2.3- divinylcyclopentane

C-C bond formation mediated by silane.6,6a 6f With respect to the development of intramolecular variants, these seminal studies lay fallow until 1990, at which point the palladium- and nickel-catalyzed reductive cyclization of tethered 1,3-dienes mediated by silane was disclosed. As demonstrated by the hydrosilylation-cyclization of 1,3,8,10-tetraene 21a, the /rarcr-divinylcyclopentanes 21b and 21c are produced in excellent yield, but with modest stereoselectivity.46 Bu3SnH was shown to participate in an analogous cyclization.46 Isotopic labeling and crossover experiments provide evidence against a mechanism involving initial diene hydrosilylation. Rather, the collective data corroborate a mechanism involving oxidative coupling of the diene followed by silane activation (Scheme 15). 

In a related cobalt-catalyzed transformation, 1,3-dienes tethered to ally lie ethers engage in Et2AlCl-mediated reductive cyclization.463 Exposure of benzylic ether 22a to Co(acac)3-PPh3 in the presence of Et2AlCl results in formation of divinylcyclopentane 22b with excellent /raar-diastereoselectivity. As demonstrated by the conversion of 23a to 23b, this method is also applicable to the stereocontrolled formation of six-membered rings (Scheme 16). 

Allylic chloride survives the (—)-sparteine-mediated deprotonation of allylic carbamates by w-BuLi. When the ( , )-9-chloro-2,7-nonadienyl carbamate ( , )-319 was treated with two equivalents of n-BuLi/(—)-sparteine (11) at —90°C in toluene, the cis-divinylcyclopentane 321 was formed with an enantiomeric ratio of 90 10 (equation 85). The epimerization to form (/ )-320 (which leads to ent-321) is much slower than the cycloalkylation step under the reaction conditions. 321 was converted in few steps into (- -)-(3/ ,4/ )-dihydromultiliden . ... 

A study of the stereochemistry of the cyclization (197) -> (198) + (200) showed a kinetically controlled 2 1 preference for the formation of the rran5-divinylcyclopentane (200) (independent of the ene /Z configuration, cf. Scheme 48) which increased to 9 1 when more catalyst and a longer reaction time were employed (Scheme 41, Table 12). This useful predominance of the thermodynamically more stable... 

The normal equilibrium can be reversed, of course, by using the oxy Cope rearrangement of a 1,2-di-vinylcyclopentanol one example was shown in Scheme 7 (62 - 63) in which irreversible tautomeriza-tion of the enol gave a fused cyclononenone. Two additional examples are shown in equations (67) and (68), the latter a key step in a synthesis of phoracantholide. ° The fused divinylcyclopentane shown in equation (69) undergoes anionic oxy-Cope rearrangement to a bridged cyclononenone. 

In this series, as well as the divinylcyclopentanes, the equilibria can be shifted completely in favor of the 10-membered ring by using the irreversible oxy-Cope rearrangement of 1,2-divinylcyclohexanols the prototype (64 65) was shown in Scheme 7. This reaction type has found wide application in the... 

Ring strain effects may also play an important role in the positioning of the equilibrium in Cope rearrangements. Whereas in the equilibrium of strain-free 1,2-divinylcyclopentane and 1,2-di-vinylcyclohexane derivatives the substrate side is favored, the equilibrium of bridged bicyclic systems and particularly that of cyclopropane- and cyclobutane-derived substrates usually lies more or less completely on the less-strained product side, as shown by the following examples a) cyclopropane derivatives formation of l756. 2757 375s 759, 4760. and 5761. 

The position of the Cope equilibrium in 3,4 rearrangements of 1,2-divinylcycloalkanes largely depends on the ring size. In the case of 1,2-divinylcyclopropanes and cyclobutanes, the equilibrium lies on the side of the enlarged rings. For 1,2-divinylcyclopentanes, cyclohexanes, and cycloheptanes, the equilibrium of the Cope rearrangement is reversed, but use of the irreversible... 

In contrast to the normal Cope rearrangement, the irreversible oxy-Cope rearrangement allows cyclononene derivatives to be synthesized from 1.2-divinylcyclopentanes, e.g., formation of ( >5922 and 7923. 

As already discussed for 1,2-divinylcyclopentanes, formation of the 10-membered ring can be forced by using the irreversible oxy-Cope rearrangement. 

Polynorbomene is more difficult to degrade. Ethene in the presence of a tungsten carbene catalyst does bring about some reduction in MW (factor of five) but no 1,3-divinylcyclopentane is detected (Wagener 1991a). This is another example of the protection against metathesis offered by substituents on the carbons adjacent to the double bonds, in this case in the form of cyclopentane rings. 

In cyclization of conjugated dienes, typically butadiene, coordination of two molecules of butadiene gives rise to the bis-jr-allyl complexe 12. The distance between the terminals of two molecules of butadiene becomes closer by 7r-coordination to Pd(0), and the oxidative cyclization is thought to generate either the l-pallada-2,5-divinylcyclopentane 13 or l-pallada-3,7-cyclononadiene 14. 

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