Transmetallation from zirconium to boron: The synthesis and conjugate addition of mixed unsymmetrical tricoordinate organoboranes containing phenyl and methyl groups.

摘要

The hydrozirconation of various alkenes and alkynes with Cp2Zr(H)Cl followed by transmetallation with chlorodiphenylborane produced high yields of B-alkyl- and B-alkenyldiphenylboranes along with Cp2ZrCl2 as a precipitate which was recycled. Using this mild methodology no disproportionation was observed in most cases. In the case of cycloalkenes disproportionation was observed, the problem was resolved by having an excess of the cycloalkene present during the transmetallation. Organoboranes containing three different organic groups, (phenyl, alkyl and alkenyl) were also successfully produced using transmetallation.; In an alternative method to produce B-alkyldiphenylboranes, zirconocene hydrochloride was used as a mild reducing agent converting chlorodiphenylborane into diphenylborane, which in the presence of excess alkene underwent hydroboration to produce the desired product. Sterics seemed to limit the synthesis of B-alkyldiphenylboranes to the use of terminal and disubstituted alkenes.; The B-alkyldiphenylboranes were found to undergo 1,4-additions with methyl vinyl ketone. The alkyl group reacted exclusively with no sign of phenyl group addition. The ketone products were obtained in very high regio-purity. The reaction proceeded faster with the slow addition of air into the solution, suggestive of the radical nature of the reaction. The B-alkenyldiphenylboranes also underwent 1,4-additions with methyl vinyl ketone. The vinyl group reacted preferentially producing the gamma,delta-enones in high yield, with retention of configuration. The addition of air into the solution was not a factor in this reaction.; The phenyl group was found to be a good auxiliary group in the conjugate addition reaction. However, due to sterics the number of different alkenes we could work with was limited. A process was developed to produce B-alkyldimethylboranes. Dimethylzirconocene was prepared via the reaction of two equivalents of methyllithium with zirconocene dichloride. In the reaction of dimethylzirconocene with alkyldichloroboranes, the methyl groups were found to undergo transmetallation to produce the B-alkyldimethylboranes. The smaller size of the methyl groups allowed for the use of trisubstituted alkenes in the process. The conjugate addition of the B-alkyldimethylboranes with methyl vinyl ketone proceeded as expected with the alkyl group reacting exclusively over the methyl groups.