Olefins metal-mediated reactions

In spite of the remarkable improvement upon previously existing methodology, there is one disadvantage that remains. For the synthesis of an olefin, a second olefin has to be sacrificed. It is obvious that a process that would enable dehydrogenation to occur in the absence of sacrificial reagents would be highly desirable. Moreover, the selectivities that can be obtained at high turnovers are still too low for practical applications. Neither turnover frequencies nor turnover numbers of the catalysis are sufficient to be useful for industrial processes. These limitations are less of an issue in total synthesis, provided that the quality of the metal-mediated reaction justifies the use of stoichiometric processes. 

In many catalytic processes and transition metal mediated reactions, a-bor-ane complexes have been shown to be intermediates. The bis(borane) complex Cp2Ti( 72-HBcat/)2 (HBcaT = HBcat-4-f-Bu) is a highly active catalyst for the hydroboration of vinylarenes [37]. A mechanism, shown in Scheme 3, has been proposed for the Ti-catalyzed hydroboration on the basis of a detailed mechanistic study [37]. Theoretical calculations provided further support to the proposed reaction mechanism and showed that the reductive elimination step, giving the product molecules, is rate-determining [38]. In the Cp2Ti(CO)2 catalyzed hydroboration of alkynes [36,37], the proposed reaction mechanism (Scheme 4) also involves a a-borane complex similar to 11 and 14. In the titanium-catalyzed decaborane-olefin hydroborations [47,48], a-borane complexes were also considered as intermediates. In the Cp2MH (M = Nb, Ta) mediated hydroboration reactions of olefins [39,41], Smith and his coworkers observed several interesting cr-borane complexes, such as 21-23 discussed above. 

To date a number of reactions have been carried out in ionic liquids [for examples, see Dell Anna et al. J Chem Soc, Chem Commun 434 2002 Nara, Harjani and Salunkhe Tetrahedron Lett 43 1127 2002 Semeril et al. J Chem Soc Chem Commun 146 2002 Buijsman, van Vuuren and Sterrenburg Org Lett 3 3785 2007]. These include Diels-Alder reactions, transition-metal mediated catalysis, e.g. Heck and Suzuki coupling reactions, and olefin metathesis reactions. An example of ionic liquid acceleration of reactions carried out on solid phase is given by Revell and Ganesan [Org Lett 4 3071 2002]. 

In this chapter, metal-mediated asymmetric oxygen-, nitrogen-, and carbon-atom transfer reactions were described. The substrates were limited to olefins. Owing to limited space, examples of the reactions were collected with the priority given to enantioselectivity, except for some germinal... 

Although transition metal-mediated P-H addition across ordinary alkenes proceeds well only with five-membered cyclic hydrogen phosphonates, addition across the olefinic linkage of a,P-unsaturated compounds occurs readily with a range of phosphorus species and catalytic agents. Of particular note are the reaction systems involving platinum,96-107 palladium,108-115 and the lanthanides.116-122... 

Thus far, we have discussed the transition metal complex-catalyzed hydrogenation of C=C, C=0, and C N bonds. In this section, another type of transition metal complex-mediated reaction, namely, the hydroformylation of olefins, is presented. 

The reaction mechanisms of these transition metal mediated oxidations have been the subject of several computational studies, especially in the case of osmium tetraoxide [7-10], where the controversy about the mechanism of the oxidation reaction with olefins could not be solved experimentally [11-20]. Based on the early proposal of Sharpless [12], that metallaoxetanes should be involved in alkene oxidation reactions of metal-oxo compounds like Cr02Cl2, 0s04 and Mn04" the question arose whether the reaction proceeds via a concerted [3+2] route as originally proposed by Criegee [11] or via a stepwise [2+2] process with a metallaoxetane intermediate [12] (Figure 2). 

Carbene and related complexes have been shown to be important players in a number of catalytic processes, including olefin metathesis (see Organic Synthesis Using Metal-mediated Metathesis Reactions). The following briefly surveys some of the structural types found in iridium chemistry. 

Free radical promoted hydrosilation of olefins typically gives poorer conversions than are produed by hydrosilation mediated by metal catalysts. However, they are not subject to isomerization of the olefin, a side reaction that can be a nuisance in metal-catalyzed hydrosilations. This is due to the extremely efficient hydrogen atom transfer reaction from Si-H to carbon radicals and the resulting very short lifetimes of the latter. The generally accepted mechanism for these processes is illustrated in Scheme 1. 

An alternative method for producing P4 macrocycles is via transition metal-mediated addition of P—H functions across olefinic double bonds as in Scheme 6. Treatment of the resulting complex with H202 and CN gives the corresponding phosphine oxide macrocycle.2,24 Various reactions at the coordinated PNEt2 function in the coordinated macrocycle have also been investigated.25... 

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