Elimination—addition decarboxylation

Phenylthioacetylene has been prepared by elimination of thiophenol and dehydrobromination of cis-1,2-bis(phenylthio)ethylene5 and cis-1-bromo-2-phenylthioethylene,2 7 respectively. The latter was obtained by addition of thiophenol to propiolic acid in ethanol and subsequent one-pot bromine addition, decarboxylative dehalogenation, and careful distillation to remove the trans isomer.2.7 on the other hand, cis-1,2-bis(phenylthio)ethylene was prepared by double addition of thiophenol to cis-1,2-dichloroethylene.5a d Although these procedures can provide useful amounts of phenylthioacetylene, they were found to be somewhat less satisfactory in our hands as far as operation and/or overall yields are concerned. Furthermore, we have encountered problems with regard to the reproducibility of one-pot dehydrobrominations of phenylthio-1,2-dibromoethane.6 However, the stepwise execution of the double dehydrobromination, as described in the modified procedure reported here, provides preparatively useful quantities of phenylthioacetylene in a practical manner. 

The reaction is thought to occur by nucleophilic addition of the N-alkyl-hydroxylamine to the keto acid as if forming an oxime (Section 19.8), followed by decarboxylation and elimination of wrater. Show the mechanism. 

Step 4 of Figure 29.12 Oxidative Decarboxylation The transformation of cr-ketoglutarate to succinyl CoA in step 4 is a multistep process just like the transformation of pyruvate to acetyl CoA that we saw in Figure 29.11. In both cases, an -keto acid loses C02 and is oxidized to a thioester in a series of steps catalyzed by a multienzynie dehydrogenase complex. As in the conversion of pyruvate to acetyl CoA, the reaction involves an initial nucleophilic addition reaction to a-ketoglutarate by thiamin diphosphate vlide, followed by decarboxylation, reaction with lipoamide, elimination of TPP vlide, and finally a transesterification of the dihydrolipoamide thioester with coenzyme A. 

The reaction proceeds at room temperature and is rationalized invoking oxidative addition of a Pd(0) species upon the allylic C - O bond of 67, followed by decarboxylation to form an oxapalladacyclopentane intermediate 66 (Pd in place of Ni), which undergoes a facile b-C elimination to finally give an co-dienyl aldehyde 68 (Scheme 17). Recently, it has been revealed that a combination of Ni(cod)2 and a phosphine ligand also catalyzes the same... 

Treated with a base, l-(arylacetyl)benzotriazoles 954 eliminate benzotriazole to form ketenes 955. When no other reagent is added, ketene 955 is acylated by another molecule of 954 to produce ot-ketoketene 956 which upon addition of water and decarboxylation during the work-up is converted to symmetrical dibenzyl ketone 957... 

These enzymes invariably involve a cofactor, pyridoxal phosphate (vitamin B6). In addition, pyridoxal phosphate is also required for most decarboxylations, racemizations, or elimination reactions in which an amino acid is a substrate. Pyridoxal phosphate is not involved in decarboxylations in which the substrate is not an amino acid. So if a question... 

In this context we postulated that the shift reaction might proceed catalytically according to a hypothetical cycle such as Scheme I. There are four key steps in Scheme I a) nucleophilic attack of hydroxide or water on coordinated CO to give a hydroxycarbonyl complex, b) decarboxylation to give the metal hydride, c) reductive elimination of H2 from the hydride and d) coordination of new CO. In addition, there are several potentially crucial protonation/deprotonation equilibria involving metal hydrides or the hydroxycarbonyl. The mechanistic details have been worked out (but only incompletely) for a couple of the alkaline solution WGSR homogeneous catalysts. In these cases,... 

The addition product of ethyl acetoacetate and methyl a-methoxyacrylate was hydrolyzed, and the resulting dicarboxylic acid was treated with dimethylamine hydrochloride and aqueous formaldehyde. The product of the Mannich reaction was decarboxylated, reesterifed, and finally treated with methyl iodide to supply quaternary salt 469 as the main product. During the above one-pot process, elimination also took place, yielding unsaturated ketone 470, which was later utilized as its hydrogen bromide adduct 471. Reaction of 3,4-dihydro- 3-car-boline either with 469 or 471 furnished the desired indolo[2,3-a]quinolizine derivative 467 as a mixture of two diastereomeric racemates. 

The effect of additives betrays the intricacy of the balance of rate effects even more. The addition of cholesterol to catalytic bilayers has been found to be beneficial for the Kemp eleminiation but to inhibit the decarboxylation of 6-NBIC. In general, the effects of additives on the decarboxylation of 6-NBIC appear to subtly depend on the structure of the hydrophobic tail and hydrophilic headgroup of additives. Similarly subtle effects were found for the Kemp elimination and nucleophilic attack by Br and water on aromatic alkylsulfonates depending on the choice of additive, hydrogen bonding effects, reactivity of partially dehydrated OH , and local water concentrations all played a role and vesicular catalysis could be increased or decreased. 

Ikegami has devised an interesting approach based upon 1,3-cyclooctadiene monoepoxide as starting material (Scheme LX) Transannular cyclization, Sharpless epoxidation, and silylation leads to 638 which is opened with reasonable regioselec-tivity upon reaction with l,3-bis(methylthio)allyllithium. Once aldehyde 639 had been accessed, -amyllithium addition was found to be stereoselective, perhaps because of the location of the te -butyldimethylsilyloxy group. Nevertheless, 640 is ultimately produced in low overall yield. This situation is rectified in part by the initial formation of 641 and eventual decarboxylative elimination of 642 to arrive at 643. An additional improvement has appeared in the form of a 1,2-carbonyl transposition sequence which successfully transforms 641 into 644... 

Substituted cyclopropyl rings conjugated with a triple bond system have recently received attention as C5 building blocks. The procedure described here is a modification of the decarboxylation-elimination reaction for the preparation of a.3 acetylenic acids from enol sulfonates of acyl malonates. Addition of aqueous alkali to the enol sulfonate of diethyl cyclopropyl carbonyl malonate gives cycl opropyl propiol ic acid, but the yield is 1 ow. 

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