Non-metal-templated approaches to bis(borane) derivatives of macrocyclic dibridgehead diphosphines via alkene metathesis.

Two routes to the title compounds are evaluated. First, a ca. 0.01 M CHCl solution of HB·P((CH)CH=CH) (·BH) is treated with 5 mol % of Grubbs' first generation catalyst (0 °C to reflux), followed by H (5 bar) and Wilkinson's catalyst (55 °C). Column chromatography affords HB·P(CH) (1%), HB·((CH)H)(-CH) (8%; see text for tie bars that indicate additional phosphorus-carbon linkages, which are coded in the abstract with italics), HB·((CH)H)((CH))((CH)H)·BH (·2BH, 10%), -HB·P((CH))P·BH (-·2BH, 4%) and the stereoisomer (/)-·2BH (2%). Four of these structures are verified by independent syntheses. Second, 1,14-tetradecanedioic acid is converted (reduction, bromination, Arbuzov reaction, LiAlH) to HP((CH))PH (; 76% overall yield). The reaction with HB·SMe gives ·2BH, which is treated with -BuLi (4.4 equiv) and Br(CH)CH=CH (4.0 equiv) to afford the tetraalkenyl precursor (HC=CH(CH))(HB)P((CH))P(BH)((CH)CH=CH) (·2BH; 18%). Alternative approaches to ·2BH (e.g., via ) were unsuccessful. An analogous metathesis/hydrogenation/chromatography sequence with ·2BH (0.0010 M in CHCl) gives ·2BH (5%), -·2BH (6%), and (/)-·2BH (7%). Despite the doubled yield of ·2BH, the longer synthesis of ·2BH vs ·BH renders the two routes a toss-up; neither compares favorably with precious metal templated syntheses.