9-Iodo-9-borabicyclo nonane

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... 

Hydroboration of terminal acetylenes with boron halides permits the stereospecific synthesis of ( )-l-chloro(or bromo)alk-l-enes919. BBr3 adds to terminal acetylenes in a regio- and stereoselective manner to yield /Lhalovinylboranes, which are very useful syn-thons92a 923. B-bromo- and B-iodo-9 borabicyclo[3.3.1]nonane react similarly924"926. 

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. 

Sih (38) has described the reduction of E-l-iodo-l-octen-3-one with Penicillium decumbens to give the desired S-alcohol. Based on optical rotation, the e.e. was about 80%, An asymmetric chemical reduction of this same ketone, using lithium aluminum hydride that had been partially decomposed by one mole each of S-2,2 -dihydroxy-l,T-binaphthol and ethanol (42), gave the desired alcohol in 97% e.e. This reagent also reduced l-octyn-3-one in 84% e.e. to the corresponding alcohol (43). A 92% e.e. could be obtained with B-3-pinanyl-9-borabicyclo[3.3.1]nonane as the reducing agent (44). 

Bromo-9-borabicyclo[3.3.0]nonane (9-Br-BBN), CH2CI2, reflux, 87-100% yield. 9-Br-BBN also cleaves diaUcyl ethers, allyl aryl ethers and methylenedioxy groups. 9-Iodo-9-borabicyclo[3.3.0]nonane has also been used effectively and does not cause haloboration of an alkene. ... 

B-bromo- or B-iodo-9-borabicyclo[3.3.1]nonanes add regioselectively to terminal alkynes to give useful intermediates for organic synthesis ... 

The described reactions start from the compounds bis-9-bora-bicyclo[3.3.1]nonane (9H-9-BBN)2, elemental sulfur (Sg), and dihydrogen sulfide or from the easily preparable 9-iodo-9-borabicyclo[3.3.1]nonane. The procedures are simple and practicable, and the yields are nearly quantitative. The (9H-9-BBN)2-sulfur reaction can be followed not only by NMR spectroscopy but also by volumetric measurements of the evolved gas (H2). It is also remarkable that the BC bonds of the 1,5-cyclooctanediylboryl group are completely stable toward sulfidation at temperatures up to about 150 . The three compounds described here can be prepared easily on a scale of some 100 g of high purity. If necessary, the purification is possible, involving distillation or sublimation under vacuum without any decomposition. 

Starting from 9-iodo-9-borabicyclo[3.3.1]nonane and elemental sulfur (method B), one obtains the colorless disulfide in boiling toluene under elimination of iodine after 5 h according to the above equation. Because further 9-iodo-9-BBN reacts only extremely slowly with the disulfide with formation of the monosulfide, the pure disulfide can be isolated nearly quantitatively. In contrast to the reduction of the disulfide with the iodide, the monosulfide is oxidized by bromine in hot toluene, forming the disulfide. ... 

Method B. The disulfide is synthesized from 9-iodo-9-borabicyclo[3.3.1]-nonane and sulfurin a 1-L flask with a magnetic stirrer, a thermometer tube, and a reflux condenser, combined with an inert-gas bypass and a bubbler. 9-I-9-BBN (60.36 g, 243 mmol) and sulfur (7.81 g, 243 mmol) in toluene (350 mL) are slowly warmed to reflux temperature. After heating for 5 h under reflux an intensive violet-colored suspension is formed, from which the solvent is removed under vacuum (12 torr bath < 40°). After sublimation of about 30 g of iodine at 0.001 torr (bath <70°) a colorless, pure disulfide (33 g, 89%) is collected mp 134-135° (DSC 133.5°), bp 90-120° at 0.001 torr. 

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). 

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