Ionic-type reactivity of 1,3-dibora-2,4-diphosphoniocyclobutane-1,3-diyls: regio- and stereoselective addition of hydracids.
Hydrogen chloride and trifluoromethane sulfonic acid readily add to the symmetrically substituted 1,3-dibora-2,4-diphosphoniocyclobutane-1,3-diyl (1) and unsymmetrically substituted 1,3-dibora-2,4-diphosphoniobicyclo[1.1.0]butane (3) under mild conditions, substantiating some formal analogies between the PBPB diradicaloids D and alkenes. X-ray diffraction analyses carried out on all the resulting 1,3-diborata-2,4-diphosphoniocyclobutanes (2, 4-6) revealed that the reactions proceed with complete stereo- and regio-selectivity, and that an unusual t-Bu --> i-Bu isomerization can occur at boron. DFT calculations shed more light on the factors controlling the regioselectivity of the additions, its concerted versus stepwise character, and support an original two-step mechanism for the t-Bu --> i-Bu isomerization, involving a carbocationic sigma-borane adduct as a key intermediate.
Fuks
Gad
G
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957), Department of Chemistry, University of California Riverside, Riverside, California 92521-0403, USA.
Saffon
Nathalie
N
Maron
Laurent
L
Bertrand
Guy
G
Bourissou
Didier
D