SYNTHESES, STRUCTURES, AND PROPERTIES OF 2,5-DIHYDRO-1H-1,2,5-PHOSPHADIBOROLES

Cyclocondensation of the (Z)-1,2-bis[chloro(dialkylamino)boryl]ethenes 6a-c and 1,2-bis[chloro(diisopropylamino)boryl]benzene (8a) with the lithium phosphides Li2P-C6H5, Li2p-C6F5, Li2P-C6H2(tBu)3, [LiPH2 . dmel, and P(SiMe3)3, yields the 2,5-dihydro-1H-1,2,5-phosphadiboroles 1a-c, 2b, c, 3a-c, 4c, 5c, 9a. Compounds 4c, 5c, 9a are characterized by an X-ray structure analysis. Due to the B-N pi-bond, these derivatives are monomeric with little contribution of B-P pi-interaction. However, low-temperature H-1-NMR studies show that the P-inversion barrier is distinct lower (10-15 kcal mol 1) than for PH3 and organophosphanes (ca. 35 kcal mol-1). This may be a result of favorable B-P pi-interaction in the transition state of the inversion at phosphorus. Reaction of the B-methyl-substituted 1,2-bis(chloromethylboryl)benzene (8b) with P(SiMe3)3 gives the 1,2-dihydro-1H-1,2,5-benzophosphadiborole 9b which forms the dimer (9b)2 below 40-degrees-C by an intermolecular B-P acceptor-donor bond. The structure of (9b)2 is confirmed by an X-ray crystal structure analysis. Treatment of 1c with oxygen, sulfur, or selenium leads to the corresponding 1,2,5-chalcogenadiborole derivatives 10, 11 and 12. By this process, the phenylphosphorus group is eliminated as the pentamer (C6H5-P)5 in 25% yield. Reaction of 1c with Me2BBr gives the Lewis acid-base adduct 13 bearing an lambda-3-sigma-4-coordinated phosphorus and one four-coordinated boron atom. Pi-complexation of lc with the (CO)3Fe fragment leads to the iron complex 15 (X-ray diffraction analysis). Deprotonation of 3c with MeLi gives the P-Li-substituted phosphadiborole [3c-H]Li which reacts with Mel, Me2SiCl2, Me3SiCl to form the P-methylated and -silylated derivatives 14a, 14b, 2c. Reaction of 2b with one equivalent of MeOH at -78-degrees-C in pentane does not give the expected P-unsubstituted compound 3b. Instead, B-P ring cleavage is observed with formation of H2P-SiMe3 and (Z)-1,2-bis[(diethylamino)methoxyboryl]ethene.