Dichloroborane, formation

Cowley and coworkers have reported two other reactions. One is the reaction of the silylphosphide 11a with dichloroborane, which afforded the diphosphadiboratane, resulting in the formation of the phosphaborene by flash-vacuum pyrolysis (equation 101)135. The other is the reaction of the silylphosphide 11a with ( j5-C5H4Me)2WCl2, which gave compounds 14 and 15 (equation 102)136. A phosphinidene tungsten complex was assumed to be a reactive intermediate. 

Ground-state oxygen alone rarely oxidizes organic compounds. A classical example is the autoxidation of benzaldehyde to benzoic acid, a usually undesirable reaction that takes place even in the absence of light. Other examples of autoxidation without illumination are oxidations at the a positions with respect to aromatic rings or at tertiary carbons [47, 48, 49, 50] and the formation of alkyl hydroperoxides from alkyl dichloroboranes [57]. Some oxidations take place when a compound is treated with oxygen in the presence of bases [9, 52, 53]. 

Oxidations by oxygen and catalysts are used for the conversion of alkanes into alcohols, ketones, or acids [54]-, for the epoxidation of alkenes [43, for the formation of alkenyl hydroperoxides [22] for the conversion of terminal alkenes into methyl ketones [60, 65] for the coupling of terminal acetylenes [2, 59, 66] for the oxidation of aromatic compounds to quinones [3] or carboxylic acids [65] for the dehydrogenation of alcohols to aldehydes [4, 55, 56] or ketones [56, 57, 62, 70] for the conversion of alcohols [56, 69], aldehydes [5, 6, 63], and ketones [52, 67] into carboxylic acids and for the oxidation of primary amines to nitriles [64], of thiols to disulfides [9] or sulfonic acids [53], of sulfoxides to sulfones [70], and of alkyl dichloroboranes to alkyl hydroperoxides [57]. 

Hawkins et al. observed that the reaction of a, -unsaturated esters with cyclopentadiene in the presence of catalytic IR, 2R)-2-naphthalenyl-cyclohexyldichloroborane leads to the enantio-selective formation of Diels-Alder adducts in high yield and ee (87-97% ee). The borane reagent is prepared via hydroboration of l-(l-cyclohexenyl)naphthalene with dichloroborane to yield the racemic product, which was later resolved via its crystalline complex with (—)-menthone to furnish >99% de after one recrystallization. The menthone-free catalyst was obtained by methanol-ysis, followed by rechlorination with BCI3 (eq 21). 

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