Journal of Molecular Catalysis, 34 (1986) 221 - 228 221

HYDROGENATION OF ARENES BY THE RhCls-ALIQUAT@ 336 CATALYST PART 2. REDUCTION OF NAPHTHALENE DERIVATIVES TO TETRALINS

IBRAHIM AMER, HAMDULLAH AMER and JOCHANAN BLUM*

Department of Organic Chemistry, The Hebrew University, Jerusalem (Israel)

(Received March 28,1985; accepted July 15, 1985)

Summary

Hydrogenation of naphthalene in the presence of the RhCls-Aliquat@ 336 catalyst was investigated. The reaction was shown to take place at 30 “C at atmospheric hydrogen pressure and to give decalin-free tetralin. Methyl- naphthalenes, naphthols and ethyl naphthoates were found to be hydro- genated mainly on the non-substituted ring. The naphthols gave tetralones in addition to the expected tetralols. Chlorinated naphthalenes proved to undergo partial hydrogenolysis even before ring hydrogenation. Further reduction of the tetralins to decalins occurred only after complete consump- tion of the starting material. &-hydrogenation was found to be the main stereochemical course of the catalytic process.

Introduction

Only a few soluble transition metal compounds have been reported to catalyze benzene-ring hydrogenation at room temperature under atmospheric hydrogen pressure [ 1 - 41. Naphthalene was found either to react very slowly [4 - 61 or to yield a mixture of tetralin and decalins [3, 51.

In this paper we report the application of the rhodium trichloride- Aliquat@ 336 catalyst [l, 71 for smooth reduction of naphthalene and related compounds to give exclusively tetralins.

Experimental

General procedure for hydrogenation of naphthalene derivatives A 10 ml three-necked flask equipped with a neoprene seal, two gas-tight

taps and a small magnetic bar was charged with a mixture of 10 mg (3.79 X

*Author to whom correspondence should be addressed.

0304-5102/86/$3.50 0 Elsevier Sequoia/Printed in The Netherlands

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lo-’ mmol) of RhC1s*3HS0, 1.0 ml of triple-distilled water, 22 mg (5.4 X

lo-’ mmol) of Aliquat@-336, 100 ~1 (0.23 mmol) of tri-n-octylamine and 0.4 - 3.0 ml of 1,2-dichloroethane. The mixture was stirred at constant rate at 30 f 0.5 “C for 10 min, and 1.0 mmol of freshly chromatographed aro- matic substrate was added. The reaction flask was connected both to an iden- tical vessel via an efficient sintered glass filter coated with a sheet of Milli- pore paper, and to a hydrogenation apparatus. The air in the system was removed and replaced by 1 atm hydrogen. The gas absorption was recorded and samples were withdrawn periodically for GC analysis. After 90 min, the stirring was interrupted and the reaction mixture was freed from a small amount of black precipitate by forcing it through the Millipore filter into the second flask, followed by washing with 0.1 - 0.5 ml of 1,2-dichloro- ethane. All measures were taken to avoid any introduction of air into the system during this operation. The stirring was now resumed in the second flask and the progress of the reaction monitored both by the H2 uptake and by periodic GC analysis of the mixture. After a total of 23 - 144 h (see Table 1) the organic layer was filtered through acidic alumina (pH 4.5), separated and analyzed either on a 10% Carbowax 20M/Chromosorb W or on a 10% OV-l’I/Chromosorb W column. Typical results for naphthalene reduc- tion and for some of its derivatives are summarized in Table 1. The products were identified either by comparison with authentic samples or by their dis- tinguishing ‘H NMR spectra given in Table 2.

R’

1 R’ = R2 = Rs = R’5 = H

2 R2 = Rs = R6 = H R’ = CH3 3 R’ = R2 = R6 = H R5 = CH3 4 R’ = R5 = R6 = H R2 = CHJ 5 R’ = R2 = Rs = H Rb = CHJ 6 R2 = Rs = R6 = H R’=Cl , R’ = R2 = R6 = H Rs = Cl 8 R’ = Rs = R6 = H R2 = Cl 9 R’ =R2 =Rs =H R6 =Cl

19 R2 = Rs = R6 = H R’=OH 11 R’ = Rs = R6 = H R2=OH 12 R2 = Rs = R6 = H R’ = COOC2HS 13 R’=Rs=R6=H R2 = COOCzHS

6

14 R2=H R’=OH 15 R’ = H R2=OH 16 R2=H R’ = COOC2H5 17 R’ = H R2 = COOC2Hs

18 19

OH

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TABLE 1

Hydrogenation of naphthalene derivatives in the presence of the RhC1s.3HzO-Aliquat@ 336 system

Expt. Substrate Reaction Conversion Products time (h) (%) (yield, W)

1 naphthalene 23 81

2 1 -methylnaph- thalene

137 31

3 2methylnaph- thalene

144 17

4 1 chloronaph- thalene

25 74

5 2chloronaph- thalene

25 68

6 1 -naphthalenol 45 100

7

8

9

10

2-naphthalenol 48 88

1 -naphthalene- carboxylic acid ethyl ester

26 82

2naphthalene- carboxylic acid ethyl ester

26 85

2,3-dimethyl- naphthalene

120 35

1,2,3,4ketrahydronaphthalene (1) (81)

1,2,3,4-tetrahydro-l-methylnaphtha- lene (2) (9); 1,2,3,4-tetrahydro-5- methylnaphthalene (3) (22)

1,2,3,4-tetrahydro-2methylnaphtha- lene (4) (1); 1,2,3,4-tetrahydro-6- methylnaphthalene (5) (16)

naphthalene (9); 1,2,3,4tetrahydro- naphthalene (1) (46); 5chloro-1,2,3,- 4-tetrahydronaphthalene (7) (10)

naphthalene (12); 1,2,3,4tetrahydro- naphthalene (1) (32); 6chloro-1,2,3,- 4-tetrahydronaphthalene (9) (24)

1,2,3,4-tetrahydro-1-naphthalenol (10) (19); 5,6,7,8-tetrahydro-l-naph- thalenol (14) (64); 3,4-dihydro-l- (2H)naphthalenone (17)

1,2,3,4-tetrahydro-2-naphthalenol (11) (4); 5,6,7,8-tetrahydro-2-naph- thalenol (15) (70); 3,4-dihydro-2- (lH)-naphthalenone (14)

1,2,3,4-tetrahydro-l-naphthalene- carboxylic acid ethyl ester (12) (23); 5,6,7,8-tetrahydro-l-naphthalene- carboxylic acid ethyl ester (16) (59)

1,2,3,4-tetrahydro-2-naphthalene- carboxylic acid ethyl ester (13) (8); 5,6,7,8-tetrahydro-2-naphthalene- carboxyiic acid ethyl ester (1’7 ) (76)

cis-1,2,3,4-tetrahydro-2,3-dimethyl- naphthalene (18) (12); 1,2,3,4tetra- hydro-6,7_dimethylnaphthalene (19) (23)

Reaction system: 1 mmol substrate, 3.79 X 10M2 mmol RhCls*3H20 in 1 ml triple- distilled water, 3.4 x 10d2 mmol Aliquat @ 336, 0.23 mmol tri-n-octylamine, 0.5 ml (CH2Cl)z (in expt. 10, 1 ml was used and in expts. 6 and 7,3 ml were needed); hydrogen pressure 1 atm; temperature 30 + 0.5 “C.

Repetition of the experiments without filtration of the initially formed precipitate was found to lead to the same results in terms of yield and reac- tion profiles, provided the filtration time was taken into consideration.

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TABLE 2

200 MHz ‘H NMR data of some tetralins listed in Table 1

Compound Chemical shifts (ppm), coupling constants and proton assignment

2 [81

3 191

5 [91

7 [lOI

9 [Ill

12 [ 121

13 [13]

14 [14]

15 [15]

16 [16]

17 a

18[ 171

19 [18]

1.315 (d,3,J = 7.0 Hz, C&), 1.565 (m,2,H2), 1.924 (m,2,H3), 2.778 (t,2, 53.4 = 5.7 Hz, H4), 2.918 (m,l,Hl), 7.030 - 7.174 (m,H5, H6, H7, H8)

1.910 (m,4,H2,H3), 2.306 (s,3,C!&), 2.717 (t,2,J3.4 = 5.7 Hz, H4), 2.569 (t,2,J1,2 = 5.8 Hz,Hl), 7.039 (m,3,H6,H7,H8)

1.776 (m,4,H2,H3), 2.279 (s,3,C&), 2.726 (m,4,Hl,H4), 6.930 (m,3, H5,H7,H8)

1.777 (m,4,H2,H3), 2.749 (t,4,J1,2 = J3,4 = 6.0 Hz,Hl,H4), 6.947 (dd,l, J6,a = 1.8 Hz, J7,* = 7.1 Hz,H8), 7.011 (t,1,J6,7,8 = 7.1 Hz; H7); 7.158 (dd,l,J6,, = 7.1 Hz, J6.8 = 1.8 Hz,HG)

1.772 (m,4,HZ,H3), 2.725 (m,4,Hl,H4), 6.992 (s,l,H5), 7.038 (m,2,H7,

H8)

1,291 (t,3,J= 7.2 Hz,C&), 1.951 - 2.154 (m,H2,H3), 2.825 (m,2,H4), 3.831 (t,l,J1,* = 4.7 Hz,Hl), 4.208 (q,2,J= 7.2 Hz, C&CH3), 7.163 (m,4,H5,H6,H7,H8)

1.287 (t,3,J = 7.0 Hz, C&), 2.231 (m,2,H3), 2.715 (m,l,H2), 2.863 (m, 2,H4), 3.051 (d,2,J = 7.4 Hz,Hl), 4.191 (q,2,J = 7.0 Hz,C&CH3), 7.100 (s,4,H5,H6,H7,H8)

1.823 (m,4,H6,H7), 2.650 (t,2,J,,s = 6.0 Hz,H8), 2.758 (t,2,JS,6 = 6.0 Hz, H5), 4.502 (br s, 1, Oa), 6.620 (d,l,J2,3 = 6.5 Hz,H2); 6.710 (d,l, J3,‘, = 6.5 Hz, H4), 7.006 (t,1,JZ,3,4 = 6.5 Hz, H3)

1.751 (m,4,H6,H7), 2.663 (m,4,H5,H8), 6.530 (d,l,Jr+3 = 2.4 Hz,Hl), 6.630 (dd,l,J1,3 = 2.4 Hz,J3/, = 8.3 Hz,H3), 6.920 (d,l,J3,q = 8.3 Hz,H4)

1.377 (t,3,J= 7.0 Hz,C&), 1.783 (m,4,H6,H7), 2.828 (dist t,2,H5), 3.03 (dist t,2,H8), 4.352 (q,2,J= 7.0 Hz,C&CH3), 7.162 (m,2,H3,H4), 7.630 (dd,l,Jz,3 = 4.3 Hz,J~,~ = 1.3 Hz,H2)

1.380 (t,3,J= 7.0 Hz,C&), 1.801 (m,4,H6,H7), 2.788 (m,4,H5,H8), 4.332 (q,2,J= 7.0 Hz,C&CH3), 7.068 (d,l,J3/, = 8.6 Hz,H4), 7.683 (d, 1,J3,4 = 8.5 Hz,H3), 7.712 (s,l,Hl)

0.921 (d,3,J = 7.0 Hz,C&), 2.003 (m,2,H2,H3); 2.512 and 2.850 (dd,2, J gem ~16.3 Hz,J,~~ = 7.1 Hz,Hl,H4 and dd,2,J,,, = 16.3 HZ, Jvic = 5.0 Hz,Hl’,H4’), 7.085 (s,4,H5,H6,H7,H8)

1.763 (m,4,H2,H3), 2.203 (s,G,CB3), 2.687 (m,4,Hl,H4), 6.850 (s,2,H5,

H8)

aIR (neat) 1695 cm-‘; found: C, 76.63; H, 7.96%. C13H1602 requires: C, 76.47; H, 7.84%.

Results and discussion

In contrast to Muetterties’ ally1 cobalt phosphites that are short lived [2], and to amino acid-rhodium complexes that tend to decompose into the free metal [4], the solvated ion pair [(CsH17)3NCH3]+[RhC14]- [l] gives a perfectly stable arene hydrogenation catalyst that, under exclusion of air,

225

can be recycled in subsequent runs. However, as even the latter complex forms some solid material during the very initial stage of the process, it should be questioned whether the hydrogenation of naphthalenes takes place by homogeneous or heterogeneous catalysis. The fact that both the filtered and the non-filtered reaction mixtures gave identical results in respect to rates and yields, is in favor of a homogeneous catalyst system. Moreover, the filtered solution proved (i) to give a negative Maitlis test for heterogeneity [19] throughout the entire process, (ii) not to scatter He-Ne laser beams [ 201, not to be poisoned by mercury [21], but to be strongly affected by the specific inhibitor of homogeneous systems - dibenzo[a,e]cycloocta- tetraene [ 211. Strongest support for the homogeneous nature of the catalysis was, however, provided by the highly reproducible kinetic measurements for various substituted naphthalene derivatives.

Under the conditions given in the Experimental section, selective hydro- genation of one of the naphthalene rings could easily be obtained. In all sys- tems studied, reduction of the second aromatic ring could be detected only after complete conversion of the starting material into tetrahydro derivatives. An example for such a stepwise reaction is illustrated in Fig. 1.

Table 1 indicates that all substituted naphthalenes studied are reduced preferentially at the non-substituted ring. We attribute this phenomenon to a steric effect on the coordination of the substrate to the catalyst. The inter- ference is particularly pronounced when the substituent is in the /3 position. Thus, while a-methyl-, a-hydroxy- and a-carbethoxy-naphthalene (expts. 2, 6 and 8) yielded tetralins 2 and 3,9 and 14,12 and 16 in ratios 0.409,0.297 and 0.390, respectively, the corresponding ratios of tetralins 4/5,11/15 and 13/17 (expts. 3, 7 and 9 with P-substituted naphthalenes) were only 0.062, 0.057 and 0.105. Introduction of two P-methyl groups seems however, to be less interfering than just one substituent, since they may participate in ally1 coordination to the metal after hydrogen abstraction (cf., e.g. [22]). Conse- quently, 2,3dimethylnaphthalene formed 34% of the tetralin in which the substituents were on the hydroaromatic ring.

time , h

Fig. 1. Reaction profiles for hydrogenation of l-naphthalenecarboxylic acid ethyl ester under conditions of Table 1. The induction period is not shown. (A) starting material; (0) 12; (0) 16; (A) 22.

226

While alkyl, hydroxy and carbethoxy groups proved not to be affected during ring saturation, the chlorine atoms in expts. 4 and 5 were found to undergo partial hydrogenolysis prior to, and probably also during, the ring hydrogenation process. The tetralin formed in these experiments might therefore be the reduction product of both the initially formed naphthalene as well as the chlorinated tetralins 6 and 8. Chlorotetralins 7 and 9, in which the aromatic halogen atoms are less labile, proved to survive at least part of the catalysis.

It is remarkable that although methyl benzoate was reduced by our catalyst system to a 1:9 mixture of cyclohexylmethanol and methyl hexa- hydrobenzoate [ 11, no ester reduction took place in expts. 8 and 9.

In analogy to the formation of cyclohexanone from phenol [ 11, l- and 2-naphthalenols formed substantial amounts of 3,4-dihydro-l(W)- and 3,4- dihydro-2( lH)-naphthalenone, respectively. As these compounds are the keto tautomers of enols 20 and 21, the transformation of naphthalene to tetralin is assumed to involve the intermediacy of dihydronaphthalene pre- cursors e ore’ (see Scheme 1).

Cis-hydrogenation was found to be the preferential stereochemical course of the process. 2,3_Dimethylnaphthalene gave in expt. 10 cis-1,2,3,4- tetrahydro-2,3dimethylnaphthalene free of the trans isomer. Moreover, when the tetralin derivatives were allowed to be further hydrogenated the final fully saturated products consisted either mainly or entirely of cisdecalins.

Scheme 1.

227

For example, both ethyl 1,2,3,4- and ethyl 5,6,7,8-tetrahydro-1-naphthoate (12 and 16, respectively) formed in expt. 8 truns-l-carbethoxy-cis-decahydro- naphthalene (22) as the only product (See Fig. 1).

22: IR (neat) 1710 cm-‘; 200 MHz ‘H NMR (CDC13) 6 1.249 (t, 3, J = 7.1 Hz, CHs), 1.377 - 1.845 (m, 15,H2, H3, H4, H4a, H5, H6, H7, H8), 2.104 (m, 1, HSa), 2.432 (dt, 1, J1+8a = 11.6 Hz, Hl), 4.121 (q, 2, J= 7.1 Hz, C&CH,); 50.32 MHz 13C{ ‘H} NMR (CDCls) 14.295, 20.819, 21.594, 22.156, 24.946, 25.574, 26.546, 32.342, 36.757, 38.796, 47.575, 59.899, 175.190 ppm; Elemental analysis found: C, 74.55; H, 10.65%. Ci3Hz202 requires: C, 74.29; H, 10.48%.

Though we have not yet performed detailed kinetic studies, our results are compatible with the mechanism outlined in Scheme 1. It has the general features of the mechanism proposed by Bleeke and Muetterties [22] for arene hydrogenation by x-CsH5Co(P(OR)s)s.

WhenR10rR2 # H,pathwaya+b+c+d+e+f+gistobefavored over pathway a --f b + c’ + d’ -+ e’ + f--f g, as the latter route suffers more seriously from the steric constraints of the substituents. Bulky R2 groups that interfere with the formation of n-ally1 intermediates c and d cause the metal coordination and subsequent reduction to take place mainly on the unsubstituted aromatic ring.

Acknowledgement

We are grateful to the U.S.-Israel Binational Science Foundation for financial support of this study.

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