Metallocarbenes, formation

Hydrocracking, 30 48-52 behavior, thermal, 29 269 catalytic, 26 383 deethylation, 30 50 demethylation, 30 50 metallocarbene formation, 30 51-52 of f -decane, 35 332-333 primary coal liquids, 40 57 procedure, 40 66-67 product distribution, 30 49 reactions, over perovskites, 36 311 suppression by sulfur, 31 229 zeolite-supported catalysts, 39 181-188... 

In all of the examples cited in Section 1.2.2.3.2.3.1, the diazo compounds are arranged such that none has a 3-hydride available. It could be expected that if a simple a-diazo ketone with fi-C — H bonds were exposed to the rhodium catalyst, metallocarbene formation would proceed as usual, but that /S-hydride elimination would compete with the desired 1,5-insertion. Such a /3-hydride elimination could, in fact, be viewed as a 1,2-insertion, i.e., 1 to 2. 

It could be expected that if a simple a-diazo ketone with P-C—H bonds were exposed to the rhodium catalyst, metallocarbene formation would proceed as usual, but that P-hydride elimination would com-... 

Metallocarbene formation by hydrogen shift explains the observed selectivity in the 1,5-dehydrocyclization of 3-methylhexane on Pt/AljOj (41). Three cyclic intermediates may be formed from this molecule, 1,2-dimethylcyclopentane (4), 1,3-dimethylcylopentane (5), and ethylcyclopentane (6). By using several selectively C-labeled 3-methylhexanes, the contribution of each parallel pathway both in cyclic type isomerization and in dehydro-cylization to gaseous cyclic molecules was determined. Relative rates of 3 2 1 were observed for 1-5, 2-6, and 6-7 ring closure (giving 5, 4, and 6, respectively) (Scheme 49 and Table VII), whatever the dispersion of the platinum (2-10%) and the temperature (32O°-38O°C). 

Figure 2.1 Potential energy surface of the [(PDI)Ca(THF)3]-catalyzed unsubstituted NjCHj diazocarbene decomposition and metallocarbene formation reaction, as well...

Table 2.2 MPA spin densities of the selected atoms/groups (as labeled in Figure 2.1) for the structures involved in the metallocarbene formation reaction 4 + N2CH2 ...

Figure 2.3 Potential energy surface of the reaction, as well as schematic presentation of [(PDIjCalTHFjj] catalyzed donor-donor (D/D)- the reactants, intermediates, transition states, substituted NjCPhj diazocarbene decom- and product of this reaction. Energies are position and metallocarbene formation presented as A/fgjjCAGgjjIfAGjoi]...

Transition metal-catalyzed carbenoid transfer reactions, such as alkene cyclopro-panation, C-H insertion, X-H insertion (X = heteroatom), ylide formation, and cycloaddition, are powerful methods for the construction of C-C and C-heteroatom bonds [1-6]. In contrast to a free carbene, metallocarbene-mediated reactions often proceed stereo- and regioselectively under mild conditions with tolerance to a wide range of functionalities. The reactivity and selectivity of metallocarbenes can be... 

The mechanistic similarity between olefin and alkane metathesis, even if intuitive, has needed time to become obvious in the laboratory for various reasons olefin formation starting from alkanes is thermodynamically disfavored, especially at low temperature, and, as a consequence, the proposal of olefinic intermediates was not obvious. Several facts lead to the metallocarbenic mechanistic assertion ... 

As discussed above, accumulated data demonstrate that the catalyst, the substrate structure, and other competing metallocarbene pathways significantly affect the ylide formation and the subsequent rearrangement process. West and co-workers have recently studied selectivity in rearrangement via five- or six-membered oxonium ylides by... 

To explain the enantioselectivity obtained with semi-stabilized ylides (e.g., benzyl-substituted ylides), the same factors as for the epoxidation reactions discussed earlier should be considered (see Section 10.2.1.10). The enantioselectivity is controlled in the initial, non-reversible, betaine formation step. As before, controlling which lone pair reacts with the metallocarbene and which conformer of the ylide forms are the first two requirements. The transition state for antibetaine formation arises via a head-on or cisoid approach and, as in epoxidation, face selectivity is well controlled. The syn-betaine is predicted to be formed via a head-to-tail or transoid approach in which Coulombic interactions play no part. Enantioselectivity in cis-aziridine formation was more varied. Formation of the minor enantiomer in both cases is attributed to a lack of complete control of the conformation of the ylide rather than to poor facial control for imine approach. For stabilized ylides (e.g., ester-stabilized ylides), the enantioselectivity is controlled in the ring-closure step and moderate enantioselectivities have been achieved thus far. Due to differences in the stereocontrolling step for different types of ylides, it is likely that different sulfides will need to be designed to achieve high stereocontrol for the different types of ylides. 

Metallocarbenes derived from diazoacetates and the appropriate transition metal are generally thought to be electrophilic in nature that is, they are susceptible to nucleophilic addition. This reactivity manifold has been exploited with the formation of ylide species on reaction with a number of nucleophiles providing intermediates that can undergo either rearrangements (e.g., 41 > 42, Scheme 8.8)25 or cycloadditions (e.g., 43 > 44, Scheme 8.8)26 depending on the type of ylide formed... 

In conclusion, the group VIII metals may be classified according to their increasing capacities to form metallocarbenes, and it is worth mentioning again that the above classification Pd < Pt < Ir < Ni < Co, parallels that suggested by the multiple exchange of methane for adsorbed methylene formation (90). 

A last question that arises now is the mode of generation of the metallocarbenes. Besides the indirect formation by C-C bond rupture, in metallocyclobutane dismutation, for instance (Scheme 59a), two other possible ways are a-hydrogen elimination (Scheme 59b) and hydrogen shift in ii-adsorbed olefin (Scheme 59c) (see also Scheme 48). 

The reactions of alkylcycloheptanes of Pt-C result in ring contraction and yield l-methyl-l-alkylcyclohexanes, and monoalkyl- and dialkylbenzenes 10, 12). These results may be interpreted by the mechanisms shown in Scheme 65. The predominance of the products resulting from Path A over those resulting from Path B is explained by the easier formation of metallocyclobutanes involving tertiary carbon atoms. Furthermore, the attack of the metallocarbene on a disubstituted carbon atom in Path A might be faster than on a monosubstituted carbon atom in Path B. 

From the above discussion, it ensues that a number of reactions involving carbon-carbon bond rupture and formation may be accounted for by a limited number of adsorbed species and elementary steps. The predominant precursor species in skeletal rearrangements are metallocyclobutanes and metallocarbenes, which can be further dehydrogenated to metallocarbynes. 

This reaction probably involves the participation of more than two carbon atoms and the extension of a delocalized n-electron system. The results are readily explained by the unequal stabilities of the two possible intermediates, involving carbene formation on the methyl group and on the alkyl group, respectively (Scheme 74). In the first case, demethylation easily occurs by metallocarbyne formation, while in the second case, stabilization by carbene-olefin isomerization (very fast compared to hydrocracking) prevents dealkylation. However, when the a-carbon atom in the alkyl group cannot dehydrogenate to give a metallocarbene, as for 1-methyl-1-terf-butylcyclohexane, dealkylation prevails over demethylation. 

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. 

A-Phthaloyl-protected (S)-phenylalanine has been used as a ligand for rhodium in the formation of metallocarbenes from diazo compounds for C-H insertion reactions (Section D.1.2.2.3.2.). Ar-Sulfonyl-protected (S)-alanine and (S)-valine are efficient ligands for chiral Lewis acids used in the Diels-Alder reaction (Section D.1.6.1.1.1.3.). A -Sulfonyl-pro-tected (S)-phenylalanine methyl ester has been used for the enantioselective protonation of lactone enolates (Section D.2.I.). The terf-butyl ester of (S)-valine readily forms imines with carbonyl compounds which are used for the highly efficient alkylations of their azaenolates (Sections D.1.1.1.4.1D.1.5.2.4.). All these derivatives can be obtained by the standard methods described in Houben-Weyl3. 

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