Mechanistic Aspects of the Reaction

Enolates of aldehydes, ketones, and esters and the carbanions of nitriles and nitro compounds, as well as phosphorus- and sulfur-stabilized carbanions and ylides, undergo the reaction. The synthetic applications of this group of reactions will be discussed in detail in Chapter 2 of Part B. In this section, we will discuss the fundamental mechanistic aspects of the reaction of ketone enolates with aldehydes md ketones. 

There has been relatively little work on the mechanism of arylation of heterocyclic compounds, but the arylation of homocyclic compounds has been studied in great detail. Those mechanistic aspects of the reactions with heterocycles which have been examined parallel closely the corresponding aspects of the reactions of homocycles, and there is... 

Synthetic and mechanistic aspects of the reactions of the alkali-metal derivatives of organotin compounds, RaSnM ("organostannylan-ionoids ) have been reviewed (79, 80). They may be prepared by reactions of the types shown in the following equations. 

Ford, P.C. and Lorkovic, I.M. (2002) Mechanistic aspects of the reactions of nitric oxide with transition-metal complexes, Chem. Rev., 102, 993, and references therein. 

However, more recent evidence contradicts the above described mechanism of the reaction of BENA with nucleophiles. The mechanistic aspect of the reactions of polyazoles and their N -silyl derivatives with BENA has been studied in most... 

Thermal rearrangement of propadienylcyclopropanes to methylenecyclopentenes has been examined in several cases however, selective transformation to the product has not necessarily been easy due to the harsh reaction conditions required for the rearrangement. The first example of this type of reaction was reported by Dewar, Fonken, and co-workers in a paper on the kinetics of the thermal reaction of 3-cyclopropyl-l,2-butadiene (44), and the reaction was found to proceed much faster (activation energy difference 8.2 kcal) than that of the corresponding vinylcyclopropane [25]. Several examples have appeared since this initial work, most of which have dealt with the mechanistic aspect of the reaction, but none of them has reached a synthetically useful level [26]. For example, thermal reaction of 3-(2-methylcyclopropyl)-1,2-butadiene (45) gives a mixture of five products, as shown in Scheme 20 [27]. 

The dearomatising cyclisation of A-benzoyl oxazolidines 448200 has opened up the possibility that non-benzylated starting materials may prove valuable in syntheses based on dearomatising cyclisation, and also reveals some deeper mechanistic aspects of the reaction. Amide 448 is ortholithiated by r-BuLi to give 449, as indicated by the fact that deuteration after only 30 min gives 453. However, with time or on warming, 449 cyclises to 452 and... 

Tanaka has recently reviewed the hydrogenation of ketones with an emphasis on the mechanistic aspects of the reaction.233 Numerous references related to this subject can be found in his article. Deuteration of cyclohexanones and an application of NMR spectroscopy to the analysis of deuterated products have revealed that on ruthenium, osmium, iridium, and platinum, deuterium is simply added to adsorbed ketones to give the corresponding alcohols deuterated on the Cl carbon, without any deuterium atom at the C2 and C6 positions, while over palladium and rhodium the C2 and C6 positions are also deuterated.234 A distinct difference between rhodium and palladium is that on rhodium deuterium is incorporated beyond the C2 and C6 positions whereas on palladium the distribution of deuterium is limited to the C2 and C6 carbons.234,235 From these results, together with those on the deuteration of adamantanone,236 it has been concluded that a Tt-oxaallyl species is formed on palladium while deuterium may be propagated by an a, 3 process237 on rhodium via a staggered a, 3-diadsorbed species. 

In situ infrared reflectance spectroscopy investigation of the oxidation reaction of ethanol appears thus as an efficient method to elucidate some mechanistic aspects of the reaction. However, the quantitative analysis of the reaction products remains difficult due to different parameters the characteristic absorption band may not be monopolar (this is the case for carbon monoxide for example) and the difficulty to obtain a quantitative relationship between infrared extinction coefficients and concentration for reaction products and by-products. 

All the work mentioned in this section is silent on the mechanistic aspects of the reactions, except for two species observed by Schultz and Bellstedt (75) which are hydrido tri- or tetranuclear complexes, namely, [H4Ru4(CO),2] and [H4Ru3(CO),o]. 

Investigations of the mechanistic aspects of the reactions of organomagnesium compounds, in general, and of Grignard reagents, in particular, that have led to our modern insight into these aspects started only rather recently (i.e., in the early 1960s). 

An obvious way to obtain a better insight into the importance of the Schlenk equilibrium on the mechanistic aspects of the reactions of Grignard reagents with a wide variety of substrates is to first study these reactions for the simpler dialkyl- (or for that matter diaryl-) magnesium species. 

To conclude this section, the mechanistic aspects of the reaction of methylmagnesium bromide with (a substituted) benzophenone will be discussed in detail, as it was investigated in the same period [18,19,93]. 

Provided the starting materials are easy to obtain, the interesting reductive substitution of aromatic ni-troso compounds (Scheme 54) is also a useful preparative technique. The mechanistic aspects of the reaction sequence have been discussed. ... 

Plaquevent and coworkers re-examined and sharply improved this method for a rapid access to enantioenriched P-trifluoromethyl-P-amino acid [47]. Nine cinchona-based catalysts were screened, and the best result was obtained using (DHQ)2PHAL 80. The reaction was performed starting with the p-nitrobenzyl enaminoester 81 at 80 °C and afforded the expected imine ester 82 in 90% isolated yield and 71% ee (Scheme 7.36). The authors put a special emphasis on the mechanistic aspect of the reaction using a deuterated substrate. According to the results, the deprotonation is both rate and asymmetric determining step. 

Shortly after the discovery of the Lewis acid-mediated Mukaiyama aldol addition reaction of enol silanes the general mechanistic aspects of the reaction were intensely investigated [30a, 30b, 30c, 30d]. These processes are considered to proceed by electrophilic activation of the aldehyde towards addition by the nucleophilic enol silane. However, aldol addition processes that proceed by alternative mechanistic pathways have been documented and studied. It is worth considering those systems that have been developed for catalytic, enantioselec-tive aldehyde addition reactions through metaiioenoiate intermediates. 

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