Tetracoordinate Organoboranes

Thermally or Chemically Induced Migrations with Retention of Boron 

Much of the chemistry of such coordinated organoboranes has been recently and ably surveyed (82). Characteristic thermal and chemically induced migrations have already been cited in foregoing discussions [Eqs. (6), (7), (16), and (17)]. Of interest here are electronic and steric factors determining these migrations. Generally considered, the interaction of a carbanion R with an a,/8-unsaturated borane 34 could lead to three modes of attachment (77) [Eq. (20), 35-37]. Coordination at boron 

However, if E in 38 is phenyl or if E is methyl and phenyllithium is used, no rearrangement occurs. Possibly the attachment of one phenyl group to boron in 40 causes too much congestion in the transition state (42, where the B-phenyl and orf/jo-phenyls would impede overlap of the boron 2pz-orbital and the 77-cloud). 

As corroboration that the reaction of pentaphenylborole with di-p-tolylacetylene leads to the mixture of 7-borabicyclo[2.2.1]heptadienes 

The extensive and informative studies of the photochemistry of tetracoordinate boron have usually been done in the presence of oxygen or protolyzing solvents (90, 91), where the organoborane products did not survive. By a study of mesitylborate salts, however, the interesting rearrangement leading to 43 was observed (58). Labeling of the mesityl 

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The mechanism of these alkylations involves a tetracoordinate boron intermediate formed by addition of the enolate of the a-bromo ester to the organoborane. The migration then occurs with displacement of bromide ion. In agreement with this mechanism, retention of configuration of the migrating group is observed.23... 

As discussed in the introduction (Section 3.2.1), derivatives of the diboracyclo-propane 1C are non-classical organoboranes having 8 SE, and according to the 2n+2 SE rule may be classified as the simplest doso-carboranes of the series CH(BH) H ( = 2). Compounds IB and 1C have been computed [5] to be 17.5 and 47.6 kcal mol-1 lower in energy, respectively, than the classical diboracyclopropane 1A. They are 2e aromatics and possess planar-tetracoordinate centers in IB this unusual geometry is found at the carbon and one boron atom, in 1C at both boron atoms (Scheme 3.2-2). 

Organoboranes thus obtained usually exist as electron-deficient trigonal species with the empty p-orbital on the boron atoms and act as Lewis acids or electrophiles. Therefore, they have a strong tendency to react with bases or nucleophiles to form the corresponding tetracoordinate organoborates ( ate complexes, 6) (Eq. 2). As X"s, both hetero-atom anions and carbanions are employed. Although the... 

The review problems below present a few additional examples of organoborane reactions. You ll find many more in standard advanced organic texts (see Appendix B). The common feature of all the reactions is that a nucleophile adds to the tricoordinate boron of the organoborane to yield an anionic tetracoordinate boron center, from which the migrating alkyl group takes off. In the chapters to follow, we ll encounter a variety of other platforms, based on other relatively electropositive p-block elements, from which alkyl and aryl groups may migrate. 

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