 1,6-enynes with tributyltin hydride in the presence of a catalytic amount of Pd(OAc)2.263 The active catalytic species is... 

The product possesses a homoallylic stannane moiety, which can be utilized as a useful synthon for cyclopropane formation (Scheme 68). Upon treatment of the homoallylstannane with HI, destannative cyclization takes place to give cyclopropylmethylsilane.271,272 A Lewis acid-catalyzed reaction with benzaldehyde dimethyl acetal affords vinylcyclopropane.273... 

To enhance the efficiency of the cyanide addition, these workers subsequently reported a three-component asymmetric synthesis of amino nitriles that avoids the use of the previously mentioned undesirable stannane [74], Thus, as illustrated in Scheme 6.23, treatment of the requisite aniline and aldehyde with HCN (toxic but cheap and suitable for industrial use) at —45°C in the presence of 2.5 mol% 65 leads to the formation of 67 with 86 % ee and in 80 % yield. As was mentioned above in the context of catalytic asymmetric three-component alkylations of imines (see Scheme 6.18), the in situ procedure is particularly useful for the less stable aliphatic substrates (cf. 71—73, Scheme 6.23). The introduction of the o-Me group on the aniline is reported to lead to higher levels of asymmetric induction, perhaps because with the sterically less demanding aliphatic systems, the imine can exist as a mixture of interconverting cis and trans isomers. 

Comparable treatment of the allenic stannane resulted in a similar outcome, producing a mixture of regioisomers from the in situ silylation and near exclusive formation of the allenylsilane from the sequential addition (Eq. 9.8). 

The steric course of this two-step process was examined with several chiral secondary propargylic phosphates (Eq. 9.35) [40], The derived propargylic stannanes were found to be formed with net inversion of configuration. Based on previous evidence that the initial formation of the allenyltitanium intermediate occurs with inversion, it can be deduced that stannylation proceeds by a syn pathway. This surprising result was attributed to coordination between the chlorine substituent of the Bu3SnCl and the electropositive titanium center (Scheme 9.11). 

---