Diradicaloid transition state

Photoelectron, UV and NMR spectroscopy, as well as MNDO calculations all predict a reduced HOMO-LUMO gap for alkenes with captodative substitution, and an enhanced reactivity of the p carbon [43]. This explains their radicophilic behavior and their high reactivity in cycloaddition processes, the diradicaloid transition states of which are stabilized when the mechanism is asynchronous cf. Sec. 3.3.7). 

Captodative dienes, such as the 2-cyano-l-aza-1,3-butadienes 128 and 129, also undergo hetero Diels-Alder reactions, both inter- and intramolecularly (Scheme 23). Reactivity and regio- and stereo-selectivity in these processes have been accounted for in terms of a concerted asynchronous mechanism involving a diradicaloid transition state [69]. The intramolecular cases illustrated here have been shown to be successful for the preparation of indolizidine 130 and quinolizidine 131 ring systems. 

In the diradicaloid region of the surface, the following critical points have been found (a) an extended diradicaloid transition state for the formation of the first bond (b) a diradical intermediate, which exists in a trans and a cis form and (c) a second diradicaloid transition state which connects the cis form of the diradical intermediate with the product isoxazole. On the basis of these results, it seems likely that the transition state for the formation of the first bond connects the reactants with the cis form of the diradical intermediate and the trans form represents a subsidiary minimum accessible via an in-plane inversion process. 

The PES s of the [2 + 2]-cycloaddition were also computed for the following reactions formaldehyde + formaldehyde and ethylene + singlet oxygen [10]. The results of these computations indicate that for a [2 + 2]-cycloaddition only step-wise reaction paths exist involving diradicaloid transition states and intermediate gauche- and trans-forms. 

The nature of the transition state of nucleophilic reactions with LL [low lowest unoccupied molecular orbital (LUMO)] substrates is analyzed and reviewed. In cation-anion combination reactions, a partial radical character is developed on both the nucleophile and the substrate. Examination of a simple state diagram shows that this diradicaloid character is increased as the LUMO of the substrate is lowered. The model is further extended to other LL substrates such as carbonyl functions and activated olefins. Three empirical manifestations of the diradicaloid character of the transition state are discussed (1) the correlation between the ionization potentials of the nucleophiles and their nucleophilicity toward LL substrates (2) the a-effect phenomenon and (3) the variations in the positional selectivity of 9-nitromethylenefluorene in nucleophilic reactions as a function of the solvent. 

In the following discussion nucleophilic reactions with LL substrate will be analyzed by making use of basic principles of electronic states. We will show that (a) under certain circumstances the transition state of nucleophilic reactions must be diradicaloid and (b) the diradicaloid nature of the transition state will in general increase with the increase in the electrophilicity of the substrate. 

To account for these unusual results and the role of the free-radical character, a mechanism that implies that NH—CO—PTM- is a good leaving group in 8, 2 reactions has been proposed. It is based on the quantum-mechanical approach used to account for the kinetics of nucleophilic substitution in benzyl halides, which involves a diradicaloid configuration in the relevant transition state. The latter would be particularly stabilized by the radical character of the labelled glycine. 

Dewar [147,183,184] include zwitterionic forms together with the diradicals (model of the diradicaloids (Fig. 2), which considers the fact that most Diels-Alder additions, although being concerted, form the two new a bonds in a non-synchronous fashion in their transition states). Applied to 51 (and 43), this model predicts a para regioselectivity (52) for the Diels-Alder addition of methyl vinyl ketone due to favorable homoconjugative participation of the non-conjugated alkene moiety [185-189], as shown with the zwitterionic form 52. ... 

Fig. 2 Interpretation of the Diels-Alder regioselectivity of 2,3,5-trimethylidenebicyclo-[2.2.1]heptane applying the diradicaloid model for the transition state (A = electron-withdrawing substituent)...

Because of the sharply avoided crossing in the region of the transition state for the [2, + 2 J concerted reaction and of the diradicaloid character of the critical points involved in the non-concerted process, the computation of this surface requires computational methods that transcend the SCF method. Recently this surface, and in particular the transition structure region, has been investigated in detail by ab initio molecular-orbital methods. The calculations have been performed at the MC-SCF level with minimal (STO-3G) and extended (4-31G) basis sets. The various critical points have been fully optimized with MC-SCF gradients and characterized by computing the corresponding Hessian matrices. 

In fig. 2 we show the potential energy diagram in the r/a space illustrated in fig. 1. In this subspace the synchronous supra-supra reaction path passes over a local maximum and there is a synchronous channel involving a coplanar syn diradicaloid structure. When rotation about the CC bond is considered, the syn diradicaloid structure turns out to be a local maximum. True transition states exist only for two reaction paths associated with... 

This choice is appropriate for the motions investigated here since it allows proper dissociation of tetramethylene into two ethylenes and also permits the correct description of a diradicaloid intermediate or transition structure. In this valence space, a complete Cl for the singlet state has 20 configurations. 

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