Para hydrogen induced polarization in hydrogenation reactions

8564 zyxwvu zyxwvu zyxwvutsrq zyxwvuts zyxwvutsr J. Am. Chem. SOC.1988, 110, 8564-8566 electrochemical methods, to furnish 38-52% yields of a blue-green intermediate 6. The optical spectrum [A, 303 (e 15 300), 380 (45 300), 646 (inf; SOOO), 704 nm (9300)] was almost identical with that of the previous intermediate; the greatly simplified proton N M R spectrum showed three methine peaks (6.26, 5.35, 5.00 ppm), two NHs (13.84, 13.22 ppm), nine methyl resonances (2.03-1.77, 1.40 ppm), and an AB quartet [2.98, 2.52 ppm (each d, JAe= 15.3 Hz)] (Figure 3). Insert A in Figure 3 shows the methine protons of the intermediate from the unsymmetrical a,c-biladiene 3 and demonstrates the presence of unequal amounts of two isomeric structures depending upon which of the two terminal methyls in 3 forms the macrocyclic bridging carbon. Irradiation of the methyl singlet in 6 at 1.40 ppm gave a nuclear Overhauser enhancement at the upfield doublet (2.52 ppm) and also at a methyl resonance (1.77 ppm). On the basis of this evidence, we propose structure 6 for the intermediate, with proton N M R assignments as annotated. High resolution FAB mass spectrometryI6 confirmed the expected molecular weight. The mechanism shown in Scheme I is proposed for the decamethyl-apbiladiene 5 electrocyclization; following deprotonationI7 the conjugated tetrapyrrole suffers two-electron oxidation and macrocyclization to give the intermediate 6. Nucleophilic attack,18 presumably by the electrolyte, causes formation of the phlorin 7 which undergoes spontaneous oxidation19 to give porphyrin. Thin-layer spectroelectrochemistry (not shown) indicates that the order of the nucleophilic attack/oxidation steps may be reversed in the electrochemical conversion of 6 into porphyrin. Acknowledgment. We thank the US.Office of Naval Research (N00014-85-C-0317), Aquanautics Corporation, and the National Science Foundation (CHE-86-19034) for support of this research. The mass spectrometer was purchased with funds provided in part by the National Institutes of Health (RROl460-01). W M Figure 1. P H I P in the IH NMR spectra of R~H.,(PPh~)~-catalyzed hydrogenations in C6D6under -3 atm para-enriched H 2 a t room temperature for (a) styrene&, (b) phenylacetylene, and (c) methyl acrylate: v = CHjCHZCOOCH3, w = P h m H , x = P h C H e H 2 , y = C6DSCDHCHD2, and z = solvent impurities. The resonance at d 4.45 ppm corresponds to H2 (while para H2 is NMR silent, ortho H2 is not). happens fast relative to proton relaxation, then the transferred protons will reflect initially the nuclear spin populations of the starting dihydrogen and give rise to polarized or enhanced transitions for the product resonances. The occurrence of PHIP is thus definitive evidence for pairwise hydrogen transfers. In this paper, we describe studies including the observation of PHIP for hydrogenation reactions catalyzed by ruthenium phosphine complexes. The tetrahydride species R u H ~ ( P P ~is, )a~known hydrogenation catalyst which readily exchanges H2and has recently been shown to be a dihydrogen c o m p l e ~ . When ~ R u H ~ ( P Pis~used ~)~ to catalyze hydrogenation of styrene-d8 in benzene-d6 under 2-3 atm of para-enriched hydrogen, a strong absorption/emission pattern characteristic of PHIP is seen in the 'H resonances of the C6DSCHDCHD2product as shown in Figure la.s The polarization is observable for up to 2 min and decays exponentially with a first-order rate constant of -0.044 s-l. During this period the broad hydride resonance of RuH4(PPh3), at 6 -7.52 ppm is observable and remains unchanged. Hydrogenation of C2D4using RuH4(PPh3), under these conditions also yields para hydrogen induced polarization in the CHD2CHDz product identical with that reported previously.2 When methyl acrylate and the alkynes Ph-H, t-BumH, and MeOCH2C=CH are employed as the substrate in these hydrogenations, the nature of the polarization changes dramatically. This is shown for PhC=-CH and CH,=CHCOOMe in Figure 1 (parts b and c, respectively), in which the initial product resonances (styrene in part b and methyl propionate in part c zyxwvutsrqpon (16) Mass spectra were measured on a VG Analytical ZAB-HS-2F instrument by using fast atom bombardment and a tetraethylene glycol matrix. Compound 6, found 438.2787. Calcd for CZ9H3,N4438.2784. (17) The precise order of deprotonation and oxidation steps cannot be defined at this point in time. (18) Johnson, A. W. In Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; Elsevier: Amsterdam, 1975: DD 741-743. (19) Hopf, F. R.; Whitten, D. 8:In Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; Elsevier: Amsterdam, 1975; pp 678-680, and references therein. Para Hydrogen Induced Polarization in Hydrogenation Reactions Catalyzed by Ruthenium-Phosphine Complexes zyxwvutsr zyxwvut Rein U. Kirss, Thomas C. Eisenschmid, and Richard Eisenberg* Department of Chemistry, University of Rochester Rochester, New York 14627 Received June 17, 1988 Para hydrogen induced polarization (PHIP) leading to enhanced IH N M R absorptions and emissions has recently been reported for hydrogenation and hydrogen addition reactions.'V2 The basis of PHIP, which was presented initially by Weitekamp, involves pairwise transfer of para-enriched H2 to ~ u b s t r a t e . ~ -If~ this (1) (a) Bowers, C. R.; Weitekamp, D. P. J. Am. Chem. SOC.1987, 109, 5541. (b) Bowers, C. R.;Weitekamp, D. P. Phys. Rev. Lett. 1986,57, 2645. (c) Pravica, M. G.;Weitekamp, D. P. Chem. Phys. Lett. 1988, 145, 255. (2) Eisenschmid, T. E.; Kirss, R. U.; Deutsch, P. P.; Hommeltoft, S. I.; Eisenbertg, R.; Bargon, J.; Lawler, R. G.; Balch, A. L. J . Am. Chem. SOC. 1987, 109, 8089. (3) (a) Weitekamp suggests the acronym PASADENA for 'parahydrogen and synthesis allow dynamically enhanced nuclear alignment". We prefer the shorter, less geographically specific PHIP. (b) The term "pairwise" means that both transferred hydrogen atoms originate from the same H2 molecule. Pairwise transfer need not be concerted or synchronous; for PHIP it must be short relative to loss of spin correlation (relaxation) and requires that the protons maintain coupling throughout the hydrogenation process. 0002-7863/88/1510-8564$01.50/0 (4) (a) Komiya, A,; Yamamoto, A. Bull. Chem. Soc. Jpn. 1976, 49, 2553. (b) Cole-Hamilton, D. J.; Wilkinson, G. N o w . J . Chim. 1977, I, 141. (e) Sanchez-Delgado, R. A,; Bradley, J. S.; Wilkinson, G.J . Chem. SOC., Dalton Trans. 1976, 399. (d) Crabtree, R. H.; Hamilton, D. G. J . Am. Chem. Soc. 1986, 108, 3124. (e) Linn, D. E.; Halpern, J. J . Am. Chem. SOC.1987, 109, 2969. (5) These experiments were carried out in 5-mm NMR tubes equipped with a Teflon valve. Solvents (0.5 mL) and substrates (10 rL) were vacuum transfered to an NMR tube containing -3 mg of the ruthenium complex. The tubes were stored at -196 "C. Para enriched hydrogen (prepared by storing H2 over a Fe,O,/silica/C catalyst at -196 "C for 3-4 h) was added just prior to thawing the tube and insertion into the magnetic field. 0 1988 American Chemical Society zyx zyx zyx zyx J . Am. Chem. SOC., Vol. 110, No. 25, 1988 8565 Communications to the Editor (Figure 1)) exhibit “net” polarization with individual resonances entirely in emission or enhanced absorption. The basis for this change in polarization is a magnetic field dependent effect recently discussed by Weitekemplcwhich arises when transfer of para H2 to substrate occurs outside the field of the NMR spectrometer followed shortly thereafter by transport into the probe. Since hydrogenation reactions involving styrene-d,, methyl acrylate, and the alkyne substrates are performed under virtually identical conditions with sample placement into the NMR probe immediately after thawing of the frozen CsD6 reaction solutions, the difference in polarization reflects the fact that methyl acrylate and alkynes hydrogenate more rapidly with RuH4(PPh3), than does styrene. When the RuH4(PPh3),-catalyzedhydrogenation of styrene-ds using para-enriched H2 is carried out in halogenated solvents (CDC13 or CD2CI2),the intensity of PHIP is greatly diminished. Concurrently, the solution changes from colorless to purple-red, and the hydride resonance of RuHCI(PPh3), is observed to grow in. Thus in halogenated solvents, the nature of the catalyst system changes to that of R U H C I ( P P ~ ~ ) ~ . The complex RuHCI(PPh3), is purportedly a very active homogeneous hydrogenation catalyst,*8 and while its mechanism of catalysis is not established definitively, it is thought to function via phosphine loss, olefin coordination, insertion into Ru-H, and hydrogenolysis (H2 addition and alkane reductive eliminati~n).~ In this mechanism, the two hydrogen atoms transferred to substrate originate on different H2 molecules. Therefore, catalysis by R u H C ~ ( P P H , )would ~ be anticipated to produce no PHIP. However, when RuHC1(PPh3), is used to catalyze hydrogenation of styrene-d8in CDCI, or CD2C12under para H2,A/E polarization of the C6DSCHDCHD2resonances occurs similar to, but much weaker than, that seen using R U H ~ ( P P ~ , ) ,The . ~ polarization decays within 90 s but can be regenerated by evacuation and addition of more para-enriched H2. This cycle can be repeated as for up to 15 min of total reaction time. With eth~1ene-d~ substrate, polarization of the CHD2CHD2product resonances is also seen. The Occurrence of PHIP in these reactions establishes that for at least some fraction of product, hydrogenation takes place with pairwise transfer of H2 to substrate. In these experiments, the hydride resonance of R u H C I ( P P ~ , )exhibits ~ no evidence of polarization, unlike the case using RhCI(PPh3),.lb To probe further the mechanism of hydrogenation using RuHC1(PPh3),, the hydride resonance of this complex was examined under different reaction conditions as shown in Figure 2. Under N2, R u H C I ( P P ~ , )exhibits ~ a hydride resonance which is a sharp quartet in either CD2C12(6 -18.22; JPH= 26 Hz) or CDC13 (6 -17.85; JPH= 26 Hz). At low temperature this resonance shifts to -18.55 ppm in CD2C12and appears as a doublet of triplets with couplings of 34 and 22 Hz. Under H2 at 298 K, the hydride resonance of RuHC1(PPh3), appears broad and without coupling, while under D2 (-3 atm), it disappears within seconds, indicative of facile exchange. When examined under hydrogenation catalysis conditions, the hydride resonance shows strikingly different behavior. In the presence of styrene in CD2C12under -3 atm H2 or D2, the hydride resonance appears as a quartet similar to that of the complex under 4 Lf M 1’4 e C zyxwvutsr zyxw (6) (a) Evans, D.; Osborn, J. A.; Jardine, F. H.; Wilkinson, G. Nature 1965, 208, 1203. (b) Hallman, P. S.; Evans, D.; Osborn,J. A.; Wilkinson, G. J. Chem. Soc., Chem. Commun. 1967,305. (c) Hallman, P. S.; McGarvey, B. R.; Wilkinson, G. J . Chem. SOC. A 1968, 3143. (7) (a) James, B. R. Homogeneous Hydrogenation; J. Wiley and Sons: New York, 1973; and references therein. (b) James, B. R. Adu. Organomet. Chem. 1979,17,319 and references therein. (c) James, B. R.; Markham, L. D.; Wang, D. K. W. J . Chem. Soc., Chem. Commun. 1974,439. (8) (a) Parshall, G. W.; Knoth, W. H.; Schunn, R. A. J . Am. Chem. SOC. 1969, 91, 4990. (b) Parshall, G. W. Acc. Chem. Res. 1970, 3, 139. (c) Levinson, J. J.; Robinson, S. D. J. Chem. SOC.A, 1970, 639. (d) Knoth, W. H.; Schunn, R. A. J . Am. Chem. SOC. 1969, 91, 2400. (e) Keim, W. J . Organomet. Chem. 1967, 19, 161. (f) Hudson, B.; Taylor, P. C.; Webster, D. E.; Wells, P. B. Discuss. Faraday SOC. 1968, 46, 37. (g) Jardine, I.; McQuinn, F. J. Tetrahedron Lett. 1966, 4871. (h) Stolzenberg, A. M.; Muetterties, E. L. Organometallics 1985, 4, 1739. (9) RuHCI(PPh3)l was synthesized and isolated from RuCI2(PPh3),+ H2 + Et3N in refluxing toluene.&8b a I I I I I I I I I I I I I I I I I I I I I Figure 2. Hydride resonances of RuHCl(PPh,),: (a) under N2in CDCI,, (b) under -3 atm H2 in CDCl,, (c) under -3 atm Dzin CDCl,, (d) during the hydrogenation of styrene-d8 under -3 atm para-enriched H2 in CDCI,, (e) in CD2C12at -66 “C, and (f) in CDzClz containing 27 mM styrene-d8 at -66 OC. N2 (Figure 2d). Moreover, in the experiment using D2 there is no reduction in hydride intensity for up to 2 h during which several turnovers of ethylbenzene-d2 are noted. It is only after styrene has been consumed that loss of the hydride resonance is observed. It thus appears that substrate suppresses hydride/H2 (or D2) exchange seen under H2 (or D2) alone. Suppression of exchange, however, does not occur by formation of a coordinatively saturated R~HCl(PPh~)~(olefin) complex. The ‘H NMR spectrum of R u H C I ( P P ~ ~ ) substrate ~ (styrene-d8, methyl acrylate, or 1hexene) in CD2C12at -66 OC shows the same hydride resonance as seen in the absence of substrate under N2 (cf., Figure 2, parts e and f, for styrene-d,). While these results-i.e., substrate suppresses hydride/H2 exchange and substrate does not bind to RuHC1(PPh3),-appear at first contradictory, they indicate that R u H C I ( P P ~ is ~ )not ~ the active hydrogenation catalyst nor is it connected to the active catalyst(s) by equilibria rapid on the NMR time scale. A species capable of hydrogenation by pairwise hydrogen transfer and therefore of yielding PHIP is RuH2(PPh3), which forms readily from R U H ~ ( P P ~and , ) ~ can be generated by dehydrohalogenation from R u H C I ( P P ~ ~ This ) ~ . latter pathway has in fact been proposed previously,10and the species RuH2(PPh3), has been invoked as an intermediate in R~H~(PPh,)~-catalyzed hydrogenation^.^^,^ We therefore suggest that even in halogenated solvents, if PHIP is observed, a small and undetectable amount of R U H ~ ( P Pis~present ~ ) ~ as an active catalyst. The qualitative differences in the magnitudes of PHIP, large for R u H ~ ( P P ~ , ) ~ catalysis in C6Ds and weak for RuHC1(PPh3), in halogenated solvents, support this notion. While PHIP establishes a mechanism based on pairwise H2 transfer, at least one other mechanism for the RuHCI(PPh,), catalyst precursor system exists. When olefins with electron-withdrawing groups such as acrylonitrile and tet- + - (10) (a) Strathdee, G.; Given, R. Can. J . Chem. 1975, 53, 106. (b) Hampton, C.; Dekleva, T. W.; James, B. R.; Cullen, W. R. Inorg. Chim. Acta 1988. 145, 165. 8566 zyxwvu J. Am. Chem. SOC.1988, 110, 8566-8567 racyanoethylene are hydrogenated using R u H C I ( P P ~ ~the ) ~ re, action solutions change color rapidly, the Ru-H resonance disappears within 15 s, and hydrogenation is observed after 15 min, but no PHIP is detected. While the present study does not resolve the complexities of the R u H C I ( P P ~catalyst ~ ) ~ system, it does show that a pairwise hydrogen transfer pathway exists, most probably via R u H ~ ( P P ~ ~ ) ~ . In addition, the change in polarization with change in substrate Figure 1. Porphycenes; Du symmetry on the ESR and N M R time scale: H2PC1 (parent compound), R I = R2 = H; H2PC2, Rl = C3H7,R2 = H; using R u H ~ ( P Pin~ CsD6 ~ ) ~ suggests that PHIP may be useful H2PC3, R, = H, R2 = C,H,; PdPC2, the two central H's are replaced in establishing relative rates. This aspect is under continuing study. by Pd, R1 = C3H7, R2 = H . Acknowledgment. We thank the Petroleum Research Fund, administered by the American Chemical Society, and the National Science Foundation (CHE 86-05033) for support of this work, the Johnson Matthey Co., Inc. for a generous loan on ruthenium salts, and Prof. Alan Stolzenberg and Prof. Brian James for helpful discussions. zyxwvut R@try NO.RuH,(PPh,),, 31275-06-6; RuHCI(PPh,)j, 55102-19-7; ethylene-d4, 683-73-8; styrene-d,, 1936 1-62-7; phenylacetylene, 536-74-3; methyl acrylate, 96-33-3. B VH "N Anion Radicals of Porphycenes: First ESR and ENDOR Characterization I Jenny Schliipmann,t Martha Huber,* Moshe Toporowicz,* Matthias Kiicher,I Emanuel Vogel,**L Haim Levanon,*-s and Klaus Mobius*Vt zyxwvut zyxwvuts Institut fur Molekiilphysik, Freie Universitat Berlin Arnimallee 14, 1000 Berlin 33 Federal Republic of Germany Institut fur Organische Chemie, Freie Universitdt Berlin, Takustrasse 3, 1000 Berlin 33 Federal Republic of Germany Department of Physical Chemistry and The Fritz Haber Research Center for Molecular Dynamics The Hebrew University of Jerusalem Jerusalem, 91 904 Israel Institut fur Organische Chemie, Universitat Koln Greinstrasse 4, 5000 Koln Federal Republic of Germany Received July I I, I988 It has been established that porphyrinoid systems are essential chromophores in many photochemical and photobiological processes. This includes a new class of porphyrin isomers, named porphycenes (Figure l), which have been recently synthesized and characterized.'*2 Investigations of their role in photophysical and photochemical processes have been The difference in molecular design and symmetry between porphyrins and porphycenes leads to different spectroscopic behavior as found in recent studies on the photoexcited singlet and triplet states.s5 The doublet state radical ions of porphycenes should also be of considerable interest in comparison with those of analogous porphyrins Institut fur Molekiilphysik, Freie UniversitPt Berlin. * Institut fiir Organische Chemie, Freie Universitgt Berlin. Hebrew University of Jerusalem. Universitiit KBln. ( 1 ) Vogel, E.; Kkher, M.; Schmickler, H.; Lex, J. Angew. Chem. 1986, 98, 262; Angew. Chem. Int. Ed. Engl. 1986, 25, 257. (2) Kkher, M. Ph.D. Thesis, UniversitPt KBln, Federal Republic of Germany, 1988. (3) Ofir,H.; Regev, A,; Levanon, H.; Vogel, E.; Kkher, M.; Balci, M. J . Phys. Chem. 1987, 91, 2686. (4) Levanon, H.; Toporowicz, M.; Ofir, H.; Fessenden, R. W.; Das, P. K.; Vogel, E.; Kkher, M.; Pramod, K. J . Phys. Chem. 1988, 92, 2429. (5) Toporowicz, M.; Ofir, H.; Levanon, H.; Vogel, E.; Kkher, M.; Fessenden, R. W., submitted for publication. 1The - 1 1 1 2 3 I 12 I I 14 1 1 10 I I v[MHzl Figure 2. (a) ESR spectrum of H2PC1'- in THF at 240 K. (b) 14N-and H-ENDOR spectrum of H2PClw in THF at 193 K; experimental Conditions: see note b of Table I. in view of anticipated relationships between electronic structure and reactivity of these compounds. In this communication we report on liquid phase ESR, electron nuclear double resonance (ENDOR), and electron nuclear nuclear triple resonance (TRIPLE) measurements of isotropic interaction parameters such as g factor and H- and 14N-hyperfine coupling constants (hfc's) of the unsubstituted free-base porphycene (H2PC1) anion radical. An extended study on the other porphycenes shown in Figure 1 will be presented in a forthcoming publication. The anion radicals of the porphycenes were prepared chemically by reduction with sodium metal under high vacuum conditiom6 Tetrahydrofuran (THF) was used as a solvent, and the porphycene concentration was about 5-104 M. The radicals were shown to be stable over at least several weeks. The anion radicals of H2PCl were also generated by potentiostatically controlled electrolysis in T H F by using tetra-n-butylammonium perchlorate (TBAP) as the supporting electrolyte.' In this case the porphycene concentration was lV3M. Optical spectra of the neutral and anion radical porphycenes were measured in a 3-mm flat cell. The ENDOR and TRIPLE experiments were performed with a self-built computer-controlled X-band ~pectrometer?~ while for ESR a commerical spectrometer (Bruker ER 200D) was used. The UV-vis spectra of H2PCl were recorded with a Cary-219 spectrophotometer. (6) Paul, D. E.; Lipkin, D.; Weissman, S. I. J . Am. Chem. SOC.1956, 78, 116. (7) Lubitz, W.; Lendzian, F.; Mobius, K. Chem. Phys. Lett. 1981,81, 235; 1981, 84, 33. ( 8 ) Mobius, K.; Plato, M.; Lubitz, W. Phys. Rep?. 1982, 87, 171. (9) Lendzian, F. Ph.D. Thesis, Freie Universitiit Berlin, Federal Republic of Germany, 1982. 0002-7863188115 10-8566%01.50/0 Q 1988 American Chemical Society