Homogeneous catalysis for direct functionalization of hydrocarbons under irradiation

Solar Energy Materials 24 (1991) 406-412 North:Holland

Solar Energy Materials

Homogeneous catalysis for direct functionalization of hydrocarbons under irradiation

Masato Tanaka * and Toshiyasu Sakakura National Chemical Laboratory for Industry, Tsukuba, lbaraki 305, Japan

Photo-assisted homogeneous catalysis by rhodium complexes has proved tc, be a powerful method to directly functionalize "inert" hydrocarbons. In particular, RhCI(CO)(PM%) 2 under near UV irradiation was found to promote a broad range of functionalizations including the carbon monoxide insertion into the C-H bond and dehydrogenation of paraffins. In addition, when RhCI(CH2=CH2)(PM%) 2 was used as the catalyst, irradiation with visible light could be successfully applied to dehydrogenation of paraffins.

1. Introduction

In current organic chemical industry , a b u n d a n t h y d r o c a r b o n resources are first t r ans fo rmed into more reactive basic chemicals like olefins, which are subjec ted to secondary synthet ic processes leading to useful end products . This is because paraf f ins which are the major componen t s of na tu ra l h y d r o c a r b o n resources have been supposed to be very. inert , and canno t be di rect ly ut i l ized as the s ta r t ing

IParaffin] I

ICracking (> 800°C)~...~ I [Ethylenel jSeparation of ] [olefrin mixture] [01igomerization ]

I l(A1fene process)]

[Terminal olef inl ~ ' ' ' ~ ' ' ~

I [H2+ COl [Hy..ii' oformylatio, l

I I Separation of I aldehyde mixture I

I l L i n ~a~..aJ._~"_Y__d -~I

Fig. 1. Scheme for the synlhesis of linea- aldehydes.

0165-!633/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

M. Tanaka, T. Sakakura / Homogeneous catalysis for functionalization of hydrocarbons 407

materials in usual chemical reactions. As exemplified by the synthesis of linear aldehydes (fig. 1), some of the primary and secondary processes are operated under drastic conditions and are non-selective. Accordingly, a tremendous amount of energy is required for the process operation and the separation of the products. Besides economic reasons, the high energy consumption should be circumvented in view of protection of global environment as well which is a major contemporary concern.

With the aim of development of alternative chemical technologies, we have found a new concept that a C - H bond of hydrocarbons, which is usually inert, can be readily cleaved by rhodium complexes under irradiation [1]. Application of the basic concept allowed us to directly functionalize aromatic and aliphatic hydrocarbons.

2. Strategy in designing the capable catalyst system

Cleavage of a C - H bond by transition metal complex compounds has been long known since the unexpected discovery by Chatt et al. [2]. In 1982, several groups in North America rediscovered the same type reactions, in which they started with dihydrido- or carbonyl complexes [3]. Upon UV irradiation, these complexes kick out dihydrogen or carbon monoxide, respectively, to generate coordinatively un- saturated and therefore highly reactive species. The C-H bond of the solvent is cleaved by these species to give hydrido(organo)metal complexes:

h,a RH (C5Me5)MLL, _..-. . , , . (C5Me5)ML ~ (C5Me5)M(R)(H)L (a)

-L'

M = Rh, I r ; L = PMe3; L' = (H) 2

M : I r ; L : L ' = CO

However, attempted reactions of the resulting complexes with reagents to functionalize the C-H bond, for instance carbon monoxide to carbonylate, were not successful. In our view, this is due to ihe complexes being so-caUed 18-electron species. In organometallic chemistry, it is widely accepted that a d-block metal comple× is stabilized when it fulfills the 18-electron configuration. In other words, when the complex has the 18-electron configuration, it does not coordinate with the incoming ligand required to functionalize the C - H bond. The consideration sug- gested the working hypothesis that the hydfido(organo)metal complex resuliing from the C - H bond cleavage should have 16-electron configuration so that it could accorranodate one more 2-electron donor ligand. In the beginning, we were inter- ested in the synthesis of linear aldehydes from paraffins and carbon monoxide. The Vaska type rhodium complex RhCI(CO)(Pg3)2 appeared promising. This is because 1) the complex is quite sure to generate a 14-electron species through dissociation of the carbonyl ligand upon irradiation [4], 2) the resulting 14-electron species may be reactive toward a C - H bond, and a 16-electron hydrido(organo)rhodium complex which is capable of ligation of carbon monoxide is expected to be generated [5], and

408 M. Tanaka, T. Sakakura / Homogeneous catalysis for functionalization of hydrocarbons

3) rhodium is one of the best transition metal in terms of facile CO insertion into carbon-metal bond [6].

3. Direct carbonyiation of the C-H bond

When a benzene solution of RhCI(CO)(PMe3) 2 was irradiated with a high-pressure mercury lamp through a Pyrex filter, benzaldehyde and some other byproduct were found by gas chromatography to be formed [7]:

hv, RhCl(CO)(PMe3) 2 (0.7 mN) C6H 6 + CO ---

I atm, room temp, 16.5 h

C6H5CHO + C6H5CH20H + C6H5COC6H5 + C6H5C6H5 (b)

Turnovers 65 7 7 2 The catalytic efficiency was 101 turnovers in terms of C-H bond cleavage for 16.5 h at room temperature. The phosphorus ligand bound to the rhodium center pro- foundly affected the efficiency of the catalyst. The guidelines to be kept in mind in selecting the ligand include: 1) the ligand should be a strong electron donor; 2) it should not be sterically demanding; 3) it should not have a C-H bond which readily undergoes intramolecu!ar bond cleavage by the central metal. Accordingly, trialkylphosphines perform better than triarylphosphines and phosphites, and among trialkylphosphines, the performance decreased in the order of PMe3 > PEt 3 > PBu 3. As to the guideline 3), even the C-H bond in PMe3 ligating to the rhodium seems to be cleaved intramolecularly. An evidence came from the carbonylation of benzene-d6 with the Rh-PMe3 complex. Thus, when we looked at the recovered catalyst by GC-MS, it turned out that deuterium was incorporated, and that the deuterium content was increasing with the progress of the carbonylation reaction. The H - D exchange may well proceed through intramolecular C-H bond cleavage via four- membered agostic interaction [8] with the rhodium center. If this is the case, phosphines having a longer alkyl chain are expected to more readily undergo such C-H bond cleavage in view of the less serious ring strain in the agostic interaction.

The same procedure could be successfully extended to linear paraffins [9]. The great feature lies in its very high linear selectivity. As shown by eq. (c),

~ +CO hP, Rh(CO)CI(PMe3) 2

= CH3CHO + room temp,, 16.5 h

Turnovers 22

CHO

+ + +

93 1 27 ~ 1 not detected

(c)

M. Tanaka, T. Sakakura / Homogeneous catalysis for functionalization of hydrocarbons

Table 1 Comparison of the C-H bond carbonylation with hydroformylation (OXO) process

409

Hydroformylation C-H bond carbonylation Starting material Olefin Paraffin "~" Hydrogen Necessary Unnecessary Temperature 100-200°C Room temperature Pressure 30-300 atm 1 atm Linear selectivity Moderate - 100% Irradiation Unnecessary Necessary

the reaction of pentane formed n-hexanal together with a very small amount of the branched isomers. As stated in the beginning, linear aldehydes are commercially produced via hydroformylation of olefins. Comparison of the process with the new procedure via C-H bond activation is given in table 1. The drawbacks of hydroformylation are: 1) the starting olefin has to be prepared in advance; 2) the process requires hydrogen gas as one of the reactants~ which has to be obtained via steam reforming of hydrocarbons or related processes; 3) the reaction conditions are rather severe; and 4) the linear selectivity is not so high. Note that the branched isomer formed as a byproduct in propylene hydroformylation is used as fuel to drive the plant. On the other hand, the new process also has some disadvantageous aspects such as the need of UV irradiation and the low productivity which remain to be overcome.

In the meantime, we found that the pentane reaction was producing other products, which were isomeric butenes and acetaldehyde (eq. (c)) [10]. The very high terminal selectivity in the butenes formation is a very exciting feature. The formation of 1-butene is due to the well established Norrish type II reaction of the initially formed n-hexanal. This is supported by the simultaneous formation of acetaldci~yde, To summarize the carbon~/lation of paraffins, it provides an entirely new route to lineai" aldehydes, terminal olefins, and acetaldehyde, all of which are very important commodity chemicals in the current petrochemical industry.

4. Dehydrogenation of saturated hydrocarbons

As mentioned in the previous section, the carbonylation of the C-H bond in paraffins proceeds through the hydrido(alkyl)metal intermediate. Alkyl-metal complexes are usually unstable owing to the/~-hydride elimination. Accordingly, when a solution of the rhodium complex in saturated hydrocarbons was irradiated without carbon monoxide, dehydrogenation smoothly proceeded to give corresponding olefins and hydrogen [11]:

h~, RhCI(CO)(PMe3)2= @

0.7 mM, ro~m temp N 2 flow, 6 h

+ dimers + H 2 (d)

236 turnovers


Table 2 Isomerization of l-hexene during cyclohexane dehydrogenation

PMes/Rh Time n-C 6 Distribution (%) T.O.N. oi

2 6 57 4 0 39 18

2 22 2 35 0 63 71

5 22 7q 0 0 26 21

|-Hexene (8 equiv, to Rh) was added. Room temp.

The catalytic efficiency appeared higher than in carbonylation, and could be even higher when the reaction was carried out at higher temperatures. An interesting feature of the reaction is that it could take place even under a hydrogen atmosphere. As is well recognized, dehydrogenation of paraffins is highly endothermic so that it cannot be expected to occur smoothly. However, under the so-called photo-sta- tionary state where we are supplying energy to the system through irradiation, the reaction can proceed out of the thermodynamic limitation in the dark.

An obvious target in dehydrogenation is the synthesis of terminal olefins starting with linear paraffins. As table 2 shows, isomerization from the terminal to inner olefins was taking place. However, the isomerization was suppressed by an addition of an extra phosphine to tbc reaction system. Accordingly, the selectivity to the terminal olefin c o r d be improved at the expense of the reaction rate when the reaction was effected in the presence of excess phosphine:

n-C6H~4 h~, RhCI(CO)(PMe3) 2 + x PMe3 -

v

room temperature

n-C4H9CH:CH 2 + n-C3H7CH:CHCH3 + C2H5CH=CHC2H5

= 0 7% 77% 15%

x = 3 70 24 6

(e)

5. Inert solvent under lhe dehydrogenation condi~,ons

One of the major drawbacks of the present procedure for C - H bond functionalization is the lack of inert solvents. Usually, the C - H bond of the solvent is also cleaved by the rhodium complex. Accordingly we have to run the reaction neat. This prevents the application to high-melting and/or expensive compounds and achieve- ment of a high conversion. The latter is a serious problem m the dehydrogenation ~ince the separation of the starting material and the products is not so easy. To circumvent these problems, inert solvents have to be found to be used in the foregoing reactions.

Obviously, perfluorohydrocarbons are the candidates. However, perfluoroalkanes

M. Tanaka. T. Sakakura / Homogeneous catalysis for functionalization of hydrocarbons 411

did not dissolve the catalyst. In addition, it is not miscible with the substrate paraffins. Perfluorobenzene formed a homogeneous solution with paraffins and the catalyst. However, it reacted photochemically with the olefins formed through dehydrogenation to afford a complicated mixture of various secondary products.

Eventually we foundt that hydrocarbons having tert-butyl groups were the solvent of choice in dehydro~,enation [12]. For instance, dehydrogenation of eyclooctane proceeded over 90% conversion in 3 h in dineopentyl:

t p,

O"v'=="°°"'='"'O 0 ,.+ooo t .Bu, , , , , t-Bu ~" + + 0.65*•,

1,5-COD (2 vol.%) 100°C, 3 h Yield 59% 27% 0.18%

Conv. 92% ~dimers 5.5%

(f)

We did not see any evidence of dehydrogenation of the solvent. Likewise, 1,3,5-tri- tert-butylbenzene also worked as an inert solvent. However, mesitylene, methyl pivalate, and t-butyl methyl ether were not good inert solvents because of either too high reactivity of their C - H bonds or the C-O bond cleavage.

6. Possible utilization of solar irradiation

The foregoing reactions take place only under UV irradiation. All attempts to thermally drive the catalytic cycle failed. Therefore, at least one photon seems to be required to carry one catalytic cycle. Since photon cost is qu~te high, the next step of the development is to search for another catalyst which is able to effect the reaction under solar irradiation, the cheap photon source.

When we were looking at the reactivity of [RhCI(PMe3)2]2, we learned that RhCI(CH2=CH2)(PMe3) 2 was readily formed upon the treatment of the dimer with ethylene, and that it exhibited an absorption band at 416 nm. This finding led us to the dehydrogenation experiment of cyclooctane with the ethylene complex catalyst. Irradiation with effective wavelength higher than 375 nm efficiently promoted the reaction in the presence of ethylene as compared with the R.hCl(CO)(PMe3)2-cata- lyzed reaction [13]:

O bY (~t~ " 375 gin) _ h. O "~'emD" "]'O [~ Rate - RhCI(CH2=CH2)(PMe3)2 700C 160 h "1 (g)

CH2=CH2 (1 atm) 30 77 (of. RhCI(CO)(PMe3)2, 30°C, TON Rate 1 h "1)

7. Summary

Besides the carbonylation and dehydrogenation, the rhodium catalyst system has proved to be capable of catalyzing a broad range of C - H bond functionalizations as summarized in fig. 2 [14]. Even though we have a long way to go yet, the research


-Ha -,,.- Olefin (R = alkyl)

[M]-L

CI.~R i f PMe3 Me3P ~'L

[M]-L

L = CO or CH2=CH2

h v = [M] -L

-Ha

Dehydrogenation If]'CH=CH2R'aSiHR,cHO "H2-H2

R-R (R = aryq R-C=C'R'

H H

R-SiR'3

"=" R'CH2OH + Olefin

I Inser t ion

c-=o o 4" R-C-H

f N'R' FI'N-C II • .,,.- R-C-H

R'C~-CH R • ,.- R,;C---CH2

Fig. 2. Scheme for C-H bond functionalizations.

and development along this line will open up an entirely new and hopefully solar chemical industry.

References

[1] For reviews on C - H bond activation, see A.E. Shilov, "The activation of saturated hydrocarbons by transition metal complexes", D. Reidel Publishing Co., Dordrecht (1984); M. Tanaka, Yuki Gosei Kagaku Kyokaishi 46 (1988) 832.

[2] J. Chatt and J.M. Davidson, J. Chem. Soc. (1965) 843. [3] A.J. Janowicz and R.G. Bergman, J. Am. Chem. Soc. 104 (1982) 352;

J.K. Hoyano and W.A.G. Graham, ibid. 104 (1982) 3723; W.D. Jones and F.J. Feher, ibid. 104 (1982) 4240.

[4] D. Wink and P.C. Ford, J. Am. Chem. Soc. 109 (1987) 436. [5] C.T. Spillett and P.C. Ford, J. Am. Chem. Soc. 111 (1989) 1932. [6] I. Tkatchenko, in G. Wilkinson (Ed.): Comprehensive Organometallic Chemistry, Pergamon Press,

Elmsford, N.Y. (1982) p. 101. [7] T. Sakakura and M. Tanaka, Chem. Lett. (1987) 249;

T. Sakakura, T. Sodeyama. K. Sasaki, K. Wada, and M. Tanaka, 3. Am. Chem. Soc. 112 (1990) 7221. [8] M. Brookhart and M.L.H. Green, J. Organomet. Chem. 250 (1983) 395. [9] T. Sakakura and M. Tanaka, J. Chem. Soc. Chem. Commun. (1987) 758.

[10] T. Sakakura, T. Hayashi, and M. Tanaka, Chem. Lett. (1987) 859. [11] T. Sakakura, T. Sodeyama, and M. Tanaka, New J. Chem. 13 (1989) 737. [12] T. Sakakura, K. Ishida, and M. Tanaka, Chem. Lett. (1990) 585. [13] T. Sakakura, F. Abe and M. Tanaka, Chem. Lett. (1991) 297. [14] M. Tanaka, Chemtech. (1989) 59;

M. Tanaka and T. Sakakura, Pure Appl. Chem. 62 (1990) 1147.

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Homogeneous catalysis for direct functionalization of hydrocarbons under irradiation