Organometallics 2011, 30, 369–371 DOI: 10.1021/om1007314
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Addition of Dissimilar Carbenes across an Unsymmetrically Substituted Alkyne: Regio- and Stereoselective Synthesis of Trisubstituted 1,3-Dienes Joseph M. O’Connor,*,† Ming-Chou Chen,*,†,‡ Ryan L. Holland,† and Arnold L. Rheingold† †
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 28, 2010
Summary: The formal addition of dissimilar carbenes across an unsymmetrically substituted alkyne has been achieved for the first time by a two-step sequence in which a cobalt-alkyne complex undergoes reaction with a carbene addend (CHR1) to give a metallacyclobutene complex, followed by reaction with a second carbene addend (CHR2) to give a mixture of diene complexes. In situ treatment of the diene mixture with fluoride leads to desilylation and conversion to a single cobalt-diene complex with a high degree of regio- and stereoselectivity. In 1978 Hong and co-workers reported the first addition of two carbenes across an alkyne to give tetrasubstituted 1,3-diene products (Scheme 1).1 The conversion of cobaltalkyne complex 1 to cobalt-dienes 2 was accompanied by the formation of either one or two dicobalt complexes (e.g., 3). The product ratio depended on the concentration of diazocarbonyl reagent, with the highest yields of 2-EE (12%) and 2-EZ (36%) observed at a 10:1 ratio of ethyl diazoacetate to cobalt-alkyne. The authors proposed the formation of an unobserved metallacyclobutene intermediate, 4, which was trapped by ethyl diazoacetate to give 2, in competition with a dimerization/hydrogen shift process to give 3. Intrigued by this novel chemistry and the proposed metallacyclobutene intermediate, we found that cobalt-alkyne complex 5 underwent reaction with ethyl diazoacetate to give the isolable cobaltacyclobutene complex 6, as well as a mixture of cobalt-diene products (7, Scheme 2).2,3 Employing an excess of ethyl diazoacetate gave only cobalt-diene
Scheme 1. Hong’s Formal Addition of Two Carbenes Across an Alkyne1
products, whereas added PPh3 led to the exclusive formation of cobaltacyclobutene 6.2a Fortunately, the three cobaltdiene complexes were readily converted to a single isomer (8) upon desilylation with fluoride ion.2b,4 Oxidation of 8 with two equivalents of ceric ammonium nitrate gave the liberated diene in 86% yield.2b,5a The observation that a stable cobaltacyclobutene could be generated from 5 and ethyl diazoacetate has now led to the first demonstration of a [1þ2þ1] addition of dissimilar carbenes across an unsymmetrically substituted alkyne. The tetrasubstituted diene products are readily desilylated and isomerized in situ to a single trisubstituted diene with excellent regio- and stereochemical control.6,7 In a preliminary study, we examined the reaction of cobaltacyclobutene 6 with butenyl diazocarbonyl, N2CH(CdO)OCH2CH2CHdCH2, in benzene-d6 at reflux (Scheme 3).
*To whom correspondence should be addressed. E-mail: jmoconnor@ ucsd.edu,
[email protected]. (1) Hong, P.; Aoki, K.; Yamazaki, H. J. Organomet. Chem. 1978, 150, 279. (2) (a) O’Connor, J. M.; Ji, H.; Iranpour, M.; Rheingold, A. L. J. Am. Chem. Soc. 1993, 115, 1586. (b) O'Connor, J. M.; Chen, M.-C.; Rheingold, A. L. Tetrahedron Lett. 1997, 38, 5241. (c) O'Connor, J. M.; Chen, M.-C.; Frohn, M.; Rheingold, A. L.; Guzei, I. A. Organometallics 1997, 16, 5589. (3) For leading references to synthetic routes toward cobalt-diene complexes and cobalt-based synthetic applications of diene complexes see: (a) Mannathan, S.; Cheng, C.-H. Chem. Commun. 2010, 46. (b) Sharma, R. K.; RajanBabu, T. V. J. Am. Chem. Soc. 2010, 132, 3295. (c) Holland, R. L.; Bunker, K. D.; Chen, C. H.; Di Pasquale, A. G.; Rheingold, A. L.; Baldridge, K. K.; O'Connor, J. M. J. Am. Chem. Soc. 2008, 130, 10093. (d) Pidaparthi, R. R.; Welker, M. E.; Day, C. S. Organometallics 2006, 25, 974. (e) Malacria, M.; Aubert, C.; Renaud, J.-L. Sci. Synth. 2002, 1, 439. (f) Grotjahn, D. B.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1990, 112, 5653. (g) O'Connor, J. M.; Fong, B. S.; Ji, H.-L., Hiibner, K. J. Am. Chem. Soc. 1995, 117, 8029. (4) The mechanism for fluoride-catalyzed isomerization of cobaltdiene complexes has yet to be elucidated. Our current speculation involves reversible attack of fluoride at cobalt to give anionic cobaltallyls of the type CpCo(F){(η 3 -(CO 2 R)CHdC(SO 2 Ph)CH-CHd C(OR)O-}.
(5) (a) For oxidative demetalation of dienes from CpCo(diene) complexes: O’Connor, J. M.; Johnson, J. A. Synlett 1989, 57, and ref 2c. (b) In a preliminary study, oxidation of 13-Me with I2 led to the formation of the free diene, CH(CO2Et)dCHC(SO2Ph)dCH(COMe), as a single isomer in 92% yield: 1H NMR (CDCl3) δ 1.28 (t, 3H, CO2CH2CH3, J = 6.9 Hz), 2.41 (s, 3H, COCH3), 4.18 (q, 2H, CO2CH2CH3, J = 6.9 Hz), 6.66 (d, 1H, CH(CO2Et), J = 16.1 Hz), 7.42 (s, 1H, CH(COMe)), 7.56 (t, 2H, m-SO2Ph, J = 8.0 Hz), 7.65 (d, 1H, p-SO2Ph, J = 7.5 Hz), 7.66 (d, 1H, CHC(SO2Ph), J = 16.1 Hz), 7.85 (d, 2H, o-SO2Ph, J = 8.0 Hz); HRMS (EI): calcd [M þ Na] 331.0611; found 331.0612. (6) For isomerization of cobalt-diene complexes: (a) Baldridge, K. K.; O’Connor, J. M.; Chen, M.-C.; Siegel, J. S. J. Phys. Chem. 1999, 103, 10126. (b) King, J. A., Jr.; Vollhardt, K. P. C. J. Organomet. Chem. 1993, 460, 91. (c) Eaton, B.; King, J. A., Jr.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1986, 108, 1359. (e) Ref 2b. (7) Dixneuf and co-workers have reported the development of a related ruthenium-catalyzed addition of two similar carbenes (N2CHTMS) across a terminal alkyne. (a) Le Paih, J.; Derien, S.; € Ozdemir, I.; Dixneuf, P. H. J. Am. Chem. Soc. 2000, 122, 7400. (b) Le € Paih, J.; Bray, C. V-L.; Derien, S.; Ozdemir, I.; Dixneuf, P. H. J. Am. Chem. Soc. 2010, 132, 7391.
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O’Connor et al.
Scheme 2. Isolation of a Cobaltacyclobutene and Control of Diene Stereochemistry
Scheme 3. Regio- and Stereoselective Addition of Two Different Carbenes across an Alkyne
Analysis of the crude reaction mixture by 1H NMR spectroscopy indicated the expected formation of multiple [(η5C5H5)Co]-containing products. We therefore undertook a preparative-scale reaction of 6 and butenyl diazocarbonyl. After 12 h at 70 °C, the volatiles were removed in vacuo and the remaining oily red residue was immediately, without purification, dissolved in acetone/methanol and treated with CsF. A chromatographic workup and recrystallization (see Supporting Information) led to the isolation of (η5C5H5)Co[η4-(E,E)-CH(CO2R)dC(SO2Ph)CHdCH(CO2Et)] (9, R = C4H7) as a red-black crystalline solid in 73% yield (Scheme 3). The regio- and stereochemical assignment for 9 (8) Emmerson, G. F.; Mahler, J. E.; Kochhar, R.; Pettit, R. J. Org. Chem. 1964, 29, 3620.
Figure 1. ORTEP drawing of (η5-C5H5)Co(PPh3)[κ2-C(SO2Ph)dC(TMS)CH(CO2C4H7)] (10). Hydrogen atoms are omitted for clarity. Selected bond distances (A˚) and angles (deg): Co-C(1) 1.940(8), Co-C(3) 2.061(7), C(1)-C(2) 1.336(10), C(2)-C(3) 1.542(12), C(1)-S 1.760(8), C(2)-Si 1.896(8), Co-P 2.187(3); C(1)-Co-C(3) 67.0(3), Co-C(1)-C(2) 102.5(5), Co-C(3)-C(2) 90.4(4), C(1)-C(2)-C(3) 100.1(7).
was based on the chemical shifts and coupling constants of the three diene hydrogens in the 1H NMR spectrum.8 In order to generate the opposite regioisomer, it was necessary to prepare a new cobaltacyclobutene complex from cobalt-alkyne 5 and butenyl diazocarbonyl. Using a procedure analogous to that for the preparation of 6, cobaltacyclobutene 10 was prepared and isolated as an air-stable, dark red crystalline solid in 67% yield (Scheme 3). The characteristic 1H and 13C NMR spectroscopic signatures for 10 are nearly identical to those for 6, and the crystalline nature of 10 allowed for an X-ray crystallographic analysis (Figure 1). The structural data confirm that the metallacycle ring of 10 is planar and that the diazocarbonyl has coupled selectively to the alkyne carbon bearing the TMS substituent. Of particular note, there was no evidence for insertion of the pendent alkene into the metallacyclobutene ring of 10.9 Heating a benzene solution of 10 and ethyl diazoacetate at 70 °C followed by treatment with CsF led to the isolation of cobalt-diene 11 as a red-black crystalline solid in 68% yield. For 11, the three hydrogen resonances of the diene are observed in the 1H NMR (CDCl3) spectrum at nearly identical chemical shift values to those observed for 9. To address the effect of steric bulk in the diazo ester reagent, the reaction of 6 with 2,6-di-tert-butyl-4-methylphenyl diazoacetate was also examined (Scheme 3). Following the procedure utilized for the preparation of 9 and 11, the cobalt-diene complex 12 was isolated as a red-black crystalline solid in 67% yield. The solid-state structure of 12 was determined by X-ray crystallography (Figure 2). While the quality of the structural data is not high, it does permit an unambiguous assignment of connectivity and stereochemistry. This structure, taken together with the close similarity of the NMR data for 9, 11, and 12, confirms the structural (9) Cobaltacyclobutene 6 undergoes intermolecular reactions with alkenes and nitroso compounds; see: (a) Holland, R. L.; Bunker, K. D.; Chen, C. H.; DiPasquale, A. G.; Rheingold, A. L.; Baldridge, K. K.; O’Connor, J. M. J. Am. Chem. Soc. 2008, 130, 10093. (b) Holland, R. L.; O'Connor, J. M. Organometallics 2009, 394.
Communication
Figure 2. ORTEP drawing of (η5-C5H5)Co(PPh3)[η4-CH(CO2Ar)d C(SO2Ph)C(H)dC(CO2Et)] (12, Ar=2,6-di-tert-butyl-4-methylphenyl). Only the syn- and anti-hydrogens of the diene are shown for clarity. Selected bond distances (A˚) and angles (deg): Co-C(1) 2.054(7), Co-C(2) 1.953(7), Co-C(3) 1.990(7), Co-C(4) 2.037(7), C(1)-C(2) 1.439(10), C(2)-C(3) 1.407(10), C(3)-C(4) 1.420(10), C(4)-C(5) 1.462(10), C(1)C(19) 1.480(11); C(1)-Co-C(4) 87.0(3), C(2)-C(1)-C(19) 123.2(6), C(3)-C(4)-C(5) 116.9(6).
assignments for all three diene complexes. The observed (E,E) stereochemistry is presumably controlled by steric interactions between the diene substituents. The butenyl ester substituent occupies an anti-position in order to minimize steric interactions with the sulfone, and the ethyl ester occupies a syn-position in order to avoid an unfavorable steric interaction with the anti butenyl ester at the opposite end of the 1,3-diene. In an effort to expand the scope of this novel transformation, we sought to incorporate carbene addends from diazoketones. Since we have thus far been unable to isolate a metallacyclobutene complex from the reactions of 5 with diazoketones, we examined the use of diazoketones as the second carbene addend. Preliminary small-scale reactions of
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6 with 1-diazopropane-2-one and 2-diazo-1-phenylethanone were analyzed by 1H NMR spectroscopy, and, once again, the formation of multiple [(η5-C5H5)Co]-containing products was observed. We therefore carried out preparative scale reactions and in both cases treated the crude cobaltdiene mixture with tetrabutylammonium fluoride (TBAF) in the hope of effecting a desilylation and in situ isomerization to a single diene isomer. In the case of 1-diazopropane-2-one, (η5-C 5H5)Co[η4 -(E,E)-CH(COMe)dC(SO2Ph)CHdCH(CO2Et)] (13-Me) was isolated as an air-stable, redblack crystalline solid in 60% yield. From the reaction with 2-diazo-1-phenylethanone, (η5-C5H5)Co[η4-(E,E)-CH(COPh)dC(SO2Ph)CHdCH(CO2Et)] (13-Ph) was isolated in 40% yield, also in the form of air-stable red-black crystals. A single-crystal X-ray crystallographic analysis of 13-Me (Figure S1) confirmed the regio- and stereochemical assignments for both 13-Me and 13-Ph. In summary, we have reported herein the first method for the addition of dissimilar carbenes across an unsymmetrically substituted alkyne to give trisubstituted 1,3-diene products with excellent control of regio- and stereoselectivity. This conceptual approach to the rapid, stereocontrolled synthesis of conjugated dienes may ultimately find application in the construction of highly functionalized carbocycles via traditional cycloaddition chemistry.
Acknowledgment. Financial support of the National Science Foundation (CHE-0518707 and CHE-0911765; and instrumentation grant CHE-9709183) is gratefully acknowledged. The authors thank Dr. Louise M. LiableSands for assistance with the X-ray crystal structure determinations. Supporting Information Available: Experimental details and characterization data for compounds 9-13. CIF files giving details of the crystal structure determinations of 10, 12, and 13-Me. This material is available free of charge via the Internet at http://pubs.acs.org.
