Use Of Carbenes Research Articles

Transition metal-free [2,3]-sigmatropic rearrangement in the reaction of sulfur ylides with allenoates.

An unprecedented transition metal free [2,3]-sigmatropic rearrangement involving stabilized sulfur ylides and allenoates has been thoroughly established. The scope and utility of this reaction have been extensively studied resulting in C-C bond formation under mild conditions with greater than 20 examples reported. A highlight of the work is the simple and fully operational process that does not involve the use of carbenes or the associated hazardous and sensitive reagents. The reaction can be performed at room temperature and using an open flask. Interestingly, the new C-C bond formation reaction is gram scalable, and the obtained isomers are readily separable, affording interesting building blocks that can be used in the preparation of complex molecules.

Competitive Reactivity of SO2 and NO2 with N-Heterocyclic Carbene: A Mechanistic Study.

Recent DFT based molecular engineering to obtain stable oxathiirane S-oxide derivatives evokes the recommencement of the use of carbenes for the sequestering of SO2, which has been kept separate so far. Carbene is one of the key chemicals for the sequestering of various premier greenhouse gases like CO2, CO, N2O, etc. In this respect, a comparative study of the reactivity of carbenes with variant greenhouse gases is highly demanding. The present investigation is engrossed in the comparative reactivity of SO2 and NO2 with carbenes. All three selected carbenes are highly susceptible to SO2 and NO2. Through an immaculate mechanistic study, we are able to corroborate that the end product of the carbene-SO2 reaction is an adduct which has a preferable structure having a six-membered ring with hydrogen bonding instead of ketone and SO with higher thermodynamic stability than the corresponding oxathiirane S-oxide derivative. Carbene reacts with NO2 to form a stable carbene N, N-dioxide derivative which forms vibrationally excited oxaziridine N-oxide which rapidly dissociates to form a ketone derivative. The formation of carbene S, S-dioxide and carbene N, N-dioxide is a barrierless process. The dissociation of oxaziridene N-oxide is also a barrierless process.

DFT based engineering of N-heterocyclic carbenes to exacerbate its activity for SO2 fixation and storage

Carbene compounds are very reactive to SO2 which assert their candidature to seize this greenhouse gas. Unfortunately, most of the carbenes which produce S,S-dioxide with SO2, undergo dissociation to yield ketone derivative and SO through an intermediate formation of oxathiirane S-oxide derivative. Thus, carbenes are excluded from the list of catalyst for SO2 fixation and storage technology. To eradicate this retardation, the stability of different oxathiirane S-oxide derivatives obtained from SO2 and 56 carbenes of various structures are studied using Density Functional Theory (DFT). Through our study, we are able to find out three oxathiirane S-oxide derivatives which have positive ΔG values for their decomposition to the respective ketone derivatives. This study corroborates that proper engineering of carbene leads to produce a stable oxathiirane S-oxide derivative as a stable product. We observed that carbenes are highly efficient to nab SO2 at room temperature. This finding should necessitate the recommencement of the use of carbene for SO2 fixation technology. We also found three carbenes which are able to produce sulfene derivative (C − S bond length is less than 1.7 Å) on reaction with SO2

Rapid Generation and Safe Use of Carbenes Enabled by a Novel Flow Protocol with In-line IR spectroscopy.

A powerful new continuous process for the formation and use of donor/acceptor-substituted carbenes is described. The safety profile of diazo group transfer on methyl phenylacetate was determined including kinetic studies in batch and in flow using in-line IR analysis. Batch work-up and liquid chromatography were circumvented by developing an optimized liquid/liquid flow separation method providing aryl diazoacetates in high purity. Fast screening of reaction conditions in flow with in-line IR analysis allowed rapid reaction optimization. Finally, a multistep process of diazo group transfer, extraction, separation and subsequent diazo decomposition combined with multiple X-H insertion reactions was established.

N-Heterocyclic carbenes: Advances in transition metal and organic catalysis

The use of N-heterocyclic carbenes (NHCs) in organic and transition metal chemistry is reviewed. This report presents the developments that have occurred in the last 12 months, describing the use of carbenes as ligands in transition metal chemistry and the catalytic uses of such transition metal complexes. Furthermore, NHCs can act as catalysts in their own right, and recent developments in this growing area of catalysis are described.

10 Carbenes: reactivity and catalysis

Since the first report of a stable carbene by Arduengo in 1991, the use of N-heterocyclic carbenes (NHC) in organic synthesis has increased dramatically. This is principally due to their unique properties. NHC are known to form strong bonds to late transition metals since they are excellent σ-donors and possess (in general) minimal π-back donation capabilities. In view of this, NHCs have rapidly been used as a new class of ligand, capable of mimicking the well-known phosphines without the classical problems associated with phosphine use. The recent use of NHC is not restricted to catalysis using late transition metals, but they are also used as organic catalysts or as reactants in multi-component reactions, as is described in later sections of this report. Studies have shown that nucleophilic N-heterocyclic carbenes represent a very versatile class of ligands due to their interesting steric and electronic properties. These ligands have several advantages over the commonly utilized phosphines: (i) stabilizing effect, (ii) high thermal stability, (iii) resistance to dissociation from the metal center. Therefore an excess of the ligand is not required in order to prevent aggregation of the catalyst to yield bulk metal. As a consequence, an increasing number of catalytic reactions make use of nucleophilic carbenes as catalyst modifiers. Specific examples include the use of metal–carbene complexes in hydrosilylation, Ru-catalysed furan synthesis and olefin metathesis. In this report, attention will be focused on the latest highlights in the field of use of carbenes. Undoubtedly, NHC ligands are currently important in a wide range of reactions and the reader will find here a summary of current applications.

Nucleophilic Carbenes: An Incredible Renaissance

Over thirty years have passed since the pioneering work of H.-W. Wanzlick on nucleophilic carbenes. A much investigated model is the imidazolidinylidene 1, which can be generated from dimer 2 and other substrates. The isolation of the stable carbene 3 by A. J. Arduengo III in 1991 sparked incredible developments: addition reactions with increases in the covalency and coordination of the carbene carbon atom dominate the chemistry of such molecules. Recently, Herrmann et al. reported on the use of carbenes as ligands for Pd catalysts.