1779
J. Am. Chem. SOC.1984, 106, 1779-1789
Photoextrusion of SO2 from Arylmethyl Sulfones: Exploration of the Mechanism by Chemical Trapping, Chiral, and CIDNP Probes' Richard S. Givens,* Borys Hrinczenko, Jerry H.-S. L k f Bogdan Mat~szewski,~ and Joan Tholen-Collison4 Contributionfrom the Department of Chemistry, The University of Kansas, Lawrence, Kansas 66045. Received June 30, 1983 Abstract: The photochemistry of eight benzylic sulfones, all of which efficiently extrude sulfur dioxide, was studied by a variety of methods. Optically active sulfones (S)-(-)-8, (S)-(-)-11, (S)-(-)-12, (S)-(+)-13, and (R)-(+)-14 were employed to measure the extent of the "hidden" return of the initial photogenerated intermediates by the stereoequilibration of the chiral center. By comparison of the regioisomeric methyl sulfones 11 vs. 13 and 12 vs. 14, preferential C-S bond fragmentation on the naphthyl side was established for the singlet excited sulfones. Added nucleophilic and proton trapping agents had no effect on the course of the reaction, ruling out ionic intermediates. The three hydrocarbon products of photodesulfonation from the unsymmetrical sulfones 8-14 provided a measure of the cage effects, which were highly structure and multiplicity dependent. Rate constants for reaction, and fluorescence, as well as the energy transfer rates from benzophenone were determined. The reaction rate constant for singlet reactivity from the 1-naphthyl sulfones was about three times greater than that for the 2-naphthyl derivatives. C I D N P studies showed a strong product signal for the 1-naphthyl sulfones from desulfonation of a triplet caged radical pair. Naphthaldehyde formation, a very minor, triplet process, was also detected by the C I D N P study of sulfones 9-12.
Extrusion of sulfur dioxide from alkyl a n d aryl sulfones has proven to b e a useful synthetic method for the construction of a wide variety of organic structures such as c y c l o p h a n e ~ steroids,6 ,~ and b e n z o c y c l o b ~ t e n e s . ~Both thermally and photochemically initiated SO2extrusion reactions have been employed, each method having particular advantages a n d limitations. For example, the thermal expulsion of SOz generally requires high-temperature (>400 "C) vacuum methods but appears not t o be limited by structural requirements,* whereas the milder photochemical process requires the presence of a chromophore situated /3 to the sulfonyl group for efficient extrusion with ordinary light s o ~ r c e s . ~ Despite the extensive interest in the thermal and excited state chemistry of sulfones, very little has appeared concerning t h e mechanistic aspects of these reactions, especially the photochemical extrusion of SO2. O u r interest in photoextrusion reactions in genera19a,'0 a n d in photodesulfonylation in p a r t i c ~ l a rhas ~~~~ stimulated the in-depth study of the mechanistic parameters including multiplicity and stereochemical factors a s well a s the
Scheme I. General R o u t e to Sulfones 7 through 14 ( 1 ) NHzCSNHz
ArCH2X
(2) KOH/H20
2
la, Ar = 1-naphthyl b, A r = 2-naphthyl X = CI or Br t
ArCH2SH
2a, Ar = 1-naphthyl b, Ar = 2-naphthyl c, A r = phenyl
NaOEt
PhCHRX
EtOH
3a, R = H b, R = CH, X = C1 or Br
ArCHZSCHRPh
4a, b, c, d, e,
Ar Ar Ar Ar Ar
= Ph; R = H = Ph: R = CH3((S)-(-)-4b) = 1-naphthyl; R = H = 2-naphthyl; R = H = 1-naphthyl; R = CH,
( (S)(-)-4e)
f , Ar = ?-naphthyl; R = CH,
((SI-(1-4f1 (1) (a) Paper 27 in the series. For paper 26, see: Matuszewski, B.; Burgstahler, A. W.; Givens, R. S. J . Am. Chem. Soc. 1982,169,6838-9. (b) A preliminary report has appeared: Givens, R. S.; Matuszewski, B. Tetrahedron Lett. 1978, 865. (c) Reported in part at the 184th National Meeting of the American Chemical Society, Kansas City, MO, Sept 14, 1982. Abstract number: ORG 67. (2) Taken in part from the Ph.D dissertation of J. H . 3 . Liu, University of Kansas, 1980.(3) On leave from the Institute of Chemistry, A. Mickiewicz University, Poznaii, Poland. (4) An Undergraduate Research Participant, University of Kansas, summer 1979. ( 5 ) (a) Boekelheide, V. Acc. Chem. Res. 1980, 13, 65. (b) Misumi, S.; Otsubo, T. Ibid. 1978, 11,251. (c) Rebafke, W.; Staab, H. A. Angew. Chem. 1973,85, 831. (d) Vogtle, F.; Rossa, L. Angew. Chem., Int. Ed. Engl. 1979, 18, 515. (6) (a) Nicolaou, K. C.; Barnette, W . E. J . Chem. SOC.,Chem. Commun. 1979, 1119. (b) Nicolaou, K. C.; Barnette, W. E.; Ma, P. J. Org. Chem. 1980, 45, 1463. (c) Oppolzer, W.; Roberts, D. A,; Bird, T . G. C. Helu. Chem. Acta 1979, 62, 2017. (d) Gokel, G. W.; Gerdes, H. M.; Miles, D. E.; Hufnal, J. M.; Zerby, G. A. Tetrahedron Lett. 1979, 3375. (e) Gerdes, H. M.; Gokel, G. W. Ibid. 1979, 3379. (7) Cava, M. P.; Schlessinger, R. H.; Van Meter, J. P. J. Am. Chem. SOC. 1964,86, 3173. (8) For a comparison of the structural requirements for one example of a sulfone extrusion reaction, see: Longone, D. T.; Kiisefoglu; Gladysz, J. A. J . Org. Chem. 1977, 42, 2787 and 9a. (9) (a) Givens, R. S. Org. Phofochem. 1981, 5 , 227, review. (b) Givens, R. S.; Olsen, R. J.; and Wylie, P. L. J. Urg. Chem. 1979.44, 1608. (c) Givens, R. S.; Wylie, P. L. Tetrahedron Lett. 1978, 865. (10) Givens, R. S.; Levi, N. In "The Chemistry of the Functional Groups. Acid Derivatives"; Patai, S., Ed.; Wiley: London, 1979; Supplementary Vol. B, p 641.
0002-7863/84/1506-1779$01.50/0
t
ArCH(CH3)CI
Ar = 1-naphthyl ((/?)-(-)-Sa) Ar = 2-naphthyl ((S>(-)-Sb) PhCH2SH
NQOEI E,OH
*
A r C H ( C H 3 ) S C H2 P h
6a, Ar = ]-naphthyl ((S)-(-)-6a) b, Ar = 2-naplithpl ( ( R)-(+)-6b) ArCHRISCHRZAr'
4or6
HzOz AcZo
ArCHRISO&HRZAr'
7, Ar. Ar' = Ph; R , , R, = H 12, Ar, Ar' = 2-naplithy1, P h : 8, AI-. Ar' = 1%: R , , K , = C l i 3 , H R , , R, = H, CH, )- a n d ((t )- and (S)-(-).8) (S)-(-)12) 9, Ar. Ar' = I-naphiliyl, Ph; 13, Ar, Ar' = I-naplitliyl. PI]: R , , R, = H R , , R, = CH H ((+ )- and 10, Ar, AT'= ?-naphthyl, PIi; (S)-(+ )- 13 ) R,, R, = H 14, A r , A r ' = 2-naphthyl. Ph; 11, Ar, Ar' = 1-naphthyl, PIi; R , , R, = CH,, H ( ( + ) - a n d R , , R, = H, CH, ( ( + ) - a n d ( RI-(+ )- 14) (S)-(-)-Il) ( ( 7
nature of the bond-breaking step in the photochemistry of arylmethyl sulfones. Results Synthesis and Exploratory Photolysis of Sulfones. T h e eight benzyl- and naphthylmethyl sulfones were synthesized by the
0 1984 American Chemical Society
1780 J. Am. Chem. SOC.,Vol. 106, No. 6, 1984
Givens et al.
Fso2m
Table I. Optical Rotations of Reactmt Sulfones and the Mdjor Coupling Product
[.I
sulfone (S1-(- 1- 8 (S)-(-)-ll (S)-(-)-12
is>(+ k 13 (R)-(+j-14
T5893u
deg
5% ecb
-29.1 -30.9 -107.5 +1.6 +67.0
24% 54% 54% 4.8% 44%
Scheme 11. Synthetic Photochcniistry for Sulfones 7, 9, and 10
major product
T [a1 5 8 9 ,
(R)-(-)-15 (-)-16 (-)-17
-2.9 -2.8 -5.7
(+)-I8
+8.6
2537
- M p . b a n i t n o ( 5 5 % conrersionJ
deg
7
* Rotations of the sulfones and the major photoproduct were determined in ethanol at 30 "C (8, 15) or chloroform at 20 "C. Percent enantiomeric excess.
19
Chart 1. Photoextrusion Products from Naphthyl Sulfones 11-14 and Benzyl Sulfonc 8 ~~
Sulfone ArCHR, SO, CHR,Ar'
~
photoproducts (Ar'CHR,),
ArCHR,CHR,Ar'
9
(ArCHR,),
20
8 d, 1-24 meso-25 21
11
d,l-24 meso-25 d,l-16
12
21
d,1-24 meso-25 d,l-17
13
22
9 9 & d,1-26 meso-27
d,l-28
sulfone
d,l-18
7e ge 9 10 11 12 13 14
19
&ph
general procedure given in Scheme I and a r e detailed in the Experimental Section. Optically active 1-phenylethyl sulfones" ( S ) - ( - ) - 8 ,(S)-(-)-11, a n d (S)-(-)-12 were also synthesized by the same sequence employing (I?)-( +)-1-phenylethyl chloride ( [ a ] 2 0 5+56.1° 8s (c 0.79, E t O H ) , 54% optical purity) obtained from the (S)-(-)-1-phenylethyl alcohol12 (of the same optical purity). T h e optically active 1-( 1- or 2-naphthy1)ethyl benzyl sulfones, (S)-(+)-13 and (R)-(+)-14, were derived from the resolved 1-( 1-naphthy1)ethyl alcohols by the same procedure used for the resolution and synthesis of the phenylethyl chloride.12 Optical purities and rotations a r e given in Table I. Preliminary photochemical studies of the sulfones (eq 1) showed
7) i' - 7)T2
A~CHSO~CHA~'
benzene
ArCHCHAr
23
Table 11. Quantum Efficiencics for Disappearance and Racemization of Sulfones 7-14
not isolated
14
23
7'
f (ArCH-
4'
t (Ar'CH--fi-
00' 0.29 0.52 0.1 1 0.014 0.1 1 0.028 0.099 0.033
asensb
Osc
@TC
0.27 0.25 0.23 0.003 0.18 0.0066 0.39 0.031
0.16 0.40 0.0 0.012 0.0 0.023 0.0 0.014
0.13 0.12 0.1 1 0.002 0.1 1 0.005 0.099 0.017
(Orac)- (Orac)directd scnsd 0.098
0.0
0.0088 0.0019 0.096 0.014
0.0 0.0
f' 0.0
* Direct (D) irradiations in benzenc: 7 and 8 at 254 niii; 9-14 Sensitized (?ens) irradiations: 7 and 8 with acctonc at 300 nm. as sensitizer and solvent at 300 n m ; 9-14 in benzene with benzoSinglet (S) and triplet iT) phenone as the scnsitizcr at 350 nm. contribution to the dircct irradiation disappearance cfficicncy. See cq 2 in the tcxt. Raccniization (rac) quantum cfficicncics for the direct or scnsitized reactions werc determined by mcasuring the percent of racemization of unrcacted sulfone at 95% of light at 350 nm and sulfone concentration was high enough to ensure efficient quenching of a sensitizer triplet state. Results are given in Table 11. 1-Naphthylmethyl 1-Phenylethyl Sulfone ((f)-11) in Benzene: (a) Direct irradiation, initial sulfone concentration 1.31 X lo-’ M, 6 RPR 3000-8, lamps, light output 2.40 f 0.06 mE/h. (b) Benzophenone-senM, benzositized irradiation initial sulfone concentration 1.32 X phenone concentration 7.27 X M, 6 RPR 3500-8, lamps, light output 1.74 f 0.03 mE/h. (c) Acetophenone-sensitized irradiation, initial M, acetophenone concentration 0.50 sulfone concentration 1.35 X M, 6 RPR 3500-8, lamps, light output 1.74 i 0.03 mE/h (Table IV). 2-Naphthylmethyl I-Phenylethyl Sulfone ( ( i ) - 1 2 ) in Benzene: (a) Direct irradiation, initial sulfone concentration 1.33 X M, 6 RPR 3000-8, lamps, light output 2.40 i 0.06 mE/h. (b) Benzophenone-sensitized irradiation, initial sulfone concentration 1.35 X M, benzoM, 15 RPR 3500-8, lamps, light phenone concentration 5.86 X output 3.37 i 0.13 mE/h. (c) Acetophenone-sensitized irradiation, initial M, acetophenone concentration 0.45 sulfone concentration 1.38 X M, 15 RPR 3500-8, lamps, light output 3.37 0.13 mE/h (Table IV). 1-Phenylethyl Benzyl Sulfone (8) in Benzene. Initial sulfone concentration 3.32 X lo-’ M (in benzene), 4 RPR 2537-8, lamps, light output 1.125 mE/h (Table 11). 1-Phenylethyl Benzyl Sulfone (8) in Acetone. Initial sulfone concentration 3.24 X M (in acetone), 16 RPR 3000-A lamps, light output 1.017 mE/h (Table 11). 1-(1-Naphthy1)ethyl Benzyl Sulfone (13) in Benzene. Initial sulfone concentration 1.31 X M (in benzene), 16 RPR 3000-8, lamps, light output 0.899 mE/h (Table 11). 1-(2-NaphthyI)ethyl Benzyl Sulfone (14) in Benzene. Initial sulfone concentration 1.51 X IO-’ M (in benzene), 16 RPR 3000-8, lamps, light output 0.899 mE/h (Table 11). Benzophenone-Sensitized Irradiation of 1-( 1-Naphthy1)ethylBenzyl M (in benzene), Sulfone 13. Initial sulfone concentration 1.25 X benzophenone concentration 7.29 X lo-’ M, 15 RPR 3500-8, lamps, light output 1.49 mE/h (Table 11). Benzophenone-Sensitized Irradiation of 1-(2-Naphthyl)ethyl Benzyl M (in benzene), Sulfone 14. Initial sulfone concentration 1.21 X benzophenone concentration 7.95 X IO-’ M, 15 RPR 3500-8, lamps, light output 1.49 mE/h (Table 11). E. Emission Spectroscopy. Quenching of Acetophenone and Benzophenone Phosphorescenceat Room Temperature. Argon deaerated carbon tetrachloride (spectrograde) solutions of acetophenone or benzophenone were analyzed at room temperature (20 “C) for phosphorescence emission. The deaeration %as continued until the phosphorescence reached a constant intensity. For acetophenone, 0.002 M solutions were excited at 345 nm and the full emission was monitored employing the ratio mode of the spectrofluorimeter; for benzophenone, 0.01 M solutions were excited at 385 nm. The spectra obtained were in good agreement with those reported by Saltiel et al.13aand by Steel et al.’3b The Stern-Volmer quenching of the emission was determined for piperylene, naphthalene, or sulfones 7, (*)-9, or (*)-IO as quenchers. The average value obtained from duplicate experiments and their standard deviations are presented in Table 111. Fluorescence Measurements for Sulfones 9-14. Fluorescence emission spectra were determined in a :-cm quartz cell with an Aminco-Bowman spectrofluorimeter using the IP 28 photomultiplier with an excitation wavelength of 310 nm. Bcth benzene and cyclohexane were used as solvents, Samples were deaerated with argon. Before recording the fluorescence emission, the UV spectra of the sulfones were always taken
*
(26) Hatchard, C. G.; Parker, C. A. Proc. R . SOC.London, Ser. A 1956. 235, 518.
J. Am. Chem. SOC.1984, 106, 1789-1793 by using the Cary 14 spectrophotometer. For fluorescence quantum efficiency measurements, the concentrations of naphthalene, as a standard, and of all sulfones were adjusted so that the absorptions of the systems were practically the same at A,, 310 nm. The measured fluorescence intensities (Z,) at the wavelength corresponding to the maximum of fluorescence were used to calculate the quantum efficiencies (Z,) relative to sulfone 9. Small variations, in the absorbances values (A) at A 310 nm were included in the calculation. The quantum yields (Table V) were then calculated according to the following expression:*'
a, - Z,(1 - lo-"') _ 0;
z;(1 - 10-A)
Fluorescence Decay Time ( T J . The fluorescence decay times were calculated from the Stern-Volmer relationship for oxygen quenching of the fluorescence emission in cyclohexane" according to the following expression: L J L = 1 T,kq[Q]
+
where L and Lo are the fluorescence intensities with and without air, T~ is the mean decay time of the deaerated solution, kq is the quenching rate constant for oxygen, and [ Q ] is the concentration of dissolved oxygen. The value of k , [ Q ] was taken as 6 X IO' s-l.]' The fluorescence intensity measurements were first obtained with samples deaerated with argon and then repeated after saturation with air, to give the L, and L values. The results, together with kf values calculated from the expression kf = b ~ ; ' , are presented in Table V. Fluorescence of the Sulfones 8, 13, and 14. The spectra were obtained in EPA (ether:isopentane:ethanol = 5:5:2). The emission maxima were observed at 317 nm for 8 (Aex 254 nm), 348 for 13 (Aex 310 nm), and 343 nm for 14 (Aex 310 nm). Fluorescence of the Sulfones 7,9, and 10. The spectra were obtained in cyclohexane at excitation wavelength of 310 nm (Table V). (27) Calvert, J. G.; Pitts, J. N., Jr. 'Photochemistry"; Wiley: New York, 1966; p 800.
1789
Phosphorescence of the Sulfones 7,9, and 10. The phosphorescence spectra were observed in chloroform glass at 77 'C. The emission maxima were found at 468 (onset 405) nm for 7 (broad), 480 nm for 9, and 475 nm for 10. (Table VI). Phosphorescence of the Sulfones 8, 13, and 14. The phosphorescence spectra were observed in EPA glass at 77 K. The emission maxima were found at 410, 433, and 460 nm for 8 (Aex 254 nm), 495, 530, 570, and 620 nm for 13 (Aex 310 nm), and 484, 518, 557, and 602 nm for 14 (Aex 310 nm) (Table VI). F. CIDNP Investigations. All CIDNP measurements were performed on a Bruker WP 80-MHz FT N M R spectrometer with a 'H probe modified for irradiation of the sample through a lens-fiber optic light pipe arrangement (see Figure 3). The unfiltered light of a Varian 300 W EIMAC VIX 300 UV Xe-Hg lamp irradiated the sample. The spectra were obtained from Fourier transformations of 5-10 free induction decays recorded at ca. 20"-60" flip angles and 2-s pulse delay. Approximately 10 mg of the sample to be analyzed was dissolved in 0.5 mL of a deuterated solvent, usually C6D6and placed in either a Pyrex or a quartz N M R tube. Certain additives were used to elucidate the nature of the photochemical mechanism. Tri-n-butyltin hydride and bromotrichloromethane were added as radical scavengers. Benzophenone (ET= 68.6 kcal/mol),16 acetophenone (ET= 73.7 kcal/mol),I6 and xanthone (ET= 74.0 kcal/mol)16 were added as triplet sensitizers. The results are listed in Table VI1 and in Figures 1 and 2. Acknowledgment. T h e support for this work was provided by the National Institutes of Health (GM 1661 1) and the University of Kansas General Research Fund. Support for t h e purchases of the Bruker W P 8 0 FTNMR with the photo-CIDNP probe and the PE 555 UV-vis spectrometers (CHE 77-07826 and 78-03307) is gratefully acknowledged. T h e assistance of Prof. G. W . Everett in the CIDNP experiments is especially appreciated. B.M. thanks the International Research a n d Exchange Program for partial support.
Mechanistic Studies of the Photodecomposition of Arylmethyl Sulfones in Homogeneous and Micellar Solutions Ian R. Could,+Chen-ho Tung,? Nicholas J. Turro,*t Richard S. Givens,' and Bogdan Matuszewski* Contribution from the Department of Chemistry, Columbia University, New York, New York 10027, and the Department of Chemistry, University of Kansas, Lawrence, Kansas 66044. Received June 30, 1983
Abstract: The mechanism of photodecomposition of aryl sulfones has been investigated by using both steady-state and time-resolved techniques. Direct evidence for radical and triplet-state intermediates is provided. A correlation is found between the dynamics of reaction of the intermediate triplet states with the stabilities of the intermediate radicals.
T h e photoactivity of aryl sulfones was first reported b y Cava et al.' who found that upon irradiation these compounds efficiently lose molecular sulfur dioxide and yield products typical of radical coupling reactions. T h e q u a n t u m yields for reaction of a series of aryl sulfones have been determined under both direct photolysis and triplet-sensitized reaction conditions.2 I t was found that both the singlet and triplet states of these compounds a r e photoreactive a n d t h a t t h e relative amount of reaction from each state was structure dependent. For example, the photocleavage of dibenzyl sulfone (DBS) la was found to occur both from singlet and triplet states. For t h e naphthyl benzyl sulfone 2a reaction was found
p - R P h C H 2 S 0 2CHR ' Ph
l a , R = R' = H lb, R = CH,; R' = H IC, R = H ; R' = CH,
2a, R = R' = H 2b, R = CH,; R' = H 2c, R = H ; R' = CH,
to occur mainly from the triplet state; however, for the @-naphthyl sulfone 3a the singlet state was determined to be t h e more pho~
t Columbia
University. *University of Kansas.
~~
~
(1) Cava, M. P.; Schlessinger, R. H.; VanMeter, J. P. J . Am. Chem. SOC. 1964, 86, 3113.
0002-7863/84/1.506-1789$01.50/0
0 1984 American Chemical Society
