Borane, with silyl-alkynes

Triphenylsilyl ethers are typically prepared by the reaction of the alcohol with triphenylsilyl chloride (mp 92-94 °C) and imidazole in DMF at room temperature. The dehydrogenative silylation of alcohols can be accomplished with as little as 2 mol% of the commercial Lewis acid tris(pentaf1uorophenyl)borane and a silane such as triphenylsilane or triethylsilane [Scheme 4.98]. Primary, secondary, tertiary and phenolic hydroxyls participate whereas alkenes, alkynes, alkyl halides, nitro compounds, methyl and benzyl ethers, esters and lactones are inert under the conditions. The stability of ether functions depends on the substrate. Thus, tetrahydrofurans appear to be inert whereas epoxides undergo ring cleavage. 1,2- and 1,3-Diols can also be converted to their silylene counterparts as illustrated by the conversion 983 98.4. Hindered silanes such as tri-... 

The monohydroboration of terminal alkynes with dicyclohexylborane, disiam-ylborane, catecholborane, and dimesitylborane, and complexes of dihaloboranes and 9-BBN provides the simple and efficient route to fra s-vinylboranes [1,2]. Unlike other dialkyl(vinyl)boranes, vinyl-9-BBN derivatives exhibit remarkable stability [3, 4]. However, with 1 1 stoichiometry of terminal alkyne 9-BBN, 9-BBN adds twice to give a significant amount of diboryl adduct (Eq. 5.19) from which boron stabilized carbanion can be made [5, 6]. The partial solution to dihydroboration is achieved either by employing 100% excess of the alkyne [3] or using silylated derivatives [4e, h]. 

Soderquist and coworkers have reported [19] that hydroboration with 9-BBN of 1-silylacetylenes, having bulkier groups on the Si, place the boron exclusively at the internal position of the alkyne as a kinetically less preferred process. Consequently, triisopropylsilyl-substituted alkynes afford the novel P-silylated vinyl-boranes, which on oxidation with alkaline hydrogen peroxide affords the corresponding ketones (Eq. 7.11) in excellent yields, a truly noteworthy example of the stability imparted to this functionality by the triisopropyl substitution on silicon [20]. It should be noted that earlier studies have shown failures to obtain (3-ketosilanes from suitable organoprecursors due to their known sensitivity to bases and nucleophiles [21]. 

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