Catalysis of a Diels−Alder Reaction by Amidinium Ions

Abstract

Amidines and guanidines are important functional groups in molecular recognition and host-guest chemistry. Here it is shown that lipophilic amidinium ions catalyze a cycloaddition reaction representing the key step of the Quinkert-Dane estrone synthesis. Hydrogen-bond-mediated association with the organic cation leads to an electrophilic activation of the dienophile and to enhanced rates of the Diels-Alder reaction. The observed effects are similar to those expected from mild Lewis acids. In competition experiments, amidinium catalysis favors the reaction of the less electron deficient dienophile.

Quantum Mechanical Investigations of Organocatalysis: Mechanisms, Reactivities, and Selectivities

“…270 Numerous applications have followed, including in catalysis of the Diels-Alder reaction, 271 (aza)-Henry reaction, 272,273 amination of 1,3-dicarbonyl compounds, 274 and ring-opening-metathesis polymerization. 275 …”

A C2‐Chiral Bis(amidinium) Catalyst for a Diels−Alder Reaction Constituting the Key Step of the Quinkert−Dane Estrone Synthesis

“…[6] Thus, soluble amidinium salts have been shown to accelerate the cycloaddition of compounds 2 and 3a by more than two orders of magnitude. [7] Furthermore, the ratio of constitutional isomers is shifted in favour of adduct 5a. [7,8] In the presence of axially chiral amidinium ions, 5a could be obtained with considerable enantioselectivity.…”

“…[7] Furthermore, the ratio of constitutional isomers is shifted in favour of adduct 5a. [7,8] In the presence of axially chiral amidinium ions, 5a could be obtained with considerable enantioselectivity. [8] In this case no transition metal is involved in the catalysis.…”

“…Tetrakis [3,5-bis-(trifluoromethyl)phenyl]borate has been shown in previous studies to reduce unwanted ion-pair interactions to a very low level. [7] The bis(amidinium) salt 12 was prepared from 5-(tertbutyl)isophthalic acid (6) (Scheme 2). Reaction of 6 with PCl 5 , followed by NH 3 , afforded diamide 8.…”

“…[12] To measure the rates, yields and product ratios in the DielsϪAlder reactions of 2 and 3a or 3b, our previously reported methods have been applied. [7,8] Solutions of diene (45 m), dienophile (30 m), catalyst 12 (30 m) and 2-methoxy-6-methylnaphthalene (10 m) as an internal standard were kept in dry CH 2 Cl 2 at the temperature given in Table 1. Rates based on the disappearance of diene 2 were obtained from repeated HPLC analyses of the reaction mixture.…”