Metallotropic shifts

Additional evidence for the involvement of metal carbenes in these processes was obtained in the reaction of dimeric substrates with a cationic Au(I) catalyst to give disubstituted alkynes (equation 39). These reactions can be explained by isomerization of the initially formed cyclopropyl gold carbene by a [1,3] metallotropic shift, followed by intramolecular trapping of the gold carbene by the alkene. Other examples of [1,3] metallotropic shifts in gold chemistry have been observed recently. ... 

Figure 4.2 The -allylgold(l) species is the transition state of the 1,3-metallotropic shift. 4.4...

Intramolecular trapping of the transient carbene postulated above has also been achieved yielding polyunsaturated bicycles (47) from the diene-diyne precursor (46).42 The intramolecular cyclopropanation is preceded by a [l,3]-metallotropic shift. 

The same authors have demonstrated that 1,3-diynes behave in predictable yet distinctive manners compared to simple enynes under electrophilic transition metal-mediated reaction conditions. This characteristic behaviour of 1,3-diynes is presumably caused by the slightly electron-withdrawing nature of the alkynyl substituent, which not only renders preferentially the formation of 5-exotype alkylidenes but also allows for the subsequent [l,3]-metallotropic shift. Several salient features of reactions with this functionality include the following (a) an acetate is more reactive than the tethered alkene as an initiator, generating [l,2]-acetate migrated alkylidene intermediate, whereas an alkene is a better terminator than an acetate/bromide to generate the cyclopropane moiety (b) allene products are not formed at all under current reaction conditions (c) 5-exo/6-endo-type alkylidene formation depends on the heteroatom substituent in the tether (d) facile metallotropic [1,3]-shift of the intermediate alkylidenes occurred whenever possible. 

Pyrazolylpyrimidines (pzpm) also provide a method for detecting the Pd-N bond rupture (see equation 16). This has been extended to tri-substituted triazines, wherein Pd-N bond rupture can be detected, but which also allows the metal to pass around the entire ligand via a series of (1) metallotropic shifts involving retention of the triazene N-Pd bond and (2) pyrazolyl ligand assisted metal hurdling from one triazene N to another (see equation 17). The subject of metals shuttling... 

Scheme 2.57 Dependence of the metallotropic shift on the nature of the 1,3-diyne substituent.

RCM reaction and concomitant metallotropic shift, and they applied this strategy for the synthesis of panaxytriol (145, Scheme 24.37). Panaxytriol 145 was a component of Red Ginseng and exhibits an antitumor activity. The key process involves the generation of ruthenium carbene species 146 by the relay RCM of 142 and the subsequent CM reaction of alkene 143 with carbene 147, which is formed by two consecutive metallotropic [1,3]-shifts of the carbene... 

SCHEME 24.37. Synthesis of panaxytriol by RCM/metallotropic shift/CM methodology. 

Simple diastereoselection in the reactions of 2-butenylboron compounds and aldehydes is critically dependent on the configurational stability of the reagentslb. As a general rule, most 2-bulenylorganometallics arc sensitive to sequential 1,3-metal shifts (1,3-metallotropic rearrangements) that result in E- to Z-olefin isomerization via the l-methyl-2-propenylmetal isomer. 

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