RhI-Catalyzed [6+2] Cycloaddition of Alkyne-Allenylcyclobutanes: A New Entry for the Synthesis of Bicyclo[6.m.0] Skeletons

Rhodium-Catalyzed [7 + 1] Cycloaddition of Exocyclic 1,3-Dienylcyclopropanes and Carbon Monoxide

A rhodium-catalyzed [7 + 1] reaction of exocyclic 1,3-dienylcyclopropanes and carbon monoxide has been developed to synthesize eight-membered carbocycle-embedded bicyclic and tricyclic molecules. In addition, ab initio calculations were conducted to reveal the reaction mechanism.

Cleavage of Carbon-Carbon σ-Bonds of Four-Membered Rings

This article reviews synthetic transformations involving cleavage of a carbon-carbon bond of a four-membered ring, with a particular focus on the examples reported during the period from 2011 to the end of 2019. Most significant is the progress of catalytic reactions involving oxidative addition of carbon-carbon bonds onto transition metals or β-carbon elimination of transition metal alkoxides. When they are looked at from synthetic perspectives, they offer unique and efficient methods to build complex natural products and structures that are difficult to construct by conventional methods. On the other hand, β-scission of radical intermediates has also attracted increasing attention as an alternative elementary step to cleave carbon-carbon bonds. Its site-selectivity is often complementary to that of transition metal-catalyzed reactions. In addition, Lewis acid-mediated and thermally induced ring-opening of cyclobutanone derivatives has garnered renewed attention. On the whole, these examples demonstrate unique synthetic potentials of structurally strained four-membered ring compounds for the construction of organic skeletons.

Substituent-dependent generation of tricyclic frameworks by the rhodium-catalyzed cycloisomerization of homopropargyl allene-alkynes: a theoretical study

Polycyclic compounds having biological activities can be modified by employing different substituents.

Synthesis and structure of 2,4,6-tri-cyclo-butyl-1,3,5-trioxane

The synthesis and structure of 2,4,6,-tri-cyclo-butyl-1,3,5-trioxane, C15H24O3 1, is described. It was formed in 39% yield during the work-up of the Swern oxidation of cyclo-butyl-methanol and may serve as a stable precursor of the cyclo-butane carbaldehyde. The mol-ecule of 1 occupies a special position (3.m) located at the center of its 1,3,5-trioxane ring. The latter is in a chair conformation, with the symmetry-independent O and C atoms deviating by 0.651 (4) Å from the least-squares plane of the other atoms of the trioxane ring. All three cyclo-butane substituents, which have a butterfly conformation with an angle between the two planes of 25.7 (3)°, are in the cis conformation relative to the 1,3,5-trioxane ring. Inter-molecular C-H⋯O inter-actions between the 1,3,5-trioxane rings consolidate the crystal structure, forming stacks along the c-axis direction. The crystal studied was refined a as a racemic twin.

Transition metal catalyzed [6 + 2] cycloadditions

The [6 + 2] cycloaddition reactions are one of the important synthetic transformations to construct eight membered carbo-/heterocyclic systems. The present review is an attempt to update readers on transition metal catalyzed [6 + 2] cycloaddition reactions of various 6π-contributing substrates such as cycloheptatrienes (CHT), cyclooctatetrenes (COTT), allenals, vinylcyclobutanones, fulvene etc. employing rhodium, cobalt, titanium, copper, platinum, ruthenium, rhenium and diphenylprolinolsilyl ethers etc. as catalysts. The transition metal catalyzed [6 + 2] cycloaddition reactions with a variety of functionalized substrates provide straightforward access to eight membered cyclic and/or 5/8, 6/8 etc. condensed carbo-/heterocyclic molecules in moderate to good yields.

Enantioselective Rhodium-Catalyzed Cycloisomerization of 1,6-Allenynes to access 5/6-Fused Bicycle[4.3.0]nonadienes

Transition-metal-catalyzed cycloisomerization of 1,n-allenynes represents a powerful synthetic tool to rapidly assemble complex polycyclic skeletons from simple linear substrates. Nevertheless, there are no reports of the asymmetric version of these reactions. Moreover, most of these reactions proceed through a 6-endo-dig cyclization pathway, which preferentially delivers the distal product (via 5/5 rhodacyclic intermediate) rather than the proximal one (via 6/5 rhodacyclic intermediate). Herein, we report an enantioselective rhodium(I)-catalyzed cycloisomerization of 1,6-allenynes to provide the proximal product 5/6-fused bicycle[4.3.0]nonadienes in good yields and with excellent enantioselectivities. Remarkably, this chemistry works perfectly for 1,6-allenynes having a cyclic substituent within the allene component, thereby affording synthetically formidable tricyclic products with excellent enantioselectivities. Moreover, extensive DFT calculations suggest an uncommon pathway involving 5-exo-dig cycloisomerization, ring-expansion, rate-determining alkene isomerization involving C_sp3-H activation, C-C activation of the cyclobutene moiety and finally reductive elimination. Deuterium labeling experiments support the rate-determining step involving the C–H bond activation in this transformation.

Cycloisomerization of 1,n-allenynes allows to efficiently assembly polycyclic skeletons, however its asymmetric variant remains elusive. Here, the authors report the enantioselective Rh(I)-catalyzed cycloisomerization of 1,6-allenynes and disclose an uncommon 5-exo-dig pathway via DFT calculations.

Rhodium(I)-Catalyzed Ring-Closing Reaction of Allene-Alkene-Alkynes: One-Step Construction of Tricyclo[6.4.0.02,6 ] and Bicyclo[6.3.0] Skeletons from Linear Carbon Chains

Treatment of dodecatrienyne derivatives with [RhCl(CO)₂ ]₂ in refluxing toluene effected the cycloisomerization to produce tricyclo[6.4.0.0^2,6 ]dodecadienes. The one-carbon shortened undecatrienyne derivatives, however, afforded bicyclo[6.3.0]undecatriene derivatives instead of tricyclic compounds, the latter of which are well known as a basic skeleton of naturally occurring octanoids. On the basis of two experiments with deuterated substrates, a plausible reaction mechanism for the construction of these products was proposed.

Rhodium(I)-Catalyzed Cycloisomerization of Homopropargylallene-Alkynes through C(sp3 )-C(sp) Bond Activation

Upon exposure to a catalytic amount of [RhCl(CO)₂ ]₂ in 1,4-dioxane, homopropargylallene-alkynes underwent a novel cycloisomerization accompanied by the migration of the alkyne moiety of the homopropargyl functional group to produce six/five/five tricyclic compounds in good yields. A plausible mechanism was proposed on the basis of an experiment with ¹³ C-labeled substrate. The resulting tricyclic derivatives were further converted into the corresponding bicyclo[3.3.0] skeletons with vicinal cis dihydroxy groups.

Creation of Novel Cyclization Methods Using sp-Hybridized Carbon Units and Syntheses of Bioactive Compounds

Some recent results on the development of new and reliable procedures for the construction of diverse ring systems based on the chemistry of sp-hybridized species, especially allene functionality, are described. This review includes: (i) synthesis of the multi-cyclic skeletons by combination of the π-component of allene with suitable other π-components such as alkyne, alkene, or additional allene under Rh-catalyzed conditions; (ii) synthesis of heterocycles as well as carbocycles by reaction of the sp-hybridized center of allene with some nucleophiles in an endo-mode manner; and (iii) total syntheses of natural products and related compounds from the sp-hybridized starting materials.

Chemo- and Regioselective Rhodium(I)-Catalyzed [2+2+2] Cycloaddition of Allenynes with Alkynes

A highly chemo- and regioselective partially intramolecular rhodium(I)-catalyzed [2+2+2] cycloaddition of allenynes with alkynes is described. A range of diverse polysubstituted benzene derivatives could be synthesized in good to excellent yields, in which the allenynes served as synthetic equivalent to the diynes. A high regioselectivity could be observed when allenynes were treated with unsymmetrical alkynes.

Construction of Azabicyclo[6.4.0]dodecatrienes Based on Rhodium(I)-Catalyzed Intramolecular [6+2] Cycloaddition between Azetidine, Allene, and Alkynes

Treatment of the allenylazetidine-alkynes with a catalytic amount of [RhCl(CO)dppp]2 (dppp: 1,3-bis(diphenylphosphino)propane) effected the intramolecular hetero-[6+2]-type ring-closing reaction via the C-C bond cleavage of the azetidine ring to produce azabicyclo[6.4.0]dodecatriene derivatives in good to excellent yields. The formation of the oxa analogue could also be achieved.

Reactivity and chemoselectivity of allenes in Rh(I)-catalyzed intermolecular (5 + 2) cycloadditions with vinylcyclopropanes: allene-mediated rhodacycle formation can poison Rh(I)-catalyzed cycloadditions

Allenes are important 2π building blocks in organic synthesis and engage as 2-carbon components in many metal-catalyzed reactions. Wender and co-workers discovered that methyl substituents on the terminal allene double bond counterintuitively change the reactivities of allenes in [Rh(CO)2Cl]2-catalyzed intermolecular (5 + 2) cycloadditions with vinylcyclopropanes (VCPs). More sterically encumbered allenes afford higher cycloadduct yields, and such effects are also observed in other Rh(I)-catalyzed intermolecular cycloadditions. Through density functional theory calculations (B3LYP and M06) and experiment, we explored this enigmatic reactivity and selectivity of allenes in [Rh(CO)2Cl]2-catalyzed intermolecular (5 + 2) cycloadditions with VCPs. The apparent low reactivity of terminally unsubstituted allenes is associated with a competing allene dimerization that irreversibly sequesters rhodium. With terminally substituted allenes, steric repulsion between the terminal substituents significantly increases the barrier of allene dimerization while the barrier of the (5 + 2) cycloaddition is not affected, and thus the cycloaddition prevails. Computation has also revealed the origin of chemoselectivity in (5 + 2) cycloadditions with allene-ynes. Although simple allene and acetylene have similar reaction barriers, intermolecular (5 + 2) cycloadditions of allene-ynes occur exclusively at the terminal allene double bond. The terminal double bond is more reactive due to the enhanced d-π* backdonation. At the same time, insertion of the internal double bond of an allene-yne has a higher barrier as it would break π conjugation. Substituted alkynes are more difficult to insert compared with acetylene, because of the steric repulsion from the additional substituents. This leads to the greater reactivity of the allene double bond relative to the alkynyl group in allene-ynes.

New pentacoordinated rhodium species as unexpected products during the in situ generation of dimeric diphosphine-rhodium neutral catalysts

Dimeric rhodium complexes of the type [Rh(PP)(μ2 -Cl)]2 (PP=diphosphine) are often used as precatalysts and are generated "in situ" from the corresponding diolefin complexes by exchange of the diene with the desired diphosphine. Herein, we report that the "in situ" procedure also leads to unexpected monomeric pentacoordinated neutral complexes of the type [RhCl(PP)(diolefin)], for the first time herein characterized by NMR spectroscopy and X-ray crystallography for the ligands 1,4-bis(diphenylphosphino)propane (DPPP), 1,4-bis(diphenylphosphino)butane (DPPB), and 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP). The pentacoordinated complexes are in equilibrium with the dimeric target compound [Rh(PP)(μ2 -Cl)]2 . The equilibrium is influenced by the rhodium-diolefin precursor, the solvent and the temperature. Based on the results of NMR and UV/Vis spectroscopic analysis (kinetics) it could be shown that the pentacoordinated complex [RhCl(PP)(diolefin)] may arise both from the "in situ"-generated neutral complex [Rh(PP)(μ2 -Cl)] by reaction with the free diolefin and, more surprisingly, directly from [Rh(diolefin)(μ2 -Cl)]2 and the diphosphine.

Rhodium diphosphine complexes: a case study for catalyst activation and deactivation

The present work provides an overview of possible activation and deactivation phenomena in homogeneous catalytic processes promoted by different types of rhodium complexes containing diphosphine ligands.