Metalation with simultaneous rearrangement

Too intensively stabilized by delocalization, 9-allylthio-9-fluorenyllithium proves inert. In contrast, allyl benzyl sulfide, after metalation with -butyllithium, rearranges smoothly at -15 °C. Several pathways are taken simultaneously as an in-depth investigation by Jean Biellmann et has revealed (Scheme 1-298). The species... 

Various unsaturated cyclic 7t-ligands undergo, within a transition-metal complex, cycloaddition reactions and rearrangements with simultaneous formation of cyclopropane subunits. This is observed with cycloheptatriene ehromium and iron complexes such as 4 which give cycloaddition products, e.g. 

There are three possible routes for the formation of a Schiff base chelate (LXX) (a) There can be an equilibrium between the heterocycle and the Schiff base. In this event, a metal chelate could be formed with the Schiff base. (b) There can be an equilibrium between the heterocycle and the two starting materials. The latter can react with the metal ion in a stepwise manner to give the Schiff base chelate, (c) Alternatively, the metal ion can react directly with the heterocycle inducing a rearrangement reaction which results in the Schiff base chelate. From a kinetic study of the reactions of 2,2 -bisbenzothiazoline and its 2,2 -dimethyl derivative with Cd(II) and Zn(II), it was determined that the pathway (b) was inconsistent with the experimental results in all cases. The most probable course of the reaction involves a metal ion-induced rearrangement reaction, although it is possible in some instances that pathway (c) might simultaneously contribute to the formation of these Schiff base chelates (109). 

Tautomeric rearrangements of transition-metal complexes with azole ligands are relatively scarce. The fluxional behavior of the rhodium complex 43 with a neutral 3,5-dimethylpyrazole was explained as the result of rapid processes of metallotropy and prototropy occurring simultaneously (Scheme 24) [74JOM(C)51],... 

Ring enlargement reactions mediated by metals, silicon, or phosphorous are not treated in this survey because of the tremendous amount of material. Rearrangements of bicyclic compounds with a simultaneous contraction and enlargement of the two rings are also excluded. 

The dianions [ Mo(CO)4(/t-SR) 2] (R = Bu or Ph) are oxidized to the neutral dimers in a chemically reversible two-electron step. The 37 mV peak separation in the cyclic voltammogram is attributed to the simultaneous transfer of two electrons, caused by metal-metal bond cleavage coupled with a molecular rearrangement (204) (Section III,C). 

Fig. 4. Chain reaction of lipid peroxidation. An oxidant removes an electron from a PUFA (step 1) to form a lipid radical. Molecular rearrangement causes formation of a reactive conjugated diene (step 2). This can react with active singlet molecular oxygen ( 02), which is in an excited state rather than the ground state to form a peroxyl radical (step 3). Also, transition metals can react with oxygen to produce potent metal-containing oxidants that may allow simultaneous binding or bridging of a biomolecule and oxygen (B6, K4, W5). The peroxyl radical can be detoxified by an antioxidant to a lipid peroxide (step 4) or the peroxyl radical can act as an oxidant to remove an electron from another PUFA (step 5), effecting a chain reaction of autooxidation. PUFA, polyunsaturated fatty acid (R5). Dot indicates unpaired electron in radical forms.

In the initial coordination, the 7r-electron cloud of the olefin overlaps with the outer bond orbital of the metal cation. This causes stretching and eventual rupture of the R-M (metal) bond. An intramolecular rearrangement follows with the migration of the caibanion (R to the most electron-deficient carbon atom of the double bond. A new covalent bond and a new caibanion are formed simultaneously ... 

---