Concurrent Cationic Vinyl-Addition and Coordination Ring-Opening Copolymerization via Orthogonal Propagation and Transient Merging at the Propagating Chain End

Controlled cationic vinyl-addition polymerization of an alkyl vinyl ether (VE) and ring-opening polymerization of ε-caprolactone (CL) simultaneously proceeded using HfCl₄/Hf(OBu)₄ as a dual-role catalyst for both mechanisms, yielding a graft copolymer consisting of a poly(VE) main chain and several poly(CL) side chains. The copolymer of conventionally incompatible monomers was generated via the unprecedented mechanisms consisting of orthogonal propagating reactions and transient merging. Specifically, the poly(CL) chains were incorporated into a poly(VE) chain through an exchange reaction between the VE-derived alkoxy group and the propagating poly(CL) chain at the acetal moiety of the propagating end of the poly(VE) chain. An appropriate ratio of HfCl₄ and Hf(OBu)₄ was indispensable for both the simultaneous vinyl-addition and ring-opening polymerizations and the alkoxy group exchange reaction.

Enantioselective Lewis Acid-Catalyzed Mukaiyama−Michael Reactions of Acyclic Enones. Catalysis by allo-Threonine-Derived Oxazaborolidinones

allo-Threonine-derived O-aroyl-B-phenyl-N-tosyl-1,3,2-oxazaborolidin-5-ones 1g,n catalyze the asymmetric Mukaiyama-Michael reaction of acyclic enones with a trimethylsilyl ketene S,O-acetal in high enantioselectivity. A range of alkenyl methyl ketones is successfully employed as Michael acceptors affording ee values of 85-90% by using 10 mol % of the catalyst. The use of 2,6-diisopropylphenol and tert-butyl methyl ether as additives is found to be essential to achieve high enantioselectivity in these reactions. The effects of the additives are discussed in terms of the retardation of an Si(+)-catalyzed racemic pathway, which seriously deteriorates the enantioselectivity of asymmetric Mukaiyama-Michael reactions. A working model for asymmetric induction is proposed based on correlation between catalyst structures and enantioselectivities.

Asymmetric catalysis of Nozaki-Hiyama allylation and methallylation with a new tridentate bis(oxazolinyl)carbazole ligand

This work describes the development of a new tridentate ligand effective for the asymmetric catalysis of Nozaki-Hiyama allylation and methallylation. Various aldehydes were allylated or methallylated with good enantioselectivity (86-96%), and a key intermediate of calcitriol lactone synthesis was also obtained with excellent diastereoselectivity (97% de, 91%). The enantioselective reaction catalyzed by this Cr-ligand complex is applicable to a broad range of aldehydes and has great potential for natural product synthesis. Another remarkable feature of this ligand is the stability of the Cr-ligand complex which was recovered after the enantioselective reaction and recycled twice without diminishing the enantioselectivity and yield.

Inter- and intramolecular differentiation of enantiotopic dioxane acetals through oxazaborolidinone-mediated enantioselective ring-cleavage reaction: kinetic resolution of racemic 1,3-alkanediols and asymmetric desymmetrization of meso-1,3-polyols

Acetals derived from racemic 1,3-alkanediols undergo kinetic resolution in chiral oxazaborolidinone-mediated ring-cleavage reaction with nucleophiles such as enol silanes and allylic silanes. Enantioselectivity of the reaction is affected by nucleophiles, the N-sulfonyl groups of oxazaborolidinones, and the substituents attached to the acetal carbon. For disubstituted acetals rac-1 and for trisubstituted acetal rac-2, derived from syn-2,4-dimethyl-1,3-pentanediol, satisfactory enantioselectivity is obtained by using methallylsilane 7b,c as a nucleophile in combination with N-mesyloxazaborolidinone 4a. On the other hand, enantioselective reaction of trisubstituted acetal rac-3b, derived from anti-2,4-dimethyl-1,3-pentanediol, is realized by using silyl ketene acetal 5e in combination with N-tosyloxazaborolidinone 4b. The reaction conditions optimized for the kinetic resolution, or enantiomer differentiating reaction, of the racemic acetals are successfully applied to asymmetric desymmetrization of meso-1,3-polyols through intramolecular differentiation of the enantiotopic acetal moieties of the bis-acetal derivatives. The utility of the ring-cleavage reaction as a method for enantioselective terminal differentiation of prochiral polyols is demonstrated in convergent asymmetric synthesis of pentol derivative 35 corresponding to the C(19)[bond]C(27) ansa-chain of rifamycin S.

Enantioselective ring cleavage of dioxane acetals mediated by a chiral Lewis acid: application to asymmetric desymmetrization of meso-1,3-diols

[reaction: see text]. Phenylalanine-derived B-aryl-N-tosyloxazaborolidinones selectively activate one of two enantiotopic oxygen atoms in prochiral anti dioxane acetals derived from meso-1,3-diols, leading to enantioselective formation of ring-cleavage products. The reaction is utilized as a key step in asymmetric desymmetrization of meso-1,3-diols.

Scope and limitations of chiral B-

A new chiral oxazaborolidine catalyst was prepared in situ from 3, 5-bis(trifluoromethyl)phenylboron dichloride and N-(p-toluenesulfonyl)-(S)-tryptophan. This catalyst is much more active than Corey's original catalyst for the Mukaiyama aldol reaction of aldehydes with silyl enol ethers. The observed syn selectivities and re-face attack of silyl enol ethers on carbonyl carbon of aldehydes imply that the extended-transition state model is applicable.