Carbonyl base-promoted

Hydroxy-substituted iron-acyl complexes 1, which are derived from aldol reactions of iron-acyl enolates with carbonyl compounds, are readily converted to the corresponding /i-methoxy or /1-acetoxy complexes 2 on deprotonation and reaction of the resulting alkoxide with iodomethane or acetic anhydride (Tabic 1). Further exposure of these materials to base promotes elimination of methoxide or acetate to provide the a,/ -unsaturated complexes (E)-3 and (Z)-3 (Table 2). 

It now appears that at least two mechanisms exist for the base-promoted homogeneous water gas shift reaction, differing in the method of hydride formation. The "associative mechanism", first proposed by Pettit and co-workers (1,4), involves nucleophilic attack on a carbonyl ligand and it has two variations. 

Analogous carbonylation reactions using nickel and iron carbonyl based systems also produce alkanecarboxylic acids [11, 13, 14]. The mechanism of the conversion of benzyl halides into arylacetic acids using iron pentacarbonyl is not as well defined as it is for reactions promoted by nickel or molybdenum carbonyl complexes. Iron... 

Sulfonium ylides generated through base-promoted deprotonation of sulfonium salt have been extensively studied. The reaction of sulfides with a diazo carbonyl compound in the presence of a transition metal catalyst is an alternative approach to obtain sulfonium ylides. Sulfonium ylides are more stable than the corresponding oxonium ylides. Stable sulfonium ylides generated by the reaction of an Rh(ii) carbene complex with thiophene have been reported (Figure 5). ... 

Lower yields of Na3[Mn(CO)4] were obtained from the direct reduction of Mn2(CO)10 in Na-HMPA, because the neutral dimer underwent slow disproportionation in this medium to form [Mn(HMPA)Jt][Mn(CO)5]2, in contrast to Re2(CO)10, which showed no tendency to react with HMPA at room temperature. Of all neutral binary carbonyl dimers known, Re2(CO)10 appears to be the most resistant toward Lewis base-promoted disproportionation reactions. The slightly lower yields of Na3[Re(CO)4], compared to those of Na3[Mn(CO)4], may have arisen from the fact that Re2(CO)10 does not cleanly reduce to [Re(CO)5] in HMPA or other solvents (22). It should... 

The carbonyl group in 24 could be temporarily protected as the dioxolane,16 or somewhat more efficiently as the dimethyl acetal 25.17 Subsequent conventional hydroboration led stereoselectively to the P alcohol 26. Although our earliest considerations called for ultimate base-promoted elimination of the OR substituent in intermediates more advanced than 16, we later came to recognize that a methoxy derivative was not going to fulfill this role to our satisfaction. Alternatively, the tactic of forming the benzyl ether at this point ultimately proved well suited to the eventual return to an unmasked hydroxyl at this site.17,18... 

Carboxylic acid esters have been studied widely and detailed mechanisms have been worked out for these reactions. Equation (4) shows the overall reaction of a carboxylic add ester with hydroxide via the nucleophilic attack of hydroxide at the carbonyl oxygen, with the subsequent loss of the alcohol moiety (HOR). Esters generally have larger ly, than kH+ values, with the result that they hydrolyze by base-promoted reactions at pH 5-6 (Mabey and Mill, 1978). 

The Darzens reaction is the base-promoted generation of epoxides XIII from aldehydes (or ketones) XI and alkyl halides XII, the latter carrying an electron withdrawing group, for example the carbonyl, nitrile, or sulfonyl, in the a-position (Scheme 6.83) [188, 189]. It is, formally, addition of a carbene to the C=0 double bond (Scheme 6.83, path B) and thus complements oxygen atom transfer to olefins... 

This suggests that in the case that both intramolecular C- and 0-arylations are of geometrically equal possibility, the former preferentially takes place. The reaction of o-bromobenzaldehydes with dibenzylketones gives l,3-diaryl-2-naphthols that may be useful materials as bulky 0-ligands (Eq. 24) [62]. This cyclization involves palladium-catalyzed a-arylation and base-promoted dehy-drative condensation. Treatment of o-iodoanilines with ketones produces indole derivatives (Eq. 25) [63]. In this case, dehydrative condensation between the amino group and the carbonyl group occurs initially and the subsequent car-bopalladation (mechanism A) leads to the products. 

Finally, a base-promoted E2 elimination of water occurs to give the product. This elimination is driven energetically by the formation of a double bond, which is stabilized by conjugation with both a phenyl group and a carbonyl group. 

The alcohol can undergo base-promoted elimination to give two possible products, 5-58 or 5-59. The new double bond in each of these products is stabilized by conjugation with the carbonyl group and at least one of the aromatic rings. 

Preparation of alkylidene cyclopropanes is feasible by alkylation of lithium carbenoids followed by base-promoted elimination (equation 13or by addition of carbonyl compounds, silylation and reductive elimination (equation 132). Similarities with the chemistry displayed by sulphur-substituted cyclopropanes (Section VI.D.l) are obvious. 

Furthermore, several syntheses of a,p-unsaturated carbonyl compounds have involved the alkylation of suitably functionalized sulfones, followed by the base-promoted elimination of the sulfenate moiety on related alkyl sulfones bearing an oxygen atom at the -y-position (Scheme 106, entries b-d Scheme 110, entry c Scheme 105, entry b). - 46i Particularly interesting is the synthesis of nuciferal (Scheme 106, entry d) which involves sequential alkylation of y,-y-dioxy sulfones, deblocking of the acetal moiety and sulfenate elimination. In this and related reactions y, y-dioxy-a-metalloalkyl sulfone has played the role of a masked p-metallo-a,p-unsaturated carbonyl compound (M(>aC—C—C=0), which is not a directly available synthon. 

A simple and versatile method for the synthesis of aziridinium salts involves the reaction of a diazoalkane with an iminium salt (82TL285, B-83MI 102-01). The aziridinium salt thus formed was oxidized to an aminoaldehyde by DMSO (Scheme 39). A key step in the preparation of the oxindole alkaloid gelsemine (104) involves the carbonylation of the unsaturated aziridinium ion (103) with disodiumtetracarbonyl picrate-dioxane complex under a CO atmosphere followed by base-promoted epimerization and deprotonation (Scheme 40) <92JOC1035>. 

The second chapter, by David A. Oare and Clayton H. Heathcock, deals with the stereochemistry of uncatalyzed Michael reactions of enamines and of Lewis acid catalyzed reactions of enol ethers with a,/ -unsaturated carbonyl compounds. It is effectively a continuation of their definitive review of base-promoted Michael addition reaction stereochemistry that appeared in the preceding volume of the series. 

There have been several reports of the use of cyano-acetamides as C-C-N synthons in pyridone synthesis. The C-C-C component that is required for the formation of a six-membered ring may be either an 0 /3-unsaturated carbonyl compound or a dicarbonyl compound. Alternatively, pyridones have been formed by base-promoted cyclocondensation of cyano-acetamides with a-keto-ketene 5,A -acetals (Scheme 10). ... 

Activated hollow spheres have been found to be advantageous for the hydrogenation of carbonyl compounds, nitriles, aromatics, and unsaturated C-C bounds. In the case of carbonyl conqrounds, promoters (e.g.. Mo and Cr) that exist as surface cations were found to be the most effective. In the case of nitriles, the use of promoters to stabilize the residual A1 content of the catalyst so that it can be used with base modifiers was found to be the most useful combination. An example of this was the improved performance of the LiOH treated Cr / Ni promoted Co hollow spheres for the hydrogenation of adiponitrile to hexamethylenediamine. Some reactions were found to be more sensitive to the type of promoter they require. In the case of l,4-dihydroxy-2-butyne, it was found that Mo worked satisfactory as a promoter while the Cr / Fe combination led to worse results. Nonetheless, for all of the reactions studied here it was found that the activate hollow spheres were more active than the activated tablets on both a volume and weight basis, thereby allowing increased flexibility in the use of promoters and other selectivity enhancing additives. 

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