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.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

A series of half-sandwich complexes [Cp*MLX2] (L = four optically pure kappa(2)-salicyloxazolines, M = Ti, Zr; X = Cl, Me) have been prepared via salt elimination and protonolysis routes. X-ray crystallographic analyses demonstrate that the complexes contain stereogenic metal centers as a result of diastereoselective coordination of the N-O ligand. This chirality persists in solution; NMR spectroscopic investigations revealed that for most members of both the Ti and Zr series a single diastereomeric species is present at all accessible temperatures. Systems with incomplete diastereoselection are also accessible by notionally moving the C-stereogenic center on the oxazoline ring to a position where its chirality is less well expressed in the structure of the complex. In one such case (for M = Zr) a lower limit barrier to epimerization of 80 kJ mol(-1) is estimated at high temperature, while for an analogous Ti complex no exchange between epimers could be observed on the NMR chemical shift time scale. In contrast, unsubstituted cyclopentadienyl series CpTiLX2 gives diastereomeric mixtures in most cases and undergoes thermally accessible exchange between epimers.

REDUCTIONS OF CARBONYLES Hydrogenation Transfer-Hydrogenation Metal Hydrides Hydroboration Hydrosilylation Enzymatic REDUCTIONS OF ALKENES Hydrogenation Hydroboration Hydroalumination Hydrosilylation Organocatalysis REDUCTIONS OF IMINES Hydrogenation Transfer-Hydrogenation Hydroboration Hydrosilylation Organocatalysis Bronsted Lewis Base REDUCTIONS OF ALKYNES Hydrogenation Hydroboration Hydrosilylation Hydroalumination REDUCTIVE AMINATIONS Organocatalysis Boranes REDUCTIVE CROSS COUPLINGS Imines and Aldehydes REDUCTIVE HECK COUPLINGS REDUCTIVE COUPLINGS Beta-lactams NA-NAPHTHALENE KINETIC RESOLUTIONS HYDROGENOLYSIS

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.

Enantioselective epoxidations of alkenes (12 examples) were achieved using a Shi-type carbohydrate-derived hydrate and Oxone. The chiral platform provided by the catalyst tolerates a wide range of substituents providing high yields and enantioselectivities (80-95.5% ee). However, styrene derivatives were only converted with poor selectivities (11-26% ee).

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The Kharasch reaction, or atom-transfer addition of polyhalogenated alkanes to alkenes is known to be catalyzed by a number of Ru( II) complexes. The easily prepared [ RuCl2( PPh3)(3)] was used to investigate the reaction scope. A number of halogenated alkanes were added to a range of alkenes with good to excellent regioselectivities.

Treatment of chiral non-racemic salicyloxazoline proligands HL with tetrabenzylzirconium(IV) gave species [L2Zr(CH2Ph)(2)]. Reactions of KL with ZrCl4(THF)(2) gave similar chloro complexes. One example of a benzyl complex was shown to exist as the Lambda-trans,cis,cis diastereomer by X-ray crystallography. DFT calculations showed the observed isomer to be the most stable by 35 kJ mol(-1), indicating that thermodynamic diastereoselection for this species is excellent. Examination of the chiral environment about the benzyl co-ligands indicates however that the degree of expression of the chirality of the structure in what would be the site of metal-based reactions is poor in comparison to related systems. Variable temperature H-1 NMR data are consistent with this in that the low temperature spectrum exhibits a very small chemical shift difference between the chemically inequivalent benzylic CH atoms, and at higher temperatures dissociation of the portion of the ligand that contains the chiral information, i.e., the oxazoline unit, leads to apparent equivalence. (c) 2005 Elsevier B.V. All rights reserved.

Relatively air and moisture tolerant cationic iridium complexes, with chiral non-racemic N,P-ligands and weakly coordinating counter ions, are efficient catalysts in the asymmetric hydrogenation of olefins. Unfunctionalised olefins are particularly difficult substrates because, in general, a polar group adjacent to the alkene bond is required for high catalytic activity and enantioselectivity. The applicability towards a variety of substrates and choice of ligands reported thus far in the literature, as well as the mechanism, selectivity issues and the importance of the anion are also discussed.

The first use of vinylsilanes as substrates in the asymmetric iridium-catalysed hydrogenation is reported, providing products with enantioselectivities of up to 98%.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

A new class of zirconium and hafnium half-sandwich complexes bearing Cp* and salicyloxazolinato ( L) ligands has been prepared by salt elimination and protonolysis routes. The analogous Cp and indenyl compounds are generally inaccessible, as are the titanium compounds. The molecular structures of four examples [Cp*MLX2] (variously M = Zr, Hf; X = Cl, Me) reveal chiral-at-metal structures which persist in solution, according to variable-temperature NMR studies; Delta G(298)(double dagger) for the racemization process was found to be ca. 75 kJ mol(-1). Treatment of these compounds with MAO, [Ph3C][B(C6F5)(4)], or [PhNMe2H][B(C6F5)(4)] leads to catalysts for alkene polymerization, the nature of which depends on the cocatalyst chosen. The anilinium salt smoothly produces a single chiral species, [Cp*ZrLMe][B(C6F5)(4)], detected also by H-1 NMR spectroscopy, which is a highly active single-site catalyst for polymerization of ethene ( and less active for copolymerization of ethene/hexene). The trityl activator produces the same catalyst and at least one other catalytically competent species, as evidenced by NMR spectroscopy and polymer modality. The use of MAO leads to a less well-defined catalyst system. The steric demand of the salicyloxazoline ligand affects the catalyst performance significantly, and computational studies show that access of ethene to either of two inequivalent coordination sites is restricted. This stability of the species [Cp*ZrLMe](+) with respect to addition appears to be the limiting factor for catalytic activity. Catalyst stability is addressed, and the steric and electronic factors affecting this are consistent with a mechanism of catalyst death by salicyloxazoline ligand loss.

This thesis contains seven papers dealing with iridium and ruthenium based catalytic asymmetric reductions, either of ketones into chiral alcohols, or olefins into chiral alkanes. The first part of the thesis describes how we have designed and evaluated new bicyclic ligands containing either N,S or N,N chelating atoms. The ligands have been evaluated in the asymmetric Ir-catalyzed transfer hydrogenation of acetophenone. The complexes evaluated induced good enentioselectivity of the product. Moreover we have also utilized a commercially available chiral diamine (QCD-amine) as a ligand in the Ru-catalyzed hydrogenation of prochiral ketones, with excellent enantioselectivity for some of the substrates used. As part of this work we investigated, both theoretically and experimentally, the mechanism of this hydrogenation. Based on these results we have proposed a new reaction mechanism for this type of hydrogenations which involves active participation of the solvent in the catalytic cycle. The last part of the thesis describes the design, synthesis and evaluation of N,P and N2C-carbene,N ligands for the Ir-catalyzed hydrogenation of carbon-carbon double bonds. The selectivities obtained in these investigations are among the best reported so far for a broad variation of substrates. A selectivity model for this hydrogenation has been derived and used in the rationalization of the results. As a part of this work we have synthesized and evaluated a new class of substrates, vinyl silanes, and showed that the scope of the hydrogenation reaction can be expanded to this new substrate class.

A chiral zirconium alkyl cation catalyses the cyclisation of certain aminoalkenes with enantioselectivity up to 82%, the highest thus far observed for such a process.

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.

A chiral zirconium alkyl cation catalyses the cyclisation of certain aminoalkenes with enantioselectivity up to 82%, the highest thus far observed for such a process.