Betweenanenes

As previously mentioned, double bonds in relatively small rings must be cis. A stable trans double bond first appears in an eight-membered ring trans-cyclooctene, p. 134), though the transient existence of tran -cyclohexene and cycloheptene has been demonstrated. Above 11 members, the trans isomer is more stable than the cis. It has proved possible to prepare compounds in which a trans double bond is shared by two cycloalkene rings (e.g., 104). Such compounds have been called [m.n]betweenanenes, and several have been prepared with m and n values from 8 to 26. The double bonds of the smaller betweenanenes, as might be... 

Among a variety of compounds whose molecular characteristics fall into this category, our current interests have further limited our discussion to the doublebond systems which may conveniently be classified as follows 1. chiral ( )-cyclo-alkenes, 2. chiral anti-Bredt rule compounds, 3. trans-doubly bridged ethylenes ( betweenanenes ), 4. bridged allenes, and 5. overcrowded olefins. 

In 1977, almost simultaneously, groups from Osaka University 54a a) and Northwestern University S5a 8) independently reported their respective syntheses of 61a and 61b, which, according to Marshall s proposal55a), can be called [10.8] and [10.10] betweenanenes. 

The isolated [8.8]betweenanene (D2-bicyclo[8.8.0]octadec-l(10)-ene) (61c), bp 125-127 °C/0.1 mmHg, was found to be converted back to the (Z)-(E) isomeric mixture by UV irradiation, and was characterized again by its inertness toward dichlorocarbene and catalytic hydrogenation. 

As the first step in extending this novel and facile betweenanene synthesis to the [lO.lOjseries of compounds, they prepared the (Z)[10.10] precursor 73 from 1,2-cyclododecanedione (65), using McMurry s ring closure of the dialdehyde 72b as the key step. 

Addition of 4-pentenyllithium to the dione 65 gave the ds-diol 71 which was converted to the (Z)-l,2-disubstituted cyclododecene 72a. Hydroboration-oxidation and chromium trioxide oxidation provided the dialdehyde 72b whose McMurry ring closure, followed by partial catalytic hydrogenation gave the (Z)[10.10] precursor 73. Treatment of this (Z)-olefin 73 with HzS04-Ac0H in benzene was reported to effect conversion into [10.10]betweenanene (61b) of 95% purity and high yield. 

Marshall s original but rather involved [10.10]betweenanene synthesis has been improved in their second approach which reportedly presented a sixfold improvement in overall yield55d). 

Finally, a brief comment on the approach 58a,b) of a group at Bologna University to thia-betweenanenes will be presented here to conclude this section. 

Although various aspects of the chemical differences between (Z)[m.n] precursors and [m.n]betweenanenes have been already presented in the preceding sections, this section will summarize these characteristics and add some data, taken mainly from Marshall and Black s short communication 55e). 

The inertness of [10.8]betweenanene toward dichlorocarbene was the first reaction 54a) observed which distinguishes between (Z)[10.8] precursor and ( )[10.8] isomer, and this distinction was further observed in the [8.8]series of compounds 54b). 

Inertness of betweenanenes toward catalytic hydrogenation was first observed in [10.8] and [8.8]betweenanenes 54a,b) and confirmed in [11.11] compound 57). 

Hydroboration with excess BH3 in THF followed by oxidation with alkaline H202 converts the (Z)-99 (n = 20, 22, and 24) into the (Z)-fused alcohols 100, and betweenanenes (101) (n = 20, 22, and 24) into the ( )-fused alcohols 102 55e). The reaction rates were found to be slightly affected by ring size, but almost no effect was observed between cis and trans isomers. 

The (Z)-(E) discrimination with this reagent was most pronouncedly demonstrated in [10.10]series 55e), where (Z)[10.10] reacted within 1 min, while ( )[10.10] was recovered unchanged after three weeks. Although this difference in reactivity was further observed in the [20.10] and [22.10] series but to a lesser extent, take out no difference in reactivity between (Z)[26.10] precursor and [26.10]betweenanene was observed. 

This oxidation converts the (Z)- and ( )[m.n] compounds into allylic alcohols after treatment with LiAlH to reduce the initially formed hydroperoxides. [10.10]Betweenanene was recovered unchanged after 20 h the reaction of (Z)[26.10] precursor was completed within 1 h while that of (E)[26.10] required 6 h. 

All [m.n]betweenanenes so far prepared in Marshall s laboratory were found to exhibit a wine-red color reaction with tetracyanoethylene5Sg), while the corresponding (Z) isomers did not. 

IV.3 Preparation and Chiroptical Properties of Optically Active Betweenanenes... 

The Osaka University group 54c d was the first to announce the preparation of betweenanene in an optically active modification and the assignment of the absolute configuration. 

Since Marshall s strategy55 in preparing optically active betweenanenes was asymmetric epoxidation by (-t-)-monoperoxycamphoric acid, this approach would be solely effective in [m.n]betweenanenes where the m ring is large enough to sway away, so exposing the buried unsaturated center. 

Epoxidation of [22.10] (105b) and [26.10]betweenanenes (105c) with this chiral reagent was quenched at 50 % conversion, and the optically active [22.10] and [26.10] betweenanenes, isolated by SiOz gel chromatography, exhibited [a]n2 —32.4° (hexane) and —24.7° (hexane) respectively60. ... 

Both these optically active betweenanenes exhibited a (—)-Cotton effect47 at 195 nm, indicating their (R)-configuration. Hydroboration-oxidation of these specimens gave the ( )-fused alcohols 106 whose respective optical purities (7.6 and 6.0%) were estimated by means of the NMR chiral shift reagent method. 

Preparation of the almost optically pure (-f-)-enantiomer of [ 10.10]betweenanene (61b) as well as its (Reconfiguration were reported in Marshall s recent paper 55f). 

A hypothetical doubly-bridged allene 123 of D2 symmetry was first formulated in Cahn, Ingold, and Prelog s classical paper 11 > in which they summarized their novel proposal for specifying various molecular chiralities. Despite the close structural resemblance between 123 and [m.m]betweenanene, Cahn, Ingold and Prelog s rules shows that the chirality of these two classes of compounds are to be specified as axial and planar respectively. 

These rotatory strengths, when combined with the reported optical purities, give [a]aba 426° and 412° for [22.10] and [26.10] betweenanenes, respectively. Since these values automatically indicate their ridiculously high [M],bs ca. 2000, the [a]D apparently are to be reduced to 1/10 th of the reported values. 

The so-called [m][n]betweenanenes 136 and 137 (for a review see Ref. 18)) are ( )-cycloalkenes with the double bond situated between the two ( )-bridges. With two bridges of equal lengths (m = n) they have Z)2-symmetry similar to the topologically related [m][n]paracyclophanes (see Section 2.6.1). 

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