Enynes reactivity mechanism

The palladium-catalyzed stannylboration (90) [124] or silylboration (87) [109, 114] succeeds in the intramolecular carbocyclization of diynes and enynes (Scheme 1-27). It is interesting that a very strained four-membered cycUzation of hexa-l,5-diyne proceeds without any difficulties, similarly to five- or six-membered cycUzation. The boryl group is selectively introduced into the more reactive C=CH rather than C=C for enynes and into the terminal C=CH rather than the internal C=CR for diynes, again suggesting a mechanism proceeding through the first insertion into the Pd-B bond in preference to the Pd-Sn or Pd-Si bond. 

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]. 

The catalytic intramolecular coupling of two C=C bonds at a ruthenium site leads to cyclization reactions. For example, although generally less reactive than a,co-diynes or enynes, 1,6-dienes react with [RuC12(COD)] in 2-propanol, leading to exo-methylenecyclopentanes in excellent yields [13] (Eq. 8). The mechanism suggests the formation of the ruthenacyclopentane(hydrido) intermediate 19. 

Until recently, intermolecular enyne metathesis received scant attention. Competing CM homodimerisation of the alkene, alkyne metathesis and polymerisation were issues of concern which hampered the development of the enyne CM reaction. The first report of a selective ruthenium-catalysed enyne CM reaction came from our laboratories [106]. Reaction of various terminal alkynes 61 with terminal olefins 62 gave 1,3-substituted diene products 63 in good-to-excellent yields (Scheme 18). It is interesting that in these and all enyne CM reactions subsequently reported, terminal alkynes are more reactive than internal analogues, and 1,2-substituted diene products are never formed thus, in terms of reactivity and selectivity enyne CM is the antithesis of enyne RCM. The mechanism of enyne CM is not well understood. It would appear that initial attack is at the alkyne however, one report has demonstrated initial attack at the alkene (substrate-dependent) is also possible, see Ref. [107]. 

We and others recently reported the metal-catalyzed addition of diboron compounds to alkenes and alkynes.15,16 Subsequent work has improved the catalysts and extended the scope of the substrate to include disubstituted alkenes, enynes, and dienes.17 Stoichiometric reactivity studies support a mechanism which involves oxidative addition of the B-B bond to the metal center, followed by insertion of the substrate into the M-B bond, and product-forming B-C reductive elimination (Scheme l).18... 

Shortly after the discovery of enyne metathesis, Trost began developing cycloisomerization reactions of enynes using Pd(ll) and Pt(ll) metallacyclic catalysts (429-433), which are mechanistically divergent from the metal-carbene reactions. The first of these metal catalyzed cycloisomerization reactions of 1,6-enynes appeared in 1985 (434). The reaction mechanism is proposed to involve initial enyne n complexation of the metal catalyst, which in this case is a cyclometalated Pd(II) cyclopentadiene, followed by oxidative cyclometala-tion of the enyne to form a tetradentate, putative Pd(IV) intermediate [Scheme 42(a)]. Subsequent reductive elimination of the cyclometalated catalyst releases a cyclobutene that rings opens to the 1,3-diene product. Although this scheme represents the fundamental mechanism for enyne metathesis and is useful in the synthesis of complex 1,3-cyclic dienes [Scheme 42(fe)], variations in the reaction pathway due to selective n complexation or alternative cyclobutene reactivity (e.g., isomerization, p-hydride elimination, path 2, Scheme 40) leads to variability in the reaction products. Strong evidence for intermediacy of cyclobutene species derives from the stereospecificity of the reaction. Alkene... 

The endiyne and enyne-allene have attracted the attention, as they undergo facile cycloaromatization to produce reactive biradicals [1], which could mimic the DNA-cleaving mechanism and properties of the new class of very potent antitumor antibiotics calicheamicins [2], esperamicins [3], neocarzinostatin [4], and dynemicins [5]. 

The proposed mechanism involves the regioselective c/s-hydroboration of the 4-bromo-l,3-enyne as observed by Zweifel [2] followed by addition of hydride, originating from t-BuLi [3], to initiate a 1,2-metalate rearrangement forging the C-C bond with inversion at the vinylic center (Scheme 2). This key step enables the stereospecific character of the whole process. To circumvent the low reactivity of (Z)-vinylborane toward aldehydes, the corresponding (Z)-vinylzinc was prepared by transmetalation with diethylzinc and reacted successfully with carboxaldehyde. The isolation of aUylic alcohols in high yields was subordinated to a careful selection of the... 

This type of cyclopropanation reaction catalyzed by a gold(I) complex produced cyclopropylmethyl carbene complex 321, which is reactive toward external alkenes or nucleophiles. The reaction depended on the ligand of the gold complex as well as the substituted patterns of enyne compounds. Echavarren and coworkers reported a cyclopropanation reaction mechanism. The cyclopropane gold complex intermediates 322 and 323 were trapped by external alkenes to give cyclopropanes 324 and 325, respectively (Scheme 1.157) [227]. 

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