Nucleophilic Catalysis of Reactions of Tricarbonyl(trimethylsilylarene)chromium Complexes with Electrophiles

For the nitroand chlorophenyl(trimethyl)silanes, the relative reactivity correlates well with the al-substituent cons tan t~[~"] , but diverges significantly from the order of stabilization of the phenyl anions which would be essential intermediates for a nucleophilic d e ~ i l y l a t i o n l ~ ~ ~ . The ui correlation holds also for the other arylsilyl substrates (Ic)-(l j) in Table 1; the remarkable increase in reactivity from ( ld ) to (lj) is similarly mirrored in the substituent constants. This clearly indicates that the electrophile participates significantly in the rate-limiting step of the reactions reported here-in contrast to normal nucleophilic d e ~ i l y l a t i o n ' ~ ~ ~ . Since "hard" bases have proven especially effective nucleophilic catalysts (KOC(CH3),, KF, CsF, tetraalkylammonium fluoride, KOAc)@I, one must assume that interaction of the catalyst with the silicon is the decisive factor for the reaction. We have also extended the reaction to substituted benzaldehydes and aliphatic aldehydes, to ketones, acyl fluorides and carboxylic acid anhydrides as well as to carbon dioxide; the respective products, secondary (4) and tertiary benzyl alcohols (5), aryl ketones (6) and substituted benzoic acids (7), are obtained, at least in part, in excellent yields. The scope of the procedure can be further widened by variation of the arylsilyl substrate. From 2-trimethylsilylbenzthiazole and benzaldehyde, for instance, the substitution product is formed in 78% yield, only after heating to 160 "C for 40 h16bl; in the presence of 1 mol-% of potassium tert-butanolate, 91% of the product is obtained after 15 min at 60 "C. Since the heteroaryl(trimethy1)silanes are readily accessible uia cycloaddition reactions[31, this method allows the introduction of substituents into heterocyclic substrates with high regiosele~tivity~~~. The reaction will be of special preparative advantage in those cases where organometallic derivatives cannot be employed because of the presence of reactive functional groups in the molecule (nitro or carbonyl compounds) or because of secondary reactions, e. g. formation of dehydroarenes from halogen compounds or isomerization to more stable anions.