Reactions of methylcubane (
1)
with the
tert-butoxyl radical (
t-BuO
),
withcytochrome P-450 enzymes,and
with a methane monooxygenase (MMO) system have been studied.For the purpose of product characterization,authentic samples of 2-methylcubyl and 4-methylcubyl derivatives
wereprepared. 2-Methylcubanecarboxylic acid(
9b) is a ne
w compound prepared from cubanecarboxylic acid.The key synthetic reactions
were (1) metalation andsubsequent iodination of the 2-position of(diisopropylcarbamoyl)cubane to effect the initialfunctionalization, (2)lithium-for-iodine exchange and methylation follo
wed by reduction togive 2-methyl-1-[(diisopropylamino)methyl]cubane, and (3) dimethyldioxirane oxidation of this amine to give
9b. The kno
wn 4-methylcubanecarboxylicacid(
9d)
was prepared here by a route related to that employedfor
9b. Reactions of acids
9b and
9d with methyllithiumgave the corresponding methyl ketones
which
were oxidized by
m-chloroperoxybenzoic acid to provide authenticsamples of 2- and 4-methylcubanol acetates (
3b and
3d). Reaction of
1 with
t-BuO
in the presence of 2,2,5,5-tetramethylisoindole-
N-oxyl radical (TMIO
) at40-55
C gave mainly cube-substituted products in confirmationofthe report (Della, E. W.; Head, N. J.; Mallon, P.; Walton, J. C.
J. Am. Chem. Soc. 1992,
114, 10730)that hydrogenatom abstraction by the electrophilic alkoxyl radical at lo
wtemperature occurs at the cubyl C-H positions. In acompetition experiment at 42
C, methylcubane
was at least 3.5 timesmore reactive to
ward
t-BuO
thancyclohexane,indicating that the cubyl positions in
1 are
40 times morereactive than the methyl positions in
1 (perhydrogen)to
ward the alkoxyl radical. Oxidation of
1 by enzymesgave alcohol products that
were converted to their acetatederivatives for identification and quantitation. Microsomalcytochrome P-450 enzymes from rat and the rat purifiedP-450 isozyme CYP2B1 hydroxylated
1 at all positions,
whereas the reconstituted MMO system from
Methylococcuscapsulatus (Bath) hydroxylated
1 only at the methylposition. The differences in regioselectivity suggest thatthetransition states for hydrogen abstraction by the alkoxyl radical andfor enzyme-catalyzed hydroxylation differconsiderably. The results are consistent
with a model forconcerted enzyme catalyzed hydroxylation of
1involving"side-on" approach to the C-H bond of substrate.