L,n-enynes

The intramolecular Alder-ene reaction (enyne cydoisomerization reaction) with alkynes as the enophiles has found wide application compared with diene systems. The reason may be the ready chemo-differentiation between alkene and alkyne functionality and the more reactive alkyne moiety. Furthermore, the diene nature of the products will promote further applications such as Diels-Alder reactions in organic synthesis. Over the past two decades the transition metal-catalyzed Alder-ene cycloisomerization of l,n-enynes (typically n= 6, 7) has emerged as a very powerful method for constructing complicated carbo- or heterocydic frameworks. The transition metals for this transformation indude Pd, Pt, Co, Ru, Ni-Cr, and Rh. Lewis acid-promoted cydoisomerization of activated enynes has also been reported [11],... 

A remarkable cyclization of a 1,9-enyne (equation 43) to form a 10-membered ring has been disclosed. The aromatic ring may play a role in the cyclization, but still this result shows that formation of large rings from l,n-enynes (n > 7) by using gold chemistry is possible. Cyclizations of 1,8-enynes that probably proceed by a different mechanism have also been reported. ... 

Scheme 15 Mechanisms of the palladium-catalyzed cycloisomerization of acyclic l,n-enynes (n = 6 7) [66-68]...

With this foundation and historical perspective of transition metal-promoted intramolecular enyne metathesis reactions of l,n-enynes in place, the remainder of this section will describe specific metal-catalyst systems with examples of their utilities for cycloisomerization reactions involving various substrates. 

Cyclopentadienyl compounds of the form CpCo(CO)2 are also known to convert l,n-enynes into the tranditional 1,3-diene cycloisomerization products (486, 487). Remarkably, the metathesis reaction is independent of tether length between both unsaturated functional groups, and leads to five-membered ring carbocycles exclusively (Scheme 60). 

Iridium-catalyzed intramolecular l,n-enyne metathesis has been studied as a unique tool for the synthesis of various types of cyclic compounds. Reactions of this type depend on both the structure of substrates and the nature of catalyst systems used (411). Recently, the cycloisomerization of various 1,6-enynes have been shown to be catalyzed by [Ir(cod)Cl]2/dppf (494). These reactions are highly stereoselective, and generate the (Z)-isomer preferentially over the ( )-isomer (Scheme 63). The proposed mechanism (Scheme 64) involves oxidative cyclization of the enyne at Ir(I) to give the trivalent iridacyclopentene. The intermediate undergoes (3-hydride elimination to give the irida-1,3-diene, which experiences steric repulsion between the metal fragment and the cis substituent on the... 

Diver, S. T Giessert, A. J. Chem. Rev. 2004,104,1317. The term enyne metathesis originally referred to ring-closing metatheses of l,n-enynes and ene-yne metathesis to intermolecular versions of this process. Enyne metathesis is now often used for both types of reactions, and is used in this way in this text. 

Zhang, D. H., Zhang, Z. and Shi, M. 2012. Transition metal-catalyzed carbocycUzation of nitrogen and oxygen-tethered l,n-enynes and diynes Synthesis of five or six-membered heterocychc compounds. Chem. Commun. (Camb.) A% %3) 10271-10279. 

Bertrand recently described the Au(I)disubstituted pyrroles 269 and 270 (Scheme 8.98) [293]. 

Scheme 1.13 Different pathways for the metal-catalyzed cycloisomerization of l,n-enynes...

General reviews for the behavior of l,n-enynes in the presence of transition metals [152-156]. 

Gold(I)-catalyzed cyclization of 1,8-enynes gives cyclobutene compounds as the main product (Scheme 1.25) [145, 197]. These intermediates can also to give isomerization or fragmentation products after prolonged reaction times or in the presence of traces of acids. Furthermore, cyclobutene compounds can also be synthesized from certain 1,6- and 1,7-enynes [15]. The formation of cyclobutene compounds by gold-catalyzed cycloisomerization of l,n-enynes will be discussed in Chap. 2. 

One of the most emblematic reaction is the trapping of different intermediates of l,n-enynes with different nucleophiles, which allows the formation of complicated structures from simple starting materials [202]. 

Formation of Cyclobutene Compounds via Gold(I)-Catalyzed Cycloisomerization of l,n-Enynes... 

Michelet, V., Toullec, R, GenSt, J.-P. (2008). Cycloisomerization of l,n-enynes chaUenging metal-catalyzed rearrangements and mechanistic insights. Angewandte Chemie - International Edition, 47, 4268-4315. 

Alder-ene reaction of l,n-enynes can be regarded as the archetypal of cycloisomerization reaction known for over 60 years in its uncatalyzed thermal form [28]. From mechanistic point of view, the reaction involves a six-electron pericyclic rearrangement between an aUylic hydrogen (the ene) and an... 

The remarkable ability of these catalysts to cycloisomer-ize l,n-enynes has been exemplified in reaction cascades for the straightforward induction of molecular complexity (Scheme 7.28). 

Cascade Reactions of l,n-Enynes (n > 6) With the C—C double bond tethered remotely to the C—C triple bond, direct reactions between these two n-systems are often supplanted by other more facile processes. Cascade reactions of these enynes often have the participation of the C—C double bond at later stages of the reactions. 

Keywords l,n-Enynes Alkenes Alkynes Allenes Gold catalysis... 

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