Lewis acids initiation mechanism

There is evidence that Lewis acids initiate a slow polymerization in some (but not most) systems by a self-ionization process in addition to the coinitiation process [Balogh et al., 1994 Grattan and Plesch, 1980 Masure et al., 1978, 1980], Two mechanisms are possible for self-initiation. One involves bimolecular ionization... 

A variety of protonic and Lewis acids initiate the cationic polymerization of lactams [Bertalan et al., 1988a,b Kubisa, 1996 Kubisa and Penczek, 1999 Puffr and Sebenda, 1986 Sebenda, 1988]. The reaction follows the mechanism of acid-catalyzed nucleophilic substitution reactions of amides. More specibcally, polymerization follows an activated monomer mechanism. Initiation occurs by nucleophilic attack of monomer on protonated (activated) monomer (XXIV) to form an ammonium salt (XXV) that subsequently undergoes proton exchange with monomer to yield XXVI and protonated monomer. The conversion of XXIV to XXV involves several steps—attachment of nitrogen to C+, proton transfer from... 

Lewis acids initiate cationic chain-growth polymerizations. There are several possible chain propagation reactions, and the mechanism of cationic chain growth is still open to... 

Scheme 9 Mechanism of alkyl halide/Lewis acid-initiated polymerizations of alkenes. (From Ref. 57.)...

Both protonic and Lewis acids initiate ring-opening polymerization of siloxanes. In spite of the practical importance [240] and the very extensive studies of this system, the mechanism of the polymerization is not completely understood. Even the nature of ionic propagating species is still a matter of controversy. The silicenium [241] or oxonium [242,243] ion structure has been postulated. 

To summarise our views on this topic we can say that the Kennedy s mechanism is probably operative, but the overwhelming process is the one originating from the formation of highly chlorinated aluminium catalysts formed in halogen-alkyl exchange reactions and possessing a much more pronounced acidity than the Lewis acids initially added. 

Lewis acids initiate polymerization through the formation of carbonium ions, and Plesch (17) has proposed that a suitable coinitiator is necessary to produce these ions. The mechanism for the initiation of the homopolymerization of epoxy resins by BF3 complexes has been proposed by Arnold (11) to proceed as follows ... 

In case of Lewis acid initiation, a cationic ROP will only occur if the counterion is not too nucleophilic [54]. When nucleophilic counterions are present, the metal-insertion mechanism will be followed instead, as already reported for several catalytic systems including ZnCl2 [55], TiCl4 and AICI3 [56]. 

Halogenation (Section 22.1 A) The electrophile is a halonium ion formed as an ion pair by interaction of chlorine or bromine with a Lewis acid. The mechanism involves an initial reaction between Clj and FeClj to generate a molecular complex that can rearrange to give a C1+, FeCl " ion pair. The C1+ reacts as a very strong electrophile with the weakly nucleophilic aromatic 77 cloud to form a resonance-stabilized cation intermediate that loses a proton to give the aryl chloride product. 

The cationic polymerization of DIPB s has been found to produce polymers containing predominantly a polyindane structure arising from a step-growth mechanism. D Onofrio, for example, showed that soluble polyindane is produced at polymerization temperatures above 70°C using Lewis acid initiation systems. ... 

Less obvious is the mechanism of initiation in the case of Lewis acid initiators. First of all, the behavior of different Lewis acids in similar systems may be different. Thus, complexes of BF3 with ethers are stable while BCI3 may react with ethers and complexes of PF5 with ethers may undergo disproportionation (Scheme 10). ... 

Characterizing the structure of the polymers, Dworak and Penczek found a significant contribution of the AM mechanism to the chain growth. Also within the scope of this elegant work, a direct correlation between the specific initiator and the percentage of secondary hydroxyl groups attached to the polymer backbone was verified. Use of SnCU or Bp3-OEt2 as Lewis acid initiators in particular proved to promote the AM mechanism. 

Friedel-Crafts (Lewis) acids have been shown to be much more effective in the initiation of cationic polymerization when in the presence of a cocatalyst such as water, alkyl haUdes, and protic acids. Virtually all feedstocks used in the synthesis of hydrocarbon resins contain at least traces of water, which serves as a cocatalyst. The accepted mechanism for the activation of boron trifluoride in the presence of water is shown in equation 1 (10). Other Lewis acids are activated by similar mechanisms. In a more general sense, water may be replaced by any appropriate electron-donating species (eg, ether, alcohol, alkyl haUde) to generate a cationic intermediate and a Lewis acid complex counterion. 

Lactams can also be polymerized under anhydrous conditions by a cationic mechanism initiated by strong protic acids, their salts, and Lewis acids, as weU as amines and ammonia (51—53). The complete reaction mechanism is complex and this approach has not as yet been used successfully in a commercial process. 

The most important reaction with Lewis acids such as boron trifluoride etherate is polymerization (Scheme 30) (72MI50601). Other Lewis acids have been used SnCL, Bu 2A1C1, Bu sAl, Et2Zn, SO3, PFs, TiCU, AICI3, Pd(II) and Pt(II) salts. Trialkylaluminum, dialkylzinc and other alkyl metal initiators may partially hydrolyze to catalyze the polymerization by an anionic mechanism rather than the cationic one illustrated in Scheme 30. Cyclic dimers and trimers are often products of cationic polymerization reactions, and desulfurization of the monomer may occur. Polymerization of optically active thiiranes yields optically active polymers (75MI50600). 

Most other studies have indicated considerably more complex behavior. The rate data for reaction of 3-methyl-l-phenylbutanone with 5-butyllithium or n-butyllithium in cyclohexane can be fit to a mechanism involving product formation both through a complex of the ketone with alkyllithium aggregate and by reaction with dissociated alkyllithium. Evidence for the initial formation of a complex can be observed in the form of a shift in the carbonyl absorption band in the IR spectrum. Complex formation presumably involves a Lewis acid-Lewis base interaction between the carbonyl oxygen and lithium ions in the alkyllithium cluster. 

The powerful nucleophilicity of enaimnes illows the dclthcion of rutro ilkenes to take place without the presence of Lewis acids The isolanon of secondary products, which can be explained by an initial Michael addition, suggests the participation of zwitlerionic intermediates m the mechanism of the reaction fEq 8 97i... 

In all these investigations Lewis acids were used as initiators at temperatures between -30 and 60 °C. The arguments used to substantiate the validity of structure 12 are unconvincing to this reviewer particularly because of the lack of sufficient experimental evidence. A subsequent paper on this subject40 did not improve the understanding of either the polymer structure or the mechanism. 

Rashkov, I. B., and Gitsov, I., Cationic polymerization initiated by intercalation compounds of Lewis acids. II. Initiating ability and mechanism of action of the initiators, J. Polym. 

Based on the experimental results above and together with Wang s previous work a tentative mechanism was proposed (Scheme 20) [33]. Thus, intermediate A is formed initially through coordination of imine and alkyne to the Lewis acid. This coordination sets the stage for an addition of the alkyne to the imine leading to the propargylamine intermediate B, which then undergoes an intramolecular... 

The mechanism of conjugate addition reactions probably involves an initial complex between the cuprate and enone.51 The key intermediate for formation of the new carbon-carbon bond is an adduct formed between the enone and the organocopper reagent. The adduct is formulated as a Cu(III) species, which then undergoes reductive elimination. The lithium ion also plays a key role, presumably by Lewis acid coordination at the carbonyl oxygen.52 Solvent molecules also affect the reactivity of the complex.53 The mechanism can be outlined as occurring in three steps. 

In 1978, Sugasawa et al., at Shionogi Pharmaceutical Co. reported ortho-selective Friedel-Craft acylation with free anilines with nitrile derivatives [4]. Sugasawa reported that the reaction requires two different Lewis acids (BC13 and A1C13) and does not proceed when N,N-dialkyl anilines are used. He proposed that boron bridging between nitriles and anilines led to exclusive ortho-acylation but a conclusive mechanism was not elucidated. The report did not offer any reason why two different Lewis acids were required and why the reaction did not progress with N,N-dialkyl anilines. Therefore, we initiated mechanistic studies. 

Similar to the addition of secondary phosphine-borane complexes to alkynes described in Scheme 6.137, the same hydrophosphination agents can also be added to alkenes under broadly similar reaction conditions, leading to alkylarylphosphines (Scheme 6.138) [274], Again, the expected anti-Markovnikov addition products were obtained exclusively. In some cases, the additions also proceeded at room temperature, but required much longer reaction times (2 days). Treatment of the phosphine-borane complexes with a chiral alkene such as (-)-/ -pinene led to chiral cyclohexene derivatives through a radical-initiated ring-opening mechanism. In related work, Ackerman and coworkers described microwave-assisted Lewis acid-mediated inter-molecular hydroamination reactions of norbornene [275]. 

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