Activation of Small Molecules
Mark M. Jones Vanderbilt University Nashville, Tennessee
by Coordination
The use of metallic catalysts is mentioned in all elementary textbooks of inorganic and organic chemistry. In some of these a few specific reactions are discussed in detail, particularly those involving anhydrous aluminum chloride or iron(II1) chloride. One important aspect of such reactions is almost universally ignored. This is the surprising generality of the coordination process as a means of activating molecules for further reaction. This catalysis or activation can be effected by a wide variety of Lewis acids, not all of which are equally effective. On surveying these reactions, one is struck by the common patterns involving the activation of small molecules via coordination in the generation of electrophilic reagents, free radicals, and even nucleophilic reagents. It is with such examples, and the underlying principles which they illustrate, that the present discussion is concerned. It is believed that if the patterns and principles are drawn to the attention of the student a t a propitious time, his understanding of these reactions and his ability to devise neF ones of this sort will both be considerably enhanced. These patterns are interpretable, for the most part, in terms that can be presented to undergraduates. General Principles
When any molecule is brought close to a positive ion, the molecule suffers a certain amount of electronic polarization. I n many cases this is merely a distortion of the electronic distribution toward the positive ion. When the molecule and the positive ion form a coordinate bond, the resultant distortion of the electronic cloud assumes a more permanent condition and this can result in very considerable changes in the rates of many reactions which such a ligand n~olecule undergoes. 0
CHsC
'0-E
0
+ BFs-
CHr-C
+
\ do-"
BR weak acid
strong acid
//
0
CHrC-4-
+ H+
0 '
The two aspects of this which are to be considered in detail here are the ways in which the electronic distribution of a ligand is affected by coordination and how a large number of the resulting reactivity changes can be brought into a systematic arrangement.
The consequences of the electronic polarization which ensues upon coordination were first studied systematically by Meerwein (1-5) who centered his attention on organic ligands. He showed how the coordination of the hydroxy group of an organic acid to a Lewis acid resulted in a large increase in the dissociation constant of the acid. This was explained as a consequence of the depletion of electrons from the hydrogen-oxygen bond which made the release of a proton from the oxygen easier. I n considering the net effect of coordination on the ligand electron density one must consider three types of coordinate bonds and their effect on this. For a simple coordinate bond of the usual sort, a sigma bond, there will be a depletion of electrons from the ligand. Both electrons of the bond are furnished by the ligand and undergo a displacement toward the metal. For a con~plexwhich also contains pi bonds between the metal and the ligand, the situation will be more complicated. If a pi bond is formed using electrons from filled d orbitals of the metal which are shared with empty ligand orbitals, this will reduce the net charge transferred from the ligand to the metal. If a pi bond is formed in which electrons are donated from suitable ligand orbitals to empty metal orbitals, this will accentuate the depletion of charge from the ligand. The difficulties involved in assessing the relative importance of these three kinds of bonds are anything but trivial (6). The occurrence of pi bonding with ligand donation may well be involved to a slight extent in the activation of molecular halogen by species such as AlCIs. Most of the complexes utilized in organic chemistry for halogenation, Friedel-Crafts reactions and the like involve predominantly sigma type coordinate bonds. I n a very gcneral way then, it is expected that the coordination of a small n~oleculewill distort its electron density pattern to produce significant, and hopefully, useful changes in its reactivity. The molecules which will be considered here are frequently found to be nonpolar or weakly polar species prior to coordination. Coordination increases the polarity and allows more reactive ionic, ion pair (7), or highly polarized species to be generated. Some typical examples are listed in the table. It is generally found that coordination results in an increase in the polarity of the ligand-acceptor system, though in some cases stereochemical arrangements may result in a small or zero net dipole for the complex as a whole (8). The coordinate bond itself always possesses a considerable dipole moment. These dipole moments are often very large. The interpretation of the total moment of the complex in terms of bond moments is rendered difficult by our sparse information on the Volume 41, Number 9, September 1964
/
493
contribution of lone pairs to the total moment of the free ligands and by the rehybridization of the acceptor species which accompanies its increase in coordination number. This factoring of the total moment into contributions from various bond moments has been attempted in some cases, but it is only within very recent times that a large number of dipole moments of coordination compounds have become available. A considerable amount of such data is necessary if the consistency of a scheme of bond moments is to be established. Reactive Species Generated by Coordination
Orieinel Molecule
Reactive Soecies Generated
+6
-8
CIS
Cl+ or CI-CI
Bh
Brf or Br-Br
ClCN
CN or NC-CI
+6
+6
-6
H-OH
RCOCl
-6
+
+6
H1O
-6
+6
-6
+6
-6
RCO+, RC=O, or R-C-C1 R3Ct
+6
AI 0 II + C1or a less palm form
H2C=CH2 -2
+6
-6
CEO NO9+ NO.+ NO NH%+ NH. + N2-(?) +
0,-
HO. NH?. H-
+6
-6
c=o
The actual formation of stable, isolatable coordination compounds is not a necessary condition which must be met before coordination can be assigned a catalytic role in a reaction. For example, the mercury(I1) and silver(1) ions and analogous species which have a considerable tendency to coordinate haloge;l can accelerate the removal of halogen from inorganic (9) and organic (10) systems: ICo(NH&Br]"
+ Hg'+ + HzO RCI
-
+ Hgz+
-
+
ICo(NH&(H2O)I3+ HgBr'
R + + HgCl+
These reactions are commonly assumed to proceed through an unstable intermediate in which the halogen is bonded to two other atoms. Some of these have been studied in very considerable detail and a very thorough kinetic study has been carried out on the reaction of mercury(I1) nitrate with some primary and secondary alkyl bromides (11). The silver ion has also been, shown to be capable of assisting such reactions in an analogous manner (12-14). The activation process is rather similar for all species and is not dependent upon the presence of a donor 494
/
Journal o f Chemical Education
species which is already polar. One factor which must always be taken into consideration is the steric interaction with the acceptor species. Thus a small, highly charged monatomic ion will usually not present the same opportunity for steric hindrance to a reaction as, say, antimony trichloride. The amount of information available on the way these steric effects may arise is extensive, but except for boron acceptors, this is somewhat unorganized. Ideally one would hope to estimate the role of the acceptor species in changing the frequency factor and activation energy of the reaction. This can be done qualitatively only when the course of the reaction has been thoroughly characterized kinetically. The mode of activation via coordination may vary considerably. In many cases it is by means of an increase in the polar character of an electrophilic reagent to give a more effective attacking species. I n other cases, the bond weakening resulting from coordination is the critical step as it then allows the bond to be broken more readily by a third reagent, as in the splitting of coordinated ethers. I n any event, the electronic configuration of the metal ion appears to be of somewhat lesser significance than its charge and radius in such reactions. The formation of pi bonds with the ligand, which generally involves back donation of electrons, is a secondary factor which appears to be either indispensable, as in the metal carbonyls and their reactions, or impossible to sort out from other factors, as in the majority of complexes. Only when a large body of strictly comparable kinetic data is available will it be possible to sort out such interactions with any confidence. A principle stated explicitly by Nyholm (16) and of use in many of these cases is that the larger the number of donors around a central metal ion, the lower the charge donated by each to the metal. This can be used to estimate the variation in the reactive sites of a complex as the coordination number is changed as well as the relative reactivity of complexes with various metal to ligand ratios. An example of the use of this principle can be seen in the explanation of the catalytic activity of zinc dialkyldithiocarbamates and their anline adducts on the disproportionation of diethyl trisulfide (16). Here the sulfur atoms of the complex act as a nucleophilic catalytic center. When the coordination number of the zinc(I1) is increased to five or six by the addition of an amine such as piperidine, the nitrogen atoms of this base aid in the neutralization of the charge of the zinc(I1). This releases electronic charge to the sulfur atoms of the ligand which then become more strongly nncleophilic and hence more active as nucleophilic catalysts. When the reactant molecule (i.e., the one on which it is desired to carry out the substitution reaction) is also capable of forn~ingcomplexes, then the Lewis acid must be added in a much larger thau catalytic quantity so that some catalytically active amount remains free after the co~nplexationequilibrium is attained. This rationale lies behind the numerous examples where large molar ratios of Lewis acid to substrate are utilized. These include the Fries reaction (17), the Friedel-Crafts reactions of aromatic acid (IS), ketones, aldehydes, and the like, as well as the more recently discovered "Swamping-Catalyst" effect in aromatic halogenation (19-21). The kinetic pattern of these reactions was first studied
by Olivier in 1914, who examined the reaction of aromatic sulfonyl chlorides with benzene in the presence of varying amounts of aluminum chloride (22). Olivier found that the rate of the reaction increased slowly with increasing mole ratios of aluminum chloride to sulfuryl chloride as long as all the sulfonyl chloride was not complexed. After a sufficient amount of aluminum chloride was added to complex all of the substrate, the rate of the reaction increased very rapidly with further additions of aluminum chloride. This same behavior is found in many other reactions and is due to the fact that the catalysis proceeds via the generation of a n electrophilic species which requires that the Lewis acid react with the poorer of two donors in the systems. The activation will then occur in an appreciable amount only after the coordination sites of the better donor are completely or nearly saturated. I n such cases the dissociation of the complex with the better donor may occur to such an extent that it furnishes some catalyst for the generation of the attacking species. I n such a situation the relationship between the mole ratio of reactants and the rate constant need not show a discontinuity a t an integral value, although it frequently will. The Generation of Elecbophiles
Of direct interest are the closely related mechanisms in which compounds such as iron(II1) chloride, aluminum chloride, and boron triAuoride catalyze the chlorination and bromination of aromatic compounds. This is explained as a result of the polarization of the halogen which arises from coordination. The reaction of chlorine and iron(II1) chloride is postulated to lead to more reactive species either by cleavage of the chlorinechlorine bond:
The activation of molecular iodine can be effected by treatment of a solution of iodine with a silver salt. The process which occurs here is:
The positive iodine can be isoated in the form of complexes with pyridine or dipyridyl if these are present in the reaction mixture (29). The same process may also be used with other halogens or pseudohalogens, though such an activation process is usually not so necessary in effecting electrophilic substitutions with them. This type of reaction also furnishes a procedure for generating reactive carbonium ions from alkyl halides or various halogen compounds (12, SO). The general reaction for such cases is RX+Agf+R++AgX+
and both R and X can he varied widely. The prime requirement is that AgX be insoluble. A variation of this may be found in reactions of the type RX
+ AgZ-RZ + AgXJ
For example: RSOICI
+ AgSR'
-
RSOBR'
+ AgC1J
Although these reactions may be carried out with cations other than silver(I), only mercury(I1) is widely used in similar reactions. This is due to the greater insolubility of the silver compounds which are formed, compared to those of other cations (e.g., lead(II)), and their correspondingly greater effectiveness. Spectrophotometric studies on the behavior of such species as trityl chloride (CaHs)aCC1,in the presence of various Lewis acids (31) show that these assist ionization by reactions of the type: ZnCh + (C6H&CCI F? (C6H&CC ZnCb-
+
or by rending the bond more polar:
Either process leads to a predicted increase in the amount of positive halogen in the system, and it is this positive halogen which is considered to be the effective halogenatiug agent in such systems. This explanation of the catalytic activity of metal halides in such systems was first proposed by Pfeiffer and Wizinger (23-26) and has since been extended to a large number of analogous reactions. All of the halides which are effective catalysts for halogenation are known to form halo anions of the sort required by such a mechanism. Iron(II1) chloride also catalyzes the aromatic iodimtion by IC1 (26). Other catalysts for these reactions, such as iodine, may act by a different mechanism (27). The activation of a molecule like a halogen or a pseudohalogen should show a certain specificity for the metal halide which parallels the acceptor tendencies of the metal halide. Thus, thiocyanogen, (SCN)*, should be activated by species which have a marked tendency to coordinate to SCN-. Here one would expect that Hg2+ would be a much more effective catalyst than AICla. However, only AICla and A1(SCN)8.2(C2H6)20 have been studied as catalysts (28).
The kinetic study of the Friedel-Crafts synthesis of benzophenone from the benzoyl chloride-aluminum chloride complex (32) and benzene showed that the reaction of [CsHsCOCI.AICls] was quite slow in the absence of any additional aluminum chloride, but speeded up considerably when further aluminum chloride was added (33). This indicates that the aluminum chloride possesses a function as an activator which is not directly related to any coordination process of the usual sort hut may well be involved in the formation of a pi complex with benzene. The exact nature of the complexes formed by aluminum chloride with acyl halides are a point of some controversy. Cook (34) has presented evidence which shows that CH8-C=O:AIC13, and [CH3CO+][AlClr-I I
61 may both be present in a solvent of high dielectric constant such as nitrobenzene ( E = 36.1) but only the first form is present in a solvent of low dielectric constant such as chloroform (e = 5.05). The use of both silver(1) and mercury(I1) ions to assist the removal of a halide ion from an alkyl halide has been extensively studied (36-37). The rate law found for such a reaction is determined by whether or not coordination plays a role in the activated complex (38). Volume 41, Number 9, September 1964
/
495
Although there are a large nunlber of ell-characterized complexes in which the nitrosyl group is a ligand, the information on their reactions is quite scattered. The reactions of nitrosopentacyanoferrate(II1) have been studied in some detail hut many of the products are still poorly characterized (39). Acetophellone forms an oxuninoketone derivative with this ion and sulfides give a characteristic color reaction. Because of the lability of the hydrogen atoms of water even prior to coordination. effects of coordination on the reactivity of water are usually quantitative differences rather than qualitative ones. One wellestablished phenomenon is the increase of the acidic nature of water when it is coordinated. This was noted and studied in detail by the early 1930's (40). Subsequent studies have been concerned primarily with quantitative determination of the equilibrium constants (41) rather than resultant changes in the reactivity patterns of water. One of the most striking examples of the reactions of a coordinated ligand is to be seen in the chlorination of coordinated ammonia as reported by Iiukushkin in studies of a variety of ammines (42). The general type of reaction was found to he: Xs-Pt-NHi X,PtNH,Cl
-
+ Clp
+ Cb
XsPt-NH2CI XsPt-NClz-
NH20H
+ ArH
-
ArNH.
C2H4
+
'/202
+
CzH,O
The use of coordination to stabilize very reactive unsaturated hydrocarbons is also possible as may be seen in the stabilization of tetramethylcyclobutadiene in its conlplex with nickel(I1) chloride (51) :
The activation of other kinds of unsaturated molecules is also possible as can be seen from the followiug example (52).
+ HCI + H + HCI +
I n some cases the complexes in which further suhstitution had occurred were found to bevery easily prepared. This further reaction with chlorine to give di- and trichloramine is one which an~nmoniaitself undergoes if the pH is sufficiently low (43, 44). Coordination appears to stabilize the various chloraniines, especially toward thermal deconlposition. From the conditions of polarization to which the coordinated ammonia is subject, one would expect that the nominal replacement of H + by C1+ would proceed more readily in the complex. From such data as is on hand this seems to be the case. It is also of interest to note that the transformation of [Pt(NH3)sC1]C13into [Pt(NHa)3(iVC1Z)T C1]+ increases the acid strength of the hydrogens which are still bonded to the nitrogen. The aluminum chloride catalyzed aminations of aromatic systems by hydroxylamine, NH,OH (45, 461, or hydroxylanline-o-sulfonic acid (47, 48) are more clear-cut examples of activation involviug a small molecule with nitrogen donors. Typical reactions are: AlCls
The overall reaction is thus:
+ HaO
When a n~oleculeor anion is incorporated as a ligand into a complex with a positive ion, it will suffer a depletion of electronic charge and will be more electrophilic in nature than it was prior to coordination. While such a species may be difficult to think of as an electrophilic reagent, it is important to note that such a species is always more susceptible to attack by nucleophilies after coordination. Examples involving larger organic molecules have been collected by Bender (49). The activation of unsaturated inolecules via coordination has been both widely used and thoroughly studied (50). Perhaps the best known example is the Smidt reaction for the preparation of acetaldehyde from ethylene. The steps in this process are:
The number of such reactions is very large indeed and is the basis of much industrial chemistry developed in large measure from the studies of Reppe and his collaborators (53). A recently reported reaction of a slightly different sort is the silver-ion catalyzed isomerization of cis4-maleylacetoacetate and maleylacetone to their trans isomers (54). The elements which have the greatest tendency to form coordinated bonds to olefins are Cuf, Ag+, PdZ+ PtZ+,and their immediate neighbors in the periodic table. These are the species most commonly iuvolved in the catalysis of the reactions of olefins. Platinum(IV) is not commonly useful though it is capable of effectingsome reactions, e.g. (55) : RCH=CHRf
+ PtFs'- + HzO
-
One point upon which a certain amount of controversy exists is the detailed description of the nature of the complex (responsible for the increased reactivity) which is formed between a Lewis acid and an unsaturated hydrocarbon such as ethylene or benzene. One may have either sigma or pi coinplexes. A sigma complex is one in which a sigma bond is formed hetween the electron deficient species and the aromatic ring or olefin. I n the case of strong protonic acids this leads to carbonium ions derived from the aromatic systen~s. In such signla complexes one finds that the proton is bound to one specific carbon atom of the aromatic system. I n pi complexes the bonding between the two constituents in presumably less localized and involves the entire aromatic ring or the unsaturated system as a whole. These are similar in many respects to the olefin complexes of the platinum metals in which the platinum is bonded in such a fashion that the center of the etbylenic linkage is in a square plane with the
other three groups bonded to the metal. Included in the class of the pi complexes are also the molecular compounds such as quinhydrone, picrates, and as a limiting case, compounds such as bis(benzene)chromium (0). The activation of a large number of organic systems of the type RCOCl or ROR is possible if such species are coordinated to suitable Lewis acids. Such processes are an integral part of the Friedel-Crafts acylation reaction. I n recent years the reactive intermediates in a large number of aromatic hydrocarbon-Lewis acid systems have been isolated and characterized by Olah and his collaborators. The activation presumably proceeds by way of a carbonium ion: RCOCl
+ AICls
-
RCO+ + AICI;
I n the case of the toluene-boron trifluoride-hydrogen fluoride system a reactive carbonium salt has been isolated which is formed in the reaction (56):
The activation of molecular nitrogen by what may well be a coordination process is involved in the fixation of this element from the air by certain microorganisms. The N, molecule is isoelectronic with the CO molecule and would he expected to form complexes with suitable coordination centers, i.e., those capable of forming pi bonds with such a ligand. It is well established that compounds of both molybdenum and iron are essential to the fixation of atmospheric nitrogen. There are a t present no unequivocal proofs that coordination can activate nlolecnle nitrogen, however. According to one report (61), nitlogen is capable of oxidizing hemoglobill-(Fez+)to hemoglobin-(Fe3+)while hydrogen can reverse this process. The hen~oglobins used were obtained from soybean nodules and presumably are those involved in nitrogen fixation. The elementary steps possible in the fixation of atmospheric nitrogen by lower organisnls can, in principle, involve either oxidation or reduction. Present evidence favors a reduction of the nitrogen and a sequence for the iron-containing nitrogen fixing enzyme, nitrogenase, involving such a reduction has been proposed (69) and is: Hz0
nitrogenase-Fez++ N2 e nitragenme-Feat + Nlisa-+ nitrogenase-Fea+(OH-)+ NHa The use of Friedel-Crafts type catalysts has been widely exploited in the activation of unusual species. Thus cyanogen chloride, ClCN, interacts with alumiuum chloride to produce a species CN+,AlCl,(possibly as an ion pair) which reacts with benzene to produce benzonitrile (57) :
A sin~ilar effect is produced by iron(II1) chloride. The variety of the types of reactions which may be effected by such a process is surprisingly large. Analogous reactions occur when nitrosyl chloride interacts with a suitable Lewis acid to produce the nitrosonium ion, NO+, as in (58) NOCl + FeCla NOIFeClrl
-
Nitryl chloride, K02C1, and nitryl fluoride behave similarly to yield the nitronium ion, KO2+, and a complex anion (59j : NOzF + BFa NOl[BFll
-
A somewhat more unusual example is the reaction of arsenic trichloride with chlorine in the presence of aluminum chloride (60). This gives a salt of the tetrachloroarsonium ion, AsC14+: The AsCl,+ species should be an electrophilic reagent, though information on this point is presently unavailable. The Generation of Free Radicals
When coordination creates circumstances favorable to the complete trausfer of electrons, free radicals of considerable reactivity may be generated. This is often found with iron, copper, and manganese complexes because of the ease with which the central ion can assume various oxidation states.
The equilibrium constant for the nitrogenase-N2 dissociation reaction has been estimated independently by several workers and is of the order of 0.02 atm (63). The possible applications of such a nitrogen complex are very inipressive. Since hydrogen can also be activated by coordination, a method of fixing nitrogen as ammonia via a "coordination chemistry Haher process" would become possible. The use of such activated nitrogen in a variety of inorganic and organic reaction systems would also lead to many useful synthetic routes. The detailed nature of such an I i x complex has so far eluded investigators. The activation of molecular oxygen by coordination is well established in both biochemical and inorganic systems. The cations which are especially active in this respect possess two qnalifications: they can form pi bonds and they can give up an electron to the oxygen. They include Fe(II), Mn(II), Co(II), Cu(I), and related transition metal ions. These reactions are almost invariably free radical reactions. Their initiation may involve the reaction of oxygen with either a hydrated metal ion or a complex containing other ligands (6466). I n biochemical systems coordination of molecular oxygen to various complexes such as iron containing respiratory enzymes (67) and analogous systems is important in activating molecular oxygen for oxidations of various sorts including those in which H20, H202, or oxidized organic compouuds are the products. The enzymes which produce HzOor H20zare called oxidases; those in which the oxygen is directly incorporated into an oxidized organic product are called oxygenases (68). I t is possible to effect hydroxylation reactions of many systems by the use of the Udenfriend reagent (69, 70). The essential ingredients of this system are: a buffer, a suitable substrate, ascorbic acid, a suitable metal conlplex, and oxygen. A suitable substrate may he an aromatic, heterocyclic, or aliphatic compound. A suitable metal complex is Fe(I1)-EDTA. The steps involved are an initial oxidation of ascorbic acid to give monodchydroascorbic acid and hydrogen, Volume 41, Number 9, September
1964 / 497
both as radicals, and subsequent interaction of these with the substrate. Thus, when aniline is used as the substrate, the products include p and o-aminophenol; pyrene is converted to 3-hydroxypyrene (71). This system is rather similar to that used in the nonenzymic decarboxylation reaction of methionine in the presence of a buffer, iron(I1) ion, ascorbic acid, and EDTA (72). The principal difference between the nonenzymic and the enzymic reaction of this sort is that the nonenzymic reactions are generally much less specific in their demands for a substrate. Thus the enzyme, present in rat livers, which can change irphenylalanine to tyrosine is ineffective in the hydrolation of D-phenylalanine (73). Another, quite different system, which can be used to activate molecular oxygen for oxidations consists of a solution of copper(1) chloride in pyridine. This r e agent appears to be one of general utility (74). The behavior of hydrogen peroxide is somewhat different from most of the other molecules mentioned, as the usual result of the activation of this molecule is its decomposition:
This reaction has been examined by a large number of investigators. A more interesting situation is one where the hydrogen peroxide is activated for attack on other species. Many of these involving catalysts such as horse radish peroxidase, have been developed by B. C. Saunders and his students (75,76). I n some cases these enzyme catalyzed reactions furnish remarkably effective and specific synthetic routes. The activation of both hydrogen peroxide and organic peroxides can frequently be effected by Fez+ or some other metal ion. Here the essential process is the generation of reactive hydroxyl radicals or their equivalent:
Such processes are of distinct synthetic utility (77). While one might expect to find certain similarities between the behavior of hydrazine and hydrogen peroxide, these are presently known n~ostly for the metal ion catalyzed decompositions of these species. There appears to have been no obvious exploitation of a "Fenton's reagent" based on hydrazine rather than hydrogen peroxide (78). The reduction of chlorate by hydrazine, however, is found to he catalyzed by both silver and copper salts and by osmic acid (79). The Generation of Nucleophiles
Coordination can lead to the formation of reactive nucleophilies under two conditions. The first is when coordination ties up the only pair of electrons as the Ha. I n this respect the behavior of hydrogen is unique. The hydrogen n~olecule has only two electrons, if these are used to form a coordinate bond the products will he a proton (solvated) whose behavior is of lesser interest, and a coordinated hydride ion which is the center of a novel type of reactivity. The second type of reactive nucleophile is found with ligands whose usual coordination involves bonding in which the back donation of electrons from the metal is more important than the formation of the normal, sigmaAype coordinate bond. This may occur with carbon monoxide. 498 / Journal of Chemical Educofion
The activation of n~olecularhydrogen via coordination or processes in which metals split the hydrogen molecule has been very thoroughly studied by J. Halpern and his students (80). Complexes of the more usual sorts may also be used as catalysts for such reactions. The ions [CO(CN)~]~and [CO(NH~)~CI]'+ have been found to catalyze the reductive amination of cu-keto acids (81); cbromium(II1) stearate catalyzes the reduction of cyclohexene (82), and the reaction product of triethyl aluminum and nickel(l1) Z-ethylhexanoate catalyzes the hydrogenation of many organic compounds, presumably via a nickel(0)hydride (85). Some of these reactions, but not all, go via coordinated hydride. Those in which simple aquo ions are involved as activators that have been subjected to kinetic study by Halpern and his group are found t o be almost all reactions in which the heterolytic splitting of hydrogen is the rate determining step if a suitable substrate is present (84). The changes in the reactivity of carbon monoxide which arise upon coordination can he seen in the reactions of the metal carbonyls. It should be recalled that carbon monoxide has only a small dipole moment ( p = 0.1 D) in the free state; coordination will tend to make this larger and hence will assist polar reactions. The enormous number of reactions of this sort which are known prevents a complete listing, hut the 0x0 process the homologation reaction and carbon monoxide insertion are typical of these: 0x0 reaction (also called hydroformylation)
Homologation reaction
Carbon monoxide insertion reaction (85) A survey of a number of these has been published (86). The activation of carbon monoxide by coordination is a widely useful method for the preparation of many kinds of organic compounds and new types of reactions are being discovered continually. A recent example is the carboxylation of alkali halides or sulfates: [CO(COJIRX
+ CO + R'OH + Base
RCOOR'
+ (H Base)X
This reaction proceeds through the acylcobaltcarhonyls, RCOCO(CO)~(87). Aluminum chloride can also assist in the activation of carbon monoxide in some reactions (88). Conclusion
From the exan~pleswhich have been presented it is apparent that coordination may furnish a very useful route in the catalysis of a large number of reactions. It must be einphasised that the known examples by no means exhaust the possible situations where coordination can act in this way. It is useful to note
(81) MURAEAMI, M., AND KANG,J.-W., Bull. C h a . Sac. Japan, 35,1243 (1962). V. A., Zhur. Fir. Khim., 36,1617 (1962); C.A., (82) TULUPOV, 57,14471 (1962). S. J., AND SCHUETT, W. R.,J. Org. Chem., 28, (83) LAPPORTE, 1947 (1963). J. F., CICCONE, S., AND HILPERN,J., Can. J. (84) HAFLROD, Chem., 39, 1372 (1961); BECK,M. T., GIMESI,I., AND FAREAS,J., Nature, 197.73 ((1963).
(85) CALDERAZZO, F., AVD COTTON,F. A,, Inorg. C h a . , 1, 30 (1962). (86) WENDER,I., STERNBERG, H. W., F R I E D ER.~ A,, METLIN, 8. J., AND MAREBY,R.E., Bulletin 600. US.Bureau of Mines, Washington, D.C., 1962. (87) HECK,R. F., AND BREBLOW, D.S., J. Am. C h a . Soc., 85, 2779 (1967). E. W., SMITA,C. H.,AND HORN,R. C., J. (88) CRANDALL, Org. Chem., 25,329 (1960).
+--
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Journol o f Chemical Education
