Phenylrhodium species

The catalytic cycle was established by the outcome of these deuterium labeling studies. Thus, after transmetallation from boron to give the phenylrhodium species, the alkyne inserts into the phenyl-rhodium bond to form the alkenylrhodium intermediate (Scheme 4.29). It follows that rhodium moved from the vinylic position of the alkenylrhodium intermediate to the ortho position of the phenyl ring [42]. The so-formed 2-(alkenyl)phenylrhodium intermediate then undergoes hydrolysis with D2O to give the deuterio phenylation product and the active hydroxorhodium intermediate. 

Miura reported a 1,4-shift of rhodium in the reaction of norbornene with arylboronic acids (Scheme 3) where rhodium moves from an alkyl sp2-carbon to an aryl sp2-carbon, and the arylrhodium species further reacts with another molecule of norbornene.30 Polyalkylated phenyls are obtained in good yields. Cesium fluoride facilitates the transmetallation, generating the phenylrhodium complex which then coordinates to the -face of norbornene. 

Hayashi proved the validity of this catalytic cycle by the observation of aU three intermediates and their respective transformations using NMR experiments (Scheme 3.5) [16]. Transmetallation of a phenyl group from boron to rhodium takes place by addi-hon of phenylboronic acid 2 m to hydroxo-rhodium complex 16 in the presence of tri-phenylphosphine to generate the phenylrhodium complex 17. The reaction of 17 with 2-cyclohexenone la gives oxa- j-allylrhodium 18, which is converted immediately into hydroxo-rhodium complex 16 upon addition of water, liberating the phenylation product 3 am. In this NMR study, triphenylphosphine was used to stabilize the phenylrho-dium(I) complex. In the absence of triphenylphosphine, the characterization of the phenyl-rhodium species was unsuccessful. 

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