Enyne cycloisomerisation

Conjugated enynes possess several of the same transition metal binding modes exhibited by their diyne counterparts, including Jt-coordination, a-carbenoid formation, and a/jt metal complexation (Scheme 35). In some cases, these constructs are very stable and can be isolated, but they are frequently transient structures in which the metal ion acts as a catalyst for enyne cycloisomerisation and ligand transformation reactions. 

Lloyd-Jones GC (2003) Mechanistic aspects of transition metal catalysed 1,6-diene and 1,6-enyne cycloisomerisation reactions. Org Biomol Chem 1 215-236... 

Enynes 79 can also undergo cycloisomerisation reactions in presence of NHC/ transition metal complexes (Scheme 5.32). The cycloadduct 124 can be prepared either in presence of complex 110 [36] or in presence of a NHC-Ni complex (prepared in situ from a mixture of [ (COD) ] and IDTB 123 [37]). In the latter case, the active catalytic species is believed to be a Ni-H species that is generated via C-H activation of the carbene ligand. 

Gold catalysts containing NHC ligands can also promote cycloisomerisation reactions. Bicyclo[3.1.0]hexanes 137-139 can be prepared from the cycloisomerisation of 1,5-enynes bearing a propargyUc acetate (135) in the presence of catalytic amounts of [AuCl(lPr)]/AgBF (Scheme 5.36) [41]. The cycloisomerisation reaction of 135 occurs by a 1,3-OAc shift/aUene-ene cyclisation/l,2-OAc shift sequence. Experimental results with allenyl acetate 136 support this hypothesis as 139 is obtained in higher ratios than 137 and 138 [41b],... 

Scheme 5.36 Golden catalysed cycloisomerisation of 1,5-enynes bearing a propargylic acetate...

Carbene/diphosphine platinum(ll) complexes 141 or carbene/monophosphine platinum(ll) complex 142 were shown to be efficient for the cycloisomerisation of 1,6-enynes 140 (Scheme 5.37) [42]. The product 143 is obtained in higher yields and higher enantioselectivity when catalyst 142 is employed rather than catalyst 141. 

Scheme 5.37 Enantioselective cycloisomerisation of 1,6-enynes catalysed by NHC-Pt complex...

Echavarren reported the cycloisomerisation of 1,7-enynes catalysed hy a cationic gold(i) species at room temperature. The reaction proceeded using 2 mol% of XXXI in dichloromethane. Remarkably, most of the transformations required only 5 min (Scheme 16.68). 

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