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5-Methoxyl Participation in Solvolysis and Secondary Deuterium Isotope Effects

R. Eliason ; »Ruder Boskovic«, Institute, 41001 Zagreb, Yugoslavia
S. Borčić ; »Ruder Boskovic«, Institute, 41001 Zagreb, Yugoslavia
D. E. Sunko ; »Ruder Boskovic«, Institute, 41001 Zagreb, Yugoslavia

Sažetak

Specifically deuterated 4-methoxy-1-pentyl broij'ylates (IP-
1,1-d2, IP-2,2-d2) and 5-methoxy-2-pentyl brosylates (IS-2-d, 1S-
-1 ,1,1 - d3, 1S-OCD3) were prepared, their solvolysis rates measured
in 950/o ethanol and the corresponding kinetic secondary deuterium
isotope effects calculated. It was found that the a-effects are strongly
reduced in magnitude, which is in contrast with results obtained
with several compounds solvolyzing via n- or CJ-participation. The
effect found with IP-l,1-d2 (kHfkD = 0.96) is significantly different
from the one measured with IS-2-d (kHfkD = 1.06). This difference
in the magnitude of the effects must reflect a relatively small
difference in the structures of the two transition states, since both
IP and IS solvolyze via the same intermediate 2. It is concluded
that the magnitude of the u- effP.ct is a sensitive measure of the
amount of bridging only if the new bond is already quite strong
in the transition state.
B-Deuterated brosylates give small effects which differ in magnitude
(kH/kD = 0.96 with IP-2,2-d2 and 1.08 with IS-1,1,1-da)
corroborating the conclusion that the new C-0 bond is strong
in the transition state of both reactions but stronger with IP than
with IS.
Results obtained in this work are in accord with the earlier
conclusion that reduced B-effects in solvolysis of exo-2-norbornyl,
3-cholesteryl, cyclobutyl and 1-methylcyclobutyl derivatives are
due to bridging.
The inverse isotope effect obtained with IS-OCD3 (kH/kD=0.95)
is »inductive« in origin and indicates that the methoxy oxygen
bears a considerable amount of positive charge in the reaction
transition state.

Ključne riječi

Hrčak ID:

195945

URI

https://hrcak.srce.hr/195945

Datum izdavanja:

22.12.1978.

Posjeta: 879 *