The Art of Problem Solving in Organic Chemistry, 2nd Ed. Supplementary Material for Chapter I 2014 Miguel E. Alonso Amelot CHAPTER I

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1 CHAPTER I [SUPPL Chapter I # 1] Arabic compound numbers in TAPSOC, Roman numerals in Supplementary material PART I: Understanding intermediates The performance of organic compounds is critically contingent upon the functional groups in it and the stereochemical constraints. Thus, becoming familiar with the three dimensional (3D) aspect of stating materials, products and interim structures between them is frequently of essence in mechanism understanding. Scheme I.4 is the first case you come across in TAPSOC. The central issue is that two C=C bonds in opposite sides of each intermediate might be a conduit to products via an intramolecular [2+2] cycloaddition under the command of UV light. Adducts are imaginative constructions of expected cyclobutanes without regard to stereochemical limitations (just to see how far our stop-and-go scheme can go). This is when steric criteria come into play by drawing 3D renderings of (Figure SPI.1). Potentially intervening C=C bonds (emphasized in green) are ill oriented or too far apart in 11 and 12 for concerted [2+2] cycloaddition (However, see Part II of this discussion. Assuming that non-concerted interaction via radical is still feasible one can stretch the argument to create products of type 15 and 16 which are highly tense and unlikely, as shown in TAPSOC Scheme I.4.

2 FIGURE SPI.1. Three dimensional renderings of compounds Hydrogen atoms not shown. As opposed to this, compounds have a much better chance of being formed even in a transitory fashion as C=C bonds can acquire a relatively more comfortable conformation for [2+2] cycloaddition. Bear in mind, however, that all these structures are scribbled out of sheer speculation, as explained in the text.

3 PART II: About molecular flexibility to reach impossible transitions Although the proximity of reacting sections in a given molecule is essential for a favorable reaction course, that is, with adequate atomic orbital overlap to create new bonds, there are remarkable cases in which this fundamental condition is apparently not met. A case in point is the reaction of Scheme SPI.1, a mechanistic problem you may wish to solve now [1]. Reaction conditions could not be simpler, which leaves all the molecular refurbishing to compound I with the aid of a proton. Give it a try. SCHEME SPI.1 In a nutshell, one can describe this reaction like this: a furan-furan bond is created while a N-N bond is lost. Given the extended conjugation, both bonding processes may be related in the sense of an electrocyclic reaction. This picture suggests a simultaneous relocation of electrons. To meet this requirement the two furan units must be quite close (<2.8 Ǻ) at some point of the reaction coordinate and before the N-N bond gives way. However, a molecular model of I reveals that the furan rings are far apart, about 12 Ǻ (Figure

4 SPI.2). Vibrational bending would have to be brought to exaggerated extremes in order to bring the furan rings to bonding distances. FIGURE SP1.2. Three dimensional stick model of compound I remarking the large distance between the furan rings. This model was created by relaxation of the molecule at 25 ºC and molecular mechanics (MM2) minimization. Nevertheless, as this reaction is an experimental reality there ought to be a way of easing the semi-linear structure via protonation of the hydrazine section. Double protonation would lead to strong RNH (+) 2 NH (+) 2 R repulsion and lengthening (weakening) of the N-N bond. Instead of the two aromatic sections drifting apart in the reaction medium, a faceto-face complex would start to take shape pulling both furans closer as a result of a profound change in the symmetry properties of the bis protonated hydrazine zone. This might be aided by the expected solvent box enclosure. Final electron reorganization would ensue thanks to the driving force for an extended interaction involving a total of 22 electrons, as suggested by authors of this work [1]. One can picture this awesome change in two different renderings (and their conceptual bases): with plenty of arrows, one for every electron pair, as drawn in route A, run the sigmatropic rearrangement and finish with proton elimination in IV.

5 Conversely, assume a delocalized intermediate via route B signaling the sites of bond formation/rupture only, once you are convinced that the electronic balance of the di cation is maintained and sufficient electrons are there for this operation. Dotted multiple bonds used in option B to suggest delocalization are just fine but may blur electron accounting. SCHEME SPI.2 Other reaction courses are imaginable, like this one: separation of the two molecular arms after hydrazine protonation, their parallel reorientation spurred by van der Waals-type attraction, and furan to furan bonding, all within the solvent cage maintaining entropic control and preventing diffusion. Under these conditions, the coupling reaction should be faster than the diffusion rate. [1] Park KH, Kang JS. J. Org. Chem. 1997;62: