309254-29-3Relevant academic research and scientific papers
Direct transition-metal-free intramolecular arylation of phenols
Bajracharya, Gan B.,Daugulis, Olafs
supporting information; experimental part, p. 4625 - 4628 (2009/05/13)
(Chemical Equation Presented) Direct transition-metal-free, base-mediated intramolecular arylation of phenols with aryl halides has been developed. In the presence of 2.5 equiv of t-BuOK in dioxane at 140 °C, the intramolecular cyclization of 3-(2-halobenzyloxy)phenols affords 6H-benzo[c]chromenes in high yields. This reaction proceeds by an initial formation of a benzyne intermediate followed by an aromatic sp2 C-H functionalization (a formal C-H activation) to form the carbon-carbon bond.
Catalytic direct arylation with aryl chlorides, bromides, and iodides: Intramolecular studies leading to new intermolecular reactions
Campeau, Louis-Charles,Parisien, Mathieu,Jean, Annie,Fagnou, Keith
, p. 581 - 590 (2007/10/03)
A catalyst for the intramolecular direct arylation of a broad range of simple and heterocyclic arenes with aryl iodides, bromides, and chlorides has been developed. These reactions occur in excellent yield and are highly selective. Studies with aryl iodides substrates revealed that catalyst poisoning occurs due to the accumulation of iodide in the reaction media. This can be overcome by the addition of silver salts which also permits these reactions to occur at lower temperature. The utility of the methodology is illustrated by a rapid synthesis of a carbazole natural product and by the synthesis of sterically encumbered tetra-ortho-substituted biaryls via ring-opening reactions of the direct arylation products. Mechanistic investigations have provided insight into the catalyst's mode of action and show the presence of a kinetically significant C-H bond cleavage in palladium-catalyzed direct arylation of simple arenes. Knowledge garnered from these studies has led to the development of new intermolecular arylation reactions with previously inaccessible arenes, opening the door for the development of other new direct arylation processes.
Bu3SnH mediated oxidative radical cyclisations: Synthesis of 6H-benzo[c]chromen-6-ones
Bowman, Russell,Mann, Emma,Parr, Jonathan
, p. 2991 - 2999 (2007/10/03)
Attempts to synthesise 6H-benzo[c]chromen-6-ones by Bu3SnH mediated cyclisation of o-(benzoyl)aryl radicals failed because of the preferred trans conformation of the ester. This problem was overcome by using cyclisation of o-(benzyloxy)aryl and o-[(aryloxy)methyl]aryl radicals to yield 6H-benzo[c]chromenes followed by oxidation to the 6H-benzo[c]chromen-6-ones. 3-Methoxy-6H-benzo[c]chromen-6-one 1, one of the main biologically active constituents of shilajit, a herbal medicine used in countries surrounding the Himalayan mountains, was synthesised using Bu3SnH mediated cyclisation of 1-benzyloxy-2,4-dibromo-5-methoxybenzene 31 to yield 3-methoxy-6H-benzo[c]chromene 25 followed by PCC oxidation of the 6-position. In order to avoid the problems of rearrangement, the aryl radical cyclisation must be designed such that whichever way the spirodienyl intermediate rearranges, the same product is obtained. For instance, the Bu3SnH mediated cyclisation of 1-iodo- and 1-bromo-2-(3-methoxy-phenyloxymethyl)benzenes 22 and 23 respectively gave both the isomers, 1-methoxy-6H-benzo[c]chromenes 24 and 3-methoxy-6H-benzo[c]chromenes 25 via rearrangement of the intermediate spirodienyl radical. The synthesised 6H-benzo[c]chromenes were oxidised in high yield to the corresponding 6H-benzo[c]chromen-6-ones. The mechanism of the 'oxidative' Bu3SnH mediated cyclisation is discussed.
