Metal carbenes usually possess versatile reactivities that are controlled by the presence of both the metals and the ligands. Diazo compounds are commonly used for the generation of such species through elimination of nitrogen. However, they are often unstable, explosive, and toxic, which limits their applications in large-scale syntheses. Thus, identifying sustainable and safe surrogates for the generation of metal carbenes has attracted great attention. In this Review, we summarize some of the most important breakthroughs in the generation, catalytic reactions, and selectivity control of metal carbenes from non-diazo starting compounds.

New Approaches to the Synthesis of Metal Carbenes

https://web.pkusz.edu.cn/huang/files/2013/04/Traceless-Directing-Strategy-Efficient-Synthesis-of-N-Alkyl-Indoles-via-Redox-Neutral-C–H-Activation.pdf

Traceless directing strategy: efficient synthesis of N-alkyl indoles via redox-neutral C-H activation.

ConspectusSo far, over 150 natural products and pharmaceuticals containing an allene moiety have been identified. During the last two decades, allenes have also been demonstrated as synthetically v...

Allenation of Terminal Alkynes with Aldehydes and Ketones.

Platinum-catalyzed formal [5+2] and [4+2] annulations of isoxazoles with heterosubstituted alkynes enabled the atom-economical synthesis of valuable 1,3-oxazepines and 2,5-dihydropyridines, respectively. Importantly, this Pt catalysis not only led to unique reactivity dramatically divergent from that observed under Au catalysis, but also proceeded via unprecedented α-imino platinum carbene intermediates.

Highly Site Selective Formal [5+2] and [4+2] Annulations of Isoxazoles with Heterosubstituted Alkynes by Platinum Catalysis: Rapid Access to Functionalized 1,3-Oxazepines and 2,5-Dihydropyridines.

The catalyst-directed divergent synthesis, commonly termed as "divergent catalysis", has emerged as a promising technique as it allows chartering of structurally distinct products from common substrates simply by modulating the catalyst system. In this regard, gold complexes emerged as powerful catalysts as they offer unique reactivity profiles as compared to various other transition metal catalysts, primarily due to their salient electronic and geometrical features. Owing to the tunable soft π-acidic nature, gold catalysts not only evolved as superior contenders for catalyzing the reactions of alkynes, alkenes, and allenes but also, more intriguingly, have been found to provide divergent reaction pathways over other π-acid catalysts such as Ag, Pt, Pd, Rh, Cu, In, Sc, Hg, Zn, etc. The recent past has witnessed a renaissance in such examples wherein, by choosing gold catalysts over other transition metal catalysts or by fine-tuning the ligands, counteranions or oxidation states of the gold catalyst itself, a complete reactivity switch was observed. However, reviews documenting such examples are sporadic; as a result, most of the reports of this kind remained scattered in the literature, thereby hampering further development of this burgeoning field. By conceptualizing the idea of "Divergent Gold Catalysis (DGC)", this review aims to consolidate all such reports and provide a unified approach necessary to pave the way for further advancement of this exciting area. Based on the factors governing the divergence in product formation, an explicit classification of DGC has been provided. To gain a fundamental understanding of the divergence in observed reactivities and selectivities, the review is accompanied by mechanistic insights at appropriate places.

Divergent Gold Catalysis: Unlocking Molecular Diversity through Catalyst Control.

A catalytic asymmetric House-Meinwald rearrangement for the synthesis of both cyclic and acyclic ketones is disclosed. From readily accessible racemic tetrasubstituted epoxides, this approach provides efficient access to chiral ketones bearing α all-carbon quaternary stereocenters with high enantiocontrol. The observation of positive nonlinear effects and nontrivial kinetic feature provided important insights into the mechanism.

Catalytic Enantioselective House-Meinwald Rearrangement: Efficient Construction of All-Carbon Quaternary Stereocenters.

Naturally occurring carbazole alkaloids such as isomukonidine, clausine L, mukonidine, glycosinine, mukonal, and clausine V have been synthesized through a gold-catalyzed cyclization reaction of 1-(indol-2-yl)-2-methoxy-2,3-allenols, which are readily available from indolecarbaldehydes and methoxypropadiene. Compared to the reported procedures, our approach is general, atom economic, highly selective, and the starting materials are easily available.

An efficient Au-catalyzed synthesis of isomukonidine, clausine L, mukonidine, glycosinine, mukonal, and clausine V from propadienyl methyl ether

An AuCl(PPh3)/AgBF4- and PtCl4-catalyzed reaction of 1-(indol-2-yl)-3-alkyn-1-ols occurred smoothly in toluene to form a series of differently polysubstituted carbazole derivatives efficiently. The regioselectivity of the 1,2-migration may be tuned by using different metal catalysts: carbazoles 3 could be obtained exclusively in the presence of AuCl(PPh3)/AgBF4via a Wagner–Meerwein type 1,2-alkyl shift, whereas in some cases the use of PtCl4 afforded differently substituted carbazoles 4 involving a platinum–carbene intermediate.

Regiocontrolled 1,2-migration in cyclization of 1-(indol-2-yl)-3-alkyn-1-ols: (Ph3P)Au+vs. PtCl4

Herein, a facile and general electroreductive deuteration of unactivated alkyl halides (X = Cl, Br, I) or pseudo-halides (X = OMs) using D2O as the economical deuterium source was reported. In addition to primary and secondary alkyl halides, sterically hindered tertiary chlorides also work very well, affording the target deuterodehalogenated products with excellent efficiency and deuterium incorporation. More than 60 examples are provided, including late-stage dehalogenative deuteration of natural products, pharmaceuticals, and their derivatives, all with excellent deuterium incorporation (up to 99% D), demonstrating the potential utility of the developed method in organic synthesis. Furthermore, the method does not require external catalysts and tolerates high current, showing possible use in industrial applications. 

/pdf/facile-and-general-electrochemical-deuteration-of-2zx3yc34.pdf

Facile and general electrochemical deuteration of unactivated alkyl halides

Described herein is a highly regioselective and efficient N-heterocyclic-carbene-catalyzed γ-trifluoromethylation of vinylogous enolates. Control experiments and DFT calculations provided important insight into the reaction mechanism.

Dengke Ma

Papers

Catalytic Enantioselective House-Meinwald Rearrangement: Efficient Construction of All-Carbon Quaternary Stereocenters.

An efficient Au-catalyzed synthesis of isomukonidine, clausine L, mukonidine, glycosinine, mukonal, and clausine V from propadienyl methyl ether

Regiocontrolled 1,2-migration in cyclization of 1-(indol-2-yl)-3-alkyn-1-ols: (Ph3P)Au+vs. PtCl4

Facile and general electrochemical deuteration of unactivated alkyl halides

NHC‐Catalyzed Electrophilic Trifluoromethylation: Efficient Synthesis of γ‐Trifluoromethyl α,β‐Unsaturated Esters