Alkene substrates catalysts

RhCl(PPh3)3 is a very active homogenous hydrogenation catalyst, because of its readiness to engage in oxidative addition reactions with molecules like H2, forming Rh—H bonds of moderate strength that can subsequently be broken to allow hydride transfer to the alkene substrate. A further factor is the lability of the bulky triphenylphosphines that creates coordinative unsaturation necessary to bind the substrate molecules [44]. 

Recently, a new class of phosphabarrelene/rhodium catalysts has been developed, which for the first time allows for hydroformylation of internal alkenes with very high activity and which proceeds essentially free of alkene isomerization [36-38]. Two examples, results of hydroformylation of an acyclic and a cyclic internal alkene substrate, are depicted in Scheme 2. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

Another example of selective C=C bond hydrogenation has arisen from mechanistic studies on an iron m-hydride dihydrogen complex, [Fe(PP3)(FI)(H2)](BF4) [PP3 = P(CH2CH2PPh2)3], a catalyst inactive with alkene substrates. Scheme 6 shows that no decoordination of dihydrogen is required in any step of the cycle and that the vacant site is created by unfastening of one of the P-donor atoms (species (16)).50 Extensive studies on catalytic alkene hydrogenation by analogous tripodal (polyphosphine) Rh, Os, and Ir complexes have been carried by Bianchini and co-workers.51,52... 

The ability of transition-metal complexes to activate substrates such as alkenes and dihydrogen with respect to low-barrier bond rearrangements underlies a large number of important catalytic transformations, such as hydrogenation and hydroformy-lation of alkenes. However, activation alone is insufficient if it is indiscriminate. In this section we examine a particularly important class of alkene-polymerization catalysts that exhibit exquisite control of reaction stereoselectivity and regioselec-tivity as well as extraordinary catalytic power, the foundation for modern industries based on inexpensive tailored polymers. 

Based on the insight that a dissociative mechanism plays the major role along the metathesis pathway [11], these catalysts have been designed such that only one bulky phosphine, one chloride and one cumulenylidene ligand are attached to a Ru(II) center. Because arene ligands are known to be labile on such a metal fragment, they will easily liberate free coordination sites ( ) for the interaction with the alkene substrate. Although the precise mode of action of such allenyli-... 

Figure 6.1. Substrate-catalyst interactions favor a specific mode of alkene insertion into the zirconocene—alkene complex.

The high levels of enantioselectivity obtained in the asymmetric catalytic carbomagnesa-tion reactions (Tables 6.1 and 6.2) imply an organized (ebthi)Zr—alkene complex interaction with the heterocyclic alkene substrates. When chiral unsaturated pyrans or furans are employed, the resident center of asymmetry may induce differential rates of reaction, such that after -50 % conversion one enantiomer of the chiral alkene can be recovered in high enantiomeric purity. As an example, molecular models indicate that with a 2-substituted pyran, as shown in Fig. 6.2, the mode of addition labeled as I should be significantly favored over II or III, where unfavorable steric interactions between the (ebthi)Zr complex and the olefmic substrate would lead to significant catalyst—substrate complex destabilization. 

The basic solution in water containing NaCo(CO)4 is treated with sulphuric acid in the presence of syn-gas and HCo(CO)4 is regenerated. This can be extracted as is shown in the drawing from water into the substrate, alkene. The catalyst is returned to the reactor dissolved in the alkene. Compared to other schemes (BASF, Ruhrchemie) the elegant detail of the Kuhlmann process is... 

For several alkene substrates, the yield of the epoxide product was lower when the reactions were carried out in pure CH2CI2 than in a mixed solvent system consisting of [BMIM]PFg/CH2Cl2 (3 1, v/v). The catalyst was also highly active for the epoxidation of aromatic alkenes. Although PhIO is an oxidant commonly used in organic solvents, it was found that the use of PhI(OAc)2 under the same conditions in the mixed solvent led to higher yields of the epoxides. 

There continues to be an increasing level of activity centered about the use of porphyrin catalysts for the epoxidation of alkenes of various configurations. For example, the sterically encumbered fra/w-dioxoruthenium(VI) porphyrin (26) was found to catalyze the epoxidation of a variety of alkenes in yields from fair to excellent e.g., 27 -> 28). Kinetic studies on a series of para-substituted styrenes point to a mechanism which proceeds via a rate-limiting benzylic radical formation. The high degree of stereoretention in cir-alkenes was attributed to steric crowding which prevents C-C bond rotation of the intermediate radical. This same steric bulk prevents the familiar side-on approach of the alkene substrate, so that a head-on approach is postulated <99JOC7365>. 

Mechanistically, it is reasonable to regard metallacyclopentanes as intermediates in the formation of cyclobutane derivatives from two alkene substrates.5 It has been established that nickelacyclopentane not only acts as an intermediate in such a reaction but also as a catalyst.6... 

After compiling many results obtained in similar studies of different substrates (alkenes, dienes, alkynes and so on), the results cannot be correlated to draw definitive conclusions due to the wide variety of parameters that can influence the reaction (substrates, catalyst precursors, supports, pressure, temperature and so on) [9, 208-214]. This is maybe the main reason why there are no clear mechanistic explanations for this simple reaction, unlike homogeneous gold-catalyzed processes. 

The inefficiency of the platinum/hydrogen reduction system and the dangers involved with the combination of molecular oxygen and molecular hydrogen led to a search for alternatives for the reduction of the manganese porphyrin. It was, for example, found that a rhodium complex in combination with formate ions could be used as a reductant and, at the same time, as a phase-transfer catalyst in a biphasic system, with the formate ions dissolved in the aqueous layer and the manganese porphyrin and the alkene substrate in the organic layer [28]. 

Although the hydrocarboxylation of 1 -alkenes is not of interest for the synthesis of more complex organic molecules, the information obtained from the hydrocarboxylation reactions with various catalysts can be applied to the synthesis and reactions of other alkene substrates. 

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