Competition, alkylation

In a similar manner competitive alkylation may continue resulting m formation of a trialkylamme... 

In a similar manner, competitive alkylation may continue, resulting in formation of a trialkylfflnine. 

Scheme 9. Competitive alkyl exchange and coupling in the reaction of [TpBut]MgMe with 13CH3I.

Dewar,120 as well as Brown and Olah, respectively, raised the importance of initial n complexing of aromatics in alkylations.109121 Relevant information was derived from both substrate selectivities (usually determined in competitive alkylations of benzene and toluene, or other alkylbenzenes), and from positional selectivities in the alkylation of substituted benzenes. Olah realized that with reactive alkylating agents substrate and positional selectivities are determined in two separate steps. 

Figure 5-75. The alkylation of a nickel(n) thiolate complex gives a thioether complex. There is no competitive alkylation of the pyridine nitrogen atom.

As previously described, ethers can be alkylated photochemically at the carbon alpha to the ether oxygen. In compounds having both an ether and a carbonyl function a "competitive alkylation process may take place. Tetrahydro-y-pyrone can serve as a good model for this "competitive alkylation between a ketone and an ether. This molecule has a ketone function having two methylene groups at its alpha positions, and an ether function having also two adjacent methylenes. When tetrahydro-y-pyrone was left in sunlight with 1-octene the 1 1 adduct was obtained in about 70 % yield. Alkylation occurred at the carbon alpha to the carbonyl function (24) ... 

Another molecule where "competitive alkylation may occur is 2-pyrrolidone. The two active positions in this molecule are the carbon alpha to the carbonyl function and the one alpha to the nitrogen atom. The possible alkylation alpha to the carbonyl group has already been... 

In another reaction, alkynyl groups were transferable in the presence of a catalytic amount of Me3Al (Equation (78)).279 Preference for the Ph group transfer has been well featured in the competitive alkyl transfer process leading to selective imine arylation (Equation (79)). Higher organic chains such as ethyl and butyl are... 

SCHEME 85. Possible transition state structures for competitive alkylation (TSR) and proton abstraction (TSh) reactions (S = THF, L = HMPA Dx>y = DxL,ys)275... 

Tp ]Mgl + CH3CH3 Scheme 4 Competitive alkyl exchange and coupling. 

Further studies by Brown and coworkers lent additional support to this mechanism and the absence of a free alkyl cation. Olah and coworkers have applied the concept of competitive alkylation to the case of naphthalene in order to study both positional and substrate selectivities, and to clarify the nature of kinetically versus thermodynamically controlled product composition. They explained the observed results by suggesting that a ir-complex, such as (1), was the intermediate involved when highly electrophilic catalysts or strongly basic aromatics were employed, and a o-complex (as proposed earlier by Brown) was involved in reactions with weakly electrophilic catalysts or less basic aromatics. 

A novel procedure for studying the competitive alkylation of benzene and toluene involved stirring a solution of 0.125 mole of each hydrocarbon and 0.05 mole of nitrosonium hexafluorophosphate in 50 g. of nitromethane at 25° and adding 11 solution of 0.025 mole of ethylamine in 20 g. of nitromethane over a period of 20 min. The results indicate the relative reactivities of toluene (T) and benzene (B) to... 

The authors do not indicate why these conditions were chosen over the more conventional, but it may be a result of competitive alkylation at the amide NH. 

The dialkyl phosphite ion, however, can itself dealkylate the phosphonate product 300 and finally one may have to reckon with a competitive alkylation of the dialkyl phosphite ion by still unchanged phosphorous diester 301 and the use of allyl halides in the Michaelis-Becker reaction may lead also to diphosphonates owing to the ability of dialkyl phosphite ions to add to unsaturated systems (see Section 9.1).302... 

The competitive alkylation and acylation confirm the anticipated relative softness of S and Se. The observed regioselectivity in the reactions of organo-phosphorus selenothioacids (40) is of interest. 

Olah and Olah reported a competitive alkylation mechanism study of naphthalene in 1976 and postulated that the positional and substrate selectivities of napthalene F-C alkylation shed insight on the kinetically... 

Scheme 8 Competitive alkyl vs. polymeryl exchange observed in polymerizations with precatalyst 4.

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