Silyl-manganese complexes

A catalytic route using a manganese (III) complex has been developed for a-hydroxylation of ketones avoiding the use of water or a protic solvent mixtures of a-hydroxyketones and their silyl derivatives were formed in excellent yield. By using a chiral pyrrolidine-based manganese (III) complex as catalyst, asymmetric oxidation was effected, with enantiomeric excess varying from 14 to 62% [30], Another kind of a-functionalized ketones resulted from silyl enol ethers which after the addition of IOB.BF3 were treated with triethyl phosphite a-ketophosphonates were obtained in this way [31] ... 

Reddy, D. R., Thornton, E. R. A very mild, catalytic and versatile procedure for a-oxidation of ketone silyl enol ethers using (salen)manganese(lll) complexes a new, chiral complex giving asymmetric induction. A possible model for selective biochemical oxidative reactions through enol formation. J. Chem. Soc., Chem. Commun. 1992, 172-173. 

Adam, W., Fell, R. T., Stegmann, V. R., Saha-Moeller, C. R. Synthesis of Opticaiiy Active a-Hydroxy Carbonyl Compounds by the Catalytic, Enantioselective Oxidation of Silyl Enol Ethers and Ketene Acetals with (Salen)manganese(lll) Complexes. J. Am. Chem. Soc. 1998,120, 708-714. 

The reaction of metal carbonyl dimers with silicon hydrides also probably involves an initial oxidative addition step. Chiral silyl-cobalt and silyl-manganese carbonyl complexes have been obtained through the reaction of optically active organosilicon hydrides with metal carbonyls65 68 (equation 15 and 16). Phosphine-substituted cobalt complexes were similarly obtained by reaction of a chiral hydrosilane with Co2(CO)6L2 [L = PPh3, P(OPh)3, P(c-C6Hn)3]69. 

On the metal side, an electronically imsaturated metal complex is required to which the Si—H bond can be added. 16-electron complexes can easily be created in situ by UV irradiation of metal carbonyl compounds, which results in the cleavage of one CO ligand. Hydrido-silyl complexes are formed when irradiation is done in the presence of a hydridosilane R4 jcSiHjc x= 1—4). Manganese complexes of the type (tj —CsRsKCOKL)... 

Roddick DM, Tilley TD, Rheingold AL, Geib SJ (1987) Coordinatively imsaturated tris (trimethylsilyl)silyl complexes of chromium, manganese, and iron. J Am Chem Soc 109 945... 

Heyn RH, Tilley TD (2002) Coordinatively and electronically unsaturated tris(trimethylsilyl) silyl complexes of manganese and iron. Inorg Chim Acta 341 91... 

Silicon and germanium hydrides react with cobalt, manganese and rhenium carbonyls affording complexes having a silicon (or germanium)-metal bond. These reactions, described previously for inactive compounds have been used in the synthesis of optically active silyl and germyl-transition metals ... 

Ojima and co-workers first reported the RhCl(PPh2)3-catalyzed hydrosilylation of carbonyl-containing compounds to silyl ethers in 1972.164 Since that time, a number of transition metal complexes have been investigated for activity in the system, and transition metal catalysis is now a well-established route for the reduction of ketones and aldehydes.9 Some of the advances in this area include the development of manganese,165 molybdenum,166 and ruthenium167 complex catalysts, and work by the Buchwald and Cutler groups toward extension of the system to hydrosilylations of ester substrates.168... 

Silyl manganese pentacarbonyl decomposes in a more complex way to give a mixture of two phases, "MnSi (actually a silicon-rich nonstoichiometric phase of approximate composition MnSii.25) and stoichiometric Mn5Si3, such that the overall metal silicon ratio is 1 1. The analogous rhenium system behaves similarly. 

Oxidative addition of the 16-electron manganese center on the silyl ether leads to silyl-manganese complex III. Attack on III by an incoming alcohol (ROH) leads to complex TV. Pseudorotation gives all four possible structures of IV. Complex IV, having a free proton,... 

Very recently, Jprgensen s group reported the preparation of some chiral nitrido-manganese complexes, which closely resemble those synthesized by the authors group, and the application to asymmetric amination of silyl enol ethers with the chiral complexes using Carreira s method [35], These results were very similar to those of the authors [22]. 

Reaction of the Pt complex (13) with (11) gives a silylplatinum complex (14), the silyl group of which has the same configuration as that of (11). The oxidative addition is thus assumed to take place from the front side of the Si—H bond (equation 7). The hydrosilane (11) reacts with a manganese complex (15) to give rise to a complex (16 equation 8), which has a structure similar to the intermediate of the oxidative addition step. °... 

The chemistry of silicon, germanium, and tin transition metal compounds has been the subject of several reviews (12, 180). Optically active silyl ligands have been introduced in a transition metal complex by reaction of chiral functional organosilanes. However chiral silyl ligands containing complexes are limited to a few metal centers we shall discuss in turn iron, cobalt, platinum, and manganese complexes. 

Although even in the IiAUi cleavage of the Si-Pt bonds a reductive elimination process is likely to occur, since the configuration at silicon is almost completely retained, a nucleophilic displacement of the silyl ligand is still conceivable. Deuterium labeling experiments have unambiguously demonstrated this possibility in silyl-manganese complexes (213). 

The easy reductive elimination process does not seem to arise from a particular lability of hydrosilane adduct 166. Complex 166 is stable in solution. Moreover, functionalized silyl-manganese complexes were shown to undergo substitution at silicon without Si-Mn bond cleavage (251) (Scheme 58). 

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