Amidinium ions reactions

Carboxyl-related and acyl substituents. Included here are cyano, protonated amidinium ion, thionoacyl, acyl (Ar—CO, H—CO, Alkyl—CO), carboxamido, carboaryloxy, carboalkoxy, carboxy (unionized), amidino (unionized), and carboxylate anion, listed approximately in order of decreasing electron attraction or activation. The relative activation by some of these groups (e.g., ketone, aldehyde, nitrile) will change upon reversible interaction with the nucleophile, which will vary with the group and with the nucleophile (e.g., MeO , N3, NCS ). Irreversible interaction will be obvious when the reaction products in kinetic studies are characterized. 

Reactions of nitriles may be promoted by the presence of divalent Co. The conversion of sterically unhindered nitriles RCN (R = Me, Et, n-Pr, w-Bu) into the corresponding amidines RC(NH2) = NH occurs in the presence of Co(N03)2.6H20 and the ketoxime Me2C=NOH. The metal free amidinium ions were crystallized as their nitrate salts.358... 

As a true testament to the potential long-term impact of H-bonding activation, a number of ureas, thioureas, and acid catalysts are now finding broad application in a large number of classical and modem carbon-carbon bond-forming processes. On one hand, Johnston s chiral amidinium ion 28 was elegantly applied to the asymmetric aza-Henry reactions (Scheme 11.12d). On the other hand, chiral phosphoric acids (e.g., 29 and 30), initially developed by Akiyama and Terada, have been successfully employed in Mannich reactions, hydrophosphonylation reac-tions, aza-Friedel-Crafts alkylations (Scheme 11.12e), and in the first example... 

Table 3. Calculated model reactions of the amidinium ions 115 and 116 with H2 in the absence and presence ofa base ((BLYP/6-311G(2d, 2p) level).

These reactions usually are slow at room temperature, and the adhesives must be cured with heat. Tertiary amines or amidines can be used to catalyze the reaction whereby the car-boxylate anion is the nucleophile and the ammonium or amidinium ion act as hydrogen-bond donors. Dusek et al. have shown that transesterification only occurs after essentially complete consumption of the carboxylic acid in compositions initially having equivalent amounts of acid and epoxy groups.40 41 The transesterification leads to an equilibrium sol-gel composition. 

Schuster et al. reported the accelerating effect of the amidinium ion on the Diels-Alder reaction [ 15]. Reaction of the diene 59 and diketone 60a or 60b in the presence of lipophilic amidinium ion 63 (1 equiv.) gave 61 and 62 (2.5 3.2 1), with a 100-fold rate increase compared to the uncatalysed conditions. When the reaction was run in the presence of chiral amidinium compound 64, 61 and 62 were obtained in 70 94% yield (ca 3 1) with 11 50% ee. The Diels-Alder adduct 61b is a key intermediate for synthesis of (—)-norgestrel (Scheme 7.12). The reaction enhancement effect of amidinium ion can be explained in terms of the hydrogen bond mediated interaction with diketone. 

Schuster, T., Bauch, M., Diirner, G. and Gohel, M.W. (2000) Axially chiral amidinium ions as inducers of enantioselectivity in Diels-Alderreactions. Organic Letters, 2,179-181 Schuster, T., Kurz, M. and Gohel, M.W. (2000) Catalysis of a Diels-Alder reaction by amidinium ions. The Journal of Organic Chemistry, 65, 1697-1701. 

Gobel et al. reported axially chiral amidinium ion (25) (Figure 2.24) as an inducer of enantioselective Diels-Alder reaction (Scheme 2.66) [126]. They proposed the dual hydrogen bond activation model (Figure 2.25). 

Recently, Johnston and coworkers reported an impressive application of catalysis by the amidinium salts in an enantioselective iodolactonization reaction (Scheme 10.62) [162]. While the free bases were effective in catalyzing the reaction, higher enantioselectivities were obtained with the mono-protonated salts. Additionally, the authors discovered a high dependence of enantioselectivity on the nature of the achiral counter-ion, and among the anions tested the triflimidate anion was found to be the best in terms of both yield and enantioselection. With 5 mol% of the catalyst 64 HNTf2, the iodolactonization products were obtained in high yields (up to 99%) and excellent enantioselectivities (up to 98% ee). 

---