Benzaldehyde (Prepared in-house from salicylaldehyde)

tert-Dodecanethiol

1,1'-Azobis(cyclohexanecarbonitrile) (ACCN)

Chlorobenzene (Pre-dried over molecular sieves)

To a solution of the benzaldehyde (200 mg, 0.91 mmol, 1 equiv.) in dry chlorobenzene (10 mL) were added ACCN (335 mg, 1.37 mmol, 1.5 equiv.) and tert-dodecanethiol (553 mg, 2.73 mmol, 3 equiv.) (Note 1). The solution was sparged by bubbling argon through the solution for 30 minutes after which the solution was maintained under an atmosphere of argon and heated at 100 °C for 20 hours. After removal of the solvent in vacuo (ensure adequate ventilation due to pungent thiol), the resulting oil was purified by column chromatography (2 x 25 cm column of silica gel, EtOAc-PE, 1 : 3) to give the benzopyran-4-one (136 mg, 68% [92% based on recovered benzaldehyde], R_f = 0.63 in EtOAc-PE, 1 : 1) and starting benzaldehyde (52 mg, 26%, R_f = 0.55 in EtOAc-PE, 1 : 1).

The reaction between an aldehyde and Michael acceptor to form a 1,4-dicarbonyl compound is synthetically equivalent to the Stetter reaction. The polarity reversal catalyzed radical conditions used here can be considered an operationally simple complementary method to achieve this single-step transformation. We have observed that alkyl-aldehydes can be prone to competitive decarbonylation under these conditions (see Other References 2).
Note 1: Whilst mechanistically the thiol acts catalytically by firstly abstracting hydrogen from the aldehyde to form an acyl radical directly (see Other References 1), in practice it has been found that for benzaldehydes a greater conversion and less complex reaction mixture is obtained by using an excess of thiol. Other alkyl thiols can be used as catalyst for this reaction, however, tert-dodecanethiol is less volatile than lower molecular weight thiols and thus less pungent. Use of an efficient fumehood is still recommended.

¹H NMR (500 MHz, CDCl₃): δ = 2.44 (dd, J = 17.0, 8.1 Hz, 1H), 2.95 (dd, J = 17.0, 5.0 Hz, 1H), 3.31-3.37 (m, 1H), 3.74 (s, 3H), 4.30 (dd, J = 12.0, 11.2 Hz, 1H), 4.60 (dd, J = 11.2, 5.3 Hz, 1H), 6.98 (d, J = 8.4 Hz, 1H), 7.03 (apparent triplet, J = 7.5 Hz, 1H), 7.48 (apparent triplet, J = 8.4 Hz, 1H), 7.89 (d, J = 7.8 Hz, 1H)

¹³C NMR (125 MHz, CDCl₃): δ = 30.1, 42.5, 52.0, 70.2, 117.8, 120.4, 121.5, 127.3, 136.0, 161.7, 171.8, 192.5

GC-MS: (EI) 220 (M⁺, 2%), 189 (17), 147 (100), 120 (58), 92 (33)

Yoshikai, K.; Hayama, T.; Nishimura, K.; Yamada, K.; Tomioka, K. J. Org. Chem., 2005, 70, 681-683.
Santoso, H.; Casana, M. I.; Donner, C. D. Org. Biomol. Chem., 2014, 12, 171-176.

1. Roberts, B. P. Chem. Soc. Rev., 1999, 28, 25-35.

2. Aitken, H. M.; Schiesser, C. H.; Donner, C. D. Aust. J. Chem., 2011, 64, 409-415.