Isomerizations in carbonylations

Review papers have covered (81RCR336 84KGS579) the field of rotational isomerism in carbonyl-containing derivatives of five-membered aromatic heterocycles. This article aims to report a critical account of the results appearing in the literature concerning conformational information on heterocyclic acyl derivatives. 

The rotational isomerism in carbonyl-containing compounds of five-membered heterocycles has been well reviewed <77JCS(P2)l60i, 81RCR336, 84KGS579. 87AHC(4l)75>. The energy of rotation around two ip -hybridized C—C bonds is about 2.9 kcal mol , while that for two. yp -hybridized C=C... 

Olefin Isomerization. Some olefins can be isomerized in the presence of metal carbonyls. The carbonyl can act as a catalyst or a stoichiometric reagent in the reaction (137). 

The process under consideration could be discussed in terms of ring-chain tautomerism of nitronates (378=381) (see the lower part of Scheme 3.215), the more so that such examples were documented for proton analogs of similar SENA (492). However, both nitronates ((378) and (381)), which were prepared by independent syntheses, are quite stable, and therefore their isomerization in the presence of a silyl Lewis acid should involve ring-chain tautomerism of cations. Evidently, cyclization of nitronate (378) is attributed to high electrophilicity of the carbon atom of the carbonyl group, provided that this group is not involved in the conjugation chain. 

Phase transfer catalysis has more recently been applied to the important area of organometallic chemistry(18). Reported applications include both the preparation(19) and the use of transition metal catalysts in isomerizations(20), carbonylations(21) and reductions(22). 

Epoxides can be isomerized to carbonyl compounds by Lewis acids.104 105 Boron trifluoride is frequently used as the reagent. Carbocation intermediates appear to be involved, and the structure and stereochemistry of the product are determined by the factors which govern substituent migration in the carbocation. Clean, high-yield reactions can be expected only where structural or conformational factors promote a selective rearrangement. 

The singular tendency of ethylene oxides to undergo isomerization to carbonyl compounds (Eq. 433) was reoognized many years ago by a number of distinguished chemists. Wurts himself,1884 in his classic publication announcing the discovery of ethylene oxide, stated that this substance was related to acetaldehyde, with which it appeared to share certain chemical properties. Conversion of ethylene oxide into... 

The action of triethylamine may cause base-induced reactions, such as a-epimerization of carbonyl compounds isomerization of alkenes into conjugation with carbonyl groups and, elimination in carbonyl compounds posssessing a good-leaving group at the (3-position... 

The products of oxidative addition of acyl chlorides and alkyl halides to various tertiary phosphine complexes of rhodium(I) and iridium(I) are discussed. Features of interest include (1) an equilibrium between a five-coordinate acetylrhodium(III) cation and its six-coordinate methyl(carbonyl) isomer which is established at an intermediate rate on the NMR time scale at room temperature, and (2) a solvent-dependent secondary- to normal-alkyl-group isomerization in octahedral al-kyliridium(III) complexes. The chemistry of monomeric, tertiary phosphine-stabilized hydroxoplatinum(II) complexes is reviewed, with emphasis on their conversion into hydrido -alkyl or -aryl complexes. Evidence for an electronic cis-PtP bond-weakening influence is presented. 

The complex is additionally stabilised by co-ordination of the phenoxide, and possibly the carboxylate, to the metal ion, illustrating the utility of chelating ligands in the study of metal-directed reactivity. We saw in the previous section the ways in which a metal ion may perturb keto-enol equilibria in carbonyl derivatives, and similar effects are observed with imines. The metal ion allows facile interconversion of the isomeric imines. The first step of the reaction is thus the tautomerisation of 5.28 to 5.29 (Fig. 5-56). Finally, the metal ion may direct the hydrolysis of the new imine (5.29) which has been formed, to yield pyridoxamine (5.30) and the a-ketoacid (Fig. 5-57). 

Dioxiranes (32) are isomeric with carbonyl oxides (33), one of the peroxidic intermediates involved in the ozonolysis process.9 Dimethyldioxirane (DMDO) (31) epoxidizes the double bond of the protected galactal favoring the Oepoxide 8 with a selectivity of 20 1.10,11 You can use 31 likewise to transform an aldehyde into a carboxylic acid. 

A study of the role of base and ligand in the isomerization of manganese-coordinated conjugated alkynyl carbonyls to the corresponding allenyl carbonyls shows that manganese requires a ligand prior to isomerization with amine bases and the efficacy of these bases correlates with p Ka (Scheme 97). Potassium /-buloxide led to rapid isomerization in the absence of added ligand.140... 

Activation Parameters for a-f Isomerization and Carbonyl Scrambling in [M(CO)3L(ij4-dieneJ] Complexes... 

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