A (hydroxycyclopentadienyl)iron dicarbonyl hydride catalyzes the Oppenauer-type oxidation of alcohols with acetone as the hydrogen acceptor. Many functional groups are tolerant to the oxidation conditions. The same complex also catalyzes the dehydrogenation of diols to lactones. A mechanism involving the formation of iron-alcohol complexes and their rapid ligand exchange with free alcohols is proposed. The trimethylsilyl groups on the cyclopentadienyl ligand of the catalyst play a critical role in stabilizing the iron hydride and increasing the catalyst lifetime.
The compatibility of the Negishi cross-coupling reaction with the versatile B–Cl functionality has been demonstrated in the context of late-stage functionalization of 1,2-azaborines. Alkyl-, aryl-, and alkenylzinc reagents have been utilized for the functionalization of the triply orthogonal precursor 3-bromo-1-(tert-butyldimethylsilyl)-2-chloro-1,2-dihydro-1,2-azaborine (2) to furnish new 2,3-substituted monocyclic 1,2-azaborines. This methodology has enabled the synthesis of previously elusive BN-naphthalene and BN-indenyl structures from a common intermediate.
The
first example of catalytic B–H activation of azaborines
leading to a new family of stilbene derivatives through dehydrogenative
borylation is reported. Ten 1,2-azaborine-based BN isosteres of stilbenes
have been synthesized using this method, including a BN isostere of
a biologically active stilbene. It is demonstrated that BN/CC isosterism
in the context of stilbenes can lead to significant changes in the
observed photophysical properties such as higher quantum yield and
a larger Stokes shift. Direct comparative analysis of BN stilbene 3g and its carbonaceous counterpart 6g is consistent
with a stronger charge-transfer character of the excited state exhibited
by 3g in which the 1,2-azaborine heterocycle serves as
a better electron donor than the corresponding arene.
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