Catechol Lewis acidity

For the studied catechol methylation reaction the catalyst structure and surface properties can explain the catalytic behaviour As mentioned above, the reaction at 260-350°C has to be performed over the acid catalysts. Porchet et al. [2] have shown, by FTIR experiments, the strong adsorption of catechol on Lewis acid/basic sites of the Y-AI2O3 surface. These sites control the reaction mechanism. 

The kinetic experiments were not performed under true catalytic conditions, i.e. the pre-prepared [FeL(DTBC)] complexes were introduced into the reaction mixtures as reactants and excess substrate was not used. Nevertheless, the results are important in exploring the intimate details of the activation mechanisms of the metal ion catalyzed autoxida-tion reactions of catechols. In excess oxygen the reaction was first-order in the complex concentration and the first-order dependence in dioxygen concentration was also confirmed with the BPG complex. As shown in Table II, the rate constants clearly correlate with the Lewis character of the complex, i.e. the rate of the oxidation reaction increases by increasing the Lewis acidity of the metal center. 

The mechanism shown in Scheme 5 postulates the formation of a Fe(II)-semi-quinone intermediate. The attack of 02 on the substrate generates a peroxy radical which is reduced by the Fe(II) center to produce the Fe(III) peroxide complex. The semi-quinone character of the [FeL(DTBC)] complexes is clearly determined by the covalency of the iron(III)-catechol bond which is enhanced by increasing the Lewis acidity of the metal center. Thus, ultimately the non-participating ligand controls the extent of the Fe(II) - semi-quinone formation and the rate of the reaction provided that the rate-determining step is the reaction of 02 with the semiquinone intermediate. In the final stage, the substrate is oxygenated simultaneously with the release of the FemL complex. An alternative model, in which 02 attacks the Fe(II) center instead of the semi-quinone, cannot be excluded either. 

The variety of C2-bridged PBs was further extended by Muhoro via hydroboration of diphenyl(vinyl)phosphine with catechol- and pinacol-boranes (Scheme 29).56 To compensate for the low Lewis acidity of these boronates, the hydroboration reactions were carried out in the presence of 5 mol% of titanocene bis(catecholborane) as catalyst. The desired products 40g and 40h were obtained with complete anti-Markovnikov selectivity. The spectroscopic data and the crystallographic study performed on 40h showed the expected monomeric open structure. 

The use of enzymes in organic solvents can also take advantage of spontaneous air oxidations, rearrangements, or cycloadditions, such as the air-oxidation of catechol produced from phenol by tyrosinase, to form ort/zo-quinone, followed by a Diels-Alder addition to various dienophiles in chloroform.13 It is interesting that an attempt to allylate the final product with the Lewis acid Sc(CF3S03)3 and (Allyl)4Sn would not proceed unless the enzyme and excess dienophile were removed first. 

If the latter reaction proceeds through a closed transition state (e.g., 5 in Scheme 7.2), good diastereocontrol can be expected in the case of trans- and cis-CrotylSiCl3 (2b/2c) [14, 15]. Here, the anh-diastereoisomer 3b should be obtained from trans-crotyl derivative 2b, whereas the syn-isomer 3c should result from the reaction of the cis-isomer 2c (Scheme 7.2). Furthermore, this mechanism creates an opportunity for transferring the chiral information if the Lewis base employed is chiral. Provided that the Lewis base dissociates from the silicon in the intermediate 6 at a sufficient rate, it can act as a catalyst (rather than as a stoichiometric reagent). Typical Lewis bases that promote the allylation reaction are the common dipolar aprotic solvents, such as dimethylformamide (DMF) [8,12], dimethyl sulfoxide (DMSO) [8, 9], and hexamethylphosphoramide (HMPA) [9, 16], in addition to other substances that possess a strongly Lewis basic oxygen, such as various formamides [17] (in a solution or on a solid support [7, 8, 18]), urea derivatives [19], and catecholates [10] (and their chiral modifications [5c], [20]). It should be noted that, upon coordination to a Lewis base, the silicon atom becomes more Lewis acidic (vide infra), which facilitates its coordination to the carbonyl in the cyclic transition state 5. 

The ability of allyltin halides to extend their coordination sphere allowed the preparation of chiral hypervalent complexes with diamine ligands, which have been efficient in the asymmetric synthesis of homoallylic alcohols with up to 82% ee100. Similarly, a chiral hypervalent allyltin was prepared from a low valent tin (II) catecholate, chiral dialkyl tartrate and ally lie halide101. The allylation of aldehydes and activated ketones proceeded with high enantiomeric excess. Allyltins prepared from Lappert s stannylene and allylic halides were shown to be efficient as well, although the Lewis acid character of the tin atom is much less marked in that case102,103. 

EXAFS studies and the aforementioned high-resolution crystallography indicate that the catechol snbstrate binds in an asymmetric fashion to the iron(II) center with Fe-Ocatechoiate bond lengths that differ by 0.2 0.4 A. These structural parameters are in excellent agreement with those reported for synthetic iron(II)-monoanionic catecholate complexes, and, on the basis of this comparison, it was proposed that the catechol binds to the iron(n) center as a monoanion. This notion was supported by subsequent UV-resonance Raman andUV-vis studies. " The monoanionic nature of the catechol substrate in extradiol dioxygenases is in sharp contrast with the dianionic catecholate character commonly found in iron(III) complexes. This difference can be rationahzed by the differing Lewis acidities of the metal centers in their divalent and trivalent oxidation states. 

The Lewis acidic character of reactive pentacoordinate silicon compounds has been unequivocally confirmed by Corriu, Sakurai and Hosomi [90]. Allylsilicates prepared from allylsilanes and catechol can undergo allylation reaction with aldehydes in the absence of Lewis acid promoter (Sch. 51). 

The role of the catecholate group and fluoride is to delocalize negative charge and increase the Lewis acidity of the silicon center, which coordinates a carbonyl oxygen to form a hexaeoordinate silicate. The six-membered cyclic transition state in the chair conformation is consistent with high threo and erythro selectivity similar to that of allyl boronates [91]. It is interesting to see the structure-reactivity and structure-selectivity correlation shown in Sch. 53 [92]. 

Stable pentacoordinated allylsiliconates have been employed in aldehyde addition reactions. These reagents require no activation by Lewis acids or Lewis bases, but have found only limited applications in synthesis to date. The use of these agents in addition to aldehydes was first described in 1987 by Corriu [59] and Hosomi [60] and by Kira and Sakurai [61] in 1988. In these reactions, the addition of a catechol or 2,2 -biphenol-derived allylsiliconate to an achiral aldehyde led to the highly regio- and stereoselective formation of homoallylic alcohols. For example, the addition of the catechol-derived 2-butenylsiliconate 81 (90/10 E Z) provided a diastereomeric mixture of homoallylic alcohols 74 and 75 in a 90/10 ratio (Scheme 10-33) [60c]. 

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