Olefin coordination

The general catalytic cycle for the coupling of aryl-alkenyl halides with alkenes is shown in Fig. 9.6. The first step in this catalytic cycle is the oxidative addition of aryl-alkenyl halides to Pd(0). The activity of the aryl-alkenyl halides still follows the order RI > ROTf > RBr > RC1. The olefin coordinates to the Pd(II) species. The coordinated olefin inserts into Pd—R bond in a syn fashion, p-Hydrogen elimination can occur only after an internal rotation around the former double bond, as it requires at least one /I-hydrogen to be oriented syn perpendicular with respect to the halopalladium residue. The subsequent syn elimination yields an alkene and a hydridopalladium halide. This process is, however, reversible, and therefore, the thermodynamically more stable (E)-alkene is generally obtained. Reductive elimination of HX from the hydridopalladium halide in the presence of a base regenerates the catalytically active Pd(0), which can reenter the catalytic cycle. The oxidative addition has frequently assumed to be the rate-determining step. 

Iron hydride complexes can be synthesized by many routes. Some typical methods are listed in Scheme 2. Protonation of an anionic iron complex or substitution of hydride for one electron donor ligands, such as halides, affords hydride complexes. NaBH4 and L1A1H4 are generally used as the hydride source for the latter transformation. Oxidative addition of H2 and E-H to a low valent and unsaturated iron complex gives a hydride complex. Furthermore, p-hydride abstraction from an alkyl iron complex affords a hydride complex with olefin coordination. The last two reactions are frequently involved in catalytic cycles. 

The proposed catalytic cycle, which is based on experimental data, is shown in Scheme 6. Loss of 2 equiv. of N2 from 5 (or alternatively 1 equiv. of N2 or 1 equiv. of H2 from complexes shown in Scheme 3) affords the active species a. Olefin coordination giving b is considered to be preferred over oxidative addition of H2. Then, oxidative addition of H2 to b provides the olefin dihydride intermediate c. Olefin insertion giving d and subsequent alkane reductive elimination yields the saturated product and regenerates the catalytically active species a. 

Fig. 35 Explanation of the enantioselectivity by face selective olefin coordination...

Pd(OAc)2 works well with strained double bonds as well as with styrene and its ring-substituted derivatives. Basic substituents cannot be tolerated, however, as the failures with 4-(dimethylamino)styrene, 4-vinylpyridine and 1 -vinylimidazole show. In contrast to Rh2(OAc)4, Pd(OAe)2 causes preferential cyclopropanation of the terminal or less hindered double bond in intermolecular competition experiments. These facts are in agreement with a mechanism in which olefin coordination to the metal is a determining factor but the reluctance or complete failure of Pd(II)-diene complexes to react with diazoesters sheds some doubt on the hypothesis of Pd-olefin-carbene complexes (see Sect. 11). 

In order to rationalize the catalyst-dependent selectivity of cyclopropanation reaction with respect to the alkene, the ability of a transition metal for olefin coordination has been considered to be a key factor (see Sect. 2.2.1 and 2.2.2). It was proposed that palladium and certain copper catalysts promote cyclopropanation through intramolecular carbene transfer from a metal carbene to an alkene molecule coordinated to the same metal atom25,64. The preferential cyclopropanation of terminal olefins and the less hindered double bond in dienes spoke in favor of metal-olefin coordination. Furthermore, stable and metastable metal-carbene-olefin complexes are known, some of which undergo intramolecular cyclopropane formation, e.g. 426 - 427 415). 

As shown in Table 1, a remarkable variety of alkene complexes bearing metal centers in a low oxidation state have been isolated and their structures have been determined by X-ray analysis. All the C-C bond distances in olefins coordinated to early transition metals at low oxidation states are more or less elongated compared to free ethylene. These structural data, together with those from NMR studies [14], indicate a major contribution of the metallacyclo-propane structure (2), a fact which is also supported by calculation studies [15]. In the case of ethylene bridging two metal centers such as [ Cp2ZrX 2(iu-f/-C2H4)] (3), the M-C bond could be characterized as a er-bond and there is a little contribution from the / -ethylene canonical structure [16-18]. 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

The most characteristic and useful reaction is the dimerization with incorporation of certain nucleophiles. It is well-known that simple olefins coordinated by Pd2+ compounds undergo nucleophilic substitutions [Eq. (9)] or addition reactions [Eq. (10)] (16, 17). Water, alcohols, and carboxylic acids are typical nucleophiles which attack olefins to form aldehydes, ketones, vinyl ethers, and vinyl esters. 

This reaction is invariably catalyzed by transition metal compounds and its mechanism is of special interest. The first explanation for this transformation is based on the so-called pairwise mechanism , in which two olefins coordinate to a transition metal center to form a transient cyclobutane-like intermediate [2], However, this idea was later replaced... 

Synthesis of 63 and 64 supports the olefin oxidation mechanisms in Fig. 16. These mechanisms have several important and noteworthy points about Ptm chemistry (1) olefins coordinate to Ptm at the axial position, which is contrasted to the -coordination of olefins perpendicular to the square-planar coordination plane of Ptn. Olefin coordination to Pt(III) should also be contrasted to the fact that olefins do not coordinate to Pt(IV). (2) Platinum111 is strongly electron-withdrawing, and the coordinated olefins receive nucleophilic attack. (3) The alkyl ce-carbon on the Ptm undergoes nucleophilic attack in aqueous solution, whereas in aprotic solvent, aldhyde (and possibly also ketone in other cases) is produced by reductive elimination. 

In the following sections we will focus on geometric and energetic aspects of the various species which compose the mechanism of Figure 3. We will start with the species prior olefin coordination (Section 3.1), we will... 

Before concluding this section, it has to be reminded that all the above calculations represent gas-phase processes without solvent and/or counterion effects, and without unfavorable entropic contributions. Recent studies have shown that they modify substantially the olefin-coordination energetics. 

Finally, the uptake energy values only represent a contribution to the total free energy of coordination. In fact, an always unfavorable uptake entropy has to be accounted for. Although few experimental data are available, it is reasonable to assume that the -FAScontribution to the free energy of olefin coordination to group 4 metallocenes at room temperature is close to the 10 kcal/mol value observed at 300 K for Ni and Pd compounds [59], The few... 

Complex D4 is considered as the active species for both alkyne and olefin coordinations. Starting from the olefin coordinated complex (D5 ), the olefin insertion into the Pt-B bond is unfavorable because of a high activation barrier (22.9 kcal/mol). On the contrary, the acetylene insertion from the acetylene coordinated complex (D5) occurs easily with a small reaction barrier (9.0 kcal/mol). This significant difference in the reaction barriers has been used to explain the inertness of olefins for diborafion reactions. The smaller barrier from D5 to D6 coincides with the highly stable insertion product D6. In contrast, the olefin insertion product D6 is relatively unstable with respect to the olefin coordinated species D5 . 

Following olefin coordination, the Chalk-Harrod mechanism proceeds by olefin insertion into the M-H bond, whereas with the modified Chalk-Harrod mechanism, olefin coordination is followed by insertion into the M-Si bond. This step distinguishes the two mechanisms. Thus, the coordination of styrene to the hydridosilyl complex to form an olefin 7t-complex may be the first step of the catalytic cycle that discriminates between the two mechanisms. We have examined this coordination process as well as the relative energies of the many isomers of the 7i-complex that are possible. 

Figure 7. Coordination of Styrene. The lower pathway shows olefin coordination accompanied by detachment of the pyrazole ligand. The upper pathway with retention of the coordinated pyrazole ligand was not observed.

The coordination of styrene with the pyrazole detached resulted in the formation of a stable n-complex (vide infra). Therefore, throughout the catalytic cycle the pyrazole plays the role of a hemilabile ligand [55], This is an important result because experimental studies [11] suggest that olefin coordination is slow and reversible. Moreover, the tuning of the steric bulk of the pyrazole substituents could be used to enhance the activity of the... 

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