Brønsted Acid Catalyzed α′‐Functionalization of Silylenol Ethers with Indoles

A new method which enables carbon–carbon bond formation at the α′‐position of silylenol ethers by using catalytic amounts of pyridinium triflate is reported. This chemistry successfully produces, structurally challenging, highly substituted indole‐containing silylenol ethers in excellent yields with complete regiocontrol, presumably through silyloxyallyl cation intermediates. Despite the use of Brønsted acid, the silylenol ether moiety does not undergo protodesilylation, thus underscoring the very mild reaction conditions.     

Bifunctional Brønsted Base Catalyzes Direct Asymmetric Aldol Reaction of α‐Keto Amides

The first enantioselective direct cross‐aldol reaction of α‐keto amides with aldehydes, mediated by a bifunctional ureidopeptide‐based Brønsted base catalyst, is described. The appropriate combination of a tertiary amine base and an aminal, and urea hydrogen‐bond donor groups in the catalyst structure promoted the exclusive generation of the α‐keto amide enolate which reacted with either non‐enolizable or enolizable aldehydes to produce highly enantioenriched polyoxygenated aldol adducts without side‐products resulting from dehydration, α‐keto amide self‐condensation, aldehyde enolization, and isotetronic acid formation.     

Asymmetric Brønsted Acid‐Catalyzed Nazarov Cyclization of Acyclic α‐Alkoxy Dienones

A Brønsted acid‐catalyzed asymmetric Nazarov cyclization of acyclic α‐alkoxy dienones has been developed. The reaction offers access to chiral cyclopentenones in a highly enantioselective manner. The reaction is complementary to our previously reported Brønsted acid‐catalyzed electrocyclization reactions, which provided differently substituted optically active cyclopentenones with a fused tetrahydropyrane ring in good yields and with excellent enantioselectivities.     

Effect of Lewis and Brønsted acids on the homopolymerization of acrylates and their copolymerization with 1‐alkenes

The first copolymerization of acrylate and methacrylate with nonpolar 1‐alkenes in the presence of Brønsted acids as complexation agents has been reported. The addition of both homogeneous and heterogeneous Brønsted acids resulted in increased monomer conversion and 1‐alkene incorporation. Further, the heterogeneous Brønsted acids can be recycled without loss of activity. A direct correlation exists between the ability of the Lewis or Brønsted acid to bind to the ester group of the acrylate/methacrylate monomer and its ability to promote the copolymerization reaction. For Lewis acids, there is also a direct correlation between the charge/size ratio at the metal center and their ability to promote copolymerizations. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5499–5505, 2008     

Lewis and Brønsted Acid Cocatalyzed Reductive Deoxyallenylation of Propargylic Alcohols with 2‐Nitrobenzenesulfonylhydrazide

Reductive deoxyallenylation of sterically hindered tertiary propargylic alcohols was realized on reaction with 2‐nitrobenzenesulfonylhydrazide (NBSH) by the combined use of Lewis and Brønsted acid catalysts. This method features a broad substrate scope, mild reaction conditions, and good functional‐group tolerance, and affords various mono‐, di‐, and trisubstituted allenes in good‐to‐excellent yields. The synthetic utility of this method was demonstrated by the synthesis of 2H‐chromenes and 1,2‐dihydroquinolines.     

Brønsted Acid Assisted Regio‐ and Enantioselective Direct O‐Nitroso Aldol Reaction Catalysed by α,α‐Diphenylprolinol Trimethylsilyl Ether

In the presence of p‐nitrobenzoic acid, the O‐nitroso aldol reaction of nitrosobenzene with enolisable aldehydes may be promoted by commercially available α,α‐diphenylprolinol trimethylsilyl ether. The reaction proceeds with good yields and essentially complete enantioselectivity, with catalyst loadings in the 5–10 mol % range. The resulting α‐oxyaldehyde adducts may be transformed in situ into α‐oxyimines, which provide 1,2‐amino alcohols upon treatment with Grignard reagents, in good overall yield (45–59 %) and with typical diastereomeric ratios ≥95:5.     

Lewis acid promoted aldol additions of α-thiosilylketeneacetals to α-alkoxy aldehydes: diastereoselective synthesis of -α-methylene-β-hydroxy-∂-alkoxy esters.

MgBr₂ mediated addition of Methyl α-methylthio propionate silylketene acetal to α and α,β-alkoxy aldehydes is highly 3,4 -selective (18:1). -α- methylene-β- hydroxy-∂-alkoxy esters (6) and (8) are synthesized.     

Lewis Base/Brønsted Acid Co‐catalyzed Enantioselective Sulfenylation/Semipinacol Rearrangement of Di‐ and Trisubstituted Allylic Alcohols

An enantioselective sulfenylation/semipinacol rearrangement of 1,1‐disubstituted and trisubstituted allylic alcohols was accomplished with a chiral Lewis base and a chiral Brønsted acid as cocatalysts, generating various β‐arylthio ketones bearing an all‐carbon quaternary center in moderate to excellent yields and excellent enantioselectivities. These chiral arylthio ketone products are common intermediates with many applications, for example, in the design of new chiral catalysts/ligands and the total synthesis of natural products. Computational studies (DFT calculations) were carried out to explain the enantioselectivity and the role of the chiral Brønsted acid. Additionally, the synthetic utility of this method was exemplified by an enantioselective total synthesis of (?)‐herbertene and a one‐pot synthesis of a chiral sulfoxide and sulfone.     

Brønsted acid‐catalyzed polymerization of ε‐caprolactone in water: A mild and straightforward route to poly(ε‐caprolactone)‐graft‐water‐soluble polysaccharides

The polymerization of ε‐caprolactone (ε‐CL) has been assessed in water using various Brønsted acids as catalysts. The reaction was found to be quantitative at 100 °C, leading to number–average molecular weights up to 5000 g mol^?1. The Brønsted acid‐catalyzed polymerization of ε‐CL in water was further conducted in the presence of water‐soluble polysaccharides thereby affording graft copolymers. The approach enables an easy, mild access to dextran hydroxyesters. For low degree of substitution, the latter self‐assembles in water to form nanoparticles. Poly(ε‐CL)‐graft‐methylcellulose copolymers can also be obtained via a similar approach. It is noteworthy that the methodology reported herein is a one‐step route to poly(ε‐CL)‐graft‐water‐soluble polysaccharides, operating in mild conditions, that is, at low temperatures, using readily available metal‐free catalysts and water as a solvent. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2139–2145