Chiral (thio)phosphorodiamides as excellent hydrogen bond donor catalysts in the asymmetric Michael addition of 2-hydroxy-1,4-naphthoquione to nitroolefins

Abstract

A novel type of bidentate hydrogen bond donor catalysts based on (thio)phosphorodiamides catalophore has been developed for the asymmetric Michael addition of 2-hydroxy-1,4-naphthoquione to nitroolefins, affording the corresponding adducts in high yields with excellent level of enantioselectivities (97- >99% ee).

Thiophosphoramide‐Based Cooperative Catalysts for Brønsted Acid Promoted Ionic Diels–Alder Reactions

“…Consistent with this presumption, a significantly longer reaction time (24 h) was required to observe the formation of the product 8, and the reaction catalyzed by 7 e proceeded to a greater extent (33 %, entry 10) than the corresponding reaction promoted by 7 a (22 %, entry 9). Similar trends were observed for the HBr-catalyzed formation of 8 (entries [11][12][13]. HBr is stronger than both hydrochloric and p-toluenesulfonic acids (pK a = 0.9 in DMSO), and bromide is a weaker coordinating anion than chloride.…”

“…In our search for alternative HBDs, we turned our attention to the thiophosphoramides 7 c-e. [11,12] We surmised that 7 e is geometrically more suited for binding a sulfate anion and could potentially form up to three hydrogen bonds with the negatively charged oxygen atoms of sulfate (see Figure 1 for an example of such complex). [13] Gratifyingly, when used as a cocatalyst, 7 e significantly accelerated the reaction of 6 and cyclopentadiene, and the quantitative formation of 8 was detected within one hour (entry 6).…”

Thiophosphoramide‐Based Cooperative Catalysts for Brønsted Acid Promoted Ionic Diels–Alder Reactions

“…To confirm that 7 e complexes anions and to gain a better understanding of the observed effect, binding and computational studies have been conducted (Figure 3). First, the association of 7 e with sulfonate anions was confirmed by combining 7 e with known methylpyridinium tosylate (11) in different proportions. Thus, the addition of 11 to solution of 7 e resulted in a concentration-dependent downfield shift of the phosphoramide NH protons in the 1 H NMR spectrum, which is characteristic of hydrogen bonding to anion (Figure 3 A).…”

“…[10] Indeed, when used as a cocatalyst under identical reaction conditions, 7 b promoted the formation of 8 (entry 3), albeit in 15 % conversion. In our search for alternative HBDs, we turned our attention to the thiophosphoramides 7 c-e. [11,12] We surmised that 7 e is geometrically more suited for binding a sulfate anion and could potentially form up to three hydrogen bonds with the negatively charged oxygen atoms of sulfate (see Figure 1 for an example of such complex). [13] Gratifyingly, when used as a cocatalyst, 7 e significantly accelerated the reaction of 6 and cyclopentadiene, and the quantitative formation of 8 was detected within one hour (entry 6).…”

“…sulfonates, phosphates, perchlorates, etc.). However, even in the situation when there is no clear geometric preference for anion binding, 7 e outperformed 7 a as the co-catalyst ( Table 1, entries [8][9][10][11][12][13]. While the acidity of HCl (pK a = 1.8 in DMSO) is close to the acidity of sulfonic acids (pK a of MsOH in DMSO is 1.6), a chloride anion could form a tighter ion pair with an oxocarbenium than sulfonate anions, in which the negative charge is distributed across three oxygen atoms.…”