Metallacycles metallacyclobutane

Confirmation that the polymerizations proceed via metallacyclic intermediates was obtained by studying the ROMP of functionalized 7-oxanorbornadienes. These polymerize slower than their norbornene analogs, allowing NMR identification of the metallacyclobutane resonances and subsequent monitoring of ring opening to the first insertion product. In addition, the X-ray crystallographic structure of complex (212) has been reported.533... 

For a cis alkene to be formed the reaction would have to proceed through a czs-a,p-disubstituted metallacyclobutane intermediate (cis isomer of 10). Although it was unclear why there was a preference for forming a cis metallacycle, which leads to the thermodynamically less stable product, it was probably related to the small size or the electron-withdrawing properties of the nitrile group. 

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. 

Theoretical studies published since 1993 reporting computationally optimized structures for four-membered boracycles and metallacycles are listed in Table 1. Many of these investigations, however, maintain a specific focus on molecular transformations (i.e., reaction mechanisms) and no longer explicitly consider the details of metallacyclobutane structure. The most significant theoretical investigations of boracyclobutene derivatives were conducted sufficiently long ago to have been reviewed in CHEC-II(1996) <1996CHEC-II(lb)887> and are not discussed further. 

The reactions of protic acids and other electrophiles at remote Lewis basic functionality have been investigated as a pathway to both transformations and interconversions of carbonyl-substituted metallacycles. Irida-, pallada-, and platinacyclobutanones react reversibly with protic acid to yield 73-2-hydroxyallyl complexes (Equation 36), modulating between metallacyclobutane and hydroxyallyl structures (and further discussed in Section 2.12.9.3.5) <1993CC1039, 1995JOM143, 19970M1159>. 

A characteristic feature of cycloolefin ring-opening metathesis polymerisation is alteration of the metal-carbon active bonds from the metal carbene a, n bond into metallacycle a bonds, and vice versa, as polymerisation progresses. It is worth mentioning, in this connection, that metallacyclobutanes can be successfully used as catalysts for this polymerisation [36,37]. 

The stability of metal alkylidene (carbene) complexes and the corresponding metallacycles can be dependent on various factors, but it is worth noting that the kind of metal, the metal oxidation state and the ligands surrounding the metal are considered to be of essential significance. Although stable metal carbene complexes are usually obtained from W and Mo compounds whereas metallacycles are obtained from Ti compounds, systems have been found in which both the metal alkylidene complex and its precursor metallacyclobutane can be detected at lowered temperature by NMR spectroscopy [45]. 

As regards metal alkylidene and metallacycle active sites participating in metathesis polymerisation, it should be emphasised that either the alkylidene or the metallacyclobutane can be the resting state of the catalyst, depending on the catalyst used for the polymerisation [99]. 

The following procedure illustrates the use of di-Grignard reagents for the synthesis of metallacycles. In spite of the difficulty in preparing the di-Grignard reagent (see Section 3.1, p. 29), this is one of the most important routes to metallacyclobutanes [69]. 

Whereas Fischer-type chromium carbenes react with alkenes, dienes, and alkynes to afford cyclopropanes, vinylcyclopropanes, and aromatic compounds, the iron Fischer-type carbene (47, e.g. R = Ph) reacts with alkenes and dienes to afford primarily coupled products (58) and (59) (Scheme 21). The mechanism proposed involves a [2 -F 2] cycloaddition of the alkene the carbene to form a metallacyclobutane see Metallacycle) (60). This intermediate undergoes jS-hydride elimination followed by reductive elimination to generate the coupled products. Carbenes (47) also react with alkynes under CO pressure (ca. 3.7 atm) to afford 6-ethoxy-o -pyrone complexes (61). The unstable metallacyclobutene (62) is produced by the reaction of (47) with 2-butyne in the absence of CO. Complex (62) decomposes to the pyrone complex (61). It has been suggested that the intermediate (62) is transformed into the vinylketene complex... 

The mechanism of ADMET polymerization (Scheme 5) contains intermediates similar to those found in ROMP chemistry in that both polymerizations contain, inclusively, various metallacyclobutane/carbene species." Although ROMP propagates exclusively via trisubstituted metallacycles, whereas ADMET requires disubstituted metallacycles, the major difference is that ADMET step chemistry is an equilibrium process driven by condensation and ROMP chain chemistry propagates irreversibly owing to the high reactivity of the carbene with strained cycloalkenes. Therefore ROMP is much faster than ADMET simply because competing equihbria, absent during ROMP, decrease the net productive rate in ADMET chemistry. 

A very good method for the synthesis of metallacycles with two metal-carbon a bonds is the reaction of dihalogenometal complexes with a,w-di-lithio- or di-Grignard-alkanes (Scheme 7). According to this procedure, metallacyclobutanes, -pentanes, -hexanes, and -heptanes of tita-... 

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). 

Cyclopropane formation during decomplexation of metallacyclobutanes is a common reaction of these systems and will be discussed in Section 5.2.6.2. In this section, only decomplexation of isolated or isolable metallacycles bearing preformed cyclopropanes is discussed. In this case, any reaction pathway may give a cyclopropane product. 

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). ... 

Some rare cases of cyclopropane formation from metallacycles other than metallacyclobutanes have been reported. Thus thermolysis of a rhenacyclopentane 3, obtained from CpRe(CO)2H2 (1) and 1,4-diiodobutane (2), yields methylcyclopropane (4) in near quantitative yield. Based... 

The preferences of the various pathways are dependent on the catalyst used, specifically the electronic and steric factors involved. The electronic contribution is based on the preference of the metallacycle to have the electron-donating alkyl groups at either the a or the carbon of ftie metallacycle [23]. The steric factors involved in the approach of the olefin to the metal carbene also determine the re-giochemistry of the metallacyclobutane formed. These factors include both steric repulsion of the olefin and carbene substituents from each other and from the ancillary ligands of the metal complex. Paths (b), (c), and (e) in Scheme 6.10 are important to productive ADMET. The relative rates of pathways (c) and (e) will determine the kinetic amount of cis and trans double bonds in the polymer chain. Flowever, in some cases a more thermodynamic ratio of cis to trans olefin isomers is attained after long reaction times, presumably by a trans-metathesis olefin equilibration mechanism [31] (Scheme 6.11). 

Two distinct pathways have been proposed for the formation of the metallacyclobutane that differ in the orientation of the metallacycle with respect to the other ligands around Ru (Figure 2). In the bottom-bound pathway, metallacycle formation takes place with an olefin bound trans to the NHC, leading to a metallacycle on the opposite face to the NHC and the two anionic ligands (X) being trans to each other. Alternatively, in the side-bound pathway, metallacycle formation takes place with an olefin bound cis... 

---