Reaction Woodward-Hoffmann allowed

The Woodward-Hoffmann allowed reactions can be classified according to how many electrons are involved, and whether the reaction occurs thermally or photochemically, as shown in Table 15.1. 

At first sight, one could suppose that this isomerization follows the Woodward-Hoffmann-allowed cyclopropyl-allyl pathway (see above). At least in some cases, however, the required heterolytic bond rupture is quite unlikely to take place under the reaction conditions (gas phase or apolar solvents). On the other... 

The thermal rearrangement of the parent trideutero compound, for example, gave an isomer mixture that indicated only a mild preference for the Woodward-Hoffmann-allowed reaction (si + ar = 53%, sr + ai = 47%).378... 

Several reactions might be possible in certain systems. For example, the thermal reaction of fulvene with butadiene can potentially proceed through three different Woodward-Hoffmann-allowed supra-supra cycloadditions ... 

No more than a hydroboration is The transition state for a Woodward-Hoffmann-allowed reaction is aromatic in character, whereas a forbidden one is antiaromatic. However, the rules of aromaticity only apply when each atom of the annulene uses one, and only one, AO to bind to its neighbors. This is not the case here each fluorine atom uses two AOs, one for bonding to the other F atom and one for the incipient bond to C. 

Scheme 6.3 applies these rules by showing the HL structures for two cycloaddition reactions 11R and 11P are the structures for the reactants and products of the Woodward-Hoffmann forbidden 2 + 2 reaction, while 12R and 12P are the structures for the Woodward-Hoffmann allowed Diels—Alder reaction. In both cases, the difference in the HL structure is only the mode of spin coupling, and therefore the promotion energy G will involve only singlet-triplet excitations. In accord, we drew in Fig. 6.5 the corresponding... 

It is well known that the thermal decomposition of a bicyclo[1.1.0]butane gives the corresponding 1,3-butadiene probably via the Woodward-Hoffmann allowed concerted pathway, while photolysis of cyclobutene also provides 1,3-butadiene as a major product. A remarkable difference in the reaction modes between the silicon and carbon analogs of the bicyclo[1.1.0]butane/cyclobutene system would partially be a consequence of the fact that the formation of the silicon analog of a conjugated diene is highly undesirable. 

What is Symmetry Allowed and Symmetry Forbidden Reactions Woodward Hoffmann Rule o Bonds involved in Cycloaddition Reactions Some more Examples of 2 + 2 Cycloadditions Photochemical Cycloadditions 2 + 3 Cycloadditions 2, 1 Cycloaddition... 

H ° H D Figure 15.24 HOMO orbital for the ring closure of butadiene electrons are involyed, and whether the reaction occurs thermally or photochemically, as shown in Table 15.1. The state correlation diagrams giye an indication of the minimum theoretical level necessary for describing a reaction, For allowed reactions, the reactant configuration Table 15.1 Woodward-Hoffmann allowed reactions... 

Allowed reaction, Woodward-Hoffmann rules. 188 Claisen reaction, 304 CVFF force eld, 40 ... 

The 2,5-dihydrofuran elimination of hydrogen is also Woodward-Hoffmann allowed. The reaction proceeds at relatively low temperatures ( 650 °K) with a low activation energy which is characteristic of such concerted processes. Conservation of orbital symmetry requires exclusive cw-2,5-hydrogen elimination accompanied by 7t-bond migration, viz. 

However, not all reactions that follow a Woodward-Hoffmann allowed path are necessarily stereospecific. (Cf. Clark and Leigh, 1987). Irradiation... 

Analysis of the fragmentation reaction of PR5 from the point of view of the conservation of orbital symmetry led Hoffmann et al. to suggest a simple answer. The least-motion mode of concerted departure from the stable Dsh structure, coupling between an apical group and an equatorial group, is symmetry-forbidden, or WH (Woodward-Hoffmann) forbidden. On the other hand, both equatorial-equatorial and apical-apical fragmentations are symmetry-allowed, or WH (Woodward-Hoffmann) allowed (Scheme 2.3). 

Woodward-Hoffmann allowed reactions (4 + 2 or 2 + 2 + 2), where G is given by Here, there is a high barrier of 62kcal/mol for the trimerization of acetylene, where 2 A sx(Tnr ) is very large (ca. 297 kcal/mol). The barrier gets lowered to 22 kcal/mol for the Diels-Alder reaction where 2 A sx(tttt ) is comparatively much smaller (ca. 173 kcaFmol). Thus, with differences of 120kcal/ mol in G, an/factor of 0.3 (as quantified recently for radical reaction [22-24]) lowers the barrier by 40 kcaVmol. 

As pointed out in a 1996 review of the importance of dynamic electron correlation [le], in general, addition of correlation between the active and inactive electrons stabihzes the ionic terms in the CASSCF wave functions for the active electrons. Not only in the Cope TS, but also in the TSs for other Woodward-Hoffmann-allowed, pericyclic reactions [3], the ionic terms in the RHF configuration benefit from this type of stabilization. Consequently, with the inclusion of dynamic electron correlation, the weight of the RHF configuration in the wave function for the TS increases and the energy of the TS is lowered, relative to that of the reactants and products. It is for this reason that calculations which include dynamic electron correlation usually give much lower activation enthalpies for allowed pericychc reactions than either RHF or CASSCF calculations do [le,21]. 

Although the 2s + 2a reaction is Woodward-Hoffmann allowed, it is sterically so hindered that thermal 2 + 2 reactions in general are not observed. Photochemical 2 + 2 reactions, however, are well known. ... 

The conrotatory path is Woodward-Hoffmann allowed for a thermal reaction, while the corresponding photochemical reaction is predicted to occm in a disrotatory fashion. 

The direct connection of rings A and D at C l cannot be achieved by enamine or sul> fide couplings. This reaction has been carried out in almost quantitative yield by electrocyclic reactions of A/D Secocorrinoid metal complexes and constitutes a magnificent application of the Woodward-Hoffmann rules. First an antarafacial hydrogen shift from C-19 to C-1 is induced by light (sigmatropic 18-electron rearrangement), and second, a conrotatory thermally allowed cyclization of the mesoionic 16 rc-electron intermediate occurs. Only the A -trans-isomer is formed (A. Eschenmoser, 1974 A. Pfaltz, 1977). 

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