Cycloadditions metallacyclobutane

The possibility of being involved in olefin metathesis is one of the most important properties of Fischer carbene complexes. [2+2] Cycloaddition between the electron-rich alkene 11 and the carbene complex 12 leads to the intermediate metallacyclobutane 13, which undergoes [2+2] cycloreversion to give a new carbene complex 15 and a new alkene 14 [19]. The (methoxy)phenylcar-benetungsten complex is less reactive in this mode than the corresponding chromium and molybdenum analogs (Scheme 3). 

The stoichiometry of this conversion is in accordance with a carbene starting structure. An alternating alkyUdene/metallacyclobutane mechanism [102, 131-133], which has precedent in the ethylene polymerization catalyzed by a Ta(III) neopentilydene complex [134], has been proposed where the chro-miiun alkyUdenes undergo [2-1-2] cycloaddition to give chromacyclobutane intermediates (mechanism III in Scheme 7). 

Evans suggests that the catalyst resting state in this reaction is a 55c Cu alkene complex 58, Scheme 4 (35). Variable temperature NMR studies indicate that the catalyst complexes one equivalent of styrene which, in the presence of excess alkene, undergoes ready alkene exchange at ambient temperature but forms only a mono alkene-copper complex at -53°C. Addition of diazoester fails to provide an observable complex. These workers invoke the metallacyclobutane intermediate 60 via a formal [2 + 2] cycloaddition from copper carbenoid alkene complex 59. Formation of 60 is the stereochemistry-determining event in this reaction. The square-planar S Cu(III) intermediate 60 then undergoes a reductive elimination forming the cyclopropane product and Complex 55c-Cu, which binds another alkene molecule. 

Compounds containing M=C bonds can undergo [2+2] cycloadditions, and this reaction allows olefin metathesis to occur. The Mo=C bond [2+2] cycloadds to the C4=C5 bond to give a metallacyclobutane A retro [2+2] cycloaddition cleaves the C4=C5 bond and makes a Mo=C4 bond. This new bond cycloadds across another C4-C5 bond to make a new C4-C5 bond retro [2+2] cycloaddition cleaves the C4=C5 bond and completes the formation of the C4=C5 bond. The process repeats itself many times over to make the polymer. No change in Mo s oxidation state or d electron count occurs in any step. 

Olefin metathesis proceeds via reversible formation of metallacyclobutanes by [2 + 2] cycloaddition (Figure 1.7). The precise pathway for such a cycloaddition has been calculated for molybdenum complexes such as 1 (Figure 1.6) [9]. These calculations suggest that although Mo-C and C-C bond formation is concerted the Mo-C bond is formed more quickly than the C-C bond. It was also found, beautifully consistent with experimental results, that the activation barrier for [2 + 2] cycloaddition is lowered by increasingly electron-withdrawing alkoxy ligands. 

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. 

The working mechanism involves a [2 + 2] cycloaddition between the Ru=C bond of ruthenium vinylidene and olefin to form the metallacyclobutane 92, which subsequently undergoes P-hydride elimination leading to the 7i-allyl hydride complex 93 and reductive elimination to furnish the conjugated trienes 89 (Scheme 6.31), and eventually to give the observed aromatic product 90. 

It appears likely that transient metallacyclobutanes are involved in a variety of organic reactions which are catalyzed by transition metal complexes. Thus, cycloadditions of activated alkenes to strained hydrocarbons such as quadricyclane and bicyclo[2.1.0]pentane are catalyzed by complexes such as Ni(CH2=CHCN)2 and probably involve initial formation of a nickelacyclobutane (Scheme 2) (79MI12200). The nature of the organometallic intermediates in related metal-catalyzed rearrangements (72JA7757) and retro-cyclo-addition reactions (76JA6057) of cyclopropanoid hydrocarbons, e.g. bicyclo[n.l.O]alkanes, has been discussed. 

Metathesis of alkene 6 to give the new alkenes 11 and 15 is explanined by the following mechanism. The first step is [2+2] cycloaddition between metal carbene 5 and alkene 6 to generate the metallacyclobutane 7 as an intermediate. The real catalyst 8 is generated by retrocycloaddition of the metallacyclobutane 7. Reaction of 8 with alkene 6 generates the metallacyclobutanes 9 and 10 as intermediates. The intermediate 10 is a nonproductive intermediate, which reproduces 6 and 8, while 9 is a productive intermediate and yields the new alkene 11 and the real catalyst 12. Cycloaddition of 12 to alkene 6 produces the productive intermediate 14, from which the new alkene 15 and the active catalytic species 8 are formed. The intermediate 13 is a nonproductive one. 

Various cyclic compounds are prepared by the reaction of these carbene complexes with various unsaturated compounds [78-80]. The metallacyclobutane 246 is generated by [2+2] cycloaddition with electron-rich alkenes, and its reductive elimination affords the cyclopropanes 247. 

The cycloaddition of alkenes with metal alkylidene complexes remains the most common entry into the metallacyclobutane structural class. Consistent with metallacyclobutane intermediacy in the olefin metathesis reaction, the [2+2] cycloaddition is generally reversible a propensity for cycloreversion (Section 2.12.6.2.4), however, can significantly limit the utility of metallacyclobutane complexes as intermediates in other synthetic transformations. 

Despite dramatically different ancillary ligand sets, two distinct niobium and tantalum alkylidene systems provide isolable metallacyclobutanes upon reaction with ethylene. In one case, the tantalum aryldiamine pincer complex 148 reacts with ethylene to provide the cr-trimethylsilyltantalacyclobutane complex 149 (Equation 66) <19940M3259>. In a more comprehensive study, alkadiene-supported half-sandwich alkylidene complexes of both tantalum and niobium (the former isolable, the latter generated in situ) undergo [2+2] cycloaddition with a range of acyclic and cyclic alkenes, albeit in modest isolated yield (Equation 67). 

There are several different types of catalysts but the mechanistic principles are the same. The metal alkylidene species (A) undergoes [2+2] cycloaddition with the alkene to give a metallacyclobutane B which then ring-opens to give a new alkylidene and the product alkene ... 

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... 

Compounds with M=X bonds tend to undergo [2 + 2] cycloadditions, and metallacyclobutanes tend to undergo [2 + 2] retro-cycloadditions. 

Green and Rooney81 proposed an alternative mechanism (Scheme 11.13b) that also accounted for Z-N catalysis. The mechanism resembles a metathesis-like pathway by starting with a-elimination to give a metal-carbene hydride followed by cycloaddition with the alkene monomer to form a metallacyclobutane. Reductive elimination finally yields a new metal alkyl with two more carbon atoms in the growing chain. The Green-Rooney mechanism, although plausible overall, requires an a-elimination, a process that is difficult to demonstrate. 

Two of these are the cycloaddition of the methyhdene with ethylene (path E, non-productive), reaction of the methylidene with an internal olefin such that the alkyl substituent on the metallacyclobutane is in the j9-position (path H, non-productive). The other two pathways are the cycloaddition of the alkylidene with an internal olefin to give the trisubstituted metallacyclobutane (path G, frans-metath-esis, non-productive) and the reaction of the alkylidene with a terminal olefin to give the a,a -disubstituted metallacyclobutane (path F), which can be looked at as a chain transfer-type event, albeit not in the sense of a chain polymerization. In this case, the alkylidene is shifted from the end of one chain to the end of another chain. So, assuming that all pathways have somewhat similar rates, the elimination of ethylene will drive the reaction to high polymer. In the case of ADMET, these additional mechanistic pathways do not prevent the polymerization reaction, since these additional pathways are either degenerate or represent processes that do not affect the overall molecular weight distribution of the polymer. 

Paths F and H in Scheme 6.9 are based on different regiochemical orientations of the cycloaddition reactions. The olefin and the carbene must be oriented cofacially prior to the cycloaddition to form the metallacyclobutane ring, so the different stereochemical pathways possible can be modeled (Scheme 6.10). 

The olefin metathesis story started with the advent of well-defined catalysts, most notably the Grubbs ruthenium catalysts.These are ruthenium carbenes that undergo a [2 + 2] cycloaddition to olefins to produce a metallacyclobutane intermediate. Cycloreversion of the metallacyclobutane in the opposite sense leads to olefin metathesis (Scheme 1.35). 

Studies on these carbene complexes, especially those of the Schrock type, have attracted special interest in connection with the mechanism of catalytic olefin metathesis reactions. The formation of metallacyclobutane intermediate from the oxidative cycloaddition reaction between carbene complex and olefin was found to be an important key step in the catalytic cycle (eq. (5)). 

The interaction of metallocarbene (or alkylidene) complexes with unsaturated compounds via the formation of a metallacyclobutane intermediate plays an important role in several stoichiometric and catalytic reactions, e.g., metathesis, cycloaddition and polymerization. 

The reaction proceeds via metallacyclobutanes as shown in Scheme 13. A [2+2] cycloaddition occurs between the olefin substrate and the metal alkylidene catalyst to produce a metallacyclobutane. Retrocycloaddition then occurs to afford an olefin metathesis product and a new metal alkylidene 88, which works as a further catalyst. 

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