Haloboration with

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)). 

The stereodefined alkenyl halides are of prime importance due to the developments of di- or trisubstituted alkene synthesis by cross-coupling reactions between organometallics and alkenyl halides, catalyzed by transition metal compounds [1-4]. 1-Halo-l-alkenes are conveniently synthesized via hydrobo-ration-halogenation reaction [5]. However, with these methods it is not possible to synthesize 2-halo-1-alkenes. Although the halometalation reaction is considered to be a powerful tool for the synthesis of 2-halo-1-alkenes, the reaction has not been adequately developed for such purpose [6]. Lappert and coworkers [7-10] are perhaps the first to report the systematic haloboration reactions of unsaturated hydrocarbons with BX,. However, there is no systematic approach to the haloboration with B-X-9-BBN to organic synthesis [11]. The desired B-X-9-BBN is prepared [12] by the reaction of 9-BBN with phosphorous pentachlo-ride, bromine, or HI. Alternatively, B-X-9-BBN are very conveniently obtained... 

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). 

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... 

Polymerization of IB from the PS macroinitiator was accomplished with BCl3 as coinitiator in CH2Cl2 at -78 °C. Due to the living nature of the polymerization of IB, high grafting efficiencies ( 85%) were reported. The resulting 15% homoPIB was most probably due to initiation from adventitious moisture or direct initiation (haloboration). 

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... 

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).-... 

The chemistry of Lewis acids is quite varied, and equilibria such as those shown in Eqs. (28) and (29) should often be supplemented with additional possibilities. Some Lewis acids form dimers that have very different reactivities than those of the monomeric acids. For example, the dimer of titanium chloride is much more reactive than monomeric TiCL (cf., Chapter 2). Alkyl aluminum halides also dimerize in solution, whereas boron and tin halides are monomeric. Tin tetrachloride can complex up to two chloride ligands to form SnCL2-. Therefore, SnCl5 can also act as a Lewis acid, although it is weaker than SnCl4 [148]. Transition metal halides based on tungsten, vanadium, iron, and titanium may coordinate alkenes, and therefore initiate polymerization by either a coordinative or cationic mechanism. Other Lewis acids add to alkenes this may be slow as in haloboration and iodine addition, or faster as with antimony penta-chloride. 

Boron trichloride and tribromide successfully polymerize styrenes and isobutene. These Lewis acids are typically used in combination with water or alkyl chlorides, acetates, ethers, and alcohols [105,153]. In contrast to earlier reports, BC13 can self-initiate polymerization of styrene and isobutene [137] by haloboration, and subsequent activation of the resulting alkyl chlorides by excess Lewis acid. Direct initiation was confirmed by the formation of lower molecular weight polymers than pre-... 

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. 

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