Mitsuo Komatsu

Organic Reactions on Silica in Water

In nature, many biological processes occur in aqueous environments, and these fascinating in vivo reactions should prompt organic chemists to explore the potential of water as a medium for organic reactions. In addition to scientific interest in aqueous reaction media, social pressure to find environmentally benign alternatives to current organic chemical processes has made these reactions attractive from an ecological point of view. The use of aqueous media in organic reactions offers significant environmental advantages and has attracted a great deal of interest because water is a desirable solvent for the following reasons: low cost, safety, and environmental concerns. Groundbreaking studies of organic reactions in aqueous environments demonstrated that Diels-Alder reactions and Claisen rearrangement of hydrophobic reactants are accelerated in aqueous solutions. In addition to these discoveries, aqueous environments result in reactivity and selectivity that are unique from reactions in organic solvents. As a result, many versatile and efficient organic reactions have been developed. Although many organic reactions are facilitated in aqueous media, some reactions proceed very slowly because the solubility of most organic molecules in pure water is limited. Because solubility is a prerequisite for reactivity, a variety of strategies expanding the scope of water-based organic syntheses have been investigated. Most commonly, organic cosolvents, such as the lower alcohols, acetone, DMF, and acetonitrile, are used to increase the solubility of hydrophobic solutes in water. Some alternative means of achieving aqueous solubility are the use of phase-transfer catalysts and surfactants. In addition to these processes, substrate modifications, such as the addition of a positive or negative charge to an ionizable substrate or the grafting of hydrophilic groups onto insoluble reactants, have been investigated. However, these approaches change the chemical behavior of the substrates and tend to complicate the reactions. Recent studies indicate that water is a promising medium for heterogenized homogeneous catalysis. Although the efficiency of heterogeneous catalysis is generally inferior to that of homogeneous systems, the advantage of immobilized catalysts include their easy recovery from reaction mixtures and their reusability. Catalysts heterogenized through covalent and non-covalent attachment to either inorganic or organic materials, such as silica, layered clays, and polymers, have been successfully employed in aqueous media. The chemical stability of the inorganic supports is important, especially under oxidizing conditions; the mechanical and thermal stability of inorganic supports is often excellent as well. This review will focus on the use of silica as an inorganic support for organic reactions in water. The hydroxy group on the silica surface readily reacts with alkoxyor chlorosubstituted silyl compounds, leading to functionalized silica supports. This manipulation allows not only for the introduction of catalysts to the surface of the support but also for the control of surface hydrophobicity. The proteiform and unique character of silica surfaces have been utilized for organic reactions in aqueous media. This review is organized into three main parts, each devoted to one form of silica that has unique effects on organic reactions in water: heterogenized catalysts on silica (immobilization by covalent binding or adsorption of catalysts on silica), hydrophobic and fluorous reverse-phase silica, and unmodified silica. The first section is subdivided into three heterogenized catalyst systems, as follows: water phase only, water-organic biphasic systems, and water-ionic liquid systems.

Iodine-catalyzed aziridination of alkenes using Chloramine-T as a nitrogen source

Abstract Iodine was found to be an efficient catalyst for the aziridination of alkenes utilizing Chloramine-T (N-chloro-N-sodio-p-toluenesulfonamide) as a nitrogen source. For example, when two equivalents of styrene were added to Chloramine-T in the presence of a catalytic amount of iodine in a 1 : 1 solvent mixture of acetonitrile and neutral buffer, the corresponding aziridine 1 was obtained in 91% yield. The reaction could be applied to other acyclic and cyclic alkenes such as 1-octene and cyclohexene. The aziridination of p-substituted styrene derivatives 2–5 with Chloramine-T showed that electron-rich alkenes reacted faster than electron-poor ones. Several Chloramine-T analogs were also examined and were found to give the corresponding aziridines 8–10 in only moderate yields.

Nitrogen atom transfer to alkenes utilizing Chloramine-T as a nitrogen source

Abstract Aziridination of alkenes proceeds successfully using Chloramine-T (N-chloro-N-sodio-p-toluenesulfonamide). When anhydrous Chloramine-T was added to an acetonitrile solution of alkenes in the presence of various CuCl catalysts and MS-5A, the corresponding aziridines were obtained in moderate to good yields.

Novel Asymmetric and Stereospecific Aziridination of Alkenes with a Chiral Nitridomanganese Complex

The addition of pyridine N-oxide is necessary to obtain high enantioselectivities in the asymmetric aziridination of styrene derivatives through transfer of a nitrogen atom from chiral, toluenesulfonic anhydride activated nitridomanganese complex 1 [Eq. (a)]. Remarkably, high stereospecificity was observed in all the aziridinations of trans- and cis-1,2-disubstituted alkenes. R1 =H, Me, nPr, iPr; R2 =H, Me; Ts=p-toluenesulfonyl.

INITIATION OF TIN-MEDIATED RADICAL REACTIONS BY DIETHYLZINC-AIR

Abstract Diethylzinc-air system can serve as an initiator of tin hydride mediated radical reactions of organic halides.

Hydroxymethylation of organic halides. Evaluation of a catalytic system involving a fluorous tin hydride reagent for radical carbonylation

Abstract Hydroxymethylation of organic halides 2 using a catalytic amount of fluorous tin hydride 1 , CO, and NaBH 3 CN as a reducing agent, proceeded smoothly to give one-carbon homologated alcohols 5 in good yields. Three phase workup (water-dichloromethane-perfluorohexane) was conveniently performed for the separation of 1 and 5 .

Aziridination of C60 with Simple Amides and Catalytic Rearrangement of the Aziridinofullerenes to Azafulleroids

The selective formation of aziridinofullerene and azafulleroid, which are isomers of the fullerene derivatives-introduced N(1) unit, is achieved. The ionic aziridination is a very convenient and risk-free procedure compared with the conventional method with azides as nitrogen sources, and gives aziridinofullerenes from various readily available amides (carbamates, ureas, carboxamides, and phosphamides). For example, benzyl carbamate was chlorinated by tert-butyl hypochlorite (tert-BuOCl) and then reacted with C(60) in the presence of base to give N-benzyloxycarbonyl aziridinofullerene exclusively and without formation of its isomer, an azafulleroid. The reaction enabled the synthesis of functional fullerene derivatives having a trialkoxysilyl group and an amino acid moiety. Azafulleroids were obtained through the rearrangement of corresponding aziridinofullerenes by using the combination of a chloramine catalyst and MS4A. Among other chloramines used, chloramine B (CB) showed superior ability as a catalyst in the rearrangement. It was found that MS4A functions as a Lewis acid in the reaction.

Cyclizative radical carbonylations of azaenynes by TTMSS and hexanethiol leading to α-silyl- and thiomethylene lactams. Insights into the E/Z stereoselectivities

Free-radical mediated cyclizative carbonylations of azaenynes were carried out using TTMSS as a radical mediator to compare the efficiency and the stereochemistry with those using tributyltin hydride. Using a substrate concentration of 0.1 M, the reactions gave good yields of alpha-silylmethylene lactams having four to seven-membered rings. The observed E-diastereoselectivity of the resulting vinylsilane moiety is in sharp contrast to the Z-selectivity observed during the analogous carbonylation using tributyltin hydride. When hexanethiol was used as the radical mediator, alpha-thiomethylene lactams were formed with E-favoring stereoselectivity again. Ab initio and DFT molecular orbital calculations on the stability of E and Z products were carried out for a set of five-membered methylene lactams bearing SnH3, SiH3, and SMe groups. The distinct thermodynamic preference for the Z-isomer was only predicted for the Sn-bearing lactam. A steric effect due to the bulky (TMS)3Si group is proposed for the E-selectivity observed in the TTMSS-mediated reaction.

Novel generation and cycloaddition of N-silylated azomethine ylides from α-silylimidates and trifluorosilane

Abstract The Generation of N-silylated azomethine ylides was achieved by the reaction of α-silylimidates and trifluorophenylsilane. The generated azomethine ylides can be regarded as synthetic equivalents of nitrile ylides since they have an alkoxy group which may serve as a leaving group. Cycloaddition with acetylenic or olefinic dipolarophiles proceeded smoothly to give pyrrole or pyrroline derivatives in good to excellent yields. For example, α-silylimidate 1 was treated with 1.2 equiv of trifluorophenylsilane in the presence of 1 equiv of dimethyl acetylenedicarboxylate to give pyrrole 2 in 97% yield. Furthermore, when starting with a secondary α-silylamide, the one-pot synthesis of N-unsubstituted azomethine ylides could be achieved by the successive treatment with alkylation and desilylation agents.

Selective [2+1] aziridination of conjugated dienes with a nitridomanganese complex: a new route to alkenylaziridines

Conjugated dienes were successfully aziridinated using a nitridomanganese complex as a nitrogen source. The reaction proceeded selectively and in good yield via [2+1] addition to give alkenylaziridines, with no evidence for the formation of any [4+1] addition products. The first asymmetric version of the reaction was revealed in the aziridination of diene 5 with chiral complex 1.