Association of Halogen Bonding and Hydrogen Bonding in Metal Acetate-Catalyzed Asymmetric Halolactonization

Abstract

SummaryCooperative activation using halogen bonding and hydrogen bonding works in metal-catalyzed asymmetric halolactonization. The Zn3(OAc)4-3,3′-bis(aminoimino)binaphthoxide (tri-Zn) complex catalyzes both asymmetric iodolactonization and bromolactonization. Carboxylic acid substrates are converted to zinc carboxylates on the tri-Zn complex, and the N-halosuccinimide (N-bromosuccinimide [NBS] or N-iodosuccinimide [NIS]) is activated by hydrogen bonding with the diamine unit of chiral ligand. Halolactonization is significantly enhanced by the addition of catalytic I2. Density functional theory calculations revealed that a catalytic amount of I2 mediates the alkene portion of the substrates and NIS to realize highly enantioselective iodolactonization. The tri-Zn catalyst activates both sides of the carboxylic acid and alkene moiety, so that asymmetric five-membered iodolactonization of prochiral diallyl acetic acids proceeded to afford the chiral γ-butyrolactones. In the total description of the catalytic cycle, iodolactonization using the NIS-I2 complex proceeds with the regeneration of I2, which enables the catalytic use of I2. The actual iodination reagent is I2 and not NIS.

Catalysis Based on C−I⋅⋅⋅π Halogen Bonds: Electrophilic Activation of 2‐Alkenylindoles by Cationic Halogen‐Bond Donors for [4+2] Cycloadditions

“…[3] Theh ighly directional and soft character of XB complexes have been applied in crystal engineering, [4] biomolecular systems, [5] and catalysis. [6][7][8] In 2008, Bolm et al reported XB-donor-catalyzed reduction of quinolines by Hantzsch esters. [7a] After this seminal work, other catalytic reactions were successfully developed for the activation of lone-pair-possessing heteroatoms (Y) in the substrates (e.g., imine derivatives, [7a,e,h,o,q,r] carbonyl compounds, [7c,f,j,l,m,n,r] alkyl halides, [7d,g] thioamides, [7i] iodonium ylides, [7k] and N-haloamides [7p] ;F igure 1B,l eft).…”

“…[14] Among the azolium salts tested, the 2-iodoimidazolinium salt 1d promoted the reaction most efficiently,r esulting in 81 %y ield of the product (entry 4). TheN H-free indoles,w hich have substituents at the 5-position, were also suitable for the reaction, resulting in good yields of products (entries [5][6][7][8]. Chloroform was the best solvent choice (entry 10).…”

“…For N-benzyl-2-vinylindoles, introduction of electron-donating and electron-withdrawing substituents at the 5-position successfully gave dimerized products in moderate to high yields (entries 2-4). TheN H-free indoles,w hich have substituents at the 5-position, were also suitable for the reaction, resulting in good yields of products (entries [5][6][7][8]. Bulky substrates such as 4a-d were tolerated, furnishing the corresponding products as single diastereomers (entries [10][11][12][13].…”

Chiral Bifunctional Selenide Catalysts for Asymmetric Bromolactonization

“…Bromolactonization products 3 could be transformed to other chiral heterocyclic compounds. For example, product 3 a was readily converted to gem ‐disubstituted epoxide 10 possessing a quaternary carbon center without loss of the stereo‐information (Scheme ) …”

Catalysis Based on C−I⋅⋅⋅π Halogen Bonds: Electrophilic Activation of 2‐Alkenylindoles by Cationic Halogen‐Bond Donors for [4+2] Cycloadditions

“…Electrostatic noncovalent interactions between the σ‐hole of electron‐deficient halogen atoms (e.g., C−X, X=I, Br, Cl; XB donor) and a Lewis base (XB acceptor) can form XB complexes (Figure B) . The highly directional and soft character of XB complexes have been applied in crystal engineering, biomolecular systems, and catalysis . In 2008, Bolm et al.…”