Rhodium-catalyzed carbonylation of 3-acyloxy-1,4-enynes for the synthesis of cyclopentenones

Rh-Catalyzed [4 + 1] Reaction of Cyclopropyl-Capped Dienes (but not Common Dienes) and Carbon Monoxide: Reaction Development and Mechanistic Study

Transition-metal-catalyzed [4 + 1] reaction of dienes and carbon monoxide (CO) is the most straightforward and easily envisioned cyclization for the synthesis of five-membered carbocycles, which are ubiquitously found in natural products and functional molecules. Unfortunately, no test of this reaction was reported, and consequently, chemists do not know whether such kind of reaction works or not. Herein, we report that the [4 + 1] reaction of common dienes and CO cannot work, at least under the catalysis of [Rh(cod)Cl]2. However, using cyclopropyl-capped dienes (also named allylidenecyclopropanes) as substrates, the corresponding [4 + 1] reaction with CO proceeds smoothly in the presence of [Rh(cod)Cl]2. This [4 + 1] reaction, with a broad scope, provides efficient access to five-membered carbocyclic compounds of spiro[2.4]hept-6-en-4-ones. The [4 + 1] cycloadducts can be further transformed into other molecules by using the unique chemistry of cyclopropyl groups present in these molecules. The mechanism of this [4 + 1] reaction has been investigated by quantum chemical calculations, uncovering that cyclopropyl-capped dienes are strained dienes and the oxidative cyclization step in the [4 + 1] catalytic cycle can release this (angular) strain both kinetically and thermodynamically. The strain release in this step then propagates to all followed CO coordination/CO insertion/reductive elimination steps in the [4 + 1] catalytic cycle, helping the realization of this cycloaddition reaction. In contrast, common dienes (including cyclobutyl-capped dienes) do not have such advantages and their [4 + 1] reaction suffers from energy penalty in all steps involved in the [4 + 1] catalytic cycle. The reactivity of ene-allenes for the [4 + 1] reaction with CO is also discussed.

Why [4 + 2 + 1] but Not [2 + 2 + 1]? Why Allenes? A Mechanistic Study of the Rhodium-Catalyzed [4 + 2 + 1] Cycloaddition of In Situ Generated Ene-Ene-Allenes and Carbon Monoxide

Transition metal-catalyzed [4 + 2 + 1] cycloaddition of in situ generated ene/yne-ene-allenes (from ene/yne-ene propargyl esters) and carbon monoxide (CO) gives the [4 + 2 + 1] cycloadducts rather than [2 + 2 + 1] cycloadducts. Investigating the mechanism of this [4 + 2 + 1] reaction and understanding why the [2 + 2 + 1] reaction does not compete and the role of the allene moiety in the substrates are important. This is also helpful to guide the future design of new [4 + 2 + 1] cycloadditions. Reported here are the kinetic and computed studies of the [4 + 2 + 1] reactions of ene-ene propargyl esters and CO. A quantum chemical study (at the DLPNO-CCSD(T)//BMK level) revealed that the [4 + 2 + 1] reaction includes four key steps, which are 1,3-acyloxy migration (rate-determining step), oxidative cyclization, CO migratory insertion, and reductive elimination. The allene moiety in the substrates is critical for providing additional coordination to the rhodium center in the final step of the catalytic cycle, which in turn favors the reductive elimination transition state in the [4 + 2 + 1] rather than in the [2 + 2 + 1] pathway. The CO insertion step in the [4 + 2 + 1] reaction, which could occur through either the UP (favored here) or DOWN CO insertion pathway, has also been deeply scrutinized, and some guidance from this analysis has been provided to help the future design of new [4 + 2 + 1] reactions. Quantum chemical calculations have also been applied to explain why [4 + 2] and [4 + 1] cycloadditions do not happen and how trienes as side products for some substrates are generated.

Rhodium(I)-Catalyzed Three-Component [4+2+1] Cycloaddition of Two Vinylallenes and CO

Transition metal-catalyzed [4+2+1] reactions of dienes (or diene derivatives such as vinylallenes), alkynes/alkenes, and CO (or carbenes) are expected to be the most straightforward approach to synthesize challenging seven-membered ring compounds, but so far only limited successes have been realized. Here, an unexpected three-component [4+2+1] reaction between two vinylallenes and CO was discovered to give highly functionalized tropone derivatives under mild conditions, where one vinylallene acts as a C₄ synthon, the other vinylallene as a C₂ synthon, and CO as a C₁ synthon. It was proposed that this reaction occurred via oxidative cyclization of the diene part of one vinylallene molecule, followed by insertion of the terminal alkene part of the allene moiety in another vinylallene, into the Rh-C bond of five-membered rhodacycle. Then, CO insertion and reductive elimination gave the [4+2+1] cycloadduct. Further experimental exploration of why ene/yne-vinylallenes and CO gave monocyclic tropone derivatives instead of 6/7-bicyclic ring products were reported here.

Rhodium-Catalyzed (5 + 2) and (5 + 1) Cycloadditions Using 1,4-Enynes as Five-Carbon Building Blocks

Cycloaddition reactions are a hallmark in organic synthesis because they provide an efficient way to construct highly substituted carbo- and heterocycles found in natural products and pharmaceutical agents. Most cycloadditions occur under thermal or photochemical conditions, but transition-metal complexes can promote reactions that occur beyond these circumstances. Transition-metal complexation with alkynes, alkenes, allenes, or dienes often alters the reactivity of those π-systems and facilitates access to diverse cycloaddition products. This Account describes our efforts toward the design of novel five-carbon synthons for use in rhodium-catalyzed (5 + n) cycloadditions, which include 3-acyloxy-1,4-enynes (ACEs) for (5 + 1) and (5 + 2) cycloadditions and 3-hydroxy-1,4-enynes (HYEs) for (5 + 1) cycloadditions. Furthermore, this Account includes relevant computational information, mechanistic insights, and applications of these cycloadditions in the synthesis of various highly substituted carbo- and heterocycles. The (5 + n) cycloaddition reactions presented herein share the following common mechanistic features: the 1,2-migration of an acyloxy group in propargyl esters or the ionization of a hydroxyl group in propargylic alcohols, oxidative cyclization to form a metallacycle, insertion of the one- or two-carbon component, and reductive elimination to yield the final product. In conjunction with a cationic rhodium catalyst, we used ACEs for the intramolecular (5 + 2) cycloaddition with tethered alkynes, alkenes, and allenes. In some cases, an electron-deficient phosphine ligand improved the reaction yields, especially when the ACE featured an internal alkyne. We also demonstrated that chirality could be efficiently transferred from a relatively simple starting material to a more complex bicyclic product. Products derived from ACEs with tethered alkenes and allenes contained one or more stereocenters, and high diastereoselectivity was achieved in most of these cases. For ACEs tethered to an allene, the reaction preferentially occurred at the internal alkene. We also switched the positions of the alkene and the alkyne in the 1,4-enyne of our original ACE to provide an inverted ACE variant, which produced products with complementary functionalities. After we successfully developed the Rh-catalyzed intramolecular (5 + 2) cycloaddition, we optimized conditions for the intermolecular version, which required a neutral rhodium catalyst and phosphine ligand. When a terminal alkyne was used as the two-carbon component, high regioselectivity was observed. While investigating the effect of esters on the rate of the intermolecular (5 + 2) cycloadditions, we determined that an electron-rich ester significantly accelerated the reaction. Subsequently, we demonstrated that (5 + 1) cycloadditions undergo this rate enhancement as well in the presence of an ester. Aside from ACEs, we synthesized HYEs in four steps from commercially available 2-aminobenzoic acid for use in the (5 + 1) cycloaddition. Mechanistically, HYEs were designed so that the aniline nitrogen could serve as the nucleophile and the -OH could serve as the leaving group. Using HYEs, we developed a novel method to make substituted carbazoles, dibenzofurans, and tricyclic compounds with a cyclohexadienone moiety. Although the occurrence of transition-metal-catalyzed acyloxy migrations has been known for decades, only recently has their synthetic value been realized. We hope our studies that employ readily available 1,4-enynes as the five-carbon components in (5 + n) cycloadditions can inspire the design of new two-component and multicomponent cycloadditions.

Triflic Acid-Catalyzed Enynes Cyclization: A New Strategy beyond Electrophilic π-Activation

The cyclization of enynes, catalyzed by a transition metal, represents a powerful tool to construct an array of cyclic compounds through electrophilic π-activation. In this paper, we disclose a new and efficient strategy for enynes cyclization catalyzed by triflic acid. The salient features of this transformation includes a broad substrate scope, metal free synthesis, open flask and mild conditions, good yields, ease of operation, low catalyst loading, and easy scale-up to gram scale. A preliminary mechanism study demonstrated that the activation model of the reaction was σ-activation, which is different from the transition-metal-catalyzed enynes cyclization. Our strategy affords a complementary method to the traditional strategies, which use transition-metal catalysts.

Rhodium(i)-catalysed intermolecular alkyne insertion into (2-pyridylmethylene)cyclobutenes

Multiply substituted benzenes were prepared by rhodium(i)-catalysed reaction of (2-pyridylmethylene)cyclobutenes with alkynes through an intermolecular alkyne insertion into the carbon–carbon bond of cyclobutenes.

Rhodium-Catalyzed Stereoselective Intramolecular [5 + 2] Cycloaddition of 3-Acyloxy 1,4-Enyne and Alkene

The first rhodium-catalyzed intramolecular [5 + 2] cycloaddition of 3-acyloxy 1,4-enyne and alkene was developed. The cycloaddition is highly diastereoselective in most cases. Various cis-fused bicyclo[5.3.0]decadienes were prepared stereoselectively. The chirality in the propargylic ester starting materials could be transferred to the bicyclic products with high efficiency. Electron-deficient phosphine ligand greatly facilitated the cycloaddition. Up to three new stereogenic centers could be generated. The resulting diene in the products could be hydrolyzed to enones, which allowed the introduction of more functional groups to the seven-membered ring.

Gold-catalyzed 1,2-acyloxy migration/intramolecular [3+2] 1,3-dipolar cycloaddtion cascade reaction: an efficient strategy for syntheses of medium-sized-ring ethers and amines

A highly efficient strategy for the formation of medium-sized-ring ethers and amines based on a gold-catalyzed cascade reaction, involving enynyl ester isomerization and intramolecular [3+2] cyclization, has been developed. Various multisubstituted medium-sized-ring unsaturated ethers and amines were obtained through this transformation. This method represents one of the relatively few transition metal catalyzed intramolecular cycloaddition reactions for medium-sized ring synthesis.

Catalyst-dependent divergent synthesis of pyrroles from 3-alkynyl imine derivatives: a noncarbonylative and carbonylative approach

A novel Ru(0)- and Rh(I)-catalyzed noncarbonylative and carbonylative cycloisomerization of readily available 3-alkynyl imine derivatives has been developed to provide 3,4-fused or nonfused pyrrole derivatives efficiently in moderate to excellent yields. The key steps involve the formation of a ruthenium carbenoid intermediate or a rhodacycle intermediate, respectively. In these reactions, CO can serve as a ligand or a reagent.

Rhodium-catalyzed acyloxy migration of propargylic esters in cycloadditions, inspiration from the recent "gold rush"

Transition metal-catalyzed acyloxy migration of propargylic esters offers versatile entries to allene and vinyl carbene intermediates for various fascinating subsequent transformations. Most π-acidic metals (e.g. gold and platinum) are capable of facilitating these acyloxy migration events. However, very few of these processes involve redox chemistry, which are well-known for most other transition metals such as rhodium. The coupling of acyloxy migration of propargylic esters with oxidative addition, migratory insertion, and reductive elimination may lead to ample new opportunities for the design of new reactions. This tutorial review summarizes recent developments in Rh-catalyzed 1,3- and 1,2-acyloxy migration of propargylic esters in a number of cycloaddition reactions. Related Au- and Pt-catalyzed cycloadditions involving acyloxy migration are also discussed.

Rhodium-catalyzed carbonylation of cyclopropyl substituted propargyl esters: a tandem 1,3-acyloxy migration [5 + 1] cycloaddition

We have developed two different types of tandem reactions for the synthesis of highly functionalized cyclohexenones from cyclopropyl substituted propargyl esters. Both reactions were initiated by rhodium-catalyzed Saucy-Marbet 1,3-acyloxy migration. The resulting cyclopropyl substituted allenes derived from acyloxy migration then underwent [5 + 1] cycloaddition with CO. The acyloxy group not only eased the access to allene intermediates but also provided a handle for further selective functionalizations.