Benzylic C—H bond

We attributed the decreased bond dissociation energy in propene to stabilization of allyl radical by electron delocalization Similarly electron delocalization stabilizes benzyl rad ical and weakens the benzylic C—H bond... 

The benzylic position in alkylbenzenes is analogous to the allylic position in alkenes. Thus a benzylic C—H bond, like an allylic one, is weaker than a C—H bond of an alkane, as the bond dissociation energies of toluene, propene, and 2-rnethylpropane attest ... 

Interestingly, [Ee(F20-TPP)C(Ph)CO2Et] and [Fe(p2o-TPP)CPh2] can react with cyclohexene, THF, and cumene, leading to C-H insertion products (Table 3) [22]. The carbenoid insertion reactions were found to occur at allylic C-H bond of cyclohexene, benzylic C-H bond of cumene, and ot C-H bond of THF. This is the first example of isolated iron carbene complex to undergo intermolecular carbenoid insertion to saturated C-H bonds. 

ROH, since the benzylic C-H bond of the substituent in the chloride complex is activated by the strong base NaOR [45a]. 

Fig. 10. Energies of selected states during dissociation of a benzylic C—H bond in...

Indoles were synthesized by the intramolecular functionalization of a benzylic C-H bond. Hence, the reaction of 2,6-xylyl isocyanide with a ruthenium complex led to 7-methylindole (Equation (36)). 

As shown in the previous two sections, rhodium(n) dimers are superior catalysts for metal carbene C-H insertion reactions. For nitrene C-H insertion reactions, many catalysts found to be effective for carbene transfer are also effective for these reactions. Particularly, Rh2(OAc)4 has demonstrated great effectiveness in the inter- and intramolecular nitrene C-H insertions. The exploration of enantioselective C-H amination using chiral rhodium catalysts has been reported by several groups.225,244,253-255 Hashimoto s dirhodium tetrakis[A-tetrachlorophthaloyl-(A)-/ r/-leuci-nate], Rh2(derived rhodium complex, Rh2(i -BNP)4 48,244 afforded moderate enantiomeric excess for amidation of benzylic C-H bonds with NsN=IPh. 

As shown in the manganese- and ruthenium-catalyzed intermolecular nitrene insertions, most of these results supposed the transfer of a nitrene group from iminoiodanes of formula PhI=NR to substrates that contain a somewhat activated carbon-hydrogen bond (Scheme 14). Allylic or benzylic C-H bonds, C-H bonds a to oxygen, and very recently, Q spz)-Y bonds of heterocycles have been the preferred reaction sites for the above catalytic systems, whereas very few examples of the tosylamidation of unactivated C-H bonds have been reported to date. 

The palladium-catalyzed reaction of benzol]quinoline in the presence of PhI(OAc)2 as an oxidant in MeCN gives an 11 1 mixture of 10-acetoxy- and 10-hydroxybenzo[ ]quinolines in 86% yield (Equation (98)).135 This chelation-directed oxidation can be extended to the benzylic C-H bond of 8-methylquinoline. The inactivated sp3 C-H bonds of oximes and pyridines undergo the same palladium-catalyzed oxidation with PhI(OAc)2 (Equation (99)).1... 

The silylation of benzylic G-H bonds is achieved by using Ru3(GO)12 catalyst in the presence of norbornene as a hydrogen acceptor.145 The reaction of 2-(2,6-dimethylphenyl)pyridine with triethylsilane in the presence of Ru3(CO)i2 catalyst and norbornene affords mono- and disilylation products in 30% and 55% yields, respectively (Equation (106)). The reaction of 2-(2-tolyl)pyridine shows that the silylation of the aromatic C-H bond is more facile than that of the benzylic C-H bond. 

An electron-enriched 1,3-diene moiety as in the substrate 381 can act as a nucleophile toward an activated alkyne moiety (Scheme 94). Iwasawa340 has reported an elegant synthesis of a diquinane framework 382, which is catalyzed by various metals and the rhenium(i) complex appears to be the best catalyst among the metal complexes examined. Minor product 384 presumably is formed through an insertion of a carbenoid species into the neighboring activated benzylic C-H bond. The same carbenoid species can undergo a 1,2-H shift to give the major product 383. 

This transformation proceeds through coordination of the isocyanide group to the ruthenium complex (structure 172), followed by insertion of the C-bound ruthenium into the benzylic C-H bond (intermediate 173). After ruthenium-mediated addition of the benzylic carbon to the isonitrile carbon and tautomerization, the desired product was obtained via elimination of the ruthenium complex. 

The comparison of physical and chemical properties of Parylene-N and Parylene-F is shown in Table 18.4. Parylene-N is considerably less stable in air than in nitrogen as a result of oxidative degradation. However, the similarity between its behavior in air and in nitrogen suggests that Parylene-F has very good thermal oxidative stability, which is most likely the result of the high stability of the C—F bond, and provides evidence that oxidative attack starts at the benzylic C—H bonds in Parylene-N.15... 

One report of a secondary /3-deuterium KIE for a carbene insertion reaction has appeared recently. Pascal and Mischke (1991) found that the /3-deuterium KIE for the insertion of dichlorocarbene into the benzylic C—H bond of cumene (reaction (39)) was (kH/kD)p = 1.250 and 1.22 when the KIE was based on GC-MS analyses and H nmr, respectively. 

A better possibility PI1CH2C)- adds to C60 Then autoxidation of a benzylic C-H bond occurs to give the hydroperoxide. Then the C60 carbanion displaces OH- from the hydroperoxide to give the product. 

Although Parylene-N possesses an outstanding combination of physical, electrical, and chemical properties, the benzylic C—H bonds present are potential sites for thermal and oxidative degradation. It is well known that replacing a C— bond with a C—F bond not only enhances the thermal stability of the resulting polymer, but also reduces the dielectric constant. Because incorporation of fluorine is known to impart thermal and oxidative stability, it became of interest to prepare poly(a,a,a, a -tetrafluoro- p -xylylene), Parylene-F Joesten reported that the decomposition temperature of poly(tetrafluoro-j9-xylylene) is ca. 530°C. Thus, it seemed that the fluorinated analog would satisfy many of the exacting requirements for utility as an on-chip dielectric medium. 

Intramolecular C-N bond coupling in arylazides via C-H bond activation catalyzed by [Ir(COD)(OMe)]2 was recently reported [131]. The intermediate iridium nitrenoid complex (50) formed after the extrusion of dinitrogen is proposed as a reactive species, which can cleave the benzylic C-H bond to yield indoline derivatives (31). 

The tra x-[Ru (0)2(por)] complexes are active stoichiometric oxidants of alkenes and alkylaro-matics under ambient conditions. Unlike cationic macrocyclic dioxoruthenium I) complexes that give substantial C=C bond cleavage products, the oxidation of alkenes by [Ru (0)2(por)] affords epoxides in good yields.Stereoretentive epoxidation of trans- and cw-stilbenes by [Ru (0)2(L)1 (L = TPP and sterically bulky porphyrins) has been observed, whereas the reaction between [Ru (0)2(OEP)] and cix-stilbene gives a mixture of cis- and trani-stilbene oxides. Adamantane and methylcyclohexane are hydroxylated at the tertiary C—H positions. Using [Ru (0)2(i)4-por)], enantioselective epoxidation of alkenes can be achieved with ee up to 77%. In the oxidation of aromatic hydrocarbons such as ethylbenzenes, 2-ethylnaphthalene, indane, and tetrahydronaphthalene by [Ru (0)2(Z>4-por )], enantioselective hydroxylation of benzylic C—H bonds occurs resulting in enantioenriched alcohols with ee up to 76%. ... 

The C-H activation of allylic and benzylic C-H bonds has considerable application in organic synthesis. Studies by Muller [131] and Davies [130] on reactions with cyclohexene revealed that Rh2(S-DOSP)4 in a hydrocarbon solvent is the optimum system for high asymmetric induction (Tab. 14.13). Although this particular example gives a mixture of the C-H activation product 179 and cyclopropane 180, similar reactions with ethyl diazoacetate gave virtually no C-H activation product. Some of the other classic chiral dirhodium catalysts 181 and 182 were also effective in this chemistry, but the en-antioselectivity with these catalysts (45% ee and 55% ee) [131] was considerably lower than with Rh2(S-DOSP)4 (93% ee) [130]. 

Intramolecular rhodium-catalyzed carbamate C-H insertion has broad utility for substrates fashioned from most 1° and 3° alcohols. As is typically observed, 3° and benzylic C-H bonds are favored over other C-H centers for amination of this type. Stereospecific oxidation of optically pure 3° units greatly facilitates the preparation of enantiomeric tetrasubstituted carbinolamines, and should find future applications in synthesis vide infra). Importantly, use of PhI(OAc)2 as a terminal oxidant for this process has enabled reactions with a class of starting materials (that is, 1° carbamates) for which iminoiodi-nane synthesis has not proven possible. Thus, by obviating the need for such reagents, substrate scope for this process and related aziridination reactions is significantly expanded vide infra). Looking forward, the versatility of this method for C-N bond formation will be advanced further with the advent of chiral catalysts for diastero- and enantio-controlled C-H insertion. In addition, new catalysts may increase the range of 2° alkanol-based carbamates that perform as viable substrates for this process. 

From this linear correlation, and entering the experimental E value determined for 4-Me0C6H4CH20H (Table 7), a BDE value of 77 1 kcalmoH could be extrapolated for the benzylic C—H bond bearing a geminal OH group. As a matter of fact, the BDEc h of benzyl alcohols was not experimentally available or reported with reasonable confidence the extrapolated value compares well with a BDEc h of 81 1 kcalmol" that could be extrapolated for PhCH20H from data of Espenson and coworkers. Because the BDEc h of toluene is 88.5 kcalmol", the extrapolated value... 

The transfer constant for f-butylbenzene is low, since there are no benzylic C—H bonds present. Primary halides such as n-butyl chloride and bromide behave similar to aliphatics with low transfer constants, corresponding to a combination of either aliphatic C—H bond breakage or the low stability of a primary alkyl radical on abstraction of Cl or Br. The iodide,... 

Whereas, in the presence of PtBr2, internal alkynes of type 127 were cleanly transformed to vinyl naphthalenes (129), similar terminal alkynes were converted primarily to indenes (131). The authors quickly discovered that the observed C—H insertion reaction is very substrate specific. Only tertiary benzylic C—H bonds could be... 

Recently, a general catalytic method forthe conversionof 2-alkyl-l -ethynylbenzenes to indenes was disclosed by the group of Liu [41]. Their proposed mechanism involves the stepwise insertion of a ruthenium vinylidene into a benzylic C— H bond (Scheme 9.21). 

The strength of the O—H bond in ROO—H is comparable to that of allylic and benzylic C—H bonds and is ca 15-20 kcalmor weaker than the corresponding RO—H bond. Therefore, alkoxy radicals like t-BuO are suitable initiators in reactions of the type described in Scheme 37. The hazardous di-icri-butyl peroxalate (DBPO) (ti/2 ca 12 h at 20 °C in solution) (equation 9) and the safer di-fert-butyl hyponitrite (DTBN) (ti/2 ca 12 h at 40 °C) (equation 10) ° have been widely used for the generation of tert-BuO radicals under very mild conditions . 

MoO(02)2(dmpz)2, 120, containing 3,5-dimethylpyrazole (dmpz) in the coordination sphere, in the presence of H2O2, selectively oxidizes benzylic C—H bonds of several alkylbenzenes to the corresponding alcohols and ketones (see, e.g., equation 82). 

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