Chiral 2,2'-bipyridine-type N-monoxides as organocatalysts in the enantioselective allylation of aldehydes with allyltrichlorosilane

[reaction: see text] The Sakurai-Hosomi-type allylation of aromatic and heteroaromatic aldehydes can be catalyzed by the new heterobidenate bipyridine monoxide PINDOX with high enantioselectivities. The sterochemical outcome is mainly controlled by the axial chirality in PINDOX, which in turn is determined by the annulated terpene units.     

Novel thiazolidine derivatives as chiral catalysts in the enantioselective addition of diethylzinc to aldehydes

New chiral thiazolidine derivatives were synthesized conveniently from natural l-cysteine and showed good enantioselectivity (up to 90% ee) in the addition of diethylzinc to aldehydes. The catalytic efficiency of the thiazolidine derivatives is influenced by the different structures of the thiazolidine rings and the bulkiness of R moiety in the ester groups.     

Ligand-accelerated enantioselective propargylation of aldehydes via allenylzinc reagents

An enantioselective propargylation of aldehydes using an allenylzinc reagent generated in situ via a zinc-iodine exchange reaction is described. The enantioselectivity is controlled by addition of a catalytic amount of readily accessible and highly tunable amino alcohol ligand L13. A wide range of aldehydes can be propargylated to afford valuable and versatile homopropargyl alcohols in good to excellent yields with high levels of enantiopurity.     

Synthesis of 4,4′-biquinazoline alcohols as chiral catalysts in enantioselective alkynylation of aldehydes with phenyl acetylene

Optically active propargylic alcohols are important chiral-building blocks in asymmetric synthesis, while the asymmetric addition of a terminal alkyne to an aldehyde is one of the most important procedures to prepare these chiral-building blocks. In this work, a family of chiral 4,4′-biquinazoline alcohols has been conveniently prepared from the easily accessible (S)-2-acetoxycarboxylic acid chlorides by reaction sequences beginning with condensation and followed by key synthetic steps including chlorination, nickel(0)-mediated homocoupling, and deprotection in addition to being examined as potential ligands in the enantioselective addition of phenylacetylene to aldehydes. These chiral ligands can be combined with Ti(OiPr)₄ and then used to catalyze the asymmetric addition of zinc acetylide, produced in situ by the reaction of phenylacetylene with diethylzinc, to aldehydes. The best enantiomeric excess obtained in this study was 75%.     

Zinc complexes of chiral phenols as catalysts for enantioselective addition of organozinc reagents to aldehydes

A class of zinc (II) chelates with chiral tertiary amino phenolic alcohols serve as effective catalysts for the enantioselective addition of diethylzinc to aromatic aldehydes with predictable absolute stereochemistry.     

Catalytic enantioselective aryl transfer to aldehydes using chiral 2,2'-bispyrrolidine-based salan ligands

Chiral C?-symmetric diamines have emerged as versatile auxiliaries or ligands in numerous asymmetric transformations. Chiral 2,2'-bispyrrolidine-based salan ligands were prepared and applied to the asymmetric aryl transfer to aldehydes with arylboronic acids as the source of transferable aryl groups. The corresponding diarylmethanols were obtained in high yields with moderate to good enantioselectivitives of up to 83% ee.     

Chiral pyrrolidine derivatives as catalysts in the enantioselective addition of diethylzinc to aldehydes

A series of pyrrolidine derivatives with β-amino alcohol moieties prepared from (S)-proline were found to catalyze the enantioselective addition of diethylzinc to aldehydes to yield optically active secondary alcohols with high enantioselectivities. A mechanism accounting for the configurational change with the bulkiness of chiral ligands is proposed.     

New class of amino-phosphinite chiral catalysts for the highly enantioselective addition of arylzinc reagents to aldehydes

A new class of amino-phosphinite chiral ligands was prepared and applied in zinc-catalyzed addition of aryl boronic acids to aldehydes; the reaction furnished the diarylmethanols in excellent yields and with a high level of enantioselectivity (up to 93% ee).     

Synthesis of new chiral aliphatic amino diselenides and their application as catalysts for the enantioselective addition of diethylzinc to aldehydes

[reaction: see text] A set of chiral aliphatic amino diselenides have been synthesized from readily available starting materials in a straightforward synthetic route via the ring-opening reaction of the parent aziridines. These ligands have been tested as catalysts for the enantioselective addition of diethylzinc to aldehydes. The influence of the alkyl group substituents on the stereoselectivity has been studied, and in the best case, an enantiomeric excess up to 99% could be obtained by using only 0.5 mol % of the chiral diselenide 3a.     

A new synthetic route to enantiomerically pure axially chiral 2,2'-bipyridine N,N'-dioxides. Highly efficient catalysts for asymmetric allylation of aldehydes with allyl(trichloro)silanes

New axially chiral 2,2'-bipyridine N,N'-dioxides 1 were obtained in an enantiomerically pure form by way of cyclic diesters 6 or 7 which were formed by the esterification of diols 2 with (R)-2,2'-bis(chlorocarbonyl)-1,1'-binaphthalene (5). Epimerization of the kinetic products at the ester formation (R(nap),S(pyr))-6 to the thermodynamically stable isomers (R(nap),R(pyr))-7 was observed in refluxing toluene or in the presence of trifluoroacetic acid. One of the N,N'-dioxides 1a which is substituted with phenyl groups at the 6 and 6' positions was found to be highly catalytically active and enantioselective for the asymmetric allylation of aldehydes with allyl(trichloro)silane giving homoallyl alcohols.     

Chiral bispyridylamide metal complexes as catalysts for the enantioselective addition of TMSCN to aldehydes

The use of (1R,2R)-N,N′-bis(2-pyridinecarboxyamido)-1,2-diphenylethane metal complexes as catalysts for the enantioselective addition of trimethylsilyl cyanide to aldehydes is described. Enantioselectivities up to 70% ee were obtained with a Ti(IV) catalyst. Complexes with Zr(IV), Sc(III), Yb(III) and Cu(II) afforded less selective catalysts. For the Zr(IV) complex, a rate and selectivity enhancement was observed when adding 0.5 equiv. of water with respect to the catalyst. Studies of the metal complexes involved in the reaction were carried out by means of ¹H NMR spectroscopy. A Zr complex was shown by X-ray crystallography to exhibit distorted octahedral coordination, with the four nitrogen atoms of the doubly deprotonated ligand essentially in one plane.     

Supported chiral Mo-based complexes as efficient catalysts for enantioselective olefin metathesis

Syntheses and catalytic activities of seven new polymer-supported chiral Mo-based complexes are disclosed. Four of the complexes are polystyrene-based, and three involve polynorbornene supports. Studies concerning the ability of the polymer-bound chiral complexes to promote an assortment of asymmetric ring-closing (ARCM) and ring-opening (AROM) metathesis reactions are detailed. In many instances, levels of reactivity and enantioselectivity are competitive with those of the analogous homogeneous catalysts. The positive effect of lower cross-linking within the polymer backbone on reaction efficiency and asymmetric induction is detailed. The optically enriched products obtained through the use of the supported complexes, after simple filtration and removal of the supported Mo catalysts, contain significantly lower levels of metal impurities as compared to products synthesized with the corresponding homogeneous catalysts.     

Aziridine sulfides and disulfides as catalysts for the enantioselective addition of diethylzinc to aldehydes

Chiral aziridine sulfides and disulfides were synthesized from readily available and inexpensive R-cysteine by a Mitsunobu reaction; their application in the addition of diethylzinc to aldehydes provides secondary alcohols with up to 99% ee and S-configuration.     

Self-supported chiral catalysts for heterogeneous enantioselective reactions

The development of heterogeneous chiral catalysts for enantioselective reactions is highly desirable in order to overcome some drawbacks of homogeneous catalysts. Different from the conventional approaches by using various types of supports or biphasic systems for the recovery and reuse of homogeneous catalysts, a conceptually new strategy for heterogenization of homogeneous chiral catalysts, that is, a "self-supporting" approach, has been developed to use homochiral metal-organic coordination polymers generated by the self-assembly of chiral multitopic ligands with metal ions, and thus obviates the use of any support. In this concept article, the success of this "self-supporting" strategy will be exemplified in heterogeneous catalysis of asymmetric carbonyl-ene, sulfoxidation, epoxidation, and asymmetric hydrogenation reactions.     

Two novel chiral rhodium complexes as catalysts for the enantioselective allylation of arylaldehydes

Two novel chiral rhodium complexes were successfully synthesized from the reaction of a new class of bidentate nitrogen ligands with RhCl₃·3H₂O in ethanol under reflux. The crystal structure of 4a was unambiguously established by X-ray analysis. Their corresponding cationic metal complexes prepared in situ from the reaction of 4a or 4b with AgBF₄ catalyze the enantioselective allylation of arylaldehydes with allylstannane in 5–50% ee.     

3-Aminoquinazolinones as chiral ligands in catalytic enantioselective diethylzinc and phenylacetylene addition to aldehydes

A series of readily known enantiomerically pure 3-aminoquinazolinones 1a–d were synthesised from easily accessible chiral pool α-hydroxy acids and α-amino acids in only four steps without any requirement of chromatography. These quinazolinones were examined as chiral ligands for catalytic enantioselective diethylzinc and phenylacetylene additions to aldehydes. For enantioselective alkylations, the effects of temperature, solvent, diethylzinc and ligand criteria were analysed, and the desired chiral alcohols were obtained in up to 86% ee. 3-Aminoquinazolinones 1a–d were also shown to be very useful ligands in enantioselective alkynylations of aldehydes. Based upon the optimised conditions, the corresponding propargylic alcohols were obtained in up to 94% ee.     

Synthesis of chiral sulfinamido-sulfonamides and their evaluation as ligands for the enantioselective ethylation of aldehydes

A family of chiral sulfinamido-sulfonamide ligands have been synthesized from sulfinimines and has been evaluated as ligands for the enantioselective addition of diethylzinc to aldehydes with Ti(OⁱPr)₄. The structure of these diamino compounds has been systematically modified to optimize the results.     

Phosphinoyl-aziridines as a new class of chiral catalysts for enantioselective Michael addition

A series of new optically pure phosphine oxides containing chiral aziridine subunit were synthesized in good yields and applied as organocatalysts in asymmetric Michael reaction of various aliphatic aldehydes with β-nitrostyrene. The corresponding organocatalysts were synthesized starting from optically pure aziridines in a few simple efficient steps. The appropriate Michael adducts were obtained in most cases in very high chemical yield (up to 97%), excellent enantioselectivity (up to 98% of ee) and diastereoselectivity (up to 95:5 of dr).     

Enantioselective additions of diphenylzinc to aldehydes using chiral pyrrolidinylmethanol derivatives as catalysts

The enantioselective addition of diphenylzinc to aldehydes using a series of chiral ligands derived from (S)-proline afforded secondary alcohols in high yields and with high enantiomeric excesses of up to 92.6%. The configuration of the secondary alcohol enantiomer obtained was found to be dependent on the catalyst used.