Metallacyclobutenes, synthesis

Consistent with metallacyclobutene synthesis, metallacyclobutane complexes can also be prepared by y-hydogen elimination (Equation 73), despite the greater entropic disadvantage. 

Among more recent innovations, the alkylative metallacyclobutane and metallacyclobutene synthesis, involving central carbon addition and addition/elimination reactivity patterns, holds considerable promise for near-term synthetic developments (Section 2.12.12). This is equally true in both the nucleophilic and free radical versions of this process, for both catalytic and stoichiometric transformations and multistep reaction cascades. 

Although the [2+2] cycloaddition continues to dominate the methodology for the synthesis of boracyclobutene and metallacyclobutene complexes, conceptually new and potentially general alternatives have recently been introduced. In particular, the central carbon alkylation of electrophilic propargyl and allenyl complexes has significantly enriched the palette of available metallacyclobutenes, raising considerable promise for the development of new reactions of relevance to organic synthesis. 

In this section, the synthesis of four-membered metallacycles by transformations of larger or smaller rings is discussed. The transmetallation of metallacyclobutene complexes is covered in Section 2.12.6.1.2. 

Tetrametallabutadienes (59) are kinetically very unstable. Rotation around the M —M bond in the s-trans conformer to the gauche is very facile and the latter cyclizes in a highly exothermic reaction to metallacyclobutene (60b) with a reaction barrier of only 2.5 328b and 2.0 kcalmol-1336 for M = Si and Ge, respectively (Table 32). The high reactivity of 58 and of 59, M = Si, Ge towards cyclization is one of the major obstacles in the synthesis of stable metallabutadienes. Bulky substituents, which increase substantially the barrier for cyclization, are required for the synthesis of stable heavy group 14 analogs of 1,3-butadiene, as was demonstrated recently by the successful isolation of 52a and 52b327. 

In both cases, the mechanism of the reactions can be described like an ene-yne metathesis catalyzed by the Ru-NHC complex [82], followed by a Diels-Alder reaction. The mechanism of these reactions (exemplified for the synthesis of 81) is depicted in Scheme 5.59. Initial [2+2] cycloaddition between the alkyne and the olefin stemming from the metallic carbene achieves metallacyclobutene 158. Subsequently, cycloelimination and [2+2] cycloaddition with the alkene reagent give 159. A second cycloelimination generates a diene (by-product) and the active catalytic species 160, which initiates the catalytic cycle following successive cycloadditions and cycloeliminations up to afford diene 82. Finally, a Diels-Alder reaction between diene 82 and a,p-nnsatnrated carbonyls (dienophile) produces exclusively the syn-(endo-)ptod ict 81. 

In the same way, the reactions of metal carbene complexes with alkynes give metallacyclobutenes, which leads to applications in organic synthesis. Tebbe s methylene complex is in equilibrium with the metallacyclobutane by... 

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