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Syntheses, Structures, and Properties of 2,5-Dihydro-1H-1,2,5-phosphadiboroles
Frankhauser, Pascal,Driess, Matthias,Pritzkow, Hans,Siebert, Walter
, p. 1341 - 1350 (2007/10/02)
Cyclocondensation of the (Z)-1,2-bisethenes 6a-c and 1,2-bisbenzene (8a) with the lithium phosphides Li2P-C6H5, Li2P-C6F5, Li2P-C6H2(tBu)3, (LiPH2*dme>, 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 ? bond, these derivatives are monomeric with little contribution of B-P ? interaction.However, low-temperature 1H-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 ? interaction in the transition state of the inversion at phosphorus.Reaction in 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 deg 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 25percent yield.Reaction of 1c with Me2BBr gives the Lewis acid-base adduct 13 bearing an λ3?4-coordinated phosphorus and one four-coordinated boron atom. ? complexation of 1c 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 Li which reacts with MeI, Me2SiCl2, Me3SiCl to form the P-methylated and -silylated derivatives 14a, 14b, 2c.Reaction of 2b with one equivalent of MeOH at -78 deg 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-bisethene.Key Words: 1H-1,2,5-Phosphadiboroles, dihydro- / Metal complexes
