1,2-DIPHOSPHA-3,4-DIBORETANES AND 1,3-DIP HOSPHA-2,4,5-TRIBOROLANE - SYNTHESIS AND STRUCTURE AS WELL AS CALCULATIONS ON THE MOLECULAR-STRUCTURE ON THE EFFECT OF SUBSTITUENTS ON THE STRUCTURE OF 1,2-DIPHOSPHA-3,4-DIBORETANE

[2 + 2]-Cyclocondensation reactions led to the synthesis of the 1,2-di phospha-3,4-diboretanes [(t-BuP)(2)B-2(NMe(2))(2)], 1a, and [(t-BuP)(2 )B(NMe(2))B(NiPr2)]. Their molecular structures have been determined b y X-ray methods, and these are compared with the structure of [(t-Bu)P -BN(iPr(2))](2), 2a. Compounds 1 show a folded B2P2 four membered ring having tert.-butyl groups in anti-positions. Ab initio calculations o n 1,2-diphospha-3,4-diboretanes demonstrate that two conformers with a nti-orientation of the substituents at the phosphorus atoms can be exp ected. These differ by the relative orientation of the almost planar P (2)BR groups to the BP2 plane. The influence of substituents (H and NH 2 at the B atoms, and H and Me at the P atoms) on the ring conformatio n has been studied. Finally, the first derivative of a 1,3-diphospha-2 ,4,5-triborolane, 3a, is reported.