Alkene cross metathesis

We will focus on the development of ruthenium-based metathesis precatalysts with enhanced activity and applications to the metathesis of alkenes with nonstandard electronic properties. In the class of molybdenum complexes [7a,g,h] recent research was mainly directed to the development of homochi-ral precatalysts for enantioselective olefin metathesis. This aspect has recently been covered by Schrock and Hoveyda in a short review and will not be discussed here [8h]. In addition, several important special topics have recently been addressed by excellent reviews, e.g., the synthesis of medium-sized rings by RCM [8a], applications of olefin metathesis to carbohydrate chemistry [8b], cross metathesis [8c,d],enyne metathesis [8e,f], ring-rearrangement metathesis [8g], enantioselective metathesis [8h], and applications of metathesis in polymer chemistry (ADMET,ROMP) [8i,j]. Application of olefin metathesis to the total synthesis of complex natural products is covered in the contribution by Mulzer et al. in this volume. 

The examples listed in Table 3.21 illustrate the synthetic possibilities of cross metathesis. In many of the procedures reported, advantage is taken of the fact that some alkenes (e.g. acrylonitrile, styrenes) undergo slow self metathesis only. Interestingly, it is also possible to realize cross metathesis between alkenes and alkynes (Table 3.21, Entries 11-13), both in solution and on solid supports [927,928]. 

With the discovery by Grubbs of ruthenium carbene complexes such as Cl2(PCy3)2Ru=CHR, which mediate olefin metathesis under mild reaction conditions and which are compatible with a broad range of functional groups [111], the application of olefin metathesis to solid-phase synthesis became a realistic approach for the preparation of alkenes. Both ring-closing metathesis and cross-metathesis of alkenes and alkynes bound to insoluble supports have been realized (Figure 5.12). 

Metathesis of alkenes has been reviewed in terms of cross-metathesis, ring opening and closing, disproportionation, transmutation, and self-metathesis.34 A review on catalytic processes involving ft -carbon elimination has summarized recent progress on palladium-catalysed C-C bond cleavage in various cyclic and acyclic systems.35... 

Enynes without the cycloalkene moiety can also react with electron-deficient alkenes by a cascade ring-closing metathesis-cross metathesis (RCM-CM) process [23] (Scheme 10). The use of Hoveyda s catalyst is necessary, not to stop the reaction at the RCM step, but to perform the subsequent CM step. Indeed, the organic product arising from the RCM is first formed and then reacts with the alkene in the presence of the ruthenium complex to give the CM reaction. 

Ri r3 In an alkene metathesis two alkenes react with an appropriate catalyst to form two new alkenes. There are different types of alkene pj2 r4 metathesis reactions The intermolecular reaction is called cross metathesis (CM), whereas intramolecular metathesis is divided into ring-closing metathesis (RCM) and ring-opening metathesis (ROM). Also two polymerization versions of alkene metathesis exist metathesis polymerization of acyclic dienes and ring-opening metathesis polymerization (ROMP). 

For cross metathesis of alkenes, it is found that in the preparation of disubstituted alkenes with one or more aUyhc substituents Grubbs-Hoveyda catalysts possessing iV-(o-tolyl) groups in the azolecarbene unit are more efficient than those with the iV-mesityl groups. But for the formation of trisubstituted alkenes the Al-mesityl catalysts are superior due to discrimination between productive and nonproductive reaction pathways. 

Cross-metathesis of alkenes with 1,2-dichloroethene under the influence of 4B provides... 

Olefin metathesis. Cross metathesis permits a facile access to allylsilanes and Z)-a,P-unsaturated nitriles. Furthermore, employing an alkene metathesis in tandem with asymmetric allylboration provides functionalized alkenes. ... 

Silylative Coupling and Cross-Metathesis of Alkenes and Dienes with Vinyl-Silicon Derivatives — New Catalytic Routes to Synthesis of OrganosUicon Compounds ... 

Summary Recent achievements in two catalytic reactions, i.e., silylative coupling and cross-metathesis of alkenes and dienes with vinyl-silicon compounds, which resulted in new synthetic routes to organosilicon molecular and macromolecular compounds are presented. The silylative coupling, also called dehydrogenative or trans-silylation and silyl group transfer, is catalyzed by metal complexes which either contain or initiate the formation of M-H and M-Si bonds, where M = Ru, Rh, Co and Ir. Cross-metathesis, which was developed very recently, proceeds in the presence of metallacarbenes, mainly those of rathenium (e.g., Grabbs catalyst). 

In the past decade two new catalytic reactions occurring between the same parent substances have been developed, namely silylative coupUng (trans-silylation) (Eq. 1) and cross-metathesis of alkenes (Eq. 2) with vinylsilicon compounds. 

Silylative Coupling and Cross-Metathesis of Alkenes and Dienes 371... 

Ru2(II,II) complexes are known to catalyze hydrogenation of alkenes and alkynes [235], cross-metathesis of alkenes [198], and intermolecular insertion of diazo compounds into 0-H bond [236]. Ru2(II,III) complexes catalyze the oxidative transformation of secondary amines to imines [237], aerobic oxidations of alcohols... 

Scheme 6.47 Cross metathesis between alkenes and enantiomerically pure phosphine oxides.

Clark JR, Diver ST. Atom Economy in the Metathesis Cross-Coupling of Alkenes and Alkynes. Org Lett. 2011 13(ll) 2896-2899. 

Cyclopropanation and Metathesis of Alkenes In a one-pot reaction containing ethyl diazoacetate, styrene, and norbornene, Ru2(p-02CMe)4 was shown to catalyze cyclopropanation as well as the selective cross-metathesis of the two alkenes [102]. Since no metathesis occurs in the absence of ethyl diazoacetate, it is proposed that the diazoester initiates the catalyst by forming a reactive... 

The relay strategy has also been applied to selective alkene cross metathesis (CM). Very recently, Kim and coworkers reported the total synthesis of (+)-3-(Z)-isolaureatin (64) via this strategy (Fig. 20) [65]. The crucial cross metathesis of alkene for stereoselective introduction of the (Z)-enyne unit was successfully realized in 76% yield in the presence of Grubbs catalyst (67) using Lee s protocol. The relay precursor, enyne (66), was subjected to the reaction mixture to initiate the desired process. 

Royer F, Vilain C, EUcaiem L, Grimaud L. Selective domino ring-closing metathesis-cross-metathesis reactions between enynes and electron-deficient alkenes. Org. Lett. 2003 5 2007-2009. 

As stated above, olefin metathesis is in principle reversible, because all steps of the catalytic cycle are reversible. In preparatively useful transformations, the equilibrium is shifted to one side. This is most commonly achieved by removal of a volatile alkene, mostly ethene, from the reaction mixture. An obvious and well-established way to classify olefin metathesis reactions is depicted in Scheme 2. Depending on the structure of the olefin, metathesis may occur either inter- or intramolecularly. Intermolecular metathesis of two alkenes is called cross metathesis (CM) (if the two alkenes are identical, as in the case of the Phillips triolefin process, the term self metathesis is sometimes used). The intermolecular metathesis of an a,co-diene leads to polymeric structures and ethene this mode of metathesis is called acyclic diene metathesis (ADMET). Intramolecular metathesis of these substrates gives cycloalkenes and ethene (ring-closing metathesis, RCM) the reverse reaction is the cleavage of a cyclo-... 

The resulting carbene complex 41b bears a hetero substituent and shows activity in the ring-opening/cross metathesis of strained bicyclic alkenes and... 

A first evaluation of complex 71a by Blechert et al. revealed that its catalytic activity differs significantly from that of the monophosphine complex 56d [49b]. In particular, 71a appears to have a much stronger tendency to promote cross metathesis rather than RCM. Follow-up studies by the same group demonstrate that 71a allows the cross metathesis of electron-deficient alkenes with excellent yields and chemoselectivities [50]. For instance, alkene 72 undergoes selective cross metathesis with 3,3,3-trifluoropropene to give 73 in excellent yield and selectivity. Precatalyst 56d, under identical conditions, furnishes a mixture of 73 and the homodimer of 72 (Scheme 17) [50a]. While 56d was found to be active in the cross metathesis involving acrylates, it failed with acrylonitrile [51]. With 71a, this problem can be overcome, as illustrated for the conversion of 72—>74 (Scheme 17) [50b]. 

The cross metathesis of acrylic amides [71] and the self metathesis of two-electron-deficient alkenes [72] is possible using the precatalyst 56d. The performance of the three second-generation catalysts 56c,d (Table 3) and 71a (Scheme 16) in a domino RCM/CM of enynes and acrylates was recently compared by Grimaud et al. [73]. Enyne metathesis of 81 in the presence of methyl acrylate gives the desired product 82 only with phosphine-free 71a as a pre-... 

Fig. la—d Typical alkene metathesis reactions ring-closing (RCM) and ring-opening (ROM) metathesis (a), diene cross metathesis (CM, b), ROM-RCM (c), and ROM-double RCM (d) sequences (ring-rearrangement reactions, RRM)... 

Olefin cross metathesis starts to compete with traditional C=C bondforming reactions such as the Wittig reaction and its modifications, as illustrated by the increasing use of electron-deficient conjugated alkenes for the ( )-selective construction of enals and enoates. 

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