Skeletal rearrangements enynes

Hypothetical (carbene)gold(i) structures of intermediates and reaction coordinates have been calculated (B3LYP/ 6-31G and LAN2DZ levels) for (H3P)Au+-catalyzed cyclization reactions of terminal enynes. The endocyclic skeletal rearrangement reactions were found to proceed exclusively via cyclopropylcarbene complexes.240... 

The skeletal rearrangements are cycloisomerization processes which involve carbon-carbon bond cleavage. These reactions have witnessed a tremendous development in the last decade, and this chemistry has been recently reviewed.283 This section will be devoted to 7T-Lewis acid-catalyzed processes and will not deal, for instance, with genuine enyne metathesis processes involving carbene complex-catalyzed processes pioneered by Katz284 and intensely used nowadays with Ru-based catalysts.285 By the catalysis of 7r-Lewis acids, all these reactions generally start with a metal-promoted electrophilic activation of the alkyne moiety, a process well known for organoplatinum... 

Shortly after Trost s works, two investigations demonstrated the high reactivity of platinum halide salts for this type of reactions. Blum reported a PtCU-catalyzed rearrangement of allyl propargyl ethers to 3-oxabicyclo[4.1.0]-heptenes (Scheme 79).294 This series of reactions also represented the premiere entry into the versatile formation of cyclopropyl products based on skeletal rearrangements of enynes.295 This intriguing aspect is discussed further. 

Ring-closing metathesis of an enyne, which has double and triple bonds in the molecule, is a remarkable reaction which is useful in synthetic organic chemistry. In enyne metathesis, the double bond is cleaved and carbon-carbon bond formation occurs between the double and triple bonds. The cleaved alkylidene part is moved to the alkyne carbon. Thus, the cyclized compound formed in this reaction has a diene moiety [Eq. (6.77)]. The reaction is also called skeletal rearrangement and is induced by Pt, Pd, Ga, and Ru catalysts ... 

PtCl2 constitutes an efficient and practical catalyst for skeletal rearrangement reaction of enynes. This includes a formal enyne metathesis reaction delivering 1,3-dienes. Skeletal reorganization of enyne 75a having a carbon chain in... 

A mechanism proposed for the skeletal rearrangement of enynes involved the presence of gold carbenes [161]. This proposed mechanism was supported by the capture of intermediate gold carbenoids trapped by reactive alkenes in intermolecu-lar cyclopropanation reactions [162]. 

Selective activation of alkyne functions of enynes to give products either of alkoxy-cyclization or of exo- and endo-skeletal rearrangement can be achieved by using alkynophilic cationic gold(I) complexes. The endocyclic cyclization catalysed by gold(I) proceeds via a mechanism different from those known for Pd(II), Hg(II), or Rh(I) catalysts.118... 

A pathway for the formation of (66) via ring-opening of (67) is favoured by most authors.142 However, the formation of dienes (68) requires a different mechanistic rationalization and the authors of this study suggest that cyclobutenes (67) are not necessary intermediates in the skeletal rearrangement of enynes (64). 

The skeletal rearrangement of enynes proceeds through that kind of intermediates. Two basic types of products are obtained when an exo pathway is followed single cleavage and double cleavage, named owing to the fact that only the olefin or both the aUcene and the aUcyne are cleaved in the transformation. Some examples are depicted in equation (34-36). ... 

Endo-skeletal rearrangements also take place with 1,6-enynes, bnt the proposed mechanism is just a variation of the exo-single-cleavage rearrangement. Formed by endo cyclization, bicyclo[4.1.0]hept-4-ene derivatives arise in some cyclizations of 1,6-enynes by proton loss and protodemetalation ofthe endo cyclopropylcarbene. " That is the case from 1,6-enynes tethered as sulfonamides and in the intramolecular cyclization of 1,6-enol ethers with alkynes (equation 40). ... 

A few examples of single-cleavage skeletal rearrangement of 1,7-enynes have been reported by using gold. ... 

Scheme 46. Intramolecular Pt(II)-Pt(IV) enyne metathesis featuring substitution-dependent skeletal rearrangement.

In the presence of cationic gold catalysts, 1,6-enynes with a terminal double bond undergo intermolecular addition reactions to carbonyl compounds. Again, a skeletal rearrangement of the gold carbene intermediate is the key to the formation of the tricyclic heterocycles. 

Intermolecular " or intramolecular [2+2+2]-cycloadditions " of 1,6-enynes and carbonyl compounds take place in the presence of cationic gold catalysts. The latter have been used successfully in natural products synthesis. A skeletal rearrangement of a gold carbene intermediate is the key step in these transformations. 

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