Configurationally homogeneous diastereomers of a linear hexa(tertiary phosphine): Enantioselective self-assembly of a double-stranded parallel helicate of the type (P)-[Cu₃(hexaphos)₂](PF₆)₃

Abstract

Three configurationally homogeneous diastereomers of the linear hexa(tertiary phosphine) Ph 2PCH2CH2P(Ph)CH2CH2P(Ph)CH2CH2P-(Ph)CH 2CH2P(Ph)CH2CH2PPh2 (hexaphos) have been isolated in enantiomerically pure form, namely (R,S,S,R)-, (R,S,S,S)-, and (S,S,S,S)-hexaphos. The strongly helicating (R,S,S,R)-(؊) form of the ligand combines with copper(I) ions to generate by stereoselective selfassembly the P enantiomer of a parallel helicate of the type [Cu 3(hexaphos)2](PF6)3, which has been characterized by x-ray crystallography. Theoretical modeling of the cation indicates that it is the relationship between the helicities of the two 10-membered rings containing the three copper ions, each of which has the twist-boat-chair-boat conformation, and the configurations of the three chiral, tetrahedral copper stereocenters of P configuration that determines the stereochemistry of the parallel and double ␣-helix conformers of the double-stranded trinuclear metal helicate. T he synthesis of complex, hierarchical molecular architectures containing two or more metal ions by self-assembly under equilibrium conditions is an important technique in coordination chemistry. For example, oligo-2,2Ј-bipyridines and related ligands combine with kinetically labile copper(I) or silver(I) ions to give solutions of stable, di-and oligo-nuclear metal helicates, grids, metallocatenanes, rotaxanes, and knots, depending on the rigidity of the spacer groups between the chelating entities (1-3). The simplest approach to the self-assembly of a particular structural motif is by the astute and selective use of rigidity in the design of the ligand (3). This approach applies also to the construction of molecular polygons and polyhedra from rigid, angular metal chelate components, such as [Pt(PEt 3 ) 2 ] 2ϩ , which can form the corners and apices of large two-dimensional and three-dimensional structures (4).More subtle considerations apply to the thermodynamic selfassembly of polynuclear metal chelates derived from fully flexible ligands. Techniques available include mismatching the binding preferences of the ligand and metal to give a particular assembly (2), and the selective formation of stable chelate rings. Chiral elements within a ligand can also dramatically affect the stability of a particular diastereomer of a supramolecular chelate assembly. For example, freely f lexible ligands of the type LOLЈOLЈOL can generate chiral and achiral diastereomers of dinuclear metal complexes of the type M 2 (LOLЈOLЈOL) 2 , chiral at the tetrahedral metal stereocenters M(LOLЈ) 2 . The relationship between the configurational elements at donor and metal and the helicities of the metal chelate ring conformers will determine the stability of a particular stereoisomer of the complex.In previous work, we have used these techniques for the stereoselective synthesis of dinuclear metal helicates from a flexible linear tetra(tertiary phosphine) (5) and an analogous hybrid phosphine-arsine (6), with univalent Group 11 ions. These ions form stable, but labile, tetrahedral...

Anionic ring‐opening polymerization of small phosphorus heterocycles and their borane adducts: An ab initio investigation

“…The configuration at phosphorus should be preserved during the propagation step, consistent with established knowledge on the stereoselectivity of alkylation of chiral phosphido-borane groups [14][15][16]. Deboranation of the stereoregular phosphine-borane polymer can then be effected, following an established procedure [4]. One cycle of the SCHEME 1 SCHEME 2 polymerization of 7 is shown in Scheme 2.…”

“…Stereoregular polyphosphines have many potential applications, one of which is the self-assembly of supramolecular polynuclear metal complexes in which linear arrays of metal ions are surrounded by two polyphosphine strands in helical arrangements. Univalent copper, silver, and gold complexes having these structures have been isolated with use of configurationally pure tetraphosphines and hexaphosphines as the helicating agents [2][3][4]. Longer polyphosphines would facilitate the self-assembly of complexes that could be considered as "metal-ion wires" in which the polyphosphines are the insulating material.…”

“…To achieve this goal, however, a stereocontrolled synthesis of long-chain polyphosphines is needed. The current approach [2][3][4], in which stereoregular polyphosphines are isolated by the fractional crystallization or chromatographic separation of statistical mixtures of diastereomers produced in nonstereoselective syntheses, is a laborious and expensive procedure that becomes increasingly less feasible as the polyphosphine becomes longer (each added chiral phosphorus center doubles the number of possible stereoisomers of a constitutionally unsymmetrical polyphosphine). In the present work, we use ab initio calculations to examine the feasibility of a possible synthetic route to stereoregular polyphosphines, with a view to directing future experimental studies.…”

Synthesis of unsymmetrical P-stereogenic oligophosphines and chemoselective cleavage of phosphine-borane coordinate bonds

“…We attempted to remove the boranes coordinated with the phosphorus atoms in tetraphosphine (S,R,R,S)-8 ÀBH 3 by using organic bases such as morpholine 23 [11][12][13] ). When (S,R,R,S)-8 ÀBH 3 was stirred in morpholine for 48 h, all the boranes were readily removed.…”

“…Wild and co-workers reported the syntheses and coordination behaviors of optically active P-stereogenic tetraphosphines and hexaphosphines, which were obtained by separating a mixture of stereoisomers by column chromatography and successive complexation with chiral palladium complexes. [17][18][19][20][21][22][23] Our group [14][15][16] and Imamoto's group [24][25][26][27][28][29] independently succeeded in synthesizing optically active P-stereogenic tetraphosphines ( Figure 1) through different synthetic routes. Imamoto et al 28 prepared their transition metal complexes, and the obtained complexes were used as chiral catalysts for transition metal-catalyzed asymmetric hydrogenations.…”