C O M M U N I C A T I O N S
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A preliminary mechanistic analysis led us to propose the catalytic
cycle shown in Scheme 1. The deuterium atom from pinacolborane-
d1 was found at the methyl group of the hydroborated product 2a-d
exclusively. Selective deuteration is consistent with migratory
insertion into either the Fe-B or Fe-H bond via the allyliron
intermediate 29a or 29b, respectively, but cannot distinguish
between the two pathways. The proposed compounds 29a and 29b
were not observed during catalysis. The turnover-limiting step and
the reversibility of the steps of the catalytic cycle are currently
unknown and, hence, the ligand-controlled regioselectivity (e.g.,
entries 6 vs 7) could be determined during oxidative addition or
migratory insertion. When the branched isomer 2b was subjected
to the reaction conditions of hydroboration, no linear isomer 2a
was observed, which established that at least one of the steps
following the regioselectivity-determining step is irreversible. The
selectivity for double bond geometry can be rationalized by the
proposed mechanism via syn migratory insertion to Fe-allyl 29a
or 29b.
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Scheme 1. Proposed Mechanism for 1,4-Hydroboration
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(E)-γ-disubstituted allylboranes. The iminopyridine-Fe-catalyzed
reactionprovidesaccesstoallylboranessversatilebuildingblockssthat
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Acknowledgment. We thank Novartis for a fellowship for
J.Y.W., Materia for a generous donation of Grubbs II catalyst, and
the ACS PRF.
Supporting Information Available: Detailed experimental proce-
dures and spectroscopic data for all new compounds. This material is
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