Complexation of Lewis acids

Alkyl cyanides with electron withdrawing substituents are more reactive and cyclotrimer-ize under both acidic and basic conditions as well as at high pressure (Table 14). Complexes of Lewis acids and hydrogen halides are particularly valuable catalysts. Two mechanisms for the acid-catalyzed cyclotrimerization have been postulated. Grundmann et al. suggest... 

The l,T-binaphthalene-2,2 -diol complexes of Lewis acids have received considerably more attention. Mikami hrst reported the apphcation of BtNOL/ri(IV) complexes for enantioselective allylations of glyoxylates.ii Keck" and Umani-Ronchi/Tagliavinin independently devised allylation procedures with (/ )-BtN0L/Ti(0-iPr)4 and (5)-BIN0L/Ti(0-iPr)2Cl2, respectively (Scheme 5.2.83). The use of molecular sieves is essential for high reactivity and stereoselectivity. 

Adhesives which are meant to cure at temperatures of 120 or 171°C require curatives which are latent at room temperature, but react quickly at the cure temperatures. Dicyanodiamide [461-58-5], (TH INI is one such latent curative for epoxy resins. It is insoluble in the epoxy at room temperature but rapidly solubilizes at elevated temperatures. Other latent curatives for 171°C are complexes of imidazoles with transition metals, complexes of Lewis acids (eg, boron trifluoride and amines), and diaminodiphenylsulfone, which is also used as a curing agent in high performance composites. For materials which cure at lower temperatures (120°C), these curing agents can be made more soluble by alkylation of dicyanodiamide. Other materials providing latency at room temperature but rapid cure at 120°C are the blocked isocyanates, such as the reaction products of toluene diisocyanate and amines. At 120°C the blocked isocyanate decomposes to regenerate the isocyanate and liberate an amine which can initiate polymerization of the epoxy resin. Materials such as Monuron can also be used to accelerate the cure of dicyanodiamide so that it takes place at 120°C. 

In this respect, both free nitrile molecules and their complex salts with acidic catalysts can take part in the formation of the triazinc.146 Hydrogen chloride, sulfuric acid, Lewis acids and complexes of Lewis acids and hydrogen halides are generally used as catalysts the complexes usually have the best catalytic potential.147-150... 

The amination of naphthalene using primary or secondary amines is one type of modification, and the treatment of naphthol with a complex of Lewis acid and ammonia is another modification. The latter was reported before the Bucherer reaction was published. 

The reaction mechanism under the initial Pictet-Hubert condition is displayed in Scheme 1 and the modified Morgan-Wall condition is outlined in Scheme 2. It is believed that the complexation of Lewis acid (e.g., ZnCh) with oxygen polarizes the carbonyl group, which facilitates the electrophilic substitution as shown below. 

The step of cationic initiation can be subdivided into two separate reactions. The first one consists of formation of ionic species and the second one of reactions of these ionic species with the olefins, a cationization process. This reaction, termed priming by Kennedy and Marechal, is a process of ion formation in a non-nucleophilic media through (1) dissociation of protonic acids to form protons and counterions, (2) reactions of Lewis acids with Bronsted acids, (3) dissociation of dimeric Lewis acids, (4) complexation of Lewis acids with water or with alkyl halides or with ethers, and so on. These reactions may take place through a series of complicated steps. The second reaction, the cationization of the olefins, may also include several intermediate steps that will eventually lead to propagating species. 

Complexation of Lewis acids with water is another case of formation of electrophiles that can initiate chain growth. MeXn represents a Lewis acid ... 

Other initiators for tetrahydrofuran polymerizations also include Lewis acids in combinations with promoters. These are complexes of Lewis acids, like BF3, SnCU, or C2H5AICI2 with epirane compounds like epichlorohydrin. The small-ring compounds are more reactive toward many Lewis acids, or protonic acids, than tetrahydrofuran and act as promoters of the initiation reactions. The initiations in the presence of small quantities of oxirane compounds can be illustrated as follows ... 

When complexes of Lewis acids with active hydrogen compounds initiate the polymerizations, such complexes acts as protonic acids. On the other hand, etherates initiate by forming oxonium ions and may involve alkyl exchange reactions with the monomer ... 

The first enantioselective Povarov reactions were two-component processes catalyzed by chiral complex of Lewis acids, but gave only modest results in terms of conversion and diastereo- and enantioselectivities [63]. Better results were obtained by Akiyama using a chiral Brpnsted acid derived from (/ )-BINOL (79) to catalyze the reaction of alkyl vinyl ethers with aromatic imines derived from o-hydroxyanilines, which gave derivatives of 8-hydroxytetrahydroquinoline 80 in good yields, with excellent diastereoselectivities and enantiomeric excesses. The authors attribute these results to the formation of a cyclic transition state wherein the chiral catalyst coordinates with both the OH and imine nitrogen, driving the attack of the nucleophile by ih Re face of the imine (Schemes 3.22 and 3.24) [64]. 

Nakamura, E., Yamanaka, M., Mori, S. 2000. Complexation of Lewis acid with trialkylcopper (III) On the origin of BF3-Acceleration of cuprate conjugate addition. J. Am. Chem. Soc. 122 1826-1827. 

Minimze steric effects in complexation of Lewis acid... 

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