Shifts, metal-assisted hydride

Thus we tend to favor the mechanism outlined in Reactions 12 and 13 (followed by Reactions 7 and 8). The mechanisms as presented do not indicate how the N-formyl product is formed although formation of a Ru-CO-N moiety at some stage seems essential metal-assisted hydride shifts are a possibility. An alternative role of the attacking piperidine in Reaction 12 or 17 could be that of a proton acceptor as discussed by others (1, 13). For example, a plausible scheme would be the following (writing R2 for C5H10)... 

Terpene synthases, also known as terpene cyclases because most of their products are cyclic, utilize a carbocationic reaction mechanism very similar to that employed by the prenyltransferases. Numerous experiments with inhibitors, substrate analogues and chemical model systems (Croteau, 1987 Cane, 1990, 1998) have revealed that the reaction usually begins with the divalent metal ion-assisted cleavage of the diphosphate moiety (Fig. 5.6). The resulting allylic carbocation may then cyclize by addition of the resonance-stabilized cationic centre to one of the other carbon-carbon double bonds in the substrate. The cyclization is followed by a series of rearrangements that may include hydride shifts, alkyl shifts, deprotonation, reprotonation and additional cyclizations, all mediated through enzyme-bound carbocationic intermed iates. The reaction cascade terminates by deprotonation of the cation to an olefin or capture by a nucleophile, such as water. Since the native substrates of terpene synthases are all configured with trans (E) double bonds, they are unable to cyclize directly to many of the carbon skeletons found in nature. In such cases, the cyclization process is preceded by isomerization of the initial carbocation to an intermediate capable of cyclization. 

The enzyme D-xylose isomerase catalyzes the interconversion of D-xylose to E>-xylulose and D-glucose to D-fhictose by transferring a hydrogen atom between Cl and C2. Various mechanisms have been suggested including base-catalyzed transfer of a proton with a cis-ene diol as an intermediate, a hydride transfer, and a metal-assisted hydride shift [86-89]. The last of these three suggestions is the preferred mechanism at this time, but more studies of the mechanism are needed. 

As shown by reaction 7.2.3.1, formation of 1-hexene or 1-octene may take place by / -elimination followed by reductive elimination of the alkene. More likely, however, is a concerted metal atom-assisted hydride shift. Thus 3,7 shift of the H atom, as shown by reaction 7.2.3.2, will give 1-hexene, while 3,9 shift would be involved for 1-octene. Note that the metallacyclic species shown in reaction 7.2.3.2, is a transition state and not an intermediate. 

Sketch (a) the transition state for a concerted metal atom-assisted 3,9 hydride shift (b) two PNP ligands (c) the ligand used for selective dimerization of butadiene (d) a general structure for molybdenum- and tungsten-based metathesis precatalyst (e) a six-coordinate rathenium precatalyst for metathesis (f) a solid isolated from the reaction between Pd(OAc)j plus PRj (R = o-tolyl) (g) a T-shaped palladium complex and a two-coordinate palladium complex with a monodentate phosphine (h) an iron complex with a seven-membered metallacycle (i) the transition state for metal-catalyzed cyclopropanation (j) a rhodium and a copper precatalyst used in cyclopropanation reactions. 

Along with the example of Scheme 13, other It2 and compounds show analogous reactivity patterns toward H2 (Scheme 14). Remarkably, the formation of a hydride bridge that replaces the metal-metal bond is observed in all cases, which may be due to the existence of a common mechanism for the activation of dihydrogen by these complexes (Schemes 13 and 14). A concerted pathway that entails coordination of the dihydrogen molecule in cis position to the intermetallic bond as first step has been postulated. Subsequently, single-site oxidative addition assisted by a shift of the electron density at the metal-metal bond would result in the formation of a hydride-bridged bimetallic complex. 

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