Of coordinated olefins

We discussed this catalysis recently (141st National Meeting of the American Chemical Society, March 1962) in terms of an olefin insertion reaction involving a Pt(II) olefin complex (3). We found that catalysis was only accomplished by platinum compounds capable of coordinating olefins. For example, substitution by tertiary phosphines blocks coordination by olefins and greatly reduces the catalytic activity of Pt(II). The substitution by phosphines does not affect the ability of the complexes to cleave the Si—H bond, however. The hindering of a catalytic reaction by blocking coordination sites is a common occurrence and is, I think, a persuasive... 

Scheme 48 shows a mechanism for the Rh(I)-catalyzed reaction proposed by Wilkinson (11a, 107). The reaction starts with the insertion of coordinated olefin into the metal-hyidrogen bond in the hydrido-... 

All of the foregoing reactions involve a formal nucleophilic substitution of a group (e.g., H and Cl) attached to the double bond of a coordinated olefin. Under different conditions, Pd(II)-catalyzed oxidations of olefins can give products (e.g., glycol esters) resulting from formal addition to the double bond of coordinated olefins.5613... 

The first applications of NMR to the study of dynamic systems of the platinum group metals appear to have been studies on the rotation about the metal-olefin bond of coordinated olefins, and this process has been investigated by many workers. There are two reasonable orientations of an olefin with respect to the rest of a square planar complex, XXIV and XXV. 

With a two-step mechanism and a first-order propagation reaction with respect to monomer, the two cases (II) and (III) are possible, propagation rate being determined either by the rate of monomer coordination (kp = k ) or by the rate of coordinated olefin insertion and its surface concentration kp = k2ki/k-i)(r able 7). 

This somewhat incomplete picture can be coordinated with known facts of homogeneous catalysis. We may say that the monoolefin complexes of Ni are too unstable to be reactive, while the simple analogues of Pt are too stable optimum reactivity therefore resides in complexes of Pd and Rh where these opposing effects balance properly. We must defer for the present a discussion of the effects which other coordinated groups e.g., PR3, GO, SnGl.r) have on the reactivity of coordinated olefins since much more systematic information is needed for this to be feasible. The limited catalytic activity of olefin complexes e.g., Gu Ag ) is presumably caused by the absence from these complexes of any vacant orbitals at which attack by another reactant can occur. 

Qualitative photochemical studies concentrate on diene complexes because of their greater thermal inertness and ease of characterization. Formation of olefin complexes is induced in situ either by photochemical or thermal means and their presence determined by spectroscopy. The photocatalyzed hydrogenation and hydrosilation of 1,3-di-enes the photocatalyzed valence isomerization of norbomadiene to quadricyc-lane and the cis trans photoisomerization of coordinated olefins are potentially usefulHowever, these transformations are not photosubstitution reactions and are not discussed here the reader should consult ref. 1 and references cited therein. Photolysis of olefin complexes leads to olefin loss with high quantum efficiency unless the olefin is a chelating di- or polyene where, as with most chelating ligands, other reactions occur. 

The observations (29) on the homogeneous reduction of coordinated olefins confirms the possibility of mechanism (1). It seems unlikely that TT-allyl-adsorbed olefin [Structure (C)] will pass directly into an alkyl radical without either (A) or (B) intervening as in mechanism (1) or (2), although the process... 

Thus the selectivity and reactivities of coordinated olefins are highly dependent on the mechanisms and steric and electronic environments of the transition metal. 

The stereochemistry of products derived from reactions of coordinated olefins and the stereochemistry of polymers formed in reactions catalyzed by transition metals are ultimately determined by the conformational stability of 7r-complexed intermediates. For example, the cisitrans ratios and the relative amounts of 1,2- versus 1,4-polymer units obtained in diene polymerization are determined by stabilities of syn and anti isomers of vr-allyls and the relative stabilities of various orientations of substituted olefins bound to metals (105). The interconversion rates of these isomers and the thermodynamic preferences of olefin-metal conformations should explain observed product distributions and provide a rational basis for catalyst design. 

The authors concluded that the linear correlations between C for a- and 7c-bonded carbons indicated that metal d to olefin n bonding was of little importance and that <7p was the dominant term in the determination of the chemical shifts of coordinated olefinic carbons, [cf., Clark et al. (38)]... 

The same type of behavior can be found for nucleophilic attack of coordinated olefins and arenes. The C=C group of (cod)PdCl2 is attacked only by basic MeOH, while [(cod)Pd(P/ 3)Cl] is rapidly attacked even in neutral MeOH. " Hydride addition to coordinated pi-ligands tends to be controlled by the charge on the metal, for example ... 

AsPh3)2i ° It is suggested that the active carbene species is formed via a 1,2-H shift of coordinated olefin. 

Several complexes arose due to different orientations of coordinated olefin and reacting group were characterized in the theoretical study of ethylene insertion in rhodium complexes (Fig. 4) [40]. The calculations were carried out for the model system using [Rh(ri -CH2=CH2)(Z)(PMe3)] complex, where Z = CH3, NH2, and OH. 

The mechanism of olefin hydroformylation catalyzed by rhodium complexes has been extensively studied. For TPP as a ligand, it corresponds to Wilkinson s dissociative mechanism, which involves the four-coordinated active intermediate HRh(CO)L2 (L = TPP, Figure 14.2). Coordination of olefin with HRh(CO)L2 yields the 7t-complex 2. The insertion of coordinated olefin to the Rh-H bond leads to the formation of alkyl complexes 3a or 3b, respectively, via the anti-Markovnikov or the Markovnikov path. Subsequently, the alkyl migration to the CO affords the acyl complexes 4a or 4b, which leads to linear or branched aldehyde and HRh(CO)L2 via hydrogenolysis, eventually. The water-soluble catalyst HRh(CO)(TPPTS)3 is considered to react according to the dissociative mechanism [10]. However, the reaction occurs at the liquid phase or the gaseous-Hquid interface [11], and the activity and selectivity are remarkably different from those... 

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