Strained bridgehead alkene

Bicyclo[3.2.2]non-l-ene, a strained bridgehead alkene, is generated by rearrangement when bicyclo[2.2.2]octyldiazomethane is photolyzed.227... 

In carbene complexes which lack /3-hydrogens, for example (48 X = H, Y = t-Bu), alkyl migration can occur to afford trisubstituted alkene complexes (66). When this involves a polycyclic hydrocarbon, the result is a strained bridgehead alkene complex (67). ... 

If the alkene is too unstable to permit isolation, it may be trapped in situ with a suitable reagent. The highly strained bridgehead alkene (40 Scheme 16) was prepared and trapped in this way as a Diels-Alder adduct with 1,3-diphenylisobenzofuran (41). ... 

The syntheses of strained bridgehead alkenes, the majority of which are based on silver(I)-assisted cyclopropyl to allyl rearrangements of halotricyclo[m.n.l. Ojalkanes, have recently been comprehensively reviewed. This can be generalized as the transformation 18 19. 

The presence of substituted pyridines in the reaction mixture is essential for the isolation of highly strained bridgehead alkenes. Even under weakly acidic conditions the products of secondary additions and (or) rearrangements of the bridgehead alkenes are ob-... 

The method has been used to prepare strained bridgehead alkenes from halogenated [n. m. 1 Jpropellenes. 

The mechanism shows why the rearrangement happens the first-formed cation cannot eliminate H in an El reaction because loss of the only available proton would give a very strained bridgehead alkene (make a model and see ). 

Compound (49) has been prepared by a Diels-Alder cycloaddition reaction, and it may be converted into the benzyne (50) by replacement of the silyl groups by halogens and then treatment with butyl-lithium.The highly strained bridgehead alkene (51) has been obtained by treatment of (52) with fluoride ion the alkene... 

Substituted tricyclopentanols (alcohols iclated to 9) have served as precursors of pyramidal cations by protonation and deltydration [37], but the sequence proposed above requires conversion of the ketone to a caibene and the insertion of the caibene center into a CH bond (a distal CH insertion into a proxiinal CH would form a highly strained bridgehead alkene), a known process [38], A possible sequence for the conversion of 9 to the caibene is preparation of the lydrazone and oxidation of this to the diazo compound [39], followed by catalytic or photochemical expulsion of nitrogen [40] ... 

Since reactivity of alkenes increases with increasing alkyl substitution, hydration is best applied in the synthesis of tertiary alcohols. Of the isomeric alkenes, cis compounds are usually more reactive than the corresponding trans isomers but strained cyclic isomeric olefins may exhibit opposite behavior. Thus, for example, raras-cyclooctene is hydrated 2500 times faster than cis-cyclooctene (98). Similar unusually high protonation rates were also measured for strained bridgehead alkenes. This may be attributed to the high groundstate strain (in the case of bicyclo[3.3.1]non-l-ene) or the formation of the nonclassical norbomyl cation when norbornene is protonated (95). 

As is usually the case, intramolecular reactions are easier than intermolecular and often do not obey the usual rules. Some do not need the carbonyl group, some show exo rather than endo selectivity, and the cyclisation of 56 gives the meta product 58. The mechanism 57 makes it clear that the expected para product (cf. 42) cannot be formed. This is a particularly impressive example as the product 58 is a bridgehead alkene with a strained geometry.10 The alkene is cis inside the six-membered ring but trans in the outer 10-membered ring. 

Disconnection of the bicyclic diketone 21 starts reasonably to reveal 22 but the two carbonyl groups are now 1,6-related suggesting a reconnection strategy (chapter 27). But this is impossible as the bridgehead alkene 23 is too strained to exist. However, if we extrude the carbon atom in the seven-membered ring between the ketone and the branchpoint 22a, we get a new ketoester 24 with a 1,5 relationship that can be made by conjugate addition (chapter 21). 

Kinetic and computational studies by Shea and Kim on MCPBA epoxidations of a series of cyclic alkenes including bridgehead alkenes and tra/w-cycloalkenes have shown that the reactivity depends primarily on the strain energy relief in the transition state <92JA3044>. 

Strain energy of the bridgehead alkenes. A decrease in ring size favors the formation of the cyclopropylmethyl acetates. 

Synthesis of Strained trans-Cycloalkenes and Bridgehead Alkenes via Solvolysis of Bicyclic... 

Dichlorotricyclo[4.2.1.0 ]non-3-ene (20) is even more strained and rearranges on warming to room temperature. The main products were derived from reactions of the unstable bridgehead alkene, 6,9-dichlorobicyclo[4.2.1]nona-l(9),3-diene (21), which was formed as a transient intermediate. This species dimerized to give 22, or could be trapped as a cycloadduct 23 with furan. ... 

The intramolecular Diels-Alder reaction of (171) proceeds at 2I0°C in a sealed tube to give the strained bridgehead diene ester (57) (Equation (12)) <86T184I>. The yield is surprisingly good, considering that both a medium ring and two strained alkene bonds are formed. 

Shea et al. investigated whether strain involved in alkenes affects reactivity and regiochemistry of the intermolecular 1,3-dipolar cycloaddition reaction [14]. Therefore, the addition of picryl azide (18) with a series of mono-and bicyclic olefins including frans-cycloalkenes and bridgehead alkenes was studied (Scheme 5). In the cases of czs-cyclooctene (16) and ci5-cyclononene (17), decomposition of the initially formed cycloadducts 19 and 20 followed... 

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