[2 + 2] CYCLOADDITION REACTIONS OF DIALKYLAMINOBIS(TRIFLUOROMETHYL)BORANES WITH ISOCYANATES AND ISOTHIOCYANATES - CRYSTAL-STRUCTURES OF (CF3)2BNME2CON-TERT-BU, (CF3)2BNME2CSN-TERT-BU, (CF3)2BNPHC(NME2)S AND (CF3)2BNMEC(NET2)OCONME

The reactions of (CF3)2BNMe2 (D1) and (CF3)2BNEt2 (D2) with isocyanates and isothiocyanates have been studied. At or near room temperature the C = N bonds of RN = C = O and RN = C = S enter into [2 + 2] cycloadditions with the B-N bond of D1 to yield the four-membered rings (CF3)2BNMe2CONR; R = Me (I), (t)Bu (II), Ph (III), CF3 (IV), and (CF3)2BNMe2CSNR; R = Me (V), Et (VI), (t)Bu (VII), Ph (VIII) and p-FC6H4 (IX) respectively. I, V, VI, VIII and IX rearrange at approximately 60-degrees-C to form the isomers (CF3)2BNMeC(NMe2)O (XI) and (CF3)2BNRC(NMe2)S; R = Me (XII), Et (XIII), Ph (XIV) and p-FC6H4 (XV), respectively. At room temperature D2 reacts with MeN = C = O to yield (CF3)2BNMeC(NEt2)O (X) directly. X and XI reversibly incorporate MeN = C = O to form the six-membered heterocycles (CF3)2BNMeC(NR2)OCONMe; R = Me (XVI) and Et (XVII). The structures of II, VII, XIV and XVII have been established by single-crystal X-ray diffraction studies. The bond distances indicate that the pi-bonding is delocalized over OCN (sp2), SCN (sp2) and SCN2 (sp2) fragments in II, VII and XIV, respectively. Furthermore, the pi-character of the exocyclic C-N bonds in XIV and XVII, while considerable, is lower than that of the exocyclic C-O(S) bonds in II, VII and XVII. The other structural assignments were based on mass spectrometry and vibrational and multinuclear NMR spectra.