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Catalytic Transfer Hydrogenation

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Catalytic Transfer Hydrogenation Catalytic Transfer Hydrogenation GOTTFRIED BRIEGER* and TERRY J. NESTRICK Department of Chemistry, Oakland University, Rochester, Michigan 48063 Received August 20, 1973 (Revised Manuscript Received November 2, 1973) Contents In 1952, Braude, Linstead, et made the sugges- tion that catalytic hydrogen transfer from an organic I. Introduction 567 donor molecule to a variety of organic acceptors might I I. Reaction Conditions 568 be possible under mild conditions. In fact, sporadic use A. Nature of the Donor 568 had been made in the past of unsaturated compounds as B. Effect of Solvents 569 hydrogen acceptors in catalytic dehydrogenation reac- C. Effect of Temperature 569 tions. However, few systematic studies were directed D. Effect of Catalyst 570 toward the reverse process, catalytic transfer hydrogena- E. Other Variables 570 tion. I I I. Applicability 570 Knowledge of the basic reaction, however, goes back A. Reduction of Multiple Bonds 570 to the turn of the century, when Knoevenage14 first ob- 1. Olefins 570 2. Acetylenes 570 served that dimethyl 1,4-dihydroterephthaIate dispropor- 3. Carbonyl Compounds 571 tionated readily in the presence of palladium black to di- 4. Nitriles 571 methyl terephthalate and (mostly cis) hexahydroterephthal- 5. Imines, Hydroxylamines, Hydrazones 571 ate. Several years later, Wieland5 observed the same 6. Azo Compounds 571 reaction with dihydronaphthalene. Wieland predicted that 7. Nitro Compounds 571 B. Hydrogenolysis 571 the reaction would also occur with the then unknown 1, Nitriles 571 dihydrobenzenes, a prediction confirmed by the work of 2. Halides 571 Zelinski and Pavlov‘ and Corson and Ipatieff‘ in the 3. Allylic and Benzylic Functional Groups 571 1930’s. In the next decade attention was focused princi- 4. Amines 573 pally on catalytic dehydrogenation, especially through the C. Structural Selectivity 574 systematic efforts of Linstead and his students. The im- D. Special Synthetic Applications 575 portant reaction variables were determined, the prepara- IV. Mechanism 576 tions of catalysts systematized, and the applications V. Summary and Prospects 580 broadened. In 1952, Braude, Linstead, et a/.,3 reported VI, References 580 the striking discovery that ethylenic and acetylenic link- ages could be reduced in high yield and purity by reflux- ing with cyclohexene in tetrahydrofuran at 65” in the presence of palladium black. Subsequent studies estab- 1. lntroducfion lished the scope of the reaction. It was shown that, in addition to reduction of ethylenic and acetylenic linkages, The reduction of multiple bonds using hydrogen gas aliphatic and aromatic nitro groups could be reduced to and a metal catalyst is a reaction familiar to all organic primary amines. Azo, azoxy, and azomethine groups un- chemists. Far less well known is the possibility of achiev- dergo reduction as well; halides can be hydrogenolyzed. ing reduction with the aid of an organic molecule as the It was further shown that carbonyl groups are generally hydrogen donor in the presence of a catalyst, a process not susceptible to reduction unless part of a potential ar- known as catalytic transfer hydrogenation. This process omatic system, as is the case with quinones or decal- is but one of several possible hydrogen transfer reactions ones. Nitriles were initially reported as unreactive, but which were classified by Braude and Linstead’ as: later studies by Kindler and Luhrs* provided the condi- a. Hydrogen migrations, taking place within one mole- tions for their reduction as well. cule More recent work has brought the use of homoge- b. Hydrogen disproportionation, transfer between iden- neous catalysts with this reaction.’-13 Investigations have tical donor and acceptor units continued on the synthetic applications of the reaction, c. Transfer hydrogenation-dehydrogenation, occurring as well as molecular details in the donor and acceptor between unlike donor and acceptor units structures, the effect of various catalysts, and the use of Each of these reaction types in turn can be realized in different solvents. l4 principle by thermal means, homogeneous catalysis, het- Other investigations related in a general way to the erogeneous catalysis, photochemical means, or with bio- topic under consideration are studies on the dispropor- logical processes. tionation of various hydroaromatic molecule^^^-'^ and the This review will concern itself with type c reactions, to- use of simple organic acceptors such as maleic acid and gether with homogeneous or heterogeneous catalysis. acetone for the aromatization of alkaloids, heterocyclics, Only the use of organic hydrogen donors will be dis- and ~teroids.l’-~’These will not be discussed in any de- cussed, although inorganic donors also fulfill this , role. tail. The use of hydrazine in such reactions has been re- This review will present what is known about the major viewed recently.2 reaction variables, the scope of the reaction, and the 567 568 Chemical Reviews, 1974, Vol. 74, NO.5 G. Brieger and T. J. Neslrick TABLE I. Hydrogen Donor Compounds TABLE ll.14 Reduction of Cinnamic Acid in the Presence of Compound Catalyst Ref Various Donow Cyclohexe ne w 27-34,36,37,42,48 Donor Solvent (xylene) Reaction timeb Subst cyclohexenes Pd 3,26, 34,4a p-Menthane Absent 16 hr (only trace redn) 1,3-Cyclohexadie ne Pd 3, 42 A'-p-Me nt he ne Present 100 min 1,4-Cyclohexadiene Pd 3 a-Phellandrene Present 2 min trons.A2-0ctaIin Pd 15 Limonene Present 3 min 19 1"-Octalin Pd 15 a-Pinene Absent 300 min 1-Methyloctalin Pd 15 @-Pinene Absent 360 min trons-2-Methyloctalin Pd 15 Camphene Absent 360 rnin (no reaction) Tetralin Pd 14, 24, 35, 39, 42, Tetralin Present 60 min 43, 58 Decalin Absent 360 min (only trace redn) 1,6-Dimethyltetralin Pd 24 6-Meth y ketra lin Pd 24 a Reaction conditions: catalyst 10% Pd/C; temp with xylene as sol- vent approximately 144" at reflux: otherwise reflux temp of donor. d-Limonehe Pd 14, 16, 30 Reaction time refers to quantitative conversion to hydrocinnamic a-Pinene Pd 14, 31 acid, unless otherwise noted. p-Pinene Pd 14, 31 Aa-Carene Pd 47 e-Phellandrene Pd 14, 31, 38, 44 TABLE 111.14 Donor Activity with Various Acceptors" p-Phellandrene Pd 31 __-_- Acceptor sb------- Terpinolene Pd 14, 31 Donor A 8 C D 1'-p-Me n t he ne Pd 8, 14, 45, 46 a-Phellandrene 2 minc 1 2 a Cadalene Pd 16 d- Limo ne ne 3 15 3 12 16 Pulegone Pd p-Menthene 100 30 25 40 Selinene Pd 16 a-Pinene 3600 Ethanol Raney nickel 40 Tetrali n 60 20 2-Propanol Raney nickel 40 H IrCI?(MezSO)3 10,ll Reaction conditions: 2.5 mmol of acceptor: 5.0 g of donor; 0.2 g of Methanol PtCh(Ph3AS)z 9 10% Pd/C; 20 cc of xylene, reflux. *A = cinnamic acid; B = 3,4- + methylenedioxycinnamic acid; C = p-rnethoxycinnamic acid; D = SnCIz.H26 oleic gcid. Time to quantitative conversion. Diethylcarbinol Raney nickel 40 Octanol R~Clz(Ph3P)a 12 fore, tetralin or the readily available monoterpenes limo- Cyclohexanol Raney nickel 40 nene, terpinolene, or a-phellandrene are also frequently Benzyl alcohol R uClp( Ph 3 P)a 12 used. In general, however, it may be said that any hy- p-Phenyletha no1 R uClz( Ph3P)x 12 droaromatic compound capable of disproportionation can a-Phenylethano1 RUCI?(P~~P)~ 12 be utili~ed.'~The specific choice depends as well on the 0-Cyclohexylphenol Pd 41 nature of the functional group to be reduced. Thus the Formic acid R hCI(PhsP)a, 13 reduction of carbonyl groups requires the use of alcohols R uCL(P~~P)~, IrBr(CO)(Ph,P)? as donors.40 Aside from obvious reaction limitations such as solubil- ity, there are important differences in the reaction rates. mechanistic proposals for catalytic transfer hydrogena- This is illustrated in Table II. Here the times required for tion. the quantitative reduction of cinnamic acid with various donors are compared. /I. Reaction Conditions It can be seen that, in the case of the various terpene donors, increasing unsaturation leads to more rapid reac- A. Nature of the Donor tion. Thus dienes are more reactive than enes, while sat- The reaction under discussion can be generalized as in urated ring systems react slowly or not at all. However, a eq 1. The donor compound DH, can, in principle, be any note of caution is appropriate here. A careful study of the reduction of mesityl oxide with d-limonene has shown that it is not limonene but rather A'-p-menthene, formed as an intermediate by disproportionation, which actually organic compound whose oxidation potential is sutficient- serves as the donor species.30 A complex mixture of in- ly low so that the hydrogen transfer can occur under mild termediates also occurs during disproportionation of a- conditions. At higher temperatures, especially in the pinene.47 Especially notable, from a practical point of presence of catalysts, almost any organic compound can view, is the short reaction time with the more reactive donate hydrogen (catalytic cracking), but this has little donors limonene and cu-phellandrene. Similar trends are potential for controlled synthesis. Similarly, at sufficiently shown with a somewhat wider range of acceptors (Table high temperatures (>300") even benzene can serve as Ill). acceptor A and be reduced to cy~lohexane.~~Therefore The slow reaction of the pinenes is, of course, not sur- the choice of donor is generally determined by the ease prising since they cannot aromatize until the four-mem- of reaction and availability. The chosen compounds tend bered ring is cleaved. It seems likely that it is not pinene to be hydroaromatics, unsaturated terpenes, and alco- itself but a reaction intermediate which serves as actual hols. Table I lists most of the donors which have been re- hydrogen donor, as with limonene.
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