5310
J. Org. Chem. 1993,58, 531+5315
Articles Photosensitized Electron Transfer from Azoalkanes: Generation, Fragmentation, and Rearrangement of Radical Cations Structurally Related to Dicyclopentadiene Waldemar Adam’ and Jiirgen Sezldelbach Zmtitut fiir Organkche Chemie, Universitat Wiirzburg, 0-8700Wiirzburg, Am Hubland, Germany Received April 27,1993.
The azoalkanes la-c extrude nitrogen upon photosensitized electron transfer (PET) to yield 1,3radical cation intermediates 2*+,which undergo interesting transformations. Electron back transfer (BET) affords the unrearranged housanes 2, but significant rearrangement to dicyclopentadiene derivatives 3 occurs prior to BET. In some cases, fragmentation to cyclopentadienes 4 is observed, a cycloreversion that occurs at the l,&radial cation stage rather than through the corresponding l,&biradical intermediates. The diphenyl-substituted azoalkane la affords high yields of the symmetric cyclopentadiene 4a, while the alkyl-substituted l b produces only small amounts of a rearranged cyclopentadiene4’b. The housanes 2a,b are also oxidized by PET, but are more reluctant to rearrange. Strong electron acceptors such as triphenylpyrylium tetrafluoroborate (TPT) or cosenaitizationwith biphenyl (Pha)must be utilizedto induce rearrangementto3aYband fragmentation to 4 and 4’.
Introduction Previously it was reported that ground state azoalkanes can serve as electron donors and thereby be converted to radical cations.lg A method frequently used to induce electron transfer is photosensitization (PET): because a set of well-characterized sensitizersis available. Selective excitation and appropriate matching of the energies of the excited states allow exothermic electron transfer, by using azoalkanes as donors.k7 While some azoalkane radical cations have been observed by ESR spectroscopy directlyunder matrix isolation,e.g. 2,3-diazabicycl0[2.2.21&2-eneY1and in solution,e.g. trum-NJV’-di(1-norborny1)diazene? other azoalkanesexperiencerapid loss of nitrogen upon one-electron o ~ i d a t i o n Well-known .~~ for their rapid e Abstract
published in Advance ACS Abstracts, August 15, 1993. (1)(a) Williams, F.;Guo, Q.-X.; Petillo, P. A.; Neleen, 5.F.J. Am. Chem. SOC.1988,110,7887. (b) Gerson, F.;Qin, X.-Z. Helv. Chim. Acta 1988, 71, 1498. (2) (a) Blackstock, S. C.; Mendicino, M. E. J. Am. Chem. SOC.1991, 113,713. (b)Gescheidt, G.;Lamprecht, A.; RGchardt, C.; Schmittel, M. Helv. Chim. Acta 1992, 75, 351. (3) (a)Engel, P. S.;Kitamura, A.; Keys, D. E. J. Am. Chem. SOC.1985, 107,4964. (b) Engel, P. S.; Kitamura, A.; Keys, D. E. J. Org. Chem. 1987, 52, 5015. (c) Engel, P. S.; Lee, W. K.; Marachke, G. E.; Shine, H. J. J. Org. Chem. 1987,52,2813. (d) Engel, P.S., Robertson, D. M., Scholz, J. N., Shine,H. J. J.Org. Chem. 1992,57,6178. (e) Adam, W.; Grabowski, S.;Miranda, M. A.; RGbenacker, M. J.Chem. Soc. Chem. Commun. 1988, 142. (4) (a) Kitamura, A.; Karatau, T.; Hotta, H. J. Chem. SOC.Chem. Commun. 1991,1451. (b) Goodman, J. L.; Zona,T. A. Tetrahedron Lett. 1992,6093-6096. (5) (a) Adam, W.; D b , M. J. Am. Chem. SOC.1987,109, 1570. (b) Adam, W.; Miranda, M. A. J. Org. Chem. 1987,52,5498. (6) Adam, W.: Chen, G.-F.:Walter, H.: Williams, F.J. Am. Chem. SOC. 1992,114,3007. (7) Adam, W.; Denninger, U.; Finzel, R.;Kita,F.;Platach, H.; Walter, H.; Zang,G. J. Am. Chem. SOC.1992,114,5027. (8) (a) Turro,N. J.; Kavarnos, G. J. Chem.Rev. 1986,86,401. (b) Fox, M. A.: Chanon. M.. Eds. Photoinduced Electron Transfer,Part C: Elsevier: Amsterdam, 1988. (c) Kavarnos, G.J. Top. Curr..Ckem. 1990; 156, 20.
Nz extrusion upon direct and triplebsenaitizedphotolysis: 2,3-diazabicyclo[2.2.llhept-2-ene(DBH) and its derivatives denitrogenate effectively upon electron transfer to yield carbon-centered radical cation^."^ Large amounts of products with rearranged carbon skeletonsand producta which arise from nucleophilic cyclization were found.1° A number of intriguing mechanistic features were revealed. Rearrangement takes place not only at the 1,3 radical cation stage, but also at the diazenyl radical cation stage.8 One class of DBH derivatives, namely the azoalkanes la-c, looked promising for the study of fragmentation reactions (cycloreversion)of radical cations. The PET reactions of azoalkanes 1 and the corresponding housane derivatives 2,11 which possess a strained cyclopropane moiety and are prone to oxidation,12would provide access to 1,3- and 1,a-radical cations (Scheme I). Radical cations derived from dicyclopentadieneshave attracted considerableinterest.13J4 For example,the term “nonvertical”radical cations was coined to describe the intriguing structure of those intermediates, which are distinctlydifferent from the diamagneticstarting material. Interconversions into other isomers and cycloreversion were studied to show that such reactions are sensitive to the conformation and the substituents of the starting (9) (a) Engel, P. S. Chem. Rev. 1980,80,99. (b) Adam, W.; Grabowski, 5.;Wileon, R. M. Acc. Chem. Res. 1990,23, 165. (10) Sendelbach, J. Ph.D. Dissertation, University of Wiirzburg, 1993. (11) (a) Beck, K.; H 6 h , A,; H h i g , 5.;Prokschy, F.Chem. Ber. 1984, 117,517. (b) Beck, K.; H h i g , S. Chem. Ber. 1987,120,477. (c) Hiinig, S.; Kraft,P. J . Prakt. Chem. 1990,332, 133. (12) (a) Gassman, P. G. In Photoinduced Electron Tramfer,Part C; Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; p 70. (b) Boche,G.; Walboraky,H.M. In Updatesfromthe ChemietryofFunctionaI Groups: Cyclopropane Derived Reactive Intermediates; Patai, S., Rappoport, Z., Eds.; J. Wiley: Chichester, 1990; p 207. (13) (a) Roth, H. D.; Schilling, M. L. M.; Abelt, C. J. Tetrahedron 1986,42,6157. (b) Roth, H. D. Acc. Chem. Res. 1987,20,343. (c) Roth, H. D. Pure Appl. Chem. 1988,60, 933. (14) Shida, T.; Momorre, T.; Ono, N. J. Phys. Chem. 1985,89,815.
0022-326319311958-5310$04.00/0 0 1993 American Chemical Society
J. Org. Chem., Vol. 58,No.20, 1993 6311
Photosensitized Electron Transfer from Azoalkanes
9,lO-dicyanoanthracene (DCAP as PET sensitizer, no cycloreversion was observed, but minor amounts of rearrangement product 3a were observed (Table I, entry 1). GC, HPLC, and NMR analysis proved that the rearrangement product consisted in all cases (Table I, entries 1-3,9,10) of only one diastereomer, 3a. Unfortunately, ita configuration could not be established by nuclear Overhauser experiments due to the overlap of relevant NMR signals, but the pronounced high-field shift of the 4-H bridgehead proton (cf. 3b) suggests that 3a is of the R* configuration. Molecular models show that in this configuration the 4-H bridgehead proton is effectively
Scheme I
1
J
2
3
3'+
I a:R=Ph;b:R=Me;c:R=H I Scheme I1
1
+ e:
material.l3 While endolexo-dicyclopentadienedoes not fragment after electron transfer in spite of the fact that ita radical cation possesses one cleaved C-C b0nd,1~J* dispiroheptadiene suffers rapid cycloreversion (Scheme IIa).16 In contrast, the anti[4 + 41 dimer (Scheme IIb) undergoes no C-C bond cleavage at all.13a Therefore, the chemical behavior of the intermediates outlined in Scheme I cannot be predicted a priori, and detailed product studies are essential to gain mechanistic insight. We describe the results of this investigation herein.
Results In PET reactions of azoalkane la as an electron donor, rapid denitrogenation occurred to yield the housane derivative 2a,rearranged product 3a, and, in some cases, cyclopentadieneand its derivative 4a (Table I, entries 1-3). Due to the high volatility of cyclopentadiene,its propensity to dimerize, and ita difficult distinction from the solvent by GC, onlythe cyclopentadienederivative 4a was analyzed in order to quantify the extent of cycloreversion. With (15) b t h , H.D.; Schilling, M. L. M.; Abelt, C. J. J. Am. Chem. SOC. 1986,108,6098.
3a
3b
shielded by the phenyl group, which is rotationally fixed by interaction with the methano bridge. When electronically excited 2,4,6-triphenylpyrylium tetrafluoroborate (TPTP was utilized as an electron acceptor, a greater amount of rearrangement product 3a was observed and, more importantly, fragmentation yielded the symmetric cyclopentadiene4a (Table I, entry 2). As observed earlier,1° the addition of biphenyl (Ph2) as ~osensitizer'~ to the PET reactions of these azoalkanes caused important changes in the relative product yields. While DCA alone gave no fragmentation (Table I, entry l),with Phz as coeensitizer(TableI, entry 31, fragmentation was the major pathway and significant rearrangement occurred. The mass balances were close to 100% in all runs, making insignificant any other reaction paths such as oligomerizationor couplingwith the reduced sensitizer, a was observed for sterically less hindered azoalkanes.6J Hydrocarbon 2a is prone to photosensitized oxidation, as was confirmed by the PET reaction with TPT (Table I, entry 9); however, with DCA as electron acceptor, even after prolonged irradiation only very low conversion was achieved and no products were detected by GC or HPLC analysis (Table I, entry 8). Again, addition of the cosensitizer Ph2 changed the reaction drastically. Rapid conversion to rearranged product 3a and cycloreversion to 4a was the primary reaction course. Under the same PET conditions as above, azoalkane lb experienced rapid nitrogen loss to yield the corresponding tetracycle 2b,the dicyclopentadiene3b, and, in some cases, small amounts of cyclopentadiene 4'b (Table I, entries 44). Interestingly,in this casethe fragmentationproduct is a rearranged cyclopentadiene and none of the symmetrical product 4b was detected, this having been unambiguously established by GC coinjection with au(16) The ground state reduction potential End and singlet excitation energy ES are -0.89 V/SCE and 2.86 eV for DCA and -0.29V/SCE and 2.82 eV for TPT.& (17) (a) Could, I. R.; Ege, D.; Moeer, J. E.; Farid, S . J.Am. Chem. SOC. ISSO, 112, 4290. (b) Julliard, M. In Photoinduced Electron Transfer, Part B; Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988;p 216.
Adam and Sendelbach
5312 J. Org. Chem., Vol. 58, No. 20, 1993
Table I. Product Studies of the Photosensitized Electron Tranefer Reaction. of Azoalkanes la-c and Tetracyclodecener 2a,b in Acetonitrile product diatribution ( % )b" entry
substrate
2 3 4 5 6
la la la lb lb lb
7
1c
1
8 9 10 11 12 13 14
2a 2a 2a 2b 2b 2b 3b
sensitizer
time (min)
mbbd ( 5% ) 100 98 99
TPT
7
25 38 42 15 28
DCAlPhS DCA DCA TPTS DCAIPh2 DCA TPTg DCAlPh2 DCAJPh2
7
36
10 40 3 3 40 4 4 120
20 15
DCA
TPT DCAIPh2 DCA
3 3 3
con+& ( % )
7
3
2
86 67 10
90
70
75 85 12
24 9 50
0
41
88
70 20 40 55 5
85
0 63
80 0
-
h h h h h h
-
at
4
14 21 42 30 73 89
f
42 27
-
94 92
h
12 47
--
58
73
-
3
2
-
6 8
-
a See Experimental Section for reaction conditions. *Determined by quantitative capillary GC, except for la (HPLC). Error 6 1 0 % of stated value. The amount of unreacted starting material ie not included in the meee balance (mb). e Relative yields normalized to 100%,error ca. 2%. f The remainder is made UD of several volatile Droducts, which were not characterized. 8 10 mol% of 2,6-di-tert-butylpyridmewere added as base. Starting material.-
thentic material. Under PET conditions,no isomerization of cyclopentadiene 4b to 4'b was observed. The strained hydrocarbon 2b with TPT and DCA/Ph2 also yielded some unsymmetrical cyclopentadiene4'b and, as the major component, the rearrangement product 3b (Table I, entries 12, 13). As for 2a, with DCA alone no reaction was observed (Table I, entry 11). In a control experiment, dicyclopentadiene derivative 3b gave no cycloreversion to 4'b with TPT or DCA/Ph2 as PET sensitizer. No significant conversion was observed even after prolonged irradiation (120 min). Azoalkane IC decomposed upon DCA-sensitized photolysis to yield housane 2c and as many as five volatile products of similar retention time as 2c (Table I, entry 7). No cyclopentadiene or its dimethyl derivatives were detected. Due to the very low mass balance and the low yield of separable material, the new products were not isolated and characterizedexcept for housane 2c. Direct" and benzophenone-sensitizedphotolysis of the azoalkanes lb,c gave no fragmentation and, for la, only insignificant amounts ( 400 nm) for 3-4 h in the presence of DCA (30 mg, 0.114 mmol). The reaction mixture was concentrated and separated through preparative GC on a 1.5-m glass column, which was packed with 1% SE-30 on Volaspher A2 and heated from 160 "C to 200 "C at 4 "C/min at a N2 flow of 1.9 bar with injector and detector temperatures of 210 "C and 230 "C. (3ao,3u,4,9,7&7aa)- 1,3-Diphenyl-2,3-dimethyl-3a,4,7,7a-tet-
J. Org. Chem., Vol. 58, No. 20,1993 6316
rahydro-4,7-methanoindene(3a): IR (CC4) 3042,3018,2980, 2924, 2880, 1573, 1470, 1418, 1305,1290, 'H NMR (200 MHz, CDCl3) 6 0.87 (d, J = 6.6 Hz, lH), 0.92 (dt, J = 6.6,1.6 Hz, lH), 1.58 (br s, 1H), 1.59 (a, 3H), 1.73 (d, J = 2.0 Hz, 3H), 2.08 (dd, J = 7.4,1.6Hz, lH),2.52(brs,lH),3.24(brd,J = 7.4Hz,lH), 6.01 (br s, 2H), 7.18-7.39 (m, 10H); '3C NMR (50 MHz, CDCls) 6 13.5 (q), 29.4 (91, 41.4 (t), 42.7 (d), 43.1 (d), 50.3 (a), 55.1 (d), 56.4 (d), 126.4 (d), 126.9 (d), 127.2 (d), 128.1 (d), 128.8 (d), 128.9 (d), 129.0 (d), 129.1 (d), 129.8 (d), 130.6 (d), 138.4 (81, 139.2 (81, 139.6 (d), 139.8 (d), 141.8 (e), 142.9 ( 8 ) . Anal. Calcd for C&N: C, 92.24; H, 7.76. Found: C, 91.94; H, 7.97. PET Reactions with lb, 2b, and IC. The capillary GC analyses were conducted on a 30-mOV-1 column operated at 40 "C for 5 min, raised at 25 "C/min to 100 "C and then at 5 "C/min to 180 OC, and held there for 15 min. A He flow of 0.8 kp/cmz was employed at an injector and detector temperature of 180 "C and 200 "C; t~ (lb) = 23.9 min, t~ (2b) 17.7 min, t~ (3b) = 18.2 min, t R (4'b) = 7.8, t~ (4b) = 7.6 min; t~ (IC) = 19.8 min, t R (2c) = 15.2 min. For the characterization of the unknown dicyclopentadiene derivative 3b, a preparative run was conducted in in 30 mL of which 200 mg of the azoalkane l b (0.925 "01) acetonitrile was irradiated for 4 h in the presence of DCA (60mg, 0.227 mmol), and the producta were separated by preparative GC on a 1.5-m glass column, which was packed with 10% SE-30 on Volaspher A2 and operated at 120 "C at a N2 flow of 1.7 bar with injector and detector temperature of 170 "C and 190 "C. (3aa,3~,48,7B,7aa)1,2,3,3-Tetramethyl-3a,4,7,7a-tetrahydro-4,7-methanoindene(3b): IR (CC4) 3020,2920,2882,2820, 1550,1522,1425,1355,1307,1240; 'H NMR (200 MHz, CDCl3) 6 0.92 (8, 3H), 0.98 (8, 3H), 1.13 (dt, J = 8.3,1.6 Hz, lH), 1.29 (d, J = 8.3 Hz, lH), 1.48 (m,3H), 1.55 (m,3H), 1.62 (dd, J = 7.2, 1.6 Hz,lH), 2.51 (br d, J = 7.2 Hz, lH), 2.70 (br s,2H), 6.07 (br s,2H); 13C NMR (50 MHz, CDCh) S 9.9 (q), 12.4 (q), 21.2 (q), 31.5 (q), 42.3 (t),42.4 (d), 43.9 (e), 44.9 (d), 52.7 (d), 55.5 (d), 130.1 (s), 137.2 (d), 138.4 (d), 140.7 (8). Anal. Calcd for ClrH,: C, 89.29; H, 10.71. Found: C, 89.09; H, 10.45.
Acknowledgment. We are most grateful for financial support by the Deutache Forschungs-gemeinechaft and the Fonds der Chemischen Industrie for a doctoral fellowship t o J.S.
