With consecutive carbonylation reactions mechanism

Any attempt to rationalize the complex stereochemistry of olefin formation will have to be concerned with the mechanism of the Wittg reaction. Unfortunately many mechanistic details of this reaction have not been satisfactorily elucidated. Thus, it is even uncertain whether the reaction system (ylide plus carbonyl compound) passes, on its way to phosphine oxide and olefin, through an open chain zwitterion, i.e. a betaine, through a cyclic oxaphosphetane, or through both of them consecutively. The reaction sequence depicted in Figure I has no other merit than that of being believed to be the most probable one (1). 

It should be mentioned here however that products of this type are seldom reported and therefore seem not to be formed in considerable quantities. The reaction of the halide with the metal and consecutive reaction of the intermediate organometallic species (see Sect. 4.4 for a discussion of the mechanism of the Barbier reaction) with the carbonyl compound seems to be fast enough to prevent this ether formation. 

As the above description indicates, the mechanism for Yang photocychzation reactions involves at least two consecutive intermediates first an excited state and then a biradical. (As with most excited carbonyl reactions, both singlet and triplet states can react.) The quantum efficiency for cycHzation, as described by Eq. (8.1), depends on how many competitive reactions each intermediate undergoes ... 

The three-component synthesis of benzo and naphthofuran-2(3H)-ones from the corresponding aromatic alcohol (phenols or naphthols) with aldehydes and CO (5 bar) can be performed under palladium catalysis (Scheme 16) [59,60]. The mechanism involves consecutive Friedel-Crafts-type aromatic alkylation and carbonylation of an intermediate benzylpalla-dium species. The presence of acidic cocatalysts such as TFA and electron-donating substituents in ortho-position (no reaction with benzyl alcohol ) proved beneficial for both reaction steps. 

Products are olefins and the corresponding acids. These reactions are among the most widely studied and best understood of all gas phase unimolecular reactions. With few exceptions they are experimentally and kinetically well behaved cleanly first-order, no surface sensitivity, and no free radical chain complications. Reactions involve 1,5-hydrogen transfer from the f -carbon to the carbonyl oxygen, migration of the carbonyl Jt-bond, rupture of the ester (C-O) bond, and formation of a (Cg-Cf) 7t-bond. All present evidence favors a mechanism in which the above occur in a concerted manner. However, a two-step consecutive mechanism (see later) cannot be entirely ruled out at this time. 

Initially, Passerini proposed that the reaction involved an addition of carboxylic acid to carbonyl compound, giving acyloxy hemiacetal, which then reacted with isonitrile to form the final product. However, current views about this reaction favor the following consecutive processes the protonation of the carbonyl compound from carboxylic acid, followed by the attack of isonitrile to give a nitrilium ion, the nucleophilic addition of carboxylate, acyl group transfer, and the final amide tautomerization. A general mechanism for this reaction is shown here. ... 

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