Bent Cyclopenta-2,4-dienylideneketene: Spectroscopic and ab initio Study of Reactive Intermediate

Abstract

Results of an experimental and theoretical study of cyclopenta-2,4-dienylideneketene (3), a highly unstable reactive intermediate, are reported. The ketene was prepared, under matrix isolation conditions at 4.2 or 10 K, by laser photocarbonylation of 1,2-didehydrobenzene (1) photogenerated earlier from phthalic anhydride (2). FTIR polarization measurements performed on partially photooriented samples of 3 immobilized in solid neon or argon provide infrared transition moment directions for most of the observed vibrations. Experimental results confirm that the ketene is bent, as predicted by ab initio calculations. Utilizing two isotopically modified 3, 3b and 3c, on the basis of the infrared absorption spectrum alone, we have analyzed and assigned its vibrations in a way, which leaves no doubt about the bent ketene structure. This work was motivated by a long standing confusion surrounding the assignments of the vibrations in 1,2-didehydrobenzene (1), especially of its "triple" bond stretch.

High-Resolution Spectroscopic Study of Matrix-Isolated Reactive Intermediates: Vibrational Assignments for 3-Fluoro-o-Benzyne and Perfluoro-o-Benzyne∇

“…The side-products identification is based on the comparison of the observed IR absorptions and calculated vibrational spectra for structures 3 and 6 . The positions of the observed characteristic absorptions in these compounds are highly compatible with well-established absorptions of the cumulative double bond in parent 3 and carbonyl stretch in parent 6 . Simultaneous determination of the IR and UV−vis absorption spectra allows for subsequent assignment of the electronic features.…”

Natural Bond Orbital Theory of Pseudo-Jahn–Teller Effects

“…As a second example of NBO/NRT hyperconjugative analysis of PJT effects, we examine related aspects of the interesting kinking (or nonkinking) of •••CCC••• cumulene chains in C 5 H 4 CCO and related mono-and diketones (Scheme 2). 22 For the C 5 H 4 CCO case that has been explored most thoroughly by theoretical 20 and experimental 21 means, the calculated lowsymmetry equilibrium geometry and C 2v -symmetric transition state geometry (with imaginary ν Q = −82i cm −1 ) are displayed in Figure 8. Neglecting the cyclopentadienyl moiety, the leading NBO donor−acceptor interactions of the OC′C″ tail (as judged by E (2) or ΔE $DEL estimates) are found to to be of π C′C″ → σ* C′C″ (d1), σ C′C″ → π* C′C″ (d2), or n O (π) → σ* C′C″ (d3) type, as summarized in Table 2.…”

“…A related type of symmetry breaking is exhibited in the unexpected kinking deformations of cumulene ketones, , which were similarly analyzed with NBO/NRT methods to provide a simple chemical rationale of the interesting deformation patterns (Scheme ).…”

Why Do Cumulene Ketones Kink?

“…The initial suggestion of possible ketene kinking was apparently offered by Radziszewski, Hess, and Zahradnik 6 in a graphical "Scheme II" mechanism for production of cyclopentadienylideneketene. Shortly thereafter, Scheiner and Schaefer 7 provided strong theoretical evidence for such bending (in close agreement with results presented below), and Radziszewski et al 8 later presented extensive IR experimental evidence "which leaves no doubt about the bent ketene structure." Other kinked ketene moieties have been also noted in X-ray structures of more complex derivatives.…”

“…Finally, it is evident that the simple picture of ketene kinking sketched herein should apply to more complex derivatives, including the cyclopentadienylideneketene species in which such kinking was first established. 7,8 Figure 8 shows calculated structures of (a) fulvenone (fulvene ketene, C 2v ) and (b) cyclopentadienylideneketene (C s ) which may be directly compared with the acyclic species in Figure 1a,b, respectively, and would be analyzed similarly.…”