1,3‐Dipolar Cycloadditions to Strained Olefins

Abstract

SummaryThe Bredt olefins bicyclo[3.3.l]non-l-ene (2), bicyclo [4.2.l]non-l(8)-ene (3), and bicyclo [4.2. llnon-1 (2)-ene (4) react rapidly with 1,3-dipoles such as diazomethane, phenyl azide, and mesitonitrile oxide to yield mixtures of two regioisomeric cycloadducts 10, 11 and 12, respectively. On the contrary, cycloaddition to the comparable monocyclic 1-methyl-(E)-cyclooctene (5) is fairly regioselective. 2-Methylnorborn-2-ene (6) gives one isomer with mesitonitrile oxide (as do less strained olefins), but mixtures with diazomethane and phenyl azide. 'H-NMR. and 13C-NMR. spectra of the cycloadducts are reported. The results are discussed in the light of frontier molecular orbital theory.Introduction. -1,3-Dipolar cycloadditions to olefins have been extensively studied and are now well understood primarily owing to the efforts of Huisgen [l]. Experience indicates a concerted mechanism [2], and frontier molecular orbital theory has explained successfully relative rates and the regioselectivity of these cycloadditions [3]. A two-step diradical mechanism is advocated by Firestone Alkyl-substituted olefins and particularly trisubstituted olefins show low reactivity against the common 1,3-dipolar reagents [ 11 and therefore have been studied only rarely [6]. However, introduction of strain makes such compounds good candidates for 1,3-dipolar cycloadditions. Alder & Stein [7] used phenyl azide to detect and characterize the strained double bond in bicyclo [2.2.l]hept-2-enes (norbornenes). (E)-Cyclooctene (1) [8], cyclopropenes and methylenecyclopropane [9] react with a number of 1,3-dipolar reagents that are inert against normal cycloolefins. Wiseman & Pletcher [ 101 reported that the strained Bred olefin bicyclo [3.3.l]non-l-ene (2) combines with diazomethane.With a good synthesis of the methylene-bridged (E)-cyclooctenes bicyclo [3.3.1]-non-I-ene (2), bicyclo [4.2. Ilnon-1 (8)-ene (3) and bicyclo [4.2. llnon-1 -ene (4) at hand [ 1 11, cycloadditions of 1,3-dipoles to these Bredt olefins can now be examined in detail. In order to separate the influence of strain and of alkyl substituents in general on the regioselectivity of the cycloaddition, a number of other trisubstituted

[2 + 2]‐Cycloadditions to Strained Bridgehead Olefins. II. Diphenylketene

“…The simple frontier orbital picture obviously does not account for the differences observed. As pointed out earlier [3], secondary orbital interactions may be rather important in cycloaddition of Bredt olefins as a consequence of the twist and out-of-plane deformations of the strained double The concerted thermal dimerization of highly strained, unstable Bredt olefins or related (E)-cycloalkenes constitutes another [n 2 s + n 2 a]-process. In analogy to the ketene cycloaddition to bridged (E)-cyclooctenes, a single cyclobutane dimer would be expected.…”

“…The simple frontier orbital picture obviously does not account for the differences observed. As pointed out earlier [3], secondary orbital interactions may be rather important in cycloaddition of Bredt olefins as a consequence of the twist and out-of-plane deformations of the strained double …”

“…-Strained bridgehead olefins (Bredt olefins [ 11) such as bicyclo- [3.3.l]non-l-ene (l), bicyclo [4.2.I]non-l(8)-ene (2), and bicyclo [4.2.l]non-l-ene (3) undergo a variety of additions uncommon to unstrained trialkyl-substituted olefins [2] [3]. The [2 + 21-cycloaddition is of special interest, because the strained (and twisted [4]) double bond of the bridgehead olefin might prefer to react antarafacially in a concerted [ n 2 s + n 2 a]-mode, which is thermally allowed according to the Woodward-Hofmann rules [5].…”

“…When 113-dipolar cycloadditions [3] and ketene cycloadditions to strained bridgehead olefins are compared, some important differences must be noted. 1 7 ( 7 ) 0.596 ( 8 1 0.173(7) 0 .…”

Polyaddition of Azide‐Containing Norbornene‐Based Monomer through Strain‐Promoted 1,3‐Dipolar Cycloaddition Reaction

“…Generally, the reactivity of electron‐poor azide and elecron‐rich alkene is higher, such as for ethyl azidoformate . It is noted that among the starting materials, strained electron‐rich alkene can react readily at room temperature or even below . Lopez and Houk and have demonstrated by theoretical study that strained cycloalkene can accelerate the reaction by an order of magnitude, and the reactivity can be further enhanced when perfluoroaryl azide is used .…”