Catalytic, Asymmetric, Bromine-Induced Semipinacol Rearrangements at Unactivated Double Bonds

Two Green Protocols for Halogenative Semipinacol Rearrangement

Semipinacol rearrangement is a special type of Wagner-Meerwein rearrangement that involves carbocation 1,2-rearrangement to provide carbonyl compounds with an α-quaternary carbon center. It has been strategically used for natural product synthesis and construction of highly congested quaternary carbons. Herein, we report a safe and green protocol that uses oxone/halide and Fenton bromide to achieve halogenative semipinacol rearrangement. The key feature of this method is the green in situ generation of reactive halogenating species from oxidation of halide with oxone or H₂O₂, which produces a nontoxic byproduct (potassium sulfate or water). Easy operation (insensitive to air and moisture) at room temperature without using special equipment adds additional advantage over previous methods.

Ritter-enabled catalytic asymmetric chloroamidation of olefins

Intermolecular asymmetric haloamination reactions are challenging due to the inherently high halenium affinity (HalA) of the nitrogen atom, which often leads to N-halogenated products as a kinetic trap. To circumvent this issue, acetonitrile, possessing a low HalA, was used as the nucleophile in the catalytic asymmetric Ritter-type chloroamidation of allyl-amides. This method is compatible with Z and E alkenes with both alkyl and aromatic substitution. Mild acidic workup reveals the 1,2-chloroamide products with enantiomeric excess greater than 95% for many examples. We also report the successful use of the sulfonamide chlorenium reagent dichloramine-T in this chlorenium-initiated catalytic asymmetric Ritter-type reaction. Facile modifications lead to chiral imidazoline, guanidine, and orthogonally protected 1,2,3 chiral tri-amines.

Intermolecular haloamination reactions are challenging due to the high halenium affinity of the nitrogen atom. This is circumvented by using acetonitrile as an attenuated nucleophile, resulting in an enantioselective halo-Ritter reaction.

Divergent Synthesis of Trifluoromethyl Sulfoxides and β-SCF3 Carbonyl Compounds by Tandem Trifluoromethylthiolation/Rearrangement of Allylic and Propargylic Alcohols

A selenium-catalyzed trifluoromethylthiolation/[2,3]-sigmatropic rearrangement of tertiary allylic and propargylic alcohols which could provide straightforward and facile access to trifluoromethyl sulfoxides was developed. Various allylic and allenic trifluoromethyl sulfoxides were obtained with moderate to excellent yields. Meanwhile, a Lewis acid mediated trifluoromethylthiolation/1,2-rearrangement to synthesize β-SCF₃ carbonyl compounds was also accomplished. These two tandem reactions feature with mild reaction conditions and metal-free. During these two reactions, the chemoselectivity of electrophilic trifluoromethylthiolation was revealed.

Enantioselective Halolactonizations Using Amino-Acid-Derived Phthalazine Catalysts

Amino-acid-derived phthalazine catalysts have been designed and synthesized for enantioselective halolactonization of prochiral dienoic acids. The scope of the reaction is evidenced by 17 examples of spiro α-exo-methylene-halolactones with up to 99.8% enantiomeric excess. The resulting enantio-enriched spiro halolactone products are found to exhibit potent antitumor effects. In addition, both antipodes of products with equally excellent enantioselevity could be obtained since a pair of enantiomeric catalysts is guaranteed.

Asymmetric Catalytic Halofunctionalization of α,β-Unsaturated Carbonyl Compounds

Halofunctionalization methods enable the vicinal difunctionalization of alkenes with heteroatom nucleophiles and halogen moieties. As a fundamental transformation in organic synthesis, the catalytic asymmetric variants have only recently been reported. In sharp contrast to the asymmetric halocyclization of simple alkenes which involves a nucleophile-assisted alkene activation process, the asymmetric halofunctionalization of enones developed by our laboratory features an electrophile-assisted 1,4-addition pathway. Our work in this area has resulted in the development of several different types of regio-, diastereo-, and enantioselective processes, including inter- and intramolecular haloaminations, haloetherifications, and haloazidations. The scope, updated mechanism, limitations, and future perspective of these reactions are discussed.

Phosphine Oxide-Sc(OTf)3 Catalyzed Highly Regio- and Enantioselective Bromoaminocyclization of (E)-Cinnamyl Tosylcarbamates. An Approach to a Class of Synthetically Versatile Functionalized Molecules

A highly regio- and enantioselective bromoaminocyclization of (E)-cinnamyl tosylcarbamates catalyzed by a chiral phosphine oxide-Sc(OTf)3 complex is described. A wide variety of optically active aryl 5-bromo-1,3-oxazinan-2-ones can be obtained with high yield and enantioselectivity.

Chiral Phosphine Oxide-Sc(OTf)3 Complex Catalyzed Enantioselective Bromoaminocyclization of 2-Benzofuranylmethyl N-Tosylcarbamates. Approach to a Novel Class of Optically Active Spiro Compounds

An efficient enantioselective bromoaminocyclization of 2-benzofuranylmethyl N-tosylcarbamates catalyzed by a chiral phosphine oxide-Sc(OTf)(3) complex is described. A wide variety of optically active spiro benzofuran oxazolidinones can be obtained with high enantioselectivities.

Enantioselective 6-exo-Bromoaminocyclization of Homoallylic N-Tosylcarbamates Catalyzed by a Novel Monophosphine-Sc(OTf)3 Complex

A highly enantioselective 6-exo-bromoaminocyclization of (E)-homoallylic N-tosylcarbamates catalyzed by a novel monophosphine-Sc(OTf)3 complex is described, giving a wide variety of optically active oxazinanones with high enantioselectivities.