An efficient domino Pd-catalyzed transformation of simple ortho-bromobenzyl tertiary alcohols to chromenes is presented. Their formation is believed to proceed via the formation of a five-membered palladacycle, which, in turn, involves in an intermolecular homocoupling with the second ortho-bromobenzyltertiary alcohol to yield the homo-biaryl bond followed by intramolecular C-O bond formation. Interestingly, when there is an allylic substituent on the benzylic carbon atom, a chemoselective switch was observed, which preferred intramolecular Heck coupling and gave indenols. Further, it has been confirmed that the tertiary alcohol functionality is indispensible to give the coupled products, whereas the use of primary/secondary benzylic alcohols furnished the simple carbonyl products via a possible reductive debromination followed by oxidation due to the availability of β-hydrogen(s).
An efficient Pd-catalyzed domino reaction of α,α-dialkyl-(2-bromoaryl)methanols to 6,6-dialkyl-6H-benzo[c]chromenes is presented. Their formation can be explained via a five membered Pd(II)-cycle that efficiently involves a domino homocoupling with the second molecule, β-carbon cleavage, and finally intramolecular Buchwald-Hartwig cyclization. This domino process effectively involves breaking of five σ-bonds (2C-Br, 2O-H, and a C-C) and formation of two new σ-bonds (C-C and C-O). This mechanistic pathway is unprecedented and further illustrates the power of transition metal catalysis.
Anilines are one
of the important chemical feedstocks and are utilized
for the preparation of a variety of pharmaceuticals, agrochemicals,
pigments, and dyes. In this context, the catalytic reduction of nitro
functionality is an industrially vital process for the synthesis of
aniline derivatives. Herein, we report an efficient nanosized bimetallic
Pd–Au/TiO
2
nanomaterial which is proved to be quite
efficient for rapid catalytic hydrogen transfer reduction of nitroarenes
into corresponding amines. Significantly, the reduction process is
successful under solvent-free and mild green atmospheric conditions.
Bimetallic Pd–Au nanoparticles served as the active center,
and TiO
2
played as a support in hydrogen transfer from
the source hydrazine monohydrate. Typical results highlighted that
the reactions were very rapid and the products were obtained in good
to excellent yields. Significantly, the process was successful in
the presence of a very low amount catalyst (0.1 mol %). Furthermore,
the reaction showed good chemoselectivity and compatiblity with double
or triple bond, aldehyde, ketone, and ester functionalities on the
aromatic ring. Typical results indicated the true heterogeneous nature
of the Pd–Au/TiO
2
nanocatalyst, where the catalyst
retained the activity, without loss of its activity.
Recent trends in research have gained an orientation toward developing efficient strategies using innocuous reagents. The earlier reported transition-metal-catalyzed carbonylations involved either toxic carbon monoxide (CO) gas as carbonylating agent or functional-group-assisted ortho sp(2) C-H activation (i.e., ortho acylation) or carbonylation by activation of the carbonyl group (i.e., via the formation of enamines). Contradicting these methods, here we describe an environmentally benign process, [Pd]-catalyzed direct carbonylation starting from simple and commercially available iodo arenes and aldehydes, for the synthesis of a wide variety of ketones. Moreover, this method comprises direct coupling of iodoarenes with aldehydes without activation of the carbonyl and also without directing group assistance. Significantly, the strategy was successfully applied to the synthesis n-butylphthalide and pitofenone.
A protecting-group-free two-step approach for the preparation of tetrahydroquinolines has been developed. The procedure involves a highly regio-and enantioselective intermolecular iridium-catalyzed allylic amination followed by one-
Lewis acid (FeCl3) mediated dual bond (C-C and C-O) formation for synthesis of 3,4-dihydrocoumarins is presented. This method has successfully delivered a number of dihydrocoumarins containing dense functionalities on the aromatic ring. Significantly, the present method enabled achieving dihydrocoumarins with tertiary as well as quaternary carbon atoms at the benzylic position. Gratifyingly, the novel spiro-tetracyclic lactones have also been dextrously prepared using this process.
An efficient domino [Pd]-catalysis for the synthesis of isobenzofuran-1(3H)-ones is presented. The strategy shows broad substrate scope and is amenable to o-bromobenzyl tertiary/secondary/primary alcohols. Significantly, the method was applied to the synthesis of antiplatelet drug n-butyl phthalide and cytotoxic agonist 3a-[4'-methoxylbenzyl]-5,7-dimethoxyphthalide.
Herein, we present m-C–H olefination on
derivatives of phenylacetic acids by tethering with a simple nitrile-based
template through palladium catalysis. Notably, the versatility of
the method is evaluated with a wide range of phenylacetic acid derivatives
for obtaining the meta-olefination products in fair
to excellent yields with outstanding selectivities under mild conditions.
Significantly, the present strategy is successfully exemplified for
the synthesis of drugs/natural product analogues (naproxen, ibuprofen,
paracetamol, and cholesterol).
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