Cyclopropanes cleavage, with metals

Primary, secondary and tertiary benzylic halides are cleaved with zinc dust in acetic acid or under basic conditions (equations 80 and 81). Benzylic C—N bonds can also be cleaved (equation 82)," with particular ease when quaternary ammonium salts are reduced (equation 83).There has also been a recent report of cyclopropane cleavage with zinc metal and zinc chloride (equation 84). ... 

Except for some of the above reactions which proceed via radical intermediates, many of the ring cleavage reactions are likely to involve initial interaction of the metal and the cyclopropanering. The nature of this interaction between 1-siloxy-1-alkoxycyclopropane 1 with metals has been investigated by the authors in some detail, and the results are summarized below. 

When the donor character of the amino substituent at the transition-metal-carbene carbon atom is reduced, it should be possible to influence the thermal stability and reactivity in favor of the desired cyclopropanation process. Indeed, pyrrol-1 -ylcarbene complexes 18 of chromium, molybdenum and tungsten do exhibit the desired reactivity. In the last step, the pyrrole ring of 19 can be converted to the NH2 function in 20 after oxidative cleavage with ozone.In this respect, the pyrrole heterocycle represents a synthetic equivalent of the amino function. 

The cyclopropane ring is known to be susceptible to insertion reactions when treated with metal complexes. Aryl-substituted cyelopropanes were shown to displace ethene from the di-chloro(ethene)platinum dimer with formation of substituted (propane-1,3-diyl)platinum dichlo-ride. The cleavage site was the bond next to the aryl substituent. 

Several general methods have been recognized as favoring C-C cleavage with transition metal complexes. Relief of ring strain has been used to open cyclopropanes. The attainment of aromaticity can also help C-C cleavage. Proximity has also been shown to assist C-C cleavage, such as the activation of 8-acylquinolines. Even aryl-methyl bonds can be cleaved if forced into proximity with the metal [1]. 

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

Vaska s complex catalyzed the transformahon of aUenylcyclopropane into 2-alkenylidenecyclohex-3-enone under conditions of pressurized CO (Scheme 11.25) [38]. In this reaction, the jr-coordination to internal oleflnic moiety of the aUene brings the metal closer to the cyclopropane ring. Release of the cyclopropane ring strain then facilitates the oxidative addition of vinylcyclopropane moiety along with C-C bond cleavage, such that metallacyclohexene is obtained a subsequent carbonyl insertion and reductive elimination then provides the product Hence, the reaction can be recognized as a [5+1] cycloaddition of vinylcyclopropane and CO. 

Cyclopropanes are cleaved by a variety of transition metal derivatives. The first observation was that PtCl4 reacts to give a platinocyclobutane. Cyclobutanes appear to be less reactive, but cubane reacts with [Rh(CO)2Cl]2 leading to C—C bond cleavage. The subject of C—C bond cleavage has been reviewed. 

DDQ, TCNQ, etc., yield cyclopropanes stereochemical studies show that the reductive elimination is either concerted with the oxidation step or that C—C coupling following the first Ti—C bond cleavage is faster than C—C bond rotations (4/). It is currently believed that these reactions proceed via outer-sphere one-electron oxidation from a metal-ligand bond. In the case of Zr, this is followed by loss of an R radical and trapping... 

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