Metallacycles alkene metathesis

Recently, highly efficient catalysts have been developed that are one component systems and contain a preformed alkylidene or metallacycle as the catalyst initiatorSome of these complexes have shown excellent utility in the synthesis of organic molecules and will be the major topic of Ae last part of this chapter. These complexes will not only change many of the approaches in simple alkene metathesis but will also have a major influence on the future applications in organic and polymer synthesis. 

Schrock carbene complexes undergo reaction with multiple bonds via four-center metallacyclic intermediates (51). Chapter 11 will consider what occurs when alkylidenes react with alkenes, a reaction known as alkene metathesis. Below are examples of Schrock carbenes reacting with polar multiple bonds such as C-N and C=0. 

Metallacyclic complexes play an important role as reactive intermediates in catalytic cycles initiated by homogeneous transition-metal complexes. Thus, metallacyclobutanes are discussed as intermediates in alkene metathesis, isomerization of strained cyclopropane compounds and many other reactions. On the other hand, numerous examples of isolable me-tallacyclobutane complexes have been reported. These can be formed by different routes such as carbon-carbon bond cleavage of cyclopropane compounds (A), cyclometallation via C — H bond cleavage (B), nucleophilic addition to allyl complexes (C), rearrangement of metallacyc-lopentanes (D) or transmetalation of 1,3-dimetallalated carbon chains (E). ... 

Alkoxide ligands play an important spectator role in the chemistry of metal-carbon multiple bonds. Schrock and coworkers have shown that niobium and tantalum alkylidene complexes are active toward the alkene metathesis reaction. One of the terminating steps involves a j8-hydrogen abstraction from either the intermediate metallacycle or the alkylidene ligand. In each case the -hydrogen elimination is followed by reductive elimination. The net effect is a [1,2] H-atom shift, as shown in equations (73) and (74), and a breakdown in the catalytic cycle. Replacing Cl by OR ligands suppresses these side reactions and improves the efficiency of the alkylidene catalysts. ... 

Alkene metathesis occurs by way of an intermediate metallacycle 87, followed by ring opening to give either the starting materials or one of the new alkenes and a new metallocarbene complex (2.111). Further metallocycle formation using another alkene and ring-opening provides the other product alkene and recovered catalyst to continue the cycle. 

Overall, the alkene metathesis reaction between 70 and ethene is energetically feasible. The largest free-energy difference between minima and transition states was found to be between the most stable metallacycle intermediate and the transition state for ethene de-coordination, similar to what was computed for the Mo,... 

A common theme in many reactions is the generation of an equivalent of CP2M through the reduction of a tetravalent precursor metallocene with a metal alkyl such as n-butyllithium. The chemistry of olefin complexes of CP2M is characterized by a facile interconversion between formally M(II) and M(IV) manifolds, as shown in equation 34. Another central motif of metallocene chemistry, especially that of titanocenes, is the accessibility of pathways connecting metallacycles and metal alkylidene complexes, eg in alkene metathesis (eq. 35). 

The mechanism of the alkene metathesis reaction also involves a catalytic cycle. A key step involves addition of the metallocarbenoid to the alkene to give a four-membered metallacycle. This metallacycle is unstable and can either revert to starting material or eliminate an alkene in the opposite direction to give a new alkene. Addition is not regioselective consequently, all possible combinations of R, and R2 result, hr this scheme, the catalyst is R,CH=[M]. 

This method is specific for metallacyclopentanes. The alkene-coupling process is favored by metal reduction. A typical synthetic strategy is the in situ reduction of a metal halide precursor in the presence of the alkene see, for example, the synthesis of 79 in Scheme 34.1 An alkylidene precursor may also lead to a metallacycle with elimination of the car-bene ligand as in the synthesis of 81, representing a deactivation pathway for alkene metathesis catalysts. Ilie two alkenes may be generated in situ in the coordination sphere by rearrangement processes, such as intramolecular hydrogen transfer from an alkyl-vinyl precursor. I ... 

Metallacycles, cyclic alkyls L M(CH2) , are associated with two reversible reactions. In Eq. 3.22, an n = 3 metallacyclobutane rearranges to an alkene and a carbene, a key step in the important alkene metathesis reaction, and in Eq. 3.23, an n = 4 metallacyclopentane rearranges to give two alkenes. Metallacycle applications are discussed in Sections 6.8 and 12.1. 

The currently known carbometallation chemistry of the group 6 metals is dominated by the reactions of metal-carbene and metal-carbyne complexes with alkenes and alkynes leading to the formation of four-membered metallacycles, shown in Scheme 1. Many different fates of such species have been reported, and the readers are referred to reviews discussing these reactions.253 An especially noteworthy reaction of this class is the Dotz reaction,254 which is stoichiometric in Cr in essentially all cases. Beyond the formation of the four-membered metallacycles via carbometallation, metathesis and other processes that may not involve carbometallation appear to dominate. It is, however, of interest to note that metallacyclobutadienes containing group 6 metals can undergo the second carbometallation with alkynes to produce metallabenzenes, as shown in Scheme 53.255 As the observed conversion of metallacyclobutadienes to metallabenzenes can also proceed via a Diels-Alder-like... 

Like styrene, acrylonitrile is a non-nucleophilic alkene which can stabilise the electron-rich molybdenum-carbon bond and therefore the cross-/self-metathe-sis selectivity was similarly dependent on the nucleophilicity of the second alkene [metallacycle 10 versus 12, see Scheme 2 (replace Ar with CN)]. A notable difference between the styrene and acrylonitrile cross-metathesis reactions is the reversal in stereochemistry observed, with the cis isomer dominating (3 1— 9 1) in the nitrile products. In general, the greater the steric bulk of the alkyl-substituted alkene, the higher the trans cis ratio in the product (Eq. 11). 

The final stereochemistry of a metathesis reaction is controlled by the thermodynamics, as the reaction will continue as long as the catalyst is active and eventually equilibrium will be reached. For 1,2-substituted alkenes this means that there is a preference for the trans isomer the thermodynamic equilibrium at room temperature for cis and trans 2-butene leads to a ratio 1 3. For an RCM reaction in which small rings are made, clearly the result will be a cis product, but for cross metathesis, RCM for large rings, ROMP and ADMET both cis and trans double bonds can be made. The stereochemistry of the initially formed product is determined by the permanent ligands on the metal catalyst and the interactions between the substituents at the three carbon atoms in the metallacyclic intermediate. Cis reactants tend to produce more cis products and trans reactants tend to give relatively more trans products this is especially pronounced when one bulky substituent is present as in cis and trans 4-methyl-2-pentene [35], Since the transition states will resemble the metallacyclobutane intermediates we can use the interactions in the latter to explain these results. 

Metallacyclobutanes or other four-membered metallacycles can serve as precursors of certain types of carbene complex. [2 + 2] Cycloreversion can be induced thermally, chemically, or photochemically [49,591-595]. The most important application of this process is carbene-complex-catalyzed olefin metathesis. This reaction consists in reversible [2 + 2] cycloadditions of an alkene or an alkyne to a carbene complex, forming an intermediate metallacyclobutane. This process is discussed more thoroughly in Section 3.2.5. 

Metallacycles have also been prepared from two molecules of alkenes or alkynes or from palladated o-alkyl- or aryl- substituted aryls by C-H activation. Metallacycles usually have five- or six-membered ring structures, and four-membered metallacycles have been shown to be intermediates in metathesis (Chapter 6). 

The understanding of the reaction mechanism is directly related to the role of the catalyst, i.e., the transition metal. It is universally accepted that olefin metathesis proceeds via the so-called metal carbene chain mechanism, first proposed by Herisson and Chauvin in 1971 [25]. The propagation reaction involves a transition metal carbene as the active species with a vacant coordination site at the transition metal. The olefin coordinates at this vacant site and subsequently a metalla-cyclobutane intermediate is formed. The metallacycle is unstable and cleaves in the opposite fashion to afford a new metal carbene complex and a new olefin. If this process is repeated often enough, eventually an equilibrium mixture of alkenes will be obtained. 

In any chain reaction, apart from initiation steps, the termination steps are also important. In metathesis there are many possibilities for termination reactions. Besides the reverse of the initiation step, the reaction between two carbene species is also a possibility (eq. (17)). The observation that, when using the Me4SnAVCl6 system, as well as methane traces of ethylene are also observed [26] is in agreement with this reaction. Further reactions which lead to loss of catalytic activity are (1) the destruction of the metallacyclobutane intermediate resulting in the formation of cyclopropanes or alkenes, and (2) the reaction of the metallacycle or metal carbene with impurities in the system or with the functional group in the case of a functionally substituted alkene (e. g., Wittig-type reactions of the metal carbene with carbonyl groups). 

Shortly after the discovery of enyne metathesis, Trost began developing cycloisomerization reactions of enynes using Pd(ll) and Pt(ll) metallacyclic catalysts (429-433), which are mechanistically divergent from the metal-carbene reactions. The first of these metal catalyzed cycloisomerization reactions of 1,6-enynes appeared in 1985 (434). The reaction mechanism is proposed to involve initial enyne n complexation of the metal catalyst, which in this case is a cyclometalated Pd(II) cyclopentadiene, followed by oxidative cyclometala-tion of the enyne to form a tetradentate, putative Pd(IV) intermediate [Scheme 42(a)]. Subsequent reductive elimination of the cyclometalated catalyst releases a cyclobutene that rings opens to the 1,3-diene product. Although this scheme represents the fundamental mechanism for enyne metathesis and is useful in the synthesis of complex 1,3-cyclic dienes [Scheme 42(fe)], variations in the reaction pathway due to selective n complexation or alternative cyclobutene reactivity (e.g., isomerization, p-hydride elimination, path 2, Scheme 40) leads to variability in the reaction products. Strong evidence for intermediacy of cyclobutene species derives from the stereospecificity of the reaction. Alkene... 

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