Synthesis of new asymmetric substituted boron amidines – reactions with CO and transfer hydrogenations of phenylacetylene

Abstract

The syntheses of the new asymmetric substituted boron amidines [N′-(2,6-diisopropylphenyl)-N-(pentafluorophenyl)acetimidamide]bis(pentafluorophenyl)borate (2a) and [N′-(2,6-diisopropylphenyl)-N-(4-cyanophenyl)acetimidamide]bis(pentafluorophenyl)borate (2b) were achieved by reaction of one equivalent of HB(C₆F₅)₂ and the respective amidines 1a and 1b. These adducts, bearing electron withdrawing groups, showed thermally induced H₂ elimination forming the four-membered cyclic diazaborate derivatives 3a and 3b. These new species were characterized by spectroscopic methods. X-ray diffraction studies have been carried out on 2a, 2b and 3a. To prevent undesired reactions at the nitrile group, one equivalent of B(C₆F₅)₃ was added to 2b yielding the 2b–B(C₆F₅)₃ nitrile adduct 4. Compound 4 underwent thermally induced dehydrogenation to give the four-membered cyclic diazaborate derivative 5. CO was inserted into the ring systems of 2a and 2b forming the five-membered diazaborolone derivatives 6a and 6b. Phenylacetylene reacted stoichiometrically with the asymmetric substituted boron amidines 2a, 2b and 4 to give styrene by double H transfer.