1988-21-2Relevant articles and documents
The formyloxyl radical: Electrophilicity, C-H bond activation and anti-Markovnikov selectivity in the oxidation of aliphatic alkenes
Iron, Mark A.,Khenkin, Alexander M.,Neumann, Ronny,Somekh, Miriam
, p. 11584 - 11591 (2020/11/23)
In the past the formyloxyl radical, HC(O)O, had only been rarely experimentally observed, and those studies were theoretical-spectroscopic in the context of electronic structure. The absence of a convenient method for the preparation of the formyloxyl radical has precluded investigations into its reactivity towards organic substrates. Very recently, we discovered that HC(O)O is formed in the anodic electrochemical oxidation of formic acid/lithium formate. Using a [CoIIIW12O40]5- polyanion catalyst, this led to the formation of phenyl formate from benzene. Here, we present our studies into the reactivity of electrochemically in situ generated HC(O)O with organic substrates. Reactions with benzene and a selection of substituted derivatives showed that HC(O)O is mildly electrophilic according to both experimentally and computationally derived Hammett linear free energy relationships. The reactions of HC(O)O with terminal alkenes significantly favor anti-Markovnikov oxidations yielding the corresponding aldehyde as the major product as well as further oxidation products. Analysis of plausible reaction pathways using 1-hexene as a representative substrate favored the likelihood of hydrogen abstraction from the allylic C-H bond forming a hexallyl radical followed by strongly preferred further attack of a second HC(O)O radical at the C1 position. Further oxidation products are surmised to be mostly a result of two consecutive addition reactions of HC(O)O to the CC double bond. An outer-sphere electron transfer between the formyloxyl radical donor and the [CoIIIW12O40]5- polyanion acceptor forming a donor-acceptor [D+-A-] complex is proposed to induce the observed anti-Markovnikov selectivity. Finally, the overall reactivity of HC(O)O towards hydrogen abstraction was evaluated using additional substrates. Alkanes were only slightly reactive, while the reactions of alkylarenes showed that aromatic substitution on the ring competes with C-H bond activation at the benzylic position. C-H bonds with bond dissociation energies (BDE) ≤ 85 kcal mol-1 are easily attacked by HC(O)O and reactivity appears to be significant for C-H bonds with a BDE of up to 90 kcal mol-1. In summary, this research identifies the reactivity of HC(O)O towards radical electrophilic substitution of arenes, anti-Markovnikov type oxidation of terminal alkenes, and indirectly defines the activity of HC(O)O towards C-H bond activation.
Palladacycle-catalyzed carbonylation of aryl iodides or bromides with aryl formates
Chen, Guangwei,Leng, Yuting,Yang, Fan,Wang, Shiwei,Wu, Yangjie
, p. 1488 - 1494 (2014/01/06)
An efficient palladacycle-catalyzed aromatic carbonylation reaction of aryl formates with aryl iodides or bromides has been developed. Commercially available and easily prepared aryl formates were employed as carbonyl sources without the use of external carbon monoxide. The present catalytic system shows broad functional group tolerance and affords aryl benzoate derivatives in good to excellent yields. Copyright
One-pot synthesis of phenols from aromatic aldehydes by Baeyer-Villiger oxidation with H2O2 using water-tolerant Lewis acids in molecular sieves
Corma, Avelino,Fornes, Vicente,Iborra, Sara,Mifsud, Maria,Renz, Michael
, p. 67 - 76 (2007/10/03)
The heterogeneous oxidation systems Sn-Beta/H2O2 and Al-Beta/H2O2 were both active for the Baeyer-Villiger oxidation of aromatic aldehydes. With alkoxy substituents in ortho and especially in para position, the corresponding formate ester was the primary reaction product with excellent selectivity. The highest selectivity toward the ester was obtained with the Sn-Beta catalyst in dioxane as solvent. High selectivities of the corresponding phenol could be obtained by employing ethanol or aqueous acetonitrile as solvent with Sn-Beta. Al-Beta was more efficient as catalyst for both Baeyer-Villiger oxidation and ester hydrolysis and, thus achieved high yields for the corresponding alcohol. However, this was a less selective catalyst than Sn-Beta zeolite. Sn-Beta was a chemoselective catalyst when the aldehyde reactant contained olefinic groups in an alkyl chain. With these reactants, the Al-zeolite gave no activity. Other classical BV oxidants, e.g., peracids, also yielded the epoxidation products. Thus, Sn-Beta catalysts open a new entry to aromatic phenols with unsaturated alkyl substituents that are interesting intermediates in the chemical industry.