Benzylic hydrogen atoms

Instability at the chain end could then arise via the benzylic hydrogen atoms due to the possible resonance stabilization of the resulting radical [Eq. (18)] ... 

The mechanism of benzylic bromination is similar to that discussed in Section 10.4 for allylic bromination of alkenes. Abstraction of a benzylic hydrogen atom generates an intermediate benzylic radical, which reacts with Br2 to yield product and a Br- radical that cycles back into the reaction to carry on the chain. The Br2 necessary for reaction with the benzylic radical is produced by a concurrent reaction of HBr with NBS. 

Structural characterisation of 58 and 59 have demonstrated that the PF, anion is located in the cleft between the two dialkylammonium cations forming hydrogen-bonds with the benzylic hydrogen atoms of one of the cations and with one of the hydrogen atoms of a hydroquinone ring. In contrast, a polymeric as-... 

The benzylic hydrogen atom of alkylarenes is replaced by the azido group on treatment with trimethylsilyl azide in the presence of 2,3-dichloro-5,6-dicyano-l,4-benzoquinone subsequent hydrogenation yields amines (e.g. equation 5)31. 

Exchange of methyl hydrogen atoms.b Exchange of benzylic hydrogen atoms. 

Fig. 7 Reactions of toluene with HO radical. HO can abstract a benzylic hydrogen atom (a) or add to the aromatic ring at the ipso (b), ortho (c), meta (d), and para (e) positions relative to the methyl group. Each resultant radical can decompose by various pathways, depending on temperature and pressure.

In the complexes of w/Ao-substituted benzyl ethers, the benzylic hydrogen atoms are diastereotopic. Alkylation of the derived lithium compounds occurs completely stereoselective-ly anti to the tricarbonylchromium face from a rotamer in which the benzylic methoxy group is anti to the or/Ao-substituent2,3. The stereochemistry of the alkylation was confirmed unambiguously by X-ray analysis of the product3. 

In order to determine whether QMT may contribute to the overall reaction of diarylcarbenes with hydrogen atom donors in solution at ambient temperature, kinetic isotope effects for the benzylic hydrogen atom abstractions of the triplet states of several diarylcarbenes with toluene-toluene-i g in fluid solution were determined over the temperature ranges of —75 to 135 °C. The results are very much dependent on the structure of the carbene (Table 9.11)." The differential... 

Substrates that carry a replaceable benzylic hydrogen atom, or a similar hydrogen that gives rise to a stabilized radical, can be selectively chlorinated or brominated. Ethylbenzene leads to only... 

Fig. 3. Axial view of space filling model of Structure I (5 mers). The benzylic hydrogen atom of each substituent (white atoms, matked with "X s) situates directly over the imine double bond of the next repeating unit. Two imine carbon atoms are also marked...

In a study dealing with the construction of a 5 -aza-naphthacene derivative, conditions of the radical reaction did not affect the SMA framework present in the starting material. This result could be explained either by the steric hindrance of the silyl group that inhibits the abstraction of the benzylic hydrogen atom to create the corresponding radical species, or this radical, if created, is too stable to react efficiently.153... 

Suitable alkylbenzene side chains are oxidized at the benzylic position under the action of mCPBA, air, and NaHC03 to generate the corresponding ketones. The oxygen-centered radical formed from mCPBA, abstracts the benzylic hydrogen atom of the benzylic substrate (68) to form a benzylic radical, and then it reacts with molecular oxygen... 

The BF3-induced reactions of iodosylbenzene with l-trimethylsilyl-3-aryl-l-propynes in dioxane follow a divergent course and afford aryl silylethynyl ketones (equation 28)31. The ketone derived from l-trimethylsilyl-3-phenyl-l-propyne is also generated in dichloro-methane, but as a minor product. These oxidations are apparently facilitated by the presence of benzylic hydrogen atoms, since a similar alkynyl ketone is not obtained from 1-trimethylsilyl-l-decyne with PhIO/BF3 in dioxane31. 

The metallation of aromatic and benzylic hydrogen atoms is often a facile route to metallacycles 36... 

A new cortisone derivative, 3,20-dioxo-lljS,17a,21-trihydroxypregna-l,4,7-triene (298), has been prepared from the intermediate (297) by a classical combination of chemical and microbiological reactions.The chemistry of 17,20-and 20,21-acetonides, epimeric at C(20), has been described. A deprotection of masked steroidal alcohols by hydride transfer has been reported.Several benzyl ethers and bismethylenedioxy-steroids have been prepared and treated with trityl fluoroborate. The results demonstrated that the benzyl hydrogen atoms are sufficiently basic to give a benzyloxonium ion, as shown in the conversion of the 17,20 20,21-bismethylenedioxy-corticoid (299) into its free analogue (302) (see Part II, Chapter 1, refs. 120 and 323). 

A close inspection of the structure of diamorphine will show that the molecule also contains benzylic hydrogen atoms, on the CH2 adjacent to the benzene ring. This site is susceptible to oxidation and, for this reason, diamorphine should be stored in a well-closed container protected from light. 

Amides 113-115 are especially attractive, since the unsaturated chain, cyclic enone, and removable chiral auxiliary such as an a-phenethyl substituent are simultaneously attached to the nitrogen atom. However, the allylic or benzylic hydrogen atoms present in the starting molecule allow competitive hydrogen abstraction processes. In the presence of an a-phenethyl group, cycloadducts were indeed isolated but in low yields and poor diastereomeric excess, besides rearranged products [75]. 

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