Stable‐Ion NMR Spectroscopy and GIAO‐DFT Study of Carbocations Derived from Multibridged [3_n]Cyclophanes

Abstract

Mono-and diprotonation of the tri-, tetra-, and pentabridged cyclophanes [3 3 ](1,3,5)cyclophane (4), [3 4 ](1,2,3,5)cyclophane (5), and [3 5 ](1,2,3,4,5)cyclophane (6) were realized, and NMR spectroscopic studies of the resulting carbocations are reported. Unlike [2.2]paracyclophane and its fluorinated analogs that are protonated at a position ipso to the ethanobridge, the monocations derived from 4, 5, and 6 are protonated at the unsubstituted ring positions. Protonation regioselectivity in the dications is pseudo-meta for 4 and 5 at unsubstituted ring positions, but for more-crowded 6 the second protonation occurs at a position that is ipso to the trimethyl- IntroductionSynthesis, structural/conformational features, and chemical transformations of cyclophanes, in particular their electrophilic substitution chemistry and directive effects, have fascinated organic chemists for decades. Recent progress in cyclophane chemistry has been summarized in several authoritative chapters and books. [1] It is almost four decades since Hefelfinger and Cram [2] reported the direct NMR spectroscopic observation of ipso monoprotonated [2.2]paracyclophane 1H + in FSO 3 H/ SO 2 ClF (Figure 1). A decade later, Hopf and associates [3] succeeded in the generation of the doubly ipso protonated 1H 2 2+ in "magic acid"/SO 2 ClF at -110°C. Double ipso protonation in 1H 2 2+ increases angle deformation, which in turn decreases charge-charge repulsion.Two ene bridge. Transannular π-π interactions in the monocations are manifested in the observed proton deshielding in the unprotonated π-deck. DFT and GIAO-DFT were employed to study the mono-and dications for comparison with the solution studies in superacids. GIAO-derived ∆NICS (1) Protonation of 2 leads to shielding of the arenium ion protons by the fluorinated deck, showing that the direction of transannular π-electron drain can be reversed upon the 2 Ǟ 2H + transformation.K. K. Laali, T. Okazaki, T. Kitagawa, T. Shinmyozu FULL PAPERIn continuation of our studies on cyclophane cations [4][5][6][7][8] and annulenium ions [9][10][11][12][13] we report here on stable-ion NMR spectroscopy and computational studies of novel mono-and dications from tri-, tetra-, and pentabridged cyclophanes