Retro-ene fragmentation

The parent silabenzene 24 was first matrix-isolated by our group in 198035 by pyrolysis of precursors 25 and 26, which yield the expected silabenzene by retro-ene fragmentation. Later, it could be shown that in analogy to carbon chemistry the hydrogen elimination from silacyclohexadiene 27 also gives the silaaromatic 2436. This reaction is allowed by the Woodward-Hoffmann rules. In accordance with the Woodward-Hoffmann rules, it could be demonstrated that silabenzene 24 is not accessible by pyrolysis of the conjugated silacyclohexadiene 28 (equation 8). 

Competitive experiments with 2H-, 13C- and 180-labelled chorismate derivatives and three different chorismate mutase enzymes have shown that in all catalysed and non-catalysed Claisen rearrangements a non-synchronous, concerted, pericyclic transition state is involved, with C-O bond cleavage considerably in advance of C-C bond formation. Some evidence has suggested that the ionic active site of the enzymes may polarize the transition state more than occurs in solution. Similar findings apply to the retro-ene fragmentation of chorismate to 4-hydroxybenzoate.17... 

A similar thermal transformation occurs in 1-phenyl-l-methyl-2-neopentylsilene210. Relative to the retro-ene fragmentation of hydrocarbons, that of silenes is favored thermodynamically since it trades a C=Si bond for a C=C bond. 

Retro ene reaction or retro ene fragmentation involves the intramolecular thermolysis of unsaturated compounds by the transfer of a y-hydrogen to the... 

The main reaction occuring in the gas-phase pyrolysis of diallyldimethylsilane 91 is the retro-ene elimination of propene with formation of 1-silabuta-l,3-diene 81 followed by a ring closure reaction to yield the l-silacyclobut-2-ene 80 (equation 20In the presence of methanol the adduct of 81 was observed. The Arrhenius parameters for the retro-ene fragmentation of 91 (log A (s )... 

Fragmentation Group-transfers, retro-ene, cyclo-reversions. 

The ketene valence isomers of mesoionic pyrimidinylium olates have been shown to undergo three different reaction pathways (Scheme 1) either retro-ene-type fragmentation to give C3O2 and 2-aminopyridine (path A), electrocycli-zation to a naphthyridine (path B), or cycloreversion to 2-pyridyl isocyanate and a ketene (path C) <2000J(P2)1841>. The exact nature of the product varied with the substituents present. 

Without Recombination of the Fragments 3.1.1.1. Retro-Diels-Aldcr or Retro-Ene... 

Conjugated tosylhydrazoncs also can be reduced to hydrocarbons with the method depicted in Figure 14.60, that is, with NaBH(OMe)3.The C=C double bond is retained but it is shifted. Figure 14.61 exemplifies this situation for tosylhydrazone A. The sequence of initial steps A —> B —> D resembles the one shown in Figure 14.60. However, the diazo compound D undergoes a different reaction, namely, a retro-ene reaction. A retro-ene reaction is a one-step fragmentation reaction of... 

The retro-ene reaction cleaves an unsaturated compound into two unsaturated fragments. A common example of a retro-ene reaction in organic synthesis is the acid-catalyzed decarboxylation of a (B-ketoester. The ester is hydrolyzed to the (3-ketoacid by the aqueous acid, which rapidly loses carbon dioxide to form enol. The loss of CO2 drives the reaction to the right-hand side. The enol rapidly tautomerizes to the methyl ketone (Scheme 8.17). 

Another competitive situation is encountered with the chrysanthemic alcohol 72 which can choose between a methyl and a hydroxymethyl group for its migrating hydrogen. As it turns out, the methyl substituent wins over the hydroxymethyl, no 77, which could be formed via transition state 74, being produced. Rather than providing 73 (which was only isolated in < 5 %) this, 7-unsaturated alcohol fragmented (in a retro-ene mode) by way of 75 and 76 to formaldehyde and a 3 1 mixture of the dienes 78 and 79. 

One limitation of the oxy-Cope rearrangement is competing fragmenlion of the substrate via a thermal retro-ene reaction731-736. With simple 1,5-dienes, e.g., 9. fragmentation is frequently the major thermal pathway. 

The retro-ene mechanism was considered unlikely225 since the dimer of 1-methoxy-1-methylsilene, l,3-dimethoxy-l,3-dimethyl-l,3-disilacyclobutane, was not observed. However, since little is known about the effect of the methoxy substituent on the dimerization and cross-dimerization rates of silenes, we consider this argument somewhat inconclusive. It is supported, however, by the observation225 that 1-methyl-1-propylsilene, formed by the pyrolysis of 1 -propyl-1 -methyl- 1-silacyclobutane under similar conditions, fails to undergo a retro-ene-type fragmentation, although such a process should be more exothermic, as it would produce a silene and an olefin instead of two silenes. 

The removal of an allylic functionality is always accompanied by an allylic rearrangement when it proceeds by a retro-ene reaction. In most cases such a reaction requires high temperatures as in the thermolysis of allylic or, preparatively more useful, propargylic ethers (Scheme 84), but allylic sulfinic acids undergo fragmentation slightly above room temperature with the potential of high stereocontrol, as the example demonstrates (equation 46). ... 

Vogel and co-workers have used 1 as double chain elongation synthon in their synthesis of long chain polypropionate fragments (eq 48). Their approach is based on an one-pot reaction cascade which involves hetero-Diels-Alder addition of electron-rich l-alkoxy-3-acyloxy-l,2-dienes (e.g., 25) to sulfur dioxide, subsequent ionization into zwitterionic intermediates (e.g., 26), then reaction of the latter with allylsilanes. The final step of this sequence is the retro-ene desulfinylation of an intermediate allylsilyl sulfinate (e.g., 27) to produce useful synthetic intermediates containing two more stereogenic centers (e.g., 28). 

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