Benzylic bond cleavage

As already indicated in the preceding schemes, the resonance-stabilized benzyl ion, [CvHv]", initially formed by benzylic bond cleavage reversibly isomerizes to the tropylium and tolyl ion isomers. The isomerization of [CvHv]" ions has been the subject of numerous studies, [50] revealing the tropylium ion as the thermodynamically most stable isomer. [51,52]... 

Example Ethyl loss clearly predominates methyl loss in the El mass spectrum of 2-(l-methylpropyl)-phenol. It proceeds via benzylic bond cleavage, the products of which are detected as the base peak at m/z 121 and m/z 135 (3 %), respectively (Eig. 6.34a). The McLafferty rearrangement does not play a role, as the peak at m/z 122 (8.8 %) is completely due to the isotopic contribution to the peak at m/z 121. From the HR-El spectrum (Fig. 6.34b) the alternative pathway for the formation of a [M-29] peak, i.e., [M-CO-H]", can be excluded, because the measured accurate mass of this singlet peak indicates CgHgO". HR-MS data also reveal that the peak at m/z 107 corresponds to [M-CHs-CO]" and that the one at m/z 103 corresponds to [M-C2H5-H20]. Although perhaps unexpected, the loss of H2O from phenolic fragment ions is not unusual. 

But all the monometallic catalysts deactivate before they can complete the 1,000 turnovers (100% conversion of alkene). This is tied into Rh-induced phosphine orthometalations and P-Ph or P-benzyl bond cleavage reactions that lead to monometallic catalyst deactivation [4], Bisbi is especially susceptible to deactivation under these conditions, barely making it past 50% conversion of the alkene. The mainly alkylated et,ph-P4 ligand does not seem to suffer from Rh-induced phosphine fragmentations under these conditions, but does tend to lose a rhodium center and fragment losing the bimetallic cooperativity. 

Note Overall, the former two reaction lead to the typical appearance of the ion series m/z 39, 51, 65, 77, 91 in the El mass spectra of phenylalkanes. However, one needs to be aware of the two competing reaction sequences leading to this series i) IVT 91 65 - 39 and ii) IVT 77 - 51 the latter usually being of lower intensity, which is in accordance with the preference for benzylic bond cleavage. ... 

Scheme 17.12. Stepwise decompositions of protonated Aspidosperma alkaloid occurring by either (a) stereochemical D ring cleavage yielding loss of acetamide neutral or (b) ion-dipole complex formation by A-benzyl bond cleavage of the protecting group of the indolic system decomposing into odd-electron product ions. (Adapted with permission from Ref 61.)...

Primary 1,2-diamines can be obtained by hydrogenolysis of the benzylic N-auxiliary bond over Pd/C, e.g., the preparation of 249d. The cleavage is not selective in the presence of other benzylic bonds in the molecule, e.g., aryl substituents at Cl - C2. However, in the presence of methoxyphenyl or dimethoxyphenyl C substituents, the N substituents can be selectively removed, as exemplified by the preparation of the primary 1,2-diamine... 

Alkyl methacrylates, hydrolysis of polymeric ester functionality, 259 Aluminum-hydrogen bond, nucleophilic substitution, 264 Amines alkylation, 28 benzyl-group cleavage, 25 Aminomethylation chloromethylated polymers, 19 Deltfpine reaction, 19 Anionic polymerization advantages, 85... 

The main mass fragmentation of secobenzylisoquinoline alkaloids involves bond cleavage between the two benzylic carbonyls. This process is evidenced by the presence of peaks representing fragment ions at m/z 151, found in spectra of all these bases and attributed to the lower portion of the molecules, and ions at m/z 220,236, and 222, found in spectra of 159,160, and 161, respectively, formed from the upper part of the compounds. Similarly, as in the mass spectra of other secoisoquinoline alkaloids incorporating the amino-ethyl substituent, the [H2C=N(CH3)2]+ ion at mjz 58 is the base peak. 

It was found in the case of O-benzyl systems that palladium oxide is much more effective than palladium metal. No such effect was observed with the N-benzyl system.8 It is possible that the N-compounds can poison the electrophile metal ions, and the hydrogenolysis of the N-benzyl bond can take place only by the hydrogenolytic cleavage instead of the insertion mechanism. This is supported by the experimental finding that the product amine can inhibit the catalyst, and this can be minimized by buffering at a pH less than 4. 

Step 1 above requires that there be bonds in the coal that are weak enough to break in appropriate numbers at conversion temperatures and times. Table I displays some kinetic data for the cleavage of benzylic bonds in a series of increasingly aromatic compounds. In accord with expectation, an extension of the aromatic system increases the ease with which the benzylic bond is broken. 

THERMAL CLEAVAGE OF BENZYLIC BONDS IN AROMATIC SYSTEMS... 

The initial C-0 bond cleavage (by attack of a/the bromide ion at the benzylic carbon, followed by recyclization of the intermediate 301 by selective alkylation at sulfur) was suggested to be responsible for a stereospecific rearrangement of 2-substituted-1,3,2 -oxazaphospholidine-2-thiones 299 derived from (-) pseudoephedrine into... 

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