Strain-Release-Controlled [4 + 2 + 1] Reaction of Cyclopropyl-Capped Diene-ynes/Diene-enes and Carbon Monoxide Catalyzed by Rhodium

Achieving transition-metal-catalyzed reactions of diene-ynes/diene-enes and carbon monoxide (CO) to deliver [4 + 2 + 1] cycloadducts, rather than the kinetically favored [2 + 2 + 1] products, is challenging. Here, we report that this can be solved by adding a cyclopropyl (CP) cap to the diene moiety of the original substrates. The resulting CP-capped diene-ynes/diene-enes can react with CO under Rh catalysis to give [4 + 2 + 1] cycloadducts exclusively without forming [2 + 2 + 1] products. This reaction has a broad scope and can be used to synthesize useful 5/7 bicycles with a CP moiety. Of the same importance, the CP moiety in the [4 + 2 + 1] cycloadducts can act as an intermediate group for further transformations so that other challenging bicyclic 5/7 and tricyclic 5/7/5, 5/7/6, and 5/7/7 skeletons, some of which are widely found in natural products, can be accessed. The mechanism of this [4 + 2 + 1] reaction has been investigated by quantum chemical calculations, and the role of the CP group in avoiding the possible side [2 + 2 + 1] reaction has been identified, showing that the [4 + 2 + 1] is controlled by releasing the ring strain in the methylenecyclopropyl (MCP) group (about 7 kcal/mol) in the CP-capped dienes.

C-H Activation by Isolable Cationic Bis(phosphine) Cobalt(III) Metallacycles

Five- and six-coordinate cationic bis(phosphine) cobalt(III) metallacycle complexes were synthesized with the general structures, [(depe)Co(cycloneophyl)(L)(L')][BAr^F₄] (depe = 1,2-bis(diethylphosphino)ethane; cycloneophyl = [κ-C:C-(CH₂C(Me)₂)C₆H₄]; L/L' = pyridine, pivalonitrile, or the vacant site, BAr₄^F = B[(3,5-(CF₃)₂)C₆H₃]₄). Each of these compounds promoted facile directed C(sp²)-H activation with exclusive selectivity for ortho-alkylated products, consistent with the selectivity of reported cobalt-catalyzed arene-alkene-alkyne coupling reactions. The direct observation of C-H activation by cobalt(III) metallacycles provided experimental support for the intermediacy of these compounds in this class of catalytic C-H functionalization reaction. Deuterium labeling and kinetic studies provided insight into the nature of C-H bond cleavage and C-C bond reductive elimination from isolable cobalt(III) precursors.

C–Cl Oxidative Addition and C–C Reductive Elimination Reactions in the Context of the Rhodium-Promoted Direct Arylation

A cycle of stoichiometric elemental reactions defining the direct arylation promoted by a redox-pair Rh(I)–Rh(III) is reported. Starting from the rhodium(I)-aryl complex RhPh{κ³-P,O,P-[xant(PⁱPr₂)₂]} (xant(PⁱPr₂)₂ = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene), the reactions include C–Cl oxidative addition of organic chlorides, halide abstraction from the resulting six-coordinate rhodium(III) derivatives, C–C reductive coupling between the initial aryl ligand and the added organic group, oxidative addition of a C–H bond of a new arene, and deprotonation of the generated hydride-rhodium(III)-aryl species to form a new rhodium(I)-aryl derivative. In this context, the kinetics of the oxidative additions of 2-chloropyridine, chlorobenzene, benzyl chloride, and dichloromethane to RhPh{κ³-P,O,P-[xant(PⁱPr₂)₂]} and the C–C reductive eliminations of biphenyl and benzylbenzene from [RhPh₂{κ³-P,O,P-[xant(PⁱPr₂)₂]}]BF₄ and [RhPh(CH₂Ph){κ³-P,O,P-[xant(PⁱPr₂)₂]}]BF₄, respectively, have been studied. The oxidative additions generally involve the cis addition of the C–Cl bond of the organic chloride to the rhodium(I) complex, being kinetically controlled by the C–Cl bond dissociation energy; the weakest C–Cl bond is faster added. The C–C reductive elimination is kinetically governed by the dissociation energy of the formed bond. The C(sp³)–C(sp²) coupling to give benzylbenzene is faster than the C(sp²)–C(sp²) bond formation to afford biphenyl. In spite of that a most demanding orientation requirement is needed for the C(sp³)–C(sp²) coupling than for the C(sp²)–C(sp²) bond formation, the energetic effort for the pregeneration of the C(sp³)–C(sp²) bond is lower. As a result, the weakest C–C bond is formed faster.

DFT perspective toward [3 + 2] annulation reaction of enals with α-ketoamides through NHC and Brønsted acid cooperative catalysis: mechanism, stereoselectivity, and role of NHC

The DFT perspective toward the [3 + 2] annulation reaction through NHC and Brønsted acid cooperative catalysis has been investigated.