Bridgehead alkene, protonation

An NOE between the the alkene proton at 3h = 6.22 and the methyl protons at Sh = 1.17 establishes the relative eonfiguration (exo) of the respeetive methyl group. The exo attaehment of the six-membered ring in the stereostrueture I follows, in partieular, from the NOE between the methyl protons at 5h = 1.26 and the bridgehead proton at Sh = 3.22 as well as the absenee of effeets between the alkenyl proton pair with Sh = 5.44/6.22 and the bridgehead proton pair with Sh = 2.85/3.22. 

Benzvalene (18) is a tricyclic benzene isomer containing a bicyclobutane ring system bridged by an ethylene moiety its radical cation is accessible by PET or radiolysis. CIDNP indicated negative hfcs for the alkene protons (H ), strong positive hfcs for the non-allylic bridgehead protons (Hy), and negligible hfcs for the... 

Directed lithiation A new route to bridgehead alkenes based on a deprotonation reaction has been established 55 The deprotonation of 9-oxabicyclo[3.3.1]nonadiene (10) with r-RnT.i-TMF.DA occurs selectively in the allylic position. While (10) has four allyl positions, only one bridgehead proton is removed, as demonstrated by quenching with Me3SnCl, Me3SiOTf, and Me3PbCl (Scheme 6). 

Attempts have been made to prepare sterically highly crowded tertiary cations (see later discussion). The possibly most hindered trialkylmethyl cation, the tris(l-ada-mantyl)methyl ion 22, has been prepared from the corresponding chloride118 [Eq. (3.9)]. Proton loss is not favored in this case because this would lead to the formation of a bridgehead alkene. Consequently, cation 22 could be observed as a persistent, long-lived cation. 

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. 

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 ). 

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). 

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