Methane, tris cleavage

Sublimation of tetraneopentyl zirconium onto the surface of silica dehydroxylated at 500 °C results in the electrolytic cleavage of one Zr-C bond by a surface proton, with the formation of the tris neopentyl zirconium [Si]-0-Zr(CH2CMc3)3 grafted species. Reaction of this supported Zr alkyl with hydrogen at 150°C leads to the formation of a zirconium hydride. Interestingly, this reaction does not yield neopentane, but rather the formation of methane and ethane are observed. These products are in fact the result of the hydrogenolysis of evolved neopentane catalyzed by the silica-supported zirconium hydride. ... 

As aheady mentioned, it was observed that one mole of hydrogen is liberated when methane is reacted with the tantalum hydride with the formation of tantalum methyl. The reaction with methane above 150°C leads to the formation of the Ta-methyl, Ta-methylene, and Ta-methylidyne species plus H2 (M=Ta) [40-42, 54]. These observations are a proof that the first step of alkane metathesis is the formation of metal alkyl intermediate via cleavage of the C-H bond of the alkane likely by sigma bond metathesis. Further, detailed mechanistic [22, 55] and experimental kinetic studies revealed that the alkenes and hydrogen are the primary products [56]. Initially, it was believed that the active site was a bis-siloxy tanta-lum-monohydride, but progressively, evidence came in favor of an equilibrium between bis-siloxy tantalum-monohydride d and bis-siloxy-tantalum-tris-hydride d° [57], and the mechanism would fit much better with a bis-siloxy-tantalum-tris-hydride [58]. 

There are several situations where cleavage of a 1,3,2-dioxaborolane to the boronic acid and diol is useful. One of these is for removal of a chiral director and replacement by its enantiomer. The first time we encountered this problem, a pinanediol ester was converted to the boronic acid via destructive cleavage of the pinanediol with boron trichloride (14). More recently, it has proved possible to convert an (R)-DICHED a-benzyloxy boronate (20) to the free bororric acid (23) with the aid of sodium hydroxide and a tris(hydroxymethyl)methane to form water soluble derivative 21 (R = CH2OH or NH(CHi)3S03 ) plus water insoluble (R)-DICHED (22). Treatment of 23 with (S)-DICHED (24) then yielded diastereomer 25 (76%, 97-98% diastereomeric purity). Further chain extension and alkylation led to 26 and 27, and deboronation yielded 28, all of which are stereoisomers that could not be accessed directly with a single chiral director (Scheme 6). 

Similar cleavage of the Si-C bond occurs with fluoride ion. The in situ reaction with carbonyl compounds gives the corresponding alkenes. Thus the reaction of tris(trimethylsilyl)methane with nonenolizable carbonyl compounds in the presence of tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF) gives the corresponding alkenes in excellent yield (eq 5). ... 

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