C-H allylation

Scheme 14.59 Ni-catalysed C-H allylation of benzamides using allyl phosphates.

In addition to benzylations, palladium-catalyzed C-H allylation reactions have also been described, but they usually involve the electrophilic substitution of an electron-rich (hetero)arene with a 7t-allyl palladium complex and therefore deviate from the scope of this chapter [41, 42]. In contrast, the palladium/copper-catalyzed allylation of polyfluoroarenes with allyl carbonates (Scheme 19.27) has been reported to occur through a different mechanism [43]. Thus, base-induced cupration of the arene would give rise to intermediate 15 that was previously characterized by X-ray crystallography [44]. Attack of the in sitw-generated JT-allyl... 

Exposure of anilide 151 and allyl carbonate 152 to a rhodium catalyst resulted in a tandem C-H allylation/oxidative cyclization to afford indole 153.The route is fairly economical the allyl group is incorporated into the indole core and only CO2 and methanol are generated as by-products. A variety of electron-donating and -withdrawing groups as well as halogens can be present as substituents (130L4576). 

Brimble, M.A. and Heathcock, C.H., Allylic amination by the Lewis-acid-mediated ene reaction of diethyl azodicarboxylate with alkenes, /. Org. Chem., 1993, 58(19), 5261-5263. 

Bond C-C C-H Primary C-H Secondary C-H Tertiary C-H Allyl C-H Vinyl... 

The copper catalysed, direct C-H allylation of polyfluoroarenes (184) with E and Z allyl phosphoates (185) and (186) to obtain allyl arenes (187) and (188) that contained fluorinated aromatic cores of an electron-deficient nature has been described by Miura et al. (Scheme 52). ... 

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

The degree to which allylic radicals are stabilized by delocalization of the unpaired electron causes reactions that generate them to proceed more readily than those that give simple alkyl radicals Compare for example the bond dissociation energies of the pri mary C—H bonds of propane and propene... 

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... 

From this value and known C—H bond dissociation energies, pK values can be calculated. Early application of these methods gave estimates of the p/Ts of toluene and propene of about 45 and 48, respectively. Methane was estimated to have a pAT in the range of 52-62. Electrochemical measurements in DMF have given the results shown in Table 7.3. These measurements put the pK of methane at about 48, with benzylic and allylic stabilization leading to values of 39 and 38 for toluene and propene, respectively. The electrochemical values overlap with the pATdmso scale for compounds such as diphenyl-methane and triphenylmethane. 

According to these data, which structural features provide stabilization of radial centers Determine the level of agreement between these data and the radical stabilization energies given in Table 12.7 if the standard C—H bond dissociation energy is taken to be 98.8 kcal/mol. (Compare the calculated and observed bond dissociation energies for the benzyl, allyl, and vinyl systems.)... 

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 ... 

The predicted bond order for a given bond is listed at the intersection of the two atoms of interest in the bond orders table. The illustration at the left shows the predicted bond orders for this molecule (where 1.0 is a traditional single bond, 2.0 is a double bond, and so on). The C-H bonds all have predicted bond orders of about. 9, while the C-C bonds have predicted bond orders of about 1.4. The latter arc consistent with the known resonance structure for allyl cation. ... 

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