Ring size effects alkenes

An interesting extension of this reaction is shown in the asymmetric kinetic resolution of cyclic allylic ether 44 under alkene coupling conditions. Use of (R)-12 as the catalyst gives (R)-45 in > 99% ee at 58% conversion. The ethylated product 46 is also formed in the reaction in 94% ee (Eq. 7) [25]. The reaction is effective for six- to eight-membered 3-oxacycloalkenes 47 as well as for a wide variety of alkoxycycloalkenes 48 [27], with some resolution dependency on the ring size of 47 (Fig. 2) [26]. 

Ideally, the Stanger demonstration should be an expression of the stretch-bend interaction in the Bj normal mode in benzene. Thus, it is instructive to examine the nature of the stretch-bend interaction in typical monocyclic and bicyclic alkenes. The IR spectra of monocyclic and bicyclic alkenes shift hypsochromically for exomethylenes and bathochromically for endocyclic alkenes with decreasing ring size. " This effect has been shown to be dominated by mode-coupling for the monocyclic series, but consistent with a pure angular effect or the bicyclic series.Further investigation in this vein is likely to resolve the paradox initiated by Stanger s work. 

The regioselectivity in diene addition reactions can also be influenced by ring strain effects in cyclization reactions. The regioselectivity is highly predictable in those cases, in which addition to the preferred diene center forms the preferred ring size. Thus, the cyclization of radical 15 proceeds readily to form the ct s-disubstituted cyclopentyhnethyl radical 16 with high selectivity. Similarly, cyclization of 17 affords exclusively bicyclic radical 18, in which the additional cyclopentane ring has been formed by addition to the terminal position of the butadiene subunit. This preference for 5-exo cyclizations onto dienes is not even dismpted by substiments at the C1 or C4 positions of the diene system, as seen for radical 19, which cyclizes to 20 (equation lO). This is in contrast to alkyl radical cyclizations to alkenes, in which major amounts of 6-endo cyclization is observed for 5-substituted systems. ... 

Benedetti and coworkers have examined the intramolecular ring opening of epoxides by bis-activated carbanions, a process exemplifled by the rearrangement of pheny Isulfonyl epoxide 75 in a sodium ethoxide-ethanol medium to the phenylsulfony 1 cyclopentanol 76. This quantitative study on the effect of ring size in such cyclizations revealed similarities to the intramolecular radical addition onto alkenes [94JOC1518]. 

The relative rate constants for the reduction of a range of cyclic alkenes by di-imide have been determined the results agree qualitatively with those reported earlier/ The effect of ring size on the rate of hydrogenation of 2-benzylidenecycloalkanols has been examined hydrogen addition usually occurs cis- to the hydroxy-group. ... 

What effect might the ring size in the following cyclo-alkenes have on the reactivity of the C = C double bond in each ... 

However, when vinylic metalation is desired, competing allylic deprotonation may occur. In general, thermodynamic acidity and the kinetic preference for vinylic deprotonation of cyclic alkenes decrease with increasing ring size." The stable alkane-soluble reagent n-Butyllithium-Potassium t-Butoxide-TMEDA in hexane metalates Ethylene with potassium and effects selec-... 

Hoye and Promo examined the effect of ring size and alkene substitution on /Z-selectivity and rate of reaction (Table 8.2) [15]. This study also employed the diphenylsilane as the linker for the construction of symmetrical his-alkoxysilanes. 

The optimal reaction conditions involved the slow addition of catalyst [Ru]-I via a syringe pump at room temperature to avoid isomerization of the alkene, which led to truncated ring systems after RCM. Although the reaction favors the formation of the Z-isomer, the selectivity does diminish with increasing ring size (Table 8.2, entries 1-3), whereas substituted aUcenes result in lower efficiency and selectivity (entry 1 vs 4). Additional studies examined the effect of a racemic secondary and achiral tertiary allylic alcohol in the TST-RCM process. Interestingly, the former affords an equal mixture of meso- and c ,I-silaketals, indicating no obvious diastereoselectivity for this particular tether (entry 5), whereas the latter proved completely unreactive (entry 6). 

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