Bonding rearrangements

The tutorial in Section 10.3.1.8 presents some of the various ways the information in the Biochemical Pathways database can be retrieved. In this tutorial the importance of searching for the reaction center, the atoms and bonds directly involved in the bond rearrangement scheme, is emphasized, It is a prerequisite for getting a deeper understanding of chemical reactions. 

In addition to the restrictions on their mobiHty caused by steric and polar interactions between chemical groups, the protein molecules in wool fibers are covalentiy cross-linked by disulfide bonds. Permanent setting only occurs if these disulfide bonds are also rearranged to be in equiHbrium with the new shape of the fiber. Disulfide bond rearrangement occurs only at high temperature (>70° C) in wet wool and at even higher temperatures (above 100°C) in... 

Initial imine formation between PMP and cr-keioglutatate is followed by double-bond rearrangement to an isomeric imine and hydrolysis. 

Reactions that do not involve bond rearrangements are usually rapid at room temperature. 

Solanesol and other prenyl alcohols are important as metabolites in mulberry and tobacco leaves and in the synthesis of isoprenoid quinones. Hence, Sato and collaborators107 have developed a stereoselective synthesis of all-trans-polyprenol alcohols up to C50. Construction of the requisite skeletons was accomplished by the alkylation of a p-toluenesulphonyl-stabilized carbanion, followed by reductive desulphonylation of the resulting allylic sulphonyl group. This was achieved most efficiently by the use of a large excess of lithium metal in ethylamine (equation (43)), although all reaction conditions led to mixtures. The minor product results from double bond rearrangement. 

Double-bond rearrangements can also take place on treatment with acids. Both proton and Lewis acids can be used. The mechanism in the case of proton acids is the reverse of the previous one first, a proton is gained, giving a carbocation, and then another is lost ... 

Many highly strained cage molecules undergo rearrangement when treated with metallic ions such as Ag , Rh(I), or Pd(II). The bond rearrangements observed can be formally classified into two main types (1) [2 + 2] ring... 

A detailed description of the local bond rearrangement has been derived [439], using the concept of the HDOS with a low-energy tail that corresponds to the H present at weak Si —Si bonds. The width of this tail is 2 i o, i-c., twice the width of the valence band tail in the electronic density of states, which in turn is about equal to the Urbach energy Eq [442,443]. The HDOS then is [439]... 

The ability of transition-metal complexes to activate substrates such as alkenes and dihydrogen with respect to low-barrier bond rearrangements underlies a large number of important catalytic transformations, such as hydrogenation and hydroformy-lation of alkenes. However, activation alone is insufficient if it is indiscriminate. In this section we examine a particularly important class of alkene-polymerization catalysts that exhibit exquisite control of reaction stereoselectivity and regioselec-tivity as well as extraordinary catalytic power, the foundation for modern industries based on inexpensive tailored polymers. 

The abundance of accessible donor and acceptor orbitals in common transition-metal complexes facilitates low-energy bond rearrangements such as insertion ( oxidative-addition ) reactions, thus enabling the critically important catalytic potential of metals. 

These energies relate to bond rearrangement in gaseous molecules, but calculations are often performed for reactions of condensed phases, by combining the enthalpies of vaporization, sublimation, etc. We can calculate a value without further correction if a crude value of AHr is sufficient, or we do not know the enthalpies of phase changes. 

Rearrangements in which a sigma bonded group is transferred from one end of a conjugated n system to the other are called sigma bond rearrangements. Some examples are ... 

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