Abstract
Thiosquaramides are the thio analogues of squaramides that are widely applicable in the fields of asymmetric catalysis, pharmaceutical research, and chemical biology. Having four-membered ring system derived from squaric acid, thiosquaramides are feasible hydrogen bond donors and acceptors. A high affinity for hydrogen bonding is driven through a concomitant increase in aromaticity of the ring. In this review the structural properties, acidity, and lipophilicity of thiosquaramides and squaramides are compared. Different synthetic procedures starting from squarates, half-squaramides or squaramides are shown, and the main derivatization methods are outlined. Finally, the yet only few applications of this interesting family are reviewed. Considering their hydrogen bonding and aromatic switching, in combination with structural rigidity, they bear the possibility of becoming robust and tunable bifunctional organocatalysts for a range of synthetically useful transformations in the future.
Highlights
The preparation of urea was an important conceptual milestone in organic chemistry
Thiosquaramides are the thio analogues of squaramides that are widely applicable in the fields of asymmetric catalysis, pharmaceutical research, and chemical biology
Having four-membered ring system derived from squaric acid, thiosquaramides are feasible hydrogen bond donors and acceptors
Summary
The preparation of urea was an important conceptual milestone in organic chemistry. Since this versatile carbonic acid derivative has generally been used as fertilizer [1], diuretic [2], and amongst others it has medical and laboratory applications [3]. Replacement of even one of the carbonyl groups by thiocarbonyl leads to disruption of ladder formation and, as a result, improves solubility This interconversion increases the acidity of the N-H bonds, in line with the case of urea and thiourea. Experiments and computations reveal that the increased acidity of thiosquaramides results in the formation of stronger hydrogen bonds than squaramide, urea, and thiourea do. Dithiosquaric half amide half ester (D) was converted into the appropriate dithiosquaramide through a second addition – elimination reaction using aralkyl amines This year as a continuation of their work, Rombola and Rawal [33] reported a new procedure in which dithiosquaramides (B) are prepared through the thionation of dicyclopentyl squarate (Entry 5, Table 2). Scheme 1 Preparation of mono-, and dithio analogues of squarates and (half)squaramides
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