Electrophilic halofunctionalization
reactions have undergone a
resurgence sparked by recent discoveries in the field of catalytic
asymmetric halocyclizations. To build mechanistic understanding of
these asymmetric transformations, a toolbox of analytical methods
has been deployed, addressing the roles of catalyst, electrophile
(halenium donor), and nucleophile in determining rates and stereopreferences.
The test reaction, (DHQD)2PHAL-catalyzed chlorocyclization
of 4-arylpent-4-enoic acid with 1,3-dichloro-5,5-dimethylhydantoin
(DCDMH), is revealed to be first order in catalyst and chlorenium
ion donor and zero order in alkenoic acid substrate under synthetically
relevant conditions. The simplest interpretation is that rapid substrate–catalyst
binding precedes rate-limiting chlorenium attack, controlling the
face selectivity of both chlorine attack and lactone closure. ROESY
and DFT studies, aided by crystal structures of carboxylic acids bound
by the catalyst, point to a plausible resting state of the catalyst–substrate
complex predisposed for asymmetric chlorolactonization. As revealed
by our earlier labeling studies, these findings suggest modes of binding
in the (DHQD)2PHAL chiral pocket that explain the system’s
remarkable control over rate- and enantioselection-determining events.
Though a comprehensive modeling analysis is beyond the scope of the
present work, quantum chemical analysis of the fragments’ interactions
and candidate reaction paths point to a one-step concerted process,
with the nucleophile playing a critical role in activating the olefin
for concomitant electrophilic attack.
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