2,3-Bis(methylene)bicyclo[2.1.1]hexane and 3,4-bis(methylene)tricyclo[3.1.0.02,6]hexane. Interaction between a .pi. system and a cyclobutane or bicyclobutane moiety

Abstract

The He (I) photoelectron spectra of 2-bicyclo[2.1.1]hexene (l), 2,3-bis(methylene)bicyclo[2.1.1]hexane (3), and 3,4-bis(methylene)tricyclo[3.1.0.02~6]hexane (4) have been investigated. The assignment given is based on a ZDO model and semiempirical calculations. Together with the PE data of benzvalene (2), the reported data allow a comparison between 1-2 and 3-4. This yields a measure of the interactions between a cyclobutane or a bicyclobutane moiety and a double bond system within a ZDO model. The resonance integral found in the case of 1 and 3 amounts to -1.9 eV, that for 2 and 4, to -2.3 eV. The investigations furthermore reveal that the electronic factors which contribute to the higher reactivity of the bicyclobutane compounds amount to 5 kcal/mol. Highly strained hydrocarbons have always attracted the interest of organic and physical organic chemists.l An interesting question in this area of chemistry is the differences between the influences of a cyclobutane and a bicyclobutane moiety on the properties of a-systems linked together as shown in A or B below. For geometric reasons A B 1 2 3 4 5 6there are essentially three classes of compounds possible if we restrict ourselves to neutral closed shell cases; those where a stands for a two, a three, or a four carbon sp2 bridge. The most simple case for a bridge with two sp2 centers is 2-bicyclo[2.l.l]hexene (1) for model A and benzvalene (2) for model B. In the case of a butadiene bridge the ring system might be connected at the 1,4 positions or at the 2,3 positions of the butadiene fragment. The most simple examples for the latter case for A and B respectively are 2,3-bis(methylene)bicyclo[2.1. llhexane (3) and 3,4-bis(methylene)tricyclo[3.1.0.02~6]hexane (4).A comparison between the reactivity toward tetracyanoethylene of the tetramethyl derivatives of 3 and 4, 5 and 6, revealed an acceleration of a factor of 3 X lo' for 62 with respect to 5. This large difference has been rationalized by the difference in the HOMO energy of 5 and 6. Since both compounds exhibit different steric requirements due to different geometries of the methyl groups at C(5,6) and C(1,6), respectively, part of the large difference in reactivity observed might be due to steric factor^.^ This assumption is based on the observation that 3 reacts by a factor of 8.5 X lo2 faster with TCNE than 5.3 The importance of electronic effects can be deduced from the fact that 4 reacts 3 X lo2 faster than 3. 4 To contribute to this discussion on the influence of a cyclobutane ring as compared with that of a bicyclobutane

Magnetic circular dichroism of an unaromatic planar [8]annulene

“…This substantial body of work has been thoroughly reviewed relatively recently and need not be described here in any detail. We merely note that after it was realized that the double bond in [2.1.1]bicyclohexene is destabilized, demonstrated that its triple annelation to benzene strongly favors the Kekule structure in which the double bonds are exo to the annelated bicyclic systems, and shown that the concept is generalizable, synthetic, and computational work demonstrated that quadruple annelation of [2.1.1]bicyclohexene onto 1 to form the object of present interest, tetrakis(bicyclo[2.1.1]hexeno)[2’,3’:2,3][2”,3”:4,5][2’“,3’“:6,7][2”“,3”“:8,1]cycloocta‐1,3,5,7‐tetraene ( 2 ), produces an intriguing D 4 h planar cyclic π‐electron system with alternating bond lengths that is neither antiaromatic nor aromatic but non‐aromatic. According to a nomenclature devised for a classification of cyclic π‐electron systems according to the relation of their excited states to those of a perimeter rather than on ground state properties, it would be called unaromatic, ranking with species such as pentalene and biphenylene.…”

Experimental Assessment of the Effect of a Bicyclo[1.1.0]butane System in Strain‐Induced Localisation of Aromatic π‐Bonds

“…tions that convert norbornene (8), benzvalene (6), and bicyclo[2.1.1]hex-2-ene (4) with buta-1,3-diene into ethene as well as 2,3-bis(methylene)norbornane (9), 3,4-bis(methylene)-tricyclo[3.1.0.0 2,6 ]hexane (7), and 2,3-bis(methylene)bicyclo[2. formally substituted by units derived from indane, cyclopentene, 2,5-dihydrofuran, cyclohexane, or 9,10-dihydroanthracene, have been examined with respect to bond fixation. [5] A substantial enhancement of the strain energy in the annulated σ-bond system is conceivable by introduction of a bicyclo[1.1.0]butane skeleton as in 3 (Scheme 1).…”

“…[6] Scheme 2. Heats of the isodesmic reactions of cisoid-buta-1,3-diene with bicyclo[2.1.1]hex-2-ene (4), benzvalene (6), norbornene (8), and cyclopentene with formation of ethene and the 2,3-bridged buta-1,3-dienes 5, 7, 9, or 1,2-bis(methylene)cyclopentane, respectively; the upper values were obtained by Jorgensen and Borden (refs. [6,7] ) employing extended Hückel calculations; the lower values have been determined in the present work by using a CCSD(T)/ccpVDZ procedure; the energetics of the conversion of cyclopentene were obtained by using the DFT method Computational methods have improved dramatically since the 1970s, which is why we have reanalysed the reactions of Scheme 2 performing DFT [B3LYP/6-31G(d)] and Coupled Cluster calculations [CCSD(T)/cc-pVDZ].…”

“…Within a ZDO model, the resonance integrals β amount to Ϫ2.3 (6, 7) and Ϫ1.9 eV (4, 5), respectively. [8] Obviously, cyclobutane must be much more strongly biased, as compared to bicyclobutane, toward bridging butadiene across the C2ϪC3 bond and against bridging ethene and, thus, can overcompensate the effect of the resonance integrals, resulting in the greatest exothermicity of its isodesmic reaction in Scheme 2.…”

“…Since the CϪCϪC bond angles formed by the bridged aromatic bond and the adjacent carbon atoms of the σ-system increase from 102.6°via 105.1°and 106.6°to 111.0°in the series 13Ϫ16, the changes of bond lengths correspond to Stanger's prediction. On the other hand, an intense interaction of the pseudo-π-orbitals of the σ-systems with the aromatic π-system is suggested by the photoelectron spectra of 4Ϫ7 [8] and 11 [16] as well as the energies of the singlet excited states of 11. [17] …”