Two-phase reactions in the chemistry of carbanions and halocarbenes. A useful tool in organic synthesis
Abstract
Synthese mehrfach cyclopropyl‐substituierter Benzole2)
Fur 1,3,5-Trivinyl-, -isopropenyl- und -athinylbenzol (3a,b,5) werden einfache Laborsynthesen beschrieben. Cyclopropanierung von Di- und Trialkenylbenzolen nach Simmons-Smith sowie mit Chloroform/Kalium-tert-butylat gelingt nur bei Isopropenylverbindungen mit befriedigenden Ausbeuten; bei der Umsetzung mehrfach vinyl-substituierter Aromaten entstehen unter diesen Bedingungen uberwiegend Polymere. Bei der Cyclopropanierung mit Chloroform/Natronlauge/Triathyl(benzyl)ammoniumchlorid erhalt man in allen Fallen hervorragende Ausbeuten an (2,2-Dichlorcyclopropyl)benzolen, die durch reduktive Enthalogenierung in die mehrfach cyclopropyl-substituierten Benzole (6a, b; 7a–d) ubergefuhrt werden konnen. Einige dieser Verbindungen lassen sich auch uber 1,3-Eliminierung aus (1,3-Dibrompropyl)benzolen gewinnen. 
 
 
 
Synthesis of Polycyclopropyl-substituted Benzenes2) 
 
 
 
Simple laboratory syntheses are described for 1,3,5-trivinyl-, -isopropenyl-, and -ethynylbenzene (3a, b, 5). Introduction of the cyclopropane moiety by the Simmons-Smith reaction or with chloroform/potassium tert-butoxide is feasible only in the case of isopropenyl benzenes; with polyvinyl-substituted arenes, only polymeric material is formed under these conditions. With chloroform/aqueous NaOH/triethyl(benzyl)ammonium chloride, (2,2-di-chlorocyclopropyl)benzenes are obtained in excellent yield in all cases. By reductive dehalogenation of these compounds, di- and tricyclopropyl benzenes (6a, b; 7a–d) are formed. 6a and 7a may also be synthesized via 1,3-elimination from (1,3-dibromopropyl)benzenes.
Diagnosis of the σ-, π- and (σ+π)-Aromaticity by the Shape of the NICSzz-Scan Curves and Symmetry-Based Selection Rules
The NICSzz-scan curves of aromatic organic, inorganic and “all-metal” molecules in conjunction with symmetry-based selection rules provide efficient diagnostic tools of the σ-, π- and/or double (σ + π)-aromaticity. The NICSzz-scan curves of σ-aromatic molecules are symmetric around the z-axis, having half-band widths approximately less than 3 A with the induced diatropic ring current arising from Tx,y-allowed transitions involving exclusively σ-type molecular orbitals. Broad NICSzz-scan curves (half-band width approximately higher than 3 A) characterize double (σ + π)-aromaticity, the chief contribution to the induced diatropic ring current arising from Tx,y-allowed transitions involving both σ- and π-type molecular orbitals. NICSzz-scan curves exhibiting two maxima at a certain distance above and below the molecular plane are typical for (σ + π)-aromatics where the π-diatropic ring current overwhelms the σ-type one. In the absence of any contribution from the σ-diatropic ring current, the NICSzz(0) value is close to zero and the molecule exhibits pure π-aromaticity.