Metallocycles, reactions

When the aromatic group of the sulfoxide is replaced by a heteroaromatic group (e.g., N-methylimidazole), the internal coordination between Li—N to form a five-membered metallocycle apparently predominates over Li—O coordination to form a four-membered metallocycle . Reaction of imidazole (S)-sulfoxide 16 with benzaldehyde produces aldol 17 as the major product in which the a-H and the sulfoxide lone pair are syn (equation 14) imidazole (R)-sulfoxide 18 reacts similarly (equation 15). The stereochemical outcome of these reactions is rationalized in terms of a-lithiosulfoxides in which the reactive diastereomer (i.e., 20 and 21) is that having one diastereotopic face of the five-membered Li—N metallocycle carrying both H and sulfoxide lone pair. 

Metallocycles as intermediates in synthesis of heterocycles by transition metal-catalyzed coupling reactions under C—H bond activation 99AG(E)1698. 

Metathesis is a catalyzed reaction that converts two olefin molecules into two different olefins. It is an important reaction for which many mechanistic approaches have been proposed by scientists working in the fields of homogenous catalysis and polymerization. One approach is the formation of a fluxional five-membered metallocycle. The intermediate can give back the starting material or the metathetic products via a concerted mechanism ... 

From the foregoing, however, it should not be concluded that the approach of Mango and Schachtschneider is appropriate for the understanding of the metathesis reaction. The main difficulty is the supposition that the metathesis is a concerted reaction. If the reaction is not concerted, it makes no sense, of course, to correlate directly the orbitals of the reactants with those of the products. Recently, non-concertedness has been proved probable for several similar reactions, which were formerly believed to be concerted. For instance, Cassar et al. (84) demonstrated that the Rh catalyzed valence isomerization of cubane to sj/w-tricyclooctadiene proceeds stepwise. They concluded that a metallocyclic intermediate is formed via an oxidative addition mechanism ... 

The most plausible inference from these studies is to consider for the metathesis reaction a nonconcerted reaction mechanism involving a five-membered metallocyclic intermediate. In the case of the metathesis of propene, this can be visualized as follows ... 

Grubbs and Brunck (86) have recently reported experimental evidence supporting this mechanism. They have made an attempt to synthesize the proposed metallocyclic intermediate for the metathesis of ethene. Starting from the assumption that a mixture of WC1 and two equivalents of (C4H9)Li forms an active metathesis catalyst (49), they treated WC1 with 1,4-dilithio-2,3-dideuterobutane. One may expect that the following reaction would take place ... 

It is clear that a detailed mechanism for the metathesis reaction of alkenes cannot yet be given with certainty. In view of the fact that, for similar reactions which are formally cyclobutane-dialkene transformations, a nonconcerted reaction pathway has been demonstrated, a concerted fusion of two alkenes to form a cyclobutane complex and its decomposition in the same way with a change in the symmetry plane is less probable. On the basis of the information on the two other mechanisms to date, the mechanism involving a metallocyclic intermediate is more plausible than a mechanism involving carbene complexes. 

For the cyclotrimerization of alkynes, several mechanisms have been proposed. The most plausible ones are a concerted fusion of three ir-bonded alkyne molecules, and stepwise processes involving a cyclobutadiene complex or a five-membered metallocyclic intermediate (98). In the case of the cyclotrimerization of a-alkynes it is possible to discriminate between a reaction pathway via a cyclobutadiene complex and the other reaction pathways, by analysis of the products. If cyclotrimerization proceeds via a cyclobutadiene complex and if steric factors do not affect the reaction,... 

If, on the other hand, the reaction proceeds via a concerted fusion of three alkyne molecules, the 1,2,3-isomer cannot be formed. Similarly, in the case of a metallocyclic intermediate, it is to be expected that the 1,3,5- and the 1,2,4-isomers will be formed exclusively (98). 

From the above, a parallel appears to exist between the metathesis of alkenes and alkynes. Both reactions result in a redistribution of, respectively, alkylidene and alkylidyne groups. Moreover, the results obtained so far suggest that in both cases the reaction might proceed via a metallocyclic intermediate. 

The compound [Cl2SiFe(CO)4]2 is obtained from Fe(CO)j and HSiClj, in a sealed tube reaction (148). It is presumably a member of the family of Group IVB-iron metallocycles of structural type (XI) (40, 246). Two Ni... 

A3-Pyrroline formation with carbonyl insertion also occurs during the reaction of /V-sulfinylarylamines with diphenylcyclopropenone in the presence of nickel carbonyl (Scheme 36).64 Phenyl isocyanate does not give a pyrroline product under these reaction conditions, hence the SO-CO exchange probably occurs within an intermediate metallocycle. The reaction... 

The formation of l,3,4-triarylpyrroline-2,5-diones from the reaction of diphenylcyclopropenone and N-sulfinylarylamines with nickel carbonyl has been described earlier (see Scheme 36 in Section IV,A,2).64 In contrast with this result, 2,4,5-triphenyl-3-isothiazolone 1-oxide is produced in this type of process using iron pentacarbonyl, and the analogous cyclohexyl derivative is formed in a nickel carbonyl-mediated reaction (Scheme 115).64 It is probable that metallocyclic species (cf. 93) are intermediates in these transformations. 

In contrast to the reaction of an i72-CS2-rhodium complex with dimethyl acetylenedicarboxylate which gives rise to a metallocycle,186 the iron complexes 103 are converted by activated acetylenes into air-sensitive carbene complexes 104. Decomposition of the latter in air provides an unusual synthetic route to substituted tetrathiofulvene derivatives (Scheme 121).187... 

Whether the metal in the metallocycle possesses a hydride atom remains unsolved. This may dictate the route to termination of the chain reaction. 

Reaction pathways apparently analogous to d and f of Eq. (26) yield a mixture of propylene and cyclopropane. Only when photochemical activation was employed were the major products olefins derived from metathesis-decomposition of the metallocycle. The failure to form metathesis olefins under moderate conditions is significant. It may be that either unimolecular dissociation of the olefin from the complex (in the absence of excess olefin to restabilize the carbene) is energetically unfavored, or the metallocyclobutane structure in the equilibrium given by steps a and b in Eq. (26) is highly stabilized and favored. These results... 

One of the most intriguing features of olefin metathesis, and an aspect which continues to receive increasing attention, is the stereochemistry of the reaction. Rationalizations of stereochemical peculiarities have undergone a metamorphosis which parallels advances of mechanistic theories from the pairwise scrambling schemes to the now-popular car-bene-to-metallocycle scheme. As yet there exists no unified stereochemical model which can adequately account for all the observed results. 

Basset and co-workers have formulated a reaction model (81. 82) wherein the structure of the intermediate metallocycle is predetermined... 

In this scheme, a highly puckered metallocycle was envisioned, possessing pseudoaxial and equatorial substituents, and reaction pathways were said to be favored which minimized the following effects (a) 1,3-diaxial interactions of substituents on the two a-carbons (b) axial substituent interactions with juxtaposed ring carbons and (c) 1,2-diequatorial interactions. This scheme predicts relatively nonstereospecific metathesis of rra/j.v-olefins but highly stereospecific metathesis of ra-olefins. For example, the following pathways for reactions of m-olefins were proposed ... 

Casey and co-workers visualized the stereochemistry of 2-pentene metathesis in terms of the relative stabilities of the various substituted metallocycle derivatives (14). As in Katz s scheme, a puckered ring was assumed, but Casey emphasized only that reaction pathways were favored which provided conformations possessing the fewest axial substit-... 

Heptadiene and zirconocene, generated from zirconocene dichloride and butyllithium, form an intermediate, presumably the metallocycle 222, which is transformed into fraws-l,2-di(bromomethyl)cyclopentane (223) by the action of bromine at —78°C. In contrast, a similar reaction of 1,6-heptadene with Cp ZrCl (Cp = pentamethylcyclopentadienyl) (from Cp ZrCl3 and sodium amalgam) gives solely the c -isomer 225 via the complex 224 (equation 114)117. 

Depending on the nature of the substrates, selectivity could be completely reversed between the two isomeric products. For example, switching R1 group between Buc and Ph gave high yields of the first and second product structures, respectively. The authors noted that the reaction did not proceed if the imine contained an ortho-MeO group at R2 or if the imine was replaced with an aldehyde, oxime, or hydrazone. The catalytic cycle is initiated by C-H activation of the imine, that is, the formation of a five-membered metallocycle alkyne insertion affords the intermediate drawn in Scheme 69. It is noteworthy that this is the first report of catalytic synthesis of indene derivatives via a C-H insertion mechanism (C-H activation, insertion, intramolecular addition). 

A mechanistic pathway is proposed based upon the observed regioselectivities and other results that were obtained during the exploration of the scope and limitations of the Alder-ene reaction.38 Initially, coordination of the alkene and alkyne to the ruthenium catalyst takes place (Scheme 5). Next, oxidative addition affords the metallocycles 42 and 43. It is postulated that /3-hydride elimination is slow and that the oxidative addition step is reversible. Thus, the product ratio is determined by the rate at which 42 and 43 undergo /3-hydride elimination. 

The bis(trimethylphosphino)zirconium complex 92 undergoes unusual reactions with alkynes.114 With diynes, metallocyclic products 93 are formed, whereas acetylene affords 94 whose nonplanar boratanaphthalene ligand is... 

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