Alkenyl mechanism

Depending on the nature of surface chain growth species, on the other hand, one is confronted mainly with the alkyl mechanism,6 based on the insertion of a methylene species C CHj) into the metal-alkyl bond, or with the alkenyl mechanism,2 wherein a surface vinyl species ( CH=CH2) reacts with a surface methylene ( CII2) to form an allyl species ( CH2CH=CH2). 

The mechanism with the widest support in the literature is the consecutive alkenyl mechanism (10-18). According to this mechanism, once butane has adsorbed onto the vanadium phosphate, it is transformed via adsorbed alkenyl intermediates into MA. A summary of the mechanism is shown in Figure 3. 

FIGURE 3 Consecutive alkenyl mechanism of n-butane oxidation to MA (77-74). 

The consecutive alkenyl mechanism (Figure 3) was put forward as the route for oxidation of unsaturated reactants. The weakly adsorbed intermediates are presumed to be in equilibrium with the gas phase, which enables furan to be seen as a product in butene oxidation (22,24,27). In contrast to the previous work, this study included an examination of the fact that none of the alkene intermediates desorb from the catalyst. It was proposed that the reaction proceeded via more strongly adsorbed alkoxide intermediates that would remain on the surface for the whole oxidation sequence (Figure 5). 

FIGURE 5 Consecutive alkenyl mechanism for butene oxidation proposed by Zhang-Lin et al. (25,26). 

The competitive experiments and KIE reactions show that C-H bmid cleavage is a rate-determining step and leads to a SeAr-like metallation. A five-membered ruthenacycle intermediate was proposed via an ortho C-H deprotonation directed by the carbonyl group with ruthenium catalyst by analogy to the alkenylation mechanism (Scheme 16). Then alkyne insertion into the resulting Ru-C bond gave a seven-membered intermediate, and the hydroarylated product was produced by protonolysis of the Ru-C bond (Scheme 17) [163, 164]. 

I.l.IJ Reactions nitlr 1,2-, 1.3-. ami 1.4-dienes. The reaction of conjugated dienes with aryl and alkenyl halides can be explained by the following mechanism. Insertion of a conjugated 1.3-diene into an aryl or alkenylpalladium bond gives the T-allvlpalladium complex 243 as an intermediate, which reacts further... 

The o-keto ester 513 is formed from a bulky secondary alcohol using tricy-clohexylphosphine or triarylphosphine, but the selectivity is low[367-369]. Alkenyl bromides are less reactive than aryl halides for double carbonyla-tion[367], a-Keto amides are obtained from aryl and alkenyl bromides, but a-keto esters are not obtained by their carbonylation in alcohol[370]. A mechanism for the double carbonylation was proposed[371,372],... 

The mechanism includes two single electron transfers (steps 1 and 3) and two proton transfers (steps 2 and 4) Experimental evidence indicates that step 2 is rate determining and it is believed that the observed trans stereochemistry reflects the dis tribution of the two stereoisomeric alkenyl radical intermediates formed in this step... 

McDowell and Stirling194 studied electronic effects upon the reactivity of aryl vinyl sulfones towards amines. Rate constants for t-butylamine addition in ethanol at 25 °C were well correlated by the Hammett equation, with p = 1.59. Comparison of this with p values for H-D exchange mentioned above191 suggested considerable carbanionic character in the transition state, perhaps in a concerted mechanism. Rates of addition of amines to alkenyl, allenyl and alkynyl p-tolyl sulfones have also been measured195. 

Fischer alkenylcarbene complexes undergo cyclopentannulation to alkenyl AT,AT-dimethylhydrazones (1-amino-1-azadienes) to furnish [3C+2S] substituted cyclopentenes in a regio- and diastereoselective way along with minor amounts of [4S+1C] pyrrole derivatives. Enantiopure carbene complexes derived from (-)-8-(2-naphthyl)menthol afford mixtures of trans,trans-cycloipentenes and ds,ds-cyclopentenes with excellent face selectivity [75]. The mechanism proposed for the formation of these cyclopentene derivatives is outlined in Scheme 28. The process is initiated by nucleophilic 1,2-attack of the carbon... 

The general mechanism of coupling reactions of aryl-alkenyl halides with organometallic reagents and nucleophiles is shown in Fig. 9.4. It contains (a) oxidative addition of aryl-alkenyl halides to zero-valent transition metal catalysts such as Pd(0), (b) transmetallation of organometallic reagents to transition metal complexes, and (c) reductive elimination of coupled product with the regeneration of the zero-valent transition metal catalyst. 

Acetylation of acetals or ketals can be accomplished with acetic anhydride and BF3-etherate. ° The mechanism with acetals or ketals also Involves attack at an alkenyl carbon, since enol ethers are intermediates. Ketones can be formylated in the a position by treatment with CO and a strong base. ... 

When NBS is used to brominate non-alkenyl substrates such as alkanes, another mechanism, involving abstraction of the hydrogen of the substrate by the succinimidyl radical " 14 can operate. " This mechanism is facilitated by solvents (such as CH2CI2, CHCI3, or MeCN) in which NBS is more soluble, and by the presence of small amounts of an alkene that lacks an allylic hydrogen (e.g., ethene). 

The plausible mechanism is based on the proposal by Jana and coworkers (Scheme 14). In this case, the sp-hybridized vinyl cation can be attacked by halide, instead of water, to give the ElZ isomer of the alkenyl halide. Compared with the systems using stoichiometric Lewis acid and strong base to prepare substituted alkenyl halides, the present method would provide an excellent alternative due to the environmentally benign system and atom efficiency. 

Intermolecular hydroalkoxylation of 1,1- and 1,3-di-substituted, tri-substituted and tetra-substituted allenes with a range of primary and secondary alcohols, methanol, phenol and propionic acid was catalysed by the system [AuCl(IPr)]/ AgOTf (1 1, 5 mol% each component) at room temperature in toluene, giving excellent conversions to the allylic ethers. Hydroalkoxylation of monosubstituted or trisubstituted allenes led to the selective addition of the alcohol to the less hindered allene terminus and the formation of allylic ethers. A plausible mechanism involves the reaction of the in situ formed cationic (IPr)Au" with the substituted allene to form the tt-allenyl complex 105, which after nucleophilic attack of the alcohol gives the o-alkenyl complex 106, which, in turn, is converted to the product by protonolysis and concomitant regeneration of the cationic active species (IPr)-Au" (Scheme 2.18) [86]. 

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