Boron bromides reactions with alkenes

Direct coupling of two vinyl groups remains an important job for the Suzuki reaction. In their synthesis of the antibiotic (+)-fostriecin, Reddy and Falck46 coupled the Z-vinyl bromide 276 with the Z-boronic ester 277 to make the triene 278 with Z,Z, -alkenes. Free OH groups obviously do not interfere. 

If boron of an alkylborane could be replaced with a halogen, the product would be an alkyl halide. However, reaction of alkylboranes (neat) with chlorine, bromine, or iodine is very difficult. a when halogenation is done with bromine or iodine dissolved in dichloromethane, however, the reaction is fast and is synthetically useful.A simple example is the reaction of alkenes with boranes followed by addition of bromine, which leads to the alkyl bromide. An example is taken from the synthesis of 2-bromobutane (70) from 2-butene in 88% yield. 0 jhe bromination occurs by a free radical mechanism. Initial reaction with bromine generates a... 

Insertion Reactions. Trimethylsilyldiazomethane undergoes net insertion between the B-C bond of borinate and boronate esters. Thus, olefin hydroboration, followed by treatment with TMSCHN2, oxidation, and desilylation offers a method for hydroxymethylation of alkenes in fair to moderate yield (eq 54). Stable, chiral allenylboranes (IR and IS) are prepared by reaction of TMSCHN2 with 5-MeO-9-BBN followed by resolution with pseudoephedrine and reaction with allenyl magnesium bromide (eq 55). Compounds IR and IS are useful for the asymmetric allenylboration of aldehydes with predictable absolute stereochemistry. The precursors to 1 are then readily recovered during work-up. ... 

Isopropoxy-4/f-l,3,2-benzodioxaborin (18) has been synthesized and found to react with a variety of hydroxy compounds to replace the isopropoxy group on boron. Aryl boric acids, RB(OH)2, reportedly condense with diols of the type HO(CH a)nOH to yield oligomeric rings (19) of varying size. The reaction of allenyl borates (20) with hydrogen bromide to yield alkenes and alkynes has been published. ... 

Oxidative addition of aryl halides to [Pd(dba)2] in the presence of tetramethylethylenediamine or 2,2 -bipyridine to produce cis- [Pd(N-N)(Ph)X] proceeds easily when iodides are used but with more difficulty with bromides. Methyllithium reacts with the iodo complexes to give the mixed dialkyls. Cross coupling of alkenylboronic and arylboronic acids with alkenes is catalysed by palladium acetate in acetic acid. The reaction is believed to occur by oxidative addition of the carbon-boron bond across an in situ Pd(0) centre. 35... 

Allylic zinc reagents are highly reactive reagents which are prone to undergo carbozincation of weakly activated alkenes . Thus, the addition of the mixed methallyl(butyl)zinc 393 with the vinylic boronate 394 provides the intermediate zinc reagent 395. After the addition of an extra equivalent of ZnBri, CuCN 2LiCl and allyl bromide, the reaction mixture was worked up oxidatively providing the alcohol 396 in 83% yield. ... 

Catalytic hydroboration of vinylic ethers, acetals, and esters with pinacolborane takes place smoothly in the presence of transition metal catalysts. However, a noticeable exception is the catalytic hydroboration of vinyl bromides 59 which do not furnish the expected hydroborated product under these conditions. The reaction of vinyl bromides with pinacolborane initially affords the expected /3-boronoalkylbromide 60. A fast. -elimination ensues to furnish the terminal alkene 61 and 7 -bromopinacolborane 63. The alkene 61 undergoes hydroboration with unreacted pinacolborane to provide the debrominated boronate 62. The intermediate 5-bromopinacolborane 63 cleaves the ethereal C-O bond in the solvent (THF) to provide 4-bromobutyl borate 64 as a side product (Scheme 11) <1996JA909, 2000CSP14505>. 

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