J. Org. Chem. 2000, 65, 2863-2869
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Further Studies of the Thermal and Photochemical Diels-Alder Reactions of N-Methyl-1,2,4-triazoline-3,5-dione (MeTAD) with Naphthalene and Some Substituted Naphthalenes Gary W. Breton* and Kristy A. Newton Berry College, Department of Chemistry, P.O. Box 495016, Mount Berry, Georgia 30149-5016 Received April 19, 1999
MeTAD thermally reacted with naphthalene (2) and methylated naphthalenes to give equilibrium mixtures of starting materials and [4 + 2] cycloadducts. Methyl substitution on the naphthalene ring generally increased both the amount of cycloadduct formed and the rate of cycloaddition relative to 2. The isolated cycloadducts were all thermally labile and quantitatively reverted to the parent naphthalene in the presence of 2,3-dimethyl-2-butene as a trap for liberated MeTAD. The rates of the cycloreversion reactions were affected by substitution patterns but not appreciably by solvent. A mechanism for the cycloaddition reaction is presented that proposes the involvement of a chargetransfer complex. Photochemically, MeTAD demonstrated lower regioselectivity in its reactions with substituted naphthalenes relative to the corresponding thermal reactions. Several years ago Sheridan described the photochemical reaction of MeTAD (1) with a number of aromatic compounds including benzene and naphthalene (2).1,2 The reactions with benzene and 2 afforded novel Diels-Alder cycloadducts (eq 1) that have served as precursors to several theoretically interesting molecules.2-4
While more reactive in a Diels-Alder sense than benzene, naphthalenes are generally poor diene components in the classical Diels-Alder cycloaddition reaction.5 Cycloaddition has been reported, therefore, only with strong dienophiles such as maleic anhydride and singlet oxygen in addition to MeTAD (eq 2).6,7 Herzog and co-
workers reported that equilibria were established between starting materials and Diels-Alder cycloadducts when naphthalene and substituted naphthalenes were heated (typically 100 °C for 24 h) in the presence of excess maleic anhydride.6 Cycloadducts were generally formed in poor yields with simple mono- and disubstituted naphthalenes (1-47%),6a,b but higher conversions were reported at high pressures (∼9000 atm) or with polyalkylated naphthalenes.6c,d The cycloadducts reverted to starting materials upon heating. Singlet oxygen does not react with 2, but it does react readily with naphthalene compounds that are at least disubstituted with alkyl groups (e.g., 1,4- or 1,8-dimethylnaphthalene).7 While the photochemical reactions of MeTAD with 2 and some substituted naphthalenes have already been reported,1,3,4 the corresponding thermal reactions of triazolinediones with naphthalenes have only been briefly mentioned in the literature.4 Herein we provide a more detailed report on the thermal and photochemical reactions of MeTAD with 2 and a series of methylated naphthalenes (compounds 3-9 in Chart 1). We also report on our studies of the thermal cycloreversions of the [4 + 2] adducts. Results 1. Thermal Reactions of MeTAD with Naphthalene and Substituted Naphthalenes. A. Reaction with Naphthalene (2). When MeTAD is mixed with 2 in CDCl3 (initially 0.40 M each) at 29 °C in the absence
* To whom correspondence should be addressed. e-mail: gbreton@ berry.edu. office: (706)-290-2661. fax: (706)-238-7855. (1) (a) Kjell, D. P.; Sheridan, R. S. J. Am. Chem. Soc. 1984, 106, 5368. (b) Kjell, D. P.; Sheridan, R. S. J. Photochem. 1985, 28, 205. (2) Hamrock, S. J.; Sheridan, R. S. J. Am. Chem. Soc. 1989, 111, 9247. (3) Kjell, D. P.; Sheridan, R. S. J. Am. Chem. Soc. 1986, 108, 4111. (4) Burger, U.; Mentha, Y.; Thorel, P. J. Helv. Chim. Acta 1986, 69, 670.
(5) Biermann, D.; Schmidt, W. J. Am. Chem. Soc. 1980, 102, 3163. (6) (a) Kloetzel, M. C.; Herzog, H. L. J. Am. Chem. Soc. 1950, 72, 1991. (b) Kloetzel, M. C.; Dayton, R. P.; Herzog, H. L. J. Am. Chem. Soc. 1950, 72, 273. (c) Plieninger, H.; Wild, D.; Westphal, J. Tetrahedron 1969, 25, 5561. (d) Oku, A.; Ohnishi, Y.; Mashio, F. J. Org. Chem. 1972, 37, 4264. (7) (a) Wasserman, H. H.; Larsen, D. L. J. Chem. Soc., Chem. Commun. 1972, 253. (b) Adam, W.; Prein, M. Acc. Chem. Res. 1996, 29, 275. (8) For comparison, Burger et al. (ref 4) reported a value of Keq ) 0.4 for the reaction of N-phenyl-1,2,4-triazoline-3,5-dione with naphthalene at 25 °C in CDCl3.
10.1021/jo9906429 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/27/2000
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J. Org. Chem., Vol. 65, No. 10, 2000
Breton and Newton
Chart 1
Table 1. Reaction of MeTAD with Symmetrically Substituted Naphthalenesa cmpd
solvent
Keq b
yield of cycloadduct, %c
2
CDCl3 C6D6 CD3CN (CD3)2CO CDCl3 (CD3)2CO CDCl3 (CD3)2CO CDCl3
2.0 0.3 0.1 0.1 2.8 0.1 17.7 0.2 351
34 (21) 10 4 4 40 3 69 (63) 7 89 (69)
3 4 5 a
Conducted at 29 °C with initial concentrations of naphthalene and MeTAD at 0.40 M each. b Determined by relative integrations of appropriate signals in the 1H NMR spectrum. Values are reliable to within (5% c Determined by 1H NMR spectroscopy. Values in parentheses are isolated yields.
of light, an equilibrium is established within 18 h in which a 34% yield (Keq ) 2.0) of cycloadduct 2C (where C denotes a [4 + 2] cycloadduct with MeTAD) is formed as the only product (eq 3 and Table 1).8 No change in
the equilibrium ratio was observed even after an additional 7 days of reaction time. The equilibrium constant for the reaction was determined at a series of temperatures over the range 26-50 °C. A van’t Hoff plot (Figure 1) afforded ∆H° ) -44 kJ/mol and ∆S° ) -141 J/mol. The identity of the reaction solvent had a strong impact on the extent of the reaction (Table 1). Reaction was observed to readily take place in nonpolar (e.g., benzene) and weakly polar (e.g., CDCl3) solvents, but strongly polar solvents (e.g., acetone) discouraged adduct formation. Adduct 2C quantitatively reverted to starting material (as a solution in CDCl3) in the presence of 2,3-dimethyl2-butene as a trap for liberated MeTAD.9 B. Reaction of MeTAD with Symmetrically Substituted Methylated Naphthalenes. Reactions of MeTAD with symmetrically substituted naphthalenes 3-5 to afford cycloadducts 3C-5C were conducted in the dark at 29 °C in CDCl3, and were followed by periodic monitoring of the reaction using 1H NMR spectroscopy. The results are summarized in Table 1. The reactions were monitored at least 24-48 h past the time required (9) The alkene 2,3-dimethyl-2-butene reacts rapidly and quantitatively with MeTAD in an ene reaction to afford a relatively inert urazole product. See: Ohashi, S.; Leong, K.-W.; Matyjaszewski, K.; Butler, G. J. Org. Chem. 1980, 45, 3467.
Figure 1. van’t Hoff plot for the reaction of MeTAD with naphthalene (2). Table 2. Thermal Reaction of MeTAD with Unsymmetrically Substituted Naphthalenesa yield of cycloadduct, %c naphthalene
solvent
6
CDCl3 (CD3)2CO CDCl3 (CD3)2CO CDCl3 (CD3)2CO CDCl3 (CD3)2CO
7 8 9
Keq
b
2.1 0.1 4.5 0.1 12.8 0.2 110 0.8
CA
CB
10 d 6 d 58 (39) 8 86 (60) 21
29 5 43 (21) 4 6 d d d
a Conducted at 29 °C with initial concentrations of naphthalene compound and MeTAD at 0.40 M each. b Determined by relative integrations of appropriate signals in the 1H NMR spectrum. Values are reliable to within (5%. c Determined by 1H NMR spectroscopy. Values in parentheses are isolated yields. d No cycloadduct observed in the 1H NMR spectrum.
for attainment of the maximum yield of cycloadduct to ensure that an equilibrium had been established. Reaction of MeTAD with 1,5-dimethylnaphthalene (3) afforded an equilibrium mixture of starting materials and cycloadduct 3C with Keq ) 2.8 within 10 min. Reaction with 2,6-dimethylnaphthalene (4) required approximately 8 h to attain equilibrium although the exent of the equilibrium in favor of cycloadduct was much greater than that of compound 3. When the reactions of 3 and 4 were conducted in acetone-d6 as solvent, lesser equilibrium amounts of cycloadduct were observed. Reaction of MeTAD with 2,3,6,7-tetramethylnaphthalene (5) in CDCl3 afforded the greatest yield of cycloadduct (89%) observed with all of the naphthalenes investigated requiring approximately 5 h of reaction time to reach equilibrium. Cycloadducts 4C and 5C were isolated by quenching excess MeTAD in the reaction mixture with 2,3-dimethyl2-butene and isolating the product via preparative thinlayer chromatography. The adducts were thermally labile and reverted quantitatively back to starting naphthalene in the presence of 2,3-dimethyl-2-butene as a trap for the liberated MeTAD. Adduct 3C was especially labile, however, and resisted all of our efforts at its isolation employing preparative TLC as well as HPLC (see below). Rate studies on its cycloreversion indicated a half-life of approximately 9 min at 29 °C. A more detailed discussion of the cycloreversion process of the adducts is presented below. C. Reaction of MeTAD with Unsymmetrically Substituted Methylated Naphthalenes. Unsymmetri-
Diels-Alder Reactions of MeTAD
cally substituted naphthalenes (6-9) present two nonequivalent aromatic rings as possible sites of attack by MeTAD. For the purposes of discussion and comparison, cycloadducts CA and CB will be defined as the [4 + 2]-cycloadducts resulting from reaction at the substituted ring (1,4-addition) and the unsubstituted ring (5,8addition), respectively (eq 4). The results of these reac-
tions are summarized in Table 2. The reaction of MeTAD with 1-methylnaphthalene (6) in the dark at 29 °C in CDCl3 was followed by periodic monitoring of the reaction using 1H NMR spectroscopy. Within the first 10 min of reaction a 15% yield of cycloadduct 6CA was observed. As the reaction proceeded, however, the yield of 6CA dropped slightly while cycloadduct 6CB began to form. The yield of 6CB increased over the course of 24 h to a maximum yield of 29%, while 6CA was afforded with a final yield of 10%. No further change was observed with extended reaction times. The combined yield of cycloadduct corresponded to Keq ) 2.1 which was comparable to the equilibrium constant obtained with naphthalene itself (Keq ) 2.0), but slightly lower than that obtained with compound 3 (Keq ) 2.8). A lesser equilibrium amount of cycloadduct was observed when the reaction was conducted in acetone-d6. A similar trend was observed during the course of reaction of MeTAD with 1,4-dimethylnaphthalene (7). At very short reaction times (
