Bridgehead proton acidity

Note that by the investigators design, the bridgehead protons ( ) are not acidic, since any resulting carbanion would be orthogonal to the boron 2pz-orbital and thus incapable of 7r-bonding stabilization. 

NMR spectra of the compound 13a showed a broad singlet at 4.41 for the clibyl protons, whereas compound 13b showed two multiplets. Conversion of bridgehead carboxylic acids to the corresponding halides using Pb(OAc)4 and iodine in refluxing benzene under illumination is reported. This is considered to be an alternative to Barton s method, because of its simplicity and ease of preparation, but it involves toxic lead compounds. 

Another reactivity domain in bicyclobutane is the bridgehead proton. As can be predicted from the large s character of its bond to the bridgehead carbon, this proton is relatively highly acidic. As will be shown in Section V.I, this feature is often used to prepare certain bridgehead derivatives of bicyclobutane. 

A rough correlation between the relative rates of deprotonation and the Jq h for the bridgehead protons in the series 7, 8 and 9, was observed by Gloss and Larrabee. Taking the C-H coupling constant as a measure of the acidity of hydrocarbons, the acidity of the proton in bicyclobutane is expected to be between that of acetylene and ethylene (Jq-h values for acetylene, ethylene and bicyclobutane are 248,156 and 205 Hz respectively). 

The generation of 2,4-alkylidene bridged bicyclo[1.1.0]but-l(3)-enes became possible when the acidity of the bridgehead proton in compounds of type 110 was detected (124 127). The deprotonation by lithium diethylamide and similar bases is reminiscent of the formation of 1,2-dehydrobenzene from... 

In bicyclo[1.1.0]butane (5), the C - H coupling constant for the bridgehead C-H groups is 202 Hz. Calculate the % s character in the bond from carbon to hydrogen at this position. Would you expect the acidity of the bridgehead protons to be greater or less than that of the protons in acetylene ... 

In addition the structure of the 1,2-azathiabenzene 78 was also confirmed by chemical evidence as shown in Scheme 10. Protonation of 78a (R1 = R2 = Me) with 70% perchloric acid yielded the corresponding cyclic amino sulfonium salt 82a in 87% yield, but not the starting sulfonium compound 76a, suggesting predominance of sulfilimine structure 78a rather than cyclic sulfonium ylide stmcture 80a. Thus, compound 78 could be recognized as the first example of a 1,2-azathiabenzene having sulfur at a bridgehead position. A proposed mechanism for the formation of 78 and 79 is shown in Scheme 9. The most acidic proton adjacent to sulfur in 76 is deprotonated with... 

Similar studies have also been carried out on condensed heterocyclic systems with a bridgehead nitrogen. Indolizine [175] and its 2-methyl-, 1,2-, 2,6- and 2,8-dimethyl- and 1,2,3-trimethyl derivatives protonate preferentially on C-3 in trifluoroacetic acid, giving cations [176] (Fraser et al., 1962). In general, when position 3 is... 

The Ni(II) complexes l-3g with hexaaza and pentaaza macrocyclic ligands as well as the Ni(II) cyclam complex display a low-spin square-planar and high-spin octahedral complex interconversion in aqueous solution (7, 12-14). However, complexes l-3a favor formation of the high-spin form upon addition of acid, which is in contrast to the cyclam complex. The protonation of tertiary nitrogen atoms at the bridgehead position facilitates the axial coordination of anion or water because of hydrogen-bonding between them 12, 14a). 

It has been suggested429 that the observed acid-catalysed isomerization of perhydro[2.2]paracyclophane (369) to (370) is initiated by protonation of a bridgehead... 

A derivative of the (bpy.bpy.bpy) cryptand, obtained by modifying one of the chains, Lbpy, forms a di-protonated cryptate with EuCb in water at acidic pH, [EuCl3(H2Lbpy)]2+ in which the metal ion is coordinated to the four bipyridyl and two bridgehead nitrogen atoms, and to the three chlorine ions (Fig. 4.25). The polyamine chain is not involved in the metal ion coordination, due to the binding of the two acidic protons within this triamine subunit. In solution, when chlorides are replaced by perchlorate ions, two water molecules coordinate onto the Eu(III) ion at low pH and one at neutral pH, a pH at which de-protonation of the amine chain occurs, allowing it to coordinate to the metal ion. As a result, the intensity of the luminescence emitted by Eu(III) is pH dependent since water molecules deactivate the metal ion in a non-radiative way. Henceforth, this system can be used as a pH sensor. Several other europium cryptates have been developed as luminescent labels for microscopy. 

Ordinarily, p-diketones are acidic because they can form enolates that can be stabilized by delocalization over both carbonyl groups. In this case, loss of the proton at the bridgehead carbon doesn t occur because the strained ring system doesn t allow formation of the bridgehead double bond. Instead, enolization takes place in the opposite direction, and the diketone resembles acetone, rather than a P-diketone, in it pKa and degree of dissociation. 

Gust, Moore, Moore and coworkers covalent cartenoid-porphyrin-quinone molecular triads 55-60 contain a cyclized hydrogen bond within the quinone acceptor framework [143], The naphthaquinone moiety of 55 is fused to a norbomene system whose bridgehead position bears a carboxylic acid, which can hydrogen bond to an adjacent quinone. Photoinduced electron transfer from the porphyrin to the quinone leads to a marked p/fg increase of the latter, resulting in a fast proton transfer ( pt 10 s ) to form the semiquinone. Back electron transfer from the semiquinone is attenuated as a consequence of the proton-stabilized charge-separated species. This leads to a two-fold increase in the quantum yield of the charge-separated state of 55, as compared to those of the reference triads 56 and 57 (see Volume III, Part 2, Chapter 2). 

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