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 CH(2)Cl(2) solution of H(3)B·P((CH(2))(6)CH=CH(2))(3) (1·BH(3)) is treated with 5 mol % of Grubbs' first generation catalyst (0 °C to reflux), followed by H(2) (5 bar) and Wilkinson's catalyst (55 °C). Column chromatography affords H(3)B·P(n-C(8)H(17))(3) (1%), H(3)B·P((CH(2))(13)CH(2))(n-C(8)H(17)) (8%; see text for tie bars that indicate additional phosphorus–carbon linkages, which are coded in the abstract with italics), H(3)B·P((CH(2))(13)CH(2))((CH(2))(14))P((CH(2))(13)CH(2))·BH(3) (6·2BH(3), 10%), in,out-H(3)B·P((CH(2))(14))(3)P·BH(3) (in,out-2·2BH(3), 4%) and the stereoisomer (in,in/out,out)-2·2BH(3) (2%). Four of these structures are verified by independent syntheses. Second, 1,14-tetradecanedioic acid is converted (reduction, bromination, Arbuzov reaction, LiAlH(4)) to H(2)P((CH(2))(14))PH(2) (10; 76% overall yield). The reaction with H(3)B·SMe(2) gives 10·2BH(3), which is treated with n-BuLi (4.4 equiv) and Br(CH(2))(6)CH=CH(2) (4.0 equiv) to afford the tetraalkenyl precursor (H(2)C=CH(CH(2))(6))(2)(H(3)B)P((CH(2))(14))P(BH(3))((CH(2))(6)CH=CH(2))(2) (11·2BH(3); 18%). Alternative approaches to 11·2BH(3) (e.g., via 11) were unsuccessful. An analogous metathesis/hydrogenation/chromatography sequence with 11·2BH(3) (0.0010 M in CH(2)Cl(2)) gives 6·2BH(3) (5%), in,out-2·2BH(3) (6%), and (in,in/out,out)-2·2BH(3) (7%). Despite the doubled yield of 2·2BH(3), the longer synthesis of 11·2BH(3) vs 1·BH(3) renders the two routes a toss-up; neither compares favorably with precious metal templated syntheses.