Modified Guanidines as Chiral Auxiliaries

Guanidine Chemistry

“…We found that modified guanidines prepared using our original methods [4][5][6] were effective not only in catalytic [7][8][9][10][11][12][13] but also in stoichiometric asymmetric syntheses. 14,15) The concept of modified guanidines as chiral auxiliaries in asymmetric synthesis is illustrated in Chart 2, [16][17][18] in which reactive, but resonance-stabilized, guanidinium salts 5 could be produced from guanidines 4 by quarternarization with a reagent (A-B). The salts 5 may react with activated unsaturated substrates (CϭD) to give addition products such as in the Michael reaction, trimethylsilyl (TMS) cyanation, or nucleophilic epoxidation, in which guanidines should act as catalysts (route A in Chart 2).…”

Catalytic Reactions of Chiral Guanidines and Guanidinium Salts

“…While there have been several attempts at the development of general chiral guanidines as chiral Brønsted bases, 8,9 we were mainly inspired by a paper published by Corey and Grogan which described a bicyclic guanidine catalyzed Strecker reaction (Scheme 2). 10 It demonstrated that a chiral bicyclic guanidine can act as an efficient chiral Brønsted base catalyst delivering high levels of enantioselectivity.…”

“…Both electron-withdrawing and -donating substituents on the aryl group in the arylmethanimines worked well (Table 7, entries 1-7). Various bketo N-acyloxazolidin-2-ones containing different aliphatic chains also worked equally well (entries [8][9][10][11][12][13][14].…”

Physico‐Chemical Properties of Organosuperbases

“…Guanidine is a stronger base than other nitrogen compounds containing one potentially basic site (N-imino or N-amino) linked to a carbon atom, for example, pyridines, amines, and amidines, and also those with two basic nitrogen atoms, for example, diamines. Basic properties of guanidines have been used for catalysis of various organic reactions [62]. The unusual thermodynamic stability of acyclic guanidines and their monoprotonated forms is ascribed to the following factors: resonance stabilization; Y-aromaticity [63,64]; favourable distribution of positive charge that leads to a favorable Coulomb interaction; aromaticity; stabilization by intramolecular hydrogen bonding; and the effect of solvation on the stability of the protonated form (strong hydrogen bond between the cation and solvent molecules in solution).…”