Charles R. Ross

Enantioselective Diels-Alder reactions: novel constrained bis(oxazoline) ligand-metal triflate catalysts

Abstract Five chiral 1,4-bis(oxazoline) ligands, each bearing a bicyclic backbone, were prepared and examined in Mg(OTf)2-, Zn(OTf)2-, and Cu(OTf)2-catalyzed Diels-Alder reactions. The cycloadditions were carried out at room temperature and afforded enantiomeric excess up to 88%. Surprisingly, a non-C2-symmetric bis(oxazoline) bearing a meso backbone was among the more efficient ligands examined.

Chemical insight from crystallographic disorder: structural studies of a supramolecular β-cyclodextrin/coumarin photochemical system

From a combination of X-ray crystallography, solid-state NMR, and theoretical calculations, a model of a β-cyclodextrin (β-CD)–coumarin inclusion complex has been developed which characterizes the complex as a 2∶3 β-CD–coumarin system with the coumarin molecules located in β-CD dimer ‘reaction nano-tubes’; the model explains the reported yield for the coumarin photodimerization reaction carried out in powdered solids.

Synthesis of phenyldiazenido, phenylhydrazido and diorganohydrazido complexes of molybdenum and tungsten and their reactions with acids: A comparison with analogous unsubstituted hydrazido (NNH2) complexes. The crystal structure of [WCl2(NN(H)Ph)(PMe2Ph)3]

Abstract The phenyldiazenido complexes [MCl(NNPh)(PMe 2 Ph) 4 ] (M = Mo,W) and phenyl- and diorganohydrazido(2-) complexes [WCl 2 (NN(H)Ph)(PMe 2 Ph) 3 ], [MCl 2 (NNRR′)(PMe 2 Ph) 3 ]] (M = Mo, W ; R = R′ = Me, R = R′ = Ph ; M = W, R = Me, R′ = Ph), [MCl 2 (NNPh 2 )(dppe)(PMe 2 Ph)] (M = Mo,W; dppe = Ph 2 PCH 2 CH 2 PPh 2 ), [MCl(NNPh 2 )(dppe) 2 ]Cl (M = Mo,W), and [WCl(NN(Me)Ph)(dppe) 2 ]Cl were synthesized, characterized and their reactions with acids studied. These complexes were prepared in a series of reactions starting with WCl 6 or (MoCl 5 ) 2 involving (i) introduction of the organohydrazido(2 -) group (NNRR′) using the corresponding hydrazine (R = R′ = Me) or hydrazine hydrochloride (R = R′ = Ph; R = Ph,R′ = H or Me), and (ii) incorporation of phosphine ligands under reducing conditions. The particular product formed in the last step depended upon the reduction time. In almost all cases reaction of the complexes with acid (1 M HCl in Et 2 O or H 2 SO 4 in MeOH) resulted in the formation of some amine and a correspondingly smaller amount of ammonia. For example, the yields of aniline and ammonia when [MCl(NNPh)(PMe 2 Ph) 4 ]Cl reacted with H 2 SO 4 in MeOH were ca 0.2 mol and ca 0.1 mol per mol of W, and 0.70 mol and 0.65 mol per mol of Mo, respectively. A comparison of acid reactions of organodiazenido and organohydrazido(2- ) is made with those of unsubstituted hydrazido(2 -) complexes prepared from coordinated dinitrogen. The crystal structure of reddish-brown [WCl 2 (NN(H)Ph)(PMe 2 Ph) 3 ] revealed a distorted octahedral complex in which the hydrazido(2 -) moiety is trans to a chlorine atom and the hydrazido(2 -) phenyl group is coplanar with the plane containing W, N, N and C(Ph) atoms. The second chlorine atom, cis to the hydrazido(2 -) ligand, has a close intermolecular contact (2.62 A) with the hydrazido(2 -) hydrogen atom of a neighbouring molecule and close intramolecular contact (2.76 A) with the endo o -phenyl hydrogen atom of the hydrazido(2 -) ligand. Comparison of structural data with those for [WCl 2 (NNR 2 )(PMe 2 Ph) 3 ] (R 2 = Me 2 or Ph 2 ) indicates significant differences in W-N(1) and W-Cl( trans ) distances between the dimethyl-, and the phenyl- and diphenyl-hydrazido(2 -) complexes, respectively.

4,6-Bis(trifluoromethyl)-N,N′-di-tert-butyl-1,3-phenylenebis(aminoxyl) and its bis(hexafluoroacetylacetonato)manganese(II) complex: synthesis, X-ray crystallography, and magnetism

Abstract Synthesis, crystallography, and magnetic characterization of bis(aminoxyl) 1 and its 2:1 complex with Mn(hfac)2 (2) are described. X-ray structure of 2 (at 20 K) provides one of the rare examples of discrete molecules found for the complexes of Mn(hfac)2 and bis(aminoxyls). The differences in conformations of the bis(aminoxyl) moieties in 1 and 2 lead to ferromagnetic and antiferromagnetic exchange coupling through the 1,3-phenylene moiety, in 1 and 2, respectively, as determined by SQUID susceptometry. DFT calculations (at the crystallographic geometries) give the triplet and the singlet ground states for 1 and for bis(aminoxyl) in 2, respectively. Two analogous conformations of the bis(aminoxyl) are proposed to be responsible for the unprecedented S-shaped time dependence for the conversion from the singlet to the triplet state of 1 in 2-methyltetrahydrofuran.

Singlet-triplet bistability in a 1,3-phenylene-based bis(aminoxyl) diradical

Rapid quenching of the title bis(aminoxyl) 1 in 2-methyltetrahydrofuran from ambient temperature to cryogenic temperatures (5 K or below) produces 1 in its singlet state, which slowly converts at low temperatures, with an S-shaped time-dependence, to the triplet ground state.

Preliminary X-ray crystallographic study of methyltetrahydrofolate: corrinoid/iron sulfur protein methyltransferase from Clostridium thermoaceticum

Methyltetrahydrofolate:corrinoid/iron sulfur protein methyltransferase from Clostridium thermoaceticum has been crystallized in two polymorphic forms and characterized by X-ray diffraction measurements. Form I displayed orthorhombic symmetry with a = 63.9, b = 53.8, c = 164.0 A. Form II also displayed orthorhombic symmetry with a = 63.5, b = 87.1, c = 117.9 A. Crystals of form I diffract to approximately. 3 A resolution; those of form II diffract to approximately 2.7 A.