Alkylation alternative mechanism

Two alternative mechanisms are possible for esterification, one is dependent upon an acyl-oxy process (R —CO- -OH + H OR) and the other an alkyl-... 

The rearrangement discovered by Kolosova et al. probably involves such reactivit (159). This reaction provides a good preparative method for various 5-amino-methylthiazoles (Scheme 43). No mechanism is proposed in the report, and it is not easy to understand how the C-5 enamine-like position competes with the very nucleophilic thiocarbonyl group of the formed A-4-thiazoline-2-thione. An alternative mechanism could start with ethanol addition at C-2. leading to the A-4-thiazoline (90) (Scheme 44). In this intermediate, C-5 nucleophilic reactivity would be favored bv the true enaminic structure. After alkylation on C-5,... 

A possible alternative mechanism for the formation of fragment alkyl ions comes immediately to mind—namely, beta fission of the Ci8 ion formed in Reactions 6 and 7 to form a smaller alkyl ion and an olefin. Thus we write as a typical example ... 

It seems likely that aryl, vinyl, and cyclopropyl halides react by an alternative mechanism, since the corresponding radicals are less stable than alkyl radicals. It has been suggested that these halides may react through a dianion.13... 

Various alkyl and aryl azides have been transformed into the corresponding nitriles using bromine trifluoride in moderate to good yields (30-60%).25 The reaction is general and gives positive results with aliphatic, aromatic, cyclic, functionalized, and optically active azides. Two alternative mechanisms have been proposed. 

A mechanistic rationale for the Pd-catalyzed addition of a C-H bond at nitriles to allenes is outlined in Scheme 3. The oxidative insertion of Pd(0) into the C-H bond of nitrile 1 produces the Pd(II) hydride species 16 (or alternatively a tautomeric structure E E2C=C=N PdH Ln may be more suitable, where E = H, alkyl, aryl and/or EWG). Carbopalladation of the allene 2 would afford the alkenylpalladium complex 17 (carbopalladation mechanism), which would undergo reductive coupling to give the addition product 3 and regenerates Pd(0) species. As an alternative mechanism, it may be considered that the hydropalladation of allenes with the Pd(II) intermediate 16 gives the jr-allylpalladium complex 18 which undergoes reductive coupling to afford the adduct 3 and a Pd(0) species (hydropalladation mechanism). 

The data seem to support an alternative mechanism compared with the so-called Lyons mechanism in which O2 is activated forming a metal-oxo (M=0) species.1831 Via reaction of a second metalloporphyrin with a primarily formed superoxo (Me-OO0) or peroxo species, a metal-oxo is formed, reacting eventually with an alkane according to the oxygen rebound mechanism. Alternatively, radicals present in solution, upon reaction with dioxygen, may form alkyl hydroperoxides that are decomposed by metalloporphyrins.[83]... 

Since both the alkyl halide and the carbonyl substrate can be reduced independently by Sml2 or Sml2 HMPA, mechanistic studies have been performed to determine whether the reaction occurs through the selective reduction of the alkyl halide to produce an organosamarium (cf. 2 in Scheme 3.2) or whether the reaction is the result of an alternative mechanism involving the... 

Several years ago it was noted that the behavior of dioxetanes substituted with large resonance groups (for example the dioxetanes postulated as intermediates in many bioluminescent reactions) is quite distinct from that of alkyl-, alkoxy-, or simple aryl-substituted dioxetanes (T. Wilson, 1976). The decomposition products of these dioxetanes are often highly fluorescent, possessing lowest excited ttti states, the yields of excited singlet states are often remarkably high, and the dioxetane stability is apparently low. It was suggested at that time that an alternative mechanism, perhaps a truly concerted mechanism, was operative. 

Cyclohexanone and some alkyl derivatives can ring contract to the corresponding cyclopentanecarboxyhc acid the reaction is promoted by thallium(lll). Alternative mechanisms which have been proposed by (i) Wiberg and Koch and (ii) McKillop et a/. involve a 2-thaUyl ketone hydrate which either (i) loses TU and a proton in ring contraction or (ii) loses T1+ first, giving rise to a protonated hydroxyoxirane... 

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