Nucleophilic catalysis by 4-(dialkylamino)pyridines revisited--the search for optimal reactivity and selectivity.

4-(Dimethylamino)pyridine (4-DMAP, 1) is well known as a catalyst for the esterification of alcohols by acid anhydrides and for various other synthetically useful transformations involving acyl transfer. 2] Its catalytic potential was first discovered by the groups of Litvinenko and Steglich in the late 1960s and its synthetic utility and that of its congeners, including polymeric variants, have been reviewed. Recently, attention has been focused on the development of enantiomerically pure chiral 4-(dialkylamino)pyridines for the kinetic resolution of alcohols and related enantioselective transformations. As a result of this interest, the detailed mechanism of catalysis by 4-(dialkylamino)pyridines and the factors that influence their reactivity have come under renewed scrutiny. In particular, Steglich and co-workers reported pyridonaphthyridine 3 as being the most catalytically active 4-DMAP analogue yet prepared for the acetylation of tertiary alcohols, and work by Kattnig and Albert has illustrated the key role of the anion and general base catalysis in regulating the rate and regioselectivity of polyol acetylation by 1 (Scheme 1). Herein, the detailed mechanism of catalysis of esterification by 4-(dialkylamino)pyridines is reexamined in light of these findings, and the complexity associated with this apparently straightforward process is highlighted. That pyridine and 4-substituted derivatives act primarily as nucleophilic rather than general base catalysts for alcohol esterification follows from the dramatic loss of activity that accompanies 2-alkyl substitution despite the relatively marginal effect that this substitution has on the pKa value of these derivatives. Such steric inhibition of catalysis was first shown to be characteristic of nucleophilic catalysis in work by Gold and Jefferson in the early 1950s on the hydrolysis of Ac2O with a series of methyl-substituted pyridines. The effect was quantified by Litvinenko and co-workers in 1981 for the catalysis of benzoylation of benzyl alcohol with BzCl. In addition to confirming the nucleophilic nature of the catalysis, this work also highlighted the particularly high catalytic activity of 1, which exhibits a rate of 3.4 ; 10 relative to the uncatalyzed reaction (Scheme 2). The high catalytic reactivity of 1 had previously been noted by Litvinenko and co-workers in the benzoylation of 3chloroaniline and subsequently, but independently, 1 was shown by Steglich and co-workers to enable esterification of even hindered tertiary alcohols with Ac2O. [4] Esterification reactions of tertiary alcohols are relatively slow and particularly susceptible to steric factors and therefore proved to be useful for exploring structure–activity relations for catalysis by 4-DMAP analogues. Accordingly, Hassner et al. found that 4pyrrolidinopyridine (4-PPY, 2) was the most efficient of a series of 4-aminopyridine derivatives, including 1, for the acetylation of 1-methylcyclohexanol with Ac2O (Scheme 3). [17] Hassner et al. noted the lack of correlation between the pKa value and the catalytic activity; they suggested that the relative efficiencies of the various catalysts reflected the stabilities of the respective derived acyl pyridinium intermediates in a mechanistic scenario involving equilibrium formation of these salts followed by rate-determining reaction with the alcohol (Scheme 4). Additionally, Hassner et al. noted that the order of catalytic activity of 4aminopyridine derivatives [4-pyrrolidino (2)> 4-dimethylamino (1)> 4-piperidino (5)> 4-morpholino (7)] mirrored the order of reactivity of cyclohexanone-derived enamines towards electrophiles. This order has been rationalized as a balance of stereoelectronic (nN!p*C=C) and steric effects (A strain) which dictates the efficiency with which the lone pair of electrons on the enamine nitrogen atom interacts with the C C double bond. By analogy with the enamine series, Hassner et al. noted that there was a qualitative correlation between the degree of shielding of the pyridyl b-hydrogen atoms in the H NMR spectra of the catalytically active 4-aminopyridine derivatives and their catalytic efficiency (see Scheme 3). They inferred that the extent of electronic communication between the lone pair of electrons of the exocyclic nitrogen atom and the carbonyl function through the pyridyl ring was a key factor in stabilizing the acyl pyridinium intermediate. The design of pyridonaphthyridine 3 (Scheme 1), recently disclosed by Steg[*] Dr. A. C. Spivey, S. Arseniyadis Department of Chemistry South Kensington Campus Imperial College London SW7 2AZ (UK) Fax: (+44)20-7594-5841 E-mail: [email protected] Highlights