Abstract
The molecular electron density theory study of this [3 + 2] cycloaddition reveals that the strain present in the cyclic allene changes its reactivity to that characteristic of highly reactive radical species.
Highlights
The use of strained species such as benzyne 5 and cyclic alkynes such as cyclopentyne 8 in organic synthesis permits their participation in organic reactions in which linear alkynes do not react
The global electron density transfer[39] (GEDT) is computed by the sum of the natural atomic charges (q), obtained by a natural population analysis (NPA),[40,41] of the atoms belonging to each framework (f) at the transition state structure (TS); global electron density transfer39 (GEDT) 1⁄4 Sqf
The present Molecular Electron Density Theory (MEDT) study has been divided into four parts: (i) in the rst one, an analysis of the CDFT reactivity indices at the ground state (GS) of the reagents involved in the 32CA reactions of nitrone 15 with allenes 10 and 18 is performed; (ii) the reaction paths associated the 32CA reaction of nitrone 15 with CHDE 10 are explored and characterised; (iii) in the third part, a topological analysis of the Electron Localisation Function (ELF) of the simplest allene 18 and strained CHDE 10 is performed in order to characterise their electronic structures; and (iv) nally, a BET study of the 32CA reaction of nitrone 15 with CHDE 10 is carried out in order to establish the origin of the high reactivity of the strained allene CHDE 10
Summary
One appealing procedure that provides a straightforward connection between the electron density distribution and the chemical structure is the quantum chemical analysis of the Becke and Edgecombe's Electron Localisation Function (ELF).[4]. Houk proposed that the predistortion of CHDE 10 into geometries similar to those of the TSs for cycloadditions could be responsible for the low activation energy of these reactions.[16] to benzyne 5,2 the strain present at the sp hybridised C5 carbon of CHDE 10 could provide some pseudoradical character to the C5 carbon, enabling CHDE 10 to experience a different reactivity pattern to that of linear allenes, and the corresponding reaction paths will be non-comparable These different interpretations about the role of the strain in the reactivity of strained species prompted us to revisit the 32CA reaction of the strained CHDE 10 with nitrone 15 within the recently proposed Molecular Electron Density Theory (MEDT)[21] (see Scheme 3). Both quantities may be approached in terms of the one-electron energies of the frontier molecular orbitals HOMO and LUMO, 3H and 3L, as m z (3H + 3L)/2 and h z (3L À 3H), respectively.[47,48] The global nucleophilicity index,[49,50] N, based on the HOMO energies obtained within the Kohn–Sham scheme,[51] is de ned as N 1⁄4 EHOMO(Nu) À EHOMO(TCE), where tetracyanoethylene (TCE) is the reference
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