Indoles. Part 1 (Chemistry of Heterocyclic Compounds, Volume by William J. Houlihan

By William J. Houlihan

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Extra resources for Indoles. Part 1 (Chemistry of Heterocyclic Compounds, Volume 25a)

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Nuclear Magrietic Resonance Proton magnetic resonance (pmr) spectra of indoles have been thoroughly investigated, and interpreted in considerable detail. Since the indole nucleus bears relatively few hydrogens, and their environment changes significantly according to the position on the nucleus, it is frequently possible to assign every proton in the spectrum of a simple indole and to determine the positions of substitution on the nucleus. Subtle effects in longe-range coupling and in the solvent and concentration dependence of chemical shifts have been carefully studied.

38 The peak near 280 m p is shifted to around 295 mp and an additional peak appears at 310-320 mp. E). The chromophore of indoline is essentially that of an alkyl aniline, and it possesses a strong electron transfer (8) band at 254 m p (E 25,000) characteristic of this system. ~' a-Methyleneindoline (27) is isoelectronic with indole, but has a different chromophore. '~ Both oxindoles and N-acylindolines have the chromophore of the alkyi acetanilide system. However, the acylindolines may possess steric inhibition of resonance, which will cause the spectrum to more nearly resemble that of a xylene.

The two main uncharged contributors and nine contributors with separation of charge are depicted in Scheme 14. These charged contributors bear the 91 H H QJ92 I l H it 94 93 /J I H QTI -0g H -0707 96 97 I H 99 Scheme 14. 95 - I 98 ii I 1 H 100 H 101 Resonance contributors to thc indole hybrid structure. positive charge on nitrogen, reflecting the known electron-donating properties of the indole nitrogen. Not shown are contributors bearing positive charge on Chapter I 56 carbon and contributors with interactions across the rings (Dewar structures).

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