Benzylic methylene

In view of the restrictions on the mode of approach of the radical to the double bond, significant strain develops at the transition state, and this requires rotation of the benzylic methylene group out of its preferred coplanar alignment. 

Oxidative degradation begins at lower temperatures in air (<300°C) and oxidation occurs most readily on benzylic methylene carbons since they are the most... 

In the presence of a very strong base, such as an alkyllithium, sodium or potassium hydride, sodium or potassium amide, or LDA, 1,3-dicarbonyl compounds can be converted to their dianions by two sequential deprotonations.79 For example, reaction of benzoylacetone with sodium amide leads first to the enolate generated by deprotonation at the more acidic methylene group between the two carbonyl groups. A second equivalent of base deprotonates the benzyl methylene group to give a dienediolate. 

Oxidation of the benzylic methylene group in cyclazocine to a ketone is also consistent with analgesic activity. Acetylation of benzomorphan 62 affords the diacetate 63. Selective hydrolysis of the phenolic acetate (64) followed by methylation of the thus uncovered phenol affords intermediate 65. 

Benzylic methylenes, 1,3-diketones such as diaroylmethanes and aroylacetones, alkenes and alkynes are all known to give 1,2,5-thiadiazoles when treated with a variety of sulfur sources and much of this work has been reviewed in CHEC(1984) and CHEC-II(1996). Recent developments are outlined below. 

The situation is different when the benzyl methylene of [23] is replaced by oxygen. Relief of one 1,5- and one 1,4-CH HC repulsion is clearly responsible for the significant oxygen-atom effect observed in this case (Table 12). 

Figure 25. X-Band EPR spectra of electrochemically generated [Com(L BuMel )(Ph2acac)]2+ (top) and its at the benzylic methylene deuterated analogue (bottom) the hyperfine coupling constants are given in millitesla (mT). [Adapted from (152).]...

Chromium-mediated oxidation of benzylic methylene groups to the corresponding oxo derivatives has also been reported [4], Tri-n-butylstannyl chromate appears to be the best co-oxidant and a trace of 4-toluenesulphonic acid also aids the oxidation. 

Chromate-mediated percarbonate oxidation of primary and secondary alcohols and of benzylic methylene groups... 

In the presence of a severalfold excess of (i )-TFAE, an (5)-enriched sample of lactone 38a shows lowfield nonequivalence for the benzyl methylene hydrogens and highfield nonequivalence for the methyl group, in accord with the model. However (5)-enriched samples of the nitrated lactones afford different results not predicted by the usual model (solvates 39). The senses of methyl nonequivalence for... 

Shtamburg and coworkers have reported that A,A -dialkoxy-A,A -dicarboalkoxyhydra-zines (219) have lower barriers to amide isomerization and weaker anomeric interactions . They measured a barrier to amide isomerization of only 9.8 kcalmoD. Furthermore, benzylic methylenes in A-benzyloxy systems were isochronous down to at least —90°C. These results are in line with observations for the A,A -diacyl-A,A -dialkoxyhydrazines since, in the carboalkoxy systems, the nitrogen lone pairs are lowered in energy by the additional electron demand, thereby reducing both amide conjugation and anomeric overlap. 

All of the chrestifoline alkaloids have a UV spectrum characteristic of a carbazole chromophore. The H-NMR spectra of all these alkaloids showed signals for an NH proton at b 10.16 or 10.31, an aromatic methoxy group at b 3.84 or 3.85, and a 2H singlet at b 4.78-5.80 assigned to the benzylic methylene protons connected to... 

Muzart and coworkers investigated the CrOs/TBHP catalyzed oxidation of various benzylic methylenes and the allylic oxidation of A - and A -cholesten-3-one to the corresponding ketones in CH2Cl2 as well as benzotrifluoride (BTF) as solvent (Scheme 130) °. It could be shown that BTF in most cases improved the results even though the reactions were carried out with less TBHP than in CH2CI2 (4 eq. compared to 7 eq.). Yields in CH2CI2 varied from 21 to 94% and those in BTF from 40 to 99%. 

The benzyl methylene groups in 115 are diastereotopic (J = 13.5 Hz) restricted rotation of the dimethylamino group is apparent in 111. ... 

The reaction of aliphatic and aromatic ketone oximes 97 with a dialkyl carbonate 98 in the presence of K2CO3 at 180-190 °C yields 3-alkyl-4,5-disubstituted-2(3//)-oxazolones 104 in 22-48% yields. Mechanistically, it is proposed that N-alkylation of the initially formed oxime O-carbonate 99 yields 100, which affords the enamine 101 in the presence of base. A [3,3] sigmatropic rearrangement ensues to produce 102, which then cyclizes to 104. In cases where 97 contains two methylene groups in proximity to the C=N bond, one of which is benzylic, the above reaction sequence is regioselective for the benzylic methylene group (Fig. 5.25 Table 5.6, Fig. 5.26). ... 

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