Transmetallation Methods

The treatment of trialkylsilyl derivatives with boron halides described in Section 

1 Electrophilic trapping of alkenylmetal intermediates with borates X i. R M B(0R )2 

3 Transition metal catalyzed coupling between aryl halides/triflates and diboronyl reagents 

2 Indirect trans-hydroboration using alkynyl bromides 

4 3 Transition metal-catalyzed cis-hydroboration of alkynes HBX2 

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On the other hand, transmetalation methods such as the tin-lithium exchange protocol or reductive Uthiations of 5, 5 -acetals were also developed by Emde and Briickner. They have shown the significant difference in the stereoselectivity between the Sn Li exchange and SAr —Li exchange protocols (Table 7). 

Selected 1-alkenylboronates synthesized by Equation (56) are shown in Scheme 17. The reaction afforded cyclic and acyclic vinylboronic esters possessing various functional groups, which are not available by conventional hydro-boration of alkynes or by a transmetallation method, for example 160-167. 

The palladium-catalyzed reaction of diboron with allyl acetates or chlorides is a convenient alternative to transmetallation method for the synthesis of functionalized boronates. The reaction of diboron 119 with allyl acetates309-312 smoothly occurred without the assistance of a base, whereas the presence of AcOK was critical for the coupling with allyl chlorides313 (Equation (57)). The boron atom coupled with the less-hindered terminal carbon giving the thermally stable (if)-allyl boronates. Thus, the (E)- and (Z)-cinnamyl acetate, chloride (entries 1-4), and their secondary derivatives (entries 7 and 8), all afforded an (if)-cinnamyl boronate. The borylation of prenyl acetate was slow (entry 6), but the corresponding chloride (entry 5) and tertiary derivative (entry 9) worked well for the same purpose (Equation (58)).309,310... 

This transmetallation method also gave (9) Bi2(C6H4)3, but only in amounts analyzable by infrared and mass spectrometry. These low yields result from a simultaneous thermal decomposition which occurs during synthesis—both compounds are stable indefinitely in air at room temperature. Transmetallations with perfluoro- and perchloro-phenylenemercury give much better yields of the very thermally stable perhalotriptycenes ... 

Via transmetallation (method 1) Metal-bismuth exchange reaction occurs between triarylbismuthine and alkyllithium or alkylsodium reagents. 

The redox transmetallation method is a simple and convenient preparative route. Cp2Hg and cyclopentadienyl complexes of T1(I) are used as a source of cyclopentadienyl ligands [19]. The reactions of dicyclopentadienylmercury are carried out in THF at 0-20 C with 4-5-fold excess of lanthanoid finely divided and activated by mercury chloride. 

Using the redox transmetallation method Beletskaya, Bregadze and coworkers [25] have obtained lanthanoid derivatives of carborane with B-Yb and B-Tm groups. The bormercurated carboranes have been used as the initial compounds ... 

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