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Carbene Transfer Reactions from Chromium(0) to Gold(I): Synthesis and Reactivity of New Fischer-Type Gold(I) Alkenyl Carbene Complexes Martı´n Fan˜ana´s-Mastral and Fernando Aznar* Instituto UniVersitario de Quı´mica Organometa´lica “Enrique Moles”, Unidad Asociada al CSIC, UniVersidad de OViedo, Julia´n ClaVerı´a 8, E-33006, OViedo, Spain ReceiVed December 2, 2008 Summary: The synthesis of new cationic alkenyl Fischer-type gold(I) complexes through a carbene transfer reaction of chromium and tungsten alkenyl Fischer carbene complexes with LAuICl complexes in the presence of AgSbF6 is reported. The stability of the resulting gold(I) carbene complexes highly depends on the nature of the ligand (L). We haVe used unstable gold(I) carbene complexes for deVeloping some catalytic reactions of Fischer carbene complexes. Heteroatom-stabilized transition-metal carbene complexes, particularly those derived from group 6, are recognized as versatile intermediates in organic synthesis.1 Doubtless the most important feature of these carbene complexes lies in their ability to transfer the alkoxycarbene ligand to different unsaturated substrates.2 However, the transfer of a carbene ligand from a metal carbene complex to another metallic center represents an interesting concept that allows access to carbene complexes of late transition metals.3 Since Fischer and Beck reported the first example of a stoichiometric Fischer-type carbene ligand transfer from molybdenum to iron complexes,4 the carbene ligand in pentacarbonyl carbene complexes of chromium, molybdenum, and tungsten has also been transferred to other metals such as gold,5-7 cobalt,8 rhodium,6,9,10 nickel,10-12 palladium,6,7,13 platinum,6,7 copper,6b,12,14 and silver.6b With regard to gold complexes, Au(I) bis-carbene complexes have been prepared * To whom correspondence should be addressed. E-mail:
[email protected]. (1) For recent reviews, see: (a) Barluenga, J.; Toma´s, M.; Santamarı´a, J. Chem. ReV. 2004, 104, 2259–2283. (b) Do¨tz, K. H. Metal Carbenes in Organic Synthesis; Springer: Berlin, 2004; Topics in Organometallic Chemistry, Vol. 13. (c) Barluenga, J.; Ferna´ndez-Rodriguez, M. A.; Aguilar, E. J. Organomet. Chem. 2005, 690, 539–587. (d) Wu, Y.-T.; Kurahashi, T.; de Meijere, A. J. Organomet. Chem. 2005, 690, 5900–5911. (e) Herndon, J. W. Coord. Chem. ReV. 2006, 250, 1889–1964. (2) For the transfer of the carbene ligand to alkenes, see: (a) Brookhart, M.; Studabaker, W. B. Chem. ReV. 1987, 87, 411–432. (b) Harvey, D. F.; Sigano, D. M. Chem. ReV. 1996, 96, 271–288. For the transfer of the carbene ligand to oxygen, nitrogen, and carbon through the corresponding ylides, see: (c) Alcaide, B.; Casarrubios, L.; Domı´nguez, G.; Sierra, M. A. Curr. Org. Chem. 1998, 2, 551–574. (3) (a) Liu, S.-T.; Reddy, K. R. Chem. Soc. ReV. 1999, 28, 315–322. (b) Go´mez-Gallego, M.; Manchen˜o, M. J.; Sierra, M. A. Acc. Chem. Res. 2005, 38, 44–53. (4) Fischer, E. O.; Beck, H.-J. Angew. Chem. 1970, 82, 44-45; Angew. Chem., Int. Ed. Engl. 1970, 9, 72-73. (5) (a) Aumann, R.; Fischer, E. O. Chem. Ber. 1981, 114, 1853–1857. (b) Fischer, E. O.; Bo¨ck, M.; Aumann, R. Chem. Ber. 1983, 116, 3618– 3623. (c) Fischer, E. O.; Bo¨ck, M. Monatsh. Chem. 1984, 115, 1159–1164. (d) Fischer, E. O.; Bo¨ck, M. J. Organomet. Chem. 1985, 287, 279–285. (6) (a) Liu, S.-T.; Hsieh, T.-Y.; Lee, G.-H.; Peng, S.-M. Organometallics 1998, 17, 993–995. (b) Ku, R.-Z.; Huang, J.-C.; Cho, J.-Y.; Kiang, F.-M.; Reddy, K. R.; Chen, Y.-C.; Lee, K.-J.; Lee, J.-H.; Lee, G.-H.; Peng, S.-M.; Liu, S.-T. Organometallics 1999, 18, 2145–2154. (7) Kessler, F.; Szesni, N.; Maass, C.; Hohberger, C.; Weibert, B.; Fischer, H. J. Organomet. Chem. 2007, 692, 3005–3018. (8) Jordi, L.; Moreto´, J. M.; Ricart, S.; Vin˜as, J. M.; Mejias, M.; Molins, E. Organometallics 1992, 11, 3507–3510.
from the reaction of tungsten diaminocarbene complexes with (Me2S)AuCl.6 In addition, the pyrazolin-3-ylidene ligand is readily transferable from chromium to gold(I) and gold(III).7 A different carbene transfer from tungsten to gold(I) was reported by Fischer and co-workers in the reaction of tungsten aryl- and alkylcarbene complexes with HAuCl4. Following this strategy, chlorocarbene gold(I) and trichlorocarbene gold(III) complexes can be prepared.5 However, these methodologies only allow the preparation of chlorocarbenes or symmetric diaminocarbene gold complexes. Herein we describe a facile procedure for the synthesis of a new nonsymmetric bis-carbene gold(I) complex which bears a NHC ligand and a methoxyalkenylcarbene ligand and different phosphane carbene gold(I) complexes. Taking into account the carbene transfer procedure described by our group for the preparation of Fischer-type rhodium(I) complexes,9 we selected the known complex [(IPr)AuCl] (2; IPr ) 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene)15 as the appropriate starting gold complex for the preparation of biscarbene 3. When complex 2 was treated with AgSbF6 at room temperature in CH2Cl2 in the presence of 1 equiv of Fischer alkenyl carbene complex 1, the reaction gave rise to the formation of the bis-carbene complex 3 with quantitative yield (Scheme 1). Despite the large number of (NHC)AuI complexes
(9) (a) Barluenga, J.; Vicente, R.; Lo´pez, L. A.; Rubio, E.; Toma´s, M.; ´ lvarez-Ru´a, C. J. Am. Chem. Soc. 2004, 126, 470–471. (b) Barluenga, J.; A Vicente, R.; Lo´pez, L. A.; Toma´s, M. J. Organomet. Chem. 2006, 691, 5642–5647. (10) (a) Barluenga, J.; Vicente, R.; Barrio, P.; Lo´pez, L. A.; Toma´s, M. J. Am. Chem. Soc. 2004, 126, 5974–5975. (b) Barluenga, J.; Vicente, R.; Barrio, P.; Lo´pez, L. A.; Toma´s, M.; Borge, J. J. Am. Chem. Soc. 2004, 126, 14354–14355. (11) Barluenga, J.; Barrio, P.; Lo´pez, L. A.; Toma´s, M.; Garcı´a-Granda, ´ lvarez-Ru´a, C. Angew. Chem. 2003, 115, 3116-3119; Angew. Chem., S.; A Int. Ed. 2003, 42, 3008-3011. (12) del Amo, J. C.; Manchen˜o, M. J.; Go´mez-Gallego, M.; Sierra, M. A. Organometallics 2004, 23, 5021–5029. (13) (a) Sierra, M. A.; Manchen˜o, M. J.; Sa´ez, E.; del Amo, J. C. J. Am. Chem. Soc. 1998, 120, 6812–6813. (b) Sierra, M. A.; del Amo, J. C.; Manchen˜o, M. J.; Go´mez-Gallego, M. J. Am. Chem. Soc. 2001, 120, 6812– 6813. (c) Albe´niz, A. C.; Espinet, P.; Manrique, R.; Pe´rez-Mateo, A. Angew. Chem. 2002, 114, 2469-2472; Angew. Chem., Int. Ed. 2002, 41, 23632366. (14) (a) Barluenga, J.; Lo´pez, L. A.; Lo¨ber, O.; Toma´s, M.; Garcı´a´ lvarez-Ru´a, C.; Borge, J. Angew. Chem. 2001, 113, 3495Granda, S.; A 3497; Angew. Chem., Int. Ed. 2001, 40, 3392-3394. (b) Barluenga, J.; Barrio, P.; Vicente, R.; Lo´pez, L. A.; Toma´s, M. J. Organomet. Chem. 2004, 689, 3793–3799. (15) (a) de Fre´mont, P.; Scott, N. M.; Stevens, E. D.; Nolan, S. P. Organometallics 2005, 24, 2411–2418. (b) Fructos, M. R.; Belderrain, T. R.; de Fre´mont, P.; Scott, N. M.; Nolan, S. P.; Dı´az-Requejo, M. M.; Pe´rez, P. J. Angew. Chem. 2005, 117, 5418-5422; Angew. Chem., Int. Ed. 2005, 44, 5284-5288.
10.1021/om801146z CCC: $40.75 2009 American Chemical Society Publication on Web 01/15/2009
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Organometallics, Vol. 28, No. 3, 2009 667
Scheme 1. Preparation of Cationic Gold(I) Bis-Carbene Complex 3
that have been described,16 to the best of our knowledge, this complex represents the first example of a gold bis-carbene complex which bears an NHC ligand and a Fischer-type carbene ligand. Complex 3 is stable at room temperature, and it can be exposed to air. The structure of bis-carbene complex 3 was characterized by spectroscopic and elemental analysis and could be determined by X-ray diffraction analysis (Figure 1). As for other Fischer-type gold carbene complexes,17 the C-Au bond length falls within the range of a typical single bond between an sp2-hybridized carbon atom and an AuI center.18 In spite of the different electronic characters of both ligands, it is important to note that both C-Au bonds have a similar lengths (2.049 and 2.010 Å) and are comparable with those found for cationic bis-carbene (NHC) gold(I) complexes described in the literature.19 In addition, complex 2 shows the expected linear coordination of the two carbene ligands (C(28)-Au(1)-C(1) ) 177.6(5)°). Following the same procedure, the cationic tricyclohexylphosphine gold(I) carbene complex 4 could be isolated in 92% yield from the reaction between the Fischer carbene complex 1 and the gold(I) complex (PCy3)AuCl (Scheme 2). This complex is also air stable at room temperature and could be characterized by NMR spectroscopy. When other (phosphine)gold(I) complexes (PR3 ) PPh3, PEt3) were used, the corresponding cationic gold(I) carbene complexes could not be isolated. In these cases we observed the formation of 5, the dimerization product of the Fischer carbene complex 1. Considering these results, we studied the catalytic version of this process. Thus, the same reaction using 15 mol % of the (phosphine)gold(I) complex and 15 mol % of AgSbF6 afforded (16) For some recent examples, see: (a) Laitar, D. S.; Mu¨ller, P.; Gray, T. G.; Sadighi, J. P. Organometallics 2005, 24, 4503–4505. (b) de Fre´mont, P.; Stevens, E. D.; Fructos, M. R.; Dı´az-Requejo, M. M.; Pe´rez, P. J.; Nolan, S. P. Chem. Commun. 2006, 2045–2047. (c) de Fre´mont, P.; Stevens, E. D.; Eelman, M. D.; Fogg, D. E.; Nolan, S. P. Organometallics 2006, 25, 5824– 5828. (d) Ray, L.; Shaikh, M. M.; Ghosh, P. Organometallics 2007, 26, 958–964. (e) Jothibasu, R.; Huynh, H. V.; Koh, L. L. J. Organomet. Chem. 2008, 693, 374–380. (f) Frey, G. D.; Dewhurst, R. D.; Kousar, S.; Donnadieu, B.; Bertrand, G. J. Organomet. Chem. 2008, 693, 1674–1682. (17) (a) Schubert, U.; Ackermann, K.; Aumann, R. Cryst. Struct. Commun. 1982, 11, 591–594. (b) Raubenheimer, H. G.; Esterhuysen, M. W.; Timoshkin, A.; Chen, Y.; Frenking, G. Organometallics 2002, 21, 3173– 3181. (18) The reported C-Au lengths are almost identical with that of the C(sp2)-Au single bond in some arylgold complexes such as [AuC6H5(Ph3P)] (2.045(6) Å); see: Ferna´ndez, E. J.; Laguna, A.; Olmos, M. E. AdV. Organomet. Chem. 2005, 52, 77–141. (19) (a) Nemcsok, D.; Wichmann, K.; Frenking, G. Organometallics 2004, 23, 3640–3646. (b) Wang, H. M. J.; Vasam, C. S.; Tsai, T. Y. R.; Chen, S.-H.; Chang, A. H. H.; Lin, I. J. B. Organometallics 2005, 24, 486– 493. For a review, see: (c) Lin, I. J. B.; Vasam, C. S. Can. J. Chem. 2005, 83, 812–825.
Figure 1. ORTEP diagram for bis-carbene complex 3 shown with 50% displacement ellipsoids and with the atomic numbering scheme depicted. Hydrogen atoms and the SbF6- counterion are omitted for clarity, and only one crystallographically independent molecule is shown. Selected bond lengths (Å) and angles (deg): Au(1)-C(1), 2.049(10); Au(1)-C(28), 2.010(10); C(28)-C(30), 1.382(17); C(30)-C(31), 1.370(18); C(28)-Au(1)-C(1), 177.6(5); C(30)C(28)-Au(1), 125.8(9). Scheme 2.
Preparation of Cationic Gold(I) Carbene Complex 4
Scheme 3. Gold-Catalyzed Dimerization of Fischer Carbene Complex 1
the similar dimerization20 product 5 after 10 h at room temperature (Scheme 3). When the same reaction was carried out using the tungsten(0) Fischer carbene complex 1W, the cyclopentenone derivatives 6 and 6′ were obtained in good yield. The formation of these products could be explained through a Nazarov cyclization21 of the dimer in which the tungsten pentacarbonyl moiety would (20) For some similar dimerization processes, see refs 11-13. (21) For a similar Nazarov-type cyclization see ref 14b.
668 Organometallics, Vol. 28, No. 3, 2009 Scheme 4. Formation of Cyclopentenone Derivatives 6 and 6′
Scheme 5. Catalytic Processes with Methyl Acrylate
probably function as the catalyst. It is important to note that when (PPh3)AuCl complex is used a mixture of 6 and 6′ is obtained, while when (PEt3)AuCl was used cyclopentenone 6′ was exclusively obtained (Scheme 4). Taking into account these results, we decided to study the reaction in the presence of an unsaturated substrate such as methyl acrylate. When Fischer carbene complex 1 was treated with 1.1 equiv of methyl acrylate in the presence of a catalytic amount (15 mol %) of (PPh3)AuCl and AgSbF6 at room temperature, the reaction afforded cyclopropane 7 as a mixture of cis and trans isomers in good yield (Scheme 5). Complex (PEt3)AuCl also catalyzed the reaction with methyl acrylate. However, in this case the [2 + 1] reaction was not observed
Communications
and cyclopentanone 8 was exclusively obtained as a result of a [3 + 2] reaction with total diastereoselectivity (Scheme 5). It is important to note that thermal reactions of the phenylsubstituted complex 1 and acrylates provide only alkenylsubstituted cyclopropanes. Only the use of a pyrrolyl β-substituted alkenylcarbene complex22 or alkenyl oxazolines as the electron-deficient olefins23 give rise to the formation of the [3 + 2] cyloadducts under thermal conditions. Unfortunately, when other unsaturated substrates were employed we did not observe any evolution of the reaction and only observed the formation of the dimerization product in some cases. In addition, complexes 3 and 4 were not active in these types of processes. In conclusion, we have reported a facile route for the preparation of Fischer-type alkenyl gold(I) carbene complexes. As far as we know, the first bis-carbene complex containing a NHC ligand and a methoxy Fischer-type carbene ligand have been described. Phosphine gold(I) carbene complexes can also be prepared using this methodology. The nature of the phosphine plays a very important role in the stability and therefore in the reactivity of these kinds of carbene complexes.
Acknowledgment. Financial support from the DGICYT (Nos. CTQ2004-08077-C02-01/BQU and CTQ2007-61048/ BQU) is gratefully acknowledged. Supporting Information Available: A CIF file giving crystallographic data for compound 3 and text and figures giving experimental details of the synthesis and characterization data for all new compounds. This material is available free of charge via the Internet at http://pubs.acs.org. OM801146Z (22) Hoffmann, M.; Reissig, H.-U. Synlett 1995, 625–627. (23) Barluenga, J.; Toma´s, M.; Sua´rez-Sobrino, A. L. Synthesis 2000, 935–940.
