The First Example of Polymer-supported Alkenyl Iodonium Salts: Efficient Synthesis of β-Functional Enamines

Abstract

Polymer-supported alkenyl(phenyl) iodonium salts were synthesized, which served as effective alkenyl transfer reagents for the preparation of β-functional enamines; the polymer support poly(4-iodostyrene) can be regenerated and recycled.

Key words

polymer-supported alkenylphenyl iodonium salts - poly(4-iodostyrene) - β-functional enamines

References

• 1a Suto MJ. GayoFung LM. Palanki MSS. Sullivasn R. Tetrahedron  1998,  54:  4141 
• 1b Gayo LM. Suto MJ. Tetrahedron Lett.  1997,  38:  513 
• 1c Parlow JJ. Tetrahedron Lett.  1996,  37:  5257 
• 1d Nicholas EJ. Scott KA. J. Org. Chem.  2000,  65:  4770 
• 1e Togo H. Sakuratani K. Synlett  2002,  1966 
• 2a Stang PJ. Zhdankin VV. Chem. Rev.  1996,  96:  1123 
• 2b Zhdankin VV. Stang PJ. Chem. Rev.  2002,  102:  2523 
• 2c Stang PJ. J. Org. Chem.  2003,  68:  2997 
• 2d Wirth T. Hirt UH. Synthesis  1999,  1271 
• 2e Varvoglis A. Hypervalent Iodine in Organic Synthesis   Academic Press; London: 1997. 
• 3a Ochiai M. In Chemistry of Hypervalent Compounds   Akiba K. VCH Publishers; New York: 1999.  Chap. 13. p.359-387  
• 3b Okuyama T. Rev. Heteroatom. Chem.  1999,  21:  257 
• 3c Okuyama T. Acc. Chem. Res.  2002,  35:  12 
• 4a Ochiai M. Sumi K. Takaoka Y. Kunishima M. Nagao Y. Shiro M. Fujita E. Tetrahedron  1988,  44:  4095 
• 4b Gronheid R. Lodder G. Ochiai M. Sueda T. Okuyama T. J. Am. Chem. Soc.  2001,  123:  8760 
• 4c Papoutsis I. Spyroudis S. Varvoglis A. Tetrahedron  1998,  54:  1005 
• 5a Ochiai M. Kitagawa Y. Yamamoto S. J. Am. Chem.Soc.  1997,  119:  11598 
• 5b Zhdankin VV. Stang PJ. Tetrahedron  1998,  54:  10927 
• 6a Huang X. Zhu Q. Synth. Commun.  2001,  31:  111 
• 6b Huang X. Zhu Q. Tetrahedron Lett.  2001,  42:  6373 

7

Preparation of Polymer-supported Alkenyl Iodonium Tosylate 3: To a solution of methyl-3-amino-crotonate (2a; 1.15 g, 10 mmol) or ethyl-3-benzylamino-crotonate (2b; 2.19 g, 10 mmol) in CH₂Cl₂ (30 mL), poly{[4-hydroxy(tosyloxy)iodo]styrene} (1.65 g, 3 mmol) was added. The suspension was stirred at 0 °C for 6 h or 2 h, respectively. After the reaction was complete, the CH₂Cl₂ was evaporated in vacuum to give oily residue. Et₂O (20 mL) was added to precipitate the product, which was filtered off and washed with acetone (10 mL × 2) and Et₂O (10 mL × 2) to afford the products. Compound 3a: white powder, 1.61 g, N%, 3.09. IR (KBr): ν = 3176, 3076 1629, 1448, 1404, 1263, 1187, 1125, 1035, 1009, 814, 684 cm^-1. Compound 3b: white powder, 1.78 g, N%, 2.44. IR (KBr): ν = 3043, 2920, 1625, 1487, 1449, 1170, 1124, 1034, 1009, 814, 683 cm^-1.

8

General Procedure for the Synthesis of β-Functional Enamines 4: To a suspension of 3a (1.0 g, N: 2.21 mmol/g) in CHCl₃ (10 mL) was added the solution of KSCN (0.1 g, 1 mmol) in water (3 mL). After the reaction was complete, the organic layer was separated and dried over anhyd MgSO₄. The solvent was removed under vacuum and Et₂O was added to the resulting oil to precipitate poly(4-iodostyrene). The solution was submitted to preparative TLC on silica gel with n-hexane-EtOAc (2:1, v/v) as the eluent to afford 143 mg (83%) of 4a, white powder, mp 77-78 °C. IR (KBr): n = 3410, 3312, 2998, 2949, 1672, 1628, 1506, 1434, 1371, 1264, 1185, 1134, 1071, 778, 648 cm^-1. ¹H NMR (400 MHz, CDCl₃): d = 9.26 (s, 1 H), 5.78 (s, 1 H), 3.77 (s, 3 H), 2.39 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃): d = 168.8, 168.7, 113.5, 75.6, 52.0, 23.4. MS (EI): m/z (%) = 172 (38.9) [M⁺], 113 (71.4), 87 (67.7), 59 (49.5), 42 (100). Anal. Calcd for C₆H₈N₂O₂S: C, 41.85; H, 4.68; N, 16.27. Found: C, 41.78; H, 4.59; N, 16.39.