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

Catalytic Site-, Diastereo-, and Enantioselective Cascade Iodocyclization of 2-Geranylarenols

A chiral amidophosphate-N-iodosuccinimide cooperative catalysis has been developed for the site-, diastereo-, and enantioselective iodocyclization of 2-geranylarenols with molecular iodine to give the corresponding iodo-containing polycyclic compounds with good levels of selectivity. This is the first example of a catalytic enantioselective iodocarbocyclization. A reactive chiral iodonium species is generated from molecular iodine via the dual halogen-bonding interactions with a chiral Lewis base and Lewis acid. The sterically demanding 3,3'-substituents of the chiral BINOL-derived amidophosphate are critical to induce the site-selective iodination at the less-hindered terminal alkenyl moiety of 2-geranylarenols.

Intermolecular Catalytic Asymmetric Iodoetherification of Unfunctionalized Alkenes

A newly prepared trinuclear Zn₃-(R,S,S)-aminoiminobinaphthoxide complex (triZn-II) catalyzed the first general intermolecular asymmetric iodoetherification of unfunctionalized alkenes. Using triZn-II, the iodoetherification reaction of unfunctionalized alkenes with o-nitrophenols proceeded smoothly to give the products with up to 92.5:7.5 er, and diene substrates were converted to the products with up to 99:1 er with the formation of a meso-isomer (dl/meso = 78/22). The chiral iodoethers gave a new platform for the synthesis of chiral morpholines.

Mechanistic Details of Asymmetric Bromocyclization with BINAP Monoxide: Identification of Chiral Proton-Bridged Bisphosphine Oxide Complex and Its Application to Parallel Kinetic Resolution

The mechanism of our previously reported catalytic asymmetric bromocyclization reactions using 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP) monoxide was examined in detail by the means of control experiments, NMR studies, X-ray structure analysis, and CryoSpray electrospray ionization mass spectrometry (ESI-MS) analysis. The chiral BINAP monoxide was transformed to a key catalyst precursor, proton-bridged bisphosphine oxide complex (POHOP·Br), in the presence of N-bromosuccinimide (NBS) and contaminating water. The thus-formed POHOP further reacts with NBS to afford BINAP dioxide and molecular bromine (Br₂) simultaneously in equimolar amounts. While the resulting Br₂ is activated by NBS to form a more reactive brominating reagent (Br₂─NBS), BINAP dioxide serves as a bifunctional catalyst, acting as both a Lewis base that reacts with Br₂─NBS to form a chiral brominating agent (P═O⁺─Br) and also as a Brønsted base for the activation of the substrate. By taking advantage of this novel concerted Lewis/Brønsted base catalysis by BINAP dioxide, we achieved the first regio- and chemodivergent parallel kinetic resolutions (PKRs) of racemic unsymmetrical bisallylic amides via bromocyclization.

The applications of catalytic asymmetric halocyclization in natural product synthesis

Catalytic asymmetric halocyclization of olefinic substrate has evolved rapidly and been well utilized as a practical strategy for constructing enantioenriched cyclic skeletons in natural product synthesis.

Efficient asymmetric syntheses of α-quaternary lactones and esters through chiral bifunctional sulfide-catalyzed desymmetrizing bromolactonization of α,α-diallyl carboxylic acids

Asymmetric halolactonizations are powerful methods for the syntheses of chiral lactones. Catalytic and highly enantioselective halolactonizations of α-allyl carboxylic acids, however, continue to present a formidable challenge. Herein, we report the chiral bifunctional sulfide-catalyzed desymmetrizing bromolactonizations of α,α-diallyl carboxylic acids. These reactions efficiently produced chiral α-quaternary lactones and esters.

A Catalyst-Controlled Enantiodivergent Bromolactonization

A catalyst-controlled enantiodivergent bromolactonization of olefinic acids has been developed. Quinine-derived amino-amides bearing the same chiral core but different achiral aryl substituents were used as the catalysts. Switching the methoxy substituent in the aryl amide system from meta- to ortho-position results in a complete switch in asymmetric induction to afford the desired lactone in good enantioselectivity and yield. Mechanistic studies, including chemical experiments and density functional theory calculations, reveal that the differences in steric and electronic effects of the catalyst substituent alter the reaction mechanism.

Enantio- and diastereoselective double Mannich reaction of malononitrile with N-Boc imines using quinine-derived bifunctional organoiodine catalyst

A chiral quinine-derived organic base catalyst with halogen bond donor functionality was used to catalyze the asymmetric double Mannich reaction of malononitrile with N-Boc and N-Cbz imines to afford 1,3-diamines in excellent yields with high enantio- and diastereoselectivities. With 2.2 equiv. of a single imine electrophile, symmetrical 1,3-diamines were obtained, whereas, with two different imine partners, unsymmetrically substituted 1,3-diamine was obtained. The monohydration of the double Mannich product was also achieved.

A Hypervalent Cyclic Dibenzoiodolium Salt as a Halogen-Bond-Donor Catalyst for the [4+2] Cycloaddition of 2-Alkenylindoles

A stable, hypervalent cyclic dibenzoiodolium salt acted as a strong halogen bonding (XB)-donor catalyst for [4+2] cycloaddition of 2-alkenylindoles, and not as an oxidizing agent. The cross-[4+2] cycloaddition of 2-vinylindoles with 2-alkenylindoles was catalyzed smoothly by the hypervalent cyclic dibenzoiodolium triflate catalyst to give the tetrahydrocarbazoles in up to 99 % yield with 17 : 1 diastereoselectivity. The hypervalent cyclic dibenzoiodolium salt was also applicable to the Povarov reaction of 2-vinylindole with N-p-methoxyphenyl (PMP) imine to give the indolyl-tetrahydroquinoline in 83 % yield.

Bromonium salts: diaryl-λ3-bromanes as halogen-bonding organocatalysts

Bromonium salts have been typically but infrequently used as good leaving groups owing to their high nucleofugality. Herein, we report the synthesis of stable bromonium salts and their first catalytic application, with excellent product yield.

Beyond hydrogen bonding: recent trends of outer sphere interactions in transition metal catalysis

The implementation of interactions beyond hydrogen bonding in the 2^nd coordination sphere of transition metal catalysts is rare. However, it has already shown great promise in last 5 years, providing new tools to control the activity and selectivity as here reviewed.

Supramolecular Halogen Bonds in Asymmetric Catalysis

Halogen bonding has received a significant increase in attention in the past 20 years. An important part of this interest has centered on catalytic applications of halogen bonding. Halogen bond (XB) catalysis is still a developing field in organocatalysis, although XB catalysis has outgrown its proof of concept phase. The start of this year witnessed the publication of the first example of a purely XB-based enantioselective catalytic reaction. While the selectivity can be improved upon, there are already plenty of examples in which halogen bonds, among other interactions, play a crucial role in the outcome of highly enantioselective reactions. This paper will give an overview of the current state of the use of XBs in catalytic stereoselective processes.

Catalytic Asymmetric Chlorination of β-Ketoesters Using N-PFB-PyBidine-Zn(OAc)2

A PyBidine-Zn(OAc)2 complex catalyzed asymmetric chlorination of β-ketoesters. With assistance of NaHCO3, a newly developed N-pentafluorobenzyl-PyBidine (N-PFB-PyBidine)-Zn(OAc)2 catalyst promoted the reaction of α-benzyl-β-ketoesters with N-chlorosuccinimide (NCS) to give the chlorinated products with up to 82% ee. Results of a mechanistic study suggested that zinc-enolate of β-ketoesters was formed on the basic (N-PFB-PyBidine)-Zn(OAc)2 catalyst. The α-chlorinated-β-ketoester was successfully transformed into the chiral epoxide through sequential asymmetric chlorination/cyano-epoxidation in a one-pot synthesis.

Halogen-Bonding Interaction between I2 and N-Iodosuccinimide in Lewis Base-Catalyzed Iodolactonization

The halogen-bonding interaction between I₂ and N-iodosuccinimide (NIS) stabilized by a Lewis base (LB) has been explored. ¹H NMR, nuclear Overhauser effect (NOE), and diffusion-ordered NMR spectroscopy (DOSY) suggest the generation of a 1:1:1 assembly, LB-I₂-NIS. In contrast, when N-iodotrifluoromethanesulfonimide (INTf₂) is used instead of NIS, LB-I₅⁺-LB is generated. On the basis of these results in combination with density functional theory (DFT) calculations, we propose a mechanism for the formation of I₂-NIS and the subsequent generation of an active iodinating species LB-I⁺.

Application of Halogen Bonding to Organocatalysis: A Theoretical Perspective

The strong, specific, and directional halogen bond (XB) is an ideal supramolecular synthon in crystal engineering, as well as rational catalyst and drug design. These attributes attracted strong growing interest in halogen bonding in the past decade and led to a wide range of applications in materials, biological, and catalysis applications. Recently, various research groups exploited the XB mode of activation in designing halogen-based Lewis acids in effecting organic transformation, and there is continual growth in this promising area. In addition to the rapid advancements in methodology development, computational investigations are well suited for mechanistic understanding, rational XB catalyst design, and the study of intermediates that are unstable when observed experimentally. In this review, we highlight recent computational studies of XB organocatalytic reactions, which provide valuable insights into the XB mode of activation, competing reaction pathways, effects of solvent and counterions, and design of novel XB catalysts.

Catalytic Asymmetric Synthesis of Chiral Bis(indolyl)methanes Using a Ts-PyBidine-Nickel Complex

A chiral tosyl-substituted bis(imidazolidine)pyridine Ts-PyBidine-nickel complex was an efficient catalyst for Friedel-Crafts reaction of indoles with methylene indolinones to give bisindolylmethane compounds having differently oxidized indole units with high enantioselectivities. Alkylation of the products proceeded smoothly in a highly diastereoselective manner, providing an all-carbon quaternary carbon center without significant loss of enantiomeric excess.