Table 1 Selected results from the reaction optimisation

From: Triplet state reactivity of iminium ions in organocatalytic asymmetric [2 + 2] photocycloadditions

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Entrya

Catalyst 2

TFA (equiv.)

Yield (%)b

e.e. (%)c

1d

2a

1

<5

n.d.

2d

2b

1

64

63

3d

2c

1

67

89

4d

2d

1

62

91

5

2d

2.5

97

92

6

No catalyst

2.5

<5

n.d.

7

2d, no light

2.5

<5

n.d.

8

2d, air

2.5

44

n.d.

  1. n.d., not determined
  2. aSubstrate 1 (0.1 mmol), organocatalyst (20 mol%, 0.02 mmol), TFA (equiv.), CHCl3 (0.4 ml) and inert atmosphere.
  3. bReaction yields were measured by 1H NMR analysis of the crude reaction mixture using dibromomethane as the internal standard.
  4. cThe e.e. values were determined by chiral stationary phase ultra-performance convergence chromatography (UPC2) after derivatization of the [2 + 2] cycloadduct with (carbethoxymethylene)triphenylphosphorane.
  5. d1,2-Dichloroethane was used in place of CHCl3.
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