Haloboration

In the previous section we have discussed that 1-halo-1-alkenes can be conveniently synthesized. However, with these methods it is not possible to synthesize 2-halo-l-alkenes. Although the halometallation reaction would be a powerful tool for the preparation of 2-halo-1-alkenes, the reaction has not been adequately developed for such purpose 183). Recently, it has been reported that B-bromo-9-borabicyclo[3.3.1]-nonane (B-Br-9-BBN) and B-iodo-9-borabicvclo[3.3.1] nonane (B-I-9-BBN)1841 react with 1-alkynes, stereo-, regio- and chemoselectively, and after protonolysis, 2-halo-1-alkenes are obtained in excellent yields (Eq. 118)1851. 

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). 

As the haloboration occurs stereospecifically, the reaction is extended for a stereo-defined synthesis of (Z)-l-alkynyl-2-halo-l-alkenes (Eq. 121)186). 

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 . 

The haloboration reaction are also applicable for the preparation of N-phenyl-(i-bromo-a,(3-unsaturated amides1891 and 2-bromoalkanals 190). 

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In addition to hydroboration, haloboration and phenylboration have also been used in the synthesis of 7r-conjugated organoboron polymers (9)29 (Fig. 9) and polymers containing B—N bonds.30... 

Figure 9 The -77-conjugated organoboron polymer (9) produced by haloboration-phenylboration polymerization between 2,7-diethynylflourene and Ph2BBr. (Adapted from ref. 29.)...

Conjugated organoboron polymers were prepared by haloboration-phenylboration polymerization between diyne monomers and bromodiphenylborane (scheme 29).54 The polymerization was carried out by adding a slight excess of bromodiphenylborane to a tetrachloroethane solution of diynes at room temperature... 

Hall-Petch relationship, 13 497 Haloacetic acids (HAAs), removal from drinking water, 17 806—807 Haloacetones, 1 163 Haloalkylating agents, 12 167 Haloalkylation, 12 166-168 Haloalkylbenzenes, intramolecular alkylation of, 12 169 IV-Halo-a-amino acids, 13 107 Haloaromatics, 13 573 Halobenzenes, as Friedel-Crafts arylating agents, 12 171 Halobismuthines, 4 28-29 Haloboration, 14 270 Halobrom (BCDMH,... 

SYNTHESIS OF 4-(2-BROMO-2-PROPENYL)-4-METHYL-v-BUTYRO-LACTONE BY THE REACTION OF ETHYL LEVULINATE WITH (2-BROMO-ALLYL)DIISOPROPOXYBORANE PREPARED BY HALOBORATION OF ALLENE... 

Synthesis of 4-(2-Bromo-2-Propenyl)-4-Methyl-Y-Butyrolactone by the Reaction of Ethyl Levulinate with (2-Bromoailyl)diisopropoxyborane Prepared by Haloboration of Allene. 

A well-defined product could be isolated from the haloboration of the aminoiminoborane CgHigNBNfBu with BCI3 or BBr3, since the primary product is stabilized by intramolecular BN coordination [Eq. (31a)j (16) the corresponding bromoboration yields compound III... 

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

Boron trihalides, particularly BBr3, add to alkenes in a reaction (haloboration) that is usually more complicated than hydroboration.465 Several compounds can be isolated as a result of secondary transformation of the primary addition product 466... 

B-Bromo and B-iodo-9-borabicyclo[3.3.1]nonane add similarly in a cis fashion to terminal triple bonds 471 They do not react, however, with alkenes and internal acetylenic bonds. In contrast to the results mentioned above, phenyl-substituted chloroboranes (PhBCl2, Ph2BCl) do not participate in haloboration. Instead, the C—B bond adds across the multiple bond to form phenylalkyl-(alkenyl) boranes.466,468... 

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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