Transfer Hydrogenation - PDF document

Transfer Hydrogenation GOTTFRIED BRIEGER* Chemistry, Oakland University, Rochester, Received August Manuscript Received Contents Reaction Conditions B. Effect D. Effect A. Reduction of Multiple Bonds I I Carbonyl Compounds Benzylic Functional D. Special Synthetic Applications IV. Mechanism V. Summary and Prospects VI, References 567 568 568 569 569 570 570 570 570 570 570 571 571 571 571 571 571 571 571 571 573 574 575 576 580 1. lntroducfion using hydrogen Hydrogen migrations, taking place one mole- Hydrogen disproportionation, transfer between iden- and acceptor Transfer hydrogenation-dehydrogenation, between unlike and acceptor units will concern gether with homogeneous heterogeneous catalysis. hydrogen donors be dis- cussed, although donors also fulfill this in such reactions Braude, Linstead, made the hydrogen transfer organic acceptors be possible sporadic use the basic the turn the century, served that tionated readily and (mostly ate. Several years observed the with dihydronaphthalene. Wieland the reaction would also prediction confirmed Investigations have topic under organic acceptors maleic acid the aromatization be discussed any de- present what about the the scope the reaction, 567 G. Brieger and T. J. Neslrick 568 TABLE I. Hydrogen Donor Compounds Chemical Reviews, 1974, Vol. 74, NO. 5 Compound 1,3-Cyclo hexadie he ne Methanol Benzyl alcohol p-P henyletha Formic acid Catalyst Ref w Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd nickel Raney nickel H IrCI?(MezSO)3 SnCIz. H26 Raney nickel Raney nickel PtCh(Ph3AS)z + R~Clz(Ph3P)a P h R hCI(PhsP)a, R uCL(P~~P)~, IrBr(CO)(Ph,P)? 27-34,36,37,42,48 3, 42 3 15 15 15 15 14, 24, 35, 39, 42, 24 24 14, 16, 30 14, 31 14, 31 47 14, 31, 38, 44 31 14, 31 8, 14, 45, 46 16 16 16 40 40 10,ll 9 3,26, 34,4a 43, 58 40 12 40 12 12 12 41 13 catalytic transfer hydrogena- be generalized The donor organic compound oxidation potential that the hydrogen transfer can occur higher temperatures, especially catalysts, almost donate hydrogen even benzene and be donor is generally determined chosen compounds tend hydroaromatics, unsaturated have been Cyclohexene, because the preferred hydrogen donor. However, temperature available with cyclohexene is not reduction at an Cinnamic Acid Donor Solvent (xylene) Reaction timeb p-Menthane Absent (only trace redn) n t Present 100 min a-Phellandrene Present 2 min Limonene Present 3 min a-Pinene Absent 300 min @-Pinene Absent 360 min Camphene Absent 360 rnin (no reaction) Tetralin Present 60 min Decalin Absent (only trace redn) temp with vent approximately to quantitative acid, unless otherwise noted. Various Acceptors" __-_- Acceptor sb------- Donor A 8 C D a-Phellandrene 2 minc 1 2 a d- Limo ne ne 3 15 3 12 p-Menthene 100 30 25 40 a-Pinene 3600 Tetrali n 60 20 Reaction conditions: 2.5 mmol of acceptor: 5.0 g of donor; 0.2 g of 10% Pd/C; 20 cc of xylene, reflux. *A = cinnamic acid; B = 3,4- methylenedioxycinnamic acid; C = p-rnethoxycinnamic acid; D = oleic gcid. Time to quantitative conversion. the readily available monoterpenes or a-phellandrene also frequently droaromatic compound capable functional group be reduced. carbonyl groups requires the limitations such important differences illustrated in Here the times required quantitative reduction cinnamic acid be seen the case donors, increasing unsaturation leads Thus dienes appropriate here. mesityl oxide with d-limonene has shown it is not A'-p-menthene, formed disproportionation, which actually serves as termediates also during disproportionation reaction time with the donors limonene and Similar trends are shown with pinenes is, not sur- cannot aromatize it is not intermediate which serves as hydrogen donor, with limonene. phellandrene and limonene shown with of the relationship has been Ill). Catalytic Transfer Hydrogenation Chemical Reviews. 1974, Vol. 74, No. 5 569 TABLE IV.2"34 Effect of Substitution in Cyclohexene on Donor Activity" % succinic % hydrocin- R after 48 hr after 72 hr toluidine namic acid H 96.3 100 95 CHa 75 50 CH3CH2 15 4.85 (CH3)zC H 24 4 0 CHdCHA 24.5 2.5 2.13 Cyclo hexyl 10 Trace 2.19 Phenyl 34.5 9.35 'I Reaction conditions: 0.01 mol of maleic or cinnamic acid; 0.02 mol of donor: 50 rng of 5% Pd/C; 25 rnl of THF, reflux; 0.0167 mol ofp-nitro- toluene; 0.05 mol of donor; 100 mg of 5% Pd/C; 82". bulk of clearly reduces the rate substituents such interesting question this effect is hydrogen transfer step the differ- reflects the relative ease of dehydrogenation various cyclohexenes. the former likely may the fact that after the same conditions, but without acceptor, cyclohex- -ethylcyclohexene, and 6. Effect of Solvents the course the reaction been studied Greater sensitivity the reduction This is special effec- suggest that such large differences between solvents do not as a-phellandrene was also following general conclusions seem warranted. Below a on the hydrogen transfer reaction rate be independent the nature far as hydrocarbons, acids, involved. Alcohols and amines, which have the capability themselves with the catalyst retard the reaction somewhat. using alcohols it must that these also serve donors, espe- possible that hydrogen transfer takes place from reducing the reaction also dehydrogenated by Raney have been observed early noted that which readily reduced p-methoxycinnamic nevertheless evolved ing the reaction. methoxycinnamic acid the methoxy Solvent on Transfer Hydrogenation p-Methoxycinnamic Acida Reaction Solvent Bp, "C time, min ha ne 102 330 Isovaleric acid Isobutyric acid ofp-methoxycinnamic acid; solvent refluxed Transfer Hydrogenation Solvent BP, "C a b Acetic acid 120 105 104 85 118 60 70 39 56 80 49.8 20.75 28.25 73(16) 55.6 85 0 9.6 77 cyclohexene donor; yield is was not the evolution drogen greatly decreased. Similarly, Braude, functional groups a solvent. Within wide range including carboxylic can be C. Effect of Temperature be a transfer hydrogenation. higher temperatures, can be mentioned earlier.p3 Generally such tions have been such a shown below I I 570 Chemical Reviews, 1974, Vol. 74, No. 5 Brieger and TABLE VII. Catalysts for Transfer Hydrogenation Catalyst ~ Pd black Pd/alumina Ni/al umi na Ni/Kieselgur Raney nickel I r H C12( M e2SO)a IrBr(CO)(Ph3P)a PtCI2(PhaAs)? + SnCI2, H20 P’d IC RuCl*(PhaP)a R hCI(Ph8P)a Ref 4, 5, 6, 32 41 23 24-39, 42-46, 40, 49-51 D. Effect of Catalyst number of different catalysts have hydrogen transfer reactions. Some were undoubtedly subtle differences catalyst preparation. which have listed in the table, all reported work has been done with palladium. standardized conditions effective, neither nor rhodium catalysts work, temperatures below desirable, however, Recently catalysts used homogeneous hydrogena- have been used for transfer hydrogenation. These in- clude the the rhodium and the platinum is not nickel occupies somewhat ambiguous transfer hydrogenation because of the demonstrated the catalyst during generation assertions that reduction with hydrogen transfer a donor have been There is also evidence some hydrogen have shown that hydrogen transfer possible with clear that plays only a not a donor oxidation/hydrogen transfer Again further work the catalyst-bound hydrogen, hydride, plays major role the quantitative serves mainly the catalyst. For a the results presented by Braude, Clearly palladium catalysts are the most fective. With other donors, especially hydrazine, work equally note that the dispropor- and cyclohexane ineffective in hydrogen trans- fer to other than system, with as donor and namic acid platinum and of palladium hydrogen transfer due, at mobilizing action bonds. For TABLE V111.48 Comparison of Catalysts for Transfer Hydrogenation“ of p-Nitrotoluene Yield of Amount Time, p-toluidine, Catalyst mg hr % 10% Pd/C 190 7 95 1% Pd/CaC08 100 18 95 0.1% Pd/A1203 18 64 PdC12 20 17 1 Pd/Pt* 10 13 100 Pt black 20 7 18 10% Pt/C 350 7 13 PtOz 45 17 0 Raney nickel 100 22 2 W 7 Raney nickel 100 100 2 a Reaction conditions: 2.5 g ofp-nitrotoluene; 15 ml of cyclohexene; catalyst as per table; time reflux as per table. * Pd deposited elec- trolytically on platinum foil. stance, palladium more effective than rhodium causing rearrangements during regular catalytic hydrogenation substituted cyclohexenes. great deal the re- the commercially available catalysts is the transfer hydrogenations. E. Other Variables the case with heterogeneous is very important. Linstead has shown that free ebullition the catalytic tetralols and product distribution, naphthols produced, catalytic transfer hydro- are carried technique de- the reaction greater selectivity desired. Alternatively I 11. Applicability have chosen present typical applications transfer hydrogenation with various functional groups special section applications with general synthetic interest. A. Reduction of Multiple Bonds 1. Olefins different olefins have transfer hydrogenation. listed in indicates that considerable variety compounds has been hydrogenated transfer hydrogenation. Nitriles are generally removed, including halide be discussed further 2. Acetylenes work done with only very limited data are appears that it is control the Catalytic Transfer Hydrogenation Chemical Reviews, 1974, Vol. 74, No. 5 571 good yield conventional hydroge- Further, the stereochemical addition of hydrogen. As a fact, in the case iridium complex definite inter- been isolated (see section Carbonyl Compounds with the Pd/C, although complete hydro- in the steroid se- shown in With isolated exceptions, can be claimed successful catalyst results achieved with partial reduction p-quinone and not surprising carbonyl compounds. Neither cyclopentanone heptanal could and Pd/C, 4. Nitriles imines has not been transfer hydrogenation, except with as donor., nitriles are completely reduced methyl groups under the conditions normally transfer hydrogenation. general then, the cyano group an aromatic proceeds quite satisfactorily other hand, aliphatic reduced with difficulty, and reduction. Trialk- are formed mediate alkylamines DH DH2 RCN &+ RCH=NH RCH,NH2 Pd C RCHZNH, + RCH-NH - RCH-NH, DHJ- I NHCH,R RCH=NCH,R + NH3 OH2 RCH=NCH,R (RCH,),NH Pd C RCHGNH + (RCH,),NH - RCH-NH, - DH2 I N(cH,R), (RCH,),N + NH3 5. imines, Hydroxylamines, Hydrazones The few reported examples carbon-nitrogen dou- listed in only relatively stable systems studied, and eral applicability in the case the ni- the reaction intermediate reduc- stages, but proceeds directly alkane, as conventional hydrogenolysis Azo Compounds and Pd/C as Dimethoxyazobenzene gives azoxybenzene, again under the same conditions, one Nitro Compounds Nitro compounds have been investigated rather exten- Table XIV. that many commonly encoun- tered functional are compatible the reducing can be the corresponding amino N-acetamido, nitrile, and phenolic hy- droxyl groups. the re- duction, probably blocking the catalyst.48 note the group with been previously discussed, nitrile group with the reported, the Certainly these technical improvement over the rather messy traditional with metals catalytic transfer than regular B. Hydrogenolysis been pointed out the previous section, hydro- genolysis frequently accompanies the reduction bonds. The systematic studies 1. Nitriles methyl group (see Tables There is, definite indication the intermediate formation amines, according can be synthetically exploited heterocycles as secondary amines. The details in section I I 2. Halides frequently removed under conditions of transfer hydrogenation. cited in have been indicated that aromatic ring can be One would that the same applies examples were reported. Fluorine, on other hand, the reduction acid chloride only a Allylic and Benzylic Functional Groups enhanced reactivity it is groups bound these systems would hydrogenolyze rather easily conventional hydrogenation. the case transfer hydrogenation well. Table gives examples reported examples not numerous, G. Brieger and T. J. Nestrick 572 Chemical Reviews, 1974, Vol. 74, No. 5 TABLE IX. Catalytic Transfer Hydrogenation of Olefins Acceptor Catalysta Donor* Product (yield, Hydrocarbons Heptene-1 Octene-1 Octene.1 Allyl benzene @-Methylstyrene a-Methylstyrene Styrene cis-Stil bene frans-Stil bene Stilbene 1,l-Diphenylethylene Anthracene Acenaphthylene But-3.enoic acid But-2-enoic acid Crotonic acid 3-Methylbut-2-enoic acid Fumaric acid Maleic acid Itaconic acid Sorbic acid Muconic acid Oleic acid ryl acryl ic Cinnamic acid p-Chlorocinnamic acid p-Methoxycinnarnic acid p-Ethoxycinnamic acid roxycin narn ic roxycin narn ic p-Methylcinnarnic acid a-Methylcinnamic acid a-Phenylcinnarnic acid p-CH,PhCH=C( Ph)COOH PhCH=C(NHCOCH,)COOH 3,4.(CH30)aPhCH=C(NHCOPh)aCOOH N H Atropic acid Mesityl oxide PhCH=CHCOCH3 PhCH=CHCOP h P hC P h 2 1 2 1 2 1 1 3 3 1 1 3 4 1 3 5 1 1 5 1 5 5 5 1 3 1 5 1 1 5 5 5 1 5 5 5 5 5 5 5 5 5 5 5 5 3 5 5 5 5 5 3 3 5 5 6 6 6 3 5 5 5 B A B A B A A C A A C D A C E A Acids A F A F F F A C A F A A F F E A G .E E G F G, H G G F F G G C G G G G G G C Ketones C G G I E I C G G F he ne Butyric acid Butyric acid Butyric acid Succinic acid Hydrocinnamic acid Hydrocinnamic acid rocin narn p-Ethoxyhydrocinnamic acid rocin narn ic a-Methylhydrocinnamic acid a-Phenylhydrocinnamic acid H C PhCHaCHaCOCHZ (70-100) P h P h hCHiCHiC0 Ph 32 13 13 32 13 32 32 10 10 32 32 10 40 10 39 32 14 32 14 14 14 32 10 32 14 32 32 14 14 43 14 43 43 14 44 14 14 14 14 14 14 14 10 57 57 57 14 14 14 10 10 30 14 12 12 12 11 14 14 14 Catalytic Transfer Hydrogenation Chemical Reviews, 1974, Vol. 74, No. 5 573 Acceptor Catalysta Product (yield, p-CHaPhCH=CHCOPh-p-OCHa p-CHaOPhCH=CHCOPh-3,4-(CH3CH20)2 PhCH=CHCOC(CH,), ' P hCH=CHC H=CH COP h hCH=CH CH=CH CO lsophorone Pulegone C holestenone 17~~-Acetoxy-6-methylenepregn-4-ene-3,20-dione 21-Acetoxy-6-methylenepregn~4-ene-3,20-dione 4 5 5 6 3 5 3 5 3 5 6 1 4 5 5 D G F I C F C F C F I e f A A P h P h C 20-one 5 21-Acetoxy-6~formyl-3-methoxypregna-3,5-dien~2O~one 5 21-Acetoxy-6-formyl-17- hyd roxy-3-rnethoxypregna- 5 3,5- d ie ne-11,20. d ione PhCH=CHCOO R (R = CzHj, CdHe, C~HSCHZ) Methyl linoleate PhCH=CPhCN 3,4-(CH,O)zPhCH=CHCN p.CH30PhCH=C(p-CHaPh)CN p-CHaPhCH=CPhCN 3,4-Methylenedioxy-PhCH=CPhCN P hCH=CHC H=CP hC N PhCH=C(COOCH2CH,)CN PhCH=C(COOCH(CH,)2)CN p.CH,PhCH=C(COOCH2CH,)CN ~,~.(CH~O)ZP~CH=C(COOCH~CH~)CN Ph(CHa)C=C(COOCHzCH,)CN Ph(C HaCHz)C=C(COOCH&H3)C N Barbituric acids 5 5 5 5 5 5 5 5 5 5 5 5 Aldehydes C Butyraldehyde (40) C PhCHZCHzCHO (0-1) I P hC H2C H (C H3)CH 0 (61) A 17~~Acetoxy-6~-methylpregn-4-ene.3,20-dione (?)r A 21-Acetoxy-6~-methylpregn-4-ene-3,2O~dione (?)c 6-~0Methylcortisone acetate A, F PhCHzCHzCOOR (100) G PhCHzCHzCOOCHs (90)' 9 Monoene ester (51) Nitriles H H H H H H H H H PhCHZCH PhCHa H P h 40 14 14 12 11 14 11 14 11 14 12 16 40 29 29 10 10 12 29 29 29 31 14 9 8 8 8 8 8 8 45 45 45 45 45 45 Miscellaneous Cornpou nds 5 F Succinic anhydride (d) 14 5 G Dihydrocou rnarin 5 F, G, H Corresponding saturated barbituric acids Ri Rz Ri Rz allyl allyl allyl isopropyl allyl phenyl allyl ethyl 1-cyclohexenyl ethyl Catalysts: 1, Pd black; 2, RhCh(Ph3P) ; 3, IrCls(Me2SO)a; 4, Raney nickel; 5, Pd/C; 6, RUCI~(P~~P)~; 7, PtCh(Ph3As)? + SnC12. *Donors: A, cyclo- hexene; B, HCOOH/HCOOLi; C, isopropyl alcohol; D, diethylcarbinol; E, tetralin; F, wphellandrene; G, limonene; H, AI-p-rnenthene; I, PhCH?OH. Concurrent hydrogenolysis. Specific yields not indicated, but approaching quantitative.'* Pulegone (disproport.). Cyclohexanol. v Meth- anol. benzylic functional reported reductions hydrocarbons (Table alcohols. Certainly benzylic amines, have been of amines benzylic examples These include phenethylamine, both in approximately Brieger and T. 574 Chemical Reviews, Transfer Hydrogenation Acceptor Catalyst Donor Product (yield, %) Ref Pd black Cyclohexene 2-Propanol/H+ cis-Stilbene ‘I Reaction time, 3 hr. Reaction time, 23 hr. TABLE XI. Catalytic Transfer Hydrogenation of Carbonyl Compounds Acceptor Catalyst Donor Product (yield, %) Ref Benzophenone Benzil Benzoin Desoxybenzoin Benzoquinone Ethyl n.benzoylbenzoate (CiiHz3)CO p-CH30PhCHzCOPh-p-OCHa p-CHIOPhCh=CHCOPh-p-OCH3 Cholestanone Coprostanone Cholestenone Butyraldehyde p.Methoxybenzaldehyde 9-Anthraldehyde 21-Acetoxy-6-formyl-17- hydroxy-3-met ie n-20-one Raney nickel Raney nickel Pd/C Raney nickel Raney nickel Pd/C Raney nickel Raney nickel Raney nickel Raney nickel Raney nickel Raney nickel Raney nickel Raney nickel I rCI3(Me2SO), I rC13(MezSO), Raney nickel Pd/C Pd/CI Pd/CI Diethylcarbinol 2. Propa no1 Cyclohexadiene Cyclohexanol Cyclohexanol Cyclohexadiene 2. Propa no1 2-Propanol 2-Propanol Diethylcarbinol Diethylcarbinol Cyclohexa no1 Cyclohexanol 2-Propanol 2-Propanol 2-Propanol Cyclohexene Cyclohexene Cyclohexene Concurrent hydrogenolysis. After saponification. in section C. Structural Selectivity comments can selectivity. Carbonyl groups conditions with Pd/C. Therefore, ketones, acids, and amides strongly adsorbed catalyst, and may therefore Free amino groups but this may be over- groups are, as expected, carbonyl compounds, double bond double bond and not clear, however, determine candi- partial reduction double bond advantage was differential reactivity the reduction benzylic esters which could good yields Pd C CGHSCHCN - C,H,CH,CN (ca. 70%) (7) I OCOC,H, reducing the nitrile.58 Nitro groups are nitriles (Table Dip hen ha ne 0-Benzylbenzoic acid Dihydrocholesterol (50) Epicoprostanol (20) D i h y d roc h o I e st e r o I (10) Butanol (5) 21-Acetoxy-6.methyl-17- hyd roxy-3-meth. (CuHz?)zCHOH (80) p-CHaOPhCHzCHzCH2Php.OCHa (80). p-CH30PhCHzOH (9) 40 40 42 40 40 42 40 40 40 40 40 40 40 40 10 10 40 29 29 29 various steroid 5% Pd’C cyclohexene duced to 6-methyl steroids without affecting other func- Again in the ,steroid selective reduction exocyclic double bond in preference to endocyclic has been in the series is process which transfer hydrogenation takes place Another type found in the reduction polynitrobenzenes. With hydrogenation, only Transfer Hydrogenation Reviews, 1974, 575 Acceptor Donor Product (yield, %) Ref be nzoni 9-Cyanop hena Tridecanonitrile Tetradecanonitrile Pentadecanitrile Hexadecanitrile Octadecanitrile nopyri d i ne 3-Cya no pyri d i ne 4-Cya nopyrid ine 3-Pyridylacetonitrile 4-Cyanoqu inoline 6-Methoxy.4.cyanoquinoline P h CHzC HzCHzCH P hC N he ne yl p h re b e Tridecane (58), tritridecylamine (8) Tetradecane (53), tritetradecylamine (12) Pentadecane (41), tripentadecylamine (21) Hexadecane (44), trihexadecylamine (26) Octadecane (35), trioctadecylarnine (20) 2-Methylpyri d ine (85-90) 3.M et h ylpyrid i ne (85-90) 4-M e t h yl pyrid i ne (85-90) 3-Ethylpyridine (85-90) 4.M et h yl q u i no1 i ne (85-90) 6-M et hoxy-4-rnethylq u inoline (85-90) HzC HzC 8 8 8 8 8 8 8 8 8 8 8 45 45 45 45 45 8 8 8 8 8 8 a,$-unsaturated nitriles Concurrent hydrogenolysis (Pd/C Catalyst) Acceptor Donor process has tne appropri- as shown in PhCH=NPh Cyclohexene PhNHz (44) 42 PhCH=NCH&HzPh Tetra Ii n CHaCHzPh, 43 CHsPh (?) PhCHzCH=NCH2CH2Ph Tetralin CHaCHzPh (60) 43 Cyclohexene PhCHzPh PhCO(Ph)C=NNHPh Cyclohexene PhCOCHzPh 33 PhCHz(Ph)C=NOH Cyclohexene PhCH2CHzPh 33 N H P hexene P (78) (40) (45) regular catalytic hydro- An Synthetic Applications particularly inter- useful applications catalytic transfer hydroge- k OH k The tendency same reaction n + "3 (12) Raney nickel NH,(CH,),NH, 80% N' H .. H the appropriate amines according is probably Raney nickel (14) 2RCHpNHp - (RCH,),NH + NH, 70-90% that invoked alkylamines during I I the intermedi- imine must give excellent epinin dimethyl ether 576 74, No. G. Brieger and T. J. Nestrick Transfer Hydrogenation Acceptor Donor Product (Yield, %) Ref Nitropropane Nitrobenzene 0-Nitrotoluene m-Nitrotoluene p. Nitrotoluene n itrob p-Nitrobenzoic acid N itroa p-Dinitrobenzene 2,4.Dinitrotoluene 2,4.Dinitro-tert- bu tyl benzene 1,8-Dinitronaphthalene 2,4-Dinitrophenol acid Picric acid 1,3,5-Trinitrobenzene 3-Nitro-N-methyl-2-pyridone 3- Nit r0.N- m ethyl-4-pyri d on e Cyclohexene Cyclohexene Cyclohexene a-Phellandrene Cyclohexene Cyclo h exe n e a-Phellandrene Cyclohexene Cyclohexene Cyclohexene Cyclohexene a-Phellandrene Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cycl o h exe n e Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cyclohexe ne Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cyclohexene a-Phellandrene Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cyclohexene Cyclo hexene Cyclohexene Cyclohexene OCOC,H, I p h e nisid ine p h 0-Aminobenzoic acid b e P h e n y n e d Nitroanil ine 48 48 48 38 48 48 38 48 48 48 48 48 38 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 38 48 48 48 48 48 48 48 48 36 36 CH,O’ CH,O’ CH,O’ CH,O’ OCH, OCH, Catalytic transfer hydrogenolysis been used of branched lected cleavage chromenone was understood at transfer reduction not simply regular catalytic hy- unusual applications with organic Catalytic Transfer Hydrogenation Chemical Reviews, 1974, Vol. 74, No. 5 577 (Pd/C Catalyst) Ref Compound Donor Product (yield, %) p-Fluorobenzoic acid Limonene Benzoic acid p-Chlorobenzoic acid Limonene Benzoic acid 0-Chlorobenzoic acid Limonene Benzoic acid 0.Bromobenzoic acid Limonene Benzoic acid p-Chlorocinnamic acid Limonene Hydrocinnamic acid 3-CIP hCH=CHCOOCH3 Limonene PhCHzCHzCOOCH3 (90) 14 4-CI P hCH=CH COP h a-Phellandrene PhCH&H?COPh (90) 14 0-Chlorobenzonitrile p-Menthene Toluene (85-90) 8 p-Chlorobenzonitrile p-Ment hene Toluene (85-90) 8 Benzoyl chloride Cyclo h exe ne Benzaldehyde (10) 42 and Allylic Compound Catalyst Benzyl chloride Cinnarnyl chloride Benzylamine Tribenzylamine PhCHzNHCH&HzPh 3,4-(CH,)zOPhCH(OCOPh)CN 4.CHZOPhCH(OCOPh)CN 3,4-Dioxymethylene-PhCH(OCOPh)CN Pd/C Pd/C Pd/C Pd/C Pd/C Pd/C Pd/C Pd/C CH2R CH,O 4 Pd /C Raney nickel CH2R R = -N(CH& -'N(CH& -N(CH& 170-Acetoxy-6- hydroxymethyl.3-methoxy. Pd/C Benzoin Raney nickel androstane-3,5-diend20-one 4 0 i n Cyclohexene Methanol pH 7-7.5 Cyclohexene Cyclohexanol 3,4-(CHZO)zPhCH?CN (70) 42 42 43 43 43 58 58 58 28 (75) 28 CH30 17~-Acetoxy~6-methyI-3-methoxy- 29 Bibenzyl (58) 40 androstane-3,5-dien-20.one (?) HO HO interest is mechanistic considerations for reported experimental evidence Evidence for the view that a somewhat different is at the following active catalyst for the for instance, fails such linkages under the standard conditions donors. Palladium tive under decalin, which release solvent), fail cinnamic acid is not adequate to give hydrogenation. that palladium plays an in these as previously transfer hydroge- nation have been reported. tal variables studies have 578 Chemical Reviews, 1974, Vol. 74, No. 5 G. Brieger and T. J. Nestrick the inhibition the disproportionation to be discussed later, has cused on from catalytic with gaseous this point. a palladium hydride intermediate which then added to the acceptor, DH, + Pd - PdH2 + D (1 8) HPd H HH (1 9) hydride was it was alcohols when colloidal pal- R2C-O-H or I I I H Pd-H H 1 2 (20) donor and another unit donor, are transfer directly. as an termolecular mechanism, possibly in- as shown 3 4 that some however, presented evidence a cyclohexadiene also shown that the palladium fit a second-order the specific activity with 4, 20 palladium atoms model is shown as hydrogenolysis must several pro- donor, the the donor, the and any even- tual transformation product acceptor while still the influence the determi- X = palladium atom 5, proposed model for transfer of one allylic hydrogen the disproportionation therefore turn a consideration mechanistic clues. purpose we have general mechanistic eq 21-24. H, + 2§ W 2H § § = catalyst (21) I HH a reversible cess, followed the activated the nature the adsorbed/activat- have determined the applicability a mechanism necessary to overall incorporation details are available important addition the disproportionation these proposals could cer- the results order to further the interpreta- seem desirable Catalytic Transfer Hydrogenation Chemical Reviews, 1974, Vol. 74, No. 5 579 CH,=CH, + CH,=CH,; D, + 21 --L 20 (25) t I 9 § CH,=CH2 + D CH,-CH2D + 8 (26) I I 9 t I I 8 9 CHZ-CH, + CHZECHD t CH2=CH, + CHZ-CHZD § § § t § (27) CH,-CH2D + D A DCH,-CH,D + 28 8 (28) I § I I CH,-CH,D + D, + DCH,-CH,D + D (29) I § § CH,-CH,D + CH,=CHD + § 2CH,-CH,D - (30) t I d § and T bonds, as well as the formation of u and H com- ple~es.~, the case palladium, and for the corresponding hydrides of approximate composition can be the metals hydrogen gas.64,65 hydride, based an alter- of Pd relatively unstable. therefore not at all invoke hydride intermediates proposed first is the formation surface atoms the palladium catalyst with the palladium hydride, C=C-CH-CH + Pd + donor Pd C=C-cH-cH e C-C-C-CH + PdH (31) /'t '\ Pd t I -Pd - the palladium hydride (still the catalyst and not This is by a C=C + PdH - c-c (32) acceptor II Pd H molecular reaction the hydride the reduced acceptor and the dehydrogenated donor C-C + C-C-C-CH II '4' Pd H Pd C-C + C=C-C=C + 2Pd HH (33) It palladium intermediate the original or isomerized donor plus dehydrogenated scheme is 6d C-C-CH-CH + C=C-C=C + 2Pd or CH-C-C-CH (34) anisms which have been proposed account for olefins under the influence of palladium and a-bond- palladium, and equilibration between the mechanism the scrambling 35 and PdO CH,=CHCD,CH, * CH,=CHCD,CH, t PdD CH,-CHCD,CH, (35) I1 Pd D CH,=CDCD,CH, * CH3-CDCD2CH3 I f PdH Pd CH,CD=CDCH, @ CH,CD=CDCH, + PdD (36) t PdD our proposed mechanism, it that the stereochemical requirements an important role. certain acceptors such preferential complexation stereochemical aspects is the newer results hydrogenation, however, least one aspect of hydrogenation, namely homogeneous hy- formed from and has isolated with palladium C6H&I.CC&/, + HlrCI,(Me,SO), - /C6H5 as-stilbene (37) 'IrCI,(Me,SO), 9ooo aspect, namely at a bond (hydro- been explored with optically 38. The I I COOR COOH stereospecific and Intermediate for- (with retention) no results are transfer hydroge- may summarize stating that little work has been done that those results which 580 Chemical Reviews, 1974, Vol. 74, No. 5 G. Brieger and T. J. Nestrick regular catalytic hydroge- V. Summary and Prospects have endeavored hydrogenation exists catalytic transfer hydrogenation with hydrogen donor. reaction, using predominantly occasionally also nickel, or transition metal catalysts, permits the reduction olefins, nitriles, trogen-containing unsaturated functional groups, benzylic and the replacement readily available compounds such cyclohexene and alcohols. regular hydrogenation and the reduction polyunsaturated steroids, has I I greater experimental convenience with transfer hydrogenation, 1 or elaborate apparatus. surprising that rou- use is not donors, such naphthalene, cause problems than the usual include the catalyst-donor system capable mild conditions, wider studies the ap- workers such Wieland,5 thinking within investigated these reactions biological reductions. idea well be revived, because tor model these reactions indeed have enzyme system. pears that adsorption and acceptor in stereochemically favorable relationship the catalyst, the adsorption, but nonre- reductions are generally stereospecific. 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