Cross-metathesis olefins

Amines and amides are usually poor partners in the cross-metathesis reaction because of the possible coordination of the emerging carbene to the polar functional group. To prevent this problem, Elkaim and Grimaud have found that boron-based Lewis acids are effective additives. Introduction of 0.1 equivalent of chlorodicyclo-hexyl borane as an additive seemingly prevents the coordination of the polar groups such as amides to the carbene intermediates, and provides a dramatic increase in the yields of the cross-metathesis products (131) (Equation 79) [82]. 

No Lewis acid, 28% yield Cy2BCI (0.1 equiv) 84% yield 

To simplify the presentation of the mechanism, the symbol stands for the transition metal and its ligands. Steps have been omitted in which ligands leave or become attached to the metal therefore, the number of ligands is not necessarily the same throughout a stage. 

Step 1 In this stage the -p -hybridized carbons of the alkene, with their attached groups, replace the benzylidene group of the catalyst. In the case of an unsymmetrical alkene such as propene, the two newly formed carbene complexes (A and B) are different. 

Complex B Propene adds to the double bond of B to give metallocyclobutanes E and F. Dissociation of E gives ethylene + A. Dissociation of F gives propene + B.  

Step 3 The two complexes A and B that react in Stage 2 are also regenerated in the same stage. Thus, stage 3 is simply 

One of the most widely used catalysts for olefin metathesis is the ruthenium complex shown. It is called Grubb s catalyst and abbreviated Cl2(PCy3)2Ru=CHC6H5. 

Entry R Catalyst/ mol% Liquid/ iL Grinding Auxiiiary Time/h Yield (%)  

Reproduced with permission from Do J-L, Mottiflo C, Tan D, trukil V. FriSdic T. Mechanochemical ruthenium-catalyzed olefin metathesis. J Am Chem Soc 2015 137 2470-9. Copyright (2015), TVnerican Chemical Society. 

Stage 2 Complex A Propene adds to the double bond of the complex to give metallocyclobutanes C and D. Dissociation of C gives propene + A. Dissociation of D gives 2-butene + B. 

CHEMO- AND REGIOSELECTIVITY ENHANCEMENT IN SOLID-SUPPORTED REACTIONS 

These developments were extended to multistep reactions on the solid support, for example, via immobilization of the alkene or the alkyne component as an ester. The immobilized alkyne 34 created the possibility for a cross metathesis with a soluble olefin however, as previously noted, undesired CM products generated from soluble alkenes can easily be removed by filtration and washing. After ene-yne cross metathesis to 35, the resin 

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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. 

The alkene groups in TsICH = CH2]s have allowed a wider variety of chemistry to be carried out than for either TsHs or TsPhs. For example, Feher s group have prepared a variety of unsaturated POSS molecules via olefin cross-metathesis... 

The reactivity of T8[OSiMe2CH = CH2]s is, as expected, often similar to that described for T8[CH = CH2]s. Initially, Feher et al. performed olefin cross-metathesis... 

The olefin cross metathesis (CM) can be described as the intermolecular metathesis of alkylidene fragments between two different olefins [133]. It can be farther divided into three main subtypes cross metathesis, ring opening cross metathesis (ROCM) and enyne cross metathesis (ECM) (Scheme 3.9). 

Scheme 5.13 Cleavage of an octenediol linker by olefin cross-metathesis to yield NPG63.69...

In general, electronically mismatched systems undergo cross metathesis more effectively, see New Approaches to Olefin Cross-Metathesis, H.E. Blackwell, D.J. O Leary, A.K. Chatterjee, et al, J. Am. 

Synthesis of Trisubstituted Alkenes via Olefin Cross-Metathesis, A. K. Chatter-JEE, R.H. Grubbs, Org. Lett. 1999, 1,... 

McNaughton, B. R. Bucholtz, K. M. Camaano-Moure, A. Miller, B. L. Self-selection in olefin cross metathesis The effect of remote functionality. Org. Lett. 2005, 7, 733-736. 

The use of menthol in the synthesis of important synthons for optically active methyl branched insect pheromones is discussed briefly. Applications of olefin cross metathesis in production of commercial products, including insect pheromones has been discussed. ... 

As described above in Eq. 43, simple allylboronates can be transformed into more elaborated ones using olefin cross-metathesis. " Treatment of pinacol allylboronate 31 with a variety of olefin partners in the presence of Grubbs second-generation catalyst 142 smoothly leads to formation of 3-substituted allylboronates 143 as cross-metathesis products (Eq. 104). Unfortunately, these new allylic boronates are formed as mixtures of geometrical isomers with modest E/Z selectivity. They are not isolated but rather are treated directly with benzaldehyde to give the corresponding homoallylic alcohol products in good yields (Table A). 

Table A. Functionalized homoallylic alcohols from olefin cross metathesis and subsequent allylboration reactions with benzaldehyde...

Figure 9.13 Templated olefin cross-metathesis of two groups of olefins tethered to the two duplex strands.

M.M. Vasbinder, S.J. Miller, Synthesis of the Pro-Gly dipeptide alkene isostere using olefin cross-metathesis, J. Org. Chem. 67 (2002) 6240-6242. 

Scheme 4 Equilibria involved in olefin cross-metathesis.

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