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Substituent Effects on the Stereochemistry of Gas‐Phase Intracomplex Nucleophilic Substitutions
Author(s) -
Filippi Antonello,
Fraschetti Caterina,
Renzi Gabriele,
Roselli Graziella,
Speranza Maurizio
Publication year - 2006
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200600063
Subject(s) - solvolysis , chemistry , racemization , substituent , medicinal chemistry , trifluoromethyl , ethanol , stereochemistry , atmospheric temperature range , nucleophile , gas phase , alcohol , phase (matter) , organic chemistry , hydrolysis , catalysis , alkyl , thermodynamics , physics
Abstract The mechanism and stereochemistry of the intracomplex solvolysis of proton‐bound complexes [ Y ⋅⋅⋅H⋅⋅⋅ M ] + between M = CH 3 18 OH and Y = 1‐arylethanol [( S )‐1‐( para ‐tolyl)ethanol ( 1 S ), ( S )‐1‐( para ‐chlorophenyl)ethanol ( 2 S ), ( S )‐1‐( meta ‐α,α,α‐trifluoromethylphenyl)ethanol ( 3 S ), ( S )‐1‐( para ‐α,α,α‐trifluoromethylphenyl)ethanol ( 4 S ), ( R )‐1‐(pentafluorophenyl)ethanol ( 5 R ), ( R )‐α‐(trifluoromethyl)benzyl alcohol ( 6 R ), and ( R )‐1‐phenylethanol ( 7 R )] have been investigated in the gas phase (CH 3 F; 720 Torr) in the 25–140 °C temperature range. Gas‐phase solvolysis of [ Y ⋅⋅⋅H⋅⋅⋅ M ] + ( Y = 2 S , 3 S , 4 S , and 7 R ) leads to extensive racemization above a characteristic temperature t # (e.g. at t # >60 °C for 7 R ), whereas below that temperature the reaction displays a preferential retention of configuration. Predominant retention of configuration is instead observed in the intracomplex solvolysis of [ Y ⋅⋅⋅H⋅⋅⋅ M ] + ( Y = 1 S , 4 S , 5 R , and 6 R ) with the temperature range investigated (25 ≤ T ≤ 120 °C). These results indicate that the intracomplex solvolysis proceeds through the intermediacy of the relevant benzylic cations (Bz + ), which is electrostatically coordinated to a H 2 O and a CH 3 18 OH molecule (a pure S N 1 mechanism). The obtained gas‐phase mechanism is discussed in the light of related solution data. It is concluded that the stereochemistry of unimolecular solvolytic reactions is determined by the lifetime and the dynamics of the species involved and, if occurring in solution, by the nature and the dynamics of the solvent cage as well.