[2+2+2] Cycloaddition Reactions Catalyzed by Transition Metal Complexes

Abstract

Recent progress in the synthesis of benzene and 1,3-cyclohexadiene derivatives, and heterocyclic compounds such as pyridines, pyridones, pyrans, pyrimidine diones, etc, has been reviewed. The general mechanistic aspects of the [2 + 2 + 2] cycloaddition reaction are discussed. The asymmetric variants of these reactions are also discussed along with the proposed models of asymmetric induction. Keywords: arenes; catalysis; cycloaddition; heterocycles; metallacycles; transition metals IntroductionThe significance and increasing popularity of [2 + 2 + 2] cycloaddition reactions is evident from the number of reviews that have recently appeared in the literature. [1][2][3] The [2 + 2 + 2] cycloaddition reaction is remarkable in terms of its ability to utilize various unsaturated substrates such as alkynes, diynes, alkenes, imines, isocyanates, isothiocyanates, and CO 2 in the synthesis of a broad variety of highly substituted cyclic molecules such as benzenes, pyridines, pyridones, 1,3-cyclohexadienes, pyrones, thiopyridones and cyclohexanes. Multisubstituted benzenes and pyridines have traditionally been synthesized by aromatic electrophilic substitution (AES) reactions and a variety of metal-mediated coupling reactions. Although, these reactions are extremely efficient, they generally involve multistep syntheses. The application of AES reactions in the synthesis of polysubstituted aromatic rings is limited by the effect of the substituent groups and, hence, it may be very difficult or even impossible to add various functionalities at a specific position on the aromatic ring. On the other hand, [2 + 2 + 2] cycloaddition reactions are extremely atom-efficient and involve the formation of several C À C bonds in a single step. Another important feature of the [2 + 2 + 2] cycloaddition reaction is tolerance of a myriad of functional groups such as alcohols, amines, alkenes, ethers, esters, halogens, and nitriles. Moreover, the availability of numerous catalytic systems that have Adv. Synth. Catal. 2006, 348, 2307 -2327 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2307 REVIEWS been efficient in tedious syntheses highlights the applicability of the [2 + 2 + 2] cycloaddition reaction. These are important requirements for the [2 + 2 + 2] cycloaddition reaction to become a universal synthetic tool for the synthesis of benzene, pyridine, and other cyclic derivatives. An important problem with the [2 + 2 + 2] cycloaddition reaction is the lack of chemoand regioselectivity observed in earlier reported reactions. However, significant effort has been focused on attaining a high degree of chemo-, regio-and even enantioselectivity with considerable success as evident from recent reports. Excellent reviews on [2 + 2 + 2] cycloaddition reactions are available. Kotha and co-workers have reviewed the synthesis of benzene derivatives by the cyclotrimerization of three alkyne functionalities utilizing transition metal systems as catalysts.[1] Their review is divided into: (a) intermolecular reactionsin which the three pi (alkyn...

Rhodium(I)‐Catalysed Intramolecular [2+2+2] Cyclotrimerisations of 15‐, 20‐ and 25‐Membered Azamacrocycles: Experimental and Theoretical Mechanistic Studies

“…In all three cases, starting material together with decomposition products were obtained. A stoichiometric amount of [CpCo(CO) 2 ], another typical catalyst for this chemistry, [1][2][3][4] was also tested. The macrocycle was heated under reflux in toluene and the solution was exposed to light.…”

“…Transition-metal-catalysed [2+2+2] cyclotrimerisation reactions involving alkynes, in which three carbon-carbon bonds are formed in one step, is one of the most elegant methods for the construction of polysubstituted aromatics, which have important academic and industrial uses (for recent reviews see references [1][2][3][4]) The Wilkinsons complex [RhCl-A C H T U N G T R E N N U N G (PPh 3 ) 3 ] has been widely employed in these reactions. Partially intramolecular approaches or fully intramolecular cycloaddition processes represent efficient entries into various multiple-fused ring compounds.…”

Metall‐vermittelte Synthese von 1,4‐Di‐tert‐butyl‐1,4‐azaborin

“…[2+2+2]-Cyclotrimerisierungen wurden bereits zur Benzol- [7] und Pyridinsynthese [8] eingesetzt. Die Anwendungsmçglichkeiten sind zahlreich, da eine Reihe funktioneller Gruppen toleriert wird und eine Vielzahl geeigneter Katalysatoren bereitsteht.…”

Allene formation by gold catalyzed cross-coupling of masked carbenes and vinylidenes

“…Among them are reactions that directly couple two different sp 2 -hybridized carbon centers to form an olefin (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13). Conceptually, the simplest process would be the dimerization and cross-coupling of two free carbenes (Fig.…”