Synthesis of 2-Halo-l-Alkenes

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 

Consequently, it is reported that B-bromo-9-borabicyclo[3.3.1]nonane (B-Br-9-BBN), and B-iodo-9-borabicyclo[3.3.1]nonane (B-I-9-BBN) haloborate the 1-alkynes, stereo-, regio-, and chemoselectively and after protonolysis 2-halo-1-alkenes are obtained in excellent yields (Eq. 13.4 Table 13.1) [15]. 

The experiments depicted in Chart 13.3 confirm that haloboration proceeds via Markovnikov cis addition of halogen and boron to the terminal triple bond. Moreover, it is concluded that B-Br-9-BBN attacks only the terminal carbon- 

2- Iodo-2-cyclohexylethene H C CBrlCH leCOOMe 2,3-Dibromo-1 -propene 

---