Isomeric Alkyl Cation/Arene Complexes in the Gas Phase

Abstract

The kinetics and the stereochemistry of the protonation-induced unimolecular isomerization of (S)-(+)-1-D(1)-3-(p-tolyl)butane have been investigated in the gas phase in the 100-160 degrees C range. The process leads to the almost exclusive formation of the relevant meta isomer with complete racemization and partial 1,2-H shift in the migrating sec-butyl group. These results, together with the relevant activation parameters, point to the occurrence of low-energy, tightly bound isomeric sec-butyl cation/toluene complexes of defined structure and stability along the isomerization coordinate. The existence and the eta(1)-type structure of these low-energy intermediate species are confirmed by ab initio calculations on closely related systems at the MP2(full)/6-311++G**//HF/6-31+G** level of theory. Their role in the relevant energy surface clearly emerges from the comparison of the present results with those concerning sec-butylation of toluene carried out under comparable experimental conditions.

Crucial Role of Elusive Isomeric η-Complexes in Gas-Phase Electrophilic Aromatic Alkylations

“…This mechanistic picture shows an analogy close to that proposed in our previous kinetic investigation on gasphase isomerization of protonated 1 Me S (henceforth denoted as I Me ). 23 In this study, only the para f meta isomerization was observed with complete racemization and partial H-scrambling in the moving sec-butyl moiety. The activation parameters of the isomerization without side-chain hydrogen scrambling (∆H q ) 10.3 ( 1.2 kcal mol -1 and ∆S q ) -5.3 ( 3.6 cal mol -1 K -1 ) are comparable with those measured for the analogous I f II and I f IV reactions.…”

“…This lowenergy region is characterized by the presence of several shallow potential energy wells corresponding to η 2′chelates A, B, B′, and B′′ separated from the corresponding σ-bonded intermediates, I-V, by very limited energy barriers. 23,45,46 As suggested in a previous study, 23 these low-energy η 2′ -chelates may play a crucial role in gasphase aromatic alkylation reactions as well. The pronounced difference in the ortho/meta ratios arising from I isomerization (Table 1) and from the direct sec-butylation of fluorobenzene (Table 4), carried out under comparable experimental conditions, excludes the possibility that the first process proceeds through the same loosely bound π-complex proposed by A&G as an intermediate of the latter reaction.…”

“…This sort of hydride ion anchimeric assistance seems to be much less effective in the isomerization of I Me . 23 Indeed, the para f meta isomerization of I Me involves activation parameters (∆H q ) 16.9 ( 3.1 kcal mol -1 and ∆S q ) +9.9 ( 7.0 cal mol -1 K -1 ) which suggest that the sec-butyl moiety undergoes a H-shift only after the almost complete C(R)-C(1) bond cleavage. This behavior may be explained by the hypothesis that a significant amount of positive charge must develop at the C(R) site of the sec-butyl moiety to promote the 1,2-H shift.…”

“…One of the first contributions to this crucial theme came from the study of the proton-induced isomerization kinetics of (S)-(+)-1-D 1 -3-(p-tolyl)butane (1 Me S in Chart 1) in the gas phase at 70 Torr and 100-160 °C. 23 The emerging picture, confirmed by theoretical calculations, suggests that the isomerization may proceed through the intermediacy of low-energy short-lived isomeric η-type structures (lifetime is ca. 10 -10 s).…”

“…This lack of information is particularly unsatisfactory since, in principle, the structure and dynamics of noncovalent complexes may control their evolution kinetics and the selectivity of the substitution process. One of the first contributions to this crucial theme came from the study of the proton-induced isomerization kinetics of ( S )-(+)-1-D 1 -3-( p -tolyl)butane ( 1 Me S in Chart ) in the gas phase at 70 Torr and 100−160 °C . The emerging picture, confirmed by theoretical calculations, suggests that the isomerization may proceed through the intermediacy of low-energy short-lived isomeric η-type structures (lifetime is ca.…”