The formation and use of iminyl radicals in novel and divergent hydroimination and iminohydroxylation cyclization reactions has been accomplished through the design of a new class of reactive O-aryl oximes. Owing to their low reduction potentials, the inexpensive organic dye eosin Y could be used as the photocatalyst of the organocatalytic hydroimination reaction. Furthermore, reaction conditions for a unique iminohydroxylation were identified; visible-light-mediated electron transfer from novel electron donor–acceptor complexes of the oximes and Et3N was proposed as a key step of this process.
Molecular assembly lines, where molecules undergo iterative processes involving chain elongation and functional group manipulation are hallmarks of many processes found in Nature. We have sought to emulate Nature in the development of our own molecular assembly line through iterative homologations of boronic esters. Here we report a reagent (α-lithioethyl triispopropylbenzoate) which inserts into carbon-boron bonds with exceptionally high fidelity and stereocontrol. Through repeated iteration we have converted a simple boronic ester into a complex molecule (a carbon chain with ten contiguous methyl groups) with remarkably high precision over its length, its stereochemistry and therefore its shape. Different stereoisomers were targeted and it was found that they adopted different shapes (helical/linear) according to their stereochemistry. This work should now enable scientists to rationally design and create molecules with predictable shape, which could have an impact in all areas of molecular sciences where bespoke molecules are required.
ABSTRACT:The use of the hypervalent iodine reagents in oxidative processes has become a staple in modern organic synthesis. Frequently, the reactivity of λ 3 iodanes is further enhanced by acids (Lewis or Brønsted). The origin of such activation, however, has remained elusive. Here, we use the common combination of PhI(OAc)2 with BF3·Et2O as model to fully explore this activation phenomenon. In addition to the spectroscopic assessment of the dynamic acid-base interaction, for the first time the putative PIDA·BF3 complex has been isolated and its structure determined by X-Ray diffraction. Consequences of such activation are discussed from a structural and electronic (DFT) points of views, including the origins of the enhanced reactivity.
The stereospecific
cross-coupling of secondary boronic esters with
sp2 electrophiles (Suzuki–Miyaura reaction) is a
long-standing problem in synthesis, but progress has been achieved
in specific cases using palladium catalysis. However, related couplings
with tertiary boronic esters are not currently achievable. To address
this general problem, we have focused on an alternative method exploiting
the reactivity of a boronate complex formed between an aryl lithium
and a boronic ester. We reasoned that subsequent addition of an oxidant
or an electrophile would remove an electron from the aromatic ring
or react in a Friedel–Crafts-type manner, respectively, generating
a cationic species, which would trigger 1,2-migration of the boron
substituent, creating the new C–C bond. Elimination (preceded
by further oxidation in the former case) would result in rearomatization
giving the coupled product stereospecifically. Initial work was examined
with 2-furyllithium. Although the oxidants tested were unsuccessful,
electrophiles, particularly NBS, enabled the coupling reaction to
occur in good yield with a broad range of secondary and tertiary boronic
esters, bearing different steric demands and functional groups (esters,
azides, nitriles, alcohols, and ethers). The reaction also worked
well with other electron-rich heteroaromatics and 6-membered ring
aromatics provided they had donor groups in the meta position. Conditions
were also found under which the B(pin)- moiety could be retained in
the product, ortho to the boron substituent. This protocol, which
created a new C(sp2)–C(sp3) and an adjacent
C–B bond, was again applicable to a range of secondary and
tertiary boronic esters. In all cases, the coupling reaction occurred
with complete stereospecificity. Computational studies verified the
competing processes involved and were in close agreement with the
experimental observations.
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