Energy & Fuels 1993, 7, 166-171
166
S y mp osium Geochemical Origins of Sedimentary Benzoporphyrins and Tetrahydrobenzoporphyrins Timothy D. Lash Department of Chemistry, Illinois State University, Normal, Illinois 61 761-6901 Received September 10, 1992. Revised Manuscript Received October 20, 1992
Complex mixtures of metalloporphyrins are associated with organic-rich sediments. Although some of these petroporphyrins have been correlated with known biological pigments, the origins of certain structures are obscure. Benzoporphyrins and tetrahydrobenzoporphyrins,two minor families of geological tetrapyrroles, have been isolated from numerous oil shales and petroleums but the origins of these molecular fossils are poorly understood. A number of possible pathways for the geochemical formation of benzo- and tetrahydrobenzoporphyrinsare discussed. The data presently available favors a Diels-Alder cycloaddition mechanism between putative divinylchlorophyll and quinone precursors for the formation of these compounds. As an aid to these studies, total syntheses of benzo- and naphthoporphyrins have been carried out and the spectroscopic properties of these extended chromophores are discussed. Further studies are proposed to help distinguish between the various possible pathways for the geochemical genesis of tetrahydrobenzo- and benzoporphyrins.
The presence of metalloporphyrins in organic sediments, such as oil shales, petroleum, and coal, lends support to the widely held view that these materials are derived from living organisms.' In the last 10 years, individual petroporphyrins have been isolated, primarily from oil shales, and characterized spectr~scopically.~-~ In particular, nOe difference proton NMR spectroscopy has provided a powerful tool for the structure determination of petroporphyrins. In certain cases, these structures have been further supported by synthetic s t ~ d i e s . ~ - ~ ~ Many of the metalloporphyrins present in oil shales have (1) (a) Baker, E. W.; Palmer, S. E. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol 1, pp 486-552. (b) Callot, H. J. In The Chlorophylls; Scheer, H., Ed.; CRC Press: Boca Raton, FL, 1991; pp 339-364. (2) Filby, R. H.; Van Berkel, G. J. InMetal Complexes in Fossil Fuels. Filby, R. H.; Branthaver, Geochemistry,Characterization,andProcessing; J. F., Eds.; American Chemical Society: Washington, DC, 1987; pp 2-37. (3) (a) Chicarelli, M. I.; Kaur, S.; Maxwell, J. R. In Metal Complexes in Fossil Fuels. Geochemistry, Characterization, and Processing;Filby, R. H., Branthaver, J. F., Eds.; American Chemical Society: Washington, DC, 1987; pp 40-67. (b) Keely, B. J.; Prowse, W. G.; Maxwell, J. R. Energy Fuels 1990,4,628-634. (c) Eckardt, C. B.; Keely, B. J.; Waring, J. R.; Chicarelli, M. I.; Maxwell, J. R. Philos. Trans. R. SOC.London B 1991, 333, 339-348. (4) (a) Ocampo, R.; Callot, H. J.; Albrecht, P. In Metal Complexes in Fossil Fuels. Geochemistry, Characterization, and Processing; Filby, R. H.; Branthaver, J. F., Eds.; American Chemical Society: Washington, DC, 1987; pp 68-73. (b) Callot, H. J.; Ocampo, R.; Albrecht, P. Energy Fuels 1990, 4, 635-639. (c) Verne-Mismer, J.; Ocampo, R.; Bauder, C.; Callot, H. J.; Albrecht, P. Energy Fuels 1990, 4, 639-643. (5) Fischer, H.; Hofmann, H. J. Justus Liebigs Ann. Chem. 1935,517, 274-277. (6)Sugihara, J. M.; McGee, L. R. J. Org. Chem. 1957, 22, 795-798. (7) Baker, E. W.; Corwin, A. H.; Klesper, E.; Wei, P. E. J . Org. Chem. 1968,33, 3144-3148. (8) Flaugh, M. E.; Rapoport, H. J. Am. Chem. SOC.1968, 90, 68776879. (9) Chaudry, I. A.; Clezy, P. S.; Mirza, A. H. Aust. J . Chem. 1980,33, 1095-1104. (10)Smith, K. M.; Langry, K. C.; Minnetian, 0. M. J . Org. Chem. 1984,49,4602-4609. (11) Verne-Mismer, J.; Ocampo, R.; Callot, H. J.; Albrecht, P. Tetrahederon Lett. 1986, 27, 5257-5260.
0887-0624/93/2507-0166$04.00/0
structures which may easily be correlated to known biologicalpigments. The identification of precursor-fossil pairs is a particularly exciting development; striking examples include the identification of "bacteriopetroporphyrins" related to bacteriochlorophylls d26 and the isolation of metalloporphyrins purportedly derived from chlorophylls c1, c2, and c3.13,27,28Nonetheless, many petroporphyrins show little resemblance to known bio(12) Verne-Mismer, J.;Ocampo, R.; Callot, H. J.; Albrecht, P. J.Chem. Soc., Chem. Commun. 1987, 1581-1583. (13) Verne-Mismer, J.; Ocampo, R.; Callot, H. J.; Albrecht, P. Tetrahedron Lett. 1988,29, 371-374. (14) (a) Lash, T.D.; Perun,T. J., Jr. TetrahedronLett. 1987,28,62656268. (b) Lash; T. D.; Johnson, M. C. TetrahedronLett. 1989,30,56975698. (15) Lash, T. D. Tetrahedron Lett. 1988,29,6877-6880. (16) Lash, T. D. Org. Geochem. 1989,14, 213-225. (17) Lash, T. D.; Catarello, J. J., submitted for publication. (18) Lash, T. D.; Balasubramaniam, R. P. Tetrahedron Lett. 1990,31, 7545-7548. (19) Lash, T. D.; Balasubramaniam, R. P.; Catarello, J. J.; Johnson,
M.C.;May,D.A.,Jr.;Bladel,K.A.;Feeley,J.M.;Hoehner,M.C.;Marron, T. G.; Nguyen, T. H.; Perun, T. J., Jr.; Quizon, D. M.; Shiner, C. M.; Watson, A. Energy Fuels 1990, 4, 668-674. (20) May, D. M., Jr.; Lash, T. D. J . Org. Chem. 1992,57, 4820-4828. (21) Smith, N. W.; Smith, K. M. J.Chem. Soc., Perkin Trans. I1989, 188-190; Energy Fuels 1990, 4,675-688. (22) Clewlow, P. J.; Jackson, A. H.; Roberts, I. J. Chem. SOC.,Chem. Commun. 1985, 724-726. Clewlow, P. J.; Jackson, A. H. J . Chem. Soc., Perkin Trans. 1 1990, 1925-1936. (23) Clezy, P. S.; Fookes, C. J. R.; Prashar, J. K. Aust. J . Chem. 1989, 42, 775-786.
(24) (a) Clezy, P. S.; Prashar, J. K. Aust. J. Chem. 1990,43,825-837. (b) Clezy, P. S.;Jenie, U., Prashar, J. K. Aust. J . Chem. 1990,43,839-856. (c) Clezy, P. S.; Mirza, A. H.; Prashar, J. K. Aust. J . Chem. 1990, 43, 857-866. (d) Clezy, P. S.; Van Phuc, L.; Prashar, J. K. Aust. J. Chem. 1991, 44, 1061-1075. (e) Clezy, P. S.; Fookes, C. J. R.; Prashar, J. K.; Salek, A. Aust. J . Chem. 1992, 45, 702-712. (25) Clezy, P. S. Aust. J . Chem. 1991, 44, 1163-1195. (26) Ocampo, R.: Callot, H. J.; Albrecht, P. J . Chem. SOC.,Chem. Commun. 1985, 200-201. (27) Ocampo, R.; Callot, H. J.; Albrecht, P.; Kintzinger, J. P. Tetrahedron Lett. 1984, 25, 2589-2592. (28) Verne-Mismer. J . ; Ocampo, R.; Callot, H. J.; Albrecht, P. Tetrahedron Left. 1990, 31, 1751--1754.
0 1993 American Chemical Society
Origins of Sedimentary Porphyrins
Energy & Fuels, Vol. 7, No. 2, 1993 167
logical metalloporphyrins. The 15,17-butanoporphyrins 1 (R = Et, Me, H)have been isolated from several different oil shales and appear to be derived from chlorophyll a (or related pigments) via a known cyclizationto the propionate ester side chain.29 However, the origins of the isomeric
Scheme I
Scheme I1
ROC :
R
cH3w 4
Scheme I11 -eREDUCTION
ME
II
1
R=EI
b R=Me 2
methylpropanoporphyrins 2 from Serpiano Oil shale30 are more mysterious, although a plausible pathway for the formation of these geoporphyrins has been put forward.18 It should be borne in mind when considering the possible pathways leading to petroporphyrin formation that these processes will be influenced by a variety of factors, including sedimentary environment and thermal stress. In addition, metabolic transformations undoubtedly play a significant role in the early stages of diagenesis and this further complicates the analysis of petroporphyrin origins. In the 1960s, a minor family of petroporphyrins with rhodo-type visible spectra were r e ~ o g n i z e d . ~ lOn - ~ ~the basis of mass spectrometry, Baker suggested that these "rhodoporphyrins" were in fact benzoporphyrins 3.34335 This hypothesis received substantial support from the synthetic studies of Clezy and co-workers, although this was based primarily on the similarity of the electronic spectra.36~~~ The origin of the benzoporphyrins is problematical, since no known biological pigments have structural features of this type. Baker and Palmer proposed'" that these compounds arose from the Diels-Alder condensation of naturally occurring quinones and chlorophyll a (Scheme I). Diels-Alder adducts of protoporphyrin IX have been extensively studied,3u1 and in certain cases they have been converted into the related benzoporphy(29) Falk, H.; Hoornaert, G.; Isenring, H-P.; Eschenmoser, A. Helu. Chim. Acta 1975,58, 2347-2357. (30) Chicarelli, M. I.; Wolff, G . A.; Murray, M.; Maxwell, J. R. Tetrahedron 1984,40, 4033-4039. (31) Howe, W. W. Anal. Chem. 1961,33, 255-260. (32) Thomas, D. W.; Blumer, M. Geochim. Cosmochim.Acta 1964,28, 1147-1154. (33)Millson, M. F.; Montgomery, D. S.; Brown, S. R. Geochim. Cosmochim. Acta 1966, 30, 207-222. (34) Baker, E. W. J . Am. Ckem. SOC.1966, 88, 2311-2315. (35) Baker, E. W.; Yen, T. F.; Dickie, J. P.; Rhodes, R. E.; Clark, L. F. J. Am. Chem. SOC.1967,89, 3631-3639. (36) Clezy, P. S.; Fookes, C. J. R.; Mirza, A. H. Aust. J. Chem. 1977, 30, 1337-1347. (37) Clezy, P. S.; Mirza, A. H. Aust. J . Chem. 1982, 35, 197-209. (38) Callot, H. J.; Johnson, A. W.; Sweeney, A. J . Chem. SOC., Perkin Trans. 1 1973, 1424-1427. (39) DiNello, R. K.; Dolphin, D. J. Org.Chem. 1980,45, 5196-5204. (40) Pangka, V. S.; Morgan, A. R.; Dolphin, D. J. Org. Chem. 1986,51, 1094-1 100. (41) Cavaleiro, J. A. S.; Jackson, A. H.; Neves, M. G. P. M. S.; Rao, K. R. N. J . Chem. Sac., Chem. Commun. 1985, 776-777.
H
1
HGW
c * w 4 H I
7
H
1 rins.42,43Hence, this proposal gains solid support from these model studies. Benzoporphyrins with additional exocyclic rings (BenzoDPEPs), as well as the related tetrahydrobenzoporphyrins(THBDPEPs),were suggested on the basis of mass spectrometricstudies. Since the DielsAlder hypothesis provided an elegant explanation for the origins of these petroporphyrins, this led to an expectation that the site of ring fusion would lie on the A ring relative to the five-membered exocyclic ring (structure 4; Scheme 11). In 1984, Barwise and Roberts44 first reported the tentative identification of a THBDPEP from El Lajjun (Jordan)oil shale. They suggested, on the basis of partial nOe difference proton NMR studies, that this compound had structure 5, where the six-membered exocyclic ring was fused to ring D. Since this compound could not arise from the Diels-Alder condensation discussed above, an alternative proposal for the origin of this structure was put forward (Scheme 111). Some evidence for the reduction of the propionic ester side chain of chlorophyll a was available,'l~~~ and an initial reduction to the alcohol 6 was suggested. Furthermore, porphyrin alcohols, albeit with quite different structures, have now been identified in the (42) Morgan, A. R.; Pangka, V. S.; Dolphin, D. J. Chem. Soc., Chem. Commun. 1984, 1047-1048. (43) Yon-Hin, P.; Wijesekera, T. P.; Dolphin, D. Tetrahedron Lett. 1989, 30, 6135-6138. (44) Barwise, A. J. G.; Roberts, I. Org.Geochem. 1984, 6, 167-176.
Lash
168 Energy & Fuels, Vol. 7, No. 2, 1993
R Me Messel oil shale.45 It was suggested that cyclization might then take place via the methylene tautomer 7. A variation on this mechanism involving cyclization onto the propionate ester side chain had been proposed p r e v i ~ u s l y . ~ ~ Although this imaginative scheme provided a plausible route to 5, chemistry of this type has yet to be observed in porphyrin systems. In any case, Maxwell and co-workers I2 13 11 R-ELnMC subsequently investigated the same oil shale and identified R=QmMe The two novel fused ring porphyrins 8a and on the origins of the tetrahydrobenzoporphyrins and THBDPEP 5 could not be detected by these workers, and benzoporphyrins from oil shales assume that all the carbon it seems likely that Barwise and Roberts were in fact atoms derive from the original biological porphyrin handling the “diDPEP” 8a. precursor. The bacteriochlorophylls d (14) have been tentatively suggested as precursors to the benzoporphyrins 9.47 Cyclization of a propyl side chain might lead to the formation of the THBporphyrin system (Scheme IVa), which might be further dehydrogenated to give benzoporphyrins 9. Porphyrins related to bacteriochlorophylls d W were isolated from the Messel oil shale.26 However, since 3 R R=El 5 b R=Me benzoporphyrins have been detected in relatively immature sediment^,^^^^^ such a pathway does not seem likely. If it does occur, one might anticipate that the isobutyl CH, NH N$ $ r HC NH Nside chain, also found in bacteriochlorophylls d , could cyclize similarly to give a methyl THBporphyrin 15 Y N HN (Scheme IVb). Perhaps five-membered rings could also CHI \ CHI CHI \ CH, be formed from the more abundant ethyl substituents CHICH CHjCH (Scheme IVc). In addition, one might speculate that 10 9 R = El Me examples of THB- and benzoporphyrins with 12-ethyl R = Et 01Me substituents or 20-methyl groups (the latter from In 1986, Maxwell and co-workers reported the isolation bacteriochlorophyllsc or e ) would also be isolated. Characof two benzoDPEPs 9 from Boscan crude oil (Cretaceous, terization of structures of these types would go a long way Vene~uela).~’In this case, the structures were firmly toward supporting this pathway. supported by nOe proton NMR studies. However, the Nearly all naturally occurring porphyrins are biochemposition of the benzo ring fusion was conclusively shown ically derived from uroporphyrinogen I11 (16, Scheme V). to be at the B position in these structures and this led to With the possible exceptions of bacteriochlorophylls c, d, a dismissal of the Diels-Alder route as an explanation for and e,j3the natural chlorophylls appear to be formed from the origins of sedimentary benzoporphyrins. French a later biological intermediate, protoporphyrin IX (17). It worked2isolated the related tetrahydrobenzoporphyrins is possible, however,that certain chlorophylls (from extinct 10 from the Moroccan Timahdit oil shale, together with organisms or presently unrecognized)retained the acetate/ structure 11, which may possibly derive from chlorophyll propionate substituents on ring B. If this were the case, c. Complex mixtures of benzoporphyrins were subsethe formation of tetrahydrobenzo- and benzoporphyrins quently shown to be present in a variety of organic-rich might be readily explained as being due to a Dieckmann sediments.48 condensation (Scheme VI).4b737This type of chemistry Many of the known porphyrin structures from oil shales was utilized by Clezy and co-workers in the total synthesis can be correlated directly to known biological precursor of b e n z o p ~ r p h y r i n s .However, ~ ~ ~ ~ ~ it does stretch credulity pigments, and their formation can usually be rationalized somewhat to assume that a chlorophyll c analog also as involving straightforward degradative and rearrangebearing acetate/propionate side chains gave rise to ment processes alone. Hence, little credence has been petroporphyrin 11. (It is also possible that the original given to intermolecular condensation reactions in the chlorophyll structures bore six-membered rings, presumformation of petroporphyrins. Methylethanoporphyrins ably owing their biogenesis to chemistry of this type.) U3and the related fused ring “diDPEPs” 1349are possibly The possibility that certain petroporphyrins were debest rationalized as being formed by microbial methylation rived from classes of chlorophylls that were associated processes, and high molecular weight p e t r o p ~ r p h y r i n s ~ ~ with extinct groups of organisms cannot be completely are unlikely to have carbon atoms solely derived from the dismissed. However, if the benzoporphyrins and their biological precursors. However, most current speculations tetrahydro analogs were derived from chlorophylls that are no longer found in the living world, one would expect (45) Prowse, W. G.; Maxwell, J. R. Geochim. Cosmochim. Acta 1989, that these structures would be absent from recent sedi53, 3081-3083. (46) Prowse, W. G.; Chicarelli, M. I.; Keely, B. J.; Kaur, S.; Maxwell, ments and this is patiently not the case. Indeed, benJ. R. Geochim. Cosmochim.Acta 1987,51,2875-2877. See also Chicarelli, zoporphyrins have been detected in sediments whose M. I.; Maxwell, J. R. Tetrahedron Lett. 1986, 27, 4653-4654. I
,
‘, 01
(47) Kaur, S.;Chicarelli, M. I.; Maxwell, J. R. J. Am. Chem. SOC. 1986,
108. ~-~ 1347-1348. ~ ~
I
~
-
(48) Kaur, S.; Gill, J. P.; Evershed, R. P.; Eglinton, G.; Maxwell, J. R. J. Chromatogr. 1989, 473, 135-151. (49) Boreham, C. J.; Fookes, C. J. R. J . Chromatogr. 1989,467, 195208. (50) Quirke, J. M. E.; Cuesta, L. L.; Yost, R. A,; Johnson, J.; Britton, E. D. Org. Geochem. 1989, 14,43-50.
(51) Baker, E. W.; Louda, J. W. Org. Geochem. 1986,10, 905-914. (52) Quirke, J. M. E.; Dale, T.;Britton, E. D.; Yost, R. A,; Trichet, J.; Belayoumi, H. Org. Geochem. 1990,15, 169-178. (53) Smith, K. M. In Biosynthesis of Tetrapyrroles; Jordan, P. M., Ed.; Elsevier: New York, pp 237-255. (54) Ping’an, P.; Eglinton, G.; Jiamo, F.; Guoying, S. Energy Fuels 1992, 6, 215-225.
Origins of Sedimentary Porphyrins
Energy & Fuels, Vol. 7, No. 2, 1993 169
Scheme IV
Scheme VI
R = Fsmeryl. etc. R' = Et. "R.'"Bu R'
0rNcopsntyl R? =
~ 01e~t
9 -9 i
COIR 14
Bacteriochlorophyll-c X = CH3; R'= Me Bactedochlomphyll-d X = CH,: R1= H Bacteriochlorophyll-c X = C H O R'= Me
o
Scheme VI1
Scheme V. Biosynthetic Pathway from Uroporphyrinogen-I11 to the Hemes and Chlorophylls
Scheme VI11 AI
A.
Uroporphyrinogen.111
Coproporphyrinogcn.ii1
B.
Ar-N.O
/ N
Chlorophylls
origins span vast expanses of geological time including the Permian (e.g., Marl slate,'@Chinese marine sediment LMK654),Jurassic (e.g., Kimmeridge shale49,Cretaceous (e.g., Boscan crude El Lajjun oil Julia Creek oil shale49, and Palaeocene (Gafsa chert48) ages. In addition, the presence of benzoporphyrins in recent sedimentary materials is also well documented.52 Hence, if the sedimentary benzoporphyrins were indeed derived from Dieckmann cyclizations in unique chlorophyll structures as depicted in Scheme VI, it would be seem very probable that these unusual chlorophylls still persist in marine ecosystems. While much remains to be learned about marine phytoplankton and bacterioplankton,55 the (55) Bidigare, R. R.; Kennicutt, M. C., 11, Ondrusek, M. E.; Keller, M. D.; Guillard, R. R. L. Energy Fuels 1990, 4, 653-657.
\
HN
, /
CH
Protoporphyrin-I1
/ \
CH
+ monoadduct
19
existence of this particular group of unusual chlorophyll structures should be treated with skepticism. In this author's opinion, the Diels-Alder pathway has been prematurely dismissed. Although chlorophyll a only has a vinyl group on the A ring, the divinyl analog 18 is common in marine algae.55 This divinylchlorophyll might undergo Diels-Alder condensations at the A or B rings, but there is strong experimental evidence to suggest that condensation at the B ring would be preferred (Scheme VII). Jackson and co-workersfound that nitrosobenzenes underwent Diels-Alder cycloaddition reactions with protoporphyrin IX to give monoadducta (Scheme VIIIA), but further addition could not take place to give the bisad-
Lash
170 Energy & Fuels, Vol. 7, No. 2, 1993
Scheme IX 6
q
' H
\\
Scheme X
Wavelength (nm) + Wavelength (nm) + Figure 1. Electronic spectra (Soret bands omitted) for (A) On the other hand, protoporphyrin I1 (Scheme VIIIB) condensed with nitrosobenzenes to give a mixture of the monoadduct and the bisadduct 19.41p56It seems likely that the chlorin chromophore of 18 would be preferenti.ally converted to a bacteriochlorin 20, rather than the less stable isobacteriochlorin 21 (Scheme VII). Initial formation of a bacteriochlorin intermediate readily explains the selectivity for the B ring in these condensations. Chlorophyll cz might be the precursor to 11, although the origin of the regioselectivity in this case is not clear. Possibly, the presence of the exocyclic ring also effects the selectivity of these processes. In chlorins, the reduced D ring enhances the double bond character of the &carbons in the "antipodal" B ring and this leads to the specificity discussed above in DielsAlder cycloaddition reactions. Crossley and co-workers have noted similar regioselectivities in the bromination of meso-tetraarylporphyrins.57 One can speculate that other condensations involving the enhanced double bond character of the 0-carbons in the B ring could also be possible. Chlorophyll b can be invoked as a precursor to the benzoporphyrins and tetrahydrobenzoporphyrins. Michael addition of an enolate ion (Scheme IX, using acetone as an example), followed by cyclization onto the formyl group, would give a six-membered ring. Further degradation might lead to 9 and 10. Similarly, chlorophyll c358 might give rise to the tetrahydrobenzoporphyrin 11. A further possibility for the formation of the required six-membered ring system involves a cycloaddition reaction with chlorophyll b.52 Quirke and his collaborators suggested that a tautomer of chlorophyll b might undergo a Diels-Alder cycloadditionwith a suitable dienophile (e.g., a quinone; Scheme X) to form the six-membered ring structure. Further degradation might lead to benzo- or tetrahydrobenzoporphyrins. If this mechanism is correct, one might anticipate that the additional methyl group (56)Recent studies by Dolphin and co-workers also support this hypothesis. Yon-Hin, P.; Wijesekera, T. P.; Dolphin, D. Tetrahedron Lett. 1991, 32, 2875-2878. See also: Pandey, R: K.; Shiau, F.-Y.; Ramachandran, K.; Dougherty, T. J.; Smith, K. M. J.Chem. SOC.,Perkin Trans. 1 1992,1377-1385. (57)Crossley, M. J.; Burn, P. L.; Chew, S. S.; Cuttance, F. B.; Newsom, I. A. J. Chem. Soc., Chem. Commun. 1991,1564-1566. (58)Fookes, C. J. R.; Jeffrey, S. W. J. Chem. SOC.,Chem. Commun. 1989,1827-1828.
y?J
yJ
benzoporphyrin 23a and (B) naphthoporphyrin 27.
Scheme XI
/
Me
-
HN
D*
/ Et
Et 22a
Me
/
Me
/
Me
Et
Et 23a
would be retained in many petroporphyrins but the major benzoporphyrin 9 (R = Et) from oil shales48is not so substituted. Hence, this route is unlikely to constitute a major pathway to sedimentary benzoporphyrins. The data currently available does not allow us to fully distinguish between these proposals. Indeed, it is possible that more than one of these pathways may play a role in the formation of benzo- and tetrahydrobenzoporphyrins. Future resolution of this issue will be dependent on the identification of additional structures, probably by using a combination of proton NMR spectroscopic analysis and comparisons to synthetic standards. We have developed new routes for the synthesis of geochemically significant porphyrins bearing carbocyclic rings.14-z0~59~60 Recently, we completedz0 the total synthesis of four isomeric tetrahydrobenzoporphyrins 22a-d. These compounds have very similar proton NMR spectra, and the resonances due to individual substituents do not resolve well enough to succumb to nOe analysis. Two of these structures (22a,b) might arise from heme (or protoporphyrin 1x1by the Diels-Alder pathway. The Barwise and Robert's proposal (Scheme 111) might provide a pathway to the isomeric porphyrins 22c,d. Hence, the identification of tetrahydrobenzoetioporphyrinscould give helpful clues about the origins of these compounds. It is noteworthy that the tetrahydrobenzoporphyrin22a was easily oxidized to the corresponding benzoporphyrin 23a by treatment with 2.1 equiv of DDQ in refluxing toluene (Scheme XI), (59) (a) Lash, T. D.; Bladel, K. A.;Johnson, M. C. Tetrahedron Lett. 1987,28,1135-1138.(b)Lash,T.D.;Bladel,K.A.;Shiner,C.M.;Zajeski, D. L.; Balasubramaniam, R. P. J. Org. Chem. 1992,57,4809-4820. (60)Lash, T.D.; Denny, C. P. Manuscript in preparation.
Origins of Sedimentary Porphyrins
Energy &Fuels, Vol. 7, No. 2, 1993 171
and this provides an exceptionally simple route for the synthesis of sedimentary %hodoporphyrins”. The spectroscopic properties for 23a were in good agreement to those previously reported for the benzoporphyrin system36J7 and 23a gave a characteristic pseudorhodo visible spectrum (Figure 1A). Figure 2. Partial 300-MHzproton NMR spectrum of naphthoporphyrin 27.
MewRi Scheme XI1
NH
N-
1 H‘
O W
CHO El
M M I-ee
t
/
J
@eM DDQ CHICIl
* Me
I
\
,
E:
Me
t
EI
27 25
26
Chromium trioxide oxidation of crude petroporphyrin mixtures yields a variety of substituted maleimides, phthalimide, and methylphthalimide.61 The latter product suggests that methyl substituted benzoporphyrins 24 may be present in oil shales and petroleum. The identification of alkylbenzoporphyrins (or the related THBporphyrins) would be of some value. The Diels-Alder pathway, for instance, might give rise to benzoDPEPs 24, where R’ and R2 are alkyl groups, but R3 and R4 should always be hydrogens since this unit is derived from the original vinyl moiety. The Michael condensation-cyclization route (Scheme IX) might give alkyl substituents for R2and R4, but R1 and R3are likely to be hydrogens. Cycloadditions to the chlorophyll b tautomer shown in Scheme X might give alkyl substituents for R2or R3;R4would most likely remain a methyl group and R1 should be a hydrogen. Cyclization of the alkyl substituents from bacteriochlorophylls d (Scheme IV) would be most likely to give a methyl substituent for R3, as discussed previously. Dieckmann condensations (Scheme VI) could conceivably give rise to methyl groups for R1, or possibly R3, due to the reduction of carboxylic acid (or ester) substituents. Hence, the position of alkyl substitution in sedimentary benzoporphyrins will give important insights into their origins. Quinone Diels-Alder adducts might also conceivablyyield naphtho[l,2-blporphyrins25, and naphtho[2,3-blporphyrins 26 could potentially arise from Scheme X. Identification of extended aromatic structures of this type could also be informative. As an aid to these studies, we (61) Barwise, A. J. G.; Whitehead, E. V. In Aduances in Organic Geochemistry 1979; Douglas, A. G.; Maxwell, 3. R., Eds.; Pergamon Press: New York, 1980; pp 181-192. (62) Lash, T.D.; Roper, T. J. Unpublished work.
recently completed the synthesis of a naphtho[l,2-b]porphyrin 27 from the dipyrrolic precursor 28 using the MacDonaId and apbiladiene routes described in Scheme XII.60 The electronic spectrum for 27 is distinctive, showing a pronounced hyperchromic shift to band 3 in the visible or Q band region (Figure lb). The proton NMR spectrum for 27 is also well resolved and shows a series of characteristic resonances in the aromatic region (Figure 2). Further synthetic studies on porphyrins with fused benzenoid rings are presently in progress.G2 The origins of the benzo- and tetrahydrobenzoporphyrins from organic sediments remain mysterious. No likely precursors have yet been identified a t the early stages of diagenesis and the definitive structures determined to date do not allow us to completely differentiate between the various pathways discussed in this paper. Stunning progress has been made in the field of porphyrin geochemistry over the last few years and the identification of relevant petroporphyrin structures can only be a matter of time. At this stage, Baker and Palmer’s Diels-Alder pathway remains the most likely explanation for the origin of these geoporphyrins. However, new chlorophyll pigments continue to be identified and a new discovery in this area may yet provide alternative possibilities for precursor structures to these unusual molecular fossils. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant No. CHE-9201149, the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Organized Research Fund of IllinoisState University. We also thank the National Science Foundation (Grant No. CHE-9001175) for providing funds to purchase a Varian 300-MHz NMR spectrometer.
