Thiourea-Catalyzed Enantioselective Hydrophosphonylation of Imines: Practical Access to Enantiomerically Enriched α-Amino Phosphonic Acids

Journal of the American Chemical Society

Volumn 126, Issue 13, 2004, Pages 4102-4103

  Joly, Guy D ^a   Jacobsen, Eric N ^a  

^a HARVARD UNIVERSITY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

IMINE; PHOSPHORAMIDIC ACID; THIOUREA DERIVATIVE;

ARTICLE; CATALYSIS; CATALYST; CHEMICAL REACTION; CYCLIZATION; ENANTIOMER; ENANTIOSELECTIVITY; PROTEIN DEGRADATION; TEMPERATURE DEPENDENCE;

EID: 1842555199     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja0494398     Document Type: Article

References (36)

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29. Other nucleophilic phosphorus reagents examined included diethyl phosphite (6% conversion, 22 h), trimethyl phosphite (0% ee), and dimethyl tert-butyldimethylsilyl phosphite (no reaction, 16 h). The poor reactivity of dimethyl tert-butyldimethylsilyl phosphite implies a critical role for the proton of the dialkyl phosphites. This is further supported by the somewhat counterintuitive increase in reaction rate with less electron-rich dialkyl phosphite nucleophiles. A correlation between reactivity and the acidity of the phosphite proton is suggested.
30. For example, la catalyzes the addition of diphenyl phosphite 3b to imine 2a in 59% ee under the same conditions employed using 1b to provide product in 77% ee (Table 1). Also, catalyst 1a affords 4c in 85% ee as compared to 90% ee using catalyst 1b under the conditions described in Table 2. Additional urea and thiourea catalysts were examined. See the Supporting Information.
31. These studies were guided by the hypothesis that catalysts 1 activate imines to nucleophilic addition via hydrogen bonding to the thiourea protons. See ref 9d.
32. A crossover experiment revealed that racemization occurred via a retroaddition pathway.
33. For the preparation of 3h, see the Supporting Information.
34. Optimization revealed very little solvent dependence; nonpolar ethereal solvents give the highest enantioselectivities, with diethyl ether providing optimal results. The reaction also displays a minimal dependence upon concentration, but best results are achieved at 0.4 M. When possible, lowering the temperature to 4 °C has a beneficial effect upon enantioselectivity.
35. Unbranched aliphatic imines are not useful substrates due to their rapid decomposition under the reaction conditions.
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