Cage Complexes of Carbenium and Silylium Cations with an Aromatic Base. Is the η⁶Coordination Type Realizable?

Abstract

The cage cations [E((CH2)3)3C6R3]+ (E = C (1), Si (2); R = H (a), Li (b), F (c)) were studied theoretically using the MP2/6-311++G(d,p) approach. The migration of the electrophilic center E and a proton around the perimeter of the arene ring in the cations [E((CH2)3)3C6H3]+ was considered. Our results are indicative of the π type of E←arene coordination in the intramolecular complexes [E((CH2)3)3C6R3]+. The cation [Si((CH2)3)3C6H3]+ (2a), which corresponds to a peak (M – CH3)+ at m/z 229 in the mass spectrum of methylsilacyclophane, exists in a firmly established η1π form, as opposed to the case for the known complexes of the trivalent silicon atom with aromatic bases. The species [C((CH2)3)3C6F3]+ (1c), as well as 4a, obtained by connecting the equatorial carbon atoms in [Si((CH2)3)3C6H3]+ by a methylene bridge, are the first representatives of stable “face” complexes of carbenium and silylium cations with a benzene ring. The process of deprotonation of the complexes [C((CH2)3)3C6H3]+, [Si((CH2)3)3C6H3]+, and 4a was found to be energetically unfavorable, even in the presence of a strong base such as NEt3. The effect of the counterions BX4 – (X = F, C6F5) and the polarity of solvents on the structure of the above cations was investigated using the examples of toluene and DMSO.

An analysis of electrophilic aromatic substitution: a “complex approach”

“…It can be seen from Scheme 5 that the EAS reaction also goes through the formation of p-complexes and s-complexes, [16][17][18][19][20][21][22][23] which explains the gradual destabilization of the aromatic system. First, a p-complex is formed, which is thermodynamically less stable and rapidly isomerizes to the corresponding scomplex.…”

“…Peaks that occur at 260-367 ppm in 13 C NMR spectra correspond to new highly magnetically deflected sites. 18 However, peaks located at 150-190 ppm were seen in the 13 C NMR spectra of the s-complexes, which can be explained by the presence of appropriate substituents that stabilize the positive charge and also by the structure of these complexes. 21,31 Their appearance in 13 C NMR spectra is characterized by the appearance of moderately strong or weak singlets, even when using distortionless enhancement by polarization transfer 13 C NMR and similar techniques.…”

“…In addition to these two methods, X-ray spectroscopy can also provide insights into the structure of the s-complex formed. 18 Characterization of p-complexes in electrophilic aromatic substitution…”

“…41 If the concept of unsaturation is introduced into the thesis of hapticity in EAS, a clear trend can be seen where the increase in the index of hydrogen deficiency (or the double bond equivalents) is accompanied by an increase in the hapticity, along with a general proportional increase in the thermodynamic stability of these p-complexes. Indeed, in the general interpretation of the stability of p-complexes, this trend is manifested, but only in hyperhaptive p-complexes; on the other hand, in the case of 'regular' hapticity complexes, this trend is not so precisely and correctly defined, 18 even when general considerations of stability are introduced. 43 That is to say, complexes with Z of 3, 4, and 5 are less frequently characterized and are more likely to be characterized when there are multiple electron-donating substituents on the benzene nucleus, wherein this number is generally Z3 row-linked substituents, in strong activating groups (e.g., OH, OR, NR 2 , CR 2 À , M; R = H, alkyl), or more than three sequentially or alternately arranged substituents, in moderately strong groups (e.g., O-esters, N-amides, OCN, SR; R = H, alkyl).…”