Synthetic Applications of Vinyl Ruthenium Carbenes Derived from Diazoalkanes and Alkynes

Recent advances in transition-metal-catalyzed Büchner reaction of alkynes

Medium-sized ring-containing organic molecules, especially seven-membered rings, are significant structural motifs. However, such frameworks are considered difficult structures to access owing to entropic effects and transannular interactions. Compared to the construction of five and six-membered rings, the synthesis of seven-membered rings can be more challenging through traditional cyclization pathways. Büchner reactions are particularly attractive and efficient synthetic strategies to construct functionalized seven-membered ring products from the benzenoid double bond with carbene. In recent years, the field of transition-metal-catalyzed Büchner ring expansion reactions of alkynes has experienced a speedy development and a diverse array of efficient synthetic procedures have been disclosed under mild experimental conditions, as the synthesis of synthetically challenging seven-membered rings is easily achieved. In this review, we will focus on the recent progress in the transition-metal-catalyzed Büchner reaction of alkynes and the mechanistic rationale is depicted where possible, with the reactions being sorted according to the type of catalyst.

Recent Advances in Catalytic Alkyne Transformation via Copper Carbene Intermediates

As one of the abundant and inexpensive metals on the earth, copper has demonstrated broad applications in synthetic chemistry and catalysis. Among these copper-catalyzed advances, copper carbenes are versatile and reactive intermediates that can mediate a variety of transformations, which have attracted much attention in the past decades. The present review summarizes two different reaction models that take place between a copper carbene intermediate and alkyne species, including the cross-coupling reaction of copper carbene intermediate with terminal alkyne, and the addition of copper carbene intermediate onto the C-C triple bond. This article will cover the profile from 2010 to 2021 by placing emphasis on the detailed catalytic models and highlighting the synthetic applications offered by these practical and mild methods.

C-H Insertion via Ruthenium Catalyzed gem-Hydrogenation of 1,3-Enynes

gem-Hydrogenation of an internal alkyne with the aid of [Cp*RuCl]₄ as the precatalyst is a highly unorthodox transformation, in which one C atom of the triple bond is transformed into a methylene group, whereas the second C atom gets converted into a ruthenium carbene. In the case of 1,3-enynes bearing a propargylic steering substituent as the substrates, the reaction occurs regioselectively, giving rise to vinyl carbene complexes that adopt interconverting η¹/η³-binding modes in solution; a prototypical example of such a reactive intermediate was characterized in detail by spectroscopic means. Although both forms are similarly stable, only the η³-vinyl carbene proved kinetically competent to insert into primary, secondary, or tertiary C–H bonds on the steering group itself or another suitably placed ether, acetal, orthoester, or (sulfon)amide substituent. The ensuing net hydrogenative C–H insertion reaction is highly enabling in that it gives ready access to spirocyclic as well as bridged ring systems of immediate relevance as building blocks for medicinal chemistry. Moreover, the reaction scales well and lends itself to the formation of partly or fully deuterated isotopologues. Labeling experiments in combination with PHIP NMR spectroscopy (PHIP = parahydrogen induced polarization) confirmed that the reactions are indeed triggered by gem-hydrogenation, whereas kinetic data provided valuable insights into the very nature of the turnover-limiting transition state of the actual C–H insertion step.

Recent progress towards the transition-metal-catalyzed Nazarov cyclization of alkynes via metal carbenes

In recent years, transition-metal-catalyzed tandem cyclization reactions of alkynes, especially those involving a metal carbene intermediate, have received worthwhile interest, as this type of reaction does not require the use of risky and potentially explosive diazo compounds as starting materials for carbene generation. A significant and general strategy for the stereospecific synthesis of 5-membered cycles is Nazarov cyclization based on the 4π-conrotatory electrocyclization of a conjugated pentadienyl cation to afford a cyclopentenyl cation. In this review, we introduce an overview of recent advances in the transition-metal-catalyzed Nazarov cyclization of alkynes via a metal carbene intermediate, and categorize these reactions according to the structure of the metal carbene. Our aim is to accelerate advancements in this enchanting area of research.

Hydrogenative Metathesis of Enynes via Piano-Stool Ruthenium Carbene Complexes Formed by Alkyne gem-Hydrogenation

The only recently discovered gem-hydrogenation of internal alkynes is a fundamentally new transformation, in which both H atoms of dihydrogen are transferred to the same C atom of a triple bond while the other position transforms into a discrete metal carbene complex. [Cp*RuCl]4 is presently the catalyst of choice: the resulting piano-stool ruthenium carbenes can engage a tethered alkene into either cyclopropanation or metathesis, and a prototypical example of such a reactive intermediate with an olefin ligated to the ruthenium center has been isolated and characterized by X-ray diffraction. It is the substitution pattern of the olefin that determines whether metathesis or cyclopropanation takes place: a systematic survey using alkenes of largely different character in combination with a computational study of the mechanism at the local coupled cluster level of theory allowed the preparative results to be sorted and an intuitive model with predictive power to be proposed. This model links the course of the reaction to the polarization of the double bond as well as to the stability of the secondary carbene complex formed, if metathesis were to take place. The first application of "hydrogenative metathesis" to the total synthesis of sinularones E and F concurred with this interpretation and allowed the proposed structure of these marine natural products to be confirmed. During this synthesis, it was found that gem-hydrogenation also provides opportunities for C-H functionalization. Moreover, silylated alkynes are shown to participate well in hydrogenative metathesis, which opens a new entry into valuable allylsilane building blocks. Crystallographic evidence suggests that the polarized [Ru-Cl] bond of the catalyst interacts with the neighboring R3Si group. Since attractive interligand Cl/R3Si contacts had already previously been invoked to explain the outcome of various ruthenium-catalyzed reactions, including trans-hydrosilylation, the experimental confirmation provided herein has implications beyond the present case.

Cu(I)-Catalyzed Oxidative Cyclization of Enynamides: Regioselective Access to Cyclopentadiene Frameworks and 2-Aminofurans

An efficient Cu(I)-catalyzed oxidative cyclization of alkynyl-tethered enynamides for the construction of fused bicyclic cyclopentadiene derivatives is disclosed. The cascade proceeds through alkyne oxidation, carbene/alkyne metathesis, and formal (3 + 2) cycloaddition. Employing aryl-tethered enynamides as starting materials, substituted 2-aminofurans can be exclusively formed.

Rhodium-Catalyzed Formal C-O Insertion in Carbene/Alkyne Metathesis Reactions: Synthesis of 3-Substituted 3 H-Indol-3-ols

An efficient and novel rhodium-catalyzed formal C-O insertion reaction of alkyne-tethered diazo compounds for the synthesis of 3 H-indol-3-ols is described. A type of donor/donor rhodium carbene generated in situ via a carbene/alkyne metathesis (CAM) process is the key intermediate and terminates in a unique transformation different from donor/acceptor carbenoids. In addition, ¹⁸O-labeling experiments indicate that intramolecular oxygen-atom transfer from the amide group to the carbon-carbon triple bond occurs during this transformation.