Bromides hydroboration-bromination

This procedure illustrates the bromopropylation of an alcohol by allylation and then hydroboration/bromination,7 a clean, selective procedure compared with other approaches via 3-substituted propyl bromide derivatives. Conversion to the tetraalkyl tin, then brominative cleavage, is the standard sequence for preparation of trialkyl tin derivatives.8 The standard lithium aluminum hydride (UAIH4) reduction of the tin bromide was not usable here because of contamination of the product by UAIH4 byproducts that were not easily separated, and the polymethylhydrosiloxane reduction method9 was not successful. However, use of NaBH4 in f, 2-dimethoxyethane was effective and convenient.10... 

An alternative synthesis of (Z)-l-halo-l-alkenes involves hydroboration of 1-halo-l-alkynes, followed by protonolysis (246,247). Disubstituted ( )-and (Z)-a1keny1 bromides can be prepared from ( )- and (Z)-a1keny1 boronic esters, respectively, by treatment with bromine followed by base (248). 

Synthesis of (A) started with the combination of 2,4,6-trimethylphenol and allyl bromide to give the or/Ao-allyl dienone. Acid-catalyzed rearrangement and oxidative hydroboration yielded the dienone with a propanol group in pore-position. Oxidation of the alcohol and bromolactonization (see p. 273) followed. Bromine and the lactone ring were trans in the product as expected (see p. 273). Treatment with aqueous potassium hydroxide gave the epoxy acid, which formed a crystalline salt with (R)-l-(a-naphthyDethylamine. This was recrystallized to constant rotation. 

Acid-Catalyzed Hydration of 2-Methylpropene 227 Hydroboration of 1-Methylcyclopentene 233 Oxidation of an Organoborane 235 Bromine Addition to Cyclopentene 237 Epoxidation of Bicyclo[2.2.1]-2-heptene 240 Free-Radical Addition of Hydrogen Bromide to 1-Butene 243... 

Vinyl bromides, however, do not provide the expected hydroboration product under these conditions. Instead, initial hydroboration occurs /3- to the bromine atom and is followed by a fast sy -elimination to furnish the terminal alkene. This then undergoes hydroboration to provide the debrominated boronate. The intermediate B-bromopinacolborane cleaves the ether C-0 bond in the solvent THF to provide 4-bromobutanol upon oxidation4 (eqll). 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the acetylene. Catechol borane (l,3>2-benzodioxaborole), which is prepared from equimolar amounts of catechol (1,2-dihydroxybenzene) and borane, is a particularly useful reagent for hydroboration of acetylenes.Protonolysis of the adduct with acetic acid results in reduction of the original alkyne to the corresponding c/5-alkene. Oxidative workup with hydrogen peroxide gives ketones via an enol intermediate. Treatment of the vinyl borane with bromine and base leads to the vinyl bromide. The net anh-addition has been rationalized on the basis of anh-addition of bromine followed by a second z/tr/-elimination of bromide and boron but there are exceptions to this generalization. 

After the development of catalyst-transfer condensation polymerization of polythiophene, the block copolymer of polythiophene and poly(alkyl acrylate) was prepared more easily. Vinyl-terminated polythiophene was first prepared. The vinyl group was converted to the 2-hydroxyethyl group by hydroboration, followed by esterification with 2-bromopropionyl bromide to give a macroinitiator for ATRP (Scheme 50) [142]. The allyl-terminated polythiophene was also converted to a macroinitiator for ATRP, which led to block copolymers of polythiophene and poly (aUcyl methacrylate) [143] or poly(acrylic acid) [144]. This allyl-terminated polythiophene has a bromine atom at the other end, which has an adverse effect on the purity of block copolymers prepared by ATRP. Hawker, Kim, and coworkers reported that replacement of the bromine with a phenyl group, followed by functionalization of the allyl group for the ATRP initiator unit, allowed access to narrower molecular weight distribution diblock copolymers of polythiophene and ATRP-derived vinyl block [145]. 

The Brown group developed a multi-step procedure including sy -hydroboration of terminal alkynes and frans-bromination followed by frans-elimination to access Z-afkenyl bromides (Fig. 16) [39]. 

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