Highly modular P-OP ligands for asymmetric hydrogenation: synthesis, catalytic activity, and mechanism.

A library of enantiomerically pure P-OP ligands (phosphine-phosphite), straightforwardly available in two synthetic steps from enantiopure Sharpless epoxy ethers is reported. Both the alkyloxy and phosphite groups can be optimized for maximum enantioselectivity and catalytic activity. Their excellent performance in the Rh-catalyzed asymmetric hydrogenation of a wide variety of functionalized alkenes (26 examples) and modular design makes them attractive for future applications. The lead catalyst incorporates an (S)-BINOL-derived (BINOL = 1,1'-bi-2-naphthol) phosphite group with computational studies revealing that this moiety has a dual effect on the behavior of our P-OP ligands. On one hand, the electronic properties of phosphite hinder the binding and reaction of the substrate in two out of the four possible manifolds. On the other hand, the steric effects of the BINOL allow for discrimination between the two remaining manifolds, thereby elucidating the high efficiency of these catalysts.

Dioxirane mediated asymmetric epoxidations: stereochemical studies via isotopic labeling.

A study, via isotopic labeling, of the stereoselective processes in a Shi-type epoxidation, has revealed that the chiral platform provided by the catalyst mediates the transfer of the pro-S "O" of the related dioxirane species to the alkene in a doubly stereoselective manner.

Aminooxazolinate; a chiral amidinate analogue.

High levels of diastereoselection with respect to chirality-at-metal are achieved at equilibrium for complexes containing a new and available range of diazaallyl ligands.

Chiral biarylamido/anisole complexes of yttrium in enantioselective aminoalkene hydaroamination/cyclisation.

A group of chiral, dibasic, biaryl-bridged amido proligands containing peripheral methoxyphenyl (anisole) ligation are developed for the synthesis of new amide complexes of yttrium and lanthanum. A potentially tetradentate bis(amidoanisole) system gives, on reaction with [Y[N(SiMe(2)H)(2)](3)(THF)] a crystallographically-characterised bis complex [Y(H)] presumably as a result of low steric demand, since a more bulky version gives the target [Y[N(SiMe(2)H)(2)](THF)]. The molecular structure of the latter reveals a similar cis-alpha structure to our recently reported Schiff-base analogue. Variable-temperature NMR studies are consistent with low rigidity in the molecular structure. A potentially tridentate, amidoanisolyl/amido proligand gives complexes [M[N(SiMe(2)H)(2)](THF)(n)](M = Y, n= 1; M = La, n= 2). Chiral non-racemic versions of the above complexes were tested in the hydroamination/cyclisation of 2,2'-dimethylaminopentane to the corresponding pyrrolidine. Activities were relatively low compared to recently reported examples, and ee values were in the range 20-40% despite the well-expressed chirality of the catalysts.

Arylaminopyridinato complexes of zirconium.

A range of 2-arylaminopyridines (HL) are synthesised readily from bromopyridines and amines using palladium-catalysed amination. Protonolysis reactions of these proligands with ZrX(4)(X = NMe(2), CH(2)Ph, CH(2)Bu(t)) yield zirconium complexes of the type [ML(n)X(4-n)], several of which have been characterised by X-ray crystallography. Control of metal/ligand stoichiometry and structure is pursued by investigation of the effects on substitution patterns of the pyridine and aryl rings. Some distinct patterns emerged; (i) the 6-methyl position on the pyridine appears to be particularly important with regards to control of stoichiometry, although there are co-ligand effects; (ii) structures of the metal alkyl derivatives [Zr(n)(CH(2)R)(4-n)] are dominated by aromatic pi-pi stacking, even when bulky arene substituents are employed at. This leads to the complexes adopting a C(2v)-symmetric core; (iii) the amides [Zr(2)(NMe(2))(2)] have structures for which aromatic pi-pi stacking is unfeasible, and correspondingly C(2)-symmetric or similar structures are adopted. All the structural data presented is consistent with a trans influence order at zirconium Me(2)N > RCH(2) > py.

Efficient predetermination of chirality-at-zirconium.

Only one of eight possible diastereomers of the organometallic chiral-at-metal complex [ZrL2(CH2Ph)2] (L = a bidentate, chiral non-racemic pyrdine alcoholate) is observed by NMR spectroscopy in the slow exchange regime.

Complexes of Triamidoamines with the Early Actinides. Synthetic Routes to Monomeric Compounds of Tetravalent Uranium and Thorium Containing Halide and Amide Ligands.

The reaction of the lithiated triamidoamine [Li(3)(NN'(3))(THF)(3)] [NN'(3) = N(CH(2)CH(2)NSiMe(2)Bu(t))(3)] with AnCl(4) (An = U, Th) followed by sublimation gives monomeric [An(NN'(3))Cl]. Reaction of these complexes with SiMe(3)X (X = Br, I) gives [An(NN'(3))X]. The amido derivatives [An(NN'(3))(NEt(2))] are prepared from H(3)(NN'(3)) and [U(NEt(2))(4)] and from [Th(NN'(3))Cl] and [Li(NEt(2))]. In each case, the complexes [U(NN'(3))X] (X = Cl, Br, I, NEt(2)) are shown by X-ray crystallography to contain a triamidoamine ligand disposed with 3-fold symmetry about the metal center. The structures are distorted from trigonal bipyramidal by displacement of the uranium atoms out of the equatorial plane of the three amido nitrogen atoms by ca. 0.8 Å. The ligand backbone is distorted in such a manner as to cause the tert-butyldimethylsilyl groups to encircle the equatorial plane of the metal atom rather than surround the apical coordination site as is observed in the transition metal complexes of this type. Variation of the auxiliary ligand has little effect on the orientation, bond lengths, and angles within the (triamidoamine)uranium fragment. The tert-butydimethysilyl-substituted triamidoamine ligand is thus ideally suited for coordination to large metals since it stabilizes the formation of 3-fold symmetric structures while also allowing reactivity at the fifth coordination site.