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

The strained bridgehead olefins bicyclo [3.3.l]non-l-ene (I), bicyclo [4.2. llnon-1 (8)-ene (2), and bicyclo [4.2.l]non-l-ene (3) react rapidly with I, 1-dichloro-2,2difluoroethene (5) to yield mixtures of regioisomeric dichlorodifluorocyclobutanes 8/9, 10/11 and 12/13, respectively. On the contrary, the reaction of 5 with the model compound (E)-1 -methylcyclooctene (4) is completely regioselective. The structure of the cycloadducts has been elucidated mainly by 19F-NMR. and 13C-NMR. spectroscopy.Introduction. -The bridgehead olefins bicyclo [3.3. llnon-1 -ene (l), bicyclo-[4.2.1]non-l (8)-ene (2), and bicyclo [4.2.l]non-l-ene (3) are highly strained, but still stable at RT. These so-called Bredt olefins [ 11 formally are methylene-bridged (E)-cyclooctenes [2] and therefore should be comparable in strain and reactivity to the model compound (E)-1-methylcyclooctene (4). Their high reactivity has been documented by a number of additions which are not observed with unstrained alkyl-substituted olefins [3].Bredt olefins belonging to the class of bridged (E)-cycloheptenes or (E)-cyclohexenes are unstable at ordinary temperatures and usually dimerize to mixtures of cyclobutanes by a [2+2]-cycloaddition [4]. It is therefore of interest to study [2 + 21-cycloadditions of the stable bridged (E)-cyclooctenes 1, 2, and 3, and compare these with more and with less strained olefins. 1, I-Dichloro-2,2-difluoroethene (5) was chosen as the partner, because 5 undergoes [2 + 21-cycloadditions with activated olefins and because it was anticipated that the resulting isomeric dichlorodifluorocyclobutanes could be readily identified by NMR. spectroscopy. The substitution pattern of 5 may lead to two regioisomeric cyclobutanes, thereby allowing to probe the 'polarity' of the double bond of the bridgehead olefin. Mixtures of stereoisomers are formed only if isomerization at the double-bonded C-atom a to the bridgehead occurs. Reactions of 5 with conjugated dienes and with I) *)Taken in part from the dissertation of M . K. Hohermuth,

SummaryThe strained bridgehead olefins bicyclo [3.3. llnon-1-ene (l), bicyclo [4.2. Ilnon-1 (8)-ene (2), and bicyclo [4.2. llnon-1-ene (3), and the comparable monocyclic (E)-1-methylcyclooctene (4) react with diphenylketene (6) to give a single cycloadduct 7, 8, 9 and 10, respectively, in which the diphenyl-substituted C-atom is bound to the bridgehead. The structure of the cyclobutanone 8 has been determined by X-ray analysis of a twin crystal obtained by crystallization with spontaneous enrichment of enantiomers.

Die Bredt‐Olefine (IV), (VII) bzw. (X) reagieren mit den Dipolen (I), (II) bzw. (III) unter Bildung von Isomerengemischen (V) + (VI) bzw. (VIII)+(IX) bzw. (XI) + (XII); die Cycloaddition an das monocyclische E‐Cycloolefin (XIII) ist dagegen regioselektiv [→ (XVa), (XIVb) bzw. (XIVc)].

Während das Bicyclononen (I) das Olefin (II) zu den Regioisomeren (III) und (IV) addiert und sich die Bredt‐Olefine (V) und (VI) analog verhalten, geht aus der Addition von (II) an das sterisch gehinderte, monocyclische E‐Olefin (VII) regioselektiv das Addukt (VIII) neben (IX) hervor.