Organoboron rearrangements

The preparation of a functional segmented block copolymer was also investigated (scheme ll).15 First hydroboration polymerization of the oligomer using thexylborane was carried out. Then the obtained organoboron polymer was subjected to a chain-transformation reaction (DCME rearrangement). DCME and lithium alkoxide of 3-ethyl-3-pentanol in hexane was added to a THF solution of the polymer at 0°C. 

Secondary amines have been prepared from organoboron compounds R1315, R12 BC1 or R1BCI2 (R1 = Et, Bu, /-Bu,, v-Bu, 3-hexyl, cyclopentyl etc.) by treatment with azides R2N3 (R2 = Bu, /-Bu, i-Bu, cyclohexyl, Ph etc.) and aqueous work-up. It is suggested that the reactions proceed by way of anionotropic rearrangements (equation 61)176. 

Trialkylboranes add to propellane to give a zwitterion that can rearrange to give 60 or react with another molecule of la to give 61. Both organoboron compounds were oxidized with hydrogen peroxide and isolated as the corresponding alcohols in 65% and 21% yield, respectively (equation 23). 

Hydrozirconation of alkenes. This organic metallic reagent reacts with olefins at 25-40° to form alkylzirconium complexes of the type (C5115)2 Zn(R)Cl, in which the transition metal moiety is attached to the least hindered position of the olefin. For example, when a suspension of (1) in benzene is shaken with 1-octene or either cis- or tran -4-octene, (2) is formed in quantitative yield. Thus internally metalated zirconium compounds undergo rearrangement much more rapidly than organoboron or aluminum analogs. Other examples of hydrozircon-ation are formulated. Very hindered olefins e.g., tetramethylethylene) fail to react at 40°. 

The photochemistry of silylated derivatives continues to be intensively investigated. A typical example of a synthetically useful process is the generation of silyloxycarbenes by photoisomerization of acylsilanes. In the presence of organoboronic esters these intermediates are trapped via formal B-C bond insertion. The adducts then rearrange to a boradioxepan that can be hydrolyzed to a ketone. The overall process offers a convenient, transition metal free method for the intermolecular carbon-carbon bond formation (see Scheme 22). ... 

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