J. Org. Chem. 1980,45, 2195-2201 73245-21-3; 3h, 73245-22-4; 3i, 73245-23-5; 6d, 17649-59-1; 6 f , 21483-46-5; 6g, 50919-647; 6i, 73195-01-4; 16a, 73245-24-6; 16b, 73195-02-5; 16c, 73245-215-7;16d, 73245-26-8; 16e, 73245-27-9; 16f, 73245-28-0; 16g, 73245-2!3-1; 16h, 73245-30-4; 16i, 73245-31-5; 17f,
2195
73195-03-6; 17g, 73198-06-8; 18, 73195-04-7; 19, 73195-05-8; 20, 73195-06-9; 21, 73195-07-0; 22, 73245-32-6; 23, 73245-33-7; 24, 73245-34-8; 25, 73195-08-1; dimethyl acetylenedicarboxylate, 76242-5; ethyl glycidate, 4660-80-4; ethyl lactate, 97-64-3.
Photodegradation of Some Alkyl N-Arylcarbamates’ John E. Herweh* and Charles E. Hoyle Arnzstrong Cork Company, Research and Development Center, Lancaster, Pennsylvania 17604 Received September 25. 1979
The UV p hotodegradation of a number of alkyl N-arylcarbamates in solution has been investigated. A mechanism for the photodegradation process is proposed. The initial excitation of the carbamate moiety involves a X , X * transition into an excited singlet state. Chemical change from this excited state proceeds primarily via homolytic cleavage of the nitrogen to alkoxycarbonyl bond to provide a radical pair in a solvent cage. The quantum efficiency for carbamate disappearance is low. The major identifiable products are amines and photo-Fries rearrangement products (where formation is possible). The arylaminyl radical is proposed as a major reaction intermediate. I t is suggested that the failure for the sum of quantum yields of formation for degradation products (amines and photo-Fries rearrangement products) to coincide with quantum yields for carbamate disappearance is due to reactions r:e.g., coupling) of the aminyl radicals. The quantum yield for carbamate disappearance is independent of both methyl group substitution on the phenyl ring and excitation wavelength. In contrast, the quantum yield for the disappearance of the parent arylamine, a carbamate photodegradation product, does show dependence on these factors.
The photodegradation of urethanes based on aryl isocyanates has been the subject of a number of report^.^-'^ The earliest studies directed toward identification of the products formed were reported by Trecker et a1.2 and Bellus and S ~ h a f f n e r .The ~ latter found that irradiation of ethyl N-phenylcarbamate (la)at 254 nm yielded ethyl o-aminobenzoate (lc),ethyl p-aminobenzoate (ld), and aniline (lb) (Scheme I). More recently, Beachell and Chang4 also reported on the photodegradation of la. In addition to products lb, IC, and Id they found C02. An intramolecular (concerted) rearrangement was proposed to account for the photo-Fries products (IC and la). Schwetlick and co-workersk10confirmed the results of earlier workers with respect to the primary photodegradation products of la in solution. They proposed, however, that the products were formed by N-C bond cleavage resulting in a solvent-caged radical pair. Within the solvent cage the ethoxycarbonyl radicals attacked the benzene ~
~
~~
~
(1) The nomenclature lused follows that recommended by: Fletcher, J.; Dermer, 0.; Fox, R. A h . Chem. Ser. 1974, No. 126. (2) Trecker, D.; Foote, C.; Osborn, C. Chem. Commun. 1968, 1034. (3) Bellus, D.; Schaffner, K. Helu. Chim. Acta 1968, 51(1), 221. (4) Beachell, H.; Chang, I. J . Polym. Sci., Polym. Chem. Ed. 1972,10, 503. (5) Schwetlick, K.; Notick, R.; Schmeider, G. 2. Chem. 1972,12, 107. (6) Noack, R.; Schwetlick, K. 2. Chem. 1972, 12, 108. (7) Schwetlick, K.; Noack, R. Z. Chem. 1972, 12, 109. (8) Noack, R.; Schwetlick, K. 2. Chem. 1972, 12, 143. (9) Noack, R.; Schwetlick, K. Z. Chem. 1972, 12, 140. (10) Noack, R.; Schwetlick, K. 2. Chem. 1973, 14, 99. Stumpe, J.; Melhorn, A,; Schwetlick, K. J . Photochem. 1978, 1. (11) Schultze, H. 2. Naturjorsch., E 1973, 28, 339. (12) Masilamani, D.; Hutchins, R.; Ohr, J. J . Org. Chem. 1976, 41, 3687.
(13) Schultze, H. Makromol. Chem. 1973, 172, 57. (14) Osawa, Z.; Cheu, E:.; Ogiwara, Y. J . Polym. Sci., Polym. Lett. Ed. 1975. 13. 535. (15) Schulman, S.; Griepentrog, J. A. Microchem. J . 1962,6(1),179-97; Chem. Abstr. 1962, 59, 92348d.
0022-3263/SO/l945-2195$01.00/0
Scheme I
@ @ -@ @rCozE +
la
lb
+
IC
I
C02Et
Id
ring at the ortho and para positions to give the reported photo-Fries products. Similarly, aniline was formed by diffusion of the anilinyl radical from the solvent cage followed by hydrogen abstraction. The existence of a cage effect was given credence when, upon photolysis of la in the vapor phase, no photo-Fries products were detected. Added support for a cage mechanism was obtained by Osawa et al.,14who reported ESR spectra as evidence for an anilinyl radical upon photolysis of the carbamate la. Schwetlick et aL8determined the disappearance quantum yield of l a (aD= 0.023) and the quantum yield for formation of l b (alb= 0.009), IC (a1,= O.OOS), and Id (aid = 0.00s). The formation of lb, however, was not measured directly but was determined from the difference in the quantum yield of disappearance for la and the sum of the quantum yields of formation for IC and Id [a1b= aD(1a) - (aid + @IC)]* The current investigation is directed toward the photodegradation of simple urethanes 2a-5a based upon aryl isocyanates bearing ring-substituted methyl groups. The effect of methyl-group ring placement on the photodegradation process and reaction products is of particular interest with respect to any analogies one may draw to the photodegradation of polymeric systems based on methyl phenylene diisocyanates. Photolysis of carbamates la-5a under a variety of conditions should yield interesting information concerning the 0 1980 American Chemical Society
2196
J. Org. C'hem., Vol. 45, No. 11, 1980 NHC 02RI I
R4%R2 R3
l a , R, = Et; R, = R, = R, = H 2a, R, = I+; R, = CH,; R, = R, = H 3a, R, = I'r; R, = CH,; R, = R, = H 4a, R, = I+; R, = R, = CH,; R, = H 5a, R, = 1%; R, = R, = R, = CH, photodegradation mechanism of aromatic urethanes. Since several of the possibilities for the photo-Fries rearrangement process are excluded, a u n i q u e o p p o r t u n i t y is provided to d e t e r m i n e the effect of methyl blocking groups on the overall photodegradation of the aromatic urethane.
Experimental Section General Methods. lJltraviolet spectra were recorded with a Beckman DK-SA spectrophotometer using 1-cm quartz cells. Infrared spectra were recorded with a Perkin-Elmer 451 infrared spectrophotometer. NMR spectra were recorded on a Jeolco Model JNM-4H-100 100-MHz spectrophotometer (using Me$i as an internal standard). The ultraviolet light source used was a 450-W Hanovia medium-pressure Hg lamp. Band-pass optical filters (254 and 280 nm) were from Baird-Atomic. A HewlettPackard Model 5710A GC with an FID was used for separation, detection, and quantification of products. The columns were 6 ft stainless-steel containing Carbowax 20-M on Chromosorb G.
Materials. Starting and Reference Materials. Isocyanates. Phenyl and ptolyl isocyanates obtained from Eastman Organic Chemicals were distilled prior to use; fractions with b o i i g points of 64.5 "C (25 mm) and 61 "C (3.6 mm), respectively, were retained. o-Tolyl isocyanate (Eastman Organic Chemicals) and 2,6-dimethylphenyl and 2,4,&trimethylphenyl isocyanates (Adams Chemical Co.) were used as received. Amines. Aniline (lb) and o-toluidine (2b)(Eastman Organic Chemicals) were distillecl, and fractions with boiling points of 67 "C (11 mm) and 90 "C (10 mm), respectively, were retained. p-Toluidine (3b) (Aldrich Chemical Co.) was purified by sublimation at 5 mm (bath temperature 90-100 "C), and material with a melting point of 43-45 "C was retained. 2,4,6-Trimethylaniline (5b)(Aldrich Chemical Co.) was distilled prior to use; bp 90-94 "C (1mm). 2,6-Dimethylaniline (4b) (Aldrich Chemical Co.) was used as received.
Photo-Fries Rearrangement Products and Precursors. Ethyl p-aminobenzoate (la) and methyl o-aminobenzoate from Aldrich Chemical Co. were used as received. 2-Amino-S-methyl-, 4-amino-5-methyl-, and 2-amino-5-methylbenzoicacids were obtained from Aldrich Chemical Co. 2-Amino-5-methylbenzoic acid was recrystallized two times from benzene in the presence of decolorizing charcoal and gave material melting at 180-181 "C. Internal Standards. n-Eicosane (PolyScience Corp.) was used as received. Solvents. Ethyl propionate (Aldrich Chemical Co.) was dried over magnesium sulfate (anhydrous) for 24 h and distilled a t atmospheric pressure under a positive nitrogen pressure to give material with a boiling point of 95.5 "C; its UV spectrum showed no absorption above 260 nm. Spectrograde cyclohexane from Burdick and Jackson L,iboratories was used in all photolysis reactions. Miscellaneous. Boron trifluoride etherate (Aldrich Chemical Co.) was distilled prior to use; material with a boiling point of 125 "C (760 mm) was retained for use.
Preparative Procedures. 1. Preparation and Characterization of Alkyl N-Arylcarbamates. General Procedure. Dry propanol (100% molar excess) and a catalytic (1% by weight of isocyanate) amount of pyridine were placed in a flame-dried flask under a nitrogen atmosphere. A solution of the requisite isocyanate in ethyl acetate (dry) was added dropwise with stirring to the alcohol/pyridine solution. The extent of reaction was determined by following the intensity of the isocyanate band (ca. 2270-2240 cm-') in the IR. In some cases heating a t 60 "C was
Herweh and Hoyle necessary to speed the rate of isocyanate reaction (Table I). When the isocyanate had completely reacted, the cooled reaction mixture was filtered to remove insolubles. In several cases ca. 30 mm) a t a temperature not exceeding 100 "C. The still residues were cooled to ice-water-bath temperatures. After hydrolysis of residual boron trifluoride etherate and its complexes by careful addition of ice-water to the still residues, followed by neutralization with aqueous 10% sodium carbonate, the aqueous insolubles were extracted with ether. The combined and dried ether extracts were concentrated a t reduced pressure. The crude propyl aminobenzoates were purified by fractional distillation in vacuo in the case of 2c and 3c and by recrystallization from hexane in the case of 2d (Table II). Structural assignments were confirmed by NMR spectroscopy (Table 11). PhotochemicalDegradation. 1. Description of Apparatus. A 450-W, medium-pressure, Hanovia mercury lamp was focused through an appropriate band-pass filter (280 and 254 nm) on a 1 cm path length quartz cell. A blower was used to cool the mercury-lamp source. When desired, test solutions contained in the quartz cell could be purged with oxygen or helium. This was accomplished through the use of a needle-valve assembly attached to the tappered, quartz, cell neck.
2. Methods of Quantifying UV Photodegradation Products and Computing Photochemical Efficiencies. The loss of carbamate due to photodegradation and the amounts of known photodegradation products were determined quantitatively by GC using eicosane as an internal standard. The quantum yields of loss (aP,) and of product formation (%J were calculated by standard methods.l8
Results Ultraviolet Spectra. The ultraviolet spectral data of carbamates la-5a and the resultant photoproducts expected from a process analogous to Scheme I are given i n Table 111. Concentration, absorbance, and extinction coefficients for peak maxima data are presented. For carbamates la-3a the fine structure of the Sl(n,n*) So transition of benzene is retained. The 2,6-dimethyl-subs t i t u t e d (4a) and 2,4,6-trimethyl-substituted (5a) carbam a t e s show little or no vibrational fine structure. Quantum Yield (@,,) for Alkyl N-Arylcarbamate Degradation. Air-saturated solutions of the different
-
(16) Sowa, F.; Nieuwland, J. J. Am. Chem. SOC.1936,58, 271. (17) Hallas, C. J. Chem. SOC.1965, 5770. (18) Turro, N. J. In "Molecular Photochemistry"; W. A. Benjamin: Reading, MA, 1965; p 6.
J. Org. Chem., Vol. 45, No. 11, 1980 2197
Alkyl N-Arylcarbamates
Table I. Characterization of Alkyl N-Arylcarbamates' mp or bp (mm), "C
alkyl N-arylcarbamates
chemical shifts, 6 % yield
HA
HB
HC
HD
HE ( HE')
48-50'
quantd
6.95 (br s)
4.20 ( 9 )
93.5-95.5 (0.1)
78.7e
6.46 (br s)
4.08 (t)
1.68 ( m )
0.94 ( t )
2.20 (s)
54- 5 5g
quanth
6.79 (br s)
4.08 ( t )
1.67 ( m )
0.93 ( t )
(2.26) (SI
108-109 (0.2)
83e
6.30 (br s)
4.06 ( t )
1.65 ( m )
0.93 ( t )
2.20 (s)
64-65
82e
6.14 (br s)
4.07 ( t )
1.65 ( m )
0.93 ( t )
2.19 (s) (2.24 1 (SI
1.28 ( t )
2af B
b
C
D
N H C O ~ C H ZCHpCH3
:af
0 c 3 YH C O ~ C H Z C H ~ C H ~
' Satisfactory analyt,ical data were reported for all new compounds listed in the table. Me,Si as reference, CDCI, as solvent. e Reported' melting point 52 "C. d Yield of crude product. l a was purified by sublimation at 0.2 mm ( p o t temperature 80-90 "C). e Yield of purified product. Compounds previously unreported. g Reported'$ melting point 53.5-54 "C. Yield of crude product. Purification was effected by recrystallization from an acetonitrile-hexane mixture.
'
'Table 11. Characterization of Amino- and Methyl-Substituted Propyl Benzoates' chemical shifts,
%
mp or bp (mm), "C yield c
72.5-74 (0.1)
YH*
HA
HB
5.76 (br s) 4.32 (q)
HC
HD
6
HE
1.37 ( t )
'@TCO,CHTCHi
H 1, 7.89 ( m , Ha) 7.25 (m, Ha) 6.68 (m, H,, €16)
8
t'
IC C45
5.82 (br s ) 4.20 ( t ) 1.75 ( m ) 1.00 ( t ) 2.11 (s) 7.15 (d, Ha) 7.79 (d, HP) 6.55 (t, H y )
104-105e (0.15-0.2)
53
54-57c
52 4.10 (br s) 4.20 ( t ) 1.75 ( m ) 1.00 ( t ) 2.13 (s)
83-84e(0.05-0.08)
36
2c ih:
7.74 ( m , Ha, HP) 6.59 (d, H,)
HqL; Cd,Bcdic,cH;
2d CH;
+
'
.?Ac 3 ii;:
H;
5.56 (br s) 4.19 ( t ) 1 . 7 5 ( m ) 0.98 (t) 2.18 (s) 7.65 (s, Ha) 7.05 (dd, H ) 6.53 (d, Hrf
cc;
,,;,
3c
Satisfactory analytical data were reported for all new compounds listed in this table. vent. e Compounds previously unreported. a
urethanes (la-5a)in cyclohexane were photolyzed to low conversions at 254 and 280 nm by using band-pass filters (Table IV). In the case of propyl N-0- and N-p-tolylcarbamates (2aand 3a),the quantum yield for carbamate disappearance (aD)was also determined in air-saturated
Me,Si as reference, CDCl, as sol-
solutions by using ethyl propionate as the solvent. Ethyl propionate was selected as a model solvent for photodegradation studies currently being carried out on simple urethanes in solid polymer matrixes of polymethyl and polypropyl methacrylate (PMMA and PPMA).l9 Photo-
2198 J. Org. C‘hem., Vol. 45, No. 11, 1980
Herweh and Hoyle
Table 111. UV Smctral Data
Table V. Quantum Yields (@ ) for Carbamate Photolysis Products Photolyzed a t 2g4 nm in CyclohexaneQib
concn, M l a
