Alkene and Alkyne Metathesis

The ether complexes WCl(0Ar) CH2But)(CHBu ) 0R2) are highly active olefin metathesis catalysts in the absence of a Lewis acid component.  

Another one-component catalyst is WCl 2(C0)3(AsPh3)2 although the activation of this complex by AICI3 is also reported.Replacement of AsPhj by PPh3 makes the catalyst ineffective, as does removal of CO as In WCl2(N0)2- 

WL(C0)8-02-RAlCl2 (R=Me or Et) catalyses alkene metathesis [activity, L=mesitylene benzene (CO)s lifetime, reverse order] in addition to doublebond migrations. The initial products of the reaction of c/j-pent-2-ene with the alkylidene complex (17a) are shown in Equation (18) [(17a) R=Bu (17b) 

Alkenes. —Reviews on Ziegler-Natta catalysis and the stereoregular and sequence-regular polymerization of butadiene have been published and the stereoselective oligomerizations of isoprene by lithium and palladium catalysts have been compared. Semi-empirical MO calculations suggest that Ziegler-Natta polymerization proceeds via a bis-alkene complex and a metallacyclo-pentane intermediate.  

Oligomerization of cyclohexene in the presence of WCl6 + EtAlCl2+ ROH (R=Et, Ph, or PhCHg) appears to involve a tungsten hydride intermediate. A hydride complex, i.e., [NiH P(OEt)3 4]+ is also the active 

ADcynes. —Several applications of the [CoCp(CO)3]-promoted Qrclization of diynes have been reported including the synthesis of chiral polyt cles [reaction (19), X=(CHa)a or 

The isolation of (19) [reaction (20)] lends support to the intermediacy of rhodacyclopent-2-enes in the rhodium-assisted cyclotrimerization of two alkynes with an alkene. 

Rainier of the University of Utah has put forward J. Am. Chem. Soc. 2007,129,12604) an elegant alternative to Ru-catalyzed alkene metathesis, demonstrating that an (O-aUsenyl ester such as 1 win cychze to the enol ether 2 imder Tebbe conditions. 

The particular reactivity of free alcohols in Ru-catalyzed alkene metathesis is imder-scored by the observation Tetrahedron Lett. 2007, 48,6905) by laved Iqbal of Dr. Reddy s Laboratories, Ltd., Miyapur that attempted metathesis of the ether 4a faded, but metathesis of the diol 4b proceeded efficiently. Kazunori Koide of the University of Pittsburgh has demonstrated (Organic Lett. 2007, 9, 5235) that the yields of cross-metathesis with an alkenyl alcohol could be enhanced by binding it to a trityl resin. He observed that the Grela catalyst 8 was particularly effective in this application. 

Residual Ru species do not interfere with some subsequent transformations. Rodrigo B. Andrade of Temple University has demonstrated (Tetrahedron Lett. 2007, 48, 5367) that metathesis with an a, 3-unsaturated aldehyde such as 11 can be followed directly by phospho-nate condensation to give the doubly-homologated product 12. Philip 1. Parsons of the University of Sussex has found (Organic Lett. 2007, 9, 2613) that the nitro functional group is compatible with the Ru catalyst. The product nitro alkene 15 could be cychzed (intramolecular Michael addition) to the cyclopentane 16, or (intramolecular dipolar cycloaddition) to the cyclopentane 17. 

Although alkyne metathesis is now well developed as a synthetic method, it has been limited to internal alkynes. For this reason, methyl alkynes are often specially prepared to set the stage for C-C triple bond formation by metathesis. Andre Mortreux of ENSC-Lille has now designed (Adv. Synth. Cat. 2007, 349, 2259) a binuclear catalyst 22 that works well with terminal alkynes such as 21. What remains to be determined is the functional group compatibility of 22. 

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Aside from the Ziegler-Natta polymerization, alkene and alkyne metathesis, and other reactions of Ti-methylene complexes, carbometallation reactions induced by alkyltitanium compounds have been dominated by those involving... 

The chemistry of alkylidene and alkylidyne complexes of early transition metals was developed by Schrock and co-workers and these complexes turned out to be of crucial importance to alkene and alkyne metathesis. Initially their research focused on tantalum complexes of the type CpTaCEIE, which after a-elimination (Figure 16.6) led to alkylidene complexes Cp(R)Cl2Ta=CHR [11]. 

The organic reactivity of the alkylidyne complexes has been extensively studied especially with regard to alkene and alkyne metathesis reactions.353,356... 

SCHEME 1. Types of alkene and alkyne metathesis reactions. DBC, double bond cleavage TBC, triple bond cleavage ADMET, acyclic diene metathesis RCM ring-closing metathesis ROMP ring-opening metathesis polymerization... 

The application of alkene [1] - and, more recently, enyne [2] and alkyne - metathesis to the synthesis of natural products has been triggered by the development of powerful catalysts that allow metathesis reactions to be carried out under mild conditions. Scheme 1 outlines two important cases of alkene and alkyne metathesis of particular interest to the synthesis of natural products (together with the general scheme of enyne metathesis, not discussed in this review). The metathesis products can be obtained in high yields, since ethene/2-butyne are formed as volatile products. After the alkene/alkyne metathesis, the substituents (R) of the alkenes/alkynes are located on the same multiple bond. Enyne metathesis can be considered as the more general case of alkene metathesis, because two new double bonds are again formed, albeit now connected by a single bond. 

Scheme 5. Combined application of alkene and alkyne metathesis, affording the open-chain natural product 24.

Grubbs, R. H., Alkene and Alkyne Metathesis Reactions , in Comprehensive Organometallic Chemistry, Pergamon Press, Oxford, 1982, Vol. 8, pp. 499-551. 

Enyne metathesis, which combines alkene and alkyne metathesis, is a means of forming a 1,3-diene system by interaction of a C=C and a C=C bond with an... 

In Section 24.12, we introduced alkene (olefin) metathesis, i.e. metal-catalysed reactions in which C=C bonds are redistributed. The importance of alkene and alkyne metathesis was recognized by the award of the 2005 Nobel Prize in Chemistry to Yves Chauvin, Robert H. Grubbs and Richard R. Schrock for the development of the metathesis method in organic synthesis . Examples of alkene metathesis are shown in Figure 27.3. The Chauvin mechanism for metal-catalysed alkene metathesis involves a metal alkyli-dene species and a series of [2 + 2]-cycloadditions and cycloreversions (Figure 27.4). Scheme 27.6 shows the mechanism for alkyne metathesis which involves a high oxidation state metal alkylidyne complex, L M=CR. 

C. (2008) Reactions of strained hydrocarbons with alkene and alkyne metathesis catalysts. J. Am. Chem. Soc., 130, 14078-14079. 

Insights from Computational Studies on d° Metal-Catalyzed Alkene and Alkyne Metathesis and Related Reactions... 

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