Chemistry of Metallacyclobutadienes

Abstract

Metallacyclobutadienes are analogues of cyclobutadienes in which one of the cyclobutadiene CR groups has been formally replaced by a transition-metal fragment. These metallacycles are interesting because they can play an important role in catalysis and can serve as starting materials for the syntheses of organometallic compounds such as metallabenzene, η -cyclopentadienyl, and η -cyclopropenyl complexes. Unlike cyclobutadienes, metallacyclobutadienes can be significantly more stable. A number of metallacyclobutadienes have now been isolated and thoroughly characterized, especially for those that contain transition metals of groups 5-9. Their properties have also been actively investigated. This article highlights the chemistry of metallacyclobutadienes with reference to their syntheses, reactivity, and structural properties.

Well‐Defined Alkyne Metathesis Catalysts: Developments and Recent Applications

“…[64,65] These molybdenacyclobutadienes represent rare structurally characterized MCBD examples. [32,68,69] During the early 2000s, M. J. A. Johnson and his group studied transition-metala lkylidynes and nitrides and the impacto f their ancillary ligands.…”

“…[10,86] Nonetheless,t he complexes were not able to convert terminal alkynes even at 40 8C. [85] Recently,F ürstner and his group synthesized two new multidentate trisilanol ligands (32,33), which they intended to coordinate to molybdenum alkylidyne complexes (Figure 3). [87] Initially,t hey attemptedaligand exchange reactionw ith the tribromo alkylidyne precursor 21.H owever,b ecause of low solubility of the ligand salts this proceduref ailed.…”

“…[28] This synthetic protocoli su sually referredt oa st he "high oxidation state" route.T he proposed mechanism of alkyne metathesis proceeding via metallacyclobutadiene intermediates was also confirmedb yt he isolation of such speciesf rom activea lkyne metathesis catalysts. [29,[30][31][32] Additionally,m etathesis of the bimetallica nalogue [(tBuO) 3 WW(OtBu) 3 ]w ith MeCCtBu represents ac onvenient access to the tungsten alkylidyne complex 3 (Scheme2). [33] Schrock's catalyst 3 has been studied thoroughly for many years and hasf ound broad use in ring-closing alkyne metathesis (ROAMP) and in the total synthesis of natural products.…”

Synthesis, Characterization and Electronic Structure of Dirhenadehyro[12]annulene Complexes

“…[1] Replacement of carbon atoms or CH groups with metal fragments in annulenes and dehydroannulenes results in metallaannulenes and metalladehydroannulenes, which can exhibit both organic and organometallic properties. For examples, metallacyclobutadienes [2,3] (1, Figure 1) can mediate metathesis reactions of metal carbynes and alkynes; metallabenzenes (2) [4,5] show properties like those of benzene.…”

“…Compared with the chemistry of annulenes and dehydroannulenes, that of metalla-analogs is much less developed. Reported metallaannulenes or metalladehydroannulenes are mainly small-ring metallacycles, such as metallacyclobutadienes (1), [2,3] metallabenzenes (2), [4] dimetallabenzenes (3), [6] metallabenzynes (4), [7,8] metallacyclooctatetrenes [9a] and ten-membered-ring dimetallaannulenes (5), [9b-d] despite the fact that many polycyclic metalla-aromatics (6)(7)(8)(9)(10)(11) [10] or spiro-aromatic metallacycles (12) [11] have been reported.…”