Alkynes haloboration

Scheme 3.45 Efficient and stereoselective synthesis of yellov/ scale pheromone by alkyne haloboration, Zr-catalyzed asymmetric carboalumination of alkenes (ZACA reaction), and Pd-catalyzed tandem Negishi coupling [55].

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

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... 

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... 

A series of books 7) and reviews 8) written on the utility of these reactions highlights the synthetic applicability of this field. However, there was no attempt to systematize the otherwise vast literature available for selective hydroboration, e.g., hydroboration of double bond in the presence of triple bond and vice versa, hydroboration of one of the double bonds of dienes, monohydroboration of alkynes, and selective hydroboration of carbon-carbon multiple bonds in the presence of many other functional groups. These topics are the aim of this review. Included here also is a brief account of the developing field of haloboration. 

The haloboration reaction of terminal alkynes has been recently demonstrated to provide useful synthetic procedures for various halogen derivatives, which will be discussed later. 

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

Haloboration Reactions. The haloboration of carbon-carbon triple bonds provides another entry point for the synthesis of organoboranes. A wide variety of haloboranes including BBrs, 9-BBN-Br, and 9-BBN-l has been found to react with terminal alkynes to produce (Z)-2-halo-l-aIkenylboranes. The reaction occurs in a stereo-, regio-, and chemoselective fashion specifically with terminal alkynes and has been used to synthesize numerous substituted olefins and related compounds. Diboration reactions of alkynes with B2CI4 are also well known. However, more convenient transition-metal-catalyzed procedures with the less reactive aUcoxy substituted diboranes B2(OR)4 have recently been developed. 

The (Z)-alkynylhaloalkenes (7) can be synthesized in relatively good yields from 1 -alkynes in a regio-and stereo-selective manner, following haloboration with Br-9-BBN and subsequent treatment with lithium alkynides and iodine (Scheme... 

Haloboranes add to terminal alkynes in a cis /in-Markovnikov manner however, the bromoboration of acetylene itself exceptionally provides a trans adduct [28] (eq (17)). A sequence of haloboration with boron tribromide and then cross-coupling with organozincs is useful for a formal carboboration of alkynes with various organic groups. Addition of boron triboromide to allene is convenient for the synthesis of (2-bromo-2-propeny])borate [29] (eq (18)). 

Haloboration In view of the brief discussion of the current scope and limitations of alkyne carbometallation presented above, recent development of the alkyne bromoboration-Pd-catalyzed alkenylation protocol, especially those involving the use of propyne [56a] and arylethynes [56b] highlighted earlier and summarized in... 

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