[2 + 2]‐Cycloadditions to Strained Bridgehead Olefins. I. 1, 1‐Dichloro‐2,2‐difluoroethene

Abstract

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,

Can Twisted Double Bonds Facilitate Stepwise [2 + 2] Cycloadditions?

“…Given the very low rotational and radical combination barriers, the diradical intermediates reside on a relatively flat energy surface and are susceptible to nonstatistical dynamic effects. 37−41 We further examined the reactions of 12 and 14 with 1,1dichloro-2,2-difluoroethene, previously reported by Becker and Hohermuth 21 (Figure 3b,c). As expected by an increased p character at the F-substituted carbon of 1,1-dichloro-2,2difluoroethene, a σ-bond first forms between the CF 2 fragment and either C1 (via TS1) or C2 (via TS1′) of the bicyclic compound.…”

“…19,20 By comparing the reaction of 2norbornene (16) to that of bicyclo[4.2.1]non-1-ene (12) and bicyclo [3.3.1]non-1-ene ( 14), Becker and Hohermuth showed that the strained double bonds of 12 and 14 yielded faster stepwise [2 + 2] cycloadditions with 1,1-dichloro-2,2difluoroethene and at milder conditions. 21 12 and 14 also gave facile concerted [2 + 2] cycloaddition reactions with a ketene. 22 The twisted double bonds of trans-cycloalkenes (n = 7−10, n is the number of ring atoms) exhibit higher reactivity compared to their unstrained cis-analogues.…”

“…22 The twisted double bonds of trans-cycloalkenes (n = 7−10, n is the number of ring atoms) exhibit higher reactivity compared to their unstrained cis-analogues. 21 In these examples, the reactivity of the twisted double bond mimics that of a twisted alkene (cf. the D 2d geometry of the lowest singlet and triplet excited state of ethylene).…”

“…Anti-Bredt alkenes are bicyclic compounds with a double bond at the bridgehead position, and some examples of stepwise [2 + 2] cycloaddition reactions with anti-Bredt alkenes are known (Figure ). A trapped bicyclo[3.2.2]­non-1-ene dimer, formed by facile stepwise [2 + 2] cycloaddition of the bridgehead double bonds, was first reported by Wiseman et al Dimers of adamantene form readily upon dehalogenation of 1,2-diiododamantane. , By comparing the reaction of 2-norbornene ( 16 ) to that of bicyclo[4.2.1]­non-1-ene ( 12 ) and bicyclo[3.3.1]­non-1-ene ( 14 ), Becker and Hohermuth showed that the strained double bonds of 12 and 14 yielded faster stepwise [2 + 2] cycloadditions with 1,1-dichloro-2,2-difluoroethene and at milder conditions .…”

“…12 and 14 also gave facile concerted [2 + 2] cycloaddition reactions with a ketene . The twisted double bonds of trans- cycloalkenes ( n = 7–10, n is the number of ring atoms) exhibit higher reactivity compared to their unstrained cis- analogues . In these examples, the reactivity of the twisted double bond mimics that of a twisted alkene (cf.…”