Halpern mechanism

While two phosphines are always trans to each other in all the intermediates in the Halpern mechanism, Brown et al. have very recently proposed a different mechanism, in which the cis biphosphine intermediates play an essential role (4). Their molecular modeling calculations where the van der Waals energy is calculated between substituted olefins and the Rh fragment with bulky trans phosphines have suggested that when the substituents on the olefin are bulky, the steric repulsion is too large for the olefin to coordinate. Their NMR experiments have shown the existence of the following equilibrium (Equation 1) in which an intermediate with a pair of cis phosphines can be formed. In fact, H RhClCPR ) has been detected in the catalytic system. 

In this article, we will compare the energetics of the conventional Halpern mechanism with that of the Brown mechanism. The basis functions used are the 3-21G for ethylene and hydrides, the ST0-2G for spectator ligands, PH and Cl, and valence double zeta basis functions for Rh with effective core potential replacing the core electrons (up to 4p) (5a.b.6). In addition, we carried out the MP2 calculations at selected, RHF-optimized structures with a larger basis set, which consists of uncontracted (3s,3p,4d) functions from the above valence DZ set for Rh, 4-31G for the ethyl group,... 

Since our calculations on the Halpern mechanism have been published (2) > will give a brief summary for comparison in a succeeding section. The potential energy profile shown in Figure 1 is constructed from the energetics of the elementary reactions involved in the Halpern mechanism. The optimized structures are shown in Figure 2. 

The potential energy profile is smooth without excessive barriers and too stable intermediates which would break the sequence of steps. The rate-determining step is found to be olefin insertion followed by isomerization, supporting the Halpern mechanism. Isomerization of the ethyl hydride complex is an important part of the rate-determining step. These two reactions, exothermic overall, has an overall barrier height of about 20 kcal/mol. The trans ethyl hydride complex, the product of ethylene insertion, may not be a local minimum (per MP2 calculation) and these two steps may well be a combined single step. 

Figure 1. Potential energy profile of the entire catalytic cycle in the Halpern mechanism for olefin hydrogenation, in kcal/mol at the RHF level, relative to l+C H +H. Numbers in parentheses are the MP2 energy at the RHF optimized geometries, relative to 4.

Halpern mechanism, if it is not prevented for some reason (eg. steric). 

In the Brown mechanism, setting aside the high energy required for isomerization from 3 to 3b, the final step of the reductive elimination would require the highest activation energy. Olefin insertion, the rate-determining step in the Halpern mechanism, is an easy process in this mechanism. The kinetics of the Brown mechanism is thus expected to be completely different from that of the Halpern mechanism. Therefore, in the cases in which olefin insertion has been found to be rate-determining, the Halpern mechanism is clearly more consistent and acceptable. 

One can consider that the Halpern mechanism and the cis mechanism are two extremes. The Halpern mechanism is most widely accepted and our ab initio MO calculations support this from the point of view of intrinsic electronic energy. However, there may be cases where the steric effect overshadows the electronic effect. There may also be cases where both effects are important. It may be, as Collman et al. said, that "this multistep reaction is very complicated. Like a chameleon, the dominant reaction mechanism changes when the nature of the catalyst, the ligands, or the substrate is altered" (8). 

In this work, we have compared the potential energy profiles of the model catalytic cycle of olefin hydrogenation by the Wilkinson catalyst between the Halpern and the Brown mechanisms. The former is a well-accepted mechanism in which all the intermediates have trans phosphines, while in the latter, proposed very recently, phosphines are located cis to each other to reduce the steric repulsion between bulky olefin and phosphines. Our ab initio calculations on a sterically unhindered model catalytic cycle have shown that the profile for the Halpern mechanism is smooth without too stable intermediates and too high activation barrier. On the other hand, the key cis dihydride intermediate in the cis mechanism is electronically unstable and normally the sequence of elementary reactions would be broken. Possible sequences of reactions can be proposed from our calculation, if one assumes that steric effects of bulky olefin substituents prohibits some intermediates or reactions to be realized. 

Halpern J, Okamoto T and Zakhariev A 1976 Mechanism of the chlorotris(triphenylphosphine)rhodium(l)-catalyzed hydrogenation of alkenes J. Mol. Catal. 2 65-9... 

Halpern J 1982 Mechanism and stereoselectivity of asymmetric hydrogenation Soienoe 217 401-7... 

Cassar and Halpern (85) provided evidence that the Rh -catalyzed valence isomerization of quadricyclene to norbornadiene also proceeds through a nonconcerted mechanism ... 

The last two decades have seen a growing interest in the mechanism of inorganic reactions in solution. Nowhere is this activity more evident than in the topic covered by this review the oxidation-reduction processes of metal complexes. This subject has been reviewed a number of times previously, notably by Taube (1959), Halpern (1961), Sutin (1966), and Sykes (1967). Other articles and books concerned, wholly or partly, with the topic include those by Stranks, Fraser , Strehlow, Reynolds and Lumry , Basolo and Pearson, and Candlin et al ° Important recent articles on the theoretical aspects are those by Marcus and Ruff. Elementary accounts of redox reactions are included in the books by Edwards , Sykes and Benson . The object of the present review is to provide a more detailed survey of the experimental work than has hitherto been available. 

Candlin and Halpern comment that the sequence of rapid rates observed for Cr " " as a reductant i.e. Co(NH3)5p > Co(NH3)sBr > Co(NH3)5CI > Co(NH3)5F ) is contrary to that found for the slow reactions of Fe (ref. 126) and Eu (ref. 113). All three reductants would appear to favour inner-sphere mechanisms, but in the case of Fe and Eu the order of reactivity seems to be connected with the stability of the product halide complex (FeX or EuX ) which increases in the order X = 1 to X = F . Or in other words, as pointed out by Halpern and Rabani, in the generalised inner-sphere reaction... 

The results obtained by Candlin and Halpern are given in Table 13 in all cases, it is seen that AF is positive. These results strongly suggest that the Fe(II) reductions proceed by inner-sphere routes. However, to be convincing the method requires calibration by reactions of known mechanism. 

Along with Cu ", MnO and Ag" , the two oxidation states of mercury are reduced by molecular hydrogen -Halpern " considers that hydrogen is oxidised by two general mechanisms corresponding to rate equations of the type... 

The mechanism of alkene hydrogenation catalyzed by the neutral rhodium complex RhCl(PPh3)3 (Wilkinson s catalyst) has been characterized in detail by Halpern [36-38]. The hydrogen oxidative addition step involves initial dissociation of PPI13, which enhances the rate of hydrogen activation by a factor... 

Although the latter product is a solvated mononuclear [Rh(MeOH)2(diphos)]+ cation, in the solid state it is isolated as a binuclear complex of formula [Rh2 (diphos)2](BF4)2, in which each rhodium center is bonded to two phosphorus atoms of a chelating bis(diphenylphosphino)ethane ligand, and to a phenyl ring of the bis(diphenylphosphino)ethane ligand of the other rhodium atom. This dimer reverts to a mononuclear species on redissolving. The mechanism of hydrogenation of the prochiral alkene methyl(Z)-a-acetamidocinnamate, studied in detail by Halpern [31], is depicted in Scheme 1.7. 

Halpern, J. Asymmetric catalytic hydrogenation Mechanism and origin of en-antioselection. In Morrison, J.D. (Ed.), Asymmetric Synthesis. Academic Press, Orlando, 1985, Vol. 5, p. 41. 

The reaction mechanism in phosphine-Rh complex catalyzed hydrogenation was elucidated by Halpern16 and Brown and Maddox17 on the basis of NMR and X-ray crystallographic studies of the reaction intermediates, as well as detailed kinetic analyses. The well-recognized mechanism proposed by Halpern is presented in Figure 6-3. 

M Halpern, in Phase Transfer Catalysis in Organic and Polymer Synthesis. Mechanism and Synthesis (ed. ME Halper), ACS Symposium Series 659, Am. Chem. Soc., Washington, 1997,... 

The reactions of several Co(ii) complexes have been examined (Halpern, 1974), namely, pentacyanocobaltate(n) (Chock and Halpern, 1969 Halpern and Maher, 1964, 1965 Kwiatek and Seyler, 1965,1968 Kwiatek, 1967), bis-(glyoximato)cobalt(il) (Schneider et al., 1969), cobalt(li) Schiff s base (Marzilli et al., 1970, 1971) and bis(dioximato)cobalt(ii) (Halpern and Phelan, 1972) complexes. A halogen-atom-transfer mechanism has been proposed for most halides (158, 159), with the exception of the reaction of cobalt(ii) Schiflf s... 

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