or Hydrides with Enynes as a

A new method for the preparation of dienylcobaloxime complexes which involves ... Carmen J. Tucker, Mark E. Welker, Cynthia S. Day, and Marcus W. Wrig...
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Organometallics 1996, 15, 2624-2632

Reactions of Cobaloxime Anions and/or Hydrides with Enynes as a New, General Route to 1,3- and 1,2-Dienylcobaloxime Complexes Heather L. Stokes and Mark E. Welker*,† Department of Chemistry, Wake Forest University, P.O. Box 7486, Winston-Salem, North Carolina 27109 Received January 30, 1996X

A new method for the preparation of dienylcobaloxime complexes which involves reactions of cobaloxime anions and/or hydrides with enynes is reported. This new dienyl complex preparative method leads to cobalt-substituted 1,3- or 1,2-dienes depending on the enyne substitution pattern chosen. Subsequent Diels-Alder reactions of the 1,3-dienyl complexes and demetalation reactions of cobalt-substituted cycloadducts are also reported. Introduction We have been preparing 1,3-dienylcobaloxime complexes (3) (Scheme 1) and examining the rates, regioselectivities, and stereoselectivities of their reactions with dienophiles in Diels-Alder reactions.1 Tada et al. have reported alternative preparations of some cobaloxime dienes as well as results of their cycloaddition reactions.2 Several other groups have reported that transition-metal substitution in the dienophiles can also have a pronounced effect on [4 + 2] cycloaddition outcomes.3 In this manuscript, we report an alternative method for the preparation of dienylcobaloxime complexes which involves reactions of cobaloxime anions and/or hydrides (4) with enynes (5). This new dienyl complex preparative method leads to cobalt substituted 1,3- (6) or 1,2-dienes (7) depending on the enyne substitution pattern chosen. Subsequent Diels-Alder reactions of the 1,3-dienyl complexes (6) and demetalation reactions of cobalt-substituted cycloadducts are also reported. Experimental Section General Methods. For a description of instrumentation and chromatographic adsorbents used see ref 1b. Cobalt chloride hexahydrate was purchased from Strem Chemicals and used as received. Dimethylglyoxime (dmg) was purchased from Fisher Scientific and recrystallized from 95% EtOH (12 mL/g) prior to use. 2-Methyl-1-buten-3-yne (17), 4-methyl-1pentyn-3-ol, and 2-methyl-1-hexen-3-yne (18) were purchased from Farchan. trans- and cis-2-penten-4-yne (20, 21) and 1-penten-3-yne (19) were purchased from Wiley. All alkynes † Henry Dreyfus Teacher-Scholar Awardee (1994-99). Email: [email protected]. X Abstract published in Advance ACS Abstracts, May 15, 1996. (1) (a) Smalley, T. L.; Wright, M. W.; Garmon, S. A.; Welker, M. E.; Rheingold, A. L. Organometallics 1993, 12, 998. (b) Wright, M. W.; Smalley, T. L.; Welker, M. E.; Rheingold, A. L. J. Am. Chem. Soc. 1994, 116, 6777. (c) Stokes, H. L.; Richardson, B. M.; Wright, M. W.; Vaughn, S. P.; Welker, M. E.; Liable-Sands, L.; Rheingold, A. L. Organometallics 1995, 14, 5520. (d) Wright, M. W.; Welker, M. E. J. Org. Chem. 1996, 61, 133. (2) (a) Tada, M.; Shimizu, T. Bull. Chem. Soc. Jpn. 1992, 65, 1252. (b) Tada, M.; Mutoh, N.; Shimizu, T. J. Org. Chem. 1995, 60, 550. (3) (a) Sabat, M.; Reynolds, K. A.; Finn, M. G. Organometallics 1994, 13, 2084. (b) Gilbertson, S. R.; Zhao, X.; Dawson, D. P.; Marshall, K. L. J. Am. Chem. Soc. 1993, 115, 8517. (c) Anderson, B. A.; Wulff, W. D.; Powers, T. S.; Tribbitt, S.; Rheingold, A. L. J. Am. Chem. Soc. 1992, 114, 10784. (d) Barluenga, J.; Canteli, R.-M.; Florez, J.; Garcia-Granda, S.; Gutierrez-Rodriguez, A. J. Am. Chem. Soc. 1994, 116, 6949.

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were used as received. 1-Ethynylcyclohexene4 (26) and pyr(dmg)2CoCl (9, L ) pyr)13a were prepared using literature procedures. NMR data are reported in ppm. (Z)-2-Hexen-4-yne (23). At -78 °C, lithium (0.132 g, 0.019 mol) was added to NH3 (50 mL). cis-2-Penten-4-yne (21) (1.096 g, 0.016 mol) was added dropwise, followed 2 min later by methyl iodide (0.98 mL, 0.016 mol). The reaction was warmed slowly to just below 25 °C to remove the NH3. Et2O (15 mL) was then added and decanted. The white residue was triturated 3 additional times with ether (50 mL) to remove the enyne. The Et2O was removed by distillation (760 mmHg) to yield the product (0.283 g, 3.5 mmol, 21.3%) which was identical by 1H NMR comparison to previously reported partial 1H NMR data for this compound.5 1H NMR (CDCl ): 5.87 (dq, 3 J ) 11.0, 6.5 Hz, 1H), 5.42 (dd, J ) 11.0, 2.2 Hz, 1H), 2.00 (d, J ) 2.2 Hz, 3H), 1.85 (d, J ) 6.5 Hz, 3H). (E)-2-Hexen-4-yne (22). This compound was prepared in a manner analogous to that reported for the Z isomer above using trans-2-penten-4-yne (20) (1.096 g, 0.016 mol) to yield the product (0.483 g, 6.03 mmol, 36.3%) which was identical by 1H NMR comparison to previously reported partial 1H NMR data for this compound.5 1H NMR (CDCl3) 6.05 (dq, J ) 15.5, 6.6 Hz, 1H), 5.4 (dq, J ) 15.5, 1.9 Hz, 1H), 1.90 (d, J ) 1.9 Hz, 3H), 1.75 (d, J ) 6.6 Hz, 3H). 2-Methyl-2-penten-4-yne (24). At 0 °C, POCl3 (4.00 mL, 0.042 mol) in pyridine (8 mL) was added dropwise to 4-methyl1-pentyn-3-ol (4.304 g, 0.043 mol) in pyridine (10 mL). The solution was warmed (25 °C) and stirred (1 h) whereupon it became deep red and transparent. The product was directly distilled using a vigreux column. The product was collected from 58-85 °C (760 mmHg) (1.54 g, 0.019 mol, 43.8%) as a 7.9:1 mixture of product to pyridine and used in this manner in subsequent reactions with cobaloxime complexes. The product was identical by 1H NMR comparison to previously reported partial 1H NMR data for this compound.5 1H NMR (CDCl3): 5.22 (s, 1H), 2.97 (s, 1H), 1.90 (s, 3H), 1.80 (s, 3H). 2-Methyl-2-hexen-4-yne (25). Li (0.168 g, 0.024 mol) was added to liquid NH3 (50 mL) at -78 °C. Enyne 24 (1.209 g, 0.015 mol) was added dropwise and vigorously stirred (10 min). The reaction turned from a deep blue to white during the enyne addition. MeI (1.60 mL, 0.025 mol) was added. The solution was warmed to 25 °C gradually to allow the NH3 to evaporate. The residue was dissolved in water (10 mL). The aqueous layer was extracted with ether (3 × 100 mL). The ether was removed by distillation at 760 mmHg to yield the (4) (a) Alexakis, A.; Marek, I.; Mangeney, P.; Normant, J. F. Tetrahedron 1991, 47, 1677. (b) Brandsma, L. Preparative Acetylenic Chemistry, 2nd ed.; Elsevier Science Publishers BV: Amsterdam, 1992; pp 16, 36, 44, 203. (5) Poutsma, M. L.; Ibarbia, P. A. J. Am. Chem. Soc. 1973, 95, 6000.

© 1996 American Chemical Society

1,3- and 1,2-Dienylcobaloxime Complexes

Organometallics, Vol. 15, No. 11, 1996 2625 Scheme 1

enyne 25 (0.543 g, 5.7 mmol, 38.2%) identical by 1H NMR comparison to an authentic sample.6 (3-Methyl-1,2-butadien-1-yl)(pyridine)bis(dimethylglyoximato)cobalt(III) (27). A. Preparation under Neutral Conditions Using (pyr)(dmg)2CoCl. NaBH4 (0.046 g, 1.21 mmol) in H2O (0.1 mL) was added slowly to the cobalt chloride 913a (0.505 g, 1.2 mmol) in MeOH (15 mL) at -20 °C and stirred 45 min. Enyne 17 (0.10 mL, 1.05 mmol) was then added. The solution was warmed to 25 °C overnight and then poured into ice water (25 mL). The aqueous layer was extracted with CH2Cl2 (3 × 25 mL). The organic extracts were combined and dried (MgSO4), followed by removal of the solvent under reduced pressure to yield the cobalt allene 27 as an orange powder (0.144 g, 0.33 mmol, 31.4%), mp 165167 °C dec. 1H NMR (CDCl3): 8.59 (d, J ) 6.8 Hz, 2H), 7.72 (t, J ) 8.0 Hz, 1H), 7.32 (t, J ) 6.8 Hz, 2H), 4.71 (m, 1H), 2.12 (s, 12H), 1.28 (d, J ) 2.0 Hz, 6H). 13C NMR (CDCl3): 194.0, 150.1, 148.9, 137.5, 125.1, 95.5, 22.3, 11.8. IR (C6H6): 3804.2, 3679.0, 3234.0, 3075.5, 2975.1, 2920.2, 2855.8, 2613.6, 1613.8, 1558.2, 1452.0, 1390.0, 1330.6 cm-1. Anal. Calcd for C18H26CoN5O4: C, 49.66; H, 6.02. Found: C, 49.55; H, 6.09%. B. Preparation under Acidic Conditions Using CoCl2. Cobalt dichloride hexahydrate (4.781 g, 0.02 mol) and dmg (4.818 g, 0.04 mol) were combined in ethanol (80 mL). The solution was stirred (10 min), and then NaOH (1.649 g, 0.04 mol) as a 50% aqueous solution and pyridine (1.6 mL, 0.02 mol) were added. The reaction was allowed to stir at 25 °C overnight. NaBH4 (0.108 g, 2.85 mmol) in H2O (1 mL), acetic acid (0.56 mL, 9.8 mmol), and enyne 17 (0.5 mL, 5.3 mmol) were then added. The reaction was stirred (3 h) at 25 °C and then refluxed for an additional 2 h. After cooling, the crude product was collected by vacuum filtration and then chromatographed on silica (EtOAc) to yield allene 27 as an orange solid (1.045 g, 2.40 mmol, 45.3%). C. Preparation with High Water Concentration. At 25 °C, cobalt dichloride hexahydrate (0.608 g, 2.56 mmol), dmg (0.788 g, 6.8 mmol), NaOH (0.227 g, 5.6 mmol) in H2O (1 mL), and pyridine (0.2 mL, 2.5 mmol) were dissolved in 50/50 water-ethanol solution (11 mL) and stirred overnight at 25 °C. NaBH4 (0.026 g, 1.6 mmol) in H2O (0.15 mL) was added at 0 °C. Enyne 17 (0.3 mL, 3.15 mmol) was added immediately after the reducing agent. The suspension was stirred (3.5 h) and then poured into ice H2O (50 mL). The solution was filtered to isolate the orange precipitate of 27 (0.715 g, 1.64 mmol, 52.0%). The filtrate was extracted with CH2Cl2, dried (MgSO4), and concentrated under reduced pressure. The residue from the filtrate (0.323 g, 0.74 mmol, 23.5%) was a 2.13:1 mixture of allene 27 to 2-methylbutadiene 28 as measured by the integral ratios for the allenic proton (4.71 ppm) and the single proton on the 2-methylbutadiene complex (28) (4.44 ppm).1c (6) (a) Koster, R.; Bubmann, A.; Schroth, G. Liebigs. Ann. Chem. 1975, 2130. (b) Henbest, H. B.; Jones, E. R. H.; Walls, I. M. S. J. Chem. Soc. 1949, 2692. (c) Nash, B. W.; Thomas, D. A.; Warburton, W. K. J. Chem. Soc. 1965, 2983. (d) Wilson, J. W. J. Am. Chem. Soc. 1969, 91, 12, 3228.

D. Synthesis of Allene 27 Using a Sodium Amalgam. Cobalt chloride (9)13a (1.042 g, 2.6 mmol) in THF (40 mL) was added to sodium (0.313 g, 0.014 mol) amalgam at -20 °C. The solution was stirred (3 h) between -14 and -30 °C. Enyne 17 (0.897 g, 0.014 mol) was added at -35 °C and stirred (4h). The solution was separated from the amalgam by cannula into a second flask containing H2O (0.7 mL) in THF (10 mL) at -20 °C and allowed to warm to 25 °C overnight. The aqueous-THF mixture was extracted with CH2Cl2, dried (MgSO4), and then concentrated to yield allene 27 (0.125 g, 0.29 mmol, 11.1%). (2-Methyl-2,3-hexadien-4-yl)(pyridine)bis(dimethylglyoximato)cobalt(III) (29). NaBH4 (0.145 g, 3.83 mmol) in H2O (0.2 mL) was added to cobalt chloride (9)13a (1.500 g, 3.8 mmol) in ethanol (20 mL) at -20 °C. Enyne 18 (0.325 g, 4.1 mmol) was added at -20 °C 45 min later. The solution was warmed to 25 °C overnight and then poured into ice water (100 mL). The allene 29 was collected by filtration (1.084 g, 0.23 mmol, 62.9%). The filtrate was extracted with CH2Cl2 (3 × 50 mL), dried (MgSO4), and concentrated under reduced pressure. The brown solid from the filtrate was chromatographed on silica (EtOAc) to give additional allene 29 (0.101 g, 0.02 mmol, 5.9%): total yield 68.7%; mp 119-120 °C dec. 1H NMR (CDCl ): 8.69 (d, J ) 4.6 Hz, 2H), 7.69 (t, J ) 4.6 3 Hz, 1H), 7.25 (d, J ) 4.6 Hz, 2H), 2.11 (s, 12 H), 1.8 (q, J ) 7.1 Hz, 2H), 1.25 (s, 6H), 0.86 (t, J ) 7.1 Hz, 3H). 13C NMR (CDCl3): 190.1, 150.1, 149.4, 137.5, 125.2, 95.7, 25.6, 22.9, 15.1, 12.0. IR (CDCl3): 3079.7, 3049.5, 3005.9, 2966.8, 2924.6, 2867.8, 2703.3, 2387.3, 1920.0, 1604.2, 1560.1, 1468.1, 1449.4, 1381.0, 1232.3, 1085.8 cm-1. Anal. Calcd for C20H30CoN5O4: C, 51.84; H, 6.52. Found: C, 51.62; H, 6.48%. Alternatively, 29 can also be prepared using more acidic conditions as follows. At 25 °C, NaOH (0.249 g, 6.2 mmol) was dissolved in MeOH (50 mL). Cobalt chloride (9)13a (0.749 g, 1.85 mmol) was added while nitrogen was bubbled through the solution. After NaBH4 (0.128 g, 3.4 mmol) was added (as a solid) the reaction was stirrred for 20 min at 25 °C. Enyne 18 (0.137 g, 1.45 mmol) was then added followed by glacial acetic acid (2.3 mL) (dropwise) until the solution turned orange. The orange solution was then poured into ice water (50 mL) and cooled in an ice bath. Allene 29 was collected by vacuum filtration (0.401 g, 0.87 mmol, 59.4%). (2,3-Pentadien-2-yl)(pyridine)bis(dimethylglyoximato)cobalt(III) (30). NaBH4 (0.303 g, 8.0 mmol) in H2O (0.5 mL) was added to cobalt chloride (9)13a (1.271 g, 3.2 mol) in ethanol (75 mL) at -20 °C and stirred 45 min. Enyne 19 (1.174 g (78% pure), 0.018 mol) was added at -20 °C, and then the solution was warmed to 25 °C overnight and poured into ice water (200 mL). The product was collected by filtration and then redissolved in CH2Cl2 and dried (MgSO4), and the solvent was removed under reduced pressure to yield 30 (0.781 g, 1.79 mmol, 56.8%), mp 144-145 °C dec. 1H NMR (CDCl3): 8.67 (d, J ) 4.9 Hz, 2H), 7.69 (t, J ) 4.9 Hz, 1H), 7.29 (d, J ) 4.9 Hz, 2H), 4.92 (m, 1H), 2.12 (d, J ) 4.5 Hz, 12H), 1.39 (d, J ) 4.5 Hz, 3H), 1.20 (d, J ) 8.9 Hz, 3H). 13C NMR (CDCl3): 192.9, 150.2, 149.7, 137.5, 125.2, 82.4, 12.2, 12.1. IR (CDCl3): 3080.4,

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Organometallics, Vol. 15, No. 11, 1996

3007.3, 2916.9, 1605.4, 1561.0, 1494.2, 1450.0, 1235.8, 1205.8, 1099.8, 1091.6, 1071.8, 1049.4 cm-1. Anal. Calcd for C18H26CoN5O4: C, 49.66%; H, 6.02%. Found: C, 49.45%, H, 6.11%. (3E)-(1,3-Pentadien-2-yl)(pyridine)bis(dimethylglyoximato)cobalt(III) (31). This complex was prepared as described for 30 above using NaBH4 (0.030 g, 0.8 mmol), cobalt chloride (9)13a (0.403 g, 1.0 mmol), and enyne 20 (0.104 g, 1.6 mmol). The crude product was purified by chromatography on silica (EtOAc) to yield 31 and 33 as an orange solid (0.182 g, 0.42 mmol, 42%, 10.5:1 trans-31 to cis-33) which was identical by spectroscopic comparison to previously reported material.1b,c (3E)-(1,3-Pentadien-2-yl)(pyridine)bis(diphenylglyoximato)cobalt(III) (32). At 25 °C, cobalt dichloride hexahydrate (2.448 g, 0.01 mol), diphenylglyoxime (4.797 g, 0.02 mol), NaOH (0.876 g, 0.02 mol) in H2O (1.2 mL), and pyridine (0.8 mL, 0.01 mol) were dissolved in ethanol (40 mL) and stirred (4 h). NaBH4 (0.063 g, 1.6 mmol) in H2O (0.15 mL) was added at 0 °C followed by enyne 20 (0.582 g, 8.8 mmol) 5 min later. The solution was warmed to 25 °C overnight and poured into ice H2O (100 mL). The orange precipitate was collected by vacuum filtration and washed with ether to yield 32 (1.96 g, 2.86 mmol, 32.5%), which was identical by spectroscopic comparison to previously reported material.1b (3Z)-(1,3-Pentadien-2-yl)(pyridine)bis(dimethylglyoximato)cobalt(III) (33). This complex was prepared in a manner analogous to that described for 30 above using NaBH4 (0.138 g, 3.64 mmol), cobalt chloride (9)13a (1.500 g, 3.71 mmol), and enyne 21 (0.276 g, 4.2 mmol). The orange precipitate of diene 33 was collected by vacuum filtration (1.271 g, 2.92 mmol, 69.9%). The filtrate was extracted with CH2Cl2 to yield a mixture of cis-33 to trans-31 (1.7:1 mixture of cis-33 to trans31 as measured by the single proton on the dienes, 0.126 g, 0.29 mmol, 7.8%). Complex 33 has been characterized previously.1c (3Z)-(1,3-Pentadien-2-yl)(pyridine)bis(diphenylglyoximato)cobalt(III) (34). At 25 °C, cobalt dichloride hexahydrate (2.012 g, 8.5 mmol), diphenylglyoxime (4.040 g, 0.017 mol), pyridine (0.8 mL, 9.9 mmol), and NaOH (0.700 g, 0.017 mol) in H2O (2 mL) were combined in 95% ethanol (15 mL). The solution was stirred for 1 h and then cooled to -20 °C. NaOH (0.375 g, 9.3 mmol) in H2O (2 mL) and NaBH4 (0.099 g, 2.4 mmol) in H2O (0.2 mL) were added slowly and stirred 45 min. Enyne 21 (0.495 g, 7.5 mmol) was added, and the solution was warmed to 25 °C overnight and then poured into ice water (100 mL). The orange-brown solid was collected by filtration. This crude product was chromatographed on silica (CH2Cl2) to yield a >20:1 E:Z ratio of 34:32 (1.082 g, 1.6 mmol, 21.1%). Spectroscopic data for 34 (mp 179-180 °C dec): 1H NMR (CDCl3) 8.95 (d, J ) 7.6 Hz, 2H), 7.84 (t, J ) 7.6 Hz, 1H), 7.44 (d, J ) 7.6 Hz, 2H), 7.31-7.15 (m, 12H), 7.13-7.05 (m, 8H), 6.48 (dq, J ) 11.1, 1.7 Hz, 1H), 5.58 (m, 1H), 5.29 (s, 1H), 4.89 (s, 1H), 1.62 (dd, J ) 6.8, 1.7 Hz, 3H); 13C NMR 153.6, 152.6, 151.4, 151.3, 151.0, 150.3, 150.1, 139.5, 138.1, 137.8, 130.2, 130.1, 129.9, 129.8, 129.7, 129.6, 129.2, 129.2, 129.0, 128.2, 128.0, 127.9, 126.2, 125.9, 125.6, 120.6, 117.6, 14.8; IR (CDCl3) 3691.2, 3605.3, 3064.5, 3030.5, 2932.5, 1952.3, 1603.9, 1580.1, 1530.0, 1485.8, 1448.0, 1285.4 cm-1; HRMS (m/z) Calcd for C38H34CoN5O4 (M + H)+ 684.2021, found: 684.2006. (2E,4E)-(2,4-Hexadieny-3-yl)(pyridine)bis(dimethylglyoximato)cobalt(III) (35). NaOH (0.140 g, 3.5 mmol) was added to cobalt chloride (9)13a (1.214 g, 3.02 mmol) suspended in 95% ethanol (60 mL) so that the solution was at pH ) 8. NaBH4 (0.117 g, 3.1 mmol) in water (0.2 mL) was then added at -20 °C. trans-4-hexen-2-yne (22) (0.217 g, 2.71 mmol) was added at -20 °C 45 min later. The reaction was warmed (25 °C) and stirred (5 h). The solution was then poured into ice water (200 mL), and the aqueous layer was extracted with CH2Cl2 (3 × 25 mL). The solvent was removed under reduced pressure and the residue was purified by column chromatography (silica, EtOAc) to yield the diene as a 2:1 mixture of

Stokes and Welker E:Z 35:36 (0.223 g, 0.5 mmol, 18.3%). 1H NMR for E (35) (CDCl3): 8.60 (d, J ) 6.8 Hz, 2H), 7.69 (t, J ) 6.8 Hz, 1H), 7.24 (d, J ) 6.8 Hz, 2H), 5.72 (d, J ) 14.1 Hz, 1H), 5.00 (m, 1H), 4.78 (q, J ) 6.6 Hz, 1H), 2.07 (s, 12H), 1.674 (dd, J ) 6.6, 1.24 Hz, 3H), 1.47 (dd, J ) 6.56, 1.65 Hz, 3H). IR (CDCl3): 3689.1, 3153.7, 3080.6, 3005.1, 2962.0, 2911.3, 2874.9, 2852.6, 1563.0, 1494.0, 1233.5, 1088.8 cm-1. Anal. Calcd for C19H28CoN5O4: C, 50.78; H, 6.28. Found: C, 50.61; H, 6.22. (2E, 4Z)-(2,4-Hexadien-3-yl)(pyridine)bis(dimethylglyoximato)cobalt(III) (36). This complex was prepared in a manner analogous to that described for 35 above using cobalt chloride (9)13a (1.418 g, 3.52 mmol), NaBH4 (0.117 g, 3.1 mmol), and cis-2-hexen-4-yne (23) (0.283 g, 3.54 mmol) to yield 36 (0.160 g, 0.4 mmol, 10%). 1H NMR (CDCl3): 8.57 (d, J ) 6.6 Hz, 2H), 7.66 (t, J ) 6.6 Hz, 1H), 7.25 (d, J ) 6.6 Hz, 2H), 5.74 (d, J ) 10.8 Hz, 1H), 5.31 (dq, J ) 6.6, 10.8 Hz, 1H), 4.89 (q, J ) 6.6 Hz, 1H), 2.05 (s, 12H), 1.51 (d, J ) 6.6 Hz, 3H), 1.05 (d, J ) 6.6 Hz, 3H). 13C NMR (CDCl3): 149.9, 149.5, 137.4, 134.2, 125.0, 122.6, 120.0, 16.3, 14.2, 11.9. IR (CDCl3): 3691.1, 3007.6, 2961.1, 2926.3, 2909.0, 2852.2, 2707.2, 2274.5, 1818.2, 1792.8, 1653.1, 1604.7, 1561.9, 1506.5, 1494.1, 1449.3, 1102.0 cm-1. Anal. Calcd for C19H28CoN5O4: C, 50.78; H, 6.28. Found: C, 50.84; H, 6.26. (4-Methyl-1,3-pentadien-2-yl)(pyridine)bis(dimethylglyoximato)cobalt(III) (37). At -20 °C, NaBH4 (0.206 g, 5.3 mmol) in water (0.6 mL) was added to cobalt chloride (9)13a (1.080 g, 2.68 mmol) in 95% ethanol (100 mL). The reaction was stirred at -20 °C (1 h) before pyridine (0.5 mL, 6.19 mmol) and enyne 24 (0.338 g, 4.23 mmol) were added, and the solution was allowed to warm to 25 °C overnight. The yellow suspension was poured into ice water (200 mL) and the resulting yellow precipitate collected by filtration. The precipitate was redissolved in CH2Cl2, dried (MgSO4), and concentrated under reduced pressure to yield diene 37 (0.995 g, 2.22 mmol, 82.7%), mp 171-172 °C dec. 1H NMR (CDCl3): 8.61 (d, J ) 7.9 Hz, 2H), 7.69 (t, J ) 7.9 Hz, 1H), 7.31 (d, J ) 7.9 Hz, 2H), 5.78 (bs, 1H), 4.72 (s, 1H), 4.06 (d, J ) 1.1 Hz, 1H), 2.06 (s, 12H), 1.54 (d, J ) 1.0 Hz, 3H), 1.302 (d, J ) 0.95 Hz, 3H). 13C NMR (CDCl3): 149.9, 149.5, 137.5, 133.2, 127.7, 125.1, 115.8, 25.6, 18.8, 11.8. IR (CDCl3): 3670.1, 3573.7, 2962.8, 2925.4, 2868.6, 2850.6, 1731.8, 1629.9, 1563.1, 1449.8, 1384.8, 1373.9, 1234.2, 1097.6 cm-1. Anal. Calcd for C19H28CoN5O4: C, 50.78; H, 6.28. Found: C, 50.68; H, 6.31. (2E)-(5-Methyl-2,4-hexadien-3-yl)(pyridine)bis(dimethylglyoximato)cobalt(III) (38). This complex was prepared in a manner analogous to that used to prepare 37 above using cobalt chloride (9)13a (1.074 g, 2.6 mmol) and enyne 25 (0.245 g, 2.6 mmol) to yield a yellow precipitate of diene 38 which was collected by vacuum filtration (0.630 g, 1.36 mmol, 51.0%), mp 167-168 °C dec. 1H NMR (CDCl3): 8.57 (d, J ) 5.8 Hz, 2H), 7.65 (t, J ) 5.8 Hz, 1H), 7.24 (d, J ) 5.8 Hz, 2H), 5.51 (s, 1H), 4.81 (q, J ) 6.6 Hz, 1H), 2.06 (s, 6H), 2.01 (s, 6H), 1.49 (s, 3H), 1.49 (d, J ) 6.6 Hz, 3H), 1.08 (s, 3H). 13C NMR (CDCl3): 150.0, 149.1, 137.3, 129.4, 128.3, 125.004, 122.1, 25.3, 19.2, 16.6, 11.9, 11.8. IR (CDCl3): 3745.1, 3155.1, 3115.7, 3080.4, 2972.9, 2908.1, 2850.4, 2727.2, 1560.0, 1449.2, 1233.2, 1097.0, 1092.5 cm-1. Anal. Calcd for C20H30CoN5O4: C, 51.84; H, 6.52. Found: C, 51.23; H, 6.38. (1-Vinylcyclohexen-2-yl)(pyridine)bis(dimethylglyoximato)cobalt(III) (39). This complex was prepared in a manner analogous to that described for 37 above using cobalt chloride (9)13a (1.276 g, 3.17 mmol) and enyne 26 (0.392 g, 3.7 mmol). The orange precipitate was collected by filtration and then redissolved into CH2Cl2 (200 mL). The CH2Cl2 solution was dried (MgSO4) and concentrated under reduced pressure to yield a crude diene complex which was triturated with 50/ 50 ether/hexane (3 × 3 mL) to yield complex 39 (0.824 g, 1.59 mmol, 50.2%). This complex (39) has been characterized previously.1c (anti- and syn-3,6-Dimethyl-1,1,2,2-tetracyano-4-cyclohexen-4-yl)(pyridine)bis(dimethylglyoximato)cobalt(III) (45 and 46). A 2:1 mixture of 35 and 36 (0.100

1,3- and 1,2-Dienylcobaloxime Complexes g, 0.223 mmol) was heated in toluene (10 mL) at 60 °C for 1 h with tetracyanoethylene (0.028 g, 0.22 mmol). The solvent was removed under reduced pressure to yield 45 and 46 (0.126 g, 0.22 mmol, 98%) as a 3.5:1 mixture of trans to cis cycloadducts). 1H NMR (CDCl3): 8.50 (app d, J ) 6.9 Hz, 2H), 7.72 (app t, J ) 6.9 Hz, 1H), 7.30 (app d, J ) 6.9 Hz, 2H), 5.38 (d, J ) 2.1 Hz, 1H) major, 5.24 (m, 1H) minor, 3.15 (q, J ) 6.9 Hz, 1H), 3.00 (q, J ) 6.9 Hz, 1H), 2.19 (s, 6H), 2.09 (s, 6H), 1.45 (d, J ) 6.9 Hz, 3H), 1.38 (d, J ) 6.9 Hz, 3H). IR (CDCl3): 3689.0, 3154.2, 2973.7, 2924.1, 1559.0, 1102.0 cm-1. Anal. Calcd for C25H28CoN9O4: C, 52.00; H, 4.89. Found: C, 51.96; H, 4.97. (syn-3,6-Dimethyl-1,1,2,2-tetracyano-4-cyclohexen-4yl)(pyridine)bis(dimethylglyoximato)cobalt(III) (46). Cobalt diene 36 (0.093 g, 0.21 mmol) was heated in toluene (10 mL) at 60 °C for 1 h with tetracyanoethylene (0.026 g, 0.20 mmol). The solvent was then removed under reduced pressure to yield product as a 12.5:1 mixture of cis and trans-cycloadducts (46 and 45). (0.114 g, 0.20 mmol, 95.7%). Mp: 181183 °C dec. Spectroscopic data for the major syn product (46): 1H NMR (CDCl3) 8.55 (d, J ) 6.7 Hz, 2H), 7.73 (t, J ) 6.7 Hz, 1H), 7.31 (d, J ) 6.7 Hz, 2H), 5.24 (d, J ) 2.2 Hz, 1H), 3.1 (q, J ) 6.7, 2.2 Hz, 2H), 2.19 (s, 6H), 2.09 (s, 6H), 1.42 (d, J ) 6.7 Hz, 6H); IR (CDCl3) 3689.0, 3157.4, 3026.8, 2977.2, 2933.9, 1606.6, 1559.1, 1450.4, 1234.9 cm-1. Anal. Calcd for C25H28CoN9O4: C, 52.00; H, 4.89. Found: C, 51.91; H, 4.99%. anti- and syn-3,6-Dimethyl-1,1,2,2-tetracyanocyclohex4-ene (47 and 48). The mixture of cycloadducts (0.050 g, 0.08 mmol) in THF (5 mL) (45 and 46 obtained from the reaction of 35 and 36 with tetracyanoethylene), was treated with AlMe3 (0.154 mL of a 2.0 M solution in hexanes, 0.308 mmol) at 0 °C. The reaction was warmed to 25 °C and stirred 1 h. Ice water (5 mL) was added at 0 °C. The solution was extracted with CH2Cl2 (3 × 30 mL) to yield a mixture of (pyridine)bis(dimethylglyoximato)cobalt(III) methyl and the organic cycloadduct. After removal of the solvent, the residue was washed with ether followed by solvent removal under reduced pressure to yield the organic cycloadducts as a 4:1 mixture of trans- to cis-cycloadducts (0.009 g, 0.05 mmol, 49.4%). The trans-47 and the cis-48 cycloadduct 1H NMR data were identical to authentic samples.7 The yellow solid that did not dissolve in the ether proved to be (pyridine)bis(dimethylglyoximato)cobalt(III) methyl (53) (0.031 g, 0.081 mmol, 93.3%) by 1H NMR comparison to an authentic sample.1b,c syn-3,6-Dimethyl-1,1,2,2-tetracyanocyclohex-4-ene (48). The mixture of cycloadducts (0.044 g, 0.07 mmol, obtained from the reaction of 36 and tetracyanoethylene) was treated with AlMe3 (0.135 mL of a 2.0 M solution in hexanes, 0.27 mmol) and worked up in a manner analogous to that reported for the reaction above to yield a 10:1 mixture of cis- to transcycloadducts (48:47)7 (0.005 g, 0.024 mmol, 31.2%) and (pyridine)bis(dimethylglyoximato)cobalt(III) methyl (53) (0.021 g, 0.05 mmol, 72.0%).1b,c (cis-4,4,7,7,8,9-Hexahydro-4,4-dimethyl-1,3-dioxoisobenzofuran-6-yl)(pyridine)bis(dimethylglyoximato)cobalt (III) (50). Cobalt dienyl complex 37 (0.505 g, 1.12 mmol) and maleic anhydride (1.028 g, 0.010 mol) were heated in THF in a sealed tube at 57 °C for 3 days. The solution was cooled to 25 °C and filtered, and the residue was washed with THF until the extracts were clear. The combined extracts were concentrated under vacuum. Ether (5 mL) was added, and the solution was stirred until an orange-brown precipitate formed. The precipitate was collected by filtration, redissolved in CHCl3, and filtered. After removal of the CHCl3, the residue was triturated with 75% ether/pentane mixture to yield cycloadduct 50 (0.448 g, 0.82 mmol, 72.8%) (mp 195-196 °C dec). 1H NMR (CDCl3): 8.58 (d, J ) 6.2 Hz, 2H), 7.80 (t, J ) 6.2 Hz, 1H), 7.29 (t, J ) 6.2 Hz, 2H), 5.29 (d, J ) 1.9 Hz, 1H), 3.19 (ddd, J ) 9.6, 8.6, 2.9 Hz, 1H), 2.77 (d, J ) 9.6 Hz, 1H), 2.60 (dd, J ) 17.0, 2.9 Hz, 1H), 2.34 (dd, J ) 17.0, 8.6 Hz, (7) O’Shea, K. E.; Foote, C. S. Tetrahedron Lett. 1990, 31, 841.

Organometallics, Vol. 15, No. 11, 1996 2627 1H), 2.07 (s, 6H), 2.05 (s, 6H), 1.11 (s, 3H), 0.97 (s, 3H). 13C NMR (CDCl3): 174.3, 172.1, 150.4, 150.1, 137.7, 136.6, 125.2, 51.2, 41.4, 35.5, 28.7, 28.5, 27.3, 12.2, 12.0. IR (CDCl3): 3689.0, 3649.9, 3157.8, 3081.6, 2965.0, 2926.2, 2901.2, 1844.9, 1776.8, 1605.9, 1559.7, 1234.1 cm-1. Anal. Calcd for C23H30CoO7N5: C, 50.46; H, 5.52. Found: C, 50.42; H, 5.58. Rearrangement of (cis-4,4,7,7,8,9-Hexahydro-4,4-dimethyl-1,3-dioxoisobenzofuran-6-yl)(pyridine)bis(dimethylglyoximato)cobalt(III) (50) to (cis-4,4,5,5,8,9Hexahydro-4,4-dimethyl-1,3-dioxoisobenzofuran-6-yl)(pyridine)bis(dimethylglyoximato)cobalt(III) (51). The cycloadduct 50 (0.207 g, 0.38 mmol) was photolyzed in MeI (10 mL) using a quartz filter for 3 h in a Hanovia 450W photolysis reactor. The methyl iodide was removed by passage of a steady stream of N2 through the reaction mixture in an efficient fume hood. The rearranged adduct (51) was recovered and triturated with 3:1 Et2O/pentane (0.193 g, 0.35 mmol, 93.2%). 1H NMR: 8.62 (d, J ) 6.4 Hz, 2H), 7.76 (t, J ) 6.4 Hz, 1H), 7.34 (t, J ) 6.4 Hz, 2H), 5.40 (dd, J ) 3.7, 1.2 Hz, 1H), 3.67 (dd, J ) 9.5, 3.7 Hz, 1H), 2.69 (d, J ) 9.5 Hz, 1H), 2.12 (s, 6H), 2.10 (s, 6H), 2.12-2.09 (m, 1H), 1.77 (dd, J ) 19.9 Hz, 1.2 Hz), 1.09 (s, 3H), 0.75 (s, 3H). IR (CDCl3): 3155.2, 2970.9, 2930.0, 2902.4, 2875.0, 1778.0, 1559.5, 1473.3, 1234.5, 1093.8 cm-1. HRMS (FAB): Calcd for C23H31N5O7Co, m/z 548.1555; found: (M + H)+, m/z 548.1530. (cis-4,4,7,7,8,9-Hexahydro-4,4-dimethyl-1,3-dioxoisobenzofuran (52). At 0 °C, 0.670 mL of 2.0 M AlMe3 in hexane (1.34 mmol) was added to adduct 50 (0.176 g, 0.32 mmol) in THF (10 mL), and the solution was then warmed to 25 °C. After 16 h, ice water (15 mL) was added at 0 °C followed by extraction with CH2Cl2 (2 × 50 mL) to remove cobalt bis(dimethylglyoximate) pyridine methyl (53) (0.093 g, 0.24 mmol, 75.4%). The aqueous layer was acidified with HCl to pH ) 4 and extracted with ether (3 × 50 mL) to yield 52 as an oil after removal of the solvent under reduced pressure (0.026 g, 1.4 mmol, 44.8%). This compound has been reported but not characterized previously.8 1H NMR: 5.65 (m, 1H), 5.35 (d, J ) 10.4 Hz, 1H), 2.60-2.75 (m, 1H), 2.40-2.22 (m, 1H), 3.12.8 (m, 2H), 1.12 (s, 6H). IR (CDCl3): 3157.5, 3022.7, 2968.9, 1713.0, 1472.2, 1380.9, 1095.1 cm-1. HRMS (EI): Calcd for C10H12O3 (M+), m/z 180.0786; found, m/z 180.0786. (cis-4,4,5,5,8,9-Hexahydro-4,4-dimethyl-1,3-dioxoisobenzofuran (51). At 0 °C, 0.180 mL of 2.0 M AlMe3 in hexanes (0.36 mmol) was added to rearranged adduct 51 (0.112 g, 0.2 mmol) in THF (10 mL) and then warmed to 25 °C. After 24 h, ice (10 g) was added at 0 °C followed by extraction CH2Cl2 (2 × 50 mL) to remove (pyridine)bis(dimethylglyoximato)cobalt(III) methyl (53) (0.040 g, 0.1 mmol, 51.0%). The aqueous layer was acidified with HCl to pH ) 4 and extracted with ether (3 × 50 mL) to yield 51 as an oil after removal of the solvent under reduced pressure (0.015 g, 0.08 mmol, 40.8%). 1H NMR (CDCl3): 5.95 (dd, J ) 8.5, 3.4 Hz, 1H), 5.80 (ddd, J ) 8.5, 1.7 Hz, 5.1 Hz), 3.45-3.35 (m, 1H), 2.83 (d, J ) 6.1 Hz, 1H), 2.25 (m, 1H), 1.7 (dd, J ) 18.4, 5.1 Hz, 1H), 1.12 (s, 3H), 1.05 (s, 3H). IR (CDCl3): 3155.1, 2965.4, 2904.1, 1711.9, 1261.3, 1241.9, 1096.8, 1091.1 cm-1. HRMS (EI): Calcd for C10H12O3, m/z 180.0786; found, m/z 180.0790.

Results and Discussion Hydrometalation of alkynes as a route to transitionmetal alkenyl complexes has been studied for over 20 years.9,10 Probably the most well-known alkyne hydrometalation agent is (bis)cyclopentadienylzirconium chlo(8) Goldman, N. L. Chem. and Ind. 1963, 1063. (9) Labinger, J. A. In Comprehensive Organic Syntheses, Trost, B. M., Ed.; Pergamon Press: New York, 1991; Vol. 3, pp 667-702. (10) For some more recent reports of alkyne carbo- and hydrometalation see: (a) Takahasi, T.; Kondakov, D. Y.; Xi, Z.; Suzuki, N. J. Am. Chem. Soc. 1995, 117, 5871. (b) Santos, A.; Lopez, J.; Matas, L.; Ros, J.; Galan, A.; Echavarren, A. M.; Organometallics 1993, 12, 4215l.

2628

Organometallics, Vol. 15, No. 11, 1996

Stokes and Welker Scheme 2

ro hydride (“Schwartz’s reagent”).11 Hydrometalation of the alkyne portion of an enyne to produce a 1-zirconium-substituted 1,3-diene (which was subsequently transmetalated to a tin diene11b or selenium diene11c for Diels-Alder reactions) was reported by Fryzuk et al. in 1986.11b Hydrometalation of both alkenes and alkynes involving cobaloxime chemistry has also been reported, and the product outcomes of these experiments were heavily pH dependent.12 Cobaloxime anions (10) are typically generated by one of two methods (Scheme 2): (i) in situ by way of reduction of the dark red cobalt(II) dimer (8); (ii) via reduction of the yellow-brown preformed cobalt(III) chloride (9).13 At pH g 9, these aqueous alcohol solutions contain predominantly the deep green cobaloxime anion (10), but as the pH of the solution is lowered to 7-8, the deep blue-violet cobalt hydrides (11) are formed.14 When L ) phosphine, these cobalt hydrides (11) can be isolated; however, when L ) a nitrogen base, they decompose fairly rapidly at 20 °C in these solutions.14 The cobaloxime anions (10) react with electron deficient alkenes to give products substituted by cobalt at the β carbon (Michael type addition reactions) (12), and the cobaloxime hydrides react with these same substrates to give R-substituted cobaloximes (13).14 Hydrometalation of alkynes via cobaloxime chemistry has also been reported.15 Anionic complex (10) is reported to react with phenylacetylene to yield the β-styryl complex (15) at pH g 9, whereas 11 is reported to react with phenylacetylene to yield the R-styryl complex (16) at pH ) 7 (Scheme 3). At pHs in between 7 and 9, a mixture of 15 and 16 was isolated.12,15 Likewise, when D-11 was generated by treating 8 with D2, there was almost exclusive deuterium incorporation at the β carbon in 16. When 9 (L ) pyr) was treated with NaBD4 in MeOH (pH ) 7), a 55:45 mixture of 16 and D-16 with D cis to Co was isolated and no 15 or (11) (a) Schwartz, J.; Labinger, J. A. Angew. Chem., Int. Ed. Engl. 1976, 15, 333. (b) Fryzuk, M. D.; Bates, G. S.; Stone, C. Tetrahedron Lett. 1986, 27, 1537. (c) Fryzuk, M. D.; Bates, G. S.; Stone, C. J. Org. Chem. 1987, 52, 2335. (12) For a review of cobaloxime chemistry in which many of these experiments are described see: Dodd, D.; Johnson, M. D. Organomet. Chem. Rev. 1973, 52, 1. (13) (a) Schrauzer, G. N. Inorg. Synth. 1968, 11, 61. (b) Bulkowski, J.; Cutler, A.; Dolphin, D.; Silverman, R. B. Inorg. Synth. 1980, 20, 127. (14) (a) Schrauzer, G. N.; Holland, R. J. J. Am. Chem. Soc. 1971, 93, 1505. (b) Schrauzer, G. N.; Holland, R. J. J. Am Chem. Soc. 1971, 93, 4060. (c) Schrauzer, G. N.; Windgassen, R. J. J. Am. Chem. Soc. 1967, 89, 1999. (d) Mizuta, T.; Kwan, T. J. Chem. Soc. Jpn. 1967, 88, 471. (15) (a) Naumberg, M.; N-V-Duong, D.; Gaudemer, A. J. Organomet. Chem. 1970, 25, 231. (b) Van Duong, K. N.; Gaudemer, A. J. Organomet Chem. 1970, 22, 473. (c) Johnson, M. D.; Meeks, B. S. J. Chem. Soc. B 1971, 185. (d) Dodd, D.; Johnson, M. D.; Van Duong, K. N.; Gaudemer, A. J. Chem. Soc., Perkin Trans. 2 1976, 1261.

Scheme 3

D-15 was reported.15b This experiment indicates that 10 and 11 must be present at pH ) 7 and that proton transfer or H atom transfer from 11 to solvent is relatively slow. The formation of the cis-alkene isomer (15) was postulated to arise from a short-lived vinyl anion intermediate (14).15 We were most interested in this alkyne hydrocobaltation as a possible route to dienylcobaloxime complexes (3). We had discovered that some diene geometries were difficult to prepare using our reported method of SN2′ attack of cobaloxime anions (1) on allenic electophiles (2).1,16 We postulated that hydrometalation of the alkyne portion of readily available enynes could prove a mild, general alternative to our previously reported allenic electrophile route to these complexes. Enyne Preparation. The enynes used in this study are all shown in Table 1. Enynes 17-21 were all commercially available. Enynes 22 and 23 were prepared in pure form by treating enynes 20 and 21 with lithium in ammonia followed by quenching with methyl iodide.4b,5 The gem-dimethyl enyne (24) was prepared by the dehydration of 4-methyl-1-pentyn-3-ol with POCl3 in pyridine.6 Enyne 24 was treated with lithium in liquid ammonia followed by methyl iodide to produce 25.6 1-Ethynylcyclohexene (26) was prepared by the dehydration of 1-ethynylcyclohexanol with POCl3 in pyridine according to a literature procedure.4a Reactions of Enynes with Cobaloximes. A. Results. Reactions of cobaloxime anions and hydrides with all enynes are summarized in Table 2. Enynes containing internally methylated alkenes (17 and 18) reacted to yield allenes (27 and 29) regardless of whether they were treated under basic (entry 1) or near neutral (entry 2) reaction conditions. When the solvent was changed to 1:1 EtOH/H2O (entry 3), the isolated (16) For other examples of SN2′ reactions of cobaloxime anions with propargyl and allyl electrophiles see: (a) Johnson, M. D.; Mayle, C. Chem. Commun. 1969, 192. (b) Collman, J. P.; Cawse, J. N.; Kang, J. W. Inorg. Chem. 1969, 8, 2574. (c) Cooksey, C. J.; Dodd, D.; Gatford, C.; Johnson, M. D.; Lewis, G. J.; Titchmarsh, D. M. J. Chem. Soc., Perkins Trans. 2 1972, 655. (d) Pasto, D. J.; Timmers, D. A.; Huang, N.-Z.; Inorg. Chem. 1984, 23, 4117.

1,3- and 1,2-Dienylcobaloxime Complexes Table 1. Enynes Used in Subsequent Reactions with Cobaloxime Anions or Hydrides

Organometallics, Vol. 15, No. 11, 1996 2629

yield of allene 27 improved (68%) and a small amount of dienyl complex 28 (8%) was also isolated. The improved yield may be due to product water insolubility since we routinely isolate the product diene or allene complexes by pouring the reaction solutions into ice water. We also attempted to generate cobaloxime anion in THF using Na(Hg), where no cobalt hydride should be present (entry 4). Only allene 27 was isolated, but there was also significant complex decomposition. No definitive conclusion about the involvement (or lack thereof) of cobalt hydrides in allene formation can be drawn from this experiment because of the low mass balance. (Possible mechanisms for allene and diene formation will be discussed in more detail in the next section.) Conversely, acidification to ensure the presence of a large amount of cobalt hydride had no effect on product outcome for an internally methylated alkene in an enyne (entries 5 and 6).16d,17 Enyne 19 containing a terminally unsubstituted alkene was the only other enyne to yield allene 30. The allene structure for all three of these complexes (27, 29, and 30) was indicated by the 190-200 ppm allene sp carbon resonance seen in the 13C NMR.17 Enynes with terminally substituted alkenes (20-26) lead to dienyl complexes (31-39) regardless of alkyne substitution or equatorial ligand set. Substitution of diphenylglyoxime (dpg) for dimethylglyoxime (dmg) as equatorial ligand resulted in lower isolated yields of dienyl complexes (compare entries 8 & 9 and 10 & 11). Alkene isomerization was noted for enynes (20 & 22) and will be discussed in more detail in the reaction chemistry section. Most of the reactions in Table 2 were performed using pyr(dmg)2CoCl (9, L ) pyr)13a as the cobalt source (method B). This chloride was then treated with NaBH4 to generate a cobalt-containing solution at near neutral pH. When this solution was made more basic (entries 12 and 13), isolated yields of

dienyl complexes (35 and 36) dropped considerably. This effect is presumably due to the increased solubility of these complexes in basic solution. In contrast to one reaction of an enyne with cobaloximes communicated by Pasto and Timmers, we saw no evidence for alkene hydrocobaltation for any of the enynes above.17 Complexes 31-33 and 39 had been characterized previously when prepared using allenic electrophiles.1b,c The E and Z isomers of dienyl complexes (35 and 36) were easily distinguished by their alkene proton coupling constants of 14.1 and 10.7 Hz, respectively. As with our previously reported dienyl complexes,1b,c the new dienyl complexes reported here (34-38) with R4 and/or R5 ) H show these alkene 1H NMR resonances at 4-5 ppm (CDCl3) and have no 13C resonance in the 190-200 ppm range seen for allenes (27, 29, 30). B. Possible Mechanisms for Allene and Diene Formation. Isolation of allenes (27, 29, and 30) was a surprise to us. When cobaloxime generated by method A was treated with enyne 17 and the reaction quenched 5 min after enyne addition, we isolated 27 and 28 (1.2: 1); however, the yield was quite low (3-4%). When the same reaction was quenched after 90 min, we obtained a 61% yield of 27 and 2% of 28. Most of the reactions reported in Table 2 were allowed to stir overnight, but this result indicates that these reactions are close to completion after 1.5 h (compare this result to entry 3, Table 2). Complex 27 could not be isomerized to 28 by (1) treatment with a series of bases (NaOH, lithium tetramethylpiperidide in THF, and potassium tertbutoxide in DMSO) (27 was recovered in all cases), (2) photolysis in a protic solvent, or (3) treatment with Wilkinson’s catalyst (a known 1,3-hydride shift catalyst;18 a 1,2-cobalt shift would also be required). If cobaloxime anions (1) are involved in product formation, then mechanism A (Scheme 4) could be used to rationalize allene formation and mechanism B could be used to explain diene formation. These mechanisms are analogous to ones previously proposed for related alkene- and alkyne-cobaloxime chemistry.12 It is interesting to note that only enynes that would lead to primary allenic anions (40) yield allenes (27). Those enynes that might lead to secondary or tertiary allenic anions yield dienyl complexes (31-39) rather than allenes. If cobalt hydrides (11, L ) pyr) are responsible for product formation, then formation of 2-cobalt-substituted diene (28) is easily explained by formal alkyne insertion (mechanism C) into the metal-hydrogen bond (Scheme 5). Presumably pyridine loss and recoordination (which we know is facile for the 1,3-dienyl complexes)1c would be required. In an attempt to identify the involvement of cobalt hydrides in these reactions, we prepared the stable (tributylphosphine)bis(dimethylgloximato)cobalt hydride14a and treated it with enyne 17 in THF at 25 °C for 4 days (heating caused decomposition). Both hydride and enyne were recovered unreacted. This result might be taken as an indication that cobaloxime anions are the reactive species that lead to allenes from enynes, but we could not rule out the possibility that the phosphine distorts the dmg ligands up out of the

(17) Pasto, D. J.; Timmers, D. A. Inorg. Chem. 1984, 23, 4115.

(18) Corey, E. J.; Suggs, J. W. J. Org. Chem. 1973, 38, 18, 3224.

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Stokes and Welker

Table 2. Reactions of Cobaloxime Complexes with Enynes

Co sourcea Ab

(1) (2) B (3) Ac (4) C (5) B (6) D (7) B (8) B (9) E (10) B (11) E (12) F (13) F (14) B (15) B (16) B

gly

enyne

R1

R2

R3

yield, %

dmg dmg dmg dmg dmg dmg dmg dmg dpg dmg dpg dmg dmg dmg dmg dmg

17 17 17 17 18 18 19 20 20 21 21 22 23 24 25 26

H H H H Et Et Me

Me Me Me Me Me Me Me

Me Me Me Me Me Me H

45 (27) 31 (27) 68 (27) 11 (27) 69 (29) 59 (29) 57 (30)

R4

R5

R6

H

H

Me

H H H H H H H H H

H H H H Me Me H Me H

H H H H H H H H

R7

H

Me Me H H Me(H) H Me Me -(CH2)4-

R8

H

H H Me Me H(Me) Me Me Me H

yield, %

8 (28)

43 (31), 10.5:1 E:Z 33 (32) 70 (33) 21 (34) 18 (35), 2:1 E:Z 10 (36) 83 (37) 51 (38) 50 (39)

a Co source: (A) (1) CoCl /dmg/pyr/NaOH/ROH, (2) NaBH in H O, (3) enyne; (B) (1) pyr(dmg) CoCl/ROH, (2) NaBH in H O, (3) enyne; 2 4 2 2 4 2 (C) (1) pyr(dmg)2CoCl/Na(Hg)/THF, (2) enyne; (D) B + CH3CO2H; (E) A except diphenylglyoxime (dpg) used in place of dimethylglyoxime (dmg); (F) B + NaOH to pH ) 8. b MeOH was used instead of EtOH. c Solvent ) 1:1 EtOH/H2O.

Scheme 4

Scheme 5

equatorial plane19 to such an extent that insertions are shut down due to steric effects. Deuterium labeling studies were also performed in hopes of gaining additional information about the mechanism(s) involved in allene and diene formation. When the cobaloxime species was generated under basic conditions (method A) and treated with enyne (17) but NaBD4 was used in place of NaBH4, we saw little if any deuterium incorporation at the allene methyls. Under these basic conditions, the allene (27) most likely arises from an anionic intermediate (mechanism A). When an enyne (24) (which led to diene 37 only in high yield) was treated with cobaloxime generated under basic (method A) or near neutral (method B) conditions except (19) For reviews of structural aspects of cobaloxime and related complexes see: (a) Randaccio, L.; Bresciani Pahor, N.; Zangrando, E.; Marzilli, L. G. Chem. Soc. Rev. 1989, 18, 225. (b) Bresciani-Pahor, N.; Forcolin, M.; Marzilli, L. G.; Randaccio, L.; Summers, M. F.; Toscano, P. J. Coord. Chem. Rev. 1985, 63, 1. (c) Toscano, P. J.; Marzilli, L. G. Prog. Inorg. Chem. 1984, 31, 105.

the NaBH4 was replaced with NaBD4, we saw significant deuterium incorporation (35 and 29%, respectively) on the alkene cis to cobalt (43). These experiments

indicate that, at least a significant amount of, the diene formed must arise from alkyne insertion (mechanism C). H for D solvent exchange15 may account for the less than 100% deuterium incorporation, or cobaloxime anions may also be involved in diene formation to some extent. Formation of the 1,3-dienyl complexes via radical reactions of enynes with cobalt hydrides is also certainly a possibility.14,15 [4 + 2] Cycloaddition Reactions and Subsequent Demetalations. We have previously reported highly exo selective Diels-Alder reactions of dienyl complexes (31 and 32).1b,c We also reported that 33 isomerized

1,3- and 1,2-Dienylcobaloxime Complexes

Organometallics, Vol. 15, No. 11, 1996 2631

Scheme 6

over a period of hours to days (depending on solvent) at 25 °C or above to a 2-3:1 mixture of 31:33.1c Isomerization of 33 to 31 was faster than Diels-Alder reactions of 33 at 25 °C or above.1c At low temperatures where alkene isomerization is not occurring, 33 produced a 1:1 mixture of syn and anti diastereomers in [4 + 2] cycloadditions with maleic anhydride.1c Likewise, we found in the present study that dpg dienyl complex (34) isomerized to a 3:1 mixture of 32:34 in refluxing THF (8 h), and unlike 33, 34 even isomerized to this same alkene mixture slowly at 0 °C in CDCl3. Since 34 isomerized to 32 at 0 °C, and we had previously shown that 32 was less reactive than 31,1b we did not attempt [4 + 2] cycloadditions at low temperature using 34. Isomerization of 34 to 32 in refluxing THF was found to be faster than [4 + 2] cycloaddition with maleic anhydride as we had previously noted for dmg complex 33.1c The 2:1 mixture of dienes 35 and 36 (entry 12, Table 2) and pure 36 were treated with the very reactive dienophile tetracyanoethylene (TCNE) (44), in toluene at 60 °C for 1 h (Scheme 6). The relative stereochemistries of the products (45:46) from the reaction of the 35/36 mixture were proven by subsequent demetalation to cyclohexenes (47 and 48) which had been previously characterized.7 Complex 36 reacted quite cleanly with TCNE (44) to produce a 12.5:1 mixture of 46:45 (96%), the relative stereochemistry of which was again proven by AlMe3-mediated demetallation to a 10:1 mixture of 48:47 (31%).7 As with our previously reported AlMe3 demetalations, these reactions also provide cobalt recovery in the form of pyr(dmg)2CoMe (53) (93 and 72%, respectively) which can be recycled into these cobaloxime syntheses.1b,c Less reactive dienophiles were more challenging in reactions with 35 and 36. Complex 36 did not react with neat methyl vinyl ketone at 60 °C over 72 h; instead isomerization to a 2:1 mixture of 35: 36 was observed. The mixture 35:36 will react with maleic anhydride at 70 °C in THF, but a poor diastereomeric ratio of presumed cycloadducts was isolated but not characterized. The alkene isomerization noted above is presumably mediated by the dmg OH protons at high temperature. Complex 37 proved to be quite thermally robust (95% recovery after 3 days in a sealed tube at 60 °C in THF) and not prone to the rearrangements noted previously for its organic counterpart, 2-methyl-2,4-pentadiene.8 Long reaction times (3 days at 60 °C in a sealed tube in THF) are required, but diene 37 will react with maleic anhydride (49) to yield the expected product (50) in 73% yield. Increasing the temperature to 90 °C and shorten-

ing the reaction time to 16 h resulted in some decomposition as well as the isolation of some rearranged adduct (51) (28% total yield, 10.5:1 50:51). Small amounts of the rearranged adduct (51) were also noted at the lower temperature (60 °C) at even longer reaction times (96 h). Complex 50 can be cleanly isomerized to 51 by photolysis in THF or MeI (93%). The isomeric composition and ring junction stereochemistries of both 50 and 51 were assigned using a combination of 1H NMR and demetallation experiments. COSY experiments were first used to assign the alkene position in 50 and 51. The 1H doublet (2.77 ppm, J ) 9.6 Hz) assigned to HA in 50 was coupled to HB (3.19

ppm, ddd, J ) 9.6, 8.6, 2.9 Hz). HC and HD were double doublets showing geminal coupling (J ) 17.0 Hz) as well as the expected couplings to HB. The 1H doublet (2.69 ppm, J ) 9.5 Hz) assigned to HA in 51 was coupled to HB (3.67 ppm, dd, J ) 9.5, 3.7 Hz). HB was also coupled to the alkene proton (HC, 5.40 ppm, dd, J ) 3.7, 1.7 Hz). HC exhibited allylic coupling to one of the diastereotopic methylenes. The diastereotopic methylenes (HD and HE) showed the expected geminal coupling (J ) 19.9 Hz). The ring junction coupling constants in 50 and 51 (9.6 and 9.5 Hz) are consistent with reported coupling constants for the cis ring junction protons in related ring systems.20 Cleavage of the cobalt-carbon bonds in 50 and 51 was achieved in THF using AlMe3 to introduce a proton in place of the cobalt substituent (52 and 54). Cobalt was recovered as the cobalt methyl complex (53) (Scheme 7). Terminally trisubstituted diene (38) proved thermally sensitive and unreactive toward maleic anhydride and methyl vinyl ketone. When 38 was heated in a sealed tube to 70 °C with these dienophiles in THF, only small amounts (ca. 10%) of unreacted 38 was recovered. Complex 39 was more thermally stable but equally unreactive toward these dienophiles at temperatures up to 120 °C in sealed tubes. Conclusions A variety of enynes have been shown to react with cobaloxime anions or hydrides to yield cobaloxime(20) (a) Yeh, M.-C. P.; Sheu, B. A.; Fu, H. W.; Tau, S. I.; Chuang, L. W. J. Am. Chem. Soc. 1993, 115, 5941. (b) Berbube, G.; Deslongchamps, P. Bull. Soc. Chim. Fr. 1987, 103. (c) Brown, G. M.; Dubreuil, P.; Demers, E. P. Can. J. Chem. 1968, 46, 1849. (d) Lo Cicero, B.; Weisbuch, F.; Dana, G. J. Org. Chem. 1981, 46, 914.

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Organometallics, Vol. 15, No. 11, 1996 Scheme 7

substituted 1,2- or 1,3-dienes depending on the enyne’s substitution pattern. Most enynes reacted to produce 1,3-dienylcobaloxime complexes. (Z)-dienylcobaloxime complexes were found to isomerize to (E)-dienylcobaloxime complexes under mild conditions. Many of the (E)-dienyl complexes react with dienophiles in exo selective Diels-Alder reactions. Substitutents in the 3 position of a 2-cobalt-substituted-1,3-diene (39) shut down [4 + 2] cycloaddition reactivity as did substituents on both of the diene termini “inside” positions of the s-cis conformation (38). 1,3-Dienylcobaloxime complexes

Stokes and Welker

which are terminally disubstituted (37) will participate in [4 + 2] cycloaddition reactions but terminally trisubstituted 1,3-dienyl complexes (38) decompose thermally faster than they participate in cycloadditions. Work in progress on cobalt dienyl complexes is aimed at replacing the glyoxime equatorial ligands with salen ligands to hopefully solve the Z to E isomerizations noted here. Acknowledgment. We thank the National Science Foundation (Grant CHE-9321454), the donors of the Petroleum Research Fund, administered by the American Chemical Society, the Exxon Education Foundation, and the Camille and Henry Dreyfus Foundation (Henry Dreyfus Teacher-Scholar Award to M.E.W, 1994-99) for their support. Low-resolution mass spectra were obtained on an instrument purchased with the partial support of the NSF (Grant CHE-9007366). The Nebraska Center for Mass Spectrometry (NSF Grant DIR9017262) performed high-resolution mass spectral analyses. H.L.S. acknowledges Sigma Xi for a student grant-in-aid of research. OM960058F