ˇ
Z. Petrovski et al. / Tetrahedron Letters 51 (2010) 3356–3359
3359
Chem., Int. Ed. 2007, 46, 5020–5023; Pera, N. P.; Nilsson, U. J.; Kann, N.
Tetrahedron Lett. 2008, 49, 2820–2823.
19. Tang, Y. F.; Deng, L. J.; Zhang, Y. D.; Dong, G. B.; Chen, J. H.; Yang, Z. Org. Lett.
2005, 7, 593–595; Tang, Y. F.; Zhang, Y. D.; Dai, M. J.; Luo, T. P.; Deng, L. J.; Chen,
J. H.; Yang, Z. Org. Lett. 2005, 7, 885–888.
12. Park, K. H.; Son, S. U.; Chung, Y. K. Chem. Commun. 2003, 1898–1899; Nam, D.
W.; Jung, H. Y.; Lee, H.; Song, K. H. J. Organomet. Chem. 2004, 689, 2586–2592;
Mastrorilli, P.; Nobile, C. F.; Paolillo, R.; Suranna, G. P. J. Mol. Catal. A: Chem.
2004, 214, 103–106; Park, K. H.; Chung, Y. K. Adv. Synth. Catal. 2005, 347, 854–
866; Lee, S. G.; Hong, S. D.; Park, Y. W.; Jeong, B. G.; Arias, J. L.; Cabrera, A.;
Sharma, P.; Rosas, N.; Sampere, R. J. Mol. Catal. A: Chem. 2006, 246, 237–241;
Fager-Jokela, E.; Kaasalainen, E.; Leppänen, K.; Tois, J.; Helaja, J. Tetrahedron
2008, 64, 10381–10387; Arias, J. L.; Cabrera, A.; Sharma, P.; Rosas, N.; Toscano,
A.; Martínez, E. E.; Rubio-Pérez, L. Catal. Commun. 2009, 10, 848–852; Ji, Y.;
Riera, A.; Verdaguer, X. Org. Lett. 2009, 11, 4346–4349; Ferrer, C.; Riera, A.;
Verdaguer, X. Organometallics 2009, 28, 4571–4576; Arias, J. L.; Cabrera, A.;
Sharma, P.; Rosas, N.; Toscano, A.; Martinez, E. E.; Rubio-Perez, L. Catal.
Commun. 2009, 10, 848–852.
13. Jeong, N.; Hwang, S. H. Angew. Chem., Int. Ed. 2000, 39, 636–638; Vázquez-
Romero, A.; Cárdenas, L.; Blasi, E.; Verdaguer, X.; Riera, A. Org. Lett. 2009, 11,
3104–3107.
14. Itami, K.; Mitsudo, K.; Yoshida, J. Angew. Chem., Int. Ed. 2002, 41, 3481–3484;
Itami, K.; Mitsudo, K.; Fujita, K.; Ohashi, Y.; Yoshida, J. J. Am. Chem. Soc. 2004,
126, 11058–11066.
20. A complete list of additives tested is provided in the Supplementary data.
21. We suspect that elimination is not just simple Lewis acid-assisted process. We
performed additional elimination experiments followed by simple TLC analysis
with compound 3a dissolved in toluene at 110 °C in the presence of
stoichiometric quantity of (1) Lewis acid (BF3ꢀEt2O); (2) DBU; (3) Co2(CO)8; (4)
Fe(CO)5; (5) no additive. In the sample, Lewis acid process was slower and only
partial after 16 h; in the sample containing DBU, there occurs complete
elimination only after 16 h, and Co2(CO)8 provided elimination in 6 h. Samples
containing Fe(CO)5 and no additive provided no elimination, or any change at all.
There was also one curiosity regarding Co2(CO)8-assisted process; the
elimination reaction took some time to start and then it was finished quite
rapidly. Such a pattern made us believe that possible mechanism for elimination
could involve abstraction of b-hydrogen atom from coordinated transient g2
-
ketone 3a to form cobalt hydride enolate species, just as the group IV metal
centers; see for example: Scott, M. J.; Lippard, S. J. Organometallics 1998, 17, 466–
474 and references therein. Cobalt hydride/enolate could afterwards eliminate
4a and recover the catalyst by free radical pathway. The whole process must be
rather sluggish. Free radicals themselves could abstract b-hydrogen atom from
thiourea 3a coordinated to metal and restart fragmentation cycle. As radical
concentration increases much faster radical fragmentation pathway could
become predominant. The final products of elimination are in any case 4a
and 5. To test the hypothesis we tried to trap free radicals with vinylcyclo-
propanedicarboxylate ester but failed to isolate any product.
15. Goettmann, F.; Le Floch, P.; Sanchez, C. Chem. Commun. 2006, 180–182.
16. Huang, Q.; Hua, R. Chem. Eur. J. 2009, 15, 3817–3822.
17. Antras, F.; Ahmar, M.; Cazes, B. Tetrahedron Lett. 2001, 42, 8153–8156; Kent, J.
L.; Wan, H.; Brummond, K. M. Tetrahedron Lett. 1995, 36, 2407–2410;
Brummond, K. M.; Wan, H. Tetrahedron Lett. 1998, 39, 931–934; Brummond,
K. M.; Wan, H.; Kent, J. L. J. Org. Chem. 1998, 63, 6535–6545; Brummond, K. M.;
Lu, J.; Petersen, J. J. Am. Chem. Soc. 2000, 122, 4915–4920; Brummond, K. M.;
Sill, P. C.; Rickards, B.; Geib, S. J. Tetrahedron Lett. 2002, 43, 3735–3738;
Brummond, K. M.; Kerekes, A. D.; Wan, H. J. Org. Chem. 2002, 67, 5156–5163;
Brummond, K. M.; Sill, P. C.; Chen, H. Org. Lett. 2004, 6, 149–152; Brummond, K.
M.; Mitasev, B. Org. Lett. 2004, 6, 2245–2248; Brummond, K. M.; Curran, D. P.;
Mitasev, B.; Fisher, S. J. Org. Chem. 2005, 70, 1745–1753; Brummond, K. M.;
Chen, D. Org. Lett. 2008, 10, 705–708.
18. Krafft, M. E. J. Am. Chem. Soc. 1988, 110, 968–970; Krafft, M. E.; Juliano, C. A.;
Scott, I. L.; Wright, C.; McEachin, M. D. J. Am. Chem. Soc. 1991, 113, 1693–1703;
Krafft, M. E.; Juliano, C. A. J. Org. Chem. 1992, 57, 5106–5115; Krafft, M. E.; Scott,
I. L.; Romero, R. H. Tetrahedron Lett. 1992, 33, 3829–3832; Krafft, M. E.; Scott, I.
L.; Romero, R. H.; Feibelmann, S.; Vanpelt, C. E. J. Am. Chem. Soc. 1993, 115,
7199–7207.
22. The molecular structure suggested for 8a was based on 1H NMR and IR
spectroscopy and mass spectrometry. Unfortunately, various attempts to get
suitable crystals for X-ray analysis failed.
23. General procedure for the synthesis of methylenecyclopentenones 4a–e: To the
stirred solution of allylthiourea
1
(50 mg; 0.316 mmol) and N,N,N0,N0-
tetramethylthiourea (42 mg; 0.316 mmol; 1.0 equiv) in dry toluene (5 mL)
under argon were added 1-alkyne (0.372 mmol; 1.2 equiv) and Co2(CO)8
(10.8 mg; 0.0316 mmol; 0.1 equiv, commercial). After 5 min the solution was
transferred to autoclave; the autoclave was purged several times with carbon
monoxide and pressure was adjusted to 3 bar. The reaction mixture was kept
at 110 °C for the time designated in Table 2. After cooling, the reaction mixture
was filtered on Celite, the solvent was evaporated, and the remaining oil was
purified by thin layer chromatography (eluent hexane/ethyl acetate = 7:3). The
yield of each product is shown in Tables 2 and 3 and SM1.