Aromaticity of highly bent benzene rings. An investigation by high-field deuterium NMR of [5]metacyclophane and model compounds

Abstract

1415of the constituent diabatic surfaces as well as that of the curve of intersection can be easily rationalized by using simple M O energy expressions. IV. ConclusionIn this paper we have analyzed the cycloaddition of two ethylene molecules in terms of diabatic surfaces. We have shown that this analysis provides a clear understanding of the index and origin of the various critical points. We have also shown that the behavior Registry No. Ethylene, 74-85-1.Abstract High field deuterium NMR provides a new and simple method for establishing the degree of aromaticity of a compound. The procedure is based on the determination of the magnetic susceptibility anisotropy from quadrupolar deuterium couplings of molecules in solution aligned by the magnetic field. The technique is illustrated for some deuterated dialkylbenzenes (Zd, 3e, 4e) and applied to [8,1 1-2H2] [5]metacyclophane (Id). Surprisingly, Id is found to be fully aromatic in spite of its strongly bent benzene ring.

Unusual reactivity of highly strained cyclophanes

“…In contrast, small cyclophanes are readily hydrogenated with Pd/H 2 at room temperature. 15 Thus, Li and Jones 15a reported that [7]paracyclophane is reduced to the stage of the (very) hyperstable olefin A; further reduction to B is extremely slow (Scheme 4 Not only in the thermal behavior, but also on irradiation, there appears to be a dichotomy between 1 and 2 (Scheme 5). As in the thermal reaction, the preferred process on irradiation of [5]metacyclophane is rearrangement to the ortho-isomer 7.…”

“…24 When both chlorines of 6 are replaced by bromine as in 51, the expected normal bromine-lithium exchange occurs, and it does so even twice with formation of 52; it is of interest that 52 was deuterated to give 53, the 2 H NMR spectrum of which furnished compelling evidence for the intact ring current of its bent benzene system. 7 The difference in behavior between 49 and 51 has been tentatively explained as follows: 16 with 49, obviously a single electron transfer/radical mechanism is operative leading to an intraannular radical attack at C-3 followed by radical ring closure to give 50, whereas with 51, first the easily accessible bromine at C-8 undergoes the normally very fast bromine-lithium exchange reaction with formation of 54; the negative charge associated with the organolithium functionality in 54 counteracts a rapid single electron transfer to the benzene ring so that the second bromine-lithium exchange has a chance to proceed as normal, resulting in the formation of 52. Scheme 16 Finally, two examples of base-induced reactivity from two different classes of [5]metacyclophanes may serve to demonstrate further the unforeseen but often rewarding chemistry one may encounter in this area.…”

Some Investigations on [6]Metacyclophanes

“…Not surprisingly, this bending enhances the reactivity, and in some cases it does so to a considerable extent. [3] Thus, [5]metacyclophanes (1, Scheme 1), which are the smallest metacyclophanes viable at room temperature, react with dienophiles in DielsϪAlder reactions at a rate that approaches and sometimes surpasses that of ordinary 1,3-dienes. However, this is not due to a localized cyclohexatriene structure, as physical data obtained from X-ray crystal structures, [4] NMR spectroscopy, [5] heats of hydrogenation, [6] and theoretical calculations [6,7] indicate that the benzene ring of 1 is fully delocalized and aromatic.…”

“…[3] Thus, [5]metacyclophanes (1, Scheme 1), which are the smallest metacyclophanes viable at room temperature, react with dienophiles in DielsϪAlder reactions at a rate that approaches and sometimes surpasses that of ordinary 1,3-dienes. However, this is not due to a localized cyclohexatriene structure, as physical data obtained from X-ray crystal structures, [4] NMR spectroscopy, [5] heats of hydrogenation, [6] and theoretical calculations [6,7] indicate that the benzene ring of 1 is fully delocalized and aromatic. Thus, their high reactivity is not a consequence of bond fixation, [8] but of the high strain present both in the bent aromatic rings and in the distorted bridges, which is released on reaction with dienophiles, [3] electrophiles, [3] phosphinidenes, [9] or carbenes.…”

“…Thus, their high reactivity is not a consequence of bond fixation, [8] but of the high strain present both in the bent aromatic rings and in the distorted bridges, which is released on reaction with dienophiles, [3] electrophiles, [3] phosphinidenes, [9] or carbenes. [10] Because of their extremely high reactivity, [5]metacyclophanes 1 have so far attracted most of our attention, but it seemed desirable to compare them with their next higher homologues, the less bent and less strained [6]metacyclophanes 2. In particular, our interest was roused by some seemingly contradictory reports in the literature concerning the DielsϪAlder reactivity of 2.…”