Push–pull allenes-conjugation, (anti)aromaticity and quantification of the push–pull character

doi:10.1016/j.tet.2013.01.027

Tetrahedron

Volume 69, Issue 11, 18 March 2013, Pages 2436-2445

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Abstract

Structures, ¹H/¹³C chemical shifts, and π electron distribution/conjugation of an experimentally available and theoretically completed set of push–pull allenes Acc₂C double bond CCDon₂ (Acc=F, CHO, CF₃, C triple bond N; Don=t-Bu, OMe, OEt, SMe, SEt, NCH₂R) have been computed at the DFT level of theory. Both orthogonal linear and orthogonal bent structures have been obtained. In the latter case the push–pull character could be quantified by the quotient method. The ¹³C chemical shift of the central allene carbon atom C-2 and chemical shift differences Δδ(C-1, C-2) and Δδ(C-2, C-3) of allene carbon atoms proved to be a quantitative alternative. TSNMRS of ring-closed push–pull allenes have been computed in addition and were employed to identify polar, carbene-like and carbone-like canonical structures of these molecules.

Graphical abstract

Introduction

Push–pull allenes 1 (Scheme 1)1, 2 prove to generate amphiphilic character (1B, 1C) and react both as electrophile and nucleophile at the central allene carbon atom³ but are able to perform carbene-like reactions (1A) as well.¹ Spectroscopic data so far known, however, indicate no significant carbene character,² i.a. the ¹³C chemical shift of the central allene carbon atom is expected high-frequency shifted up to 300 ppm.⁴ From ¹³C NMR spectra of other push–pull allenes was concluded that the contribution of push–pull type polarized structures (1B, 1C) in the ground state should be only little.⁵ Due to sp hybridization of the central allene carbon atom the two halves of the molecules are orthogonal and conjugation between the acceptor and the donor sides should be inhibited. For push–push allenes 2, however, in addition to the orthogonal position of the two halves of the molecules also bent structures 2A (Scheme 2) have been reported.6, 7 To prove push–pull allenes 1 accordingly, comparable theoretical studies or X-ray structures of known compounds are not published yet.

Efforts to quantify the push–pull character of push–pull allenes 1 by experimental indications remained only qualitative: barriers to rotation are too large to be studied on the NMR time scale,⁸ only Saalfrank1, 2 studied the ¹³C chemical shifts of the allene carbon atoms of 2 with respect to the degree of the push–pull character, however, the ¹³C chemical shift as a complex terminus is subjected to a number of substituent effects and not to the corresponding electron density at the carbon atoms alone⁹ to be correlated to canonical structures 1B and 1C. And theoretically studied are push–pull allenes 1 not yet.

Thus, a set of push–pull allenes synthesized by Saalfrank et al.1, 2 (completed by computed structures still unknown) and of the corresponding symmetrical analogs (3–7 in Scheme 3) was studied at the DFT level of theory,¹⁰ the ¹³C chemical shifts of the structures were computed by the gauge-including atomic orbital (GIAO) method¹¹ and occupation numbers of the allene π and π* orbitals by the Natural Bond Orbital (NBO) method¹² as well. The results of these computations were discussed in order to learn more about structure and electron distribution/conjugation in push–pull allenes and to quantify, if possible, the inherent push–pull character.

Section snippets

Structures

The structure of push–pull allenes 3–6 have been found as expected from text books: the allene framework proves to be linear and the two sides, due to sp hybridization of the central carbon atom C-2, are orthogonal to each other—thus conjugation between the Acc₂C double bond and CDon₂ parts of the molecules cannot be generated. This changes obviously if as electron-donor substituents the ring-closed imidazolyl substituent is introduced (push–pull allenes 7—Scheme 3). The two unlike parts of 7 are still

Conclusions

The structures of the push–pull allenes 3–12 studied could be, dependent on substitution and degree of push–pull character, both linear orthogonal and also bent orthogonal [increasing electron-withdrawing power of the acceptor substituents increases the bending (CHO>C triple bond N>CF₃) of the allene skeleton].

The ¹³C chemical shift of C-2 and the chemical shift differences Δδ(C-1, C-2) and Δδ(C-2, C-3), respectively, can unequivocally indicate quantitatively the push, pull character of push–pull allenes if

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Push–pull allenes-conjugation, (anti)aromaticity and quantification of the push–pull character

Abstract

Graphical abstract

Introduction

Section snippets

Structures

Conclusions

Tetrahedron Lett.

Tetrahedron Lett.

Angew. Chem., Int. Ed. Engl.

Chem. Ber.

Chem. Ber.

Acc. Chem. Res.

Tetrahedron Lett.

J. Mol. Struct. (THEOCHEM)

Isr. J. Chem.

Chem. Commun.

Kiso Yuki Kagaku Rengo Toronkai Yokoshu

Angew. Chem., Int. Ed.

J. Org. Chem.

J. Phys. Org. Chem.

Magn. Reson. Chem.