Elimination phenyl groups

A phenyl group on alkenes can serve as a leaving group via insertion/p-carbon elimination. When the P-ketoester 30 was treated with a palladium catalyst and an oxidant, cyclization took place to afford naphthalene 33 (Scheme 7.10) [13]. The authors proposed a mechanism in which the intermediary alkylpalladium species 31 undergoes syn-P-carbon elimination to produce 32 and a phenylpal-ladium species. The eliminated phenyl group Is trapped by methyl acrylate via the Heck reaction to form methyl cinnamate. Aromatization serves as a driving force for the catalytic C-C bond cleavage. 

If mutarotation were not a factor, the threo pair would give the isomer having the phenyl group and nitrogen atom in a cis relationship and the erythro pair the isomer with the phenyl and carbonyl groups cis. Since it is doubtful, at least in some cases, that steric integrity is maintained in acetic anhydride prior to elimination, the subsequent... 

The mechanism outlined above is supported by experimental findings. An intermediate 5 has been isolated, " and has been identified by and N-nuclear magnetic resonance spectroscopy. Side-products have been isolated, which are likely to be formed from intermediate 4. N-isotope labeling experiments have shown that only the nitrogen remote from the phenyl group is eliminated as ammonia. 

These results are in accordance with the findings of Boothe and coworkers26, who found that the reactions of four diastereomeric 2-bromo-3-phenylsulfinylbutanes with tributyl-tin radicals generate /3-phenylsulfinyl sec-butyl radicals (8) which eliminate PhSO radicals to form the 2-butenes in a stereoselective manner. The stereoselectivities observed in this free radical elimination must result from the fact that the rate constant for elimination is greater than that for rotation about the C—C bond. Furthermore, a neighboring phenyl group on the radical center seems to stabilize the radical enough so that the internal rotation can compete with the -elimination reaction. It is also noteworthy that the small... 

Finally, the major structural features in the substrate promoting E2 elimination are those that serve to stabilise the resultant alkene or, more particularly, the T.S. that precedes it. Such features include increasing alkyl substitution at both a- and //-carbon atoms (leading to alkenes of increasing thermodynamic stability), or introduction of a phenyl group that can become conjugated with the developing double bond. 

Dimerization as well as lithium hydride elimination can be avoided by replacing a (3 hydrogen atom in 5 by an alkyl or a phenyl group. Thus 9a as well as 9b upon the addition of lithium metal yield stable vicinal dilithiumorganic compounds 10 which as 1,2-dilithioethane derivatives interestingly neither lose lithium hydride nor add to the starting material ... 

The use of cyclic alkenes as substrates or the preparation of cyclic structures in the Heck reaction allows an asymmetric variation of the Heck reaction. An example of an intermolecular process is the addition of arenes to 1,2-dihydro furan using BINAP as the ligand, reported by Hayashi [23], Since the addition of palladium-aryl occurs in a syn fashion to a cyclic compound, the 13-hydride elimination cannot take place at the carbon that carries the phenyl group just added (carbon 1), and therefore it takes place at the carbon atom at the other side of palladium (carbon 3). The normal Heck products would not be chiral because an alkene is formed at the position where the aryl group is added. A side-reaction that occurs is the isomerisation of the alkene. Figure 13.20 illustrates this, omitting catalyst details and isomerisation products. 

Thus the additional stabilization of the eliminated free radical by the phenyl group promotes essentially quantitative cleavage. The mechanism of the production of the end-capped oligomer is probably as follows (24) ... 

If, instead of electrocyclization, electrophilic attack of the closer upper edge of the phenyl group by the carbene carbon atom occurs, a zwitterionic intermediate might result, which upon 1,4-elimination of (COljW would yield a 1-methoxy-1,3-cyclopentadiene. Suprafacial hydrogen migration would finally lead to the formation of the observed diastereomer. 

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