Phosphoric acid derivatives imines

Prior to Yamamoto s entry into this field, the scope of chiral phosphoric acid catalysis was strictly limited to electrophiUc activation of imine substrates. By designing a catalyst with higher acidity it was suspected that activation of less Lewis basic substrates might be possible. To this end, Yamamoto reported incorporation of the strongly electron accepting N-triflyl group [57] into a phosphoric acid derivative to yield the highly acidic N-triflyl phosphoramide 13 (Scheme 5.32)... 

A BINOL-derived phosphoric acid derivative has been used as a catalyst in the enantioselective synthesis of a-amino phosphonates via hydrophosphonylation of imines with diisopropyl phosphite.82... 

Tatsui G (1928) Synthesis of carboline derivatives. J Pharm Soc Jpn 48 453 159 Taylor MS, Jacobsen EN (2004) Highly enantioselective catalytic acyl-pictet-spengler reactions. J Am Chem Soc 126 10558-10559 Terada M, Uraguchi D, Sorimachi K, Shimizu H (2005) Process for production of optically active amines by stereoselective nucleophilic addition reaction of imines with C nucleophiles using chiral phosphoric acid derivative. PCT Int Appl WO 2005070875 2005-08-04... 

In 2006, Akiyama and coworkers established an asymmetric Brpnsted acid-catalyzed aza-Diels-Alder reaction (Scheme 36) [59]. Chiral BINOL phosphate (R)-3o (5 mol%, R = 2,4,6- Pr3-CgH2) bearing 2,4,6-triisopropylphenyl groups mediated the cycloaddition of aldimines 94 derived from 2-amino-4-methylphenol with Danishefsky s diene 95 in the presence of 1.2 equivalents of acetic acid. Piperidinones 96 were obtained in good yields (72 to >99%) and enantioselectivi-ties (76-91% ee). While the addition of acetic acid (pK= 4.8) improved both the reactivity and the selectivity, the use of benzenesulfonic acid (pK= -6.5) as an additive increased the yield, but decreased the enantioselectivity. A strong achiral Brpnsted acid apparently competes with chiral phosphoric acid 3o for the activation of imine 94 and catalyzes a nonasymmetric hetero-Diels-Alder reaction. The role of acetic acid remains unclear. 

In 2007, AntiUa and coworkers described the Brpnsted add-catalyzed desymmetrization of me yo-aziridines giving vicinal diamines [75]. hi recent years, chiral phosphoric acids have been widely recognized as powerful catalysts for the activation of imines. However, prior to this work, electrophiles other than imines or related substrates like enecarbamates or enamides have been omitted. In the presence of VAPOL-derived phosphoric acid catalyst (5)-16 (10 mol%) and azidotrimethylsilane as the nucleophile, aziridines 129 were converted into the corresponding ring-opened prodncts 130 in good yields and enantioselectivities (49-97%, 70-95% ee) (Scheme 53). 

Until 2006, a severe limitation in the field of chiral Brpnsted acid catalysis was the restriction to reactive substrates. The acidity of BINOL-derived chiral phosphoric acids is appropriate to activate various imine compounds through protonation and a broad range of efficient and highly enantioselective, phosphoric acid-catalyzed transformations involving imines have been developed. However, the activation of simple carbonyl compounds by means of Brpnsted acid catalysis proved to be rather challenging since the acid ity of the known BINOL-derived phosphoric acids is mostly insufficient. Carbonyl compounds and other less reactive substrates often require a stronger Brpnsted acid catalyst. 

Gridnev and Terada calculated the optimized structure of the complex derived from phosphoric acid and N-acyl imine [14], wherein the hydrogen-bond complex was observed by density functional theory (DFT) calculations. 

VAPOL-derived phosphoric acid 11 was shown to catalyze the amidation of Boc-protected N-aryl imines with sulfonamide, phthaUmide, and maleimide nucleophiles to furnish the corresponding chiral aminals in excellent yields and ee s (Scheme 5.29) [52, 53]. This represents the first general catalytic and asymmetric... 

In Ught of the recent developments in thiourea, diol, and phosphoric-acid-mediated catalysis, far fewer studies have focused on the use of chiral carboxyhc acids as suitable hydrogen bond donors. To this end, Mamoka synthesized binaphthyl-derived dicarboxylic acid 49 which catalyzes the asymmetric Mannich reaction of N-Boc aryl imines and tert-diazoacetate (Scheme 5.65) [120]. The authors postulate that catalytic achvity is enhanced by the presence of an addihonal car-boxyhc acid moiety given that use of 2-napthoic acid as catalyst provided only trace amounts of product... 

The Friedel Crafts (F C) reaction via activation of electrophiles functionalized by a nitrogen atom, such as imines, is undoubtedly the most practical and atom eco nomical approach to introduce a nitrogen substituted side chain to aromatic com pounds. The enantioselective version of the F C reaction of nitrogen substituted substrates, including imines, with electron rich aromatic compounds enables effi cient access to enantioenriched aryl methanamine derivatives [37[. Several excellent approaches to highly enantioselective F C reactions have been established using chiral phosphoric acid catalysts. 

The present F C reaction proceeded through the in situ generation of aliphatic imines that were delivered via the protonation of the enecarbamates by the phos phoric acid catalyst (Figure 3.4). Phosphoric acid functioned as an efficient catalyst for the dual transformation that involved the in situ generation of imine and the enantioselective carbon carbon bond formation with indole. This protocol offers the distinct advantage of generating in situ unstable aliphatic imines from storable and thus easily handled enecarbamates, and hence is applicable to other organic transformations. In fact, Terada et al. applied the present method to an enantiose lective direct Mannich reaction [51]. The method provides an efficient pathway to p alkyl P aminocarbonyl derivatives in optically active forms. 

Soon after these initial reports, the groups of Antilla [92] and You [93] indepen dently applied the chiral phosphoric acid catalysis to the enantioselective hydro genation of a imino esters. The method provides an alternative route to the enantioselective synthesis of a amino esters. Antilla and coworkers employed a new type of axially chiral phosphoric acid (9) derived from VAPOL originally developed by his research group (Scheme 3.42), whereas lg was used in You s case. In both cases, excellent enantioselectivities were achieved. You and coworkers further applied the method to the enantioselective reduction of a imino esters having an alkynyl substituent at the a position (Scheme 3.43) [94]. Both alkyne and imine moieties were reduced under transfer hydrogenation conditions with an excess amount of... 

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