Mechanistic Families

Similar chemical reactions often proceed by similar mechanisms. Its important to recognize these similarities when they exist because this makes it easier to compare such quantities as reaction rates and product selectivity. 

The reactions shown above are nucleophilic substitutions that involve replacement of bromine by oxygen. The reactions may or may not proceed by similar mechanisms. 

One after the other, step through the sequence of structures corresponding to the three nucleophile substitution reactions shown above (reaction 1, reaction 2, reaction 3). Decide whether loss of Br occurs with or without the assistance of RO /ROH. The nucleophile-assisted and unassisted mechanisms are called Sn2 and SnI mechanisms respectively. Label each reaction as Sn2 or SnI as appropriate. 

For each reaction, plot energy (vertical axis) vs. the number of the structure in the overall sequence (horizontal axis). Do reactions that share the same mechanistic label also share similar reaction energy diagrams How many barriers separate the reactants and products in an Sn2 reaction In an SnI reaction Based on your observations, draw a step-by-step mechanism for each reaction using curved arrows () to show electron movements. The drawing for each step should show the reactants and products for that step and curved arrows needed for that step only. Do not draw transition states, and do not combine arrows for different steps. 

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Most MCRs of type IA are 3CRs whose equilibrating products react with further educts by higher MCRs of type II. In 1960, Hellmann and Opitz published their a-Atni-noalkylierung book [7], which was a comprehensive list and discussion of all so-far-known MCRs. They realized, as did others previously, that the name reactions [2,5] S-3CR, the M-3CR [32] and many other 3CRs belong to the mechanistic family of a-aminoalkyla-tions of nucleophiles, which are usually the anions of deprotonated weak acids components. This collection of 3CRs is now also referred to as the HO-3CR [4],... 

The organization is along the lines of reaction type rather than functional groups. The first nine chapters discuss most of the important reactions presently in use in organic synthesis. Although the emphasis here is on synthesis, the reactions that are discussed in each chapter are usually members of related mechanistic families. Chapter 10 discusses synthetic tactics and strategy in general. Chapter 11 considers some of the special features of macromolecular synthesis. 

The remainder of this paper describes the division of the paraffin hydrocracking reactions into mechanistic families with a unique reaction matrix operator for each reaction family. The reaction rules and QSRCs used are then discussed for each reaction family. The technical specifications and the iteration process to find the optimum subset of the mechanistic model will also be discussed. 

Rhodium complexes with chelating bis(oxazoline) ligands have been described to a lesser extent for the cyclopropanation of olefins. For example, Bergman, Tilley et al. [32] have prepared a family of bis(oxazoline) complexes of coordinatively unsaturated monomeric rhodium(II) (see 20 in Scheme 13). Interestingly, the use of complex 20 in the cyclopropanation reaction of styrene afforded mainly the cis cyclopropane cis/trans = 63137), with 74% ee and not the thermodynamically favored trans isomer. No mechanistic suggestions are proposed by the authors to explain this unusual selectivity. 

Another important family of elimination reactions has as its common mechanistic feature cyclic TSs in which an intramolecular hydrogen transfer accompanies elimination to form a new carbon-carbon double bond. Scheme 6.20 depicts examples of these reaction types. These are thermally activated unimolecular reactions that normally do not involve acidic or basic catalysts. There is, however, a wide variation in the temperature at which elimination proceeds at a convenient rate. The cyclic TS dictates that elimination occurs with syn stereochemistry. At least in a formal sense, all the reactions can proceed by a concerted mechanism. The reactions, as a group, are often referred to as thermal syn eliminations. 

The fact that compounds having similar chemical structures often react in similar ways implies that they follow corresponding mechanisms in proceeding from reactants to products. One must have a very sound basis in experimental fact to be able to defend successfully a proposed reaction mechanism that contains elementary reactions that represent a major departure from those accepted by the scientific community as being a proper mechanistic interpretation of related reactions. On occasions such departures are necessary, and they themselves may provide the key to an improved understanding of the common family of related reactions. 

Although there are several classes of phospholipases, the phospholipase C (PLC) family is the one that has recently received intense scrutiny in the general context of signal transduction. The present account therefore details recent structural and mechanistic studies of one member of this important super family of enzymes, the phosphatidylcholine-preferring phospholipase C from B. cereus (PLCB(.). 

To resolve the issue of cyclization specificity, the x-ray crystal structure of the stilbene synthase from pine was determined to atomic resolution. This information allowed the mutagenic conversion of alfalfa CHS to a functional STS, and crystal structures of this engineered STS were solved, in the apo form and with resveratrol bound in the active site (Austin and Noel, unpublished). These experiments support a mechanistic proposal, which prompted further mutagenic and modeling experiments. This work has allowed the elucidation of the structural and mechanistic basis for cyclization specificity (aldol versus Claisen condensation) in the CHS family of type III PKSs. 

The absence of a high-resolution structure of any transporter in the NT family greatly complicates the interpretation of structure-function studies. The recent identification of the bacterial and archaeal transporters and their potential suitability for direct structural studies raises the exciting prospect of being able to test specific structural hypotheses related to transport. In the meantime, the use of the numbering scheme introduced here in a context of predicted structural properties should facilitate communication among researchers on different members of the transporter family as the mechanistic details are being probed. 

In summary, the Danheiser reactions are a family of [3 + 2] annulations that take place between allenylsilanes and diverse electrophiles. The products can be carbo-cycles or heterocycles. In all cases, the annulations proceed most efficiently when the allenylsilane has a non-hydrogen substituent on the carbon atom bearing the silicon. This is a consequence of the common mechanistic pathway that proceeds through a vinyl carbocation intermediate. 

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