(Thio)urea Organocatalyst Equilibrium Acidities in DMSO

Abstract

Bordwell's method of overlapping indicators was used to determine the pK(a) values of some of the most popular (thio)urea organocatalysts via UV spectrophotometric titrations. The incremental effect of CF(3) groups on acidic strength was also investigated. The pK(a)'s are in the range of 8.5-19.6. The results may lead to a better understanding of noncovalent organocatalysis and may aid in future catalyst development.

Organocatalyzed synthesis of optically active cyanohydrins starting from alpha-ketoesters

“…Quinine was able to perform the cyanosilylation of α-ketoester 1a with a 92% conversion after 24 hours, but without selectivity (entry 9). Thus, we have prepared both a cinchona-based urea (VIIIa) and thiourea (VIIIb) containing a 3,5-bistrifluoromethylphenyl group in order to increase the system reactivity [15]. Both catalysts were able to catalyze the cyanosilylation of 1a with good performance in toluene at -45ºC.…”

Lewis acid enhancement by hydrogen-bond donors for asymmetric catalysis

“…1A)(1,2). However, because of the generally weak Brønsted acidity of the catalysts (3,4), these approaches commonly require highly electrophilic substrates with labile carbon-heteroatom (σ or π) bonds (2). We considered whether the anion-binding principle could be applied in a fundamentally different way, to enhance the reactivity of Lewis acids such as silyl triflates through association of a chiral H-bond donor with the triflate.…”

Synthesis and determination of pKa values of new enantiopure pyridino- and piperidino-18-crown-6 ethers

“…However, the equilibrium values have their limitations for a kinetic property such as transition state stabilization. 36 According to a recent review 4 the p s K a values of H-bond donor species in small-molecule catalysis are in the range of 6-28 in DMSO.…”