Synthesis of Phosphine-Boranes and Their Metalation

Abstract

The synthesis and characterization of a range of novel phosphine-borane precursors is described via several methods: i) the reaction of PCl3, RPCl2, or R2PCl with Grignard reagents followed with BH3SMe2 to give R2P(BH3)CH2CH3 (R = iPr (110), Ph (111), Cy (112)), (PhCH2)2P(BH3)R (R = Cy (113), tBu (114)), ii) the reaction of R2PCl with Ph2P(BH3)CH2Li to give the mixed phosphine/phosphine-borane adducts Ph2P(BH3)CH2PR2 (R = Cy (115), Mes (116), iPr (117) tBu (118)). Thermolysis of either Ph2P(BH3)CH2PCy2 (115) or Ph2P(BH3)CH2PtBu2 (118) leads to migration of the borane group to the more electron rich phosphorus center and formation of Ph2PCH2P(BH3)(Cy2) (119) and Ph2PCH2P(BH3)tBu2 (120), respectively. Kinetic studies on the migration of the borane group using 31P{1 H} and 11B{1 H} NMR spectroscopy show that the migration of the borane group from the PPh2 to the Cy2 center does not follow simple kinetics, but that the migration leading to conversion of (118) to (120) proceeds via a dissociative process. The addition of one more equivalent of BH3.SMe2 to (115), (118), or (116) leads to the formation of the bis(phosphine-borane)s Ph2P(BH3)CH2P(BH3)Cy2 (121), Ph2 P(BH3)CH2P(BH3)tBu2 (122) and Ph2P(BH3)CH2P(BH3)Mes2 (123), respectively. The corresponding alkali metal complexes are formed when (110), (111), (113), (114), (115), (116), (117) or (118) are treated with nBuLi, PhCH2Na or PhCH2K in THF to give [{R2P(BH3)CHCH3}Li(THF) [M = Li, R = iPr (124), Ph (128); M = Na, R = Ph (129); M = K, R = iPr (126), Ph (130)]; [{(PhCH2)(PhCH)P(BH3)R}Li(THF)] [R = tBu, M = Li (131), Na (133), K (134); R = Cy, M = Li (134); [{Ph2P(BH3)CHPCy2}M(THF)] [M = Li (135), K (142)]; [{Ph2PCHP(BH3)Cy2}M(THF)] [M = Li (154), Na (156), K (157)]; [{Ph2P(BH3)CHPMes2}M](THF) [M = K (144), Li (137)]; [{Ph2P(BH3)CHPiPr2}M](THF) [M = Na (140), K (145)]; [{Ph2P(BH3)CHPtBu2}M](THF) [M = Li (138), Na (141)]. In the presence of co-ligands the adducts [{iPr2P(BH3)CHCH3}Na(PMDETA) (125), [{iPr2P(BH3)CHCH3}K(PMDETA) (127), [{Ph2P(BH3)CHPCy2}Li(TMEDA)] (136), [{Ph2P(BH3)CHPCy2}K(12-Crown-4)] (143), [{Ph2P(BH3)CHPCy2}Na(TMEDA)] (139) and [{(Ph2)PCHP(BH3)(Cy2)}Li(TMEDA)] (155) were isolated. An array of lithium, sodium and potassium complexes of bis(phosphine-borane)-stabilized carbanions were synthesized by treatment of (121) , (122) or (123) with one equivalent of nBuLi in THF to give [{Ph2P(BH3)CHP(BH3)R2}Li(THF) [R = Cy (146), Mes (147), tBu (148)]. The heavier alkali metal derivatives [{Ph2P(BH3)CHP(BH3)Cy2}Na(THF)] (151), [{Ph2P(BH3) CHP(BH3)tBu2}Na(THF)] (152), [{Ph2P(BH3)CHP(BH3)Cy2}K(THF) (150), [{Ph2P(BH3)CHP(BH3)Mes2}K(THF)] (151) and [{Ph2P(BH3)CHP(BH3)tBu2}K(THF)] (153) were also isolated. The reactivity of phosphine-borane-substituted organolithium compounds towards oxygen, sulfur, and nitrous oxide was investigated. This revealed novel P-C cleavage and insertion reactions. As a result, several novel compounds were identified by NMR spectroscopy and X ray crystallography, including [{Ph2P(BH3)O}Li(THF)2]2 (158), [{Ph2P(BH3)CH(Ph)S]Li(PMDETA) (159), and [{Ph2P(BH3)CH(tBu)NNO}Li(THF)]2 (160). Moreover, a novel dialkylstannylene and a series of trialkylstannate compounds, M[R3Sn], were synthesized and characterized using the ligands synthesized as part of this project. The compounds were obtained by reacting either two or three equivalents of alkyllithium complex with one equivalent of SnCl2 or SnCp2, resulting in the formation of {Ph2P(BH3)CHCH3}2Sn (161), {Ph2P(BH3)CHPh}3SnLi (162), {Ph2P(BH3)CHPiPr2}2Sn (163), {Ph2P(BH3)CHPMes2}2Sn (164) and {Ph2PCHP(BH3)Cy2}2Sn (165).