Alkylmetallation

Whereas zirconium-catalyzed alkylalumination of alkynes is at present limited to some simple alkyl groups, it has been found that carbozirconation with Cp2Zr(R)Cl considerably expands the scope of the alkylmetalation of alkynes. The reactions are initiated by catalytic amounts of additives (typically Ph3C[B(C6F5)4]), which probably serve to generate cationic zirconocene species. Regioselectivity in favor of attack at the more highly substituted carbon is observed [70,71] (Scheme 8.35). 

Mixtures that are already 99% (S,S)-diol (a7 )-a-chloro boronic ester on reaction with methylmagnesium bromide or other simple alkylmetallic compounds must undergo destruction of the 1 % (. >,.SJ-diol (o S)-a-chloro boronic ester to byproducts that have very different physical properties from the major product. Thus, this chemistry accounts for the very high diastereomeric ratios noted in Table 1 (Section 1.1.2.1.2.1.). 

Ahnlich wie Imine konnen auch Iminium-Salze durch Alkylmetall-Verbindungen reduktiv C-alkyliert werden bzw. konnen, umgekehrt gesehen, Organometall-Verbindungen durch Iminium-Salze aminomethyliert werden siehe hierzu S. 1069f. 

Any attempt to describe concisely the structure of ZN catalysts must necessarily suffer from oversimplification. ZN catalysts can be used in soluble, insoluble or coloidal form, either free or attached to a support. In principle, they consist of mixtures of salts of Group IV-VIII metals with alkylmetallic compounds of Group I—III metals (cocatalysts). 

All of the proposed mechanisms for the reduction of alkynes with metal hydride-transition metal halide combinations involve an initial hydrometallation of the ir-system by the transition metal hydride, formed by the reaction of the original metal hydride with the transition metal halide, to form the vi-nylmetallic intermediate (99 equation 38). For the reduction of alkenes, similar alkylmetallic intermediates are implied to be formed. In the case of the reduction of alkenes with NaBH4 in the presence of Co" in alcohol solution, the hydrometallation reaction appears to be reversible as evidenced by the incorporation of an excess of deuterium when NaBD4 was used in the reduction. ... 

The mechanistic dichotomy between nucleophilic addition and electron transfer may also apply to alkylmetallates derived from transition metals. For example, an... 

A number of stannyl-zinc and -cadmium compounds have been prepared by reaction of a tin-alkali metal compound with a zinc or cadmium halide, or a tin hydride with an alkyl-zinc or -cadmium compound. The coordination of a ligand such as a triphenyl-phosphine, TMEDA, or bipyridyl, or a solvating solvent such as DME, both enhances the nucleophilicity of the alkyl group in the alkylmetallic compounds and stabilises the stannylmetallic product. Thus triphenyltin hydride reacts with diethylzinc or diethylcad-mium in pentane or benzene with separation of metallic zinc or cadmium, but with a preformed complex, or in a coordinating solvent, the distannylmetallic compound is formed (e.g. equation 19-32). 

Michael addition of alkylmetallic reagents to a,j8-disubstituted nitroalkenes, followed by protonation of the intermediate nitroate anion, generally gives mixtures of svn- and anti-diastereomers with poor diastereoselectivity, for example 7124. 

Alkylmetallation The title compound effectively promotes the reaction of alkynes with alkylzirconium species derived from hydrozirconation of alkenes. 

The asymmetric Heck reaction is catalysed by enantiomerically pure palladium catalysts formed with chelatingbiphosphines, especially BINAP, and this has proved an effective method for the synthesis of sterically constrained carbon centres, including quaternary centres. This chapter concludes with a brief discussion of enantioselective alkylmetallations using Grignard and organoaluminium species, which have proved useful in the diastereo- and enantioselective synthesis of polyene systems. 

One other item of historical interest may suffice. Paneth was an inorganic chemist who reported on simple free alkyl radicals, which he generated by the pyrolysis of alkylmetallic compounds of metals such as lead and whose half-life period he measured. About 3 years before, H. Taylor (a ph)rsical chemist) published in a less generally used periodical, his ideas and experimental observations on studies concerned with catalytic reduction processes of hydrogen-ethylene mixtures. He wrote that the simplest method of liberating free alkyl radicals into a mixture of these gases was by decomposition of metal alkyls such as diethylmercury and tetraethyllead. 

A limitation of pinanediol boronic esters (15) results from the difference between the two diastereotopic faces of the trigonal boron atom. The sequential double di-astereodifferentiation observed with chiral directors having C2-symmetry is not possible. Borate anion 16 derived from 15 rearranges to (a-chloroalkyl)boronic esters 17 and 18 in a ratio that usually exceeds 50 1 (Scheme 8.3) [12]. Alkylmetallic reagents attack 17 from the less hindered side to form 19, in which the chloride to be displaced is not in a comparable steric environment to that in 16 (28). The major diastereomer 20 is produced in about the same proportion as its precursor 17 (Scheme 8.3). 

C-C cross coupling of arylhalides/tri-flates with alkylmetallates using palladium catalysis. 

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