82
J. Am. Chem. SOC.1993, 115, 82-86
The Photochemistry of Mixed Anhydrides: A Search for Selectivity in Photochemically Initiated Bond Cleavage React ions John H. Penn* and Walter H. Owens Contribution from the Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506. Received June 15, 1992
Abstract A number of mixed anhydrides (RC(O)OC(O)R’)have been synthesized for the purpose of exploring the regioselectivity of their photochemical reactions. The photochemicalreaction products, quantum yields, and reaction rates have been determined. For those compounds containing no a-hydrogens, high regioselectivity is obtained upon irradiation and can be understood on
the basis of the relative C-0 bond dissociation energies. The reaction can be quenched by intramolecular electron transfer. For those compounds containing a-hydrogen atoms, the Norrish type I1 reaction is faster than the corresponding C-0 cleavage reactions, resulting in poor regioselectivity.
Introduction Research in our group has focused on the selective cleavage of chemical bonds.’ Recent investigations have been targeted toward conversion of the carboxylic acid functional group into other functionality which may be elaborated in synthetic schemes* or fossil fuel conversion proce~ses.~ The conversion of carboxylic acids to aldehydes has received a great deal of attention by synthetic organic chemists. In all cases? derivatives of the acid (Le., acid chlorides, amides, esters) are converted to the aldehyde by selectivereduction. None of these methods are generally applicable to a variety of functional groups and yields of the desired aldehyde tend to be 580%. Since there are difficultieswith the reduction of the carboxylic acid derivatives, the standard synthetic procedure for the conversion of carboxylic acids to aldehydes involves the direct reduction of the acid followed by mild oxidation of the resulting alcohol. Recently FukuyamaZ and co-workers have used the conversion of carboxylic acids to ethyl thiol esters followed by treatment with triethylsilane and a catalytic amount of palladium to yield aldehydes. This reaction appears to be reasonably general and is sufficiently mild to be unreactive toward other potentially reducible functional groups such as amides, sulfides, and olefins. However, the yields of the desired aldehyde are dependent upon the nature of other functional groups in the thiol ester. To circumvent these problems and to develop a potential one-pot method for the conversion of the carboxylic acid group to a variety of other functional groups, we have investigated the reactions of mixed anhydrides. If the mixed anhydride could be cleaved with high regioselectivity via photochemical (or other) activation methods to the desired acyl radical, then the radical could be “tamed” by a variety of methods to functionalize as desired.5 Our strategy requires the solving of several problems. (1) The reaction of carboxylic acids to form the mixed anhydrides must proceed in high yields for a variety of attached functionalities. This was easily accomplished through the use of acyl pyridiniums.6 (2) The regiochemistry of the reaction must be controllable. Previous (1) (a) Penn, J. H.; Lin, Z. J . Am. Chem. Soc. 1991, 113, 1001. (b) Penn, J. H.; Deng, D.-L.; Aleshire, S. K. J . Org. Chem. 1988, 53, 3572. (2) Fukuyama, T.; Lin, S.-C.; Li, L. J . Am. Chem. SOC.1990,112,7050. (3) Stock, L.M. Prepr. Pup., Am. Chem. Soc., Diu. Fuel Chem. 1987.32,
463. (4) (a) Cha, J. S.;Kim, J. E.; Yoon, M. S.;Kim, Y. S. Tetrahedron Lett. 1987,28,6231. (b) Corriu, R. J. P.;Lanneau, G. F.; Perrot, M. Tetrahedron Lett. 1987, 28, 3941. (c) Brown, H. C.; Cha, J. S.; Nazer, B.; Yoon, N. M. J . Am. Chem. SOC.1984, 106, 8001. (5) (a) Barton, D. H. R.; Blundell, P.; Jaszberenvi, J. C. J . Am. Chem. Soc. 1991, 113, 6937. (b) Barton, D. H. R.; Blundell, P.; Jaszberenvi, J. C. Tetrahedron Lett. 1989, 30, 2341. (c) Barton, D. H. R.; Crich, D.; Kretzschmar, G. Tetrahedron Lett. 1984, 25, 1055. (d) Ueno, Y. Pharmazie 1984, 39, 123. (6) Penn, J. H.; Owens, W. H. J . Org. Chem., submitted for publication.
0002-7863/93/1515-82$O4.00/0
studies on the photochemical reactions of symmetrical anhydrides, cyclic anhydrides, and a variety of other carboxylic acid derivatives have shown that Norrish type I reactions and C-O cleavage reactions are the predominant excited state reaction pathways of these species,with the stability of the resultant radicals apparently controlling the reaction.’J Calculations of the bond dissociation energies of the two carbonyl C-O bonds (Note bonds B and C in Figure 1) for various substitution patterns, using Benson’s rules? showed that 5-6 kcal/mol differences could be obtained between these two bonds. With such a high difference in energy, extremely high regioselectivity should be easily accessible. (3) Most importantly, activation of the cleavage must be accomplished in a way that utilizes the anticipated regioselectivity. We report here on our efforts to initiate the cleavage via photochemical means and the resultant attempts to control the cleavage pathways in a way which might lead to a useful synthetic method. The compounds selected for this study are shown in Figure 1. These compounds were specifically chosen to vary the number of phenyl groups to systematically change the predicted bond dissociation energy of bonds B, C, and D. These changes may be reflected directly in the reaction rates and quantum efficiencies of the reactions. Compounds 1 have been classified as a group because they contain no a-hydrogens. Compounds 2 have been grouped together because they repesent a benzoic anhydride but contain a-hydrogens which might participate in Norrish type I1 reactions. Compound 3 was specifically synthesized to verify whether the high regioselectivity pattern observed in 1 might be observed in molecules containing a-hydrogens. Results Synthesis of Starting Materials. The syntheses of these mixed anhydrides have been previously described.6 In brief, the a p
propriate acyl pyridinium complex is treated with the appropriate carboxylic acid to form the mixed anhydride as shown in eq 1 (see ref 6 for complete details of the synthetic procedures). All reactions producing mixed anhydrides proceeded in 293% yield, thereby fulfilling goal 1 of our required strategy for facile transformation of carboxylic acids to other functional groups. (7) (a) Scheffer, J. R.; Wostradowsski, R. A. J . Org. Chem. 1972, 37, 4317. (b) Scheffer, J. R.;Wostradowski, R. A. Chem. Commun. 1971, 144. (8) (a) Coyle, J. D. Chem. Reo. 1978, 78, 97 and references cited therein. (b) Roof, A. A. M.; van Woerden, H. F.; Cerfontain, H. Tetrahedron 1976, 32, 2967. (c) Chamberlain, G. A.; Whittle, E. J . Chem. Soc.,Furaduy Trans. 1975, I , 1978. (d) Hiroka, H. J . Am. Chem. SOC.1973, 95, 1664. (9) Benson, S. W. Thermochemical Kinetics, 2nd ed.; John Wiley and Sons: New York, 1976.
993 American Chemical Society
The Photochemistry of Mixed Anhydrides
J. Am. Chem. SOC., Vol. 115, No.1, 1993 8 3 Table I. Photochemical Results for 1 irrad time conv product yieldd (mol W ) compd" (h) (W) PhCHO RC02H RH PhC02H RCHO la lb lbb lbc
10 26 26 26
18 9.4 10.3 9.8
97.2 96.3 98.6 99.2
7.2 2.0 1.7 2.1
101.0 99.1 100.6
20.1 1.9 1.4 1.5
Reactions with added DHA as a hydrogen source. A
Photolysis at
254 nm. 'Triplet sensitized reaction with benzophenone. dProduct
Du
B 3c
0.8 0.3 0.5
yields are estimated to be +5W.
2.: R = R' = H 2b: R = H; R' = Ph 20: R = R' = Ph
la: R = C(CIi,), lb: R = CPh,
Figure 1. Structures of the compounds selected for study together with possible bonds to cleave.
-
0
I1
PhCHO
Ph C.
Bond B 7 RC02H
O. RH
0: Ph
:o:
) ,: O ; B
0 II
.C
Bond C
R '
-
RCHO
;R
PhCOZH
c p
.O
I I
P 'h
Figure 3. Photochemical reaction pathways for anhydrides 2.
ko)l. 0
Bond D
Ph
R.
-co,
PhCHO
RH
Figure 2. Photochemical bond cleavage reactions of anhydrides 1.
Explontory Photolysis of Mixed Anhydrides with NOa-Hydrogens. Benzoic pivalic anhydride (la) and triphenylacetic benzoic anhydride (lb) were irradiated in acetonitrile solution (20 mM) using a Rayonet reactor with 300-nm lamps. The solutions were degassed by bubbling argon through the solution for 60 min prior to irradiation after which the solutions were sealed with latex septa. Since radical intermediates were expected from thesc reactions,9,lOdihydroanthracene(DHA,2 mM) was added as a hydrogen source to trap the radical species. The H-atom donor was kept at a low concentration to ensure that all the light was absorbed by the mixed anhydrides. The products from photolysis of triphenylmethyl l b were isolated. In all cases, GCMS, IR, and NMR spectra were identical with those from authentic samples. For solid compounds, mixed melting points firmly established the identities of the products. The products of tert-butyl la were identified by comparison of their GCMS spectra and matching of retention times on 2 different GC columns (carbowax and phenylmethylsilicone columns). As expected, the products resulting from photolysis of 1 can be easily rationalized by the analysis in F v 2. Bond B cleavage leads to a radical pair from which benzaldehyde and RH or RC02H can be formed by hydrogen atom abstraction. Bond D cleavage proceeds similarly in that R H and PhCHO can be formed following decarboxylation and hydrogen atom abstraction. We note that bond B and bond D cleavage pathways result in similar products. The alternative cleavage pathway (i.e., bond C) results
in PhC02H and RCHO following hydrogen atom abstraction. The quantitative results of these reactions are summarized in Table I and are shown diagrammatically in Figure 2. We estimate our error to be i5% for all compounds. The amount of conversion was intentionally kept to a minimum value to avoid total consumption of the H-atom donor. These results were obtained from GC analysis using hexadecane as a internal standard. The mass balance for photoreactions of la is not good. However, since (CH3)3CC02'is known to decarboxylate rapidly to (CH3)3C*(k = 1.6 X lo9 s-'),'O the resulting (CH3)3CHor (CH3)2C==CH2 formed by hydrogen abstraction would be sufficiently volatile as to make their analysis difficult. We have searched exhaustively for (CH3)$CH0 in the reactions of l a but have been unable to detect any. Most informative is the addition of a small amount of a standard sample following the photoreactions in order to simulate the production of this material in solution. On the basis of these experiments, we conclude that the amount of (CH3)3CCHO is
