1242
J. Org. Chem. 1983,48, 1242-1246
chloride, 563-47-3; N-bromosuccinimide, 128-08-5; 1-chloro-3(phenylmethoxy)-2-propanol, 13991-52-1; 3-(benzyloxy)propene 1,2-epoxide, 2930-05-4; 2-[ (allyloxy)methyl]-15-crown-5,6816786-2; n-bromooctane, 111-83-1; l-chloro-3-(2-methoxyethoxy)propan-2-01,18371-79-4; 2-methoxyethanol, 109-86-4;3-(2-meth0xyethoxy)propene 1,2-epoxide, 13483-49-3; 3-(2-methoxyethoxy)-l,2-propanediol, 84131-01-1; l-chloro-3-[2-(2-methoxyethdiethylene glycol monomethyl oxy)ethoxy]propan-2-o1,84131-02-2;
ether, 111-77-3;3-[2-(methoxyethoxy)ethoxy]propane1,2-epoxide, 71712-93-1; 3-[2-(methoxyethoxy)ethoxy]-l,2-propanediol, 84131-03-3; 2-[2-[2-(benzyloxy)ethoxy]ethoxy]ethylp-toluenesulfonate, 84131-04-4. Supplementary Material Available: Spectral and analytical data for compounds in this paper (4 pages). Ordering information is given on any current masthead page.
Crown Cation Complex Effects. 21. Spectral Evidence Bearing on the Interaction between Arenediazonium Cations and 21-Crown-7in Nonpolar Solutions James R. Beadle, Raj K. Khanna, and George W. Gokel* Department of Chemistry, University of Maryland, College Park, Maryland 20742
Received August 4, 1982 15-Crown-5 does not complex arenediazonium tetrafluoroborates, but 21-crown-7 is known to complex them more strongly than does 18-crown-6. Despite this, the infrared and ultraviolet band shifts for complexed vs. noncomplexed forms are far smaller for the former than for the latter. It is suggested that instead of the crown completely and tightly surrounding the diazonio function, the crown collars (nearly encircling) the diazonio group and then uses the remaining donor atom(s) either to solvate the terminal nitrogen atom or interact as a base with the r-acidic aromatic ring, providing additional stability. Since the mode by which 18-crown-6 and 21-crown-7 solvate the diazonium ion in each case differs, the spectral manifestations of this interaction differ.
Since the original observation of arenediazonium cation complexation by crown ethers,' there has been considerable interest in the nature of the interaction,24 its effect on both the crown and cation: and utilization of crown-complexed arenediazonium cations as synthetic intermediates in a variety of reaction^.^-^ Much of this work has recently been reviewed.'O It was established in the first report of the arenediazonium cation-crown interaction that crown rings containing fewer than about 18 members do not complex the salt, but a variety of larger ones do.' In later work, these observations were quantitated by kinetic studies." It was shown, for example, that the rate of thermal decomposition (by the Schiemann reaction) of 4-tert-butylbenzenediazonium tetrafluoroborate was slowed more by 21-crown-7 than by any of the other macrocycles surveyed. A tenfold difference in decomposition rate was observed for the above reaction when 21-crown-7 was compared to the next lower homologue, 18-crown-6. This difference in reaction rate was correlated by Zollinger and co-workers4 to an (1) (a) Gokel, G. W.; Cram, D. J. J . Chem. SOC.,Chem. Commun. 1973,
481. (b) Kyba, E. P.; Helgeson, R. C.; Madan, K.; Gokel, G. W.; Tarnowski, T. L.; Moore, S. s.; Cram, D. J. J.Am. Chem. SOC.1977,99,2564. (2)Krane, J.; Skjetne, T. Tetrahedron Lett. 1980, 21, 1775. (3)Bartach, R. A.; Cirsky, P. J. Org. Chem. 1980,45, 4782. Szele, I.; Zollinger, H. Tetrahedron Lett. 1981,22, (4)Nakazumi, H.; 3053. (5) Korzeniowski, S. H.; Leopold, A.; Beadle, J. R.; Ahern, M. F.; Sheppard, W. A.; Khanna, R. K.; Gokel, G. W. J . Org. Chem. 1981,46, 2153. (6)Gokel, G. W.;Korzeniowski, S. H.; Blum, L. Tetrahedron Lett. 1977, 1633. (7) Korzeniowski, S. H.; Gokel, G. W. Tetrahedron Lett. 1977, 1637. (8)Korzeniowski, S. H.; Blum, L; Gokel, G. W. Tetrahedron Lett. 1977, 1871. (9) Korzeniowski, S. H.; Gokel, G. W. Tetrahedron Lett. 1977, 3519. (10)Bartach, R.A. Prog. Macrocycl. Chem. 1981,2, 1. (11)Bartsch, R. A.; Juri, P. N. J. Org. Chem. 1980,45, 1011.
0022-32631831 1948-1242$01.50/0
Table I. Comparison of Infrared Spectral Data of Crown Complexed Arenediazonium Salts crown (equiv)g solvent notes substituentb v N E N , ~cm-l 4-Me 4-Me 4-Me 4-Me 4-Me 4-Me0 4-Me0 4-Me0 4-Me0 4-t-BU 4-t-BU 4-t-Bu 4-t-BU 4-t-BU 4-t-BU 4-t-BU 4-n-BuO 4-n-BuO 4-n-BuO 4-n-BuO 4-n-BuO
2286 2283 2286 2287 2275 2247 2261 2262 2254 2277 2306 2271, 2306 2282 2286 2287 2278 2245 2245, 2294 2262 2262 2260
none 21-C-7 (1) 21-'2-7 (1) 2 1 4 - 7 (5) 24-C-8 ( 5 ) none 21-C-7 (1) 21-C-7 (1) 24-C-8 (5) none 18-C-6 (1) 18-C-6 (1) 21-C-7 (1) 21-C-7 (1) 214-7 (5) 24-C-8 ( 5 ) none 18-C-6 (1) 2 1 4 - 7 (1) 21-C-7 (1) 21-C-7 (1)
mull mull CHCI, CHCI, CHCI, mull mull CHCI, CHC1, mull mull CHCI, mull CHCI, CHCI, CHCI, CHCI, CHCI, mull
a C C
a C
c, d 4 C
c C C
c, e a
CHCI, C C CH,CI, a The solid complexes had the following melting points C): 3, 1 1 3-115: 4, 118-119; 5, 104-104.5;6, 78-80. Benzenediazonium Cetrafluoroborate salts with the indicated substituents. The indicated components (0.2 mmol/equiv) were stirred with solvent (0.5 mL) until solution occurred. The peak intensities were approximately 2271 (1):2308 (1.36). e The peak intensities were approximately 2245 (1):2294 (1). Calibrated against the 1601.8-cm-l line of polystyrene. "C" in the abbreviations means "crown".
r
approximately tenfold difference in stability constant for the respective complexes. Although the 21-crown-7 complex of a typical arenediazonium cation is certainly stronger 0 1983 American Chemical Society
J. Org. Chem., Vol. 48, No. 8,
Crown Cation Complex Effects Table 11. Comparison of
1983 1243
NMR Spectral Data of Crown Ether Complexed Arenediazonium Salts' chemical shiftsd
compd 1 1 1 2 2 2 3 3 3 3
3* 3* 3* 4 4 4 4 4
substituent 4-Me 4-Me 4-Me 4-Me0 4-Me0 4-Me0 4-t-Bu 4-t-Bu 4-t-BU 4-t-Bu 4-t-BU 4-t-BU 4-t-BU 4-n-BuO 4-n-BuO 4-n-BuO 4-n-BuO 4-n-BuO
crown (equiv)c
ipso
ortho
meta
para
18-C-6 ( 5 ) 2142-7 ( 1 ) 24-C-8 ( 5 ) 18-C-6 ( 5 ) 21-C-7 (1) 244-8 (5) none 18-C-6 (5) 2 1 0 7 (1) 24-C-8 ( 5 ) none 18-C-6 ( 1 ) 18-C-6 ( 5 ) none 18-C-6 (1) 18-C-6 ( 5 ) 21-C-7 (1) 24-C-8 ( 5 )
113.31 112.20 110.33 105.44 104.37 102.38 110.46 112.76 112.85 110.46 110.19 113.58 113.64 101.26 104.49 104.68 104.31 102.65
131.38 131.99 131.90 132.51 135.55 135.75 132.61 129.25 132.76 132.14 132.43 130.29 129.64 135.64 133.40 132.14 135.91 136.54
129.60 130.33 130.07 116.19 115.67 115.67 128.79 127.50 127.50 126.51 128.87 128.43 128.27 117.50 116.80 116.05 116.44 116.78
151.58 152.60 152.71 166.77 167.98 168.06 166.93 163.05 165.15 164.43 167.33 164.54 164.10 168.87 167.05 165.76 167.95 168.47
a Determined in CDCl,, except for those entries marked with an asterisk, which were in CH,Cl,. I , Substituent on All chemical shifts are in parts per "C" in abbreviations means "crown". benzenediazonium tetrafluoroborate. million (6 ) downfield from internal Me@.
than that with 18-crown-6, there is no clear evidence to suggest that the interaction between crown and salt is identical in both cases. We present the results of a study bearing on this question here.
Results and Discussion Our study of the 21-crown-7 complexes of several arenediazonium salts confirms the observations made by others that this crown is a stronger complexing agent for the salts than is 18-~rown-6.~~~9" Infrared spectra (see Table I) of CHC13-soluble4-n-butoxybenzenediazonium tetrafluoroborate (4) were obtained in that solvent. The absorbances in the triple-bond stretch region are (A) a single peak at 2245 cm-' for the free salt 4 and (B)two peaks at 2294 and 2245 cm-I (ratio ca. 2:l) for 4 in the presence of 1equiv of 18-crown-6, both in CHC13solution. When a single equivalent of 21-crown-7 is added to a CHC13 solution of 4, a single absorption at 2262 cm-' is observed. When the more polar CH2C12is substituted for CHC13 as solvent, a large peak at 2260 cm-' is observed which has a small shoulder (due to solvent) on the highfrequency side. The single absorbances observed in the latter two cases confirm the stronger complexation of arenediazonium cation by 21-crown-7. In B, it appears that about two-thirds of the salt is complexed. A more surprising observation is that when 4 is complexed by 18-crown-6,the triple bond absorption is moved to higher wavenumber by 49 cm-' while when complexed by 21-crown-7 the change is only 15-17 cm-'. There is no difference between the free and 21-crown-7-complexed forms of 4-methylbenzenediazonium tetrafluoroborate (1) either when mulled in mineral oil or dissolved in CHC13. The same trend is observed in the ultraviolet spectra. Uncomplexed 4 has a maximum at 318 nm (CHCl,) and is shifted to 303 nm by addition of 1equiv of 18-crown-6 but only to 309 nm by an equivalent amount of 21-crown-7. Finally, the ipso carbon atom resonance of 4 (see Table 11) is observed at essentially the same frequency when either crown is added to the salt. All of the spectral manifestations of the crown-cation interaction seem to suggest that 21-crown-7 interacts more weakly with arenediazonium cations than does 18-crown-6. As noted above, there is convincing evidence to the contrary on this point. Since the interaction of the larger crown is clearly not weaker than that with the smaller
crown, it must somehow be different. Compelling evidence has accumulated which suggests that in the case of 18-crown-6at least, the arenediazonium cation inserts itself into the crown hole.'JO The crown "collars" the "neck" of the diazonium salt. Although no crystal structure of the crown complex has appeared, an incompletely refined structure has been obtained which clearly confirms this expectation.'2 In their CNDO/2 calculations, Bartsch and Cirsky used three dimethyl ether molecules to approximate three of the crown's donor group^.^ The choice of three donors was undoubtedly made to simplify the calculation, but it is not clear what relevance the known Dsd conformation observed with potassium salts may have to the present system, although it is noted by these authors. Even with only three oxygen atom donors arranged in a triangle about the diazonio function, evidence for electrostatic stabilization was obtained.3 Although it would be most convincing to have X-ray crystal structure data, the difficulties experienced by Haymore in pursuit of structures of these complexes suggest that other, if less satisfactory, methods must be employed.12 We have utilized C-P-K space-filling molecular models in the analysis which is presented below. From the crystal structure of benzenediazonium chloride, one can judge the cylindrical diameter of the N-N bond to be about 2.4 A.13 18-Crown-6 appears to have a minimum hole size of 2.6 A, and 21-crown-7 is obviously larger. The binding of 21-crown-7 might be expected to be stronger on the basis of a larger number of donor groups, but the stabilization is not as great with still larger crowns,'' so the simple number of donor groups cannot hold the entire answer. Bartsch has speculated that although the 21-membered ring might seem too large for effective interaction, "the greater ring flexibility may allow for relief of steric interactions between the ortho hydrogens of the benzenediazonium cation and the macrocyclic ring"." Evidence for such a conclusion might be obtained from proton NMR, but the ortho hydrogens of 4-tert-butylbenzenediazonium tetrafluoroborate differ in Me2SO-d6 solution by only 0.07 ppm in the presence of 18-crown-6 compared to the noncomplexed state.'O Futhermore, one (12) Haymore, B. C., personal communication, 1981. (13) Rsmming, C. Acta Chem. Scand. 1959,13, 1260.
Beadle, Khanna, and Gokel
1244 J. Org. Chem., VoZ.48,No. 8, 1983
might expect the larger and more flexible 24-crown-8 to be an even more accommodating binder and hold the cation with correspondingly greater strength. Krane and Skjetne2 have shown by dynamic NMR studies that the free energies of association between 4-methylbenzendiazonium cation and crowns are in the order 21-crown-7 > 18-crown-6 > 24-crown-8. In addition, dicyclohexano-24crown-8 is less stabilizing in the Schiemann reaction than the corresponding 21-membered ring and about the same as l8-crown-6. On the basis of our examination of C-P-Kmolecular models, we believe that an important difference between 18-crown-6 and 21-crown-7 is the latter's inability to assume the planar D M conformationwhich is well-known for 18-crown-6 and its derivatives. Judging from models of both 21-crown-7 and 24-crown-8, there is a noticable pucker in the ring resulting from the conformational requirements of the backbone ethylenes. Since the strongest solvation of the diazonio group will arise from the closest contact between the oxygen donors and the positive charge, it seems reasonable to assume that one of the CH2-O-CH2 units in 21-crown-7 will turn upward and away from the mean plane of the other oxygens. This will present to the diazonium ion a cavity of essentially the same size as that found in 18-crown-6 and required for the cation.I3 The remaining oxygen donor group can now interact with other electrophilic centers to provide additional stabilization to the overall complex. One possible interaction of this type is shown in eq 1.
i
In this, the remaining ether donor bonds to the terminal nitrogen of the diazonium cation bound by the other six oxygen donors. This presumably induces a rehybridization frm sp to sp2and places a positive charge on the oxygen donor group bound to nitrogen. Although the "complex" proposed in eq 1has some appealing aspects to it, there are also some possible objections. First, there is the obvious question of why the 21crown-7 complex should not simply involve a flat macroring having more donor groups arranged symmetrically about the diazonio group, albeit a t a greater distance between charge and electron pair. We have assembled models of the 21-crown-7 complexes in both forms (see Figure 1,middle and bottom) and believe that the complex represented by eq 1 (Figure 1, bottom) is of lower energy than that corresponding to the known complex with 18crown-6 (Figure 1, top). When the ethylene gauche interactions are minimized in the crown backbone, the seventh CH20CH2unit in the ring creates a strained situation compared to 18-crown-6. If this oxygen puckers upward, it might solvate the nitrogen terminus of the diazonium cation. In so doing, a resonance structure such as shown in eq 1would result. The lack of a substantial infrared shift for a complexing agent known to be stronger than l8-crown-6 might be due to the change from a triple bond to a cumulated double bond as ~ h 0 w n . l ~ In the resonance structure illustrated in eq 1,the terminal nitrogen is rehybridized, but there is no nuclear motion within the diazonium ion. Where there is a change in nuclear positions, this possibility would obviously be (14) Bellamy, C. J. "Advance8 in Infrared Group Frequencies"; M e thuen: London, 1968;Chapter 3.
t
Figure 1. C-P-KMolecular models of possible complexes between 4-methylbenzenediaium cation and crown ethers: top, complex with 18-crown-6, simple insertion model; middle, complex with 21-crown-7, simple insertion model; bottom, complex with 21crown-7, according to eq 1.
excluded from consideration. This terminal nitrogen solvation would nicely explain the special strength of the 21-crown-7 complex, and since an isolated triple bond becomes a cumulated double bond, the infrared data are accommodated14 as well. Notice also that there is a negative charge present on the para carbon in the aromatic ring. If the present structure is credible, the para carbon must show some increase in electron density relative to the uncomplexed state. In all of the compounds whose 13C NMR spectral data are shown in Table 11, the para carbon is more shielded when complexed by 21-crown-7 than when uncomplexed, although in some cases the shift is small. These shifts generally tend to be smaller than those observed when the same arenediazonium cation is complexed by 18-crown-6, and while this does not discredit eq 1, it makes the uniqueness of eq 1 less convincing. An alternative structure for the complex which forms between an arenediazonium cation and 21-crown-7 is shown in eq 2. Notice that this differs from that shown in eq 1only by the fact that the "puckered" oxygen is now turned inward (over the ring) rather than outward (see above). The same interaction between crown and cation as seen with 18-crown-6 is now possible, but the oxygen
J. Org. Chem., Vol. 48, No. 8, 1983
1245
Table 111. Comparison of UV Spectral Data of Crown-Complexed Arenediazonium Saltsa compd 1
mav overlav the electron-deficient aromatic parkpate & a donor-acceptor complex. In the first ieport of arenediazonium cation to crown interactions,l colored solutions resulted from the interaction of colorless naphthalene crowns and colorless arenediazonium salts. This color was attributed to a T-T interaction whereas the present situation would be a u base to 7~ acid interaction. The structure of eq 2 also predicts shielding of the para carbons, in which sense it is consistent with the discussion above. It is not clear, however, why the infrared shifts should be so much smaller in this case than in the case of 18-crown-6since the mechanism of stabilization proposed here is essentially similar to that invoked for the smaller ring systems. In conclusion, we must say that although each of the structures above has some appeal, neither is supported by enough evidence t o say it is the unique structure for such complexes. W e favor the former structure because of the arguments above and because of the known reduced reactivity of crown-complexed arenediazonium salts toward nucleophiles.
Summary We believe that the arenediazonium cation complexes are m u c h more variable in precise structure than has heretofore been suspected. It seems clear that a principal interaction is multiple-oxygen solvation of the positively charged a-nitrogen atom, but such solvation m a y involve more than one resonance form. We believe that the greater stabilization of arenediazonium cations by 21-crown-7 molecules compared to that provided by the smaller crowns is probably due to a combination of a-nitrogen solvaton and stabilization either of the electron deficient aromatic ring or of the @-nitrogen at the same time by Lewis base donation from the polyether oxygen used in collaring the -Nz+ group.
Experimental Section Instruments used included a Meltemp capillary device, a Perkin-Elmer Model 281 infrared spectrophotometer (calibration: 1601.8-cm-' band of polystyrene), and a Varian Associates Model EM-360 NMR spectrometer (ca. 15% solutions in CDC13). Combustion analyses were performed by F. Kasler of the University of Maryland. Solvents used were AR grade. Tetrahydrofuran (THF) was distilled from lithium aluminim hydride before use. AR grade CHC13was washed with concentrated H@04 and water, dried over Na2S04,and distilled prior to use. 18-Crown-6 was obtained from Union Carbideels Tri- and tetraethylene glycol ditosylates were prepared from tri- and tetraethylene glycol.'6 Arenediazonium Salts. The arenediazonium tetrafluoroborates were prepared by the method of Roe.16 Compounds 1-3 were prepared from commercially available anilines. 4-n-Butoxyaniline16was diazotized to give 4. All arenediazonium salts were reprecipitated from acetone-ether and air-dried immediately prior to use. 21-Crown-7. A 500-mL flask was charged with dry THF (1 L) and NaH (20.2 g, 0.84 mol) and heated to reflux. A solution of both tetraethylene glycol (81.4 g, 0.42 mol) and triethylene glycol ditosylate (192 g, 0.42 mol) in THF (1L) was added dropwise (12 h). After being stirred 12 h, the mixture was cooled and filtered. (15) We thank Dr. Leonard Kaplan of Union Carbide for a gift of 18-crown-6. (16) Dale, J.; Kristensen, P. 0. Acta. Chem. Scand. 1972, 26, 1471.
2
3 4
substituentb
max
kmnY -..I_ (18-(2-6)' .
4-Me 278 4-Me0 315 4-t-BU 280 4-n-BuO 318
268 300 270 303
Xmax
hmax
(21-C-7)'
(24-C-8)'
271 307 270 309
276 3 12 2 80 313
a Approximately 2.2 X M salt and 3.13 X M Refers crown in CHCl,; peak positions in ranometers. to benzenediazonium tetrafluoroborate. ' "C" in the abreviations means "crown".
The filtrate was reduced in volume and chromatographed (alumina, 0-2% propanol/hexanes) to give a pale yellow oil which was distilled [Kugelrohr, 180-200 "C (0.1 torr)] to afford 21crown-7 as a colorless liquid: 18.7 g (14%); NMR 6 3.73 (s); 13C NMR 6 70.73. 24-Crown-8 was prepared by the method described for 21crown-7. Addition over 12 h of a tetraethylene glycol solution (116 g, 0.60 mol) and tetraethylene glycol ditosylate (300 g, 0.60 mol) in THF (800 mL) to a slurry of NaH (29.7 g, 1.19 mol) in refluxing THF (1L) gave, after column chromatography (alumina, 0-2% 2-propanol/hexanes) and distillation [Kugelrohr, 185-200 "C (0.1 torr)], 24-crown-8 as a colorless liquid: yield 17.5 g (8%); lH NMR 6 3.67 (8); 13C NMR 6 70.39. 4-Methylbenzenediazonium Tetrafluoroborate/21Crown-7 1:l Complex (21-(3-7.1). 4-Methylbenzenediazonium tetrafluoroborate (206 mg, 1 mmol) and 21-crown-7 (308 mg, 1 mol) were dissolved in CH2C12(3 mL). After filtration, slow addition of EbO (10 mL) with stirring precipitated the solid white complex which was collected by filtration: yield 450 mg (80%); mp 113-115 "C; IR (Nujol) 2283 cm-'. Anal. Calcd for C21H36N07BF4:C, 49.04; H, 6.86; N, 5.45. Found: C, 48.79; H, 6.88; N, 5.52. 4-Methoxybenzenediazonium Tetrafluoroborate/21Crown-7 1:l Complex (21-C-7.2). 4-Methoxybenzenediazonium tetrafluoroborate (222 mg, 1 mmol) and 21-crown-7 (308 mg, 1 mol) were dissolved in CH2C12 (3 mL) and the complex was precipitated by addition of E h O yield 495 mg (89%); mp 118-119 OC; IR (Nujol) 2261 cm-'. Anal. Calcd for C21H35N208BF4: C, 47.56; H, 6.65; N, 5.28. Found: C, 47.16; H, 6.71; N, 5.38. 4-tert -Butylbenzenediazonium Tetrafluoroborate/%lCrown-7 1:l Complex (21-C-7.3). 4-tert-Butylbenzenediazonium tetrafluoroborate (264 mg, 1 mmol) and 21-crown-7 (308 mg, 1 mmol) were dissolved in CH2C12 (3 mL). The complex was precipitated by addition of Et20: yield 528 mg (95%); mp 104-104.5 "C; IR (Nujol) 2282 cm-'. Anal. Calcd for C24H41N207BF4: C, 51.81; H, 7.43; N, 5.03. Found: C, 51.87; H, 7.64; N, 5.13. 4-n -Butoxybenzenediazonium Tetrafluoroborate/21Crown-7 1:1Complex (21-C-7.4). 4-n-Butoxybenzenediazonium tetrafluoroborate (264 mg, 1 mmol) and 21-crown-7 (308 mg, 1 mmol) were dissolved in CH2C12(3 mL). Et20 was slowly added until the solution became cloudy, and then it was cooled in an acetoneldry ice bath until precipitation began. Et20 was added to complete the precipitation, and then the complex was collected by filtration: yield 440 mg (77%); mp 78-80 "C; IR (Nujol) 2262 cm-'; Anal. Calcd for C, 50.37; H, 7.21; N, 4.89. Found: C, 50.65; H, 7.11; N, 4.69. IR Studies. The 1:l complexes were mulled in Nujol and the 2400-2200-cm-' region of their IR spectra recorded by using an expanded scale. Solution spectra were obtained by stirring the arenediazonium salt (0.2 mmol) and the appropriate amount of crown ether in CHC13 (0.5 mL) until solution occurred. A sample of the homogeneous solution was placed in a solution IR cell (NaCl, 0.025" path length) and the spectrum recorded as above. UV Studies. The absorbance spectra of CHC13solutions of the diazonium salts (2.2 X lo4 M) and crown ethers (3.13 X M) were recorded on a Hitachi 100-80A W-vis spectrophotometer (Table 111). Solutions of 2142-7-3and 21-C-7.4 could be prepared directly by dilution of stock solutions of known concentrations. Arenediazonium salts 21-C-7.1 and 21-C-7.2 are only slightly soluble in CHC13, so saturated solutions of each were prepared and diluted until their absorbances at A,, were approximately
1246
J. Org. Chem. 1983,48, 1246-1250
equal to those of solutions 214-7.3 and 21-C-7.4. 13C NMR Studies. Samples were prepared by stirring the arenediazonium salt (0.5 mmol) with the appropriate amount of crown ether in CDC13 until solution occurred. I3C NMR spectra were recorded on a Varian XL-100 NMR spectrophotometer in the 'H fully decoupled mode. CDC13 provided an internal deuterium lock.
Acknowledgment. We warmly thank the National Institutes of Health for support of this work through Grants GM-26990, GM-29150, and GM-29610. Registry No. 4-Methylbenzenediazonium tetrafluoroborate, 459-44-9; 4-methoxybenzenedianium tetrafluoroborate, 459-64-3; 4-tert-butylbenzenediazonium tetrafluoroborate, 52436-75-6; 4-butoxybenzenediazonium tetrafluoroborate, 76832-54-7; 21-
crown-7/4-methylbenzenediazoniumtetrafluoroborate complex, 85048-77-7;24-crown-8/4-methylbenzenediazonium tetrafluoroborate complex, 85048-78-8; 21-crown-7/4-methoxybenzenediazonium tetrafluoroborate complex, 80440-74-0; 24-crown-8/4methoxybenzenedimnium tetrafluoroborate complex, 8504879-9; 18-crown-6/4-tert-butylbenzenediazoniumtetrafluoroborate complex, 65791-39-1; 21-crown-7/4-tert-butylbenzenediazonium tetrafluoroborate complex, 85048-80-2; 24-crown-8/4-tert-butylbenzenediazonium tetrafluoroborate complex, 85048-81-3; 18-crown-6/4butoxybenzenzenediazonium tetrafluoroborate complex, 85048-82-4; 21-crown-7/4-butoxybenzenediazonium tetrafluoroborate complex, 85048-83-5; 18-crown-6/4-methylbenzenediazonium tetrafluoroborate complex, 63281-55-0; 18-crown-6/4methoxybenzenedimnium tetrafluoroborate complex, 74317-32-1; 24crown-8/4butoxybe~nzenediazonium tetrafluoroborate complex, 85048-84-6;tetraethylene glycol, 112-60-7;triethylene glycol ditosylate, 19249-03-7; tetraethylene glycol ditosylate, 37860-51-8.
Structural Effects in Photoepoxidation Sensitized by a-Diketones Edward L. Clennan,*la David R. Speth,lb and Paul D. Bartlett*lc Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071,and Department of Chemistry, Texas Christian University, Fort Worth, Texas 76129 Received August 6, 1982
A series of a-diketones and benzils were examined for their effectiveness as sensitizers in the photoepoxidation reaction. The reduction potentials of these a-diketones were also determined at a hanging-mercury-drop electrode. The reduction potentials of the p,p'-disubstituted-benzils were successfully correlated to the s u m of the Hammett u values. No evidence could be found that electron transfer plays an important role in the epoxidation mechanism. The mechanistic possibilities are briefly discussed. Studies of the photoepoxidation reaction in a variety of solvents have also shown that the reaction is very insensitive to solvent effects.
In 1976, Bartlett and Shimizu2 reported an a-diketone-photosensitid epoxidation reaction. Prior to their report, several examples3 of photooxidations, which resulted in concurrent formation of epoxides and singlet oxygen-like products, had appeared in the literature. The a-dicarbonyl-sensitized reaction produces high yields of epoxides and is a preparatively useful photoepoxidation. Despite the high yield of epoxides, this reaction suffers from competing processes that include (a) energy transfer to oxygen, resulting in excited singlet oxygen, (b) energy transfer to the olefin, leading to isomerization, (c) direct oxidation of the a-diketone, and (d) addition of excited ketone to the olefin to form an oxetane. The plurality of these competing reactions has made it difficult to isolate the photoepoxidation reaction for a mechanistic study. Two attractive mechanistic hypotheses, which take advantage of the unique chemical and physical properties of a-diketones to explain the efficiency of these photoep-
of the demonstrated importance of electron transfer6>'in the initiation of photooxidation under conditions close to those of singlet-oxygen formation. In this report we describe the electrochemical and spectral properties of a series of p,p'-disubstituted-benzils and the Leonard series of tetramethylated a-diketones.s We also present qualitative and quantitative data on the use of these a-diketones in the photoepoxidation reaction and discuss our results in light of the two mechanisms presented above.
Electrochemistry The one-electron-reduced products of a-diketones, the semidiones, have been examined extensively by ESR.g These semidiones can be easily produced by a variety of methods including electrochemical and chemical reduction of a-diketones. Surprisingly, the electrochemical studies of this interesting redox couple are quite limited. The
oxidation sensitizers, are (1)single-oxygen-atom donation
to the carbon-carbon double bond of olefiis by acylperoxy or aroylperoxy radicals, which are easily formed from adiketones? and (2) a photoepoxidation initiated by electron transfer t o the a-diketone to produce the easily formed ~emidione.~The second mechanism is attractive in view (1) (a) University of Wyoming. (b) Dow Chemical Company, Midland, MI. (c) Texas Christian University. (2) Shimizu, N.; Bartlett, P. D. J. Am. Chem. SOC.1976, 98, 4193. 1974,96,627. (b) (3) (a) Bartlett, P. D.; Ho, M. S. J.Am. Chem. SOC. Jefford, C. W.; Boschung, A. F. H e h . Chim. Acta 1977, 60, 2673. (c) Bartlett, P. D. Chem. SOC.Reu. 1976,5, 154. (4) (a) Gream, G. E.; Paice, J. C.; Ramsay, C. C. R. Aust. J. Chem. 1969, 22, 1229. (b) Gream, G. E.; Paice, J. C. Ibid. 1969, 22, 1249.
( 5 ) Russell, G. A.; Osuch, C. E.; Sentore, G.; Mouta, T.; Yomashito, M. J. Org. Chem. 1979, 44, 2780. (6)(a) Eriksen, J. Fwte, C. S.; Parker, T. L. J.Am. Chem. SOC.1977, 99,6455. (b) Spada, L. T.; Foote, C. S. Ibid. 1980,102,391. (c) Manning, L. E.; Eriksen, J.;Fwte, C. S. Ibid. 1980,102,4275. (d) Eriksen, J.; Fmte, C. S. Ibid. 1980, 102, 6083. (7) (a) Barton, D. H. R.; Leclerc, G.; Magnus, P. D.; Nenzies, I. D. J. Chem. SOC., Chem. Commun. 1972, 447. (b) Brown-Wensley, D. A,; Mattes, S. L.; Farid, S. J. Am. Chem. SOC.1978, 100, 4162. ( c ) Schaap, A. P.; Zaklika, K.A.; Kaskar,B.; Fung, L. W.-M. Ibid. 1980,102,389. (d) Chem. Commun. 1980. 457. (e) Mattes. S. L.: Farid. S. J. Chem. SOC.. Mattes; S. L.; Farid; S. J. Am. Chem.'Soc. 1982, 104, 1454. (8) Leonard, N. J.; Latinen, H. A.; Mottus, E. H. J. Am. Chem. SOC. '
1953, 75, 3300. (9) Russell, G. A,; Ballenegger, M.; Malkus, H. L. J. Am. Chem. SOC. 1975, 97, 1900 and references therein.
0022-3263/83/1948-1246$01.50/00 1983 American Chemical Society
