Table 1 Optimization of the reaction conditions

From: Catalytic 4-exo-dig carbocyclization for the construction of furan-fused cyclobutanones and synthetic applications

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Entry

Catalyst (x mol%)

Solvent

Yield (%)a

2a

3a

1b

ZnCl2 (20.0)

DCM

<5

6

2c

AgSbF6 (5.0)

DCM

–

–

3c

JohnPhos(MeCN)AuSbF6 (5.0)

DCM

–

–

4

Cu(MeCN)4PF6 (5.0)

DCM

15

30

5

[PdCl(η3-C3H5)]2 (5.0)

DCM

21

<5

6

Rh2(OAc)4 (2.0)

DCM

44

20

7

Rh2(Oct)4 (2.0)

DCM

45

19

8

Rh2(TPA)4 (2.0)

DCM

41

15

9

Rh2(OPiv)4 (2.0)

DCM

54

10

10

Rh2(esp)2 (2.0)

DCM

43

9

11

Rh2(pfb)4 (2.0)

DCM

20

17

12

Rh2(TFA)4 (2.0)

DCM

16

14

13

Rh2(tfacam)4 (2.0)

DCM

20

65

14

Rh2(cap)4 (2.0)

DCM

NR

NR

15

Rh2(OPiv)4 (2.0)

DCE

74

13

16

Rh2(OPiv)4 (2.0)

toluene

47

12

17

Rh2(OPiv)4 (2.0)

CH3CN

71

10

18

Rh2(OPiv)4 (2.0)

EtOAc

81

<5

19

Rh2(OAc)4 (2.0)

EtOAc

80

<5

20

Rh2(OPiv)4 (1.0)

EtOAc

83

<5

21d

Rh2(OPiv)4 (1.0)

EtOAc

88 (84)e

<5

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  1. Reaction conditions: to a solution of metal catalyst (x mol%) in 1.0 mL solvent, was added 1a (0.1 mmol, 34.4 mg) in the same solvent (1.0 mL) slowly over 10 min via a syringe at 40 °C under an argon atmosphere, and the reaction mixture was stirred for additional 1.0 h under these conditions.
  2. DCM dichloromethane, DCE dichloroethane, EtOAc ethyl acetate.
  3. aYields were detected by proton NMR of the crude reaction mixture using 4-nitrobenzaldehyde as the internal standard.
  4. bLow conversions (<20%) were observed, and most of the material was recovered.
  5. cThe material 1a was decomposed into a complex mixture.
  6. d100 mg 4 Å MS was added.
  7. eIsolated yield.
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