Synthesis of Aldehydes and Ketones by the Dehydrogenation of Alcohols in the Presence of a Raney Nickel/Aluminium Isopropoxide/A

CEJC 2(1)2004 214{219 Communication

Synthesisof Aldehydes and Ketones by the Dehydrogenationof Alcohols in the Presence ofa RaneyNickel/ AluminiumIsoprop oxide/Alumina Catalyst

1 1;2 2 1;2 Ilie Ciocan Tarta ,Ioan A.Silberg ,Mircea Vlassa ¤,Ioan Oprean

1 \RalucaRipan "University, Institute ofChemistry, 30F^ ant^aneleStr., RO-3400Cluj-Napoca, Romania 2 \Babes-Bolyai"University, Departmentof Chemistry, 11Arany Janos Str., RO-3400Cluj-Napoca, Romania

Received 24June 2003; accepted 13October 2003

Abstract: Thesynthesis ofaldehydes andketones by the dehydrogenationof allylic and secondaryalcohols using aRaney-Ni-Al( i-OPr)3-Al2O3 catalyst without hydrogenacceptors is presented. c Central EuropeanScience Journals. All rights reserved. ® Keywords: dehydrogenation, alcohols, heterogeneous catalysis Raney nickel, aluminium isopropoxide, alumina

1Introduction

The oxidation of alcoholsto carbonyl compounds isa fundamental reaction in organic synthesis [1].A number of transition metal-catalyzedprocedures havebeen developed, such as those using ruthenium [2]or palladium[3].A Raney-Nicatalyst isused frequently for the dehydrogenation of secondary alcoholsto ketones because of the high yields. However,this later method has the disadvantage of requiring either high temperatures[4] or largeamounts of hydrogen acceptors ( i:e.cyclohexanone),which are required for completeconversion[5]. Sometimes, noncatalytic quantities of Raney-Nihave been used for the dehydrogenation ofprimary and secondary alcohols[6].

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Our studies show that the dehydrogenation ofthe secondary and allylicalcohols can be achievedwith acatalyticmixture of Raney-Ni:Al( i-OPr)3:Al2O3:substrate (3:5:5:100 weight) without hydrogen acceptors and at temperatures between 120-170 0C (see Table 1). The reaction isselective, producing ketones,and the yieldsobtained are quiteim- pressive.This procedure has industrial signi¯cance, as the reaction could potentially be conducted inasemi-continuous mode.

2Resultsand Discussion

The experimentalresults are shown in Table1. Secondary saturated alcoholsare oxidized to the corresponding ketones in high yields(90-98%; seeT able1: entry 1-4,7.). Primary and secondary allylicalcohols produce the corresponding saturated carbonyl compounds by redox isomerization (Table1: entry 6,7) [7].T ertiary alcohols(see T able1: entry 9) and primary alcoholsdo not react. The dehydrogenation reaction of secondary alcoholsto ketones does not occur when either Raney-Ni, Al( i-ORr)3,or alumina are absent from the catalyticmixture. No reaction isobserved if the Raney-Niis substituted with other Ni(II) salts ( i:e. NiCl2) or complexes( i:e. Ni(PPh3)2Cl2).Wealso observe that using octeneas ahydrogen acceptor has no in°uence on the yieldor the reaction time,but if air isbubbled through the reaction mixture, the reaction does not proceed atall. This mechanism issupported by the following observations: a) The formation of complex3 ispossible knowing the tendency of Alcompounds to form four-coordinate complexes( i.e.Meerwein-Pondorf-Verleyreaction). b) The transition state isfavored by Raney-Nidue to the great a±nity of Nifor hydrogen. c) The elimination of one hydride ion from the alkoxidecarbon and one proton from the oxonium ion inthe presence of Ni-Raneyvia a six-atom concerted ring transfer, leadsto the formation ofthe ketone,4. d) The inductive e®ect (+I) or conjugation e®ects (+E) of the substituents stabilize the carbocation obtained by eliminationof hydride ion.This can beexplainedbased on the high reactivityof allylicand secondary alcohols.

3Conclusions

In conclusion,our studies show that the mixture of Raney-Ni-Al( i-OPr)3-Al2O3is an excellentcatalyst for the preparation of ketones and aldehydes from secondary and allylic alcoholsin good yields.

4Experimental

1 H-NMRspectra were recorded on aGemini 300MHzspectrometer using CDCl 3 as the solvent and TMS(tetramethylsilane) as the internal standard. IRspectra were recorded 216 I.C.T artaet al. / CentralEuropean Journal of Chemistry 2(1)2004 214{219 on aPerkin Elmer 700 spectrophotometer. GCand MSanalyseswere performed on aHewlett Packard 5890(GCL)-5872(MSD) using aHP-5MS 30m x0.25 ¹mcapillary. The reactions were carried out under anitrogen atmosphere. Aluminium oxidegranules, about 1-5 mm,were obtained from Merck.All alcohols were obtained commercially.

General oxidation procedure

Toadegassed alcohol(0.1 mol),3%( weight mass) offreshly prepared Raney-Ni[8], 5%

(w/w) of Al(i-OPr)3,and 5%(w/w) of Al 2O3 were added. This mixture was magnetically stirred under nitrogen and heated for 6-50 hrs (seeT able1). The catalyst was removed by ¯ltration under an inert atmosphere and washed with petroleum ether. Subsequently, the ¯ltrate was puri¯ed by fractional distillation in vacuo,yielding52-95% of the pure carbonyl compounds (seeT able1). The catalyst removed by ¯ltration (a mixture of Ni and alumina) was reused. The mass, 1H,and IRspectra of the products corresponded with those reported inliterature.

References

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Scheme 1. Dehydrogenationmechanism of alcoholsin the presenceof

Raney-Ni-Al(-OPr) 3/Al2O3 catalyst.