2738
J . Am. Chem. SOC.1987, 109, 2738-2745
Electron-Transfer-Induced Photocyclization Reactions of Arene-Iminium Salt Systems. Effects of Cation Diradical Deprotonation and Desilylation on the Nature and Efficiencies of Reaction Pathways Followed Alexander J. Y. Lan, Robert 0. Heuckeroth, and Patrick S. Mariano*
Contribution from the Department of Chemistry, University of Maryland, College Park, Maryland 20742. Received October 23, 1986
Abstract: Photocyclization reactions of the N-xylyl-I-pyrrolinium perchlorates 3-6, induced by excited-state electron transfer, have been explored with the intent of uncovering mechanistic details and developing synthetic applications. Both of the silicon-substituted salts 4 and 6 undergo photocyclization to produce the respective benzoindolizidine products, 30 and 13, exclusively via mechanisms involving sequential electron transfer-desilylation. On the other hand, the non-silicon-containing 2-phenyl- 1-pyrrolinium perchlorate 5 undergoes conversion to both the indolizidine 13 and 2-phenyl- 1-pyrroline (10) upon irradiation. Photofragmentation generating 10 is proposed to arise by cleavage of the intermediate diradical cation 14, which occurs in competition with deprotonation and 1,6-diradical coupling to form 13. Finally, the benzopyrrolizidines 22 and 23 are produced when the 2-methyl-N-xylyl- and 2-methyl-N-benzylpyrroliniumperchlorates 3 and 21 are irradiated. Deuterium labeling studies have shown that these photocyclization reactions proceed through diradical cation coupling processes. The features of these photochemical processes whose chemical outcomes are dependent upon the nature of reaction pathways available to cation diradical intermediates are discussed. Finally, the photoinduced, diradical cyclization methodology for transformation of the silicon-containing N-xylylpyrrolinium perchlorates 4 and 6 has been compared to the alternative dipolar pathway promoted by fluoride-induced desilylation. The indolizidines 30 and 13 are formed when these salts are treated with cesium fluoride at high temperature. However, the yields are much lower than those observed for the photocyclization processes.
In the preceding paper' we have described the results of studies probing the electron-transfer-initiated, photoaddition reactions of 1-pyrrolinium salts and arene systems. The results of that effort suggested that photoreactions in these systems could be promoted by excitation of either the arene donors or iminium salt acceptors and that the intermediate cation radical pairs generated by excited-state electron transfer are transformed to products by a number of pathways including electrofugal group loss from benzylic positions, radical coupling, and cage collapse. The important conclusion drawn from this study is that product distributions in these photoreactions a r e governed by the nature of benzylic electrofugal groups which influence the rates of cation radical pair conversion to radical pair precursors of the photoproducts. Investigations of arene-iminium salt electron-transfer photochemistry have been extended to intramolecular systems with the intent of probing further the effects of benzylic electrofugal groups on the nature and efficiencies of photocyclization processes available to these systems. In addition, we felt that a n exploratory study of this type would provide preliminary information about the synthetic application of arene-iminium salt photocyclization processes in the area of N-heterocycle synthesis. At the outset, we envisaged that diradical cation intermediates 2 (Scheme I) would be formed by intramolecular electron transfer in the excited state of the linked arene-iminium salt 1. Ensuing reactions of these intermediates could occur via electrofugal group loss (path a), radical coupling (path b), or heterolytic fragmentation (path c) pathways and lead to generation of cyclization or cleavage products. In the intramolecular systems, cage escape r e s king in intermolecular processes should be highly improbable. Therefore, photoreactions of linked arene-iminium salts have the potential of displaying high degrees of chemoselectivity, controlled by the nature of benzvlic electrofugal groups. Our preliminary iri estigations in this area have focused on several N-xylylpyrrolinium perchlorates, 3-6, whose structural and electronic characteristics have the potential of revealing information about the mechanisms, scope, and synthetic utility of
Scheme I
1. hv
2. SET
R ' 'R 1
photocyclization processes initiated by intramolecular electron transfer. Owing to the electronic differences between the 2ClO,QPh
E ( I ) Borg, R. M.; Heuckeroth, R. 0.;Lan, A. J. Y . ;Quillen, S. L.; Mariano, P. S. J . Am. Chem. SOC., preceding paper in this issue. Lan, A. S. Y.; Quillen, S. L.; Heuckeroth, R. 0.;Mariano, P. S. J . Am. Chem. SOC.1984, 106, 6439.
3 (E=H) 4 (E=TMS)
0002-786318 7 11509-27 38$0 1.5010 0 1987 American Chemical Society
33 5 (E=H) 6 (E=TMS)
J . Am. Chem. SOC.,Vol. 109, No. 9, 1987 2139
Arene-lminium Salt Reaction Pathways
These substances were characterized as the known 2-phenyl-1 pyrroline ( (1 5%) and the phenylbenzoindolizidine 13 (18%).
Scheme 11"
C
O
'
H
a,b
2
E O T M S
TMS
Me
- -
OH
9
d
11
e
5 (E=H) 6 (E=TMS)
12
13
10
followed by basic workup of the crude photolysate and chromatographic separation led to isolation of two major photoproducts.
Spectroscopic data for the latter material were in full accord with the assigned structure. Especially characteristic in this regard was the mass spectrum which contained intense signals a t m / e 144 and 104 corresponding to retro [4 21 fragmentation of the benzoindolizidine nucleus. In addition, two AB quartets in the IH N M R spectra at ca. 3.0 and 3.7 ppm are fully consistent with expected resonances corresponding to the protons at C-5 and C-2 in 13. In contrast, irradiation (Corex) of methanol or acetonitrile solutions of the trimethylsilyl-substituted N-xylylpyrrolinium perchlorate 6 followed by basic workup and chromatography afforded exclusively the benzoindolizidine 13 in yields which vary between 40% and 60%. Careful inspection of the crude photolysate failed to reveal the presence of pyrroline 10 in the reaction mixture. Generation of both the indolizidine 13 and pyrroline 10 can be understood by consideration of the nature and reactivity of intermediates formed following intramolecular electron transfer in the excited state of the xylylpyrrolinium salt 5. The UV spectroscopic properties of the 2-phenyl- and 2-methylpyrrolinium perchlorates demonstrate that irradiation of 5 a t wavelengths greater than 280 nm leads to excitation of the more strongly absorbing conjugated iminium cation chromophore. In addition, considerations of electrochemical potentials and fluorescence quenching information, gained from our studies with iminium salt-arene systems,' suggest that singlet electron transfer from the arene to the excited singlet state of pyrrolinium salt grouping in 5 should be rapid. Indeed, the availability of a reversible electron-transfer pathway must be the reason why the fluorescence efficiency of 5 is negligibly small as compared to that of 1methyl-2-phenyl- 1-pyrrolinium perchlorate. The cation diradical 14 (E = H) produced by electron transfer in the singlet state of 5 can transform to the singlet biradical 15 through deprotonation a t the unsubstituted benzylic position. Homolytic fragmentation of cation diradicals related to 14 ( E = H ) is also known to be a facile process based upon our earlier studies of N-allylquinolinium salt p h o t o ~ h e m i s t r y . ~ Thus, fragmentation of 14 (E = H ) by C-N bond cleavage would give 2-phenyl- 1-pyrroline (10). In this sequence (Scheme HI),generation of photocyclization and photofragmentation products occurs by partitioning at near equal rates of the initially formed cation diradical 14 ( E = H) through deprotonation and C-N bond cleavage pathways. The exclusive production of benzoindolizidine 13 from photoreaction of the trimethylsilyl-substituted pyrrolinium salt 6 can be understood in terms of this same mechanistic scheme. The enhanced chemoselectivity occurring upon replacement of hydrogen by the SiMe3 substituent a t benzylic centers in these systems appears to be related to the relative rates of electrofugal group loss, converting cation diradicals 14 (E = SiMe,) to the neutral diradical 15 and other processes such as C-N bond cleavage available to 14. Thus, exclusive production of 13 from 6 is in accord with the greater rate for arene cation radical desilylation vs. deprotonation noted in studies with intermolecular systems.'.6 Rapid desilylation of 14 ( E = SiMe,) results in its selective transformation to diradical 15 and onward to benzoindolizidine 13.
(2) Bielawski, J.; Brandage, S.; Lindblom, L. J . Heterocycl. Chem. 1978, IS, 97. ( 3 ) Trost, B. M.; Chen, D. M. T. J . Am. Chem. SOC.1983, 105, 2315. (4) Swenton, J . S.; Shih, C. J. Org. Chem. 1982, 47, 2668.
(5) Yoon, U. C.; Quillen, S. L.; Mariano, P. S.; Swanson, R.; Stavinoha, J. L.; Bay, E.; Mariano, P. S. J. Am. Chem. SOC.1982, 104, 1204. (6) Farid, S.; Mattes, S., unpublished results from studies probing the photoaddition reactions of benzyltrimethylsilane and 9,lO-dicyanoanthracene.
"(a) n-BuLi, TMEDA, THF; (b) Me,SiCl; (c) H2S04, THF;(d) Ph3P, CBr,, Et,O; ( e ) NaI, acetone. phenyl- and 2-methyl-1-pyrrolinium cation function in these salts, activation of electron transfer can occur via selective excitation of either the arene donor groupings (in 3 and 4) or the conjugated iminium salt moieties (in 5 and 6). The presence of both hydrogen and trimethylsilyl substituents at benzylic positions of the arene groupings in these salts should provide a n opportunity to probe the chemistry of cation diradical intermediates under conditions in which both fast (in the case of SiMe3) and slow (in the case of H ) pathways for electrofugal group loss are available. Preparation of the N-Xylylpyrrolinium Perchlorates. The N-xylylpyrrolinium perchlorates 3-6 were prepared by benzylation of the corresponding l-pyrrolines,* 7 and 10, with either omethylbenzyl bromide (8) or the corresponding trimethylsilylsubstituted benzyl bromide or iodide, 9 and 11. In the case of the silicon-substituted pyrrolinium salt, 4, benzylation was promoted by use of silver perchlorate, which furnished the perchlorate salt directly. In all other cases the pyrrolinium halide salts were subjected to perchlorate ion exchange on Dowex-1 in order to produce substances having a non-electron-donating counteranion. o-((Trimethylsi1yl)rnethyl)benzyl bromide and iodide, 9 and 11, were prepared by standard procedures from the corresponding benzyl alcohol 12, which itself was generated by an 0,C-bistrimethylsilylation, 0-desilylation sequence (Scheme 11) analogous to that described by Trost3 for (trimethylsilyl)methallyl alcohol synthesis. It should be noted that the silylbenzyl alcohol 12 has been prepared previously by use of a different m e t h ~ d o l o g y . ~
KeBr 1. Et20
-
+
8
7
3
2.c104 exchange
9
7
B~ K
Ql'h'
10
1,neat
e
8
exchange
1 . THF (&,h
+
' - 6 TMS 2 . ~ 1 0 ;
exchange 10 11 Photochemistry of the 2-Phenyl-1-pyrrolinium Perchlorates 5 and 6. Irradiation (Corex) of a methanol solution of the nonsilicon-containing N-xylyl-2-phenylpyrroliniumperchlorate 5
+
~
~~~~
2140
J . Am. Chem. Soc., Vol. 109, No. 9, 1987
Lan et al.
T h e facility of desilylation of the cation diradical 14 is also reflected in the quantum yield for benzoindolizidine formation. Electrofugal group loss from 14 should be competitive with energy-wasting, back electron transfer that transforms 14 to the starting xylylpyrrolinium salt. The rate constants for electrofugal loss, k+, and back electron transfer, kesET, will be reflected in photocyclization quantum efficiencies, c#+, according to the following proportionality [4 a k-E+/(k-E+ kBSET)]. Indeed, the quantum yield for benzoindolizidine 13 formation from the silicon-containing pyrrolinium salt 6 (& = 0.12 f 0.02) is ca. 12 times larger than that for the protio analogue 5 (& = 0.010 f 0.002). Thus, the more rapid rate of desilylation vs. deprotonation of 14 has a pronounced effect not only on the reaction chemoselectivity but also on the quantum efficiencies for photocyclization. As indicated above, the singlet diradical 15 is most likely the direct precursor of benzoindolizidine 13. An alternative and more complex route for transforming 15 to 13 is possible. This involves fragmentation of the diradical 15 to produce the o-xylylenepyrroline pair followed by Diels-Alder cycloaddition. Both steps in a route of this type are reasonable, since homolytic cleavage reactions of 1,4-diradical vinylogous to 15 and [4 2]-cycloaddition reactions of o-xylylenes with imines' are both well-precedented processes. However, investigations with the specifically deuterium-labeled xylylpyrrolinium salt 18 have led to results which argue against the operation of the o-xylylene pathway as well as other more complex sequences for benzoindolizidine generation. The dideuteriated pyrrolinium perchlorate 18 was prepared by a sequence starting with ethyl toluate. Reduction of this ester with lithium aluminum deuteride provided the benzyl alcohol-d2 16, which was transformed to the iodide 17 by a route analogous to that used for preparation of its protio analogue 11 (see Scheme I1 above). Benzylation of 2-phenyl-1-pyrroline with 17 and perchlorate anion exchange then gave the crystalline salt 18.
Scheme I11
+
+
a
COzEt CH3
13 Scheme IV
18
CDzOH
L W 4
7 8(,,, 16
QPh
q
18
DD
\
19 19 Irradiation of 18 ( M e C N , Corex) followed by basic workup and chromatographic separation led to isolation of the corresponding photocyclization product. 'H NMR analysis of this substance demonstrated that it is the 2,2-dideuterioindolizidine 19 uncontaminated (
