Ring strain additivity

Cyclobutanes are cleaved less readily than are cyclopropanes, but, nonetheless, hssion occurs without difficulty if the ring has additional strain UI3,I74b,I76), adjacent unsaturation 72b,J53a), or aromatic substituents... 

Ring strain enhances alkene reactivity. Norbornene, for example, undergoes rapid addition of TFA at 0° C.12... 

The use of theoretical methods in the study of bicyclic systems with P-, As-, Sb-, or Bi- bridgehead atoms has contributed to an increased understanding of the geometry, stability, and ring-strain effects of these systems. In addition, important data relating to basicity and the interpretation of nuclear magnetic resonance (NMR) and X-ray data have been generated. A vast majority of the work done has focused on P. 

Hetero substituted 2-cyclopropylideneacetates are ring-strain activated acrylates, highly reactive dienophiles in Diels-Alder reactions, but also powerful Michael acceptors. The reactivity of these compounds is enhanced by the same strain release in the Diels-Alder cycloadditions as well as in the 1,4-additions, and indeed the borderline between tandem Michael-cyclization and Diels-Alder-type cycloaddition is not well defined in many cases. 

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... 

Simple aromatic aza-Claisen rearrangement without charge acceleration by addition of an acid required other activating factors to enable the reaction to be run at acceptable temperatures. The reduction of ring strain was found to serve as a useful promoter to induce aromatic aza-Claisen rearrangement [21]. 

Somfai enhanced the driving force of some amide enolate aza-Claisen rearrangements by choosing vinylaziridines as reactants [24]. The additional loss of ring strain offered the advantage of running most of the reactions at room temperature to synthesize unsaturated chiral azepinones. Various substitution... 

Vaska s complex catalyzed the transformahon of aUenylcyclopropane into 2-alkenylidenecyclohex-3-enone under conditions of pressurized CO (Scheme 11.25) [38]. In this reaction, the jr-coordination to internal oleflnic moiety of the aUene brings the metal closer to the cyclopropane ring. Release of the cyclopropane ring strain then facilitates the oxidative addition of vinylcyclopropane moiety along with C-C bond cleavage, such that metallacyclohexene is obtained a subsequent carbonyl insertion and reductive elimination then provides the product Hence, the reaction can be recognized as a [5+1] cycloaddition of vinylcyclopropane and CO. 

The elemental processes involved such as P-H oxidative addition and P-C reductive elimination are precedented (see above). The telomer formation may stem from the four-membered intermediate, which can relieve the ring strain upon further insertion of acrylonitrile into the Pt-C bond. 

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