Wilkinson catalyst, olefin hydroboration

It took another decade however before the idea of developing a rhodium-catalyzed olefin hydroboration process came to fruition. This occurred in 1985 when Mannig and Noth reported the first examples of such a process.8 They discovered that Wilkinson s catalyst 2 was effective for the addition of catecholborane 1 to a range of alkenes and alkynes, as exemplified by cyclopentene 4 (Scheme 2). 

Figure 1.. The two proposed reaction pathways based on experimental results for hydroboration reactions of olefins catalyzed by the Wilkinson catalyst. (O.A. Oxidative Addition Olefin Migratory Insertion R.E. Reductive Elimination)...

Hydroboration of olefins catalyzed by the Wilkinson catalyst Rh(PPh3)3Cl has been the most studied reaction by quantum chemical calculations [25-27], In the following, three representative studies are described. 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

In this chapter, theoretical studies on various transition metal catalyzed boration reactions have been summarized. The hydroboration of olefins catalyzed by the Wilkinson catalyst was studied most. The oxidative addition of borane to the Rh metal center is commonly believed to be the first step followed by the coordination of olefin. The extensive calculations on the experimentally proposed associative and dissociative reaction pathways do not yield a definitive conclusion on which pathway is preferred. Clearly, the reaction mechanism is a complicated one. It is believed that the properties of the substrate and the nature of ligands in the catalyst together with temperature and solvent affect the reaction pathways significantly. Early transition metal catalyzed hydroboration is believed to involve a G-bond metathesis process because of the difficulty in having an oxidative addition reaction due to less available metal d electrons. 

Catalytic hydroboration of olefins with catecholborane (143) in the presence of Wilkinson catalyst was first reported in 1985 [78,79,80]. Although the reaction takes place without the catalyst, it requires high temperature. The Rh-catalyzed reaction proceeds smoothly at room... 

Thus, detailed experimental and theoretical studies are highly desirable on the mechanism of the transition-metal-catalyzed olefin hydroboration reactions, as well as on the role of the transition-metal center, substrates, and electronic and steric factors in the mechanism. MMM [67] have presented the first detailed ab initio molecular orbital (MO) study of possible reaction pathways illustrated in Fig. 22 for the reaction of C2H4 with the boranes HB(0H)2 and HB02(CH2)3 catalyzed by the model Wilkinson catalyst RhCl(PH3)2. The reaction of BH3 with C2H4 catalyzed by the Rh(PH3)2Cl have been studied by MMM [68] and DS [69]. 

As expected, allylic alcohols and ethers afford v/c-diol derivatives upon hydroboration. However, in the presence of Wilkinson s catalyst, a reverse regiochemistry for the hydroboration is observed [108]. There is the possibility that boron, rather than hydrogen, is transferred from the metal to the olefin. In other words, the boron acts as a donor. 

Another application of the Wilkinson-type catalyst 18 is the rhodium catalyzed hydroboration of olefins [19]. Various alkenes 20 (internal, terminal, styrenes, etc.) have been successfully hydroborated with catecholborane (21) providing the corresponding boronic esters 22 in nearly quantitative yield. Oxidative work-up (Na0H/H202) led to the corresponding alcohols 23 in 76-90% yield Eq. (11). 

Hydrogenation of olefins, enols, or enamines with chiral Wilkinson type catalysts, e.g., Noyorl hydrogenation. Hydroboration of olefins with chiral boranes. Sharpless epoxi-dation of allylic alcohols. 

The first examples of catalytic hydroborations were reported in the 1980s. Sneddon published a series of papers on the additions of the B-H bonds in boron clusters to alk5mes catalyzed by transition metal complexes. " An example of these processes is shown in Equation 16.42. These reactions provided precursors to new boron cages and to boron-carbide materials. In 1985, Noth published the hydroboration of olefins with catecholborane catalyzed by Wilkinson s catalyst. One example of this process (Equation 16.43) shows the difference in chemoselectivity between the catalyzed and uncatalyzed processes. This report by Noth led to the development of catalytic hydroboration as a method for organic synthesis. Studies on both early and late transition metal catalysts have been conducted, and these studies included experiments to probe for differences in selectivities between catalyzed and uncatalyzed processes. 

Mechanisms of catalytic hydroborations of olefins are thought to fall into two categories, and each category can be furtiier divided into two subsets. The reactions catalyzed by Wilkinson s catalyst and by phosphine-free rhodium catalysts are thought to occur by oxidative additions, migratory insertions, and reductive eliminations through two distinct groups of intermediates. The mechanisms of the reactions catalyzed by early metals and lanthanides occur without oxidations and reductions, and two types of mechanisms for reactions catalyzed by early metals have been identified. These four mechanisms are shown in Schemes 16.11-16.14. 

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