Homogeneous Catalysis: Progress, Problems, and Prospects

11 Homogeneous Catalysis: Progress, Problems, and Prospects Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0103.ch011

ERIC W. STERN Engelhard Industries, 497 Delancy St., Newark, N. J. 07105

Research in catalysis by coordination complexes has resulted in the discovery of many new reactions and the development of several important industrial processes. Close parallels to heterogeneous and biocatalysis are indicated. Progress in reaching a detailed understanding of homogeneous catalytic reactions has been slower. Problems encountered are that nonisolable catalytic species may form in situ, kinetic studies often provide only limited mechanistic information, observed rates and products are affected strongly by media and catalyst compositions, and analogies between apparently closely related systems are frequently invalid. Difficulties in commercialization include containment of corrosive systems and product isolation from liquid media. These problems appear resolvable by currently available means. It is expected that continued research will lead to new applications and will contribute substantially to improved understanding of catalytic phenomena in general.

"p\uring the past decade homogeneous catalysis has progressed from an interesting novelty to its present status as a recognized field. In the interval a great deal has been said and written concerning the subject, much of it well beyond the scope of this discussion which is limited to an indication of what has been accomplished, an outline of the problems which have been encountered, and a forecast of future prospects. In the strictest sense, homogeneous catalysis involves catalytic reactions occurring in a single phase. However, as currently used, the term implies only that at least a portion of a particular reaction is known or suspected strongly to occur in the coordination sphere of a metal (most frequently a transition metal). Activation of substrates and likely the steric course of the reaction are then consequences of bonding in an in197 In Origin and Refining of Petroleum; McGrath, H., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

198

ORIGIN A N D R E F I N I N G OF P E T R O L E U M

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0103.ch011

termediate complex. The definition is not necessarily limited to reactions which are homogeneous in the physicochemical sense. While most reac tions included in this category are carried out in the liquid phase, slurry, gas-liquid, and even gas—solid systems are not excluded. Homogeneous reactions, such as general acid—base catalysis or radical chain processes, usually are not considered unless metal complexes play a part. Thus, designation of the area as coordination catalysis would probably be more descriptive. Progress The major accomplishment of research in homogeneous catalysis to date has been the discovery of an impressive number of new reactions. Several of these have been developed into industrially important proc esses. In addition, the list of complexes which have potential as new catalysts or as models for reaction intermediates grows steadily. While the incentive for discovering new reactions and seeking practical applica tions for these remains understandably high, increasingly serious efforts are being made to gain insight into intimate reaction detail. The following is an attempt to illustrate the variety of results ob tained. Obviously, any of the topics cited could provide the basis for extensive discussion. Reactions. OXIDATIONS. Nucleophilic Reactions of Olefins. These reactions, which over-all, involve replacement of vinyllic hydride by a nucleophile (Reaction 1), RCH=CH

2

+ H X ^l RC=CH Cu(II) ι Fd

o,

i

2

+ RCH=CH—X

(1)

are, at present, probably the best known and most widely studied ho mogeneous catalytic reactions. Indeed, the current high interest in this area of chemistry can be traced to the disclosure of the now familiar Wacker synthesis of acetaldehyde from ethylene and water by this route. The reaction which converts olefins to aldehydes and/or ketones in aque ous media (1), yields vinyl compounds in nonaqueous systems and has been demonstrated with such nucleophiles as alcohols (2, 3), carboxylic acids and their salts (2, 3), amines and amides (3), cyanide (4), and carbanions (5). Like many homogeneously catalyzed oxidations, it con sists actually of two separate stoichiometric reactions, in this instance, nucleophilic attack on olefin leading to reduction of the "catalyst" ( Reac tion 2) and re-oxidation of the metal to its active state (Reaction 3). RCH=CH

2

+ P d X + H X -> R C H = C H — X + Pd(0) + 2HX 2

In Origin and Refining of Petroleum; McGrath, H., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

(2)

11.

STERN

199

Homogeneous Catalysis Pd(0) + 2CuX -> P d X + 2CuX 2

2CuX + 2HX + l/20

CuX + H 0

2

2

2

The over-all process is catalytic both in metal and regenerating agent. While this type of reaction has been demonstrated with most of the group VIII noble metals, palladium has the highest activity and, there fore, is normally used. Regenerating agents (co-catalysts) are most fre quently CuCl , FeCl , and quinone. Reaction details are complex (6). Oxidation of Ahohoh. Conversion of alcohols to aldehydes and acids has received considerably less attention. The chemistry of this process has been explored primarily with respect to formation of stable hydrides of Pt, Ir, Ru, and Os (7,8) (Reaction 4). 2


(3)

2

3

(Et P)PtCl + EtOH + K O H - » (Et P) PtHCl 3

2

3

2

+ C H 3 C H O + KC1 + H 0 2

(4)

When the metal hydride formed is unstable (as in the case of Pd), the reaction can be used as the basis for a catalytic conversion of alcohols (9, 10) (Reaction 5). R C H O H + 1/20,

° > RCHO + H 0 20% H S0 P d S

2

4

2

2

(5)

4

Oxidative Coupling. A number of cases of this reaction type have been reported. Again, all are really two-step processes involving reduc tion of metal in the coupling step followed by in situ re-oxidation. Recent examples are coupling of aromatics (11) (Reaction 6), 2 C H + 1/20, 6

6

P d ( 0 A c )

)

2

HOAc

C H 6

5

-

C H 6

5

+ H 0 2

(6)

HC10< aromatics and olefins (12) (Reaction 7), 4>CH=CH + φΗ + Pd(OAc) -> CH=CH + Pd(0) + 2HOAc 2

2

Pd(0Ac) CH2=CH0Ac — -Î A c O — C H = C H — C H = C H — O A c

(7)

2

Cu(OAc)

< ^ C = C H - » φ—C=CH—CH—CH=C—φ 2

I CH3

(8)

2

I

I

CH3

and ^-substituted α-olefins (14,15)

CH3

(Reactions 8 and 9).


(9)

200

ORIGIN A N D R E F I N I N G OF P E T R O L E U M

Older examples are coupling of phenols catalyzed by Cu(I)-pyridine in which the relative amounts of carbon-carbon and carbon-oxygen coupling are controlled by the copper/pyridine ratio (16) (Reaction 10)


ArOH

and coupling of thiols to disulfides (17). While many aspects of oxidative coupling resemble radical processes, it is by no means clear that radical species are actually involved (18). Acetoxyfotion of Aromatics.

Reaction 11,

φΗ + HOAc + P d X -> φΟΑο + Pd(0) + 2HX 2

(11)

originally found as a side reaction in aromatic coupling, can be made highly selective by a proper choice of conditions. It has been examined both with respect to phenyl acetate and benzyl acetate formation (19, 20). Autoxidation. In this category, which refers to direct reactions of substrates with oxygen, there are numerous examples of metal catalysis of hydrocarbon oxidation. While metal participation most frequently involves catalysis of hydroperoxide decomposition (21, 22) (Reactions 12 and 13), ROOH + M+» -> R 0 • + M » - » + H+ +(

2

(12)

ROOH + Μ ^ " - » -> RO · + M+» + ΟΗparticipation in initiation (23-26) and termination (27) steps has also been found. In addition, such recently discovered cases as catalysis of the conversion of phosphines to phosphine oxides ( Reaction 13 ) (Ar P) Pd > Ar PO 3

Ar P + l/20 3

2

4

3

(13)

and of isocyanides to isocyanates (28-30) as well as oxidations of S 0 (31), C O (32), NO, and N 0 (33) should be mentioned.

2

2

REDUCTIONS. Hydrogénation of Mono- and Polyolefins and Acetylenes. Catalysis of hydrogénation of unsaturated hydrocarbons has been studied widely. Catalysis by a variety of metal complexes including those


11.

STERN

201

Homogeneous Catalysis

of Pt, Pd, Rh, Ru, Ir, Os, Cr, and Co has been demonstrated (34). Particular attention has been given to the catalytic properties of (

Homogeneous Catalysis: Progress, Problems, and Prospects

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