Transmetalation, cyclic

The mechanism of carbometallation has been explored computationally.77 The reaction consists of an oxidative addition to the triple bond forming a cyclic Cu(m) intermediate. The rate-determining step is reductive elimination to form a vinyl magnesium (or zinc) reagent, which then undergoes transmetallation to the alkenyl-copper product. 

The anti stereochemistry is consistent with a cyclic TS, but the reaction is stereocon-vergent for the E- and Z-2-butenylstannanes, indicating that isomerization must occur at the transmetallation stage. The adducts are equilibrated at 82 °C and under these conditions the anti product is isolated on workup. 

Reagent-controlled stereoselectivity can provide stereochemical relationships over several centers when a combination of acyclic and chelation control and cyclic TS resulting from transmetallation is utilized. In reactions mediated by BF3 or MgBr2 the new centers are syn. Indium reagents can be used to create an anti relationship between two new chiral centers. The indium reagents are formed by transmetallation and react... 

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... 

An enantioselective variant of the diene cydization reaction has been developed by application of chiral zirconocene derivatives, such as Brintzinger s catalyst (12) [10]. Mori and co-workers demonstrated that substituted dial-lylbenzylamine 25 could be cyclized to pyrrolidines 26 and 27 in a 2 1 ratio using chiral complex 12 in up to 79% yield with up to 95% ee (Eq. 4) [ 17,18]. This reaction was similarly applied to 2-substituted 1,6-dienes, which provided the analogous cyclopentane derivatives in up to 99% ee with similar diastereoselectivities [19]. When cyclic, internal olefins were used, spirocyclic compounds were isolated. The enantioselection in these reactions is thought to derive from either the ate or the transmetallation step. The stereoselectivity of this reaction has been extended to the selective reaction of enantiotopic olefin compounds to form bicyclic products such as 28, in 24% yield and 59% ee after deprotection (Eq. 5) [20]. 

In the Zr-catalyzed cyclic carbometallation discussed above, a tandem process consisting of (i) transmetallation and (ii) P-H abstraction provides the missing link in the catalytic cycle. In a series of recent examples reported by Takahashi [206—208] and Hoveyda [209—214], the missing link has been provided by a process consisting of (i) [5-elimination or deheterometallation (Pattern 10), (ii) transmetallation, and (iii) P-H abstraction (Scheme 1.62). Some of these reactions have been developed into enantioselective C—C bond-forming processes, as discussed below. 

Kawakami et al. have prepared optically active bifunctional l,3-dimethyl-l,3-diphenyldisiloxanes.158,159 Strohmann et al. have prepared enantiomerically enriched Si-centered silyllithium compounds, which react stereo-specifically with triorganochlorosilanes.160-162 In solution, slow racemization of the silyllithium compounds takes place, which, however, can be circumvented by transmetallation with MgBr2. Oestreich et al. prepared new Si-centered cyclic silanes adopting the strategies developed by Corriu and Sommer.163 Bienz et al. have developed enantioselective routes for the preparation of C-centered chiral allenylsilanes.156,164-166... 

A cyclic homoallylic alcohol is efficiently provided by a sequence of Jt-allylpalla-dium complex formation, transmetallation with hexa-n-butyldistannane and intramolecular allylation (Scheme 16.17) [22]. The same transformation can be conducted by means of indium (Scheme 16.18) [23]. 

A combination of Ni(cod)2 and R2Zn effects simultaneous nickel metallacycle formation and organozinc transmetallation, yielding a cyclic homoallyl alcohol (Scheme 16.87) [95],... 

The palladium-catalyzed reaction of aryl- and vinyl-tin reagents with stereochemically defined allyl chlorides proceeds with overall retention of configuration, indicating that the second step, entailing interaction of the iT-allylpalladium complex and the organotin, proceeds by transmetallation and reductive elimination (attack at Pd, retention) (equations 166 and 167).142145 Comparable results were obtained with cyclic vinyl epoxides and aryltins.143... 

Transmetallation of silylmercury compounds, which are most easily available via the reaction of a silane with (/-Bu)2llg, appears to be the most convenient method for the preparation of cyclic potassium oligosilyl compounds c-(MegSis)K (19)83c as well as c-(MenSi6)K (20)84 were obtained by Hengge and coworkers from the corresponding mercury compound by means of a Na/K alloy in THF (equation 33). 

Organoboranes are used for ketone synthesis under basic conditions. The cyclic ketone 482 is prepared from alkenyl iodode 479. Hydroboration of terminal double bond, followed by carbonylation generates 480, and the cyclic ketone 482 is formed by intramolecular transmetallation of 480 to afford 481 [236],... 

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