Reactions of Boron Amidinates with CO2 and CO and Other Small Molecules

Reaction of Piers' borane, HB(C6F5)2, with either tert-butyl or isopropyl carbodiimide cleanly affords the boron amidinates HC(RN)2B(C6F5)2 [R = iPr (1), tBu (2)]. These species undergo a variety of insertion reactions. For example, treatment of 1 with CO2 or excess carbodiimide gives HC(iPrN)2(CO2)B(C6F5)2 (3) or HC(iPrN)2C(iPrN)2B(C6F5)2 (4), respectively. Similarly, exposure of 1 or 2 to 1 atm CO gives HC(RN)2(CO)B(C6F5)2 [R = iPr (5), tBu (6)], while reaction of 1 with CNtBu gives HC(RN)2(CNtBu)B(C6F5)2 [R = iPr (7), tBu (8)]. Compounds 1 and 2 also react with benzaldehyde, resulting in the formation of HC(RN)2(PhHCO)B(C6F5)2 [R = iPr (9), tBu (10)]. Compound 1 also reacts with MeCN to give HC(iPrN)2(MeCN)B(C6F5)2 (11) and effects heterolytic C−H cleavage to afford HC(iPrN)(iPrNH)(PhCC)B(C6F5)2 (12). In contrast, the species PhC(iPrN)2B(C6F5)2 (14) derived from PhC(iPrN)2BCl2 (13) failed to react with any of the above substrates. These data, in addition to the isolation of HC(iPrN)2(C6F4)BF(C6F5) (15), the product of thermolysis of 1, provide further support for the notion that the transient "open-chain" form of these amidinates is present in solution.