Templating strategies in phosphorus macrocycle chemistry

The C3 symmetric macrocycle 1,4,7-triphosphacyclononane ( 1) is a highly desirable analogue of the ubiquitous 1,4,7-triazacyclononane ligand, and is expected to form kinetically stable transition metal complexes with potential applications in areas such as biomimicry and catalysis. Traditional synthetic strategies have not successfully produced 1 as the free ligand; a novel templating strategy employing a C3 symmetric tris(silane) tripod (derived from tris(dimethylsilyl)methane or -ethane) as a molecular framework for the preparation of 1 is therefore proposed. This synthetic route exploits the phosphorus protecting group chemistry of silicon to hold three cyclizable vinylphosphine units in close proximity and promote ring closure via three P-H addition reactions across adjacent vinyl groups. Solvolytic cleavage of the P-Si bonds should afford the free macrocycle (1).; Substitution of the bromosilane groups in RC(Me2SiBr) 3 (17, R = H; 18, R = Me) with three equivalents of LiPR'2 produced the model tris(silylphosphine) tripods 13-14 (R' = Ph) and 19-20 (R' = Et). Incorporation of the bulky dimethylsilyl "elbow" groups was found to affect the conformational preferences of the tris(silane) core in solution and in the solid state. Barriers to rotation around the tripod arms (Ccore-Si) in tripods 13-14 and 17-20 (probed by molecular modelling and variable temperature NMR spectroscopy) ranged from 6-8 kcal/mol, with higher barriers observed for compounds with larger substituents on silicon (Br < PEt 2 < PPh2). Replacement of the apical proton on the tripod core by a methyl group also increased the barrier to arm rotation. The susceptibility of compounds 13-14 and 19-20 to solvolytic cleavage of the P-Si bonds by protic reagents was studied: P-Si bond cleavage proceeded cleanly, with quantitative liberation of secondary phosphine. Molybdenum carbonyl complexes of the Lewis basic tris(silylphosphine) tripods 13-14 and 19-20 were prepared by reaction with MO(CO)6 or Mo(pip)2(CO)4. Compounds 19-20 form both kappa 2- and kappa3-coordinated metal complexes, while the more sterically congested tripods 13-14 only coordinate to the metal in a kappa2-fashion.; The phosphide transfer reagents LiPHPh (34), TMEDA•Mg(PHPh) 2 (35), LiAl(PHPh)4 (36), and "TMEDA•Zn(PHPh) 2" (37) were investigated for their ability to install reactive P-H functionalities into monohalosilanes and 1,2-bis(chlorodimethylsilyl)ethane (41). Reactions with lithium phosphide 34 gave primarily disilylated products (Si2PPh), while phosphides 35-37 were more selective for formation of silylphosphines (SiPHPh) retaining a P-H bond.; Structural trends in the 1H and 31P {lcub} 1H{rcub} NMR spectroscopic data of a number of novel silylphosphines were identified, and used to analyze the products of reactions between 17 and/or 18 and the phosphide transfer reagents M(PHR) n (M = Li, TMEDA•Mg; R = Ph, Cy, vinyl; n = 1 [Li], 2 [Mg]). When R = Ph or Cy, the products from reactions with 17-18 varied with both the metal and tripod used, while reactions of 17 and 18 with lithium or magnesium vinylphosphide reagents typically produced mixtures of partially substituted tripod species, including the tris(phosphines) RC(Me2SiPH(CH=CH2))3 (83, R = H; 86, R = Me). Reductive coupling of the vinylphosphide reagent(s) may explain the appearance of large quantities of phosphorus-containing by-products, presumed to be the triphosphine (CH2=CH)P[PH(CH=CH2)] 2 (89). Tripods 83-86 were proposed as non-metal templates for the synthesis of the P3 macrocycle 1. Treatment of mixtures containing 83-86 with AIBN suggests that hydrophosphination of vinylphosphine moieties in 83-86 is successful, and that 83-86 are viable templates for the cyclization of 1,4,7-triphosphacyclononane (1).