Synthesen, Strukturen und Eigenschaften von 2,5‐Dihydro‐1H‐1,2,5‐phosphadiborolen

Abstract

Syntheses, Structures, and Properties of 2,5‐Dihydro‐1H‐1,2,5‐phosphadiborolesCyclocondensation 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 · 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 π bond, these derivatives are monomeric with little contribution of B–P π 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 π 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°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 λ3σ4‐coordinated phosphorus and one four‐coordinated boron atom. π 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 [3c‐H]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°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.