Rhodium‐Catalyzed Cross‐Cyclotrimerization and Dimerization of Allenes with Alkynes

It has been established that a cationic rhodium(I)/binap complex catalyzes the cross‐cyclotrimerization of two molecules of a monosubstituted allene with one molecule of a functionalized alkyne to give 3,6‐dialkylidenecyclohex‐1‐enes. In contrast, the reactions involving di‐ or trisubstituted allenes and/or unfunctionalized alkynes afforded cross‐dimerization products, substituted dendralenes, through β‐hydrogen elimination from the corresponding rhodacycles.     

Iron‐catalyzed Cross‐Coupling of Propargyl Carboxylates and Grignard Reagents: Synthesis of Substituted Allenes

Presented herein is a mild, facile, and efficient iron‐catalyzed synthesis of substituted allenes from propargyl carboxylates and Grignard reagents. Only 1–5 mol % of the inexpensive and environmentally benign [Fe(acac)₃] at ?20 °C was sufficient to afford a broad range of substituted allenes in excellent yields. The method tolerates a variety of functional groups.     

Aromatic Osmacyclopropenefuran Bicycles and Their Relevance for the Metal‐Mediated Hydration of Functionalized Allenes

The aromatic osmacyclopropenefuran bicycles [OsTp{κ³‐C¹,C²,O‐(C¹H₂C²CHC(OEt)O)}(PⁱPr₃)]BF₄ (Tp=hydridotris(1‐pyrazolyl)borate) and [OsH{κ³‐C¹,C²,O‐(C¹H₂C²CHC(OEt)O)}(CO)(PⁱPr₃)₂]BF₄, with the metal fragment in a common vertex between the fused three‐ and five‐membered rings, have been prepared via the π‐allene intermediates [OsTp(η²‐CH₂=CCHCO₂Et)(OCMe₂)(PⁱPr₃)]BF₄ and [OsH(η²‐CH₂=CCHCO₂Et)(CO)(OH₂)(PⁱPr₃)₂]BF₄, and their aromaticity analyzed by DFT calculations. The bicycle containing the [OsH(CO)(PⁱPr₃)₂]⁺ metal fragment is a key intermediate in the [OsH(CO)(OH₂)₂(PⁱPr₃)₂]BF₄‐catalyzed regioselective anti‐Markovnikov hydration of ethyl buta‐2,3‐dienoate to ethyl 4‐hydroxycrotonate.     

Synthesis of new ionic liquids based on (5Z,9Z)-alkadienoic acids and choline

Efficient synthesis of ionic liquids based on stereoisomerically pure natural and synthetic higher (5Z,9Z)-alkadienoic acids and choline hydroxide was developed. The key unsaturated carboxylic acids were prepared using the stereoselective cross-cyclomagnesiation reaction of aliphatic and oxygen-containing 1,2-dienes with EtMgBr in the presence of Mg metal and Cp₂TiCl₂ catalyst.     

Recent developments in enantioselective gold(I) catalysis

The use of gold(I) complexes as catalysts for organic transformations has become increasingly common over the past decade, leading to the development of a number of useful carbon-carbon and carbon-heteroatom bond-forming processes. In contrast, enantioselective catalysis employing gold(I) complexes was, until recently, exceedingly rare, due in large part to the pronounced tendency of gold(I) to form linear, two-coordinate complexes. However, new approaches and strategies have emerged over the past two years, leading to the development of a number of effective gold(I)-catalyzed enantioselective transformations, most notably the enantioselective hydrofunctionalization of allenes. Outlined herein is an overview of enantioselective gold(I) catalysis since 2005.     

Studies on thermal reactivity of beta-(1,2-allenyl)butenolides and 2-allyl-3-allenylcyclohex-2-enones

A series of thermal pericyclic reactions of beta-allenylfuranones have been studied. It was observed that beta-allenylfuranones would undergo 1,5-hydrogen shift to afford a new type of trienes upon heating. Due to their high reactivity, these trienes would undergo subsequent pericyclic reactions based on the nature of the substituent group R: When R is an alkyl group, the intermediate 4a or 4b would undergo a further 1,7-hydrogen shift to afford a more stable conjugated triene 3; with R being phenyl or cyclopropyl group, the 1,7-hydrogen shift was inhibited and the 4-type conjugated triene would form a six-membered ring 5 via 6 pi-electrocyclization. Interestingly, introducing another C=C double bond into the triene intermediate (R = CH=CH2, the 18-type intermediate would undergo 8 pi-electrocyclization reaction to form an eight-membered ring. Such a transformation was also observed with 2-allyl-3-allenylcyclohex-2-enones. The deuterium-labeling mechanistic studies show that the alkyl groups at the allenyl moiety of 1 participated in the isomerization process via 1,7-hydrogen shifts between 18 A, 20 A, and 29 A.     

Controllable cyclization reactions of 2-(2',3'-allenyl)acetylacetates catalyzed by gold and palladium affording substituted cyclopentene and 4,5-dihydrofuran derivatives with distinct selectivity

Efficient room-temperature syntheses of cyclopentenes and 4,5-dihydrofurans with different substitution patterns were performed starting from the same materials (i.e., 2-(2',3'-allenyl)acetylacetates). Depending on the choice of metal catalyst, the Au-catalyzed reaction afforded C-attack-5-endo cyclization products 2, whereas the Pd-catalyzed one led to the formation of O-attack-5-exo cyclization products 3. The selectivity may be explained by the steric and electronic effects of the substrates and catalysts.     

PdI2-catalyzed coupling-cyclization reactions involving two different 2,3-allenols: an efficient synthesis of 4-(1',3'-dien-2'-yl)-2,5-dihydrofuran derivatives

Transition-metal-catalyzed dimeric coupling-cyclization reactions of two different 2,3-allenols afforded 4-(1',3'-dien-2'-yl)-2,5-dihydrofuran derivatives 3. 2-Substituted 2,3-allenols 1 cyclized to form the 2,5-dihydrofuran ring, whereas the 2-unsubstituted 2,3-allenols 2 provided the 1,3-diene unit at the 4-position. The reaction is proposed to proceed through an oxypalladation, insertion, and beta-hydroxide elimination process. The C=C double bond was formed with high E stereoselectivity by beta-hydroxide elimination.     

Highly diastereoselective palladium-catalyzed cyclizations of 3,4-allenylic hydrazines and organic halides -- highly stereoselective synthesis of optically active pyrazolidine derivatives and the prediction of the stereoselectivity

Pyrazolidines containing two chiral centers, an interesting class of heterocyclic compounds possessing a range of biological activities, have been prepared highly diastereoselectively (up to 95:5) through asymmetric Pd(OAc)(2)-catalyzed cyclizations between the easy available optically active allenylic hydrazines and organic halides in THF in the presence of (R,R)-Bn-Box (L2) as the ligand. It was observed 1) that in most cases (3R,5S)-pyrazolidines were obtained in good yields with very high enantiopurities (>99%) and high diastereoselectivities (up to 95:5) in the presence of (R,R)-Bn-Box (L2), 2) that aryl halides containing electron-donating or -withdrawing groups, heteroaryl, and 1-alkenyl iodides are all suitable substrates for this diastereoselective cyclization, 3) that the absolute configurations of the newly formed chiral centers in the pyrazolidines depend on the structure of substrate 1, and 4) that the enantio- and diastereopurities of the trans-pyrazolidines are co-controlled by the chiralities of the chiral catalysts and the substrates. A model for prediction of the enantiopurities of the products and the diastereoselectivities of the reactions based on an HPLC study of the starting hydrazines and the products was established.     

Titanocene(II)-promoted multicomponent reactions utilizing alkynyl sulfones, alkenyl sulfones, and carbonyl compounds: a novel method for the synthesis of vinylallenes

Titanocene(II)-promoted cross coupling of alkynyl- and (Z)-alkenyl sulfones affords alpha-(phenylsulfonyl)alkenyltitanium species. Further treatment of these species with the titanocene(II) reagent generates titanium vinylvinylidene complexes, which react with carbonyl compounds in one pot to produce substituted vinylallenes with complete stereoselectivity. By using alpha,beta-unsaturated ketones, 1,3,4,6-tetraenes are also obtained stereoselectively.