Intermediates metallacyclic

In order to gain more insight into this proposed mechanism, Montgomery and co-workers tried to isolate the intermediate metallacycle. This effort has also led to the development of a new [2 + 2 + 2]-reaction.226 It has been found that the presence of bipyridine (bpy) or tetramethylethylenediamine (TMEDA) makes the isolation of the desired metallacycles possible, and these metallacycles are characterized by X-ray analysis (Scheme 56).227 Besides important mechanistic implications for enyne isomerizations or intramolecular [4 + 2]-cycloadditions,228 the TMEDA-stabilized seven-membered nickel enolates 224 have been further trapped in aldol reactions, opening an access to complex polycyclic compounds and notably triquinanes. Thus, up to three rings can be generated in the intramolecular version of the reaction, for example, spirocycle 223 was obtained in 49% yield as a single diastereomer from dialdehyde 222 (Scheme 56).229... 

Metal carbenes are also initiators for the metathetical polymerization of alkynes by, e. g., catalysts such as M0CI5 and WClg. Reaction of the metal carbene with an alkyne gives a metallacyclobutene intermediate, followed by opening of this intermediate metallacycle into a new metal carbene that in its turn can interact with another alkyne molecule, and so on eq. (13). Metathesis of acetylenes proceeds through reactions between an alkylidyne complex and an alkyne via a metallacyclobutadiene intermediate (eq. (14)). 

Reductive coupling of alkynes or alkenes may be carried out with Cp2ZrCl2, Ti(0-i-Pr)4, or other Ti(IV) and Zr(IV) complexes. The reaction is often carried out in an intramolecular fashion to achieve better control of regiochemistry. The diyne or other polyunsaturated compound is added to a mixture of the Zr(IV) or Ti(IV) complex and a reducing agent such as BuLi. After formation of the intermediate metallacycle, the C-M bonds are cleaved by treatment with an electrophile such as H+ or I2 to give the organic product. 

Our primary interest with 43 is to develop an alternative procedure for diene cyclometallation such that the intermediate metallacycles will equilibrate at lower temperature and more efficiently than is observed with Cp2ZrCl2. It is also striking that with more substituted dienes, the trans metallacycles derived from 43 are also favored over the cis by a more substantial margin than with Cp2ZrCl2. For diene 1 (Scheme 14), for instance, the trans titanacycle 46 is calculated to be more stable than the cis titanacycle 47 by 3.3 kcal/mol. 

The reaction involves initial formation of the zirconium-imido species, followed by [2-1-2] cycloaddition with the C—C unsaturation (Scheme 13). This is consistent with the observation that bis(amidate) complexes do not mediate hydroamination with secondary amine containing substrates. The cyclic transition state of the intramolecular reaction determines the regioselectivity of the reaction followed by successive protonation of the intermediate metallacycle and release of product to regenerate the catalyticaUy active imido species. 

Direct evidence for this mechanism has been proved in several cases. Furthermore, the characterization of the organic and organometallic products of decomposition offers insight into the structure of the intermediate metallacycle. 

However, owing to the lack of j8-hydrogen in the intermediate metallacycle, 1,1-disubstituted ethylenes (2-methylpropene, methylenecyclohexane) do not undergo )S-hydrogen transfer instead, their methylene group is catalytically exchanged by a degenerated olefin metathesis reaction ... 

Among the most useful hetero-cyclocouplings (Scheme 4-25) are the CpCoL2-promoted reactions with nitriles giving pyridines [106] and the Ni(0)-catalyzed reactions with CO2 to give pyrones [107]. In both cases considerable regioselectivity can be achieved and intermediate metallacycles have been implicated. Another interesting diversion in the cyclo-... 

The subsequent decomposition of the intermediate metallacycle results in the formation of a new olefin. The u-bond metathesis [43] in which a a bond interacts with a transition-metal complex and is broken... 

ADMET polymerization reactions of this and other diene esters, also carbonates, are listed in Table 8.6. As with monoene esters, for reaction to occur there must be more than one methylene group between the double bond and the ester (or carbonate) group. The inhibition by a proximate ester group may be a polarization effect hindering the formation and/or the rearrangement of the intermediate metallacycle, or it may result from a coordination of the carbonyl oxygen to the metal centre (Patton, J.T. 1992). 

Alkoxide ligands play an important spectator role in the chemistry of metal-carbon multiple bonds. Schrock and coworkers have shown that niobium and tantalum alkylidene complexes are active toward the alkene metathesis reaction. One of the terminating steps involves a j8-hydrogen abstraction from either the intermediate metallacycle or the alkylidene ligand. In each case the -hydrogen elimination is followed by reductive elimination. The net effect is a [1,2] H-atom shift, as shown in equations (73) and (74), and a breakdown in the catalytic cycle. Replacing Cl by OR ligands suppresses these side reactions and improves the efficiency of the alkylidene catalysts. ... 

Alkene metathesis occurs by way of an intermediate metallacycle 87, followed by ring opening to give either the starting materials or one of the new alkenes and a new metallocarbene complex (2.111). Further metallocycle formation using another alkene and ring-opening provides the other product alkene and recovered catalyst to continue the cycle. 

Alternatively, if the propylene interacts with a metal-methylidene intermediate (for illustrative purposes, the deuterium label has been moved to the vinyl terminus in Scheme 10.1b), again two intermediate metallacycles are possible. The formation of a P-substituted metallacycle exchanges the termini of the olefin and degeneratively regenerates a methylidene. If, instead, an a-substituted metallacycle is formed, an alkylidene intermediate is generated upon cycloreversion, thereby productively moving the catalyst species back to the first set of pathways. 

In contrast, it is straightforward to rationalize the observed stereochemical retention if degenerate propylene metathesis occurs via a metal-ethylidene species (Scheme 10.17). In this case, the intermediate metallacycles are a,a disubstituted. Although the P-position is unsubstituted, the experiments with 2-butene (Scheme 10.13) have established that a,a -substituents prefer to be oriented in a cis-configuration. This preference explains why there is any stereoselectivity in this degenerate process and also correctly predicts the stereochemistry of the propylene-tfs that was generated. 

The RCM reactions of malonates have been defined as benchmark substrates for olefin metathesis catalysts [20]. The productive pathway begins with a crossmetathesis reaction to generate an alkylidene intermediate with a pendant olefin. A second, intermediate metallacycle is generated by the intramolecular addition of this pendant moiety, which subsequently releases the cyclized product and an equivalent of ethylene (Scheme 10.18). 

The recently reported catalytic system Pd(02CR)2/Phen, for the carbonylation of PhN02 to PhNCO [33, 90] appears to be one of the most active and selective catalysts reported so far. At least part of the catalytic cycle by which it works appears to be reasonably well understood [91, 92] and the importance of the formation of intermediate metallacycles is now clearly emerging. Moreover, in this case the intermediate formation of an imido complex could not be necessary. 

Though the detailed mechanism of olefin epoxidation is still controversial, Scheme 8 depicts possible intermediates, metallacycle (a), K-cation radical (b), carbocation (c), carbon radical (d), and concerted oxygen insertion (e) [2, 216, 217]. As discussed above, the intermediacy of metallacycle has been questioned. One of the most attractive mechanism shown in Scheme 8 is the involvement of one electron transfer process to form the olefin 7C-cation radicals (b). Observation of rearranged products of alkenes, known to form through the intermediacy of the alkene cation radicals, in the course of oxidation catalyzed by iron porphyrin complexes is consistent with this mechanism [218, 219]. A -alkylation during the epoxidation of terminal olefins is also well explained by the transient formation of olefin cation radical [220]. A Hammett p value of -0.93 was reported in the epoxidation of substitute styrene by Fe (TPP)Cl/PhIO system, suggesting a polar transition state required for cation radical formation [221] Very recently, Mirafzal et al. have applied cation radical probes as shown in Scheme 9 to... 

Depending upon the structure of the olefin, either methyl ketones or epoxides can be formed by treatment with molecular oxygen and a catalytic quantity of [PdCl(N02)(MeCN)2] (Scheme 6). With monocyclic olefins other products are formed, resulting mainly from -elimination of intermediate metallacyclic species. ... 

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