Protonation of Cobalt−Allene Constitutional Isomers: Highly Selective

Nov 11, 2010 - Synopsis. The isomeric cobalt−allene complexes 2-Z and 3-E undergo protonation with HBF4 to give the cationic π-allyl complex 4 and the...
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Organometallics 2010, 29, 6161–6164 DOI: 10.1021/om100701e

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Protonation of Cobalt-Allene Constitutional Isomers: Highly Selective Formation of Cobalt-Allyl and Oxacobaltacyclopentadiene Complexes Joseph M. O’Connor,*,† Ming-Chou Chen,*,†,‡ and Ryan L. Holland† †

Department of Chemistry and Biochemistry (0358), University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0358, United States , and ‡Department of Chemistry, National Central University, Chung-Li, Taiwan 32054, Republic of China Received July 15, 2010

Summary: The first reactivity studies of mononuclear cobaltallene complexes are reported. The isomeric cobalt-allene complexes (RS,RS,Z)-[(η5-C5H5)Co(1,2-η2-CH(SO2Ph)d CdCH(CO2Et))] (2-Z) and (SR,RS,E)-[(η5-C5H5)Co(2, 3-η2-CH(SO2Ph)dCdCH(CO2Et))] (3-E) undergo protonation with HBF4 to give the cationic π-allyl complex (η5-C5H5)Co(PPh3)[exo-η3-CH(CO2Et)CHCH(SO2Ph)](BF4) (4) and the first structurally characterized oxacobaltacyclopentadiene complex, (η5-C5H5)Co(PPh3)[κ2-OC(OEt)CHdC(CH2SO2Ph)](BF4) (5-BF4), respectively. Deprotonation of 5-BF4 regenerates cobalt-allene complex 3-E, as well as 2-Z. Protonation of metal-η2-allene complexes (I) has been demonstrated to give η3-allyl (II),1 κ1-alkenyl (III),1a,2 or 1-metallacyclopropene (IV)3 complexes, depending on the nature of the metal, ancillary ligands, reagents, and allene substituents (Scheme 1).4 For example, Werner reported that (η5-C5H5)Rh(PiPr3)(η2-H2CdC=CHMe) underwent reaction with HBF4 (followed by NH4PF6) to give the η3-allyl rhodium complex [(η5-C5H5)Rh(PiPr3)(η3-H2CCHCHMe)]PF6, whereas, protonation with CF3CO2H in the presence of NaI led cleanly to the κ1-vinyl complex (η5-C5H5)Rh(PiPr3)I(κ1-CMedCHMe).1a Pombeiro observed that protonation of (dppe)2 ReCl(η2-H2CdCdCHPh) gave a 1-metallacyclopropene complex,3a and Casey found that treatment of (η 5 -C 5 Me 5 )Re(CO)2 (η2-H2CdCdCHMe) with HBF4 at low temperature generated *Corresponding authors. E-mail: [email protected]; mcchen@ ncu.edu. (1) (a) Wolf, J.; Werner, H. Organometallics 1987, 6, 1164. (b) Gibson, D. H.; Vonnahme, R. L.; McKiernan, J. E. J. Chem. Soc. (D) 1971, 720. (2) Bowden, F. L.; Giles, R. Coord. Chem. Rev. 1976, 20, 81. (3) (a) Pombeiro, A. J. L.; Hughes, D. L.; Richards, R. L.; Silvestre, J.; Hoffmann, R. J. Chem. Soc., Chem. Commun. 1986, 1125. (b) Casey, C. P.; Brady, J. T.; Boller, T. M.; Weinhold, F.; Hayashi, R. K. J. Am. Chem. Soc. 1998, 120, 12500. (c) Kuznetsov, M. L.; Pombeiro, A. J. L.; Dement'ev, A. I. J. Chem. Soc., Dalton Trans. 2000, 4413. (4) For the reactions of metal hydrides with allenes to give allyl or vinyl complexes see: Bai, T.; Ma, S.; Jia, G. Coord. Chem. Rev. 2009, 253, 423. (5) Cobalt-allene complexes appear to serve as key intermediates in a number of cobalt-mediated reactions of allenes: (a) Greenfield, H.; Wender, I.; Wotiz, J. H. J. Am. Chem. Soc. 1956, 21, 875. (b) Nakamura, A.; Kim, P. J.; Hagihara, N. J. Organomet. Chem. 1965, 3, 7. (c) Nakamura, A. Bull. Chem. Soc. Jpn. 1966, 39, 543. (d) Furukawa, J.; Kiji, J; Ueo, K. Makromol. Chem. 1973, 170, 247, and references therein. (e) van Ommen, J. G; Stijntjes, J.; Mars, P. J. Mol. Catal. 1979, 5, 1. (f ) Caffyn, A. J. M.; Mays, M. J.; Solan, G. A.; Conole, G.; Tiripicchio, A. J. Chem. Soc., Dalton Trans. 1993, 2345. (g) Bates, R. W.; Devi, T. R. Tetrahedron Lett. 1995, 36, 509. (h) Hayes, B. L.; Adams, T. A.; Pickin, K. A.; Day, C. S.; Welker, M. E. Organometallics 2000, 19, 2730, and references therein. (i) Wu, M. S.; Shanmugasundaram, M.; Cheng, C. H. Chem. Commun. 2003, 718. ( j) Petit, M.; Aubert, C.; Malacria, M. Tetrahedron 2006, 62, 10582. (k) Chang, H. T.; Jayanth, T. T.; Cheng, C. H. J. Am. Chem. Soc. 2007, 129, 4166. (l) Park, J. H.; Kim, E.; Kim, H. M.; Choi, S. Y.; Chung, Y. K. Chem. Commun. 2008, 2388. r 2010 American Chemical Society

Scheme 1. Protonation of Metal-η2-Allene Complexes

Scheme 2. Conversion of Cobaltacyclobutene 1 to Isomeric CobaltAllene Complexes

a 1-rhenacyclopropene, which underwent rearrangement to a rhenium-allyl at -16 °C.3b To date there have been only two reports of isolated mononuclear cobalt-η2-allene complexes.5 In 1965 Nakamura and co-workers reported [(η5-C5H5)Co(CO)(η2-Ph2CdCdCPh2)], which was isolated in 2% yield.5b More recently, we reported the conversion of cobaltacyclobutene complex 16 to three isomeric allene complexes: (RS,RS,Z)-[(η5-C5H5)Co(1,2-η2CH(SO2Ph)dCdCH(CO2Et))] (2-Z), (RS,RS,Z)-[(η5-C5H5)Co(1,2-η2-CH(SO2Ph)dCdC(CO2Et)H)] (2-E), and (SR,RS, E)-[(η5-C5H5)Co(2,3-η2-CH(SO2Ph) dCdCH(CO2Et))] (3-E) (Scheme 2).7 This paper describes the first reaction chemistry of characterized cobalt-η2-allene complexes. Allene 2-Z undergoes protonation to form an π-allyl complex, whereas protonation of 3-E generates an oxacobaltacyclopentadiene complex. Deprotonation of the oxacobaltacyclopentadiene regenerates 3-E, (6) O’Connor, J. M.; Ji, H.; Iranpour, M.; Rheingold, A. L. J. Am. Chem. Soc. 1993, 115, 1586. (7) O’Connor, J. M.; Chen, M. C.; Fong, B. S.; Wenzel, A.; Gantzel, P.; Rheingold, A. L.; Guzei, I. A. J. Am. Chem. Soc. 1998, 120, 1100. Published on Web 11/11/2010

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Scheme 3. Protonation of Cobalt-Allene Constitutional Isomers

thereby demonstrating that oxametallacyclopentadienes serve as precursors to η2-allene complexes. Access to 2-Z and 3-E afforded a unique opportunity to examine protonation selectivity for a pair of isomeric allene complexes. When HBF4 (1.2 equiv) was added to a -78 °C diethyl ether solution of 2-Z, followed by warming of the solution to room temperature, the allyl complex 4 precipitated as an orange solid. In the 1H NMR spectrum (CDCl3) of 4, diastereotopic hydrogen resonances were observed at δ 4.45 (1H, m, OCH2CH3) and 4.57 (1H, m, OCH2CH3), and resonances attributed to the three allyl ligand hydrogens (CoCH) were observed at δ 2.00 (t, 1H, JPH = JHH = 10 Hz), 2.29 (t, 1H, JPH = JHH= 10 Hz), and 6.65 (t, 1H, JHH=10 Hz). For comparison, in the 1H NMR spectrum (acetone-d6) of [(η5-C5H5)Co(PPh3)(η3-CH2CMeCH2)](BF4) (6), the Hanti resonance of the allyl ligand is observed at δ 1.4 (d, JPH =13 Hz) and the Hsyn resonance at 4.43 (s).8 The relatively downfield Hanti chemical shift for 4 is attributed to the presence of the electron-withdrawing sulfone and ester substituents. The assignment of the allyl hydrogen resonances in the 1H NMR spectrum of 4 was further supported by a series of NMR experiments. Saturation of the δ 6.65 resonance resulted in the collapse of the δ 2.00 and 2.29 triplets to doublets, and saturation of either the 2.00 or the 2.29 resonance led to collapse of the 6.65 triplet to a doublet. The exo allyl orientation for 4 is supported by the observation of a NOE at the δ 6.65 resonance;but not at the 2.00 or 2.29 resonances; upon irradiation of the C5H5 hydrogen resonance. The allyl ligand chemical shift assignments were confirmed by a 2DHSQC-NMR experiment (CDCl3).9 Protonation of 3-E takes a much different course than in the case of 2-Z. Addition of HBF4 (1.2 equiv) to a diethyl ether solution of 3-E led to formation of oxacobaltacyclopentadiene complex 5-BF4, which was isolated as a brown powder (Scheme 3).10 In the 1H NMR spectrum (acetone-d6) of 5-BF4, two sets of diastereotopic hydrogen resonances were observed: δ 3.70 (1H, m, OCH2CH3) and 4.10 (1H, m, (8) Aviles, T.; Barroso, F.; Royo, P.; Noordik, J. H. J. Organomet. Chem. 1982, 236, 101. (9) 1H-13C correlations (1H, δ; 13C, δ): (1.42; 14.44), (2.00; 51.50), (2.29; 76.91), (4.45; 63.52) (4.57; 63.52), (5.70; 92.96), (6.65; 86.47), (7.52; 128.88), (7.63; 130.07), (7.67; 132.82), (7.69; 134.59). See Supporting Information for the spectrum. (10) We have not found a literature example of an oxacobaltacyclopentadiene complex for comparison to 5. For oxairidacyclopentadiene complexes, see: (a) Bleeke, J. R.; New, P. R.; Blanchard, J. M. B.; Haile, T.; Beatty, A. M. Organometallics 1995, 14, 5127. (b) O'Connor, J. M.; Hiibner, K.; Merwin, R.; Pu, L. J. Am. Chem. Soc. 1995, 117, 8861.

OCH2CH3); δ 4.87 (1H, d, JHH = 16.8 Hz, CH2SO2Ph) and 5.45 (1H, d, JHH = 17.1 Hz, CH2SO2Ph). Thus, protonation of 3-E delivered a hydrogen to the sulfone-bearing carbon of the allene ligand, rather than to the central allene carbon, as was the case for 2-Z. A 1-cobaltacyclopropene structure (IV, Scheme 1) for 5-BF4 is excluded on the basis of the 13C{1H} NMR (CD2Cl2) spectrum, which exhibited no resonances downfield of the ester carbonyl carbon signal at 188.4 ppm (s). For comparison, the 13C NMR resonances for the carbene carbons in the Pombeiro and Casey 1-rhenacyclopropenes were observed at δ 258.2 and 302.6, respectively.3 Two types of spectroscopic evidence suggest that the ester oxygen is coordinated to cobalt in 5-BF4. In the 13C NMR spectrum, the 188.4 ppm carbonyl carbon resonance is significantly downfield of the 176 ppm ester resonance observed in 3E, and no carbonyl ν(CdO) absorption is observed at wavenumbers higher than 1647 cm-1 in the IR spectrum (thin film) of 5-BF4. In addition to the above data, the R-sp2-carbon resonance of the metallacycle ring in 5-BF4 is observed at δ 174.6 (d, JPC = 25 Hz), which is significantly downfield of the corresponding R-sp2-carbon resonance (159.5, d, JPC = 4.4 Hz) for the cobaltacyclopentene (η5-C5H5)Co(PPh3)(κ2-CRd CRCHRCHR) (7-CO2Me, R = CO2Me, Figure 1).11 We attribute the downfield nature of the R-carbon resonance to its electron-deficient identity as the β-carbon of an R,β-unsaturated ester and possibly to a contribution from the cobaltafuran resonance structure 5-BF4-R (Scheme 3). The conversion of 3-E to 5-BF4 was demonstrated to be reversible by the observation that addition of n-BuLi (2 equiv) to a -78 °C THF solution of 5-BF4 (0.026 mmol, 8.7 mM) led to the formation of 3-E and 2-Z in a 9:1 ratio (78% combined yield). The structure of 5-BF4 was confirmed by a single-crystal X-ray diffraction study on (η5-C5H5)Co(PPh3)[κ2-OC(OEt)CHd C(CH2SO2Ph)](CF3CO2) (5-CF3CO2), which was prepared by protonation of 3-E with trifluoroacetic acid (Figure 2). The oxametallacycle ring defined by Co-C2-C3-C4-O1 is essentially planar, with the largest displacements of the ring atoms from the mean plane at C3 (þ 0.017(2) A˚) and C4 (-0.017(2) A˚). The conformation about the C1-C2 bond places the phenyl sulfone group away from the bulky PPh3 ligand with S1 positioned 1.17 A˚ above the plane of the metallacycle ring. (11) Wakatsuki, Y.; Aoki, K.; Yamazaki, H. J. Am. Chem. Soc. 1979, 101, 1123.

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Figure 1. Cobaltacyclopentene complexes. Scheme 4. Mechanistic Hypotheses Involving Protonation at C-3 of the Allene Ligand

Figure 2. ORTEP drawing of (η5-C5H5)Co(PPh3)[κ2-OC(OEt)CHdC(CH2SO2Ph)](CF3CO2) (5-CF3CO2) at the 50% probability level (hydrogen atoms and CF3CO2- counterion omitted for clarity). Selected bond distances (A˚) and angles (deg): CoC2 1.914(2), Co-O1 1.950(2), Co-P1 2.279(1), C1-C2 1.508(3), C2-C3 1.338(4), C3-C4 1.434(4), C4-O1 1.239(3), C2-Co-O1 84.4 (1), Co-C2-C1 126.7(2), Co-C2-C3 111.0(2), Co-O1C4 110.9(1).

The C2-C3 bond distance of 1.338(4) A˚ and the C3-C4 bond distance of 1.434(4) A˚ indicate localized carbon-carbon double and single bonds, respectively. Unfortunately, the large standard deviations for the bond distances in cobaltacyclopentene complex 7-Ph10 and cobaltaoxanorbornadiene complex 812 (Figure 1) do not permit many useful comparisons of bond distance data to those for 5-CF3CO2. However, the Co-P bond distance in 5-CF3CO2 (2.279(1) A˚) is significantly longer than in cobaltacyclopentene 7-Ph (2.232(3) A˚), and the C2-Co-O1 angle in 5-CF 3CO 2 (84.4(1)°) is larger than the corresponding (12) Stolzenberg, A. M.; Scozzafava, M.; Foxman, B. M. Organometallics 1987, 6, 769.

C-Co-C angle in 7-Ph (82.0(4)°). The 1.914(2) A˚ Co-C2 bond distance in 5-CF3CO2 is significantly shorter than the corresponding Co-C(sp2) distance in 8 (1.954(6) A˚), which is consistent with a contribution from resonance structure 5-BF4-R. By analogy to Casey’s mechanism for the conversion of rhenium-allenes to allyl complexes,3b 1-cobaltacyclopropene intermediates may be involved in the formation of both 4 and 5. Protonation at C-3 of the allene ligand in 2-Z and 3-E would give cationic 1-cobaltacyclopropene intermediates V and VI, respectively (Scheme 4). The observed reaction outcomes then require a formal 1,2-hydride shift in V and a carbon to oxygen tautomerization in VI. In the case of intermediate VI, either the tautomerization process must be faster than a 1,2-hydride shift or the 1,2-shift must be reversible. A similar tautomerization in V would be less favorable in that it would require binding of the sulfone oxygen to cobalt. A fundamentally distinct mechanism involves initial protonation at the metal center (Scheme 5). It is well established that metal-hydrides undergo reaction with allenes to give either allyl or vinyl complexes. 1,4 The allyl products result from reductive elimination of the hydride and C(2) of the allene ligand, whereas the formation of vinyl complexes results from reductive elimination of hydride and C-1 of the allene. In the case of 2-Z, protonation at the metal would generate hydride intermediate VII, from which a reductive elimination at C-2 of the allene ligand leads directly to η3-allyl 4 with the correct stereochemistry. However, in the case of 3-E, protonation at cobalt generates hydride intermediate VIII, from which there is no simple direct path to 5, since a reductive elimination at C-1 in VIII would give a vinyl complex, and elimination at C-2 would give a propargyl intermediate. In either case, a subsequent hydrogen shift is required to produce 5. The protonation reactions of cobalt-allene complexes 2-Z and 3-E proceed in high yield (93%) yet display very distinct outcomes. Allyl complex 4 is generated from 2-Z, whereas

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Scheme 5. Mechanistic Hypotheses Involving Protonation at Cobalt

the first structurally characterized cobaltafuran, 5, is formed from 3-E. The available experimental data do not allow us to rule out pathways that involve protonation at the metal or protonation at C-3 of the allene ligand. Future experiments will employ isotopic labeling studies and an examination of the less readily available allene stereoisomer 2-E in the hopes of clarifying the mechanistic details. Deprotonation of the cobaltafuran complex regenerates cobalt-allene complexes, thus providing a new synthetic route toward allene complexes.

Acknowledgment. The support of the National Science Foundation is gratefully acknowledged (Grants CHE0518707 and CHE0911765 and Instrumentation Grant CHE0741968). The authors thank Dr. Peter Gantzel for assistance with the X-ray crystal structure determination. Supporting Information Available: CIF files giving details of the crystal structure determination of 5-CF3CO2 and characterization data for 4 and 5-BF4. This material is available free of charge via the Internet at http://pubs.acs.org.