A 6π + 6π Potentially Antiaromatic Zwitterion Preferred to a Quinoidal Structure: Its Reactivity Toward Organic and Inorganic Reagents
摘要:
A straightforward synthesis of the zwitterionic benzoquinonemonoimine 8 is reported. This molecule is a rare example of a zwitterion being more stable than its canonical forms. It is shown that 8 is best described as constituted of two chemically connected but electronically not conjugated 6 pi electron subunits. Its reactivity with electrophiles such as H+, CH3+, and metal salts leads to the synthesis of new 12,7 electron molecules 12 (H+), 14 (CH3+), and 20 (Pd2+), respectively, in which one or both 6 pi electron subsystems localize into an alternation of single and double bonds, as established by X-ray diffraction. The acidity of the N-H protons of 8 can be modulated by an external reagent. Dependent on the electrophile used, the control of the pi system delocalization becomes possible. When the electrophile simply adds to the zwitterion as in 12, 14, or 15, there is no more negative charge to be delocalized and only the positive charge remains delocalized between the nitrogen atoms. Furthermore, when a reaction with the electrophilic reagent results in deprotonation, as in 17-21, there remains no charge in the system to be delocalized. DFT calculations were performed on models of 8, 12, 14, 20, and on other related zwitterions 9 and 10 in order to examine the influence of the fused cycles on the charge separation and on the singlet-triplet energy gap. An effect of the nitrogen substituents in 8 is to significantly stabilize the singlet state. The dipole moment of 8 was measured to be 9.7 D in dichloromethane, in agreement with calculated values. The new ligands and complexes described in this article constitute new classes of compounds relevant to many areas of chemistry.
Unprecedented zwitterion in quinonoid chemistryElectronic supplementary information (ESI) available: spectroscopic data for 4 and 6 and an ORTEP view of the structure of 4. See http://www.rsc.org/suppdata/cc/b1/b107828n/.
作者:Olivier Siri、Pierre Braunstein
DOI:10.1039/b107828n
日期:2002.1.30
The first 12π-electron zwitterionic structure in quinonoid chemistry is described with the N,N,O,O -molecule 6 in which the positive charge is π-delocalized between the nitrogen atoms and the negative charge between the oxygen atoms; depending on the crystallization solvent, a 1D-tape-like H-bonded network can be generated in the solid-state.
N,N,O,O -分子 6 描述了醌类化学中的第一个 12π 电子齐聚物结构,其中正电荷在氮原子之间π-分散,负电荷在氧原子之间π-分散;根据结晶溶剂的不同,可在固态中生成 1D 塔式 H 键网络。