Chemistry of Benzocyclopropenes. ¹H‐and ¹³C‐NMR. Spectra of 2, 5‐dideuterio‐1‐fluorobenzocyclopropenium ion

Abstract

Preparation of 1, 1‐difluorobenzocyclopropene (4) and of its 2, 5‐ and 3, 4‐dideuterio derivatives 4a and 4b is reported. Upon ionization in cold fluorosulfonic acid, 4 affords 1‐fluorobenzocyclopropenium ion (6). 1H‐ and 13C‐NMR. spectra of 4 and 6 are assigned on the basis of the data for the specifically deuteriumlabelled compounds 4a and 6a. Hydrolysis of 6a leads to 2, 5‐dideuteriobenzoic acid (7a).

Preparation of 3,4‐Fused Cyclopropabenzenes and Cyclopropabenzenyl Cations

“…The "C,H coupling constants at C(2)/C(5) vary in the series from 4a (170 Hz) and 4c ( I 70) to 4d (172) and 4e (175 Hz); the value of 1 is 169 Hz (251. By comparison, the 13CC,H coupling constant of the corresponding symmetrical dicyclobuta[a,d]benzene is 159.9 Hz [26] at the aromatic C-H bond, while the corresponding values for cyclopropabenzene are 168.5 (C(2)/C(5)) and 159.0 (C(3)/C(4)), which are shifted to 175 and 161 HL in 4 [24]. The change upon difluoro substitution at C(l) in 4' is thus about the same as that in going from 1 to 4e.…”

“…-The 'H-and W-NMR spectra of 3 and 4 were readily assigned upon comparison with the spectra of unsubstituted and 2,s-disubstituted compounds [4] [24]. Details are summarized in Tables 3 and 4 (Exper.…”

Allylic Reactions of Benzocyclopropenes. Discrimination of halogen substituents in 1, 1‐Dihalogenobenzocyclopropenes

“…The I3C-NMR. spectrum of 16 is very similar to that of 1 [4]. The signal at 115.7 ppm is assigned to C(2) and C ( 5 ) , while the singlet at lower field (134.4 ppm) corresponds to C (3) and C (4).…”

“…The signal for the fluoro substituent was found as triplet (JH,F = 2.0 Hz) at 97.5 ppm downfield from C6F6, in good agreement with the chemical shift observed for 1Oc. By analogy with 1,l-difluorobenzocyclopropene (1) the H, F-coupling is assigned to the protons in positions 2 and 5 [4]. The I3C-NMR.…”

“…Thus the simplest benzocyclopropenium ion 2 known to-date has been obtained by ionization of 1,l-difluorobenzocyclopropene (1) in fluorosulfonic acid [3] [4]. Attempts to generate the parent ion 3 by hydride transfer from benzocyclopropene (4) to triphenylmethyl fluoroborate failed [5].…”