Titanium Lewis Acids Research Articles

Polar Thermoplastics with Tunable Physical Properties Enabled by the Stereoselective Copolymerization of Vinyl Ethers

A series of isotactic, semicrystalline vinyl ether copolymers (up to 94% meso diads) were synthesized using a chiral BINOL-based phosphoric acid in combination with a titanium Lewis acid. This stereoselective cationic polymerization enabled the systematic tuning of both glass transition (Tg) and melting temperature (Tm) in copolymers derived from alkyl vinyl ethers (i.e., ethyl, butyl, isobutyl). Additionally, a vinyl ether comonomer bearing an acyl-protected alcohol was utilized as a platform for postfunctionalization. Copolymers containing the masked alcohols were shown to undergo deprotection and subsequent coupling with a desired acid chloride. Collectively, these results highlight the diverse material properties and expanded chemical space accessible through stereoselective cationic polymerization mediated by a chiral anion.

Catalyst-controlled stereoselective cationic polymerization of vinyl ethers.

The tacticity of vinyl polymers has a profound effect on their physical properties. Despite the well-developed stereoselective methods for the polymerization of propylene and other nonpolar α-olefins, stereoselective polymerization of polar vinyl monomers has proven more challenging. We have designed chiral counterions that systematically bias the reactivity and chain-end stereochemical environment during cationic polymerization. This approach overrides conventional chain-end stereochemical bias to achieve catalyst-controlled stereoselective polymerization. We demonstrate that this method is general to vinyl ether substrates, providing access to a range of isotactic poly(vinyl ether)s with high degrees of isotacticity. The obtained materials display the tensile properties of commercial polyolefins but adhere more strongly to polar substrates by an order of magnitude, indicating their promise for next-generation engineering applications.

The right hand lines up vinyl ethers

Polymers Well-optimized catalysts produce vast quantities of isotactic polypropylene, in which the side chains all face the same way. Add an oxygen into the monomer, though, and that degree of uniformity becomes harder to enforce. Teator and Leibfarth report a general protocol to polymerize a variety of such vinyl ethers isotactically (see the Perspective by Foster and O'Reilly). They rely on a chiral phosphoric acid in combination with a titanium Lewis acid to bias the monomer orientation during cationic polymerization. The resulting polymers show promising adhesive properties. Science , this issue p. [1439][1]; see also p. [1394][2] [1]: /lookup/volpage/363/1439?iss=6434 [2]: /lookup/volpage/363/1394?iss=6434

The first report of Lewis acid reagents in the intramolecular Rauhut–Currier reaction

The first report of Lewis acid use in intramolecular Rauhut–Currier reactions is described. Titanium Lewis acids lead to rapid Rauhut–Currier reactions in the case of two classic substrates. More importantly, titanium and tin Lewis acids were the only successful reagent for a more complex substrate, thereby illustrating the potential for Lewis acid reagents to facilitate challenging intramolecular Rauhut–Currier reactions that fail with traditional reagents.

ChemInform Abstract: Stereoselective Titanium‐Mediated Aldol Reactions of α‐Benzyloxy Methyl Ketones.

AbstractThe titanium‐mediated aldol reaction of chiral ketones is sensitive to the titanium Lewis acid used in the enolization step and represents a new approach to the substrate‐controlled acetate aldol reaction.

Metalloenzyme-like catalyzed isomerizations of sugars by Lewis acid zeolites.

Isomerization of sugars is used in a variety of industrially relevant processes and in glycolysis. Here, we show that hydrophobic zeolite beta with framework tin or titanium Lewis acid centers isomerizes sugars, e.g., glucose, via reaction pathways that are analogous to those of metalloenzymes. Specifically, experimental and theoretical investigations reveal that glucose partitions into the zeolite in the pyranose form, ring opens to the acyclic form in the presence of the Lewis acid center, isomerizes into the acyclic form of fructose, and finally ring closes to yield the furanose product. The zeolite catalysts provide processing advantages over metalloenzymes such as an ability to work at higher temperatures and in acidic conditions that allow for the isomerization reaction to be coupled with other important conversions.

Lewis acid activated synthesis of highly substituted cyclopentanes by the N-heterocyclic carbene catalyzed addition of homoenolate equivalents to unsaturated ketoesters.

Lewis acid activated synthesis of highly substituted cyclopentanes by the N-heterocyclic carbene catalyzed addition of homoenolate equivalents to unsaturated ketoesters.

Diastereoselective addition of chlorotitanium enolates of N-acyl thiazolidinethione to activated imines — A novel synthesis of β-lactams

We report a novel methodology for preparing enantiomerically pure β-lactams, starting from nitriles in diastereomeric ratios up to 10:1. The power of the methodology was demonstrated by the efficient synthesis of the cholesterol absorption inhibitor SCH 48462.Key words: β-lactam, N-metalloaldimine, titanium Lewis acids, cholesterol absorption inhibitor.

Design of Next‐Generation Chiral Acid‐Base Catalysts and Application to Fine Organic Synthesis

AbstractFor Abstract see ChemInform Abstract in Full Text.

A new, alternative, halogen-free synthesis for the fragrance compound Melonal using zeolites and mesoporous materials as oxidation catalysts

At present Melonal (2,6-dimethyl-5-hepten-1-al) fragrance is industrially produced by a Darzens reaction from 6-methyl-5-hepten-2-one, with ethylchloroacetate as reagent. We present here a novel halogen-free synthesis strategy that involves the chemoselective oxidation of Citral (3,7-dimethyl-6-octen-1-al), a common compound in the fragrance industry, with H 2O 2 and Sn-Beta or Sn-MCM-41 as catalysts. The performance of tin Lewis acid sites is compared with other potential catalytic sites, and it is found that aluminium Brønsted acid sites and zirconium or titanium Lewis acid sites are less efficient and selective than Sn. In the case of Ti, epoxidation by-products could be found. The re-usability of Sn-Beta zeolite is discussed, as is the heterogeneity of the reaction.

次世代型キラル酸塩基触媒のデザインと精密有機合成への活用

Apart from enzymes as biocatalysts, a broad repertoire of chiral reagents, auxiliaries, and catalysts can be developed in recent years. In this respect, the design of new catalysts and new reactions in a practical and an environmentally benign manner is increasingly important in the 21st century for the construction of new and useful organic molecules. Accordingly, we have developed new catalytic asymmetric reactions based on [1] the rational design of conceptually new, chiral bidentate Lewis acids (particularly, aluminum and titanium Lewis acids) in organic synthesis including catalytic asymmetric synthesis; [2] the design of environmentally benign, binaphthylmodified chiral phase-transfer catalysts as chiral organocatalysts for practical asymmetric synthesis.

Dramatic Effect of Boron-Based Lewis Acids in Cross-Metathesis Reactions

Amino compounds such as allylic carbamates or amides are poor partners in cross-metathesis reactions because of possible coordination of the emerging carbene to the polar functional group. Whereas the use of titanium Lewis acid catalysts was unsatisfactory, an impressive increase in yields was obtained with boron-based Lewis acids.

TitaniumLewis Acids for Asymmetric Catalysis: Synthesis and Structural Characterization of Dichloro[diolato(2−)-κO,κO′]bis(solvent)titanium ([TiCl2(diolato)(solvent)2]) Complexes

The complexes [TiCl2{(R,R)-TADDOLato}(DME)]⋅MeCN (3), and [TiCl2{(R,R)-1-Nph-TADDOLato}(MeCN)2]⋅CH2Cl2 (4b) (DME=1,2-dimethoxyethane; (R,R)-TADDOLato=(4R,5R)-2,2-dimethyl-α,α,α′,α′-tetraphenyl-1,3-dioxolane-4,5-dimethanolato(2−)-κO,κO′; (R,R)-1-Nph-TADDOLato=(4R,5R)-2,2-dimethyl-α,α,α′,α′-tetra(naphthalen-1-yl)-1,3-dioxolane-4,5-dimethanolato(2−)-κO,κO′) were prepared and isolated in high yield as stable crystalline materials (Scheme 1). They constitute ideally suited and easy-to-handle catalyst precursors for a large number of Ti-catalyzed asymmetric reactions, for which they have been previously generated in situ. The X-ray crystal structures of 3 and 4b show a distorted octahedral geometry around Ti with the chloro ligands in mutual trans positions (Figs. 5 and 6). The new chiral diols α-(1S,3R)-3-hydroxy-2,2,3-trimethylcyclopentyl]-α-phenylbenzenemethanol (13a), derived from camphoric acid (5), and (M)-6,6′-dimethyl-α,α,α′,α′-tetraphenyl[1,1′-biphenyl]-2,2′-dimethanol (15) were prepared (Schemes 3 and 4). These new ligands are able to form mononuclear complexes with the TiIVCl2 fragment. The corresponding complex 14 derived from 13a was characterized by X-ray as a mixed THF/MeCN adduct.

Stereoselective synthesis of α,α-disubstituted amines by a sequential nucleophilic addition to O-protected cyanohydrins

The first enantioselective double nucleophilic addition reaction to O-protected cyanohydrin was achieved in 71% yield and 82:18 enantiomeric ratio by using a bis(BINOL)titanium Lewis acid complex. New (4 S,5 S)-2,2-dialkyl-4,5-bis(hydroxymethyl)-1,3-dioxolane titanium complexes were synthesized and they displayed good levels of enantioselection in the nucleophilic addition reaction.

Toward the Molecular Imprinting of Titanium Lewis Acids: Demonstration of Diels−Alder Catalysis

This communication reports the free radical incorporation of a titanium(IV) complex containing polymerizable aryloxide ligands into a rigid and porous polystyrene/divinylbenzene-based matrix. This yields a yellow-orange insoluble polymer that can be converted into a dark red polymer with SiCl4; the latter polymer is a good catalyst for the Diels−Alder reaction.

Enantioselective Diels‐Alder Reactions: NMR Spectroscopy of a Chiral Titanium Lewis Acid under High Pressure

Abstract1H‐NMR investigations of the temperature and pressure dependence of the equilibrium of the reaction of the tartaric acid derivative 7 with titanium dichloride diisopropoxide (8) to give the chiral Lewis acid 9 show that increasing pressure and temperature shift the equilibrium to the achiral Lewis acid 8. The observed decrease in enantioselectivity of the cycloaddition reaction of isoprene (3) with 3‐crotonyl‐1,3‐oxazolidin‐2‐one (4) in the presence of 7 and 8 under high pressure may therefore be attributed at least in part to the unfavorable influence of pressure on the formation of 9.

Addition and substitution complexes of 2-pyrrolidinone with chloro and alkoxo titanium(IV) derivatives

When the γ lactam 2-pyrrolidinone (=LH) is reacted with members of the series TiCl 4−x(OR) x where R = isopropyl or ethyl, either addition or substitution may occur depending on the value of x. The adducts TiCl 4·LH, TiCl 4·2LH, TiCl 2(OR) 2·LH and TiCl 2(OR) 2·2LH corresponding to x values of 0 and 2 may be isolated. In contrast, when x = 3 or 4 the N-substituted products TiCl(OR) 2L and Ti(OR) 3L are obtained. Infra-red and NMR ( 1H and 13C) data are consistent with the involvement of the carbonyl group in the adduct formation and also possibly in dimerization of the substitution derivatives. The efficiency of, titanium Lewis acids as NMR shift reagents for lactams has been re-confirmed.