Macrocycles, strained

Stability towards heating, irradiation and exposure to air was also reported for [n]pericyclinosilanes 70-72 [20, 21], and even the highly strained 74 [24]. But, in contrast to the hydrocarbons, the pericyclinosilanes did not react with bromine [21 ]. On the other hand, the presence of the silicon atoms brings about a specific reactivity of these macrocycles making them labile and capable of changes in ring size. Thus, when a mixture of compounds 70-72 a (n = 6) was... 

Apparently, the oxidative acetylene coupling can overcome a drastic strain increase as demonstrated by the highly efficient cyclization of octamethyltetra-deca-l,4,7,10,13-pentayne 36 and its perspirocyclopropanated analogue 61 to the corresponding 14-membered macrocycles 181 (67%) and 62 (45%), respectively (Scheme 34) [18]. 

X-ray structure analysis showed that macrocycle 57 was essentially planar, with the twist angle of the benzene rings from the plane of the macrocycle being less than 2°. Most of the strain was seemingly contained in the triple bonds, as these were bent from linearity by 10.1° to 12.3°. Despite its strained nature, the macrocycle showed remarkable stability. Decomposition occurred above 300°C on attempted melting. No reaction was observed between 57 and cyclopenta-diene at room temperature. 

An X-ray structure analysis of 74 (R=C4Hg) revealed that the unsaturated portion of the molecule was planar, with the angles between adjacent acetylenic bonds deviating by 13 -15° from 180°, the value for a strain-free molecule. Since the connection of the alkyne moieties to the aromatic rings was only shifted slightly (2-3°), distortion of the acetylene linkages appears as the major source of instability in these macrocycles. 

Recrystallization of 76 (R=H) from CH2CI2 provided crystals adequate for X-ray structural determination. The molecule was found to be saddle shaped with a phenyl ring at each vertex and nadir. The alkynyl bonds were found to be essentially linear and to possess a mean length of 1.194 A, typical for the length of triple bonds in free butadiyne. Although 76 is a dehydrobenzoannulene possessing a 4n TT-electron circuit, the nonplanarity of the macrocycle alleviated much of the strain associated with a flat structure and thus precluded the possibility of anti-aromatic ring currents. 

In late 1995, a team led by Vollhardt and Youngs reported their work on the strained PAM/PDM hybrid 80 [55]. Whereas the synthesis of 80 was not remarkable [Eq. (2)1, the solid-state behavior of the molecule was. X-ray crystallography revealed that the macrocycle was moderately strained, with the monoynes bent inward toward the center of the macrocycle by 3.9 -11.5° and the diyne unit bent outward by 8.6-11.2°. More importantly, crystal packing revealed that the diyne moieties were aligned in the prerequisite fashion for topochemical diacetylene polymerization to occur. Indeed, irradiation of crystals of 80 produced a violet... 

With sp bond angles calculated to be around 162°, macrocycle 131 would be highly strained and was therefore expected to be quite reactive [79]. The octa-cobalt complex 132, on the other hand, should be readily isolable. Indeed, 132 was prepared easily from 133 in five steps, and was isolated as stable, deep maroon crystals (Scheme 30). All spectroscopic data supported formation of the strain-free dimeric structure. Unfortunately, all attempts to liberate 132 from the cobalt units led only to insoluble materials. Diederich et al. observed similar problems when trying to prepare the cyclocarbons [5c]. Whether the failure to prepare these two classes of macrocycles is due to the extreme reactivity of the distorted polyyne moiety or to the lack of a viable synthetic route is not certain. Thus, isolation and characterization of smaller bent hexatriyne- and octatetrayne-containing systems is an important goal that should help answer these questions. 

Dohm, J., Vdgtle, F. Synthesis of (Strained) Macrocycles by Sulfone Pyrolysis. 161,69-106 (1991). 

Finally, it is important to mention that there are other related publications in which porphyrin macrocycles are not directly used as dipolarophiles but are transformed into new derivatives that can react with carbonyl ylides via ACE (alkene cyclobutene epoxide) reactions. This idea arose in 1997, when Russell and co-workers found that fused ester-activated cyclobutene epoxides 86 can be ring-opened to give carbonyl ylides 87, and that these can be trapped stereospecifically by ring-strained alicyclic dipolarophiles, such as 2,5-norbomadiene, to form hetero-bridged norbomanes 88 in good yields, through ACE transformations (Scheme 31) <97CC1023>. 

Macrocycles may also promote the formation of less common coordination geometries for particular metal ions because of increased macrocyclic ring strain on coordination. Such an effect is illustrated by the variation in the structures of the nickel complexes of the 14-, 16-, 18-, and 20-mem-bered tropocoronand macrocycles of type (14) (Imajo, Nakanishi,... 

The coordinated macrocycle readily reacts with alkoxide ions to yield products of type (71) (Taylor, Urbach Busch, 1969). In so doing additional flexibility is imparted to the ring which may reduce ring strain and, in part, provide a driving force for the reaction. Thus the coordinated imine carbons appear predisposed to attack by such nucleophiles. Based on this knowledge, elegant template syntheses of three-dimensional derivatives have been performed. The syntheses involved the reaction of [M(taab)]2+ (M = Ni, Cu) with the dialkoxide ions derived from bis(2-hydroxyethyl)sulphide or bis(2-hydroxyethyl)methylamine (Katovic, Taylor Busch, 1969). The products were demonstrated to be monomeric square-pyramidal complexes of type (72). The condensation... 

In another study, Co(m) has been shown to be favoured over Co(n) as the ligand strain energy decreases in the complexes of the N4-donor aliphatic macrocycles (278 X = NH). Thus, for the 13- to 16-membered ring complexes of type /ra s-[CoCl2L]2+, it is the complex of cyclam (14-membered) which most promotes the Co(n)/Co(m) transformation (Hung et al., 1977). 

Singlet oxygen reacts with electron rich or highly strained alkenes to form 1,2-dioxetanes. These four-membered ring peroxides decompose on warming to two carbonyl compounds (or moieties), usually with appearance of light emission (chemiluminescence). The macrocyclic bis-lactone in (6.17)608>, a musk fragrance, has been synthesized via such a sequence. 

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