Transition Metal Sulfur Chemistry - American Chemical Society

Chapter 10 Catalytic Hydrogenolysis of Thiophenic Molecules to Thiols by Soluble Metal Complexes 1

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Downloaded by UNIV OF GUELPH LIBRARY on October 7, 2012 | http://pubs.acs.org Publication Date: November 29, 1996 | doi: 10.1021/bk-1996-0653.ch010

C. Bianchini , A. Meli , and R. A. Sánchez-Delgado

1Istituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione, CNR, Via J. Nardi 39, 50132 Firenze, Italy Instituto Venezolano de Investigaciones Cientificas, Caracas 1020—A, Venezuela 2

The thermally generated 16-electron fragments [(triphos)RhH] and [(triphos)IrH] react with benzo[b]thiophene (BT) and dibenzo[b,d]thiophene (DBT) by C-S bond scission to give (triphos)Rh[η3-S(C H4)CH=CH2] and (triphos)IrH(η -C,S-DBT), respectively [triphos = MeC(CH2PPh2)3]. The Rh complex is an efficient catalyst precursor for the homogeneous hydrogenation of BT, forming 2-ethylthiophenol and, to a lesser extent, dihydrobenzo[b]thiophene. The Ir complex is a catalyst precursor for the homogeneous hydrogenation and hydrodesulfurization of DBT to 2-phenylthiophenol, biphenyl and H2S. The mechanisms of these catalytic transformations have been elucidated by the isolation and characterization of key species related to catalysis combined with high pressure NMR spectroscopic studies. 2

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The development of efficient catalysts for the simple hydrogenolysis of thiophenic molecules to thiols remains an attractive goal in hydrodesulfurization (HDS) catalysis. The thiol products can then be desulfurized over solid catalysts under milder reaction conditions than those required to accomplish the overall HDS of the thiophene precursors. This aspect is particularly important for the dibenzothiophenes since the conventional catalysts can desulfurize the corresponding aromatic thiols without affecting the benzene rings, necessary to preserve a high octane rating. Some homogeneous modeling studies devoted to understanding the mechanisms through which thiophenes are degraded to thiols by transition metal complexes have recently been reported (1-10). Among these, only two examples have been described in which the hydrogenolysis reactions occur in catalytic fashion (9,10). The present article is concerned with these two homogeneous hydrogenolysis reactions, which involve benzo[£]thiophene (BT) and dibenzo[6,iflthiophene (DBT) as model compounds. Rh-Catalyzed Conversion of Benzo[£]thiophene into 2-Ethylthiophenol The most widely accepted mechanisms for HDS of BT over solid catalysts are shown in Scheme 1. Path a begins with hydrogénation to form dihydrobenzo[&]thiophene 0097-6156/96/0653-0187$15.00/0 © 1996 American Chemical Society In Transition Metal Sulfur Chemistry; Stiefel, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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TRANSITION METAL SULFUR CHEMISTRY

( D H B T ) prior to C-S bond scission and desulfurization. The second pathway (b) involves initial C-S bond scission, followed by desulfurization of the 2vinylthiophenol product, and hydrogénation of the vinyl group. The results obtained with the homogeneous metal system described below show that the hydrogenolysis of B T to 2-ethylthiophenol (ETSH) can occur only after insertion has occurred into the C-S bond and that path b may be redirected (dotted line in Scheme 1) so as to contain the hydrogénation of 2-vinylthiophenol to E T S H prior to the desulfurization step. The thermally generated 16-electron fragment [(triphos)RhH] reacts with B T by C-S bond scission to give (triphos)Rh[rp-S(C6H4)CH=CH2], (1) [triphos = MeC(CH2PPh2)3] (11). The 2-vinylthiophenolate complex, 1, forms by reductive coupling of a terminal hydride with the vinyl moiety of a metallabenzothiabenzene intermediate (Scheme 2) (7). The 2-vinylthiophenolate complex, 1, is an active catalyst precursor for the homogeneous transformation of B T into E T S H . At 160°C and 30 arm H2 in either THF or acetone, B T is converted to E T S H with an average rate of 13 (mol per mol of catalyst per hour) in the first two hours. The reaction is not fully selective as D H B T is also formed (relative rate of 0.6 ) in an independent catalysis cycle (vide infra). Catalytic runs performed under different conditions show that the rate of formation of E T S H increases significantly with the concentration of B T , while it is only slightly affected by the H2 pressure (Table I). Below 15 arm H 2 and 100°C, no appreciable transformation of Β Τ is observed. The catalytic system is truly homogeneous up to 180°C. Above 200°C, appreciable decomposition of the catalyst occurs with formation of Rh metal particles which are responsible for the observed heterogeneous HDS of BT to ethylbenzene and H2S (entries 6,14 of Table I). The catalytic mechanism has been probed by high-pressure N M R (HPNMR) spectroscopy combined with the isolation and characterization of key species of the catalytic cycle. Under catalytic conditions, ^ P ^ H ) N M R spectroscopy shows that all rhodium is incorporated into (triphos)Rh(H)2[o-S(C6H4)C2H5], (2) and [ ( η triphos)Rh{|i-o-S(C6H4)C2H5}]2, (3). Below 100°C, only the dihydride complex, 2, is present in solution. After quenching the catalytic reactions with dinitrogen, all rhodium is recovered as the bis-thiolate complex (triphos)RhH[0-S(C6H4)C2Hs]2 2

(4).

The nature of the chemical processes that connect compounds 1, 2, 3 and 4 has been elucidated by independently carrying out a variety of reactions using isolated compounds, some of which are summarized in Scheme 3. The 2-vinylthiophenolate complex, 1, reacts in THF with H2 (>15 atm) at 60°C, quantitatively converting to the dihydride, 2. This reaction has been mimicked by the sequential addition to 1 of H+, H", and H2. From this experiment, it has been concluded that the conversion of 1 to 2 is a stepwise process in which the higher activation energy step is the first H2 uptake to give the (alkyl)hydride (triphos)RhH^ -S(C6H4)CH(CH )], (5). This step may involve a heterolytic splitting of H 2 . The dihydride, 2, and the dimer, 3, are in equilibrium in THF by reductive ehmination/oxidative addition of H2, while the dimer 3 reacts with E T S H yielding the bis-thiolate complex, 4. Incorporation of all of the experimental evidence leads to the mechanism shown in Scheme 4 for the reaction between B T and H2 (15-60 atm) catalyzed by 1 in the temperature range from 120 to 180°C where the system is homogeneous. Initially, the 2-vinylthiophenolate ligand in 1 is hydrogenated to 2ethylthiophenolate (steps a-b). The unsaturated 16-electron fragment [(triphos)Rh{0S(C6H4)C2H5}] either picks up further H2 to give the dihydride 2 (step d) or dimerizes to 3 (step c), the latter path being favored at high temperature and low H2 pressure. It is the dihydride complex that, upon interaction with B T , eliminates E T S H and forms an r ^ - S - B T adduct (step e). In the r\ -S bonding mode, B T is 2

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In Transition Metal Sulfur Chemistry; Stiefel, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

10. BIANCHINI ET AL.

Catalytic Hydrogenolysis of Thiophenic Molecules HS


2

HS 2

Scheme 1. Proposed heterogeneous mechanisms for HDS of BT.


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BIANCHINI ET AL.

Catalytic Hydrogenolysis of Thiophenic Molecules

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activated in such a way that C-S insertion is followed by attack by the electron-rich Rh(I) metal on the adjacent carbon atom (via electron donation into the C-S antibonding orbital). As a result of the C-S bond scission, the rhodabenzothiabenzene hydride intermediate is formed (step f)» which regenerates the 2-vinylthiophenolate precursor 1 (step g) via hydride migration, thus closing the catalytic cycle A . The observed catalytic production of D H B T is explained by taking into account a parallel catalysis cycle, quite similar to those previously described by Fish (12-14) and Sânchez-Delgado (15-17) for the chemoselective hydrogénation of B T to D H B T (DHBT is stable under the actual reaction conditions). The occurrence of cycle Β requires that the T^-S-BT intermediate is in equilibrium with its η -2,3-ΒΤ isomer (step h), in which the C 2 - C 3 double bond is activated for accepting a migrating hydrogen. As a result, an alkyl intermediate (step i) is formed, which can oxidatively add H2 and later eliminates D H B T (steps j-k). In the proposed mechanistic picture, the real catalyst for both transformations of BT is the 16-electron fragment [(triphos)RhH] generated from the dihydride 2 by reductive elimination of E T S H . This process is apparently the rate determining step in light of the HPNMR evidence as well as the dependence on both hydrogen pressure and substrate concentration (Table I). The prevalence of hydrogenolysis of B T to E T S H over hydrogénation to D H B T is most likely driven by steric effects: although the Rh center is sufficiently electron-rich to bind the C 2 - C 3 double bond of B T , the large steric hindrance provided by the six phenyl substituents of triphos favors the η *-S coordination mode of BT, and ultimately controls the chemoselectivity of the reaction with H2.


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I r i d i u m - C a t a l y z e d C o n v e r s i o n of Phenylthiophenol, Biphenyl and H 2 S

Dibenzo[6,rf]thiophene

to

2-

There are two principal reaction pathways that have been proposed to account for the HDS of DBT over conventional heterogeneous catalysts (Scheme 5). Path a involves the hydrogénation of one of the arene rings of D B T to give tetrahydrodibenzothiophene, which, after C-S bond scission, is hydrogenated to 2cyclohexylthiophenol. Path b is quite similar to one of the mechanisms suggested for HDS of B T (see Scheme 1) as it involves the opening and hydrogénation of the substrate to give 2-phenylthiophenol prior to desulfurization. The homogeneous modeling study described below shows that the HDS of DBT can proceed via ring opening to 2-phenylthiophenol prior to desulfurization and hydrogénation and thus provides evidence that the desulfurization does not necessarily require the preliminary hydrogénation of one benzene ring of DBT. The 16-electron fragment [(triphos)IrH], generated in situ by thermolysis of the (ethyl)dihydride complex (triphos)Ir(H)2(C2H5) is capable of selectively cleaving DBT in THF at 160°C to give the C-S insertion product (triphos)IrH(Ti -C,S-DBT) (6) (10) (Scheme 6). At lower temperature, kinetic C-H insertion compounds are also (< 160°C) or exclusively (< 120°C) produced (10). Complex 6 in T H F is hydrogenated (100°C, 5 atm of H2) to the 2phenylthiophenolate dihydride (triphos)Ir(H)2(SCi2H8) (7), which gives biphenyl, H2S, and 2-phenylthiophenol upon treatment with 30 atm of H2 at 170°C (Scheme 7). In the presence of an excess of DBT this reaction is catalytic, although the rate of transformation of the thiophenic molecule is quite slow as only 10 mol of DBT per mol of catalyst precursor are converted in 24 h to either open (rate 0.25) or desulfurized (rate 0.16) products. Most importantly, the reaction is homogenous (mercury test). Based on the results of several independent reactions with isolated compounds, some of which are summarized in Scheme 8, as well as the fact that, at the end of the 2



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Table I. Catalytic Hydrogénation Experiments* reaction mixture composition (%)b


run 1 2 3 4 5 6d 7 8 9 10 11 12 13 14d 15 e

solvent acetone acetone acetone acetone acetone acetone THF acetone acetone THF acetone acetone THF THF acetone

T ( ° Q PH2 t(h) ETB ETSH (atm)

BT

160 160 160 160 160 160 160 160 160 120 100 180 220 220 160

73.2 57.4 51.8 44.6 37.6 37.9 41.3 40.2 34.3 97.6 98.1 29.0 45.8 51.1 34.0

30 30 30 30 30 30 30 15 60 30 30 30 30 30 30

2 4 8 12 16 16 16 16 16 4 16 16 16 16 16

0.2 0.2 0.3 0.4 0.4

25.5 39.8 45.1 51.0 57.4 57.6 52.8 55.3 60.2 2.0 1.4 64.2 43.3 42.6 61.2

—

0.2 0.3 0.4 — —

0.9 3.5 —

0.3

DHBT

other

ETSH rate 0

1.1 2.6 2.8 4.0 4.6 4.5 5.7 4.2 5.1 0.4 0.5 5.9 6.9 6.0 4.5

— — — — —

— — — — — —

0.5 0.3 —

a

12.7 9.9 5.6 4.2 3.6 3.6 3.3 3.5 3.8 0.5

Transition Metal Sulfur Chemistry - American Chemical Society

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