Methylene arenium

Interestingly, reaction of the QM complex 1 with strong electrophiles, such as HOTf or Me3SiOTf, did not lead to the expected rearomatization. Rather, the first stable methylene arenium complexes 4 were formed (Scheme 3.4).10... 

Following this observation, a general approach for the synthesis of pincer-type methylene arenium compounds was developed (Scheme 3.4). Upon reaction of the methyl rhodium (or iridium) complexes 5 with a slight excess of triflic acid, dihydrogen (not methane ) was evolved to form the methylene arenium complexes 4a.11 Thus, the methylene arenium form is clearly preferred over the benzylic M(III) form, in which the positive charge is localized at the metal center. 

In addition to the methylene arenium case, in which a coordinatively unsaturated positively charged metal center is stabilized by transfer of positive charge to the aromatic ring, stabilization can be accomplished by rf-C—H or rj2-C—C agostic interactions with the aromatic system (see Ref. [5]). 

As observed with the pincer-type quinone methide complexes (Section 3.2), the one-site coordinated QMs can also undergo chemical transformation to other quinonoid compounds. For instance, reaction of complex 20 with MeOTf resulted in the methylene arenium complex 21 (Scheme 3.13).22... 

Experimental observations,23 supported by high-level ab initio calculations, 24 indicate that two extreme resonance forms contribute to the general energy of the benzyl cation the aromatic form A, in which the positive charge is concentrated at the methylene group, and the nonaromatic, methylene arenium form B with a sp2 ipso-carbon atom and ring-localized charge (Scheme 3.13). Unlike benzyl cations of the form A, which were isolated and studied, especially by Olah and coworkers,23 compounds represented by the form B remained elusive. Thus, metal complexation... 

Vigalok, A. Shimon, L. J. W. Milstein, D. Methylene arenium cations via quinone methides and xylylenes stabilized by metal complexation. J. Am. Chem. Soc. 1998, 120, 477 183. 

Gauvin, R. M. Rozenberg, H. Shimon, L. J. W. Ben-David, Y. Milstein, D. Osmium-mediated C—H and C—C bond cleavage of a phenolic substrate p-quinone methide and methylene arenium pincer complexes. Chem. Eur. J. 2007, 13, 1382-1393. 

The electron-deficient complex [Rh(C0)L]CF3S03 344 (L = 3,5-bis(di-r t -butylphosphinomethyl)-2,6-dimethyl-4-methylene-3,5-cyclohexadien-l-one) was prepared." " Protonation of the rhodium complex RhCl(CH3)[C6H(CH3)2(CH2P(/-Bu)2)2] with a strong acid (HOTf, trifluoromethanesulfonic acid) resulted in clean formation of the methylene arenium complex [RhCl CH2=C6H(CH3)2(CH2P( -Bu)2)2 ]OTf which was fully characterized (Scheme 47)." " " ... 

Recent protonation studies on methylene-bridged PAHs led to the generation and complete NMR characterization of a series of persistent carbocations.17 33 Figure 51 summarizes the proton data for these non-altemant arenium ions, whereas carbon NMR data and A5 values are gathered in Fig. 52. The protonation sites... 

Fig. 51. Compilation of H NMR data for arenium ions from methylene-bridged non-alternant polyaromatic hydrocarbons (partial A<5lH values in parentheses).

Besides the above-considered interactions of aromatic compounds with electrophilic agents to generate arenium ions, there are a number of other ways. In 1958 E. Doering et al. showed 4-methylene-l,l,2,3,5,6-hexamethylcyclohexa-2,5-diene (30) to be a very strong base (pK = 1.38 and, unlike polymethylbenzenes, to be readily solved in hydrochloric acid yielding the heptamethylbenzenium ion (16a). 

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