HALOBORATION, of 1-alkynes

Generally, chloroborations seem to be vigorous reactions. The haloboration of 1-alkynes with haloorganoboranes is also known to be rapid under mild conditions (74). 

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>. 

Haloboration of terminal alkynes yielded stereodefined (Z)-2-halo-l-alkenylboron compounds e.g., 114, 115 that allowed further functionalization of the resulting C-Br or G-B bond. 2,2-Disubstituted-l-alkenylboronic esters were stereoselectively obtained when the haloboration was followed by cross-coupling with 1-alkynylzinc chlorides (Equation (18)).192 The iodoboration-conjugate addition sequence gave alkenyl iodides that were used as intermediate for the total synthesis of deoxyepothilone derivatives (Equation (19)).193-195... 

Satoh, Y, Serizawa, H., Miyaura, N., Hara, S., Suzuki, A. 1988. Organic synthesis using haloboration reactions. 11. A formal carboboration reaction of 1-alkynes and its application to di- and trisubstituted alkene synthesis. Tetrahedron Lett. 29 1811—1814. 

Haloboration. The reaction of B-Br- and B-I-9-BBN with 1-alkynes leads, after protonolysis, to 2-halo-1-alkenes in high yield (equation I).2... 

Stereoselective addition of B-Br across a terminal acetylene (haloboration) was first reported by Blackborow (66). Suzuki and coworkers examined the intermediate obtained from the haloboration of alkynes with B-bromo and 5-iodo-9-borabicyclo[3.3.1]nonanes for various organoborane reactions. Suzuki has reviewed the applications of haloboration for organic syntheses (67-69). A recent application involving the 1,4-addition of a halovinyl-9-BBN to methyl vinyl ketone (70) for the synthesis of a promising anti-cancer agent, 12,13-desoxyepothilone B due to Danishefsky and coworkers is shown in Figure 19 (77). 

A most useful haloboration agents, B-bromo-9-borabicyclo[3.3.1]nonane and BBr3, react readily with terminal alkynes via Markovnikov syn addition of the Hlg-B moiety to the carbon-carbon triple bond in an uncatalyzed process. The haloboration reaction occurs regio- and chemoselectively at terminal triple bonds, but for other types of unsaturated compounds it is nonselective 639... 

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