Substituted toluenes, benzylic metallation

A selective method for benzylic metalation of o-, m-, and p-substituted toluenes has been reported using BuLi/f-BuOKVTMP and rationalized by the ability of the mixed metal amide base to facilitate an anion migration from the kinetic (o-aryl) to the benzylic metalation site. Reaction of 2-(l-phenylethylidene)malononitrile and 2,2,2-trifluoro-1 -phenylethanone gives (2 ,4 )-2-cyano-6,6,6-trifluoro-3,5-diphenyl-hexa-2,4-dienamide via a vinylogous aldol reaction followed by rearrangement of the 2/f-pyran intermediate (Scheme 52). °°... 

The acidity of benzylic protons of aromatics complexed to transition-metal groups was first disclosed by Trakanosky and Card with (indane)Cr(CO)3 [61]. Other cases are known with Cr(CO)3 [62], Mn(CO)3 [63], FeCp+ [64, 65], and Fe(arene)2+ [31, 66] but none reported the isolation of deprotonated methyl-substituted complexes. We found that deprotonation of the toluene complex gives an unstable red complex which could be characterized by 13C NMR ( Ch2 = 4.86 ppm vs TMS in CD5CD3) and alkylated by CH3I [58] Eq. (13) ... 

Syndiotactic polystyrene was first obtained only recently by Ishihara et al. [5] in polymerisation with a homogeneous catalyst derived from a transition metal compound such as monocyclopentadienyltitanium trichloride and methylalu-minoxane in toluene. Since then, several authors have reported on the synthesis of syndiotactic polystyrene promoted by different catalysts based on metal hydrocarbyls such as benzyl compounds, half-sandwich metallocenes (e.g. monocyclopentadienyl, monopentamethylcyclopentadienyl and monoindenyl metal derivatives), metal alkoxides, metallocenes and some other compounds. These catalysts are commonly derived from titanium or zirconium compounds, either activated with methylaluminoxane or aluminium-free, such as those activated with tris(pentafluorophenyl)boron, and promote the syndiospecific polymerisation of styrene and substituted styrenes [5-10,21,48-70], Representative examples of the syndiospecific polymerisation of styrene using catalysts based on various titanium compounds and methylaluminoxane are shown in Table 4.2 [6,52,53,56,58],... 

Allyl bromide or benzyl chloride and various ring-substituted benzyltin chlorides however react with metallic tin in refluxing toluene (110 °C) to give good yields of the corresponding diorganotin dihalide (e.g. equation 4-40) 62 the tin can be activated with mercuric chloride and triethylamine 63 Pentamethylcyclopentadienyl bromide similarly reacts with tin to give the pentamethylcyclopentadienyltin bromides.64... 

Benzylic oxidation of aromatic side-chains is also a well established technology in the bulk chemicals arena, e. g. toluene to benzoic acid and p-xylene to ter-ephthalic acid [1,2]. These processes involve homogeneous catalysis by, e. g., cobalt compounds, however, and also fall outside the scope of this book. Ammoxi-dation of methyl-substituted aromatic and heteroaromatic compounds is performed over heterogeneous catalysts in the gas phase but this reaction is treated elsewhere (Section 9.5). Transition metal-substituted molecular sieves have been widely studied as heterogeneous catalysts for oxidation of aromatic side-chains in the liquid phase, but there are serious doubts about their heterogeneity [5,6]. Here again, a cursory examination of the literature reveals that supported palladium seems to be the only heterogeneous catalyst with synthetic utility [4]. 

The literature reveals that both retention and inversion of the configuration have been observed for the reactions of lithiated benzylic and related systems with various electrophiles.[14] hi most cases no solid-state structure of the corresponding lithium alkyl could be determined to confirm the absolute configuration of the stereogenic metalated carbon center. The stereochemical course of the reaction of our lithium alkyl with the electrophile iodomethane in toluene can be understood on the basis of the solid-state structure of compound (R,S)-2, which was crystallized from the same solvent as was used in the substitution reaction with the alkyl iodide, in combination with computational studies. 

The results of the reaction of benzyl and substituted benzyl halides with metallic nickel powders are summarized in Table 7.13. Benzyl chloride reacted at room temperature with metallic nickel prepared from nickel chloride to give a mixture of the coupled product bibenzyl (40%) and the reduction product toluene (60%). However, the coupled product was found to be formed mainly when the reaction was run at 70°C and when the nickel powders used were prepared from nickel iodide. Under these conditions, a yield of 86% bibenzyl was attained. The fact that iodide ion present in the system facilitates the homocoupling reaction may be ascribed to the chlorine-iodine exchange during the reaction [148]. Higher reaction temperatures such as 70°C may also accelerate the exchange reaction. 

---