Lewis acid formation

Fig. 2. Principle of Lewis acid formation by photolysis of aryldiazonium salts, MX = PFj, BFj, SbFs etc.

In addition to the formation of alkali alkoxometallates by the reactions of alkali alkoxides (strong bases) with alkoxides of a variety of metals and metalloids (Lewis acids), formation of heterometal alkoxides has been shown to occur even between alkoxides of such similar metals as aluminium and gaUium as weU as niobium and tantalum. However, the formation constant of the latter derivative has been found to be statistical, which precludes the isolation of this bimetallic alkoxide in view of the equilibrium ... 

Reaction time can also be reduced by means of microprocess technology [130]. A reaction time of 30 min was necessary for a 30 ml-batch reactor to achieve a conversion of 65% at a selectivity of 45%, whereas with the falling film microreactor only 14 s for the same performance was required. This leads also to a large space-time yield with 401 mol/(l h) forthe falling film microreactor as compared to 1.3 mol/(l h) forthe batch reactor. The impact of Lewis-acid formation on the reaction course was investigated using an iron microreactor instead of the usually used nickel microreactor. The plate surface is converted to iron chloride under the reaction conditions. The selectivity of the target product then drops from 67% to 50%. 

Unfortunately, addition of copper(II)nitrate to a solution of 4.42 in water did not result in the formation of a significant amount of complex, judging from the unchanged UV-vis absorption spectrum. Also after addition of Yb(OTf)3 or Eu(N03)3 no indications for coordination were observed. Apparently, formation of a six-membered chelate ring containing an amine and a ketone functionality is not feasible for these metal ions. Note that 4.13 features a similar arrangement and in aqueous solutions, likewise, does not coordinate significantly to all the Lewis acids that have been... 

The Fischer cyclization is usually carried out with a protic or Lewis acid which functions both to facilitate the formation of the cnchydrazine by tautomerization and also to assist the N N bond breakage. The mechanistic basis of the Fischer cyclization has been discussed in recent reviews[l,2]. 

Thus a second method was envisaged, the reaction of a nitrile, hydrogen selenide, and an a-halogenated ketone in the presence of a condensation catalyst, which can be POCl, or POCI3 with a Lewis acid such as PCI3 or anhydrous ZnCl. The use of fresh AICI3 leads to the formation of tarry side-products. 

Alkyl halides by themselves are insufficiently electrophilic to react with benzene Aluminum chloride serves as a Lewis acid catalyst to enhance the electrophihcity of the alkylating agent With tertiary and secondary alkyl halides the addition of aluminum chlonde leads to the formation of carbocations which then attack the aromatic ring... 

The boron atom in boron trifluoride is hybridized to the sp planar configuration and consequently is coordinatively unsaturated, ie, a Lewis acid. Its chemistry centers around satisfying this unsaturation by the formation with Lewis bases of adducts that are nearly tetrahedral sp [ The electrophilic properties (acid strengths) of the trihaloboranes have been found to increase in the order BF < BCl < BBr < BI (3,4). 

Evidence supporting the formation of 1 1 addition compounds is substantiated by the actual isolation of stable acyl haUde—Lewis acid complexes. 

Stabilization Mechanism. Zinc and cadmium salts react with defect sites on PVC to displace the labHe chloride atoms (32). This reaction ultimately leads to the formation of the respective chloride salts which can be very damaging to the polymer. The role of the calcium and/or barium carboxylate is to react with the newly formed zinc—chlorine or cadmium—chlorine bonds by exchanging ligands (33). In effect, this regenerates the active zinc or cadmium stabilizer and delays the formation of significant concentrations of strong Lewis acids. 

Nucleophilic Ring Opening. Opening of the ethyleneimine ring with acid catalysis can generally be accompHshed by the formation of an iatermediate ayiridinium salt, with subsequent nucleophilic substitution on the carbon atom which loses the amino group. In the foUowiag, R represents a Lewis acid, usually A = the nucleophile. 

The polymerization of ethyleneimine (16,354—357) is started by a catalyticaHy active reagent (H or a Lewis acid), which converts the ethyleneimine into a highly electrophilic initiator molecule. The initiator then reacts with nitrogen nucleophiles, such as the ethyleneimine monomer and the subsequendy formed oligomers, to produce a branched polymer, which contains primary, secondary, and tertiary nitrogen atoms in random ratios. Termination takes place by intramolecular macrocycle formation. 

Salt formation with Brmnsted and Lewis acids and exhaustive alkylation to form quaternary ammonium cations are part of the rich derivati2ation chemistry of these amines. Carbamates and thiocarbamates are formed with CO2 and CS2, respectively the former precipitate from neat amine as carbamate salts but are highly water soluble. 

Physical Properties. Both (1) and (2) are weak bases, showing 4.94 and 5.40, respectively. Their facile formation of crystalline salts with either inorganic or organic acids and complexes with Lewis acids is in each case of considerable interest. Selected physical data for quinoline and isoquinoline are given in Table 1. Reference 4 greatly expands the range of data treated and adds to them substantially. 

The irradiation of calciferol in the presence of iodine leads to the formation of 5,6-/n7 j -vitaniin D2 [14449-19-5] (31) or [22350 1-0] (32) (67,68). 5,6-/ra j -Vitainin D as well as vitamin D (2) or (4) can be converted to isovitamin D by treatment with mineral or Lewis acids. Isocalciferol (35) [469-05-6] or (36) [42607-12-5] also forms upon heating of 5,6-/ -vitamin D. Isotachysterol (33) [469-06-7] or (34) [22350-43-2] forms from isocalciferol or vitamin D upon treatment with acid, and its production appears to be the result of sequential formation of trans- and isocalciferol from calciferol. These reactions are the basis of the antimony trichloride test for vitamin D (69—72). 

Antimony pentafluoride is a strong Lewis acid and a good oxidizing and fluorinating agent. Its behavior as a Lewis acid leads to the formation of numerous simple and complex adducts. It reacts vigorously with water to form a clear solution from which antimony pentafluoride dihydrate [65277-49-8], SbF 2H2O, may be isolated. This is probably not a tme hydrate, but may well be better formulated as [H O] [SbF OH]. 

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