Inorg. Chem. 2009, 48, 1416-1422
Cobalt Induced Tandem C-H Activation/Decarbonylation of Various Aromatic Aldehydes and Related Benzylic Alcohols Forming Mononuclear Aryl Monocarbonyl Complexes Robert Beck,*,†,‡ Ulrich Flo¨rke,§ and Hans-Friedrich Klein† Eduard-Zintl-Institut fu¨r Anorganische and Physikalische Chemie, Technische UniVersita¨t Darmstadt, Petersenstrasse 18, D-64287 Darmstadt, Germany, and Anorganische and Analytische Chemie, UniVersita¨t Paderborn, Warburger Strasse 100, D-33098 Paderborn, Germany Received September 1, 2008
Reactions between Co(CH3)(PMe3)4 and various ortho-substituted aromatic aldehydes (Ar(R1,R2)CHO) and related benzylic alcohols (Ar(R1,R2)CH2OH) proceed with high selectivity to give the mono carbonyl complexes Aryl(R1,R2)(PMe3)3Co(CO): R1 ) H, R2 ) CH3 (1), R1) H, R2 ) ethyl (2), R1 ) CH3, R2 ) CH3 (3), R1 ) H, R2 ) NH2 (4), R1 ) F, R2 ) F (5), R1 ) H, R2 ) CF3 (6), and R1 ) H, R2 ) benzo (7). This tandem C-H bond activation/decarbonylation reaction provides easy and rapid access under mild conditions (-70 °C) to the first isolated trimethylphosphine stabilized aryl-monocarbonyl complexes of cobalt. X-ray diffraction studies were performed on ortho-amino substituted complex 4, ortho-trifluormethyl 6, and naphthyl derivative 7.
Introduction Stoichiometric and catalytic transformations involving C-H activation are of present interest due to the synthetic utility that the facile functionalization of unreactive C-H bonds would provide.1 The relatively low cost of cobalt complexes, as compared to their more expensive 4d and 5d congeners, provides an impetus for their utilization in catalytic or stoichiometric transformations that proceed through C-H bond activation.2 Recently, we studied the activation of aromatic, vinylic, and aliphatic C-H bonds and its high selectivity with CoCH3(PMe3)4 assisted by nitrogen3 and phosphorus4 donor functions. The activation of the C-H bond of aldehydes is * To whom correspondence should be addressed. E-mail: metallacycle@ gmail.com. † Technische Universita¨t Darmstadt. ‡ Present Address: School of Chemistry and Chemical Engineering, Shandong University, Shanda Nanlu 27, 250100 Jinan, People’s Republic of China. § Universita¨t Paderborn. (1) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879–2932. (2) Omae, I. Appl. Organomet. Chem. 2007, 21, 318–344. (3) (a) Klein, H.-F.; Camadanli, S.; Beck, R.; Leukel, D.; Flo¨rke, U. Angew. Chem., Int. Ed. 2005, 44, 975–977. (b) Klein, H.-F.; Camadanli, S.; Beck, R.; Flo¨rke, U. Chem. Commun. 2005, 381–382. (4) Klein, H.-F.; Beck, R.; Flo¨rke, U.; Haupt, H.-J. Eur. J. Inorg. Chem. 2003, 853–862. (5) Tolman, C. A.; Ittel, S. D.; English, A. D.; Jesson, J. P. J. Am. Chem. Soc. 1979, 101, 1742–1751.
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a reaction of great interest due to its connection with the catalytic decarbonylation of these substrates and its importance in organometallic and organic synthesis. The activation of aldehyde C-H bonds has been investigated by other research groups with iron(0),5 ruthenium(0),6,7 ruthenium(II),8 osmium(VI, IV, 0),9-11 rhodium(I),12-18 rhodium(III),19 iridium(I),18,20-25 iridium(III),19 (6) Ozawa, F.; Yamagami, I.; Yamamoto, A. J. Organomet. Chem. 1994, 473, 265–272. (7) Kondo, T.; Akazome, M.; Tsuji, Y.; Watanabe, Y. J. Org. Chem. 1990, 55, 1286–1291. (8) Lehmkuhl, H.; Schwickardi, R.; Mehler, G.; Kru¨ger, C.; Goddard, R. Z. Anorg. Allg. Chem. 1991, 606, 141–155. (9) Barrio, P.; Esteruelas, M. A.; Onate, E. Organometallics 2004, 23, 1340–1348. Esteruelas, M. A.; Herna´ndez, Y. A.; Lo´pez, A. M.; Oliva´n, M.; Rubio, L. Organometallics 2008, 23, 799–802. Arce, A. J.; De Sanctis, Y.; Deeming, A. J. J. Organomet. Chem. 1986, 311, 371– 378. (10) Hardcastle, K. I.; Minassiam, H.; Arce, A. J.; De Sanctis, Y.; Deeming, A. J. J. Organomet. Chem. 1989, 368, 119–130. (11) Brown, K. L.; Clark, G. R.; Headford, C. E. L.; Marsden, K.; Roper, W. R. J. Am. Chem. Soc. 1979, 101, 503–505. (12) Bianchini, C.; Meli, A.; Peruzzini, M.; Vacca, A.; Zanobini, F. Organometallics 1987, 6, 2453–2455. (13) Bianchini, C.; Meli, A.; Peruzzini, M.; Ramirez, J. A.; Vacca, A.; Vizza, F.; Zanobini, F. Organometallics 1989, 8, 337–345. (14) Marder, T. B.; Roe, D. C.; Milstein, D. Organometallics 1988, 7, 1451– 1453. (15) Milstein, D. Organometallics 1982, 1, 1549–1551. (16) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Bachechi, F. Organometallics 1991, 10, 820–823. (17) Milstein, D. Chem. Commun. 1982, 1357–1358. (18) Ko, J.; Joo, W.-C. Bull. Korean Chem. Soc. 1987, 8, 372–376.
10.1021/ic801673h CCC: $40.75
2009 American Chemical Society Published on Web 01/21/2009
Cobalt Induced Tandem C-H ActiWation/Decarbonylation Scheme 1. Five-Membered Nickel and Cobalt Acyl Metallacycles
activation/carbonyl migration deinsertion pathway. The same reaction products are observed with related benzylic alcohols in fair yields. Results and Discussion
and platinum(0).26 In some cases, the C-H activation step is followed by decarbonylation which results in a R-M-CO derivative,7,8,19-22,26 while in other examples, a stable acyl-metal-hydride is formed.9,10,18,26 The majority of research studies centering on aldehyde decarbonylation have utilized Rh(I) complexes, which have been found to be the most efficient catalysts.27-35 Activity has also been reported for cobalt(I) systems36 and for a cobalt-protoporphyrin complex,37 but no information has been given for intermediates. In a related study of aldehydic C-H bond activation with a pentamethylcyclopentadiene supported Co(I) center, Brookhart et al. described the intermolecular hydroacylation of vinylsilanes with a variety of alkyl aldehydes.38 We have previously reported on the activation of OC(R)-H bonds of salicylaldehyde derivatives (A),39,40 2-diphenylphosphinobenzaldehyde (B),41 with trimethylphosphine supported methyl nickel, and methyl cobalt complexes, where thermally stable acyl complexes were observed with fivemembered metallacycles (Scheme 1). In this report, we describe the synthesis of stable orthoaryl-substituted cobalt complexes with one terminal carbonyl group. With various aromatic aldehydes, the reaction of CoCH3(PMe3)4 follows, regioselectively, a tandem C-H (19) Gomez, M.; Kisenyi, J. M.; Sunley, G. J.; Maitlis, P. M. J. Organomet. Chem. 1985, 296, 197–207. (20) Gutierrez-Puebla, E.; Monge, A.; Paneque, M.; Poveda, M. L.; Salazar, V.; Carmona, E. J. Am. Chem. Soc. 1999, 121, 248–249. (21) Bernard, K. A.; Atwood, J. D. Organometallics 1988, 7, 235–236. (22) Bernard, K. A.; Atwood, J. D. Organometallics 1989, 8, 795–800. (23) Landvatter, E. F.; Rauchfuss, T. B. Organometallics 1982, 1, 506– 513. (24) Rauchfuss, T. B. J. Am. Chem. Soc. 1979, 101, 1045–1047. (25) Thorn, D. L. J. Mol. Catal. 1982, 17, 279–288. (26) Koh, J. J.; Lee, W.-H.; Willard, P. G.; Risen, W. M. J. Organomet. Chem. 1985, 284, 409–419. (27) Tsuji, J.; Ohno, K. Tetrahedron Lett. 1967, 23, 2173–2176. (28) Ohno, K.; Tsuji, J. J. Am. Chem. Soc. 1968, 90, 99–107. (29) Walborsky, H. M.; Allen, L. E. Tetrahedron Lett. 1970, 11, 823–824. (30) Walborsky, H. M.; Allen, L. E. J. Am. Chem. Soc. 1971, 93, 5465– 5468. (31) Suggs, J. W. J. Am. Chem. Soc. 1978, 100, 640–641. (32) Doughty, D. H.; Pignolet, L. H. J. Am. Chem. Soc. 1978, 100, 7083– 7085. (33) Baldwin, J. E.; Barden, T. C.; Pugh, R. L.; Widdison, W. C. J. Org. Chem. 1987, 52, 3303–3307. (34) Abu-Hasanayn, F.; Goldman, M. E.; Goldman, A. S. J. Am. Chem. Soc. 1992, 114, 2520–2524. (35) O’Connor, J. M.; Ma, J. J. Org. Chem. 1992, 57, 5075–5077. (36) Kovacs, I.; Szalontai, G.; Ungvary, F. J. Organomet. Chem. 1996, 524, 115–123. (37) Dennis, M.; Kolattukudy, P. E. Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 5306–5310. (38) Lenges, C. P.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 6965–6979. (39) Klein, H.-F.; Bickelhaupt, A.; Hammmerschmitt, B.; Flo¨rke, U.; Haupt, H.-J. Organometallics 1994, 13, 2944–2950. (40) Klein, H.-F.; Haller, S.; Sun, H.; Li, X.; Jung, T.; Ro¨hr, C.; Flo¨rke, U.; Haupt, H.-J. Z. Naturforsch. 1998, 53b, 587–598.
The reaction of aromatic aldehydes with CoCH3(PMe3)4 proceeds at -70 °C through C-H activation followed by a decarbonylation step according to eq 1. During warm up, the evolution of gas (CH4) is detected, and the color changes from dark red to orange-yellow.
This method provides high yields and appears to be widely applicable by tolerating different functional groups in ortho positions of the starting aldehydes, with the formation of aryl carbonyl complexes [Aryl(R1,R2)(PMe3)3Co(CO)]: R1 ) H, R2 ) CH3 (1), R1 ) H, R2 ) ethyl (2), R1 ) CH3, R2 ) CH3 (3), R1 ) H, R2 ) NH2 (4), R1 ) F, R2 ) F (5), R1 ) H, R2 ) CF3 (6), and R1 ) H, R2 ) benzo (7) (eq 1). All products crystallize from pentane or diethyl ether as orange rhombic crystals in 58-79% yield. When the reactions were all carried out in pentane, without crystallization of a product, just by filtering and washing with ice-cold pentane, a light yellow powder was obtained in almost quantitative yields, according NMR data. The complexes (1-7) show similar 1H, 13C(1H), and 31 1 P( H) NMR spectra and absorption regions in the infrared (IR) for the ν(C≡O) stretching frequencies. In the 1H NMR spectra of 1-7, the resonances for the trimethylphosphine ligands appear at 1.18-1.30 ppm as broad singlets, except for 1, which appears at 1.23 ppm as a doublet (2JP,H ) 6.1 Hz). The metallated carbon resonances (Co-C) in the 13 C(1H) NMR are observed as expected with a low field shift for complexes 1-7 from 166 to 180 ppm as broad multiplets (cobalt: I ) 7/2). The 31P(1H) NMR resonances of complexes 1-7 exhibit at δ ) 8.2-10.3 ppm broad singlets, which are assigned for a trigonal bipyramidal coordination sphere with dynamic ligand behavior (Berry type pseudorotation and ligand dissociation). In addition, when cooling down the NMR samples to -80 °C, no P,P-coupling of the different phosphorus nuclei was observed. In the IR, these complexes absorb at 1850-1886 cm-1 ν(CtO) in their IR spectra, which are typical stretching frequencies for terminal carbon monoxide ligands.42 To the best of our knowledge, 1-7 represent the first examples of simple aryl monocarbonyl cobalt complexes. Inorganic Chemistry, Vol. 48, No. 4, 2009
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Figure 1. Molecular structure of 4; selected bond lengths [Å] and angles [deg]: Co1-C1 2.016(1), Co1-C7 1.709(1), Co1-P1 2.1709(4), Co1-P2 2.1620(4), Co1-P3 2.2369(5), C7-O1 1.158(2), C1-C6 1.425(2), N1-H · · · Co 2.76(2), C6-N1 1.379(2), C7-Co1-C1 137.37(6), P1-Co1-P2 164.57(2), C1-Co1-P3 110.89(4), C7-Co1-P3 111.74(4), P1-Co1-P3 97.27(2), P2-Co1-P3 97.57(2), C6-C1-Co1 123.99(10), N1-C6-C1 120.66(12).
A single-crystal X-ray diffraction study was performed on the ortho-amino-substituted complex 4. Dark orange rhombic crystals were grown through the cooling of ether solutions of 4 to -27 °C. The molecular structure of 4 is shown in Figure 1. The cobalt atom resides in the center of a trigonal bipyramid of donor atoms, where the carbon atom of the phenyl group, a trimethylphosphine ligand, and the carbonyl group occupy equatorial positions. The two remaining PMe3 ligands reside in axial positions. The main axis connecting apical positions (P2-Co1-P1 ) 164.57(2)°) is slightly bent from idealized geometry (180°) and the angles from the equatorial (P3) to the axial phosphorus atoms are similar to 97.57(2)° for P2 and 97.27(2)° for P1, respectively. The sum of inner ligand angles in the equatorial plane (358°) is in accord with a cobalt atom centered in the trigonal plane. The Co-P bond lengths (2.1709(4) Å and 2.1620(4) Å) fall in a range also observed for other cobalt complexes containing aryl and carbonyl groups.43 The Co1-P3 distance (2.2369(5) Å) is elongated due to the trans influence of the C1 atom of the phenyl group. The complex contains two different Co-C bonds: a shorter one connected to the carbonyl ligand (Co1-C7 ) 1.709(1) Å) and a longer one connected to the phenyl group (Co1-C1 ) 2.016(1) Å), which is typical for (sp, sp2) hybridized carbon atoms.43 The bond distance between the protons of the ortho amino group and the cobalt atom is relatively short (N-H · · · Co ) 2.76(2) Å). However, no spectroscopic evidence was found (41) Klein, H.-F.; Lemke, U.; Lemke, M.; Brand, A. Organometallics 1998, 17, 4196–4201. (42) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987. (43) Klein, H.-F.; Beck, R.; Flo¨rke, U.; Haupt, H.-J. Eur. J. Inorg. Chem. 2003, 1380–1387.
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for agostic or anagostic interactions, which was reported by Brammer et al.,44 for a cobaltate anion containing a metal contact (N-H · · · Co ) 2.613 Å). In a reverse reaction pathway, a stable five-membered 2-aminoacyl palladium metallacycle was reported when carbon monoxide inserts into the palladium aryl bond.45 In the cobalt case, the balance of the hard NH2-donor and the remaining soft trimethylphosphine ligands is not suitable for stabilizing such a five-membered aminoacyl cobaltacycle. Only when a soft donor atom in the ortho position of 2-diphenylphosphinobenzaldehyde is present does the reaction with CoCH3(PMe3)4 afford stable acyl complexes as aforementioned, by the insertion of carbon monoxide observed with a phenyltris((tert-butylthio)methyl)borate cobalt complex,46 or by hydrolysis of a CF2-group in a [Cp] stabilized cobalt complex.47 The well-established and related nucleophilic reaction pathway is observed when the carbonyl-metalate complex reacts with the corresponding benzoylchloride derivatives forming stable acyl complexes,48 which can readily decarbonylate.49 This is also reported for the anion [Co(CO)4]-.50 Typically, these complexes have a central equatorial triscarbonyl motif [Co(CO)3] in common.51 To the best of our knowledge, an initial aldehyde-C-H activation reaction at cobalt centers has not been reported before. We were able to extend its scope with aryl aldehyde derivatives. The reaction pathway allows the synthesis of aryl-cobalt (monocarbonyl) complexes, which cannot be prepared easily by other routes. So far, only when π-acceptor substituents are present (e.g., allyl- or diphenylphosphino chelating substituents) were mononuclear complexes with an [Ar-Co-CO] monocarbonyl unit described.52,53 Under similar reaction conditions, complex 6 was synthesized (eq 1). Its spectroscopic data are very similar to that of complex 4 (see Experimental Method section). The molecular structure of 6 (Figure 2)shows a distorted trigonal bipyramidal coordination environment of cobalt(I), with two trimethylphosphine ligands in apical positions. The aryl ligand that lies in a plane containing the carbonyl group and (44) Brammer, L.; McCann, M. C.; Bullock, R. M.; McMullan, R. K.; Sherwood, P. Organometallics 1992, 12, 2339–2341. (45) Vicente, J.; Abad, J.-A.; Frankland, A. D.; Ramı´rez de Arellano, M. C. Chem. Eur. J. 1999, 5, 3066–3075. Vicente, J.; Abad, J.-A.; Frankland, A. D.; Ramı´rez de Arellano, M. C. Chem. Commun. 1997, 959–960. (46) DuPont, J. A.; Coxey, M. B.; Schebler, P. J.; Incarvito, C. D.; Dougherty, W. G.; Yap, G. P. A.; Rheingold, A. L.; Riordan, C. G. Organometallics 2007, 26, 971–979. (47) Hughes, R. P.; Lindner, D. C.; Rheingold, A. L.; Yap, G. P. A. Organometallics 1996, 15, 5678–5686. (48) Heck, R. F.; Breslow, D. S. J. Am. Chem. Soc. 1962, 84, 2499–2502. (49) Halle, L. F.; Crowe, W. E.; Armentrout, P. B.; Beauchamp, J. L. Organometallics 1984, 10, 1694–1706. (50) Gipson, S. L.; Bryson, L. A.; Klausmeyer, K. K. Inorg. Chim. Acta 2002, 340, 221–224. (51) (a) Galamb, V.; Pa´lyi, G.; Cser, F.; Furmanova, M. G.; Struchkov, Y. T. J. Organomet. Chem. 1981, 209, 183–195. (b) Galamb, V.; Pa´lyi, G.; Boese, R.; Schmid, G. Organometallics 1987, 6, 861–867. (c) Ungva´ry, F.; Kova´cs, I.; Hammerschmitt, B.; Cordier, G. Organometallics 1993, 12, 2849–2852. (d) Zucchi, C.; Boese, R.; Alberts, K.; Herbrich, T.; To´th, G.; Bencze, L.; Pa´lyi, G. Eur. J. Inorg. Chem. 2001, 2297–2304. (52) Albertin, G.; Antoniutti, S.; Bacchi, A.; Bordignon, E.; Pelizzi, G. Organometallics 1995, 14, 4126–4131. (53) Giesbrecht, G. R.; Gordon, J. C.; Clark, D. L.; Hijar, C. A.; Scott, B. L.; Watkin, J. G. Polyhedron 2003, 22, 153–163.
Cobalt Induced Tandem C-H ActiWation/Decarbonylation
Figure 2. Molecular structure of 6; selected bond lengths [Å] and angles [deg]: Co1-C1 2.048(6), Co1-C8 1.733(6), Co1-P1 2.2465(15), Co1-P2 2.1793(19), Co1-P3 2.1783(18), C8-O1 1.178(7), C1-C6 1.416(8), C6-C7 1.511(8), C7-F1 1.285(8), C7-F2 1.354(9), C7-F3 1.356(9), C8-Co1-C1 118.4(2), P3-Co1-P2 168.88(6), C1-Co1-P3 85.49(17), C8-Co1-P3 88.5(2), P1-Co1-P2 95.38(7), P1-Co1-P3 95.58(6), C6-C1-Co1 134.9(4). Scheme 2. Syn- and Anti-Conformation of the Ar(R)-Co-CO Unit in 4, 6, and 7 in Solid State
the third trimethylphosphine ligand occupy the equatorial positions. The Co-C and Co-P bond distances and angles closely resemble those for the related complex 4. The most interesting feature in the molecular structure is the anti-conformation of the carbonyl ligand and the trifluoromethyl group in the ortho position of 6 in a solid state (Scheme 2). Geometrical parameters as well as the unresolved disorder of the CF3 group suffer from rather bad data quality and should be understood as proof of connectivity only. Interestingly, in 6, the steric demand of the trimethylphosphine ligand is oriented toward the trifluoromethyl substituent, and the smaller carbonyl group is pointing away. An opposite arrangement would be expected with the bulky CF3substituent. Alternative Reaction Pathway. An alternative reaction pathway occurs with benzylic alcohols (2-methylbenzylalcohol, 2-aminobenzylacohol, and 1-naphthalenemethanol). When combining CoCH3(PMe3)4 with the related alcohol in diethyl ether, a slow color change from red to yellow is observed after between 24 and 48 h has passed (eq 2). With a reaction yield of 29-44%, the same products as aforementioned (eq 1) for selected benzaldehydes were achieved with the related benzylic alcohols, and additionally,
CoH(PMe3)4 is formed, identified by IR spectra ν(Co-H) ) 1935 cm-1. Ostensibly, in a prevailing side reaction, the hydrido-cobalt side product (Scheme 3) attacks the benzene system, observed in a raw product 1H NMR spectrum of 7, which exhibits proton resonances in typical olefinic area range between 5.3-5.8 ppm and other resonances of unidentified byproducts. Unfortunately, we have not been able to isolate those olefinic products. However, again, the complexes (1, 4, and 7) crystallize as orange-yellow rhombic crystals from pentane and diethyl ether solutions. The predicted structure of 7 (similar to that of 4 and 6) derived from NMR data in solution was confirmed by X-ray diffraction (Figure 3). The Co-P and P-C bond lengths are close to the average reported mean values of bond lengths.54 In complex 7, the orientation of the carbonyl ligand exhibits a syn orientation to the condensed aryl-ring in the ortho position. The distance between the cobalt atom and the neighboring hydrogen atom in the 8-position of the naphthyl ring (H1 · · · Co1 ) 2.888 Å) is relatively short, similar to that in complex 4, but, again, without spectroscopic evidence for agostic or anagostic interactions.55 All spectral data obtained from solutions are consistent with the results of the three X-ray diffraction experiments. Selected structural and NMR data for 4 and 7 are summarized in Table 1. Proposed Reaction Mechanism. The proposed reaction mechanism for the formation of complexes 1-7 is initiated by dissociation of a trimethylphosphine ligand from the starting complex [Co(CH3)(PMe3)4] (18 VE). The tetracoordinated [Co(CH3)(PMe3)3] intermediate (16 VE) proceeds in reaction with the aromatic aldehyde by an aldehydic C-H bond activation when the hexacoordinated acyl cobalt complex is formed (Scheme 1). A stable acyl cobalt complex has been isolated in a previous study, when a diphenylphosphino group was introduced into the ortho position of the aldehyde.41 Without a phosphorus donor atom in the ortho position, the hydrido-methyl-Co(III) intermediate is unstable (release of CH4) and a decarbonylation step occurs by insertion of the cobalt fragment into the aryl-acyl bond, generating a terminal carbonyl Co(I) complex. A similar tandem C-H activation/decarbonylation reaction of highly hindered tertiary aliphatic aldehydes with iridium complexes has been previously described by Alaimo et al.56 An alternative route starts from the related alcohol, which proceeds through elimination of methane, producing an (54) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D. G.; Taylor, R. J. Chem. Soc., Dalton. Trans. 1989, S1–S83. (55) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6908–6914. (56) Alaimo, P. J.; Arndtsen, B. A.; Bergman, R. G. Organometallics 2000, 19, 2130–2143.
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Beck et al. Scheme 3. Proposed Reaction Pathway of Co(CH3)(PMe3)4 with Aromatic Aldehydes and Benzylic Alcohol Derivatives
alcoholate cobalt complex (Scheme 3). β-Hydrogen elimination releases the in situ generated aldehyde and a stable hydrido-cobalt(I) complex (NMR). Afterward, still present CoCH3(PMe3)4 in solution reacts with the new formed aldehyde, which might also explain the rather low yield (
