Bridgehead alkenes strain

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

For carbenes 25 and 26 1,2-shifts of bonds a and b will lead to different bridgehead olefins, 17 and 27 for 26, and 1-norbomene 16 and bicy-clo[3.1.1]hept-l-ene (30) for 25, whereas carbene 24a will only give alkene 15. In accordance with experimental observations,19 calculations show that the more strained bond will migrate preferentially. 

Strained n/d/ -Bredt alkenes as well as -cycloalkenes will undergo [2 + 2] cycloadditions with alkenes. The bridgehead bicyclo[3.3.1]nonenones 16, produced as a transient species by the dehydrobromination of the corresponding bridgehead bromoketone, undergo cycloaddition with 1,1-dimethoxyethenc (15) to give the tricyclic ketones 17 in quantitative yields.7... 

Surprisingly, imino carbon shifts of strained 2 7/-azirines [431] do not differ substantially from those of open-chain imines (Section 4.9.3). On the other hand, the alkene carbon shifts of l-azabicyclo[2.2.2]oct-2-ene do not reflect the polarization characteristic of other enamines (Section 4.9.2) (+ )-M electron donation of the bridgehead enamino nitrogen cannot take place since the lone-pair orbital and the p orbitals of the it bond are... 

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). 

These are generally limited to what are termed kinetically stabilized alkyls, i.e. those devoid of protons p to the metal (Figure 4.5). These also include norbornyl and adamantyl examples since decomposition via p-metal hydride elimination (see below) would require the installation of an alkenic bond between the a and p carbons of the precursor alkyl. This is precluded for these two alkyls because of the prohibitive strain associated with forming a double bond to a bridgehead atom within a small cage structure (Brendt s rules). The tetrakis(norbornyl) complexes are also remarkable because some uncharacteristic oxidations states can be attained, e.g. Cr(IV), Co(IV) (low-spin e4t2°). The second factor which may confer stability is steric bulk bimolecular decomposition routes are thereby discouraged. 

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). ... 

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... 

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. 

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. 

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... 

---