Chemistry Reference
In-Depth Information
18
O Distribution in rearrangements of esters of hydroxamic acids.
SCHEME 4.15.
undergo reduction by Fe
2
þ
with large rate accelerations.
18,95
Apparently, these esters
do not efficiently complex with Fe
2
þ
.
95
Heesing and coworkers examined the extent of
18
O scrambling during
rearrangement of
O
-alkyl or
O
-arylsulfonyl-
N
-benzoyl-
N
-phenylhydroxylamines
(Scheme 4.15).
100
Outcomes that can be characterized by the percentage of
18
O
label in the phenol oxygen of the product from esters with the
18
O label originally at
the ester oxygen are summarized in Scheme 4.15. Rearrangement of the
para
-tosyl
ester in a series of nonhydroxylic solvents at 0-20
C proceeded with a label
distribution in the phenol oxygen of the product that ranged from 20% in dioxane,
25% in ether, 29% in pyridine, to 33% in CH
3
CN.
100a
This corresponds to label
randomization ranging from 61% in dioxane to 100% in CH
3
CN. Scrambling
increases to that expected for complete randomization as solvent polarity increases.
Radical processes were discounted because the label distribution was not sensitive to
UV irradiation. Results were explained by an ion pair that in low-polarity solvents
collapsed to products before complete randomization of the label could occur.
Rearrangement in methanol led to 43% of the label in the phenol oxygen of the
product.
100b
This is consistent with 85% of rearrangement proceeding via a process
leading to randomization and 15% by the equivalent of a 1,3-rearrangement. It was
suggested that H-bonding by the solvent slows down rotation of the sulfonate ion that
prevents complete label randomization.
100b
Underwood found similar results in
34a
.
86
These results could be explained by short-lived
ion pairs with incomplete label randomization, or by a concerted mechanism in
competition with an ion pair mechanism in which randomization is complete.
Ford and Scribner published the first serious attempt to calculate the properties of
aryl nitrenium ions in 1981.
101
Their MNDO calculations predicted that ions
formed during hydrolysis of
32a
64a
-
l
