Lithium metal carbon-hydrogen bonds

Alkane Carbon-Hydrogen Bond Activation), such behavior might not be expected of a monophosphine. However, in the reaction of [Rh(/u.-Cl)(COE)2]2 with four equivalents of PhCH2CH2PBu 2, this is observed as the product in (18) and conversion to (19) occurs on chloride abstraction (see also (174), Section 42). " A related situation is obtained when [Rh(/x-Cl)(Bu 2PCH2PBu 2)]2 is treated with neopentyl lithium where the highly unsaturated, 14-electron metal in the product, (20), is stabilized by an agostic interaction. ... 

Metal-catalysed biaryl formation continues to be of interest, and there has been a theoretical study of reactivity and regioselectivity in biaryl formation involving the cleavage of carbon-hydrogen bonds by a concerted metalation-deprotonation pathway. Various combinations of metal/ligand/base have been evaluated for the arylation of benzene with aryl bromides at high temperatures and pressures. The combination of cobalt(III) acetylacetonate and lithium bis(trimethylsilyl)amide proved to be effective. ... 

The metalation reaction, that is, the conversion of a relatively unuseful carbon-hydrogen bond to a synthetically advantageous carbon-metal bond, is one of the most important and widespread chemical transformations practiced today. A key intermediary tool for the preparation of pharmaceuticals, agrochemicals, perfumes/ cosmetics and fine chemicals, amongst other everyday commodities, the metalation reaction has typically been the domain of the highly polar alkali metals, nearly always lithium. Indeed, Collum emphasized this domination in 1993 stating that it would appear that well over 95% of natural products syntheses rely upon lithium based reagents in one form or another [1]. 

Although catalytic hydrogenation is the method most often used, double bonds can be reduced by other reagents, as well. Among these are sodium in ethanol, sodium and rerr-butyl alcohol in HMPA, lithium and aliphatic amines (see also 15-14), " zinc and acids, sodium hypophosphate and Pd-C, (EtO)3SiH—Pd(OAc)2, trifluoroacetic acid and triethylsilane (EtsSiH), and hydroxylamine and ethyl acetate.However, metallic hydrides, such as lithium aluminum hydride and sodium borohydride, do not in general reduce carbon-carbon double bonds, although this can be done in special cases where the double bond is polar, as in 1,1-diarylethenes and in enamines. " °... 

The details of the mechanism are poorly understood, though the oxygen of the alcohol is certainly attacking the carbon of the isocyanate. Hydrogen bonding complicates the kinetic picture. The addition of ROH to isocyanates can also be catalyzed by metallic compounds, by light, or, for tertiary ROH, by lithium alkoxides ° or n-butyllithium. ° ... 

Carbon forms bonds not only with itself and with hydrogen but also with many other elements, including strongly electron-attracting elements such as fluorine and strongly electropositive metals such as lithium ... 

The application of organometallic compounds or hydrides of the more electropositive main group metals lithium, magnesium or aluminium in organic synthesis, catalysis and polymerization is due to the polarity of the metal/carbon and metal/hydrogen bonds, i.e. to the considerably unsymmetrical electron distribution M —or —respectively. While there are... 

The reduction of a carbon-carbon multiple bond by the use of a dissolving metal was first accomplished by Campbell and Eby in 1941. The reduction of disubstituted alkynes to c/ s-alkenes by catalytic hydrogenation, for example by the use of Raney nickel, provided an excellent method for the preparation of isomerically pure c -alkenes. At the time, however, there were no practical synthetic methods for the preparation of pure trani-alkenes. All of the previously existing procedures for the formation of an alkene resulted in the formation of mixtures of the cis- and trans-alkenes, which were extremely difficult to separate with the techniques existing at that time (basically fractional distillation) into the pure components. Campbell and Eby discovered that dialkylacetylenes could be reduced to pure frani-alkenes with sodium in liquid ammonia in good yields and in remarkable states of isomeric purity. Since that time several metal/solvent systems have been found useful for the reduction of C=C and C C bonds in alkenes and alkynes, including lithium/alkylamine, ° calcium/alkylamine, so-dium/HMPA in the absence or presence of a proton donor,activated zinc in the presence of a proton donor (an alcohol), and ytterbium in liquid ammonia. Although most of these reductions involve the reduction of an alkyne to an alkene, several very synthetically useful reactions involve the reduction of a,3-unsaturated ketones to saturated ketones. ... 

---