Isomerization Intramolecular Rearrangement Process

Dihydride complexes synthesized directly using H2 generally have cis dihydride geometry, although a trans concerted addition of H2 is allowed in principle . Some data exist for trans H2 addition to fra i-IrH(CO)(PPh3)2, but the evidence was clouded by the possibility of an intramolecular rearrangement process . Also cis-trans isomerization within M(H)2[P(0R)3]4 complexes (M -I- Fe, Ru) has been documented. Some Pt(PR3)2 complexes, where R is a bulky substituent, give trans dihydrides, and trans-Mo(H)2(diphos)2 is formed from rrans-Mo(N2)2(diphos)2 . 

We define repeating unit isomerization as a process subsequent to polymerization, in which an intramolecular rearrangement of the repeating unit leads to a thermodynamically preferred structure ... 

Dynamic intramolecular rearrangements are observed for a variety of diene-metal complexes at, or near, ambient temperature. This stereochemical non-rigidity may be detected by variable temperature NMR experiments40 in which the signals observed for a static structure coalesce into time averaged signals for the fluxional process. For purposes of this section, processes with activation energies > ca 25 kcal mol 1 or which are irreversible will be considered to be isomerization phenomena and will be discussed in Section IV. 

Isomerizations are important unimolecular reactions that result in the intramolecular rearrangement of atoms, and their rate parameters are of the same order of magnitude as other unimolecular reactions. Consequently, they can have significant impact on product distributions in high-temperature processes. A large number of different types of isomerization reactions seem to be possible, in which stable as well as radical species serve as reactants (Benson, 1976). Unfortunately, with the exception of cis-trans isomerizations, accurate kinetic information is scarce for many of these reactions. This is, in part, caused by experimental difficulties associated with the detection of isomers and with the presence of parallel reactions. However, with computational quantum mechanics theoretical estimations of barrier heights in isomerizations are now possible. 

The formation of 5-hexenal (reaction 18) is believed to be an intramolecular rearrangement since the addition of oxygen does not cause its suppression. At least in a methyl substituted cyclohexanone the analogous process has been shown to occur by the transfer of a hydrogen atom from the beta position to the carbonyl group before the fission of the six-membered carbocyclic ring (29) as only one of the two possible isomeric heptenals is formed. 

An explanation of the catalytic action of sulfur dioxide in these isomerization processes has been given by de Boer el al. (3). They assumed that sulfur dioxide adds to the double bond, leading to the formation of a biradical. Intramolecular rearrangements and subsequent splitting off sulfur dioxide result in these isomerizations ... 

First, we can have an efficient process to get one, and only one, isomer, followed by a rearrangement process. However, the barriers corresponding to the intramolecular isomerization are too large 40 kcal/mol for HSiN — HNSi and 100 kcal/mol for the reverse transformation HSiN <— HNSi [61]. Under interstellar conditions, such high barriers cannot be aossed. If we neglect tunneling effects, there is one possibihty left, namely bimolecular reactions that formally provide the interconversion HSiN <-> HNSi. 

Rearrangement refers to any intramolecular processes leading to net isomerization. Some rearrangements involve C skeletal changes, while others do not. In fact, many of the intramolecnlar versions of the reactions discussed in Parts III-VIII do fit this definition and are therefore rearrangement reactions. Since it is more appropriate and convenient to discuss them as the intramolecular versions of various reactions that can also proceed in-termolecularly, they are discussed in the respective earlier parts. They include the following reactions, and their representative examples are shown in Scheme 1 ... 

General.—Possible routes for intramolecular rearrangements of four-, five-, and six-co-ordinated structures have been reviewed. The isomerization pathways are represented by Schlegel diagrams these representations are correlated, where relevant, with specific mechanisms such as those discussed by Muetterties, and the Berry pseudorotation process for five-co-ordinate species. 

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