Highly Regio- and Stereoselective Synthesis of Polysubstituted Cyclopropane Compounds via the Pd(0)-Catalyzed Coupling−Cyclization Reaction of 2-(2‘,3‘-Allenyl)malonates with Organic Halides

Deciphering the Chameleonic Chemistry of Allenols: Breaking the Taboo of a Onetime Esoteric Functionality

The allene functionality has participated in one of the most exciting voyages in organic chemistry, from chemical curiosities to a recurring building block in modern organic chemistry. In the last decades, a special kind of allene, namely, allenol, has emerged. Allenols, formed by an allene moiety and a hydroxyl functional group with diverse connectivity, have become common building blocks for the synthesis of a wide range of structures and frequent motif in naturally occurring systems. The synergistic effect of the allene and hydroxyl functional groups enables allenols to be considered as a unique and sole functionality exhibiting a special reactivity. This Review summarizes the most significant contributions to the chemistry of allenols that appeared during the past decade, with emphasis on their synthesis, reactivity, and occurrence in natural products.

Tertiary amine self-catalyzed intramolecular Csp3–H functionalization with in situ generated allenes for the formation of 3-alkenyl indolines

Tandem Pd(0) catalyzed allenylation/tertiary amine self-catalyzed C_sp3–H functionalization reaction for the synthesis of 3-alkenyl indolines.

Efficient trifluoromethylation via the cyclopropanation of allenes and subsequent C–C bond cleavage

Trifluoromethyl has been efficiently incorporated into organic skeletons via allene cyclization and C–C bond cleavage.

Reactions of ruthenium-allenylidene complexes tethering a cyclopropyl group

Two cyclopropyl allenylidene complexes [Ru]=C=C=C(R)(C(3)H(5)) ([Ru]=[RuCp(PPh(3))(2)], Cp=Cyclopentadienyl; R=thiophene (2a) and R=Ph (2b)) are prepared from the reactions of [Ru]Cl with the corresponding 1-cyclopropyl-2-propyn-1-ol in the presence of KPF(6). Thermal treatment, halide-anion addition, and palladium-catalyzed reactions of 2a and 2b all lead to a ring expansion of the cyclopropyl group, giving the vinylidene complexes 4a and 4b, respectively, each with a five-membered ring. This ring expansion proceeds by C-C bond formation between Cβ of the cumulative double bond and a methylene group of the cyclopropyl ring. In the reaction of 2a with pyrrole, consecutive formation of two C-C bonds, one between C-2 of pyrrole and Cγ of 2a and the other between C-3 of pyrrole and Cα, results in the formation of 6a. The reaction proceeds by addition of pyrrole and 1,3-proton shifts. The hydrogenation of 2a by NaBH(4) is carried out in different solvents. The cumulative double bonds are reduced regioselectively to give a mixture of 7a and 8a. Interestingly, use of different solvents leads to different ratios of 7a and 8a. Presence of a protic solvent like methanol in dichloromethane or chloroform solution increases the yield of 8a, thus revealing that both the rates of hydroboration and deboronation increase. The structures of two new complexes 4a and 6a have been firmly established by X-ray diffraction analysis.

Pd-catalyzed regio- and stereoselective cyclization-heck reaction of monoesters of 1,2-allenyl phosphonic acids with alkenes

[reaction: see text] The cyclization-Heck reactions of monoesters of 1, 2-allenyl phosphonic acids with alkenes were studied. The reaction afforded 4-(1-Z-alkenyl)-2-ethoxy-2,5-dihydro[1,2]oxaphosphole 2-oxides regio- and stereoselectively. Pd(II) was regenerated from the in situ formed Pd(0) using CaH2(cat.)/NaI/O2 or benzoquinone to furnish the catalytic cycle.

Enantioselective Synthesis of Vinylcyclopropanes and Vinylepoxides Mediated by Camphor-Derived Sulfur Ylides: Rationale of Enantioselectivity, Scope, and Limitation

By a sidearm approach, camphor-derived sulfur ylides 1 were designed and synthesized for the cyclopropanation of electron-deficient alkenes and epoxidation of aldehydes. Under the optimal conditions, the exo-type sulfonium salts 4a and 4b reacted with beta-aryl-alpha,beta-unsaturated esters, amides, ketones, and nitriles to give 1,3-disubstituted-2-vinylcyclopropanes with high diastereoselectivities and enantioselectivities. When the endo-type sulfonium salts 5a and 5b were used, the diastereoselectivities were not changed, whereas the absolute configurations of the products became the opposite to those of the reactions of 4a and 4b. An ylide cyclopropanation of chalcone derivatives with phenylvinyl bromide in the presence of catalytic amount of chiral sulfonium salts 4b and 5b has been developed. The sidearmed hydroxyl group was found to play a key role in the control of enantioselectivity and diastereoselectivity. The origins of the high diastereoselectivity and enantioselectivity were also studied by density functional theory calculations, which reveal the importance of the hydrogen-bonding between the sidearmed hydroxyl group and the substrate in determining the diastereoselectivity and enantioselectivity. The ylides 1 were also successfully applied for the epoxidation of aromatic aldehydes.

Highly Stereoselective Synthesis of Trisubstituted Vinylcyclopropane Derivatives via Arsonium Ylides

[reaction: see text] Alkylidene or arylidene malonates reacted with arsonium allylides to give trans-disubstituted cyclopropane-1,1-dicarboxylates with high stereoselectivity in high yields. The mechanism of the cyclopropanation reactions has also been investigated.