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Mechanisms and Stereochemistry of Acid‐Induced Ring Opening of Optically Active 1,2‐Propene Oxides in the Gas Phase
Author(s) -
Troiani Anna,
Filippi Antonello,
Speranza Maurizio
Publication year - 1997
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.19970031223
Subject(s) - chemistry , propene , epoxide , nucleophile , ring (chemistry) , moiety , regioselectivity , stereochemistry , medicinal chemistry , proton , photochemistry , catalysis , organic chemistry , physics , quantum mechanics
The acid‐induced ring opening of ( S )‐(—)‐1,2‐propene oxide ( 1S ) and ( R )‐(+)‐1,2‐propene oxide ( 1R ) has been investigated in gaseous CH 4 and CH 3 F at 720 torr and in the presence of a nucleophile, NuOH (Nu = H or CH 3 ). The mechanism of the ring‐opening reaction has been assessed by modulating the composition of the gaseous mixture. Two reaction pathways are operative in the gas phase, both proceeding through complete inversion of configuration of the reaction center. A first process is detectable only in the CH 3 F/H 2 O systems and takes place within a persistent proton‐bound complex generated by interaction of the epoxide with the CH 3 OH + 2 ion, formed by methylation of H 2 O with (CH 3 ) 2 F + . Such an intracomplex ring‐opening pathway proceeds through proton transfer from the CH 3 OH + 2 ion to the epoxide followed by motion of the neutral CH 3 OH moiety around the 1‐H‐oxonia‐2‐methyl‐cyclo‐propane structure (H‐ 1R or H‐ 1S ) ( k <10 8 s ‐1 ) before attacking the ring carbons from the rear. In all the other systems with added CH 3 OH, this intracomplex pathway is preceded by a faster “ extracomplex ” pathway involving the attack of an external CH 3 OH molecule on the proton‐bound adduct. The regioselectivity of the intracomplex process is similar to that of the extracomplex pathway. Both are characterized by a slight preference for the C β center of H‐ 1 R (or H‐ 1S ) ( extra‐complex path regioselectivity: α/β = 0.72±0.05; intracomplex path regioselectivity: α/β = 0.71±0.05). The regioselectivity of H‐ 1 R (or H‐ 1S ) is substantially different from that of the 1‐Me‐oxonia‐2‐methyl‐cyclopropanes (Me‐ 1 R or Me‐ 1 S ) toward the same nucleophile NuOH (α/β = 4.1±0.35 (Nu = H); 2.28±0.16 (Nu = CH 3 )). This difference is attributed to a transition structure wherein the C α –O bond rupture increases from H‐ 1 R (or H‐ 1 S ) to Me‐ 1 R (or Me‐ 1 S ) and in passing from CH 3 OH to H 2 O. The regioand stereoselectivity of the gas‐phase acid‐induced ring opening of 1 S and 1 R are compared with those of related reactions carried out in solution.