Surendra U. Kulkarni

Organoboranes : XXIV. Facile substitution and exchange reactions of 9-borabicyclo[3.3.1]nonane (9-BBN) and its B-substituted derivatives. Simple convenient preparations of B-halo derivatives☆

Abstract The exchange and substitution reactions of 9-borabicyclo[3.3.1]nonane (9-BBN) and its B -substituted derivatives with various reagents have been studied. The reaction of 9-BBN with halogens and hydrogen halides provides a simple route to B -halo-9-BBNs ( B -X-9-BBN, X = Cl, Br, I). These B -X-9BBNs can also be prepared by the facile redistribution reaction between the respective boron trihalides and 9-BBN or B -OMe-9-BBN. The alcoholysis of 9-BBN or B -X-9-BBN affords B -alkoxy-9-BBN in high yield. The exchange reaction between B -alkoxy-9-BBN and borane-methyl sulfide affords 9-BBN in quantitative yield. This study reports the simple procedures for the interconversion of a variety of B -substituted 9-BBNs.

Organoboranes for synthesis. 4 : Oxidation of organoboranes with pyridinium chlorochromate. A direct synthesis of aldehydes from terminal alkenes via hydroboration

Abstract The oxidation of trialkylboranes containing primary a1kyl groups with pyridinium chlorochromate (PCC) in methylene chloride provides the corresponding aldehydes in good yields. The stoichiometry for the oxidation of alcohols, borate esters and trialkylboranes with PCC has been examined. In view of the poor regioselectivity (only 94% primary alkyl groups) and functional group tolerance observed in the hydroboration with borane (BH 3 .THF or BH 3 .SMe 2 ), a more selective hydroborating agent, bis(3-methyl-2-butyl)borane (disiamylborane), was utilized for the preparation of aldehydes from terminal alkenes. However, the formation of 3-methyl-2-butanone as a by-product, and the requirement of six moles of PCC per mole of aldehyde are major disadvantages in this method. This difficulty was circumvented by employing monochloroborane-dimethyl sulfide for hydroboration. This reagent exhibits high regioselectivity (⪢ 99% primary alkyl groups) in the hydroboration of terminal alkenes. Oxidation of the resulting dialkylchloroborane following hydrolysis affords the desired aldehydes in satisfactory yields. Consequently, the hydroboration of terminal alkenes, followed by PCC oxidation, represents a direct convenient method for the transformation of alkenes into the corresponding aldehydes.

Organoboranes : XXV. Hydridation of dialkylhaloboranes. New practical syntheses of dialkylboranes under mild conditions

Abstract Practical methods for the synthesis of dialkylboranes (R 2 BH) via the hydridation of dialkylhaloboranes (R 2 BX) have been developed. Convenient methods available for the preparation of R 2 BX via the hydroboration of alkenes with monohaloborane complexes (H 2 BX · SMe 2 ) make this approach valuable for the preparation of various dialkylboranes, R 2 BH, many of which are not available by direct hydroboration of alkenes with borane itself. The suitability of various hydriding agents, such as borane derivatives, complex metal hydrides, and alkoxy metal hydrides, for the hydridation of R 2 BX was examined, utilizing B -halo-9-borabicyclo[3.3.1]nonane as a representative dialkylhaloborane. In this case, the unusual stability of the resulting dialkylborane, 9-BBN, permits direct estimation of the reaction products by 11 B NMR spectroscopy. The generality of the procedure has been demonstrated.

Hydroboration: LVIII. Monohydroboration of alkynes with representative dialkylboranes of varying steric requirements. A general synthesis of dialkylvinylboranes

The hydroboration of alkynes with representative dialkylboranes (R2BH) covering a range of structural types has been examined. Convenient methods now available for the preparation of a variety of R2BH, via hydridation of the corresponding dialkylchloroboranes (R2BCl), permitted a systematic investigation of the hydroboration of terminal and internal alkynes with these reagents. The monohydroboration internal alkynes generally proceeds cleanly. However, in the monohydroboration of terminal alkynes, varying amounts of dihydroborated products are also formed, the amount of such side products varying with the steric requirements of R2BH. Thus, relatively less hindered dialkylboranes, such as borinane, 9-borabicyclo [3.3.1]nonane (9-BBN), and di-n-hexylborane, produce considerable quantities of 1, 1-diboraalkanes, whereas, sterically more demanding reagents, such as bis(3-hexyl)borane, dicyclohexylborane, and disiamylborane, afford insignificant amounts of dibora derivatives, Dihydroboration is noticeably suppressed by carrying out the hydroboration at lower reaction temperatures.

Oxidation of organoboranes containing primary alkyl groups with pyridinium chlorochromate. A direct synthesis of aldehydes from terminal alkenes

Abstract Organoboranes derived from terminal olefins are oxidized by pyridinium chlorochromate to aldehydes in good yields.