Ligand carbene, rearrangement

Besides dissociation of ligands, photoexcitation of transition metal complexes can facilitate (1) - oxidative addition to metal atoms of C-C, C-H, H-H, C-Hal, H-Si, C-0 and C-P moieties (2) - reductive elimination reactions, forming C-C, C-H, H-H, C-Hal, Hal-Hal and H-Hal moieties (3) - various rearrangements of atoms and chemical bonds in the coordination sphere of metal atoms, such as migratory insertion to C=C bonds, carbonyl and carbenes, ot- and P-elimination, a- and P-cleavage of C-C bonds, coupling of various moieties and bonds, isomerizations, etc. (see [11, 12] and refs, therein). 

In the complex with two methylimidazol units one of them is N-coordinated and the other a C-coordinating carbene ligand. It seems that in the synthesis of this complex a homoleptic rearrangement of the QFs precursor complex is accompanied by a rapid migration of the proton nitrogen to the initially coordinated carbon. 

Cyclopropanation of l,3-dienes. a,0-Unsaturated carbenes can undergo [4 + 2]cycloaddition with 1,3-dienes (12, 134), but they can also transfer the carbene ligand to an isolated double bond to form cyclopropanes. Exclusive cyclopropanation of a 1,3-diene is observed in the reaction of the a,(3-unsaturated chromium carbene 1 with the diene 2, which results in a frans-divinylcyclopropane (3) and a seven-membered silyl enol ether (4), which can be formed from 3 by a Cope rearrangement. However, the tungsten carbene corresponding to 1 undergoes exclusive [4 + 2]cycIoaddition with the diene 2. 

The mechanism proposed72 involves initial nucleophilic attack at the carbene carbon by the dithiocarbamate anion, effectively resulting in addition across the metal-carbon bond. Rearrangements of the dithiocarbamate ligands then form an V-allyldithiocarbamate species Complex 53 was isolated from the reaction mixture of 51 with the diethyldithiocarbamate and identified by X-ray crystallography. 

The reaction of two equivalents of W(C=CC=CH)(CO)3Cp with Ru3(CO)io (NCMe)2 gives the RU3W cluster 149 (Scheme 30), which is also obtained from 135 and W(C=CC=CH)(CO)3Cp. The extended organic ligand is formed by coupling of two molecules of the diynyl complex with two of CO, to form a cyclopen-tadienone attached by a carbenic interaction to the cluster W atom, and featuring formylethynyl and C=CW(CO)3Cp substituents. " One of the elementary steps in the reaction mechanism may involve formal rearrangement of the diyne to a dicarbyne. 

Initiation involves coordination of the double bond of monomer with the transition metal (imino and OR ligands not shown), cleavage of the 7t-bond with formation of a 4-membered metallocyclobutane intermediate, followed by rearrangement to form a metal-carbene propagating center ... 

We can also observe reactions that lead to a more stable arrangement of the whole system very probably via primary addition and subsequent rearrangement (4 in Fig. 3). In addition, it can be established that because of the electron withdrawal of the M(CO)5 moiety, hydrogen atoms in cv-alkyl positions to the carbene carbon develop such an acidic character that their acidity corresponds to that of nitromethane (5 in Fig. 3). Finally, by cleavage of the carbene ligand from the metal complex, pathways in synthetic organic chemistry are opened (6 in Fig. 3). 

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