Energy & Fuels 1993, 7, 315-318
315
Ionic Hydrogenation of Disubstituted Naphthalenes with BF3oH20-Et3SiH M. Eckert-Maksib* and D. Margetib Department of Organic Chemistry and Biochemistry, Rudjer BoBkoviE Institute, Zagreb, Croatia Received July 17, 1992. Revised Manuscript Received December 29, 1992
A number of dimethoxy-substituted naphthalenes and their mono N(CH3)2or SCH3 analogues are reduced by using Et3SiH or EtsSiD in boron trifluoride monohydrate. Reaction mechanisms are discussed on the basic of ionic deuteration experiments. The mode of protonation of the starting compounds is rationalized in terms of proton affinities calculated by using MNDO procedure. Introduction The ionic hydrogenation of aromatic compounds has gained attention recently as a potentially useful methodology for upgrading of fossil fuels.lP2 In this respect, usage of BFsH20 as acidic component has been found to be of crucial importance.lt3t4 It is strong enough to protonate a number of compounds of potential interest in fuel processing, like, e.g., polycondensed aromatics or naphthalenes substituted with electron-donating groups. Hence, most of these compounds can be either partially or fully hydrogenated in the presence of BFrH20 and appropriate hydride donor. Particularly interesting is a behavior of alkyl aryl ethers3and their sulfur congeners3y4 which subsequent to ring hydrogenation undergo arylheteroatom bond cleavage. For instance, l-methoxynaphthalene (1) and 2-methoxynaphthalene (2) in the presence of BFyH20-Et3SiH as hydrogenating pair produce tetralin in 37 % and 26 % yield, re~pectively.~ Despite the promise engendered in the hydrogenation of some aromatic species by B F ~ O H ~ O - E ~ no ~ Ssys~H, tematic study of the influence of substituents has been carried out so far. Therefore, it seems worthwhile to determine the full scope and limitations of this potentially powerful hydrogenating system. In the current study, we focus our attention on the reactivity of disubstituted naphthalene derivatives 3-10 (Scheme I),with the specific objectives of examining: (i) the reactivity of various positional isomers of dimethoxynaphthalene, (ii) the effect of replacement of one of the methoxy groups with another electron-donating substituents, and (iii) the course of reaction in the presence of EtsSiD instead of EtsSiH. This is of considerable importance from the synthetic point of view,= itoffersapotentialroutefor preparingspecifically deuterated tetralin derivatives. Additionally, in order to evaluate intrinsic basicities of various ring positions in the naphthalene moiety, proton affinities (PA) were calculated for the most characteristic (1)Cheng, J. C.; Maioriello, J.; Larsen, J. W. Energy Fuels 1989, 3, 321-329. (2) Olah, G. A,; Bruce, M. R.; Edelson, E. H.; Husain, A. Fuel 1984, 63, 1130-1137. (3) Larsen, J. W.; Chang, L. W. J . Org. Chem. 1979,44, 1168-1170. (4) Eckert-MaksiE, M.; MargetiC, D. Energy Fuels 1991, 5 , 327-332.
0887-0624/93/2507-0315$04.00/0
representatives of the considered molecules by using semiempirical MND05 procedure.
Results and Discussion Results of ionic hydrogenation experiments for disubstituted naphthalenes 3-10, together with the previously reported results3 for 1 and 2 are summarized in Table I. A survey of the data shows that all examined compounds 3-10 react satisfactorily under conditions employed affording the corresponding tetralin derivatives in good to excellent yields. This is in remarkable contrast to the results reported for 1 and 2 (entries 1 and 8 in Table I). Not unexpectedly, j3 position isomers 6 and 7 were found to be significantly less reactive than their a-position counterparts. Thus, 6 and 7 give 6-methoxytetralin in 35% and 50% yield, respectively, upon 6 h treatment. However, when the reactions were allowed to proceed for 24 h (experiments 8 and 10) the yields on hydrogenation product increased to 95% and 80%, respectively. Of particular interest in this respect are the results obtained for compound 5. When treated with BFsH20Et3SiH for 24 h (experiment no. 6, Table I), this compound produced 657% 6-methoxytetralin as the single hydrogenated product, indicating that only the ring substituted at a-position undergoes hydrogenation. Another interesting case is provided by compound 10. This isomer was found to be least reactive among all examined naphthalene derivatives. In addition, both OCH3groups are subjected to hydrogenolysis. Thus, NMR ( 5 ) Dewar, M. J. S.; Thiel, W. J. Am. Chem. SOC.1977,99,4899-4907, 4907-4916.
0 1993 American Chemical Society
316 Energy & Fuels, Vol. 7,No. 2, 1993
Eckert-MaksiE and Margetit
Table I. Reduction of Compounds 1-10 with BF3.HzO-Et3SiH at 25 O C expt no. 1 2 3 4 5 6 7 8 9
10 11 12 13 14
starting compd 1
2 3 4
5 5 6 6 7 7 10 10 8 9
hydrogenation product tetralin tetralin 5-methoxytetralin 5-methoxytetralin 6-methoxytetralin 6-methoxytetralin 6-methoxytetralin 6-methoxytetralin 6-methoxytetralin 6-methoxytetralin tetralinc tetralinC 5-dimethylaminotetralin 5-methylthiotetralin
yields," % 376 266
100 100 49 65 35 95 50
80 65 80 100 100
reaction time, h 10
10 6 6 6 24 6 24 6 24 24 48 24 24
Yields of hydrogenation product with respect to the starting base as determined by 'H NMR and GC. *Reference 3. 2,3Dimethoxy-3,4-dihydronaphthalene and 2,3-dimethoxytetralin are the minor products.
('Hand 13C)and GUMS analyses of thereaction products obtained after 24 h treatment of 10 with BFyHzO-Et3SiH show the presence of the starting compound (35% ) along with the mixture of hydrogenated products in which tetralin (M+ = 132) appears as the principal component. In addition to tetralin, the presence of 2,3-dimethoxy3,4-dihydronaphthalene (M+ = 190) and 2,3-dimethoxytetralin (M+ = 192) is detected albeit in rather small quantities. Their portion, however, becomes insignificant upon prolonging reaction time to 48 h, suggesting that both components undergo subsequent reaction with BF3*HzO-Et3SiH themselves. Having established the role of substitution pattern of the OCH3 groups on the potential of the naphthalenic ring for reduction with BF3-Hz0-Et3SiH9two experiments (entries 13and 14 in Table I) were carried out to ascertain the susceptibility of the heterosubstituted naphthalenes for ionic hydrogenation. For this purpose 1,5-disubstituted derivatives 8 and 9 were chosen, in order to permit a direct comparison with the results obtained for 1,5-dimethoxynaphthalene (3) and 1,5-dimethylthionaphthalenes.The first among them, as shown above, undergoes hydrogenation with extreme easiness, while the second compound, as found in the preceding study," can be considered as a prototype of poorly reactive substrate. The current results show that replacement of only one of the OCH3 moieties in 3 with either N(CH3)z (leading to 8) or SCH3 (leading to 9) has slightly retarding influence on the easiness of reduction. In both instances, quantitative reduction of the ring to which OCH3group is attached is achieved within 24 h. To summarize, the present results indicate that the potential of the naphthalene ring for reduction with BFrHz0-EtSiH ionic hydrogenating pair depends strongly on the electron-donating ability of the substituents, their location, and their number. This is in full agreement with conclusions reached by Larsen's group in studying reactivity of various monosubstituted naphthalenes.1J The above-mentioned factors are also responsible for the very high degree of chemoselectivity of the process. Formation of the single hydrogenated product in all cases considered so far strongly indicates that protonation of the naphthalene moiety with BFrH20 leads to the formation of single arenium ion. In order to gain more detailed insight into the mode of their protonation, we proceed in two ways: (a) experi-
Table 11. Ionic Deuteration of Naphthalene Derivatives 1, 3,6,and 10 with EtsSiD in BFsaH20 at 25 OC, and Results for Ionic Deuteration of 1-Methoxytetralin expt no.
1 2 3 4 5
starting compd
1 3
yield," %
hydrogenation product
100 5-methoxytetralin-1,1,3-d3 100 7-methoxytetralin-1,3,3-d3 60
tetralin-l,1,3-d3
6
10 tetralin-1,.2,3,4-d4 1-methoxy- tetralin-1-d tetralin
24 100
reaction time, h
24 24 37 144 24
Yields of deuterated product with respect to the starting base as determined by 'H NMR and GC.
Scheme I1
r
1
OCH,
Scheme I11
W
O
\ \
1 oa
CH3
Xk
mentally by carrying out some of the hydrogenation experiments in the presence of Et3SiD instead of Et3SiH and (b) theoretically by calculating the intrinsic proton affinities of the various ring positions for some of the considered species. Ionic Reductions with BFrH20-Et3SiD. The results of ionic reduction experiments carried out by using Et3SiD for compounds 1,3,6, and 10 are shown in Table 11. Their perusal indicates that in each case incorporation of three deuterium atoms in the reduced ring was observed, 10 being an exception.6 Two of deuterium atoms enter the position occupied earlier by the departing methoxy group, while the third one is located meta to this position. The observed pattern is compatible with a stepwise mechanism3y7shown in Scheme I1 for the compound 3 as representative of cup-disubstituted naphthalenes. It should be stressed, however, that neither 1,5-dimethoxy3,4-dihydrotetralin nor 1,5-dimethoxytetralin derivatives could be detected among reaction products even upon considerable shortening of the reaction time. This may be understood in terms of their high reactivity under ionic hydrogenation conditions. The parent 3,4-dihydrotetralin is known to undergo ionic hydrogenation even in the presence of weaker acid than that used in the current study,* while the structurally related 1-methoxytetralin readily exchanges the OCH3 group with deuterium upon treatment with BF3*HzO-Et3SiD (entry 5, Table 11). Using the same arguments, similar reaction pathway can be postulated for the reduction of isomers carrying the OCH3 group(s) in the j3-ring position(s); the single exception is given again by 10. Its exposure to BF30H20EtsSiD leads, namely, to formation of tetralin-l,2,3,4-& (6) MargetiE, D.; Eckert-MaksiC,M., to be submitted for publication. (7) Cf.: Kursanov, D. N.; Parnes, Z. N.; Kalinkin, M. I.; Loim, N. M. Ionic Hydrogenation and related Reactions; (English translation by Wiener, L.),Horwood: London, 1985. (8) Parnes, Z.N.; Beilinson, E. Y.;Kursanov, D. N. Zh. Org. Chem. 1972,8, 2342-2346.
Hydrogenation of Disubstituted Naphthalenes
Energy & Fuels, Vol. 7, No. 2, 1993 317
Table 111. Proton Affinities (PA's) of Oxygen and Selected showing lH resonances of 1.77 (4H), 2.72 (2H) and 6.80Ring Carbons of Naphthalene Derivatives 1-3,6,6, and 10 7.20 (4H) ppm, as the principal product. Such a deuterium and Heats of Formation of the Corresponding Conjugate distribution is rather difficult to account for except if Acids (A&(BH+)) as Calculated by MNDO Method protonation occurs at the carbon atoms carrying OCH3 site of AHr(BH+), PA, kJ groups. Addition of the proton a t the a-ring position compound protonation kJ mol-' mol-' should have resulted in formation of tetralin-2,2,3,3-&. 2 694 851 One could probably invoke steric effects to rationalize 3 752 793 lowering of the intrinsic basicity of the a-ring carbon atoms a \ / I 4 679 866 adjacent to the sites of substitution. That is, due to close 11 775 770 1 proximity of the methoxy groups in 10, the CH3 groups 1 676 861 within substituents might be forced out of the aromatic 3 725 912 plane. The hybridization of oxygen atom in such a case 4 734 803 would approach the sp3 canonical state and the transfer 11 758 779 1 542 851 of electrons from the methoxy groups would become more 3 563 830 difficult. Examination of the MNDO computed formal 4 528 865 charges for the planar (loa) and perpendicular (lob) 11 605 788 conformers of 10 shows that rotation of the CH3 groups 2 509 868 influences mostly the electron densities of oxygen and C-1 3 591 786 4 497 880 and C-2 ring atoms. Oxygen becomes more negative (by 5 508 869 0.03 eV), while the latter atoms become less negatively 7 535 842 (C-1) and less positive (C-2) charged, respectively. The 8 585 792 ,,& decrease in negative charge at C-1 appears to be com11 601 776 HsCO 12 603 774 promise between two opposite effects; an increase in the 5 u-electron density and a simultaneous decrease in the 1 522 849 ?r-electrondensity. The former contribution is outweighed 3 553 818 HaCO 4 552 819 by the latter leading to the net lowering of the negative 6 11 592 779 charge density by 0.06 eV. In contrast, the C-2 atom in 1 540 (567)" 839 (659)" 10b appears to be less positive by 0.04 eV than in loa, as 2 608 (627) 771 (599) owJ 11 a consequence of an increase in both s- and p-electron 582 (602) 797 (624) 10 density. It is further interesting to note that totalelectron density at C-1 in 10b is of the same order of magnitude Values for structure 10b are given in parentheses. as the total electron densities of the carbon atoms in the unsubstituted ring, in sharp contrast to the situation The results are summarized in Table I11 and are selfencountered in loa. explanatory. However, a few general comments are in Proton Affinities of Dimethoxynaphthalenes. The place. As far as methoxynaphthalenes are concerned, high selectivity observed in the course of ionic hydrogeMNDO predicts the largest affinity for protonation at the nation of methoxynaphthalene derivatives and their C-4 atom in 1, while in 2 the highest PA is obtained for heteroatoms analogues in BFrH&-Et&H, as mentioned the C-1 atom. The predicted trend is in full agreement above, is governed by the protonation step. Since each of with experimentally established sites of protonation for the employed substrates possesses more than one basic these compounds,12showing that MNDO calculated PA's center, the outcome of protonation will strongly depend can serve as useful guide in predicting protonation sites on the intrinsic basicities of these centers. To evaluate for the class of compounds considered in this work. these values, we have calculated proton affinities (PA's) Turning to dimethoxynaphthalenes, we note that for for each of the ring carbon atoms and corresponding all molecules containing methoxy group(s) in the a-ring heteroatoms in the given molecules by using the MNDO position's protonation at the C-atom in the para position procedure. Although the PA values derived in this way to the site of substitution is predicted to be strongly favored usually depart from the experimentally determined PA's over protonation at the other basic sites in the molecule. by roughly lo%, their relative ordering for the various Similarly a-carbon atom (C-1 or C-5) has the largest PA sites within polybasic compounds is generally c o r r e ~ t . ~ J ~ of all carbon atoms in molecules possessing the methoxy The same holds for the series of structurally related group in the O-ring position. It should be noted that both molecules.s In each case, full geometry optimization was predictions are consistent with the pattern of deuterium performed in all parent compounds (B) of interest and distribution observed in ionic deuteration experiments. their protonated forms (HB+). The gas-phase proton Finally, comparison of the results obtained for 2 and 10 affinities were then calculated according to the following indicates that introduction of the second methoxy group equation in the @positionwithin the same ring significantly reduces affinity for its protonation. This trend is even more PA(B) = AHf(H') + AHf(B) - AHf(HB+) (1) pronounced for the structure in which the CH3 groups within methoxy fragments are rotated by 90° relative to The experimental value of AHf(H+)was used (1536 kJ the naphthalene ring. However, irrespective of the conmol-')" because MNDO predicts AHAH+)to be too high formation of the methoxy group, the C-1 ring position is by 171 kJ mol-l. predicted to be the most basic center. This, together with the charge distribution in the starting base is at variance (9) DeKock, R. L.; Jasperse, C. P. Inorg. Chem. 1983,22,3839-3843. with the outcome of ionic deuteration of 10. Obviously, (10)Olivella, S.;Urpi, F.; Vilarassa,J. J.Comput. Chem. 1984,5,230-
,&j I
,
aYCHa
236. (11) Lias,S.In Kinetics of Ion-Molecule Reactions; Ausloos, P., Ed.; Plenum Press: New York, 1978; p 233.
(12)Olah, G. A.; Mateescu, G. D.; Mo, J. K. J. Am. Chem. SOC.1973, 95,1865-1874.
318 Energy &Fuels, Vol. 7,No. 2, 1993
further mechanistic studies, which are beyond the scope of this paper, are desirable in order to clarify this point.6
Experimental Section Starting Materials. The starting compounds were prepared either by methylation13of the commercially available dihydroxynaphthalenes (5-7,lO)or according to the procedures described and 9l7). Boron trifluoride monohydrate earlier (3,'4 4,'5 (deuteride) was obtained by bubbling BF3 into ice-cooled water (D20).3EtaSiH was reagent 99% grade and was used without further purification. Et3SiD was prepared by reducing Et3SiCl with LiA1D4.1s Instrumentation. The purity of the starting compounds and/ or the identification of the products was accomplished by using the following methods: gas chromatography (Varian Aerograph M-180; GC conditions: 3% SE-30onVaporite 30,110/120mesh), 'Hand NMR spectrometry (Jeol FX-SOFT, solvent CDCl3, internal reference tetramethylsilane), IR spectrometry (PerkinElmer, Model 297), mass spectrometry (Varian CH-7 spectrometer),and in some cases GC/MS analysis (VarianGC spectrometer 3400 equipped with a 30-mnarrow-bore methylsilica capillary column connectedwith MAT ITD 800 Finigan MS spectrometer). Calculations were performed on Univac 1100 and Convex CP 120 at the University Computing Center in Zagreb. Stability of all starting compounds in BF3sH20 as well as the extent of deuteration of the most characteristic representatives in BF3aD20 were checked prior to hydrogenation experiments. All of the examined substances were found to be stable in the former acidic system. They also undergo considerable H/D exchange within the ring indicating that the applied acidic system is able of protonating the naphthalene ring. The hydrogenation experiments were performed by using 7-10 M excess of BF3.H20 and 3-4 M excess of Et3SiH according to the procedure outlined below. Typical Procedure for Ionic Hydrogenati~n.~.~ Dimethoxynaphthalene 3 (300 mg, 1.0 mmol) dissolved in 3-4 mL of CHzClz was added dropwise to a flask containing BF3.H20 (1.1g, 12.8 (13)Zweig, A.;Maurer, A. H.; Roberta, B. G. J.Org. Chem. 1967,32, 1322-1329. (14)(a) Rutolo, D.; Lee, S.; Sheldon, R.; Moore, H. W. J.Org. Chem. 1978,43,2304-2308. (b) Bun-Hoi, N.P.; Lavit, D.; J. Chem. SOC.1956, 2412-2415. (15)Parker, K. A.; Iqbal, T. J. Org. Chem. 1980,45,1149-1151. (16)Tarbell, D. S.;Fukushima, D. K. J. Am. Chem. SOC.1946,68, 1456-1459. (17)Watson, E.R.; Dutt, S. J. Chem. SOC.1922,121,2414-2419. (18)(a) Gilman, H.; Dunn, H. E. J. Am. Chem. SOC.1951,73,34043407. (b) Karakhanov, E.A.; Demjanova, E. A.; Skarin, E. G.; Viktorova, E.A. Khim. GeterosykL Soedin. 1975,11, 1479-1481.
Eckert-MaksiE and MargetiE mmol) at 0 "C. After the addition was completed, the mixture was stirred for 10-15 min and allowed to warm up to room temperature, and then triethylsilane (742 mg, 6.4 mmol) was added dropwise. The reaction mixture was stirred at room temperature for additional 5-6 h, neutralized by cold saturated aqueous Na2C03 solution, and extracted with CHzClz (3 X 10 mL). The combined organic extracts were washed with water (2 X 15 mL), dried (MgS04),and evaporated on a rotary evaporator to leave ca. 270 mg of brownish oil, which according to NMR and GC analysescontained ca. 90% of 5-methoxytetralin. Purification of the crude product was accomplished by column chromatography on silica gel using CHzClz/ether as eluent. Reductions with BF3.H20-Et3SiD were performed according to the procedure outlined above. The sites of deuteration, as determined by 1H NMR spectroscopic analysis, are indicated in Table 111. The relevant spectroscopic data of the deuterated products are as follows: Tetralin 1,1,3-d3: IR (KBr) 2920,2860,2160,1490,1450,780, 760 cm-1; 1H NMR (CDC13)6 1.75 (s, 3 H), 2.71-2.73 (m, 2 H), 7.04 (s,4 H); MS, m/e (relative intensity) 135 (M+,66), 134 (21), 106 (loo), 105 (48), 93 (25), 92 (18). B-Methoxytetralin-l,1,3-d3:IR (KBr) 2930,1590,1470,1440, 1255, 1100 cm-1; IH NMR (CDC13) 6 1.72 (s, 3 H), 2.00-2.66 (m, 2 H), 3.77 (s,3H), 6.57-7.12 (m, 3 H);MS, m/e (relative intensity) 165 (M+,loo), 150 (30), 136 (46), 135 (31), 134 (33), 106 (29), 93 (35). 7-Methoxytetralin-1,3,3-d3: IR (KBr) 2910,2880,2150,1615, 1505,1260,1235,1040cm-1; 'H NMR (CDC13) 6 1.72-1.78 (m, 3 H), 2.67 ( 8 , 2 H), 3.76 (s, 3 H), 6.61-7.22 (m, 3 H); MS, m/e (relative intensity) 165 (M+,loo), 164 (65),163 (21), 150 (7), 137 (28), 136 (70), 135 (40), 134 (16), 93 (18). Tetralin-1,2,3,4-d4;IR (KBr) 3060,3020,2920,2160,1495,1450 cm-l; 'H NMR (CDC13) 6 1.77 (m, 2 H), 2.73 (m, 2 H), 6.80-7.22 (m, 4 H). Tetralin-1-d: IR (KBr) 2940,2860,1505,1495,1450,745 cm-I; 'H NMR (CDCl3) 6 1.52 (m, 4 H), 2.72 (m, 3 H), 6.75-7.23 (m, 4 H), MS, m/e (relative intensity) 133 (M+, 1001, 132 (44), 115 (12), 104 (83), 91 (41), 85 (651, 83 (100).
Acknowledgment. Financial support by Grant PN 740 from the Department of Energy and Ministry of Science, Technology and Informatics of the Republic of Croatia is gratefully acknowledged. We are indebted to the Bayer AG, Organic Chemical Group, for a generous and 1,6-,2,6-,and gift of 1-amino-5-hydroxynaphthalene 2,3-dihydroxynaphthalene. We also thank Professor J. W. Larsen for valuable discussion.
