Organometallics 2010, 29, 4643–4646 DOI: 10.1021/om1005269
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Lithiation/Electrophilic Quench Sequence of “Benzylic” Position of (η5-Methylcyclohexadienyl)Mn(CO)3 Complexes Julien Dubarle Offner,† Franc-oise Rose-Munch,*,† Eric Rose,*,† Noemie Elgrishi,† and Helene Rousseliere§ †
Equipe de Chimie Organique et Organom etallique, Institut Parisien de Chimie Mol eculaire IPCM, UMR CNRS 7201, Universite Pierre and Marie Curie Paris 06, Case 181, 4 Place Jussieu, 75005 Paris, France, and §Centre de R esolution de Structures, Universite Pierre and Marie Curie Paris 06, Case 42, 4 Place Jussieu, 75005 Paris, France Received May 28, 2010 þ
BF4-
1 and Summary: [(η -Pentamethylbenzene)Mn(CO)3] [(η6-1,2,4,5-tetramethylbenzene)Mn(CO)3]þBF4- 2 complexes were prepared and reacted with nucleophiles to provide neutral exo-substituted (η5-polymethylcyclohexadienyl)Mn(CO)3 complexes 3-6. To study the regioselectivity of the deprotonation at a “benzylic” position of (η5-methylcyclohexadienyl)Mn(CO)3 complexes, compounds 3-6 were submitted to a lithiation/electrophilic quench sequence, and functionalized complexes 9-18 were obtained in good yields and with a total regioselectivity. A second sequence gave rise to the formation of unprecedented bifunctionalized (η5-1,2,4,5-tetramethylcyclohexadienyl)Mn(CO)3 and (η5pentamethylcyclohexadienyl)Mn(CO)3 complexes 19-22. 6
Introduction Coordination of the tricarbonylmanganese moiety to arenes considerably modifies their chemical properties and consequently their reactivity. Indeed, the presence of the electrondeficient Mn(CO)3þ fragment implies a decrease of the electronic density of the arene and thus dramatically enhances the electrophilic character of the ligand and the acidity of its protons.1 Furthermore, addition of nucleophiles R1 to cationic [(η6arene)Mn(CO)3]þ complexes leads to the formation of neutral exo-substituted (η5-6-R1-cyclohexadienyl)Mn(CO)3 complexes (Scheme 1, path a),2 whose chemistry has been intensively developed in the past decade to provide access to a wide range of organic and organometallic molecules. Nucleophilic addition to the (η5-cyclohexadienyl)Mn(CO)3 affords a cis-disubstituted cyclohexadiene with a complete control of stereoselectivity (Scheme 1, path b),3 and oxidation leads to the decoordination
of the manganese unit, giving access to the substituted free arene (Scheme 1, path c). If the η5 complex is substituted by a good leaving group such as R = OMe or Cl and if it is treated with a hydride and then an acid, cine or tele nucleophilic substitution occurs (Scheme 1, path d).4a-g Palladium-catalyzed coupling reactions can be achieved in (η5-halogenocyclohexadienyl)Mn(CO)3 complexes and allows the introduction on the η5 system of many different functional groups, as well as a second organometallic unit (Scheme 1, path e).4h-j Finally (η5-cyclohexadienyl)Mn(CO)3 complexes can also be easily functionalized by a lithiation/electrophilic quench sequence (Scheme 1, path f).4k,l The diversely substituted η5 complexes provide an easy access to the corresponding η6 complexes after rearomatization by exo-hydride abstraction at the sp3 carbon atom by using CPh3þBF4- (Scheme 1, path g). Having discovered the lithiation/electrophilic quench sequence methodology that allows substitution of the ring protons of (η5-cyclohexadienyl)Mn(CO)3 complexes, we wondered whether this reaction could be applied to functionalize “benzylic” positions, i.e., positions R to the η5-π system of such complexes. Indeed, the study of the benzylic position has already been reported for the cationic [(η6-arene)Mn(CO)3]þ series5 and
*To whom correspondence should be addressed. E-mail: franc-oise.
[email protected];
[email protected]. (1) (a) McDaniel, K. F. In Comprehensive Organometallic Chemistry II; Abel, E. W.; Stone, F. G. A.; Wilkinson, G.,Eds.; Pergamon Press: Oxford, UK, 1995; Vol. 6, pp 93-107. (b) Pike, R. D.; Sweigart, D. A. Coord. Chem. Rev. 1999, 187, 183. (c) Pape, A. R.; Kaliappan, K. P.; K€undig, E. P. Chem. Rev. 2000, 100, 2917. (d) Giner-Planas, J.; Prim, D.; Rose-Munch, F.; Rose, E.; Monchaud, D.; Lacour, J. Organometallics 2001, 20, 4107. (e) Rose-Munch, F.; Rose, E. Eur. J. Inorg. Chem. 2002, 1269. (f) Prim, D.; Andrioletti, B.; Rose-Munch, F.; Rose, E.; Couty, F. Tetrahedron 2004, 60, 3925. (g) Sweigart, D. A.; Reingold, J. A.; Son, S. U. In Comprehensive Organometallic Chemistry, 3rd ed.; Crabtree, R. H. Mingos, D. M. P., Eds.; Elsevier: Oxford, 2006; Vol. 5, Chapter 10, pp 761-814. (2) (a) Semmelhack, M. F. In Comprehensive Organometallic Chemistry II; Abel, E. W.; Stone, F. G. A.; Wilkinson, G., Eds.; Pergamon Press: Oxford, 1995; Vol. 12, p 979. (b) Pike, R. D.; Sweigart, D. A. Synlett 1990, 565. (3) See for example: (a) Pearson, A. J.; Bruhn, P. R. J. Org. Chem. 1991, 56, 7092. (b) Miles, W. H.; Brinkman, H. R. Tetrahedron Lett. 1992, 33, 589. (c) Pearson, A. J.; Shin, H. Tetrahedron 1992, 48, 7527.
(4) (a) Balssa, F.; Gagliardini, V.; Rose-Munch, F.; Rose, E. Organometallics 1996, 15, 4373. For Cr complexes, see: (b) Boutonnet, J. C.; Rose-Munch, F.; Rose, E. Tetrahedron Lett. 1985, 26, 3989. (c) Rose-Munch, F.; Rose, E.; Semra, A. J. Chem. Soc., Chem. Commun. 1986, 1551. (d) Rose-Munch, F.; Rose, E.; Semra, A. J. Chem. Soc., Chem. Commun. 1987, 942. (e) Djukic, J.-P.; Rose-Munch, F.; Rose, E. Organometallics 1995, 14, 2027. (f) Rose-Munch, F.; Rose, E.; Semra, A.; Garcia-Oricain, J.; Knobler, K. J. Organomet. Chem. 1989, 363, 297. (g) Rose-Munch, F.; Rose, E.; Semra, A.; Garcia-Oricain, J.; Bois, C. J. Organomet. Chem. 1989, 363, 103. (h) Auffrant, A.; Prim, D.; Rose-Munch, F.; Rose, E.; Schouteeten, S.; Vaissermann, J. Organometallics 2003, 22, 1898. (i) Jacques, B.; Tranchier, J.-P.; Rose-Munch, F.; Rose, E.; Stephenson, G. R.; Guyard-Duhayon, C. Organometallics 2004, 23, 184. (j) Schouteeten, S.; Tranchier, J.-P.; Rose-Munch, F.; Rose, E.; Auffrant, A.; Stephenson, G. R. Organometallics 2004, 23, 4308. (k) Jacques, B.; Chavarot, M.; Rose-Munch, F.; Rose, E. Angew. Chem., Int. Ed. 2006, 45, 3481. (l) Jacques, B.; Chanaewa, A.; Chavarot-Kerlidou, M.; Rose-Munch, F.; Rose, E.; Gerard, H. Organometallics 2008, 27, 626. (m) Boutonnet, J.-C.; Rose-Munch, F.; Rose, E.; Jeannin, Y.; Robert, F. J. Organomet. Chem. 1985, 297, 185. For an anti-eclipsed conformation of a bulky group, see: (n) Renard, C.; Valentic, R.; Rose-Munch, F.; Rose, E. Organometallics 1998, 17, 1587. For a staggered conformation, see: (o) Boutonnet, J. C.; Levisalles, J.; Rose, E.; Precigoux, G.; Courseille, C.; Platzer, N. J. Organomet. Chem. 1983, 255, 317. (5) (a) Johnson, J. W.; Treichel, P. M. J. Chem. Soc., Chem. Commun. 1976, 688. (b) Treichel, P. M.; Johnson, J. W. Inorg. Chem. 1977, 16, 749. (c) Treichel, P. M.; Fivizzani, K. P.; Haller, K. J. Organometallics 1982, 1, 931. (d) Bernhardt, R. J.; Eyman, D. P. Organometallics 1984, 3, 1445. (e) Bernhardt, R. J.; Wilmoth, M. A.; Weers, J. J.; LaBrush, D. M.; Eyman, D. P. Organometallics 1986, 5, 883. (f) LaBrush, D. M.; Eyman, D. P.; Baenziger, N. C.; Mallis, L. M. Organometallics 1991, 10, 1026. (g) Hull, J. W.; Roesselet, K. J.; Gladfelter, W. L. Organometallics 1992, 11, 3630.
r 2010 American Chemical Society
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Organometallics, Vol. 29, No. 20, 2010 Scheme 1. Reactivity Features of the (Cyclohexadienyl)tricarbonylmanganese Complexes
Scheme 2. Monodeprotonation of [(η6-alkylarene)Mn(CO)3]þ Complexes
has been well developed in the (η6-arene)Cr(CO)3 series.6 In the early 1990s, Eyman et al.5e and then Gladfelter et al.5f investigated the generality of deprotonation of organometallic complexes at a benzylic position using a large excess of strong basesuch as potassium hydride or potassium tert-butoxide-and worked on different [(η6-alkylarene)Mn(CO)3]þ complexes. They observed the formation of (η5-alkylbenzyl)Mn(CO)3 complexes presenting a highly activated exocyclic double bond (Scheme 2). However, to our knowledge, a deprotonation at a “benzylic” position has never been performed with (η5-cyclohexadienyl)Mn(CO)3 complexes; therefore we applied the lithiation/electrophilic quench sequence to (η5-polymethylcyclohexadienyl)Mn(CO)3 complexes in order to study the reactivity of these positions. Here we report on the synthesis of a series of η5-Mn complexes diversely substituted at a benzylic position as well as the structure of two of them.
Results and Discussion Synthesis of the Starting Materials. The (η5-cyclohexadienyl)Mn(CO)3 complexes used as starting materials in the present study were synthesized on one side from the (η6pentamethylbenzene)Mn(CO)3þBF4- complex 1 to settle the proper experimental conditions for a “benzylic” deprotonation and, on the other side, from the (η6-tetramethylbenzene)Mn(CO)3þBF4- complex 2 to determine if there is any competition between the deprotonation at the ring position and the “benzylic” position. The [(η6-pentamethylbenzene)Mn(CO)3]þBF4- 1 and [(η6-1,2,4,5-tetramethylbenzene)Mn(CO)3]þBF4- 2 complexes were readily obtained in 90% (6) (a) Davies, S. G.; Coote, S. J.; Goodfellow, C. L. In Advances in Metal Organic Chemistry; Liebeskind, L. S.,Ed.; JAI Press: Greenwich, 1991; Vol. 2, pp 1-57. (b) Davies, S. G.; McCarthy, T. D. In Comprehensive Organometallic Chemistry II; Abel, E. W.; Stone, F. G. A.; Wilkinson, G., Eds.; Pergamon Press: Oxford, UK, 1995; Vol. 12, pp 1039-1070. (c) Volk, T.; Bernicke, D.; Bats, J. W.; Schmalz, H.-G. Eur. J. Inorg. Chem. 1998, 1883–1905.
Dubarle Offner et al. Scheme 3. Nucleophilic Addition to (η6-pentamethylbenzene)Mn(CO)3þBF4- Complex 1 and (η6-tetramethylbenzene)Mn(CO)3þBF4- Complex 2
and 88% yields respectively, via the “silver(I) method” using BrMn(CO)5, AgBF4-, and the corresponding free arenes.7 Subsequently, each of these cationic complexes underwent two different nucleophilic additions, the first one with phenyl Grignard reagent at 0 °C and the other one with n-butyllithium at -78 °C.4l Two sets of η5 complexes were thus isolated: the (6-exo-substituted-η5-pentamethylcyclohexadienyl) Mn(CO)3 complexes 3 and 4 from 1 in 47% and 67% isolated yields, respectively, and the (6-exo-substituted-η5-1,2,4,5-tetramethylcyclohexadienyl)Mn(CO)3 complexes 5 and 6 from 2 in 80% and 72% isolated yields, respectively (Scheme 3). An X-ray analysis of the yellow crystals of the (6-phenyl-η5-1,2,4,5tetramethylcyclohexadienyl)Mn(CO)3 complex 5 highlights the usual conformation of a Mn(CO)3 tripod with a CO ligand eclipsing the sp3 carbon C6, which is located 41.05(9)° above the plane formed by the five sp2 carbons (Figure 1).8 Lithiation/Electrophilic Quench Sequence. We first performed a lithiation/electrophilic quench sequence on the (η5pentamethylcyclohexadienyl)Mn(CO)3 complexes 3 and 4 in order to know whether the reaction takes place or not at any benzylic position. We first followed the operating mode described in the case of deprotonation of the η5-cyclohexadienyl ring,4k,l where 1.4 to 2 equiv of nBuLi and 1.6 to 2.5 equiv of the electrophile were introduced at -78 °C, but we then slightly modified the procedure because it appeared that the deprotonation at the benzylic position was more difficult to perform than at the cyclohexadienylic position. We thus optimized the reaction by adding an excess of n-BuLi at -40 °C to a THF solution of a η5 complex and TMEDA; then the reaction was stirred for two hours at this temperature, and finally a larger excess of the electrophile was introduced. After an extra hour of stirring at -40 °C the mixture was warmed to room temperature and quenched by addition of water. We found out that the “benzylic” carbanion was stable up to -20 °C; nevertheless the best yields were observed at a reaction temperature of -40 °C. Different trials were run with various electrophiles: chlorotrimethylsilane, N,Ndimethylformamide, chlorodiphenylphosphine, and Eschenmoser’s salt. The reaction was feasible with this large range of reactants. Chromatography on silica gel afforded in each case a (7) Jackson, J. D.; Villa, S. J.; Bacon, D. S.; Pike, R. D.; Carpenter, G. B. Organometallics 1994, 13, 3972. (8) For the first X-ray analyses of (η5-cyclohexadienyl)Mn(CO)3 complexes, see: (a) Churchill, M. R.; Scholer, F. R. Inorg. Chem. 1969, 8, 1950. (b) Mawby, A.; Walker, P. J. C.; Mawby, R. J. J. Organomet. Chem. 1973, 55, C39. For the first theorical study of (η5-cyclohexadienyl)Mn(CO)3 complexes, see: (c) Hoffmann, R.; Hofmann, P. J. Am. Chem. Soc. 1976, 98, 598.
Note
Figure 1. Molecular structure of 5 with thermal ellipsoids at the 50% probability level. Scheme 4. Lithiation/Electrophilic Quench Sequence on Complexes 3 and 4
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Figure 2. Molecular structures of 9 and 13 with thermal ellipsoids at the 50% probability level. Scheme 5. Lithiation/Electrophilic Quench Sequence on Complexes 5 and 6
unique monofunctionalized complex. These new complexes 7-12 were isolated in 15% to 71% yields, and some starting material was always recovered in 10% to 42% isolated yields depending on the electrophile used (Scheme 4). Furthermore, no formation of acylmetalate, resulting from a possible nucleophilic attack of n-butyllithium on a carbonyl ligand, was detected.9 Interestingly the lithiation/electrophilic quench sequence is completely regioselective and occurred only at the equivalent carbons C9 and C10 (it affords a couple of enantiomers), whatever the electrophile. This regioselectivity was evidenced by a simple NMR study, performing a HMBC (heteronuclear multiple bond correlation) experiment between carbon C3 and protons H9 (or H10) and between C6 and protons H7 (or H8), which confirmed the position of the substituents on carbon C9 (or C10). Furthermore, crystallization of complex 9 from diethyl ether at 4 °C afforded yellow crystals suitable for X-ray analyses. In this structure, the Mn(CO)3 tripod eclipses carbons C4-C10, the sp3 carbon C6 (located 42.52°(15) above the plane described by the five sp2 carbons), and carbons C2-C9 bearing the diphenylphosphine group, which is on the opposite side from the Mn(CO)3 group (Figure 2, left). These preliminary observations do not reveal if the conformation of the Mn(CO)3-tripod favors the formation of the “benzylic” carbanions eclipsed by a Mn-CO bond and consequently the regioselective deprotonation observed in the present study or if the steric effect of the exo-phenyl group at the C6 carbon inhibits the approach of nBuLi at the C7 and C8 carbons. These results are relevant to those obtained a couple of years ago4l in the case of a (η5-6-phenylcyclohexadienyl)Mn(CO)3 complex, which could be efficiently silylated via a lithiation/ electrophilic quench sequence. Two regioisomers resulting from the lithiation at C2/C4 and at C3 were isolated in a 80:20 ratio, respectively. It is interesting to notice that only a monocarbanion is produced, even if four equivalents of n-butyllithium was introduced; double or triple deprotonation was never observed,
in contrast with the experiments reported in the literature with (η6-arene)Cr(CO)3 complexes at aromatic positions10a or at benzylic positions of benzyl allyl ethers.10b We then performed this sequence using (η5-1,2,4,5-tetramethylcyclohexadienyl)Mn(CO)3 complexes 5 and 6 and synthesized the monofunctionalized (η5-pentamethylcyclohexadienyl)Mn(CO)3 complexes 13-18 as unique products in 48% to 69% isolated yields (starting material was also recovered with yields ranging from 11% to 28%) (Scheme 5). To our surprise, no deprotonation occurred at the C3 cyclohexadienyl carbon, in good agreement with the 1H NMR spectrum, which shows the H3 proton signal around 5 ppm for each complex. The reaction was also totally regioselective and took place only at the same benzylic position as in the previous example, on carbons C9 or C10 (Scheme 5). Again this total regioselectivity might be explained by a great difference of reactivity between ring and benzylic protons. Yellow crystals of 13 suitable for X-ray analysis were grown from diethyl ether at 4 °C. The Mn(CO)3 tripod shows the typical conformation eclipsing carbons C2, C4, and C6, which is located 41.69(2)° above the plane described by the five sp2 carbons. The trimethylsilyl group is located on carbon C9, confirming the regioselectivity of this reaction, and the C9-Si bond points in the direction opposite the Mn(CO)3 entity (Figure 2, right). Such an eclipsed bulky group has been also observed in (arene)Cr(CO)3 complexes.4m Second Lithiation/Electrophilic Quench Sequence. Taking into account these results, we decided to perform a second lithiation/electrophilic quench sequence using a monofunctionalized complex from the “tetramethylbenzene series”, the most interesting one, because the yields of the first lithtiation/electrophilic quench sequence in this series were better than those observed in the “pentamethylbenzene series”. Futhermore, CD3OD was chosen as electrophile, knowing the excellent yields obtained with it in the first sequence. The reaction conditions were slightly modified, and a strict control of
(9) Sheridan, J. B.; Padda, R. S.; Chaffee, K.; Wang, C.; Huang, Y.; Lalancette, R. J. Chem. Soc., Dalton Trans. 1992, 1539. (10) (a) Gibson, S. E.; Saladin, S. A.; Sur, S. Chem. Commun. 2000, 2011. (b) Abecassis, K.; Gibson, S. E.; Martin-Fontecha, M. Eur. J. Org. Chem. 2009, 1606.
(11) (a) Duisenberg, A. J. M.; Kroon-Batenburg, L. M. J.; Schreurs, A. M. M. J. Appl. Crystallogr. 2003, 36, 220. (b) Blessing, R. H. Acta Crystallogr. 1995, A51, 33. (c) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J. Appl. Crystallogr. 1993, 26, 343–350. (d) Sheldrick, G. M. Acta Crystallogr. 2008, 64, 112–122.
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Scheme 6. Second Lithiation/Electrophilic Sequence
the monofunctionalized starting material. A total regioselectivity was still observed, and the reaction took place only on carbon C10 (and not on carbons C3, C9, and C7 or C8) (Scheme 6). In both cases, 1H NMR of the products 21 and 22 showed two sets of diastereotopic protons, H9a/H9b and H10a/H10b. Moreover, two methyl groups and two methylene groups were present in the 13C-DEPT NMR spectra. Again, these analyses were a proof that we synthesized the desired bifunctionalized compounds.
the temperature and a very slow warming to room temperature were necessary. Thus, starting from (6-phenyl-η5-2-methylenetrimethylsilyl-1,4,5-trimethylcyclohexadienyl)Mn(CO)3 complex 13 and (6-phenyl-η5-2-methylenediphenylphosphine-1,4,5trimethylcyclohexadienyl)Mn(CO)3 complex 15, complexes 19 and 20 were isolated in 65% and 43% yields, respectively (Scheme 6). In both cases, 1H NMR spectra indicated clearly the substitution of a proton atom H10 by a deuterium atom: it was exhibited by the change of integration from 3 to 2 of its singlet signal and by the two doublets for the diastereotopic H9a and H9b, which remained unchanged. Furthermore, by using 13 C-DEPT NMR spectroscopy, the CH3 signal of C10 turned into a CH2 signal. All these observations excluded a second functionalization on the same carbon C9 and were in good agreement with a substitution of the C10 carbon. The formation of the two complexes 19 and 20 validated the principle of our methodology. We thus extended the second sequence starting from complex 13 to other electrophiles, DMF and ClPPh2, and obtained bifunctionalized (η5cyclohexadienyl)Mn(CO)3 complexes 21 and 22 in low isolated yields. But in both cases, we recovered about 40% of
Conclusion We successfully developed a method of lithiation/electrophilic quench sequence for mono- and bifunctionalized (η5-polymethylcyclohexadienyl)Mn(CO)3 complexes at “benzylic” positions, stressing the strong electron-withdrawing effect of the Mn(CO)3 entity. These reactions are completely regioselective in the studied examples, as evidenced by NMR and X-ray analyses, and occur only at the methyl groups substituting the C2/C4 carbons of the cyclohexadienyl ring. This methodology opens new possibilities on the application of (arene)tricarbonylmanganese complexes in organic and organometallic syntheses.
Acknowledgment. J.D.O. thanks the CNRS for a financial grant. F.R. and E.R. thank the CNRS for financial support and Z. Ahmadi, an Erasmus student in collaboration with Prof. Dr. J. Heck (Hamburg). Supporting Information Available: Experimental part and crystal data for complexes 5, 9, and 13. This material is available free of charge via the Internet at http://pubs.acs.org.
