N-Alkoxyimidazolylidines (NOHCs): Nucleophilic carbenes based on oxidized imidazolium core

We report the first preparation of a N-alkoxyimidazolylidene (NOHC), a nucleophilic carbene based on an oxidized imidazolium core. The carbene carbon shows the highest upfield 13C NMR shift compared to...

Photochemical [2+1]-Cycloadditions of Nucleophilic Carbenes

Visible light induced singlet nucleophilic carbene intermediates undergo rapid [2+1]-cycloaddition with tethered olefins to afford unique bicyclo[3.1.0]hexane and bicyclo[4.1.0]heptane scaffolds. This cyclopropanation proceeds using only visible light irradiation, circumventing the use of exogenous (photo)catalysts or sensitisers and showcases an underexplored mode of reactivity for nucleophilic carbenes in chemical synthesis. The discovery of additional transformations including a cyclopropanation/retro-Michael/Michael cascade reaction to afford chromanone derivatives are also described.

Carbene Character in a Series of Neutral PCcarbeneP Cobalt(I) Complexes – Radical Carbenes Versus Nucleophilic Carbenes

Cobalt(I) complexes supported by a series of PCcarbeneP pincer ligands of varying donicity, differing in the aryl group linking the phosphine arms with the anchoring carbon donor, are described. Addition of the proligands to cobalt bromide results in the formation of a series of cobalt(II) tetrahedral complexes, Ln-1, which serve as excellent precur-sors to the corresponding PCalkylP and PCcarbeneP complexes. Square planar cobalt PCcarbeneP complexes, L2R-3-X (X = Cl, Br), are readily synthesized by addition of a bulky aryloxide radical to the corresponding PCalkylP complex, L1-2-Br or via addition of L2R to ClCo(PPh3)3 in the presence of trityl radical or by addition of NaHBEt3 and trityl radical to iso-lated L2R-1. For the L2NMe2 PCcarbeneP complexes, salt metathesis reactions with either CsOH·H2O, LiCH2TMS, or LiNH2 result in the corresponding hydroxo, alkyl, and amine complexes, L2NMe2-3-R (R = OH, CH2TMS, NH2). Reaction of L2NMe2-3-OH with benzoic acid affords the 2-O2CPh derivative The nature of the carbene bond in either ligand plat-form as well as the effects of the X-type capping ligand on the Co=C bond are explored computationally and show that triplet structures are relatively more stable in for the less electron donating ligand L1 while singlet Co(I) carbenes dominate for the more electron rich L2 derivatives. For L2NMe2 complexes, the effect of the trans ligand X was also probed. Pi donors imbue the carbene with singlet character while the strongly  donating alkyl derivative exhibits significant triplet character.

A DFT Study on the Molecular Mechanism of Additions of Electrophilic and Nucleophilic Carbenes to Non-Enolizable Cycloaliphatic Thioketones

The molecular mechanisms of addition of dihalocarbenes and dimethoxycarbene to thioketones derived from 2,2,4,4-tetrmethylcyclobutane-1,3-dione were examined on the basis of the DFT wb97xd/6-311g(d,p)(PCM) calculations. Obtained results demonstrated that the examined processes exhibit polar nature and in the case of electrophilic dichloro-, and dibromocarbenes are initiated by the attack of carbene species onto the sulfur atom of the C=S group. Remarkably, reactions involving more electrophilic carbenes (dichloro-, and dibromocarbene) proceeds via stepwise mechanism involving thiocarbonyl ylide as a transient intermediate. In contrast, analogous reactions with nucleophilic dimethoxycarbene occur via a single step reaction, which can be considered as the [2 + 1] cycloaddition reaction initiated by the attack onto the C=S bond. A computational study showed that difluorocarbene tends to react as a nucleophilic species and resembles rather dimethoxycarbene and not typical dihalocarbene species. Significantly higher reactivity of the thioketone unit in comparison to the ketone group, both present in 3-thioxo-2,2,4,4-tetramthylcyclobutanone molecule, was rationalized in the light of DFT computational study.

Diastereoselective Palladium-Catalyzed [4+1] Cycloadditions of Vinyl Benzoxazinanones with in situ Formed Carbenes

Herein, we present a palladium-catalyzed tandem [4+1] cycloadditions of vinyl benzoxazinanones with N-tosylhydrazones. The cycloaddition is accomplished by merging the in situ generated aza-ortho-quinone methides (aza-o-QMs) with nucleophilic carbenes. This method enables the construction of diverse indolines with broad functional group compatibility in good yields with high levels of diastereoselectivity under mild conditions.

Visible-Light Induced Singlet Nucleophilic Carbenes: Rapid and Mild Access to Fluorinated Tertiary Alcohol Derivatives

Singlet nucleophilic carbenes (SNCs) that contain only one heteroatom donor remain underexplored and underutilized in chemical synthesis. To discover new synthetic strategies that harness these SNCs as reactive intermediates, aromatic or aliphatic siloxy carbenes represent excellent model substrates as they can be readily generated photochemically from stable acyl silane precursors. We herein report the discovery that photochemically generated siloxy carbenes undergo 1,2-carbonyl addition to trifluoromethyl ketones, followed by a silyl transfer process to afford benzoin-type products. This new transformation is a rare example of the use of ketones as trapping reagents for SNC intermediates and delivers an efficient, user-friendly and scalable process to access fluorinated tertiary alcohol derivatives driven by only light, circumventing the use of catalysts or additives.

Visible-Light Induced Singlet Nucleophilic Carbenes: Rapid and Mild Access to Fluorinated Tertiary Alcohol Derivatives

Singlet nucleophilic carbenes (SNCs) that contain only one heteroatom donor remain underexplored and underutilized in chemical synthesis. To discover new synthetic strategies that harness these SNCs as reactive intermediates, aromatic or aliphatic siloxy carbenes represent excellent model substrates as they can be readily generated photochemically from stable acyl silane precursors. We herein report the discovery that photochemically generated siloxy carbenes undergo 1,2-carbonyl addition to trifluoromethyl ketones, followed by a silyl transfer process to afford benzoin-type products. This new transformation is a rare example of the use of ketones as trapping reagents for SNC intermediates and delivers an efficient, user-friendly and scalable process to access fluorinated tertiary alcohol derivatives driven by only light, circumventing the use of catalysts or additives.