Rhodium‐Catalyzed [3+2+2] and [2+2+2] Cycloadditions of Two Alkynes with Cyclopropylideneacetamides

It has been established that a cationic rhodium(I)/H₈‐binap complex catalyzes the [3+2+2] cycloaddition of 1,6‐diynes with cyclopropylideneacetamides to produce cycloheptadiene derivatives through cleavage of cyclopropane rings. In contrast, a cationic rhodium(I)/(S)‐binap complex catalyzes the enantioselective [2+2+2] cycloaddition of terminal alkynes, acetylenedicarboxylates, and cyclopropylideneacetamides to produce spiro‐cyclohexadiene derivatives which retain the cyclopropane rings.     

Divergent Reactivity of Rhodium(I) Carbenes Derived from Indole Annulations

Rhodium(I) carbenes were generated from propargylic alcohol derivatives as the result of a dehydrative indole annulation. Depending on the choice of the electron‐withdrawing group on the aniline nitrogen nucleophile, either a cyclopropanation product or dimerization product was obtained chemoselectively. Intramolecular hydroamidation occurred for the same type of propargylic alcohol derivatives when other transition‐metal catalysts were employed.     

The Divergent Synthesis of Nitrogen Heterocycles by Rhodium(II)‐Catalyzed Cycloadditions of 1‐Sulfonyl 1,2,3‐Triazoles with 1,3‐Dienes

The first rhodium(II)‐catalyzed aza‐[4+3] cycloadditions of 1‐sulfonyl 1,2,3‐triazoles with 1,3‐dienes have been developed, and enable the efficient synthesis of highly functionalized 2,5‐dihydroazepines from readily available precursors. In some cases, the reaction pathway could divert to formal aza‐[3+2] cycloadditions, thus leading to 2,3‐dihydropyrroles. In this context, the titled reaction represents a capable tool for the divergent synthesis of two types of synthetically valuable aza‐heterocycles from common rhodium(II) iminocarbene intermediates.     

Morita–Baylis–Hillman Reaction of β,β‐Disubstituted Enones: An Enantioselective Organocatalytic Approach for the Synthesis of Cyclopenta[b]annulated Arenes and Heteroarenes

The first enantioselective organocatalytic intramolecular Morita–Baylis–Hillman (MBH) reaction of sterically highly demanding β,β‐disubstituted enones is presented. The MBH reaction of β,β‐disubstituted‐α,β‐unsaturated electron‐withdrawing systems was previously considered to be unfeasible. Towards this end, designer substrates, which under simple and practical reaction conditions generate a variety of cyclopenta[b]annulated arenes and heteroarenes in excellent enantiopurities and near‐quantitative yields in remarkably short reaction times, are described. The reason for the unusually facile nature of this reaction is attributed to the synergy guided and entropically favored intramolecular reaction. Further, this strategy provides easy access to a substantial number of bioactive natural products and pharmaceutically significant compounds.     

Gold‐Catalyzed 1,2‐Oxoarylations of Nitriles with Pyridine‐Derived Oxides

We report the first success in the gold‐catalyzed oxoarylations of nitriles with pyridine‐derived N‐oxides using gold carbenes as initiators. These oxoarylations were also achieved satisfactorily in intermolecular three‐component oxidations, including diverse alkenyldiazo esters, nitriles, and pyridine‐based oxides.     

Rhodium‐Catalyzed Cyclization of O,ω‐Unsaturated Alkoxyamines: Formation of Oxygen‐Containing Heterocycles

O,ω‐Unsaturated N‐tosyl alkoxyamines undergo unexpected Rh^III‐catalyzed intramolecular cyclization by oxyamination to produce oxygen‐containing heterocycles. Mechanistic studies show that an aziridine intermediate seems to be responsible for the formation of the heterocycles, possibly via a Rh^V species.     

Rhodium‐Catalyzed Diastereoselective Cyclization of Allenyl‐Sulfonylcarbamates: A Stereodivergent Approach to 1,3‐Aminoalcohol Derivatives

A diastereoselective and stereodivergent rhodium‐catalyzed intramolecular coupling of sulfonylcarbamates with terminal allenes is described and it provides selective access to 1,3‐aminoalcohol derivatives, scaffolds found in bioactive compounds. The reaction is compatible with a large range of different functional groups, thus furnishing products with high diastereoselectivities and yields. Moreover, multigram scale reactions, as well as the application of suitable product transformations were demonstrated.     

Aniline Dearomatization and Silver‐Catalyzed [3+3] Dipolar Cycloaddition: Efficient Construction of Oxocino[4,3,2‐cd]indoles from 2‐Alkynylanilines and 2‐Alkynylbenzaldoximes

2‐Alkynylanilines are attractive starting materials in indole synthesis because of their ready availability. Herein, a one‐pot stepwise procedure is reported for efficient construction of multisubstituted oxocino[4,3,2‐cd]indoles from 2‐alkynylanilines and 2‐alkynylbenzaldoximes. The method comprises the oxidative dearomatization of 2‐alkynylanilines, the silver‐catalyzed [3+3] cycloaddition with 2‐alkynylbenzaldoximes, and subsequent thermal radical skeletal rearrangement and aromatization.     

Exploiting Distal Reactivity of Coumarins: A Rhodium‐Catalyzed Vinylogous Asymmetric Ring‐Opening Reaction

While the utility of vinylogous enolates is well established in the setting of vinylogous aldol, Mannich, and Michael chemistries, literature reports concerning γ‐reactivity are scarce for other reaction classes. Presented herein is an unprecedented example of vinylogous reactivity exemplified by the rhodium‐catalyzed asymmetric ring‐opening reaction of oxabicycles. This strategy also provides a powerful route to incorporate the biologically useful coumarin motif into the hydronapthalene scaffold.     

Rhodium(II)‐Catalyzed Annulation of Azavinyl Carbenes Through Ring‐Expansion of 1,3,5‐Trioxane: Rapid Access to Nine‐Membered 1,3,5,7‐Trioxazonines

The rhodium(II)‐catalyzed denitrogenative coupling of N‐alkylsulfonyl 1,2,3‐triazoles with 1,3,5‐trioxane led to nine‐membered‐ringed trioxazonines in moderate‐to‐good yields. 1,3,5‐Trioxane, acting as an oxygen nucleophile, reacted with the α‐aza‐vinylcarbene intermediate, giving rise to ylide formation, which was probably the key step in the reaction. Triazoles that contained aryl substituents with various electronic and steric features on the C4 carbon atom were well‐tolerated. The synthesis of trioxazonine derivatives was achieved through a one‐pot, two‐step procedure from 1‐mesylazide and a terminal alkyne by combining Cu^I‐catalyzed 1,3‐dipolar cycloaddition and rhodium‐catalyzed transformations.     

Enantioselective Intramolecular C−H Insertion of Donor and Donor/Donor Carbenes by a Nondiazo Approach

The first enantioselective intramolecular C?H insertion and cyclopropanation reactions of donor‐ and donor/donor‐carbenes by a nondiazo approach are reported. The reactions were conducted in a one‐pot manner without slow addition and provided the desired dihydroindole, dihydrobenzofuran, tetrahydrofuran, and tetrahydropyrrole derivatives with up to 99 % ee and 100 % atom efficiency.     

Rhodium Enalcarbenoids: Direct Synthesis of Indoles by Rhodium(II)‐Catalyzed [4+2] Benzannulation of Pyrroles

Disclosed herein is the design of an unprecedented electrophilic rhodium enalcarbenoid which results from rhodium(II)‐catalyzed decomposition of a new class of enaldiazo compounds. The synthetic utility of these enalcarbenoids has been successfully demonstrated in the first transition‐metal‐catalyzed [4+2] benzannulation of pyrroles, thus leading to substituted indoles. The new benzannulation has been applied to the efficient synthesis of the natural product leiocarpone as well as a potent adipocyte fatty‐acid binding protein inhibitor.     

Rhodium‐Catalyzed Tandem Cyclization/SiC Activation Reaction for the Synthesis of Siloles

Siloles represent an important emerging class of photoluminescent materials. Reported herein is a new synthetic strategy involving a tandem cyclization/Si? C activation reaction featuring high efficiency, wide substrate scope, and practical utility. This method enabled the first synthesis of benzofuran siloles as well as rapid access to conjugated siloles. During the course of the study we also uncovered an unusual yet general Si? C(sp²) activation in the presence of π acids.     

RhI‐Catalyzed Bimolecular Cyclization between Two Different 1,5‐Bisallenes: A Combinatorial One‐Step Approach to Heterosteroids and Mechanistic Implications

In this paper, a bimolecular‐cyclization reaction between two different bis(allene)s with at least one heteroatom as the tether under the catalysis of trans‐[RhCl(CO)(PPh₃)₂] is described. This protocol provides an efficient entry to different heterocyclic 18,19‐norsteroid‐like scaffolds. The tricyclic product was formed highly selectively from the cyclization reaction of bis(2,3‐butadienyl)sulfide with dimethyl 2‐bis(2′,3′‐butadienyl)malonate, which sheds light on the mechanism involving the metalla‐[4.3.0]‐bicyclic intermediate formed by the cyclometallation of the terminal and the internal C=C bonds of each of the two allene moieties in 2‐bis(2′,3′‐butadienyl)malonate.