Symmetry-forbidden processes

The photochemical cycloaddition of a carbonyl compound 1 to an alkene 2 to yield an oxetane 3, is called the Patemo-Buchi reaction - This reaction belongs to the more general class of photochemical [2 + 2]-cycloadditions, and is just as these, according to the Woodward-Hofmann rules, photochemically a symmetry-allowed process, and thermally a symmetry-forbidden process. 

The first excited state of cyclobutene (o27t ) is correlated with the upper excited state ( /J /2 /3) of butadiene making it a high energy symmetry forbidden process. 

What makes photoexcited lepidopterene and its derivatives undergo adiabatic cycloreversion with so high quantum efficiency The answer to this question must be linked with fact that the formation of lepidopterene from its cycloreversion product A is a highly efficient ground state process, viz. an intramolecular Diels-Alder reaction, which is symmetry-allowed by Woodward-Hoffmann rules. By the same token, the excited state 4jm-2ji cycloreversion of lepidopterene L is a symmetry-forbidden process. Thus, it is... 

The 1,2-elimination mechanism as shown In Equation 1 is a four-electron orbital symmetry forbidden process (25). Therefore, an alternative mechanism, such as the allowed six electron process shown in Equation 2, may be more likely. If the reaction procedes In this manner, a terminally unsaturated fatty acid carboxylate will be formed just as for the 1,2-elimination. The deuterium labeling... 

V V2 3) of butadiene making it a high energy symmetry forbidden process. 

The mechanism of this thermal valence isomerization was formulated as proceeding by an intermediate diradical. Extended Hiickel calculations have shown that the opening of the endo compounds into diradicals is a symmetry-forbidden process, in contrast to the ring opening of the exo derivatives. ... 

If the Stevens rearrangement is a concerted reaction, it is a symmetry-forbidden process based on the Woodward-Hoffmann rules. Indeed, it was shown to occur via an intramolecular hemolytic cleavage-radical pair recombination process, which explains the lack of crossover products and the observed retention of configuration at the migrating... 

The 1,4-rearrangement, though symmetry-allowed, is rarely observed. The 1,2-, 5,2- and 3,4-rearrangements are symmetry-forbidden processes which would have to take place via a suprafacial-anta-rafacial mode. They have been shown to proceed by radical pair intermediates. ... 

The first system we consider is the isotope exchange reaction for H2 and Dj. Experimental studies of this reaction are very difficult to perform since impurities allow the formation of H atoms, which then results in a radical exchange reaction. The early theoretical studies were flawed by the assumption that the reaction proceeded by a four-center symmetry-forbidden process. While the later theoretical studies focused on the symmetry-allowed termolecular process... 

Thermal [2+2]-cycloaddition reactions are less common, but photochemical [2+2]-cycloaddition reactions are very common. This fact can be explained by analyzing these cycloaddition reactions using Woodward-Hofifmann selection rules. In frontier orbital approach, the thermal reaction of two ethene molecules (one is HOMO and other is LUMO) is orbital symmetry forbidden process for its suprafacial-suprafacial [7t s+7t s]-cycloaddition, but a suprafacial-antarafacial [jt s+jt a]-cycloaddilion reaction is symmetry allowed process (Fig. 3.1). It signifies that the cycloaddilion of one two-7t electron system with another two-ji electron system will be a thermally allowed process when one set of orbitals is reacting in a suprafacial mode and other set in an antarafacial mode ( s means suprafacial and a means antarafacial). Thermal [7t s+Ji a]-reactions usually occur in the additions of alkenes to ketenes, when alkene is in the ground state and ketene in the excited state [1] (Fig. 3.2). 

In thermal reaction, bonding interaction is maintained in the suprafacial mode of 1,5-shift and hence this process is symmetry allowed, while the antarafacial shift is symmetry forbidden. The suprafacial shift also corresponds to a favorable six-electron Huckel-type transition state in thermal reaction, whereas Huckel-type TS for suprafacial [l,3]-sigmatropic hydrogen shift is antiaromatic and is a forbidden process (Fig. 4.2) [1, 2]. Photochemically, [l,5]-hydrogen shift in the suprafacial mode is a symmetry forbidden process, but antarafacial shift is a symmetry allowed process (Fig. 4.3). 

Thermal [1,3]-suprafacial hydrogen shift is orbital symmetry forbidden process, but [1,3]-suprafacial alkyl shift is symmetry allowed process with inversion of configuration of migrating alkyl carbon. For example, the thermal rearrangement of bicyclo-[3.2.0]-heptene 1 to bicyclo-[2.2.1]-heptene 2 [4]. 

The concerted suprafacial thermal addition of two olefins is a symmetry-forbidden process and thermal 1,2-cycloadditions of simple olefins have been... 

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