J. A. Bull et al. / Tetrahedron Letters 49 (2008) 1352–1356
1355
M.; Fengas, D.; Knight, C. K.; Parker, J.; Quayle, P.; Raftery, J.;
Richards, S. N. Tetrahedron Lett. 2005, 46, 7129–7134; (e) Edlin, C.
D.; Faulkner, J.; Quayle, P. Tetrahedron Lett. 2006, 47, 1145–1151; (f)
Edlin, C. D.; Faulkner, J.; Helliwell, M.; Knight, C. K.; Parker, J.;
Quayle, P.; Raftery, J. Tetrahedron 2006, 62, 3004–3015; (g) Edlin, C.
D.; Faulkner, J.; Fengas, D.; Helliwell, M.; Knight, C. K.; House, D.;
Parker, J.; Preece, I.; Quayle, P.; Raftery, J.; Richards, S. N. J.
Organomet. Chem. 2006, 691, 5375–5382.
benzannulation conditions afforded 6, 9e and 9h in 46–55%
isolated yields. Interestingly we note that exchanging the
phosphine ligand of 23 by a NHC ligand, as in the 2nd
generation Grubbs catalyst 24, apparently has a negative
effect22 upon both the rate and isolated yields of represen-
tative benzannulation reactions (Table 1). Use of catalysts
possessing a hemilabile ligand, as in the case of the
Hoveyda–Grubbs catalyst 25, also proves to be detrimental
whilst a mixed catalyst system comprising of 2 and 23 has
little effect on the overall efficiency of the benzannulation
reaction leading to 9h.
In conclusion we have discovered that readily available
NHC-transition metal carbene complexes promote a vari-
ety of ATRC reactions. This study also demonstrates the
potential benefits of microwave irradiation on metal-catal-
ysed ATRC reactions. That several of these ATRC reac-
tions do proceed more effectively under microwave
heating may be indicative of a ‘microwave effect’23 an
observation which is currently under scrutiny. The fact that
the carbene complex 2 appears to be intact at the end of the
reaction also suggests that recycling of the catalyst in these
reactions is a distinct possibility.24 We anticipate that tun-
ing25 (both electronic and steric) of the catalyst system will
enable these reactions to proceed under milder conditions,
a goal which is also the focus of current investigations. In
addition we have yet to compare the reactivity of these
pre-formed carbene complexes with the newly developed,
and highly reactive, ruthenium26 and copper27 catalysts
under our microwave conditions.
7. Bull, J. A.; Hutchings, M. G.; Quayle, P. Angew. Chem., Int. Ed. 2007,
46, 1869–1872.
8. Conducting these reactions with, for example, bipy-based ligand
systems rapidly generates green-coloured solutions, presumably due
to the generation of Cu(II) complexes. In the case of (pro)ligand 3a
this is not the case thereby leading us to speculate as to the nature of
the catalytic species in this particular case.
9. Michon, C.; Ellern, A.; Angelici, R. J. Inorg. Chim. Acta 2006, 359,
4549–4556; For a review on ‘tethered’ carbenes see: Liddle, S. T.;
Edworthy, I. S.; Arnold, P. L. Chem. Soc. Rev. 2007, 36, 1732–1744.
10. For the X-ray structure of an analogous zinc complex see: Wan, X.-J.;
Xu, F.-B.; Li, Q.-S.; Song, H.-B.; Zhang, Z.-Z. Acta Crystrallogr.,
Sect. E 2005, 61, m2174–m2175.
11. See Herrmann, W. A.; Weskamp, T.; Bo¨hm, V. P. W. Adv.
Organomet. Chem. 2001, 48, 1–69 and references cited therein.
12. Complex 2 is now commercially available from Strem Chemicals.
13. For the use of Ru carbene complexes in (i) ATRC reactions see Ref.
6a; (ii) ATRP/Kharasch reactions see: Delaude, L.; Demonceau, A.;
Noels, A. F. Top. Organomet. Chem. 2004, 11, 155–171 and references
cited therein; We are aware of a single report concerning the use of
copper carbene complexes as catalysts in ATRP recations, see: Bantu,
B.; Wang, D.; Wurst, K.; Buchmeiser, M. R. Tetrahedron 2005, 61,
12145–12152.
14. For use of microwave activation of Kharasch reactions see: (a)
´
´
Adamek, F.; Hajek, M. Tetrahedron Lett. 1992, 33, 2039–2042;
ultrasound irradiation: (b) Shvekhgeimer, M.-G. A.; Kobrakov, K. I.;
Gafarov, D. M. Dokl. Akad. Nauk. 2001, 377, 210–211.
15. Whilst there is a growing body of applications to ATRP chemistry see:
(a) Zhang, H.; Schubert, U. S. Macromol. Rapid Commun. 2004, 25,
1225–1230; (b) Leenen, M.; Wiesbrock, F.; Hoogenboom, R.;
mers.org); (c) Wiesbrock, F.; Hoogenboom, R.; Schubert, U. S.
Macromol. Rapid Commun. 2004, 25, 1739–1764; (d) Cheng, Z.; Zhu,
X.; Zhou, N.; Zhu, J.; Zhang, Z. Radiat. Phys. Chem. 2005, 72, 695–
701; (e) Delfosse, S.; Wei, H.; Demonceau, A.; Noels, A. F. Polym.
Prepr. 2005, 46, 295–296; (f) Delfosse, S.; Borguet, Y.; Delaude, L.;
Demonceau, A. Macromol. Rapid Commun. 2007, 28, 492–503;
utilisation of this technique in ATRC/group transfer cyclisation
reactions is under-represented (see: (g) Wetter, C.; Studer, A. Chem.
Commun. 2004, 174–175; (h) Ericsson, C.; Engman, L. J. Org. Chem.
2004, 69, 5143–5146. for pertinent examples).
Acknowledgement
J.A.B. and C.L. thank DyStar UK Ltd for support of
this research programme.
References and notes
1. Arduengo, A. J., III. Acc. Chem. Res. 1999, 32, 913–921.
2. Alder, R. W.; Blake, M. E.; Oliva, J. M. J. Phys. Chem. (A) 1999,
103, 11200–11211.
3. Olaf, K. Chem. Soc. Rev. 2007, 36, 592–697; Dragutan, V.; Dragutan,
I.; Delaude, L.; Demonceau, A. Coord. Chem. Rev. 2007, 251, 765–
794.
16. Isolated as a single diastereoisomer which is identical to that reported
in Ref. 6a.
17. Representative experimental procedures: Benzannulation reactions: A
solution of ester 8h (500 mg, 1.6 mmol) and the catalyst 2 (38.8 mg,
0.08 mmol) in dry DCE (6 mL) was heated in a microwave reactor18
at 200 °C under nitrogen for 2 h. Upon cooling to ambient temper-
ature the solvent was removed in vacuo and the crude product
purified by flash column chromatography (eluent petrol) to afford 1,8-
dichloronaphthalene 9h. Yield 240.0 mg (76%). IR mmax (film): 1597,
4. (a) Jurkauskas, V.; Sadighi, J. P.; Buchwald, S. L. Org. Lett. 2003, 5,
2417–2420; (b) Kaur, H.; Zinn, F. K.; Stevens, E. D.; Nolan, S. P.
Organometallics 2004, 23, 1157–1160.
´
´
5. (a) Dıez-Gonzalez, S.; Scott, N. M.; Nolan, S. P. Organometallics
2006, 25, 2355–2358; (b) Fructos, M. R.; De Fremont, P.; Nolan, S.
P.; Mar, D.-R. M.; Perez, P. J. Organometallics 2006, 25, 2237–2241;
´
´
(c) Dıez-Gonzalez, S.; Correa, A.; Cavalo, L.; Nolan, S. P. Chem. Eur.
J. 2006, 12, 7558–7564; (d) Gade, L. H.; Bellemin-Laponnaz, S. Top.
Organomet. Chem. 2007, 21, 117–157; Roland, S.; Mangeney, P. Top.
Organomet. Chem. 2005, 15, 191–229; Clavier, H.; Guillemin, J.-C.;
Mauduit, M. Chirality 2007, 19, 471–476; (e) Gillingham, D. G.;
Hoveyda, A. H. Angew. Chem., Int. Ed. 2007, 46, 3860–3864.
1553, 1501, 1359, 1324, 1195, 1153, 970, 887 cmÀ1 1H NMR
;
(500 MHz, CDCl3) d 8.20 (2H, dd, J = 8, 1 Hz), 7.83 (2H, dd,
J = 7.5, 1 Hz), 7.37 (2H, dd, J = 8, 7.5 Hz) ppm; 13C NMR
(125.75 MHz, CDCl3): 137.2, 130.9, 130.4, 128.5, 127.5, 126.2 ppm;
m/z (EI/CI): 195/197/199, 161/163, 125. Accurate mass: C10H6O35Cl2
(M+) requires 195.9841; found 195.9844. ATRC reactions: A solution
of N-allyl-2,2,2-trichloro-N-phenylacetamide, 16 (500 mg, 1.9 mmol)
and catalyst 2 (45.5 mg, 0.09 mmol) in DCE (6 mL) was heated in a
microwave reactor18 at 110 °C under nitrogen for 3 h. Upon cooling
to ambient temperature the solvent was removed in vacuo and the
6. (a) Quayle, P.; Fengas, D.; Richards, S. Synlett 2003, 1797–1800; (b)
Edlin, C. D.; Faulkner, J.; Fengas, D.; Knight, C. K.; Parker, J.;
Preece, I.; Quayle, P.; Richards, S. N. Synlett 2005, 572–576; (c)
Faulkner, J.; Edlin, C. D.; Fengas, D.; Preece, I.; Quayle, P.;
Richards, S. N. Tetrahedron Lett. 2005, 46, 2381–2385; (d) Helliwell,