Cross-ring strain

Transannular strain (Section 6.6) A type of strain that occurs in larger rings because of steric crowding of atoms on the opposite sides of the ring. Also called cross-ring strain. 

The cyclopropyl effect has controlled the regioselectivity of the cross-coupling reactions of propargylic/allenylic metallic species with electrophiles afford alkynic cyclopropanes (Scheme 9).70 Cyclopropyl ring strain, which makes the formation of vinylidenecyclopropanes unfavourable, is believed to control the regioselectivity. 

Strained rings may be opened by a ruthenium carbene-catalyzed reaction with a second alkene following the mechanism of the Cross Metathesis. The driving force is the relief of ring strain. As the products contain terminal vinyl groups, further reactions of the Cross Metathesis variety may occur. Therefore, the reaction conditions must be optimized to favour the desired product. 

The intermolecular enyne cross metathesis, and consecutive RCM, between a terminal alkyne and 1,5-hexadiene produces cyclohexadienes, by cascade CM-RCM reaction, and trienes, formed during the sole CM step. Studies of various parameters of the reaction conditions did not show any improvement of the ratio of desired cyclohexadiene product [25] (Scheme 12). The reaction with cyclopentene instead of hexadiene as the alkene leads to 2-substituted-l,3-cycloheptadienes [26]. After the first cyclopentene ROM, the enyne metathesis is favored rather than ROMP by an appropriate balance between cycloalkene ring strain and reactivity of the alkyne. 

The photochemical extrusion of nitrogen from cyclic azo compounds has been studied on a broad scale. Cyclic azo compounds, in contrast to acyclic ones can react with N2 elimination from both the Si and the T state. Ring strain as well as the lack of dsjtrans isomerisation favor photodecomposition. Since intersystem crossing is not very efficient the Ti state has mostly been populated by energy transfer. 

Lee and coworkers (17) have also investigated the solution-phase photochemistry of cyclobutanone and have found (X - 310 nm) to be l/20th of that obtained in the vapor phase. This reduction has been attributed to enhanced predissociation by the interaction of vibrationally excited with neighboring solvent molecules. If this solvent interaction in addition to ring strain already present in the cyclic ketone both facilitate a-cleavage from the vibrationally excited state, relative to intersystem crossing to T, then it is not unreasonable to propose the cyclobutanone state as the precursor to some, if not all, of the observed photoproducts. 

From this state, ring strain facilitated predissociation to a "biradical-like" transition state [135] or vibrational relaxation (k ) to S may occur. It is also conceivable that transition state [135] could be produced directly from S °. Alternatively, molecules in the S ° state could intersystem cross (kST) to the triplet manifold (T ). For 2-alkylidenecyclobutanones, reactivity is manifested in isomerization about the exocyclic carbon-carbon double bond, while for the saturated cyclobutanone derivatives studied, definitive evidence for solution-phase reactivity is not available. If analogy is again made to the vapor-phase photochemistry of cyclobutanone [21], reactivity could conceivably result in decarbonylated products. Indeed, preliminary evidence has been obtained from sensitization experiments employing m-xylene as triplet sensitizer that decarbonylation of a saturated cyclobutanone is enhanced by selective population of its state (35). ... 

This reaction is favored because of the release of ring strain. If the reaction partners do not have a mirror plane perpendicular to the double bond undergoing the cross-metathesis, a mixture of regioisomers may be obtained, whose composition is strongly dependent on the reaction conditions, the reactants, and the catalyst... 

For the selectivity of the cross-metathesis reaction with ethylene (ethenolysis) the choice of a suitable catalyst and a cycloolefin for which ring-opening is energetically favorable as well as the optimization of the reaction conditions are crucial. Cycloolefins with a norbomene skeleton were chosen for their high ring strain. The silicon-containing educts can be obtained by hydrosilylation of norbomadiene with the corresponding silanes. 

Cross Metathesis. Of the three major types of olefin metathesis, cross metathesis (CM) has been the most challenging to selectively control. CM is an intermolecular reaction between two olefins that releases ethylene gas, among a statistical mixture of combinatorial products it is a thermodynamically controlled reaction, where impurities arising from homodimerization are common. In addition, unlike ROMP, which relieves ring strain, and RCM, which forms stable 5- and 6-membered rings, CM has no enthalpic driving force (71). 

---