Carbon dioxide hydroboration

The addition of a gas to a reaction mixture (commonly the hydrogen halides, fluorine, chlorine, phosgene, boron trifluoride, carbon dioxide, ammonia, gaseous unsaturated hydrocarbons, ethylene oxide) requires the provision of safety precautions which may not be immediately apparent. Some of these gases may be generated in situ (e.g. diborane in hydroboration reactions), some may be commercially available in cylinders, and some may be generated by chemical or other means (e.g. carbon dioxide, ozone). An individual description of the convenient sources of these gases will be found under Section 4.2. 

Transition-metal-catalyzed hydroboration, a reduction using B H bonds, of styrene derivatives has been demonstrated to occur in supercritical carbon dioxide using tunable Rh(I) complexes as catalysts. In the case of vinyl anisole (eq. 2.5), significantly higher regioselectivities were reported for the reaction in this medium than in THF or perfluoro(methylcyclohexane) (Carter et ah, 2000). 

The effect of solvent and metal cation on the hydroboration of a - and /3-pinenes in the presence of acetic acid is described319 and B2H6-LiBH4 hydroboration has now been extended (cf. Vol. 3, p. 81) to 2,10-epoxypinane 320 unfortunately the discussion is poor and products are inadequately described.320 Hydroalumination of a-and /3-pinene has been reported with di-isobutylaluminium (cf. Vol. 5, p. 38) with the less-hindered sabinene, hydroalumination is less stereospecific.321 Heating (209 X = Al/3) with carbon dioxide gives the acid (209 X = C02H) (Chemical Abstracts omits the use of carbon dioxide).321... 

In general, the acetylenic triple bond is highly reactive toward hydrogenation, hydroboration, and hydration in the presence of acid catalyst. Protection of a triple bond in disubstituted acetylenic compounds is possible by complex formation with octacarbonyl dicobalt [Co2(CO)g Eq. (64) 163]. The cobalt complex that forms at ordinary temperatures is stable to reduction reactions (diborane, diimides, Grignards) and to high-temperature catalytic reactions with carbon dioxide. Regeneration of the triple bond is accomplished with ferric nitrate [164], ammonium ceric nitrate [165] or trimethylamine oxide [166]. 

Reaction medium. By virtue of its attributes supercritical carbon dioxide deserves its increasing applications as a reaction medium in synthesis. These include epoxidation" with oxygen and PhCHO, rhodium-catalyzed hydroboration, Pd(0)-catalyzed 1,4-hydroarylation of enones" and Heck reaction in the presence of Pd-C," carbonylation of aryl halides, Glaser coupling of 1-alkynes (mediated by CuCy using a solid ba.se (NaOAc)," as well as seandium(III) perfluorooctanesulfonate-catalyzed Diels-Alder and hetero-Diels-Alder reaetions. ... 

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