Spectacular new atom efficient domino cycloaddition sequences involving [4]dendralene, the simplest cross-conjugated tetraene, are reported. Up to eight stereocenters, three new rings, and six C-C bonds are generated in one synthetic operation. The site selectivity of dienophile addition to cross-conjugated trienes and tetraenes is controlled with a simple Lewis acid.

The enantiomers of the (R,R)-(+/-) diastereomer of the chelating C(2)-tetra(tertiary phosphine) (R,R)-(+/-)-1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane, (R,R)-(+/-)/(R,S)-1, spontaneously self-assemble into homochiral double-stranded disilver(I) and digold(I) helicates of the type [M(2)(tetraphos)(2)]X(2) upon reaction with appropriate silver(I) and gold(I) salts. The corresponding copper(I) complex is mononuclear. Crystal and molecular structures of Delta-(-)-[Cu{(R,R)-1}]PF(6).EtOH, Lambda-(-)-[Ag(2){(R,R)-1}(2)](PF(6))(2), and Lambda-(-)-[Au(2){(R,R)-1}(2)](PF(6))(2).CH(2)Cl(2) are reported. The structure determinations on the silver and gold complexes are the first to be performed on enantiomerically pure dimetal helicates with chiral ligands. The dinuclear silver complex crystallizes with one molecule each of the left-handed (Lambda) D(2)-double helix and C(2)-side-by-side helix conformers of the cation and associated anions in each unit cell, whereas crystals of the analogous gold complex contain only the side-by-side helix and associated anions. The absolute configuration(s) of the metal stereocenter(s) in each complex is S. Conductance measurements in acetonitrile indicated considerable rearrangement of the mononuclear copper complex into the dinuclear helicate complex, whereas the silver and gold complexes conducted as di-univalent salts under similar conditions. Energy minimization calculations of the structures of the disilver complex cation with use of the program SPARTAN 3.0 predicted the structures observed with considerable accuracy, especially the conformations of the chiral central ten-membered ring in the complex and the relationship of the helicity of this ring to the stereoselective formation of the double helix and side-by-side helix structures. The resolution of (R,R)-(+/-)-1 is the first on a tetra(tertiary phosphine). The more-soluble (R,R)-(+/-) form of the ligand was separated in high yield from the less-soluble (R,S) form by selective extraction with tetrahydrofuran, whereupon it was resolved by the method of metal complexation with the readily prepared homochiral complex (+)-di(&mgr;-chloro)bis[(R)-1-[1-(dimethylamino)ethyl]-2-phenyl-C(2),N]dipalladium(II)-1-dichloromethane, (R)-2.CH(2)Cl(2). The enantiomers of the phosphine were obtained by liberation from the diastereomeric complexes (R(C)),(R(P),R(P))- and (R(C)),(S(P),S(P))-3 (X = PF(6)) and brought to optical purity by crystallization from acetone-ethanol, giving colorless needles having mp 88 degrees C and [alpha](21)(D) +20.5 (c 1.0, CH(2)Cl(2)) (S,S enantiomer) and [alpha](21)(D) -20.5 (c 1.0, CH(2)Cl(2)) (R,R enantiomer). The crystal and molecular structures of (R(C)),(R(P),R(P))-3 (X = PF(6)) have been determined. The complete optical purity of each enantiomer of the tetra(tertiary phosphine) was confirmed in each case by the quantitative repreparation of the diastereomeric palladium complex from which it was liberated.

The phenoxonium cation of a vitamin E model compound has been crystallized using the non-nucleophilic carborane and tetrakis(pentafluorophenyl)borate counteranions. The crystal structures confirm the assignment of the unusually stable phenoxonium cation and indicate that there is a substantial shortening of the carbon-oxygen bond lengths of the para-carbon atoms in the phenolic ring and a substantial increase of the carbon-oxygen bond length at the quaternary carbon. The crystallographic data are in excellent agreement with structural predictions from molecular orbital calculations.

Reactions of the bis(bidentate) Schiff-bases N,N'-bis(6-alkyl-2-pyridylmethylene)ethane-1,2-diamine (where alkyl = H, Me, iPr) (L) with tetrakis(acetonitrile)copper(I) hexafluorophosphate and silver(I) hexafluorophosphate afforded, respectively, the double-stranded, dinuclear metal helicates [T-4-(R,R)]-(+/-)-[M2L2](PF6)2 (M = Cu, Ag). The helicates were characterized by 1H and 13C NMR spectroscopy, conductivity, microanalysis, and single-crystal X-ray structure determinations on selected compounds. Intermolecular ligand exchange and intramolecular inversion rates for the complexes were investigated by 1H NMR spectroscopy. Reversible intermolecular ligand exchange between two differently substituted helicates followed first-order kinetics. The rate constants (k) and corresponding half-lives (t(1/2)) for ligand exchange for the dicopper(I) helicates were k = (1.6-1.8) x 10(-6) s(-1) (t(1/2) = 110-120 h) in acetone-d6, k = 4.9 x 10(-6) s(-1) (t(1/2) = 40 h) in dichloromethane-d2, and k> 2 x 10(-3) s(-1) (t(1/2) < 5 min) in acetonitrile-d3. Ligand exchange for the disilver(I) helicates occurred with k > 2 x 10(-3) s(-1) (t(1/2) < 5 min). Racemization of the dicopper(I) helicate by an intramolecular mechanism was investigated by determination of the coalescence temperature for the diastereotopic isopropyl-Me groups in the appropriate complex, and DeltaG() >> 76 kJ mol(-1) was calculated for the process in acetone-d6, nitromethane-d3, and dichloromethane-d2 with DeltaG() = 75 kJ mol(-1) in acetonitrile-d3. Complete anion exchange of the hexafluorophosphate salt of a dicopper(I) helicate with the enantiomerically pure Delta-(-)-tris(catecholato)arsenate(V) ([As(cat)3]-) in the presence of Dabco gave the two diastereomers (R,R)-[Cu2L2][Delta-(-)-[As(cat)3]]2 and (S,S)-[Cu2L2][Delta-(-)-[As(cat)3]]2 in up to 54% diastereomeric excess, as determined by (1)H NMR spectroscopy. The diastereomerically and enantiomerically pure salt (R,R)-[Cu(2)L2][Delta-(-)-[As(cat)3]]2 crystallized from the solution in a typical second-order asymmetric transformation. The asymmetric transformation of the dicopper(I) helicate is the first synthesis of a diastereomerically and enantiomerically pure dicopper(I) helicate containing achiral ligands.

Dilithium phenylphosphide reacts with 1,3-dichloropropane or 1,2-dichloroethane to give 1-phenylphosphetane (1) or 1-phenylphosphirane (2), respectively, both of which can be isolated by distillation in vacuo. The phosphetane rapidly polymerizes when neat but is stable in benzene wherefrom the polymer can be selectively and quantitatively separated from 1 by the addition of trans-dichlorobis(diethyl sulfide)palladium(II). Four-membered 1 has a remarkably low-field 31P NMR chemical shift (13.9 ppm), and 2, a remarkably high-field shift (−236 ppm). The crystal and molecular structures of the potential cyclotrimerization precursor complexes fac-[Mo(CO)3(1)3] (7), fac-[Mo(CO)3(2)3] (8), and [(η5-C5H5)Fe(2)3]PF6 (9) have been determined. Both molybdenum complexes have C 3 symmetry in the solid state, and the iron complex has C 1 symmetry. An interesting feature of the three structures is that the phenyl groups of the small phosphorus heterocycles in each case are arranged in groups of three syn or anti to the auxiliary ligands.