Alkoxide mechanism

Kowalski A, Libiszowski J, Majerska K, Duda A, Penczek S (2007) Kinetics and mechanism of E-caprolactone and LJ--lactide polymerization coinitiated with zinc octoate or aluminum acetylacetonate The next proofs for the general alkoxide mechanism and synthetic applications. Polymer 48 3952-3960... 

Agaskar et al. (28) proposed a different consecutive alkoxide mechanism, based on theoretical and experimental results. In contrast to other proposals, it suggests that adsorbed oxygen is the active oxygen species (Figure 6). 

FIGURE 6 Consecutive alkoxide mechanism for butane oxidation proposed by Zhang-Lin et al. (25,26). 

FIGURE 7 Consecutive alkoxide mechanism proposed by Agaskar et aL (28). 

Consecutive Alkoxide Mechanism. A consecutive mechanism was proposed by Zhang-Lin and co-workers (213,214). The mechanism was based on kinetic data calculated for the oxidation of butane, 1-butene, 1,3-butadiene, and furan over (V0)2P207 and VOPO4 phases. Unlike the TAP reactor studies, the kinetic data suggested that furan is not an intermediate for butane oxidation but is an intermediate for butadiene oxidation. The differences observed in the oxidation of butane and the unsaturated hydrocarbons question the validity of applying butene and butadiene oxidation results to the butane system. 

Fig. 29. Consecutive alkoxide mechanism as proposed by Agaskar and co-workers (215).

The following mechanism appears reasonable (compare Section VI, 12), It assumes that the function of the aluminium ieri.-butoxide, or other alkoxide. is to provide a source of aluminium ions and that the aluminium salt of the secondary alcohol is the actual reactant. Aluminium with its sextet of electrons has a pronounced tendency to accept a pair of electrons, thus facilitating the initial coordination and the subsequent transfer of a hydride ion ... 

Wylation under neutral conditions. Reactions which proceed under neutral conditions are highly desirable, Allylation with allylic acetates and phosphates is carried out under basic conditions. Almost no reaction of these allylic Compounds takes place in the absence of bases. The useful allylation under neutral conditions is possible with some allylic compounds. Among them, allylic carbonates 218 are the most reactive and their reactions proceed under neutral conditions[13,14,134], In the mechanism shown, the oxidative addition of the allyl carbonates 218 is followed by decarboxylation as an irreversible process to afford the 7r-allylpalladium alkoxide 219. and the generated alkoxide is sufficiently basic to pick up a proton from active methylene compounds, yielding 220. This in situ formation of the alkoxide. which is a... 

Section 5 15 Dehydrohalogenation of alkyl halides by alkoxide bases is not compli cated by rearrangements because carbocations are not intermediates The mechanism is E2 It is a concerted process m which the base abstracts a proton from the p carbon while the bond between the halogen and the a carbon undergoes heterolytic cleavage... 

As crowding at the carbon that bears the leaving group decreases the rate of nude ophilic attack by the Lewis base increases A low level of steric hindrance to approach of the nucleophile is one of the special circumstances that permit substitution to pre dominate and primary alkyl halides react with alkoxide bases by an 8 2 mechanism m preference to E2... 

The reaction between an alkoxide ion and an aryl halide can be used to prepare alkyl aryl ethers only when the aryl halide is one that reacts rapidly by the addition-elim mation mechanism of nucleophilic aromatic substitution (Section 23 6)... 

A cis-elimination mechanism has been postulated for this decomposition which foUows first-order kinetics (120). The rate is accelerated by addition of lithium j iZ-butoxide [4111-46-0] and other bases, and by an increase in temperature (120). Pyrolysis of j iZ-butyUithium in the presence of added alkoxide is one-half order in alkyUithium and first order in alkoxide (120). Thermal decomposition of j iZ-butyUithium at 0.18% alkoxide at 25, 40, 50, and 60°C is 0.1%, 0.6%, 2.0%, and 6.8%/d, respectively (121). 

Mixing. In method 1, a suspension of colloidal powders, or sol, is formed by mechanical mixing of colloidal particles in water at a pH that prevents precipitation (step A in Fig. 1) (8). In method 2 or 3, a Hquid alkoxide precursor such as Si(OR)4, where R is CH (TMOS), C2H (TEOS), or C Hy, is hydrolyzed by mixing with water (eq. 2). 

The same products may be made from primary alkoxides by the violent reaction with elementary chlorine or bromine. A radical mechanism has been proposed to account for the oxidation of some of the alkoxy groups (54) ... 

The acetal might undergo ionization with formation of an alkoxide ion and a carbocation. In a second step, the alkoxide would be protonated. This mechanism is extremely rare, if not impossible, because an alkoxide ion is a poor leaving group. 

Esters react with alcohols in either aeidic or basic solution to exchange alkoxy groups (ester interchange) by mechanisms which parallel hydrolysis. The alcohol or alkoxide acts as the nucleophile ... 

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