Paracyclophanes, strain

Similar behaviour has been observed in the photoreaction of methyl a-cyano-4-[2-(2-pyridyl)ethenyl]cinnamate (7 OMe) crystals in which the yield of [2.2] paracyclophane reached 65% on irradiation at — 78°C (see Scheme 10 p. 153) (Hasegawa et al., 1989b). From the crystal structure analysis of the same type of [2.2] paracyclophane, which is topochemically derived from alkyl a-cyano-4-[2-(4-pyridyl)ethenyl]cinnamate crystals, a highly strained molecular shape is confirmed in which two phenylene rings are severely bent (Maekawa et al., 1991b). 

These results are significant in understanding the photochemical behaviour of organic crystals at low temperature and in the development of a new synthetic route to highly strained [2.2]paracyclophane derivatives. 

The crystal of 2 OPr recrystallized from EtOH/H20 solution, and the mixed crystal of the same ethyl and propyl cinnamate derivatives (2 OEt and 2 OPr), on photoirradiation for 2h at room temperature with a 500 W super-high-pressure Hg lamp, afforded the highly strained tricyclic [2.2] paracyclophane (2 OEt-2 OPr-cyclo) crystal quantitatively (Maekawa et ai, 1991b). A crystal structure analysis was carried out of a single crystal of the complex of 2 OEt-2 OPr-cyclo with HFIP (recrystallization solvent) in a 1 2 molar ratio. Fig. 13 shows the molecular structure of 2 OEt-2 OPr-cyclo viewed along the phenylene planes. The short non-bonded distances and deformation of the benzene rings, as seen in Fig. 13, are common to those of [2.2] paracyclophanes, as previously reported (Hope et ai, 1972a,b). 

Topochemical [24-2] photoreactions of diolehn crystals has been reviewed. The reactions clearly depart from typical solution chemistry crystal-lattice control offers a unique synthetic route into photodegradable polymers, highly strained [24-2] paracyclophanes, stereoregular polymers, and absolute asymmetric synthesis. However, achieving the desired type of crystal... 

A variety of other highly-strained electron-rich donors also form colored complexes (similar to homobenzvalene) with various electron acceptors, which readily undergo thermal cycloadditions (with concomitant bleaching of the color).209 For example, Tsuji et al.210 reported that dispiro[2.2.2.2]deca-4,9-diene (DDD), with an unusually low ionization potential of 7.5 eV,211 readily forms a colored charge-transfer complex with tetracyanoquinodimethane (TCNQ). The [DDD, TCNQ] charge-transfer complex undergoes a thermal cycloaddition to [3,3]paracyclophane in excellent yield, i.e.,... 

It is noteworthy that intramolecular steric interactions still distort the molecule, albeit less so, in the case of [3.3]paracyclophane (3) lx). Here the strain energy is only 7 kcal/mol, mostly contributed by the... 

The electronic spectra of the [2.2]paracyclophanes provide valuable information regarding the extent and mechanisms of transannular electronic interactions in strained 7t-electron systems. A rigid system like the [2.2]paracyclophane molecule is of great value as a model for checking theoretical data with a view to interpretating U V spectra of sterically hindered molecules ( overcrowded compounds ). 

The ion 96 carries a positive charge which can be distributed over both rings formation of 96 involves compensation of bond angle strain, leading to facilitation of n-n repulsion strain between the neutral benzene nuclei in the starting compound. The twisting of the system in 96 corresponds to the somewhat twisted crystal structure of the [2.2]-paracyclophane molecule. 

The high strain energy of [2.2]paracyclophane (see Section 2.1.) facilitates ring-opening of the molecule via cleavage of the benzyl-benzyl bonds. Pyrolysis at 400 °C affords p,p -dimethylbibenzyl (155) and p,p -di-methylstilbene 109h At 600 °C, p-xylylene (156) is formed it polymerizes spontaneously to the linear poly-p-xylylene (10) on condensation 110>. 

Ultraviolet spectra of aromatic systems are often used to probe strain-induced perturbations in the K-system. Out-of-plane deformations of the benzene ring shift the 260 nm band to the red and increase its intensity. Classical examples are [2.2]paracyclophane (286 nm) and Pascal s twisted benzenes. The for a given transition reveals changes in the energy of the filled/unfilled gap, whereas the extinction coefficient reveals the efficiency of the transition. 

Obviously carboxy derivatives such as 11-19 are simple chiral structures suitable for optical resolutions through diastereomeric salts. For this purpose carboxylic groups have been introduced into [10]- and [8]paracyclophane either by chloro-methylation and oxidation of the carboxaldehydes obtained thereof 39,44) or by lithiation and subsequent carboxylation40). Electrophilic substitution of strained paracyclophanes is not advisable since it may initiate rearrangement to the more stable metacyclophanes. Carboxy[7]paracyclophane (72) was first prepared in 1972 by ring contraction of a diazoketone derived from 4-carboxy[8]paracyclophane (75) 45). 

A reversible vinylidene insertion was proposed to explain die formation of (55) on flash vacuum pyrolysis of the anthracene derivative (56) at 1100 °C.65 The expected loss of HC1 followed by 1,2-H shift and 1,5-CH insertion of the resulting vinylidene species would give rise to the strained paracyclophane (57). This is proposed to ring open to the alternative alkylidene (58) before proceeding to the observed product (55). 

The first observation of the thermal transformation of a strained paracyclophane into its Dewar isomer has been reported.56 Hexahalobispropellane (41), on treatment with potassium t-butoxide, has been shown to afford the phenol (43) along with (44). The formation of both these compounds has been rationalized57 by invoking the intermediacy of (42) (see Scheme 10). 

The [10 11] methano[3 2]paracyclophane system (108) [229] appears less strained than 107 yet more restricted than diphenylcyclopropane. The CIDNP effects observed are closer to that of ds-diphenylcyclopropane than to that of 107. Obviously, a large fraction of the strain inherent in 107 is relieved by substituting a trimethylene bridge for one of the cyclopropane moieties [229],... 

Paracyclophane-1,9-diene is an interesting cyclic diene monomer with a very strained ring. It has been easily polymerised with a tungsten carbene complex [148] to low molecular weight poly(p-phenylene vinylene) [148] ... 

Normally, a discussion of strained cyclic alkynes (which is the focus of this section) would not include rings with more than seven or eight members.1-3 However, a notable exception is [2.2]paracyclophane-l-yne (277), formally... 

An X-ray crystal structure of 282 was obtained.108 Significant parameters are included in Table VIII. Particularly notable is the large bend-back angle of 58-59°, which is consistent with the presence of the highly strained [2.2]paracyclophane-l -yne moiety.109... 

A stereospecific synthesis of [6]paracyclophane-8-carboxylic acid (2) was reported, and its CD spectrum was compared with that of [8]paracyclophane-10-carboxylic acid (3, R = CO2H) [17]. The former afforded a pair of bisignate CD patterns in the 200-400 nm region, while only a pair of Cotton effect peaks of the same sign were observed for the latter. The CD intensity (IAsI) of 2 was in the order of 10 M 1 cm4 for the main band at 220 nm. [6]Paracyclophane (2) is one of the most strained cyclophanes, and its boat-type deformation of the benzene ring led to the red-shift of ca. 30 nm in its UV and CD spectra. The theoretical spectrum obtained at the TD-DFT-B3-LYP/TZV2P level well reproduced the experimental spectrum except for the sign of the weak 1Lh band around 320 nm [8]. The CD spectrum of structurally similar methyl 3,6-hexanooxepin-4-carboxylate was also reported [16]. 

Further approaching and deforming the benzene rings by additional short bridges ought to enhance both of the peculiarities of [2.2]paracyclophane. The recently synthesized highly strained, multiply bridged phane hydrocarbons (e.g. 2,15,16,... 

The synthesis and properties of the highly symmetrical hydrocarbon phanes 2 and 6 have been reported in 197018 X-ray data of 619 indicate a molecular strain exceeding that of [2.2]paracyclophane-diene (7). The latter was estimated by Gantzel and Trueblood to be about 39 kcal/mole (163.0 kJ/mole)20). 

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