Reaction Mechanism Strategies

Mechanism problems are easier than product-prediction problems because helpful information can be derived from the product. Given the starting and ending points you need only supply the lowest-energy path between them. You may not be able to see the answer to a long mechanism problem at the start just let a logical analysis carry you to the answer. Don t panic and try to force a quick (and incorrect) answer. 

Look at each new bond and then at the polarization of the corresponding sites in the starting materials so that you can decide what the original nucleophile-electrophile pairs might have been for those bonds. If more than one new bond has been made, consider all possible sequences of bond formation, not just the first one that occurs to you. Look at adjacent functionality to get a hint as to whether it would stabilize an electrophile s or nucleophile s partial or full charge. Ask how proton transfer might have generated the needed nucleophiles and electrophiles. Consider any reasonable proton transfer and always check the value (at best 10 ). 

Finally, do not combine steps to save drawing out the structure again. Keep to the pathways to avoid the trap of merely rearranging the lines and dots of the structures on the page. Be as rigorous in your own practice as you would be on an exam you are trying to develop good intellectual habits. The prime directive is this  

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One strategy in limiting the formation of ozone and other photochemical oxidants has been the use (in the past) of low reactivity fuels in internal combustion engines. More recently, alternate fuels (methanol, for instance) have been proposed for regions that suffer from elevated levels of photochemical air pollution. The effect of switching to such a low-reactivity fuel may be seen in Equation E2 for methanol, which has a simple atmospheric reaction mechanism. 

Optimization strategies and a number of generalized limitations to the design of gas-phase chemiluminescence detectors have been described based on exact solutions of the governing equations for both exponential dilution and plug-flow models of the reaction chamber by Mehrabzadeh et al. [12, 13]. However, application of this approach requires a knowledge of the reaction mechanism and rate coefficients for the rate-determining steps of the chemiluminescent reaction considered. 

These reactions have been studied in detail for materials such as silica, and understanding of reaction mechanisms, as well as of the role of the precursor and catalyst (acid or base), has been well documented.63,64 Similar studies have been carried out in other material systems, most notably, lead zirconate titanate [Pb(Zr,Ti)03 PZT].52,65-68 For multicomponent (mixed-metal) systems such as those noted, prehydrolysis of less reactive alkoxides is sometimes employed to improve solution compositional uniformity. Other synthetic strategies to achieve molecular level mixing of reagents have also been employed. Here, synthesis of mixed-metal alkoxides has been a focus of investigators.40-42 A key point is to restrict the amount of water and to control how it is added to form solubalizable precursor species, rather than to induce precipitation.1,52,69,70... 

The reaction itself is rather attractive and useful as a synthetic tool, because of its practical simplicity. Indeed, 1,5-dicarbonyl compounds can be readily prepared from olefin and 1,3-dicarbonyl compound in a one-pot reaction. Subsequent aldolization of the resulting 1,5-dicarbonyl compounds would furnish cyclohexenones. The reaction mechanism and stereochemistry of the de Mayo reaction has been comprehensively reviewed7), and the basic strategy of which has also been developed and extended. The following examples would demonstrate some applications and variations of the de Mayo reaction. 

Intramolecular ionic Diels-Alder reactions were carried out in highly polar media to afford carbocyclic ring systems. The strategy, which obviates the need for high temperatures and pressures, features in situ generation of heteroatom-stabUized allyl cations that undergo subsequent (4 + 2) cycloaddition at ambient temperature. Typically, reactions were complete within 1 hour after addition of substrate. Some cycloadducts were the result of a concerted process, whereas others were formed via a stepwise reaction mechanism (Grieco, 1996). 

The strategies for asymmetric synthesis are found in J. A. March, Advanced Organic Chemistry, Reactions, Mechanism, and Structure, 4th ed., Wiley, New York, 1992, pp. 116-120. See also M. B. Smith and J. A. March, Advanced Organic Chemistry, Reactions, Mechanism, and Structure, 5th ed., Wiley, New York, 2001, pp. 147-150. 

R. Sterner, Two (j3a)(8)-barrel enzymes of histidine and tryptophan biosynthesis have similar reaction mechanisms and common strategies for protecting their labile substrates, Biochemistry 2002, 41, 12032-120342. 

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