Bromoboration reaction

The experiments have shown that B-Br-9-BBN attacks only the terminal triple bond whereas internal triple bond, internal and terminal double bonds can withstand the bromoboration reaction conditions (Eqs. 119 and 120). 

Bromoboration Reactions. BBrj does not add to isolated double bonds, but reacts with allene spontaneously even at low temperature to give (2-bromoallyl)dibromoborane, which provides stable (2-bromoallyl)diphenoxyborane by the addition of anisole. The diphenoxyborane derivative reacts with carbonyl compounds to give 2-bromohomoallylic alcohols in high yields (eq 12). Bromoboration of 1-alkynes provides (Z)-(2-bromo-l-alkenyl)dibromoboranes stereo- and re-gioselectively (eq 13), which are applied for the synthesis of trisubstituted alkenes, a,p-unsaturated esters, and 7,8-unsaturated ketones, bromodienes, 1,2-dihalo-l-alkenes, 2-bromoalkanals, and (3-bromo-oi,3-unsaturated amides. ... 

Bromoboration (Z)-/>2-dihalo-l -alkenes1 Reaction of a terminal alkyne (1 -octyne) with BBr3 in CH2C12 results in a p-bromoalkenylborane, which undergoes brominolysis or iodinolysis with retention to provide (Z)-l,2-dihalo-l-alkenes (equation I). 

In an attempt to synthesize (Z)-l,2-dihalo-l-alkenes from bromoboration adducts of 1-alkynes with B-X-9-BBN, all efforts have been unsuccessful. Fortunately, it has found that the haloboration of 1-alkynes with tribromoborane, followed by the reaction with iodine or bromine chloride in the presence of sodium acetate gives the expected (Z)-l,2-dihalo- 1-alkenes stereospecifically (>98%) in good yields (Eq. 122)187>. 

Haloboranes add to terminal alkynes in a cij-Markovnikov manner however, the bromoboration of acetylene itself exceptionally provides a rrans-adduct which gives the corresponding ( )-l-alkenylborates (10) by reaction with organozinc halides [24] (Scheme... 

E and Z boronic esters (13) and (14) were made easily accessible by bromoboration of acetylene followed by esterification of the resulting dibromoboranes The Pd(0) crosscoupling reaction occurs under mild conditions at room temperature leading to a wide variety of the butadienylboronic esters (15) with all possible combination of configurations of the conjugated double bonds as related in table 1. 

As mostly discussed earlier. Type III alkenyl derivatives, that is, ( )-R CH= CHM(or X), are widely and satisfactorily generated by (i) alkyne hydrometallation (M = B, Zr or, in some cases, Al, etc.) (Table 3.2, Scheme 3.6), (ii) polar halogenation reactions ofalkynes (Eqs. (1), (2), and (7), Scheme 3.15), and additionally, (iii) anti bromoboration of ethyne [53] followed by Negishi coupling (Eq. (1), Scheme 3.12). On the other hand. Type IV alkenyl derivatives may be prepared by (i) Normant alkylcupration of ethyne [67, 68] (Eqs. (5) and (6), Scheme 3.11), (ii) Zr-catalyzed alkylalumination of ethyne, (iii) syn hydroboration of 1-halo-l-alkynes followed by hydride-induced inversion of configuration [59] (Scheme 3.9), (iv) hydroboration of 1-alkynes followed by brominolysis (but not iodinolysis) with inversion [95], and (v) syn hydrozirconation or syn hydroalumination of 1-boryl- or 1-silyl-l-alkynes followed by protonolysis of the C-Al or C-Zr bond [96-98]. 

Notably, it has been observed that bromoboration of acetylene with boron tribromide proceeds rapidly to generate the unexpected ( )-2-bromoethenylboron dibromide (Scheme 23.15). The generally accepted reaction mechanism is that the bromoboration of acetylene proceeds in a syn fashion and then the intermediate spontaneously rearranges to the trans isomer. 

Hyuga, S., Takinami, S., Hara, S., Suzuki, A. 1986. Organic synthesis using haloboration reaction. Part 9. A direct and selective synthesis of ( Z)-l-bromo-l,3-dienes and ( ,Z)-1,3-dienes by the hydrobora-tion-bromoboration sequence of two alkynes. Tetrahedron Lett. 27 977-980. 

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