Gem-bimetallics

The impetus for the development of gem-bimetallics was initially to discover alkylidene-transfer reagents akin to Tebbe s reagent [14]. Schwartz prepared bimetallic aluminum—zirconocene derivatives by the hydrometallation of various vinyl metallic compounds [15—17]. Knochel has developed zinc—zirconium gem-bimetallics by hydrozircona-tion of vinylzincs and has used them as alkylidene-transfer reagents [18], More recently, other gem-bimetallics have been developed that exhibit different reactivities of the two carbon—metal bonds. Thus, Normant and Marek have reported the allylmetallation of vinyl metals to afford zinc—magnesium and zinc—lithium gem-bimetallics, which react selectively with various electrophiles such as ClSnBu3, H20, etc. [19, and references cited therein]. However, selective and sequential cleavage of the two carbon—metal bonds... 

The formate anion reduces allylic acetates in the S 2 mode. Acquisition of allylic gem-bimetallic compounds is facilitated by this process. 

This chapter is intended to highlight the synthesis and use of geminal bimetallic compounds such as gem-metallozirconocenes, but we will concentrate on the chemistry of aluminum, boron, lithium, gallium, germanium, tin, zinc, and zirconium. 

Treatment of the optically active gem-borazirconocene alkanes with deuterium oxide followed by alkaline oxidation affords the corresponding optically active 1-deuterio primary alcohols. The enantiomeric excess of the resulting primary alcohols represents the diaster-eoselectivity of the asymmetric hydrozirconation (Scheme 7.13). Based on the cost and availability of optically active ligands, three types were explored monoterpenes, 1,2-diols, and 1,2-amino alcohols. Hydrozirconation of optically pure 1-alkenyl boranes 39 provided optically active 1,1-bimetallics 40. 

The results obtained for the fresh and aged commercial Pt/Rh and Pd/Rh TWCs are shown in Table 2. The first column contains the dispersions and calculated spherical particle sizes (in parentheses) derived from the CO methanation technique based on an assumed adsorption stoichiometry of 1 CO per exposed noble metal atom. The arbitrary choice of a stoichiometric factor of 1, rather than the value of 0.7 suggested by the EmoPt-l catalyst, was made on the basis of several factors. The main reason is that the presence of Rh in these catalysts (16% and 10% of the noble metal weight in the Pt/Rh and Pd/Rh catalysts, respectively) is likely to increase the average stoichiometric factor above 0.7 due to the presence of gem-dicarbonyl species on Rh. Bimetallic Pt/Rh particles have been found in automotive catalysts, sometimes with surface enrichment by Rh [20,21] or even bulk enrichment of selected particles as... 

Scheme 11 (a) A )i -Ti Ti -acetylide bimetallic Au catalyst (24) used in the azide-alkyne cycloaddition, (b) a gem-diaurated catalyst (25) used for the cycloaddition of dialkynes, and (c) a trimetallic gold complex (26) used in the cycloisomerisation of allenynes... 

Iron carbene complexes bearing chirality at the carbene ligand can be generated from optically pure bimetallic (chromium-iron) complexes by the addition of 1 equiv of TMSOTf in the presence of an olefin, which in situ undergoes an asymmetric cyclopropa-nation. Excellent ee s are obtained when the reaction is carried out with gem-disubstituted olefins (eq 122). ... 

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