Merry-go-round, mechanism

The nickel(II)-catalyzed polymerization of isocyanides proceeds relatively fast, a remarkable observation given the steric crowding that is introduced upon formation of the polymer chain. The driving force for the reaction is the conversion of a formally divalent carbon in the monomer into a tetravalent carbon in the polymer, yielding a heat of polymerization of 81.4 kJ moD1.169 For this polymerization reaction, a merry-go-round mechanism has been proposed. Upon mixing of the isocyanides with the Ni(II) catalyst, a square-planar complex is formed (Scheme 7), which in some occasions can be isolated when bulky isocyanides are used. Subsequent attack by a nucleophile on one of the isocyanide ligands is... 

Scheme 7. Merry-go-Round Mechanism in Ni(II)-Catalyzed Isocyanide Polymerization...

A nucleophile (X-) attacks one of the isocyano carbon atoms, generating the a-iminomethylnickel species B. Then, coordination of the fifth isocyanide to the nickel center (to form C) is followed by the migration of the imi-nomethyl group onto the neighboring isocyano carbon atom (C2), forming a dimeric intermediate D. This associative migration step is repeated to form the poly(isocyanide) E. Nolte and Drenth called this possible mechanism a merry-go-round mechanism, and it was also proposed as the key mechanism for the screw-sense selective polymerizations discussed later. 

A mechanism for asymmetric induction has been discussed based on the merry-go-round mechanism proposed for Ni(II)-catalyzed polymerization, and discussed earlier. In the initial stages, a chiral amine attacks one of the four isocyanide ligands (C1) on the nickel to form a diaminocarbene complex I, whose structure has been elucidated in detail [60]. Among the possible conformations, the one that involves weak interaction between the Ph group and Ni was presumed to have the most favorable conformation (Scheme 37). The structure of the intermediate I has also been discussed in... 

In triangular clusters of the type M3(CO)i2, M3(CO)i2 xL c equatorial CO molecules undergo exchange according to the planar merry-go-round mechanism (3.64). The CO groups which are approximately coplanar with the M3 ring rotate... 

Fluorine coordination degenerate fluorine migration in a Merry-Go-Round type mechanism... 

From the scheme described in Fig. 3, it is evident that quantum yield measurements in solids are the first step toward understanding mechanisms of solid-state photoreactions. Such measurements, however, have not been done frequently, because they require special precautions and apparatus (34). Recently we have found that quantum yields for some solid-state photoreactions can be conveniently estimated by the usual merry-go-round technique (11,29). The procedure is very simple dissolve a sample with ether in a Pyrex tube, evaporate the solvent to leave a coated crystalline film whose surface area is adjusted to be constant as precisely as possible, and then irradiate after degassing on a merry-go-round apparatus. The obtained quantum yields may not be very precise because of the reflection of light from the crystal surface and a variable surface area, but their reproducibilities are confirmed to be quite satisfactory (usually 5 %). 

A second mechanism, which Jack Richards (7) at Cal Tech has called the two-hydrogen merry-go-round, involves a hydrogen transfer without an intermediate. In this mechanism, you say, Well, you don t want an intermediate like this. You simply have some kind of a scheme whereby you push a hydrogen from a substrate into the coenzyme and simultaneously another one comes out of the coenzyme. This is called the two-hydrogen merry-go-round because one would assume this mechanism would be highly stereospecific. To use both hydrogens, as the experimental data require, each displacement must involve an inversion. It takes two turnovers to get all of the hydrogen out of the coenzyme. 

In M4(CO)j2 systems, the merry-go-round and basal face change mechanisms, shown in reactions (4.18) and (4.19), have been identified. In the first case the unbridged intermediate is shown. For the second case, the... 

Attempts to distinguish between possible mechanisms for CO ligand mobility in Group 9 carbonyl clusters, i.e. merry-go-round versus Johnson and Benfield s ligand polyhedral model, have proven inconclusive. A series of Co-NMR studies on [Co4(CO)i2] and analysis of the thermal motion of the CO ligands using new structural data for [Rh4(CO)i2] failed to provide conclusive evidence for the type of fluxional processes observed. 

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