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Concave pyridines 1 were used to catalyze the addition of primary and secondary alcohols to ketenes 4, and the kinetic data of these catalyses were determined. In inter‐ and intramolecular competitions the use of 1e–g led to improved selectivities for the acylation of primary alcohols in comparison with secondary alcohols. All primary alcohols react at comparable rates. Observed rate constants were correlated with Taft's Es values. The starting materials and products were fully characterized.

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Pseudo-first-order rate constants k(obs) for the (concave) pyridine-catalyzed addition of ethanol to diphenylketene have been determined photometrically. The catalytic activity of the concave pyridines 1 or 2 is determined by their basicity and by their size. This effects a difference in iqobs) of up to three orders of magnitude. The decrease in catalytic activity by the concave shielding can be overcome by introduction of basicity-increasing substituents into the pyridine ring (i.e. 1 q). An amine -alcohol complex is probably the reactive species in this catalysis as the slope p of 0.3 -0.5 in a Brsnsted plot supports.Concave reagents have been designed to improve the selectivity of standard chemicals in organic chemistry by combination of concave structures with standard reagents. One class of such reagents are concave pyridinebisla~tams~.~) which have already been used as proton transfer reagents in protonation reactions of nitronate anions4a5). While in these reactions the concave pyridines are used stoichiometrically (i) in form of their conjugate acids and (ii) not as catalysts, there are a number of reactions which may be pyridine-catalyzed. Due to the 2,6-disubstitution of the pyridine ring, nucleophilic catalyses as for instance pyridinecatalyzed acylations are not possible6) but in general basecatalyzed reactions concave pyridines should be applicable. As an example we have chosen the base-catalyzed addition of alcohols to heterokumulenes. In this work we report that concave pyridines 1 and 2 do catalyze the addition of ethanol to diphenylketene') and investigate the mechanism of the reaction (general base catalysis 7c) vs. nucleophilic catalysis 7d)). RFor the reaction between diphenylketene and ethanol (in excess) in the presence of different concave pyridines 1 and 2 and open-chain analogs 6-13, pseudo-first-order rate constants in dichloromethane have been determined photometrically at 25 "C by recording the disappearance of the ketene absorption. The results are listed in Table 1.Indeed as Table 1 shows, concave pyridines 1 or 2 do catalyze the ethanolysis of diphenylketene. For the uncatalyzed reaction, a maximum k(obs) value of 0.09 . lop3 s-' has been determined. By the use of concave pyridines 1 and 2, an acceleration by a factor of 3 to 2500 can be achieved. In the case of the concave carboxamides 1, it must be stated that the carboxamide bridgeheads may also catalyze this reaction as the k(obs) value of 0.7 . 10W3 s-' for N,N-dimethylacetamide (6) shows. In contrast to the pyridine nitrogen atom, the carboxamide functions are not located within the concave space of 1. Therefore, only concave pyridines 1 with a distinctly higher reactivity than the acetamide 6 can be considered concave catalysts. On the other hand, the basicity dependences of the catalytic activity of different classes of 1 argue for the pyridine nitrogen atom as the catalytic center (see Figure). In the 4-diethylamino-substituted concave pyridines l q and 2k-m, a competition between the basicity centers in the concave (pyridi...

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