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1. Generation of Diazomethyl Radicals by Hydrogen Atom Abstraction and Their Cycloaddition with Alkenes

   https://doi.org/10.1002/anie.202105472  

   Su, Yong‐Liang; Dong, Kuiyong; Zheng, Haifeng; Doyle, Michael P. (July 2021, Angewandte Chemie International Edition)

   Abstract A general catalytic methodology for the synthesis of pyrazolines from α‐diazo compounds and conjugated alkenes is reported. The direct hydrogen atom transfer (HAT) process of α‐diazo compounds promoted by thetert‐butylperoxy radical generates electrophilic diazomethyl radicals, thereby reversing the reactivity of the carbon atom attached with the diazo group. The regiocontrolled addition of diazomethyl radicals to carbon‐carbon double bonds followed by intramolecular ring closure on the terminal diazo nitrogen and tautomerization affords a diverse set of pyrazolines in good yields with excellent regioselectivity. This strategy overcomes the limitations of electron‐deficient alkenes in traditional dipolar [3+2]‐cycloaddition of α‐diazo compounds with alkenes. Furthermore, the straightforward formation of the diazomethyl radicals provides umpolung reactivity, thus opening new opportunities for the versatile transformations of diazo compounds. 

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2. Selenium-Catalyzed Regioselective Intermolecular Diamination of Dienes and Dienolate Derivatives

   https://doi.org/10.1021/acs.orglett.4c03431  

   Dohoda, Alexander F; Zottarelli, Victoria L; Hajikedir, Dureti; Michael, Forrest E (September 2024, Organic Letters)

   We report a selenium-catalyzed diamination of dienes using sulfamates as a convenient nitrogen source. This reaction proceeds regioselectively for 1,2-addition at the less substituted alkene, without the need for a tethered diamine. We also report the first diamination of dienyl phosphates and tosylates, affording synthetically useful α,β-diaminoketone derivatives with high syn diastereoselectivity. Density functional theory calculations support a mechanism proceeding via [4+2] cycloaddition of the diene with a selenium bis(imide), followed by ring-opening aminolysis and [2,3]-sigmatropic rearrangement. 

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3. Stereoisomer-dependent rate coefficients and reaction mechanisms of 2-ethyloxetanylperoxy radicals

   https://doi.org/10.1016/j.proci.2024.105578  

   Doner, Anna C; Zádor, Judit; Rotavera, Brandon (July 2024, Proceedings of the Combustion Institute)

   Cyclic ethers undergo H-abstraction reactions that yield carbon-centered radicals (Ṙ). The ether functional group introduces a competing set of reaction pathways: ring-opening and reaction with O2 to form peroxy radical adducts, ROȮ, which can result in stereoisomers. ROȮ derived from cyclic ethers can subsequently isomerize into hydroperoxy-substituted carbon-centered radicals, Q̇OOH, which can also undergo ring-opening reactions or pathways prototypical to alkyl oxidation. The balance of reactions that unfold from cyclic ether radicals depends intrinsically on the size of the ring and the structure of any substituents retained in the formation step. The present work examines unimolecular reactions of peroxy radicals from 2-ethyloxetane, a four-membered cyclic ether formed during n-pentane oxidation, and reveals stereoisomer-specific reaction pathways. Automated quantum chemical computations were conducted on constitutional and stereoisomers of ROȮ derived from O2-addition to 2-ethyloxetanyl radicals. Pressure-dependent rate calculations were conducted by solving the master equation from 300 – 1000 K and from 0.01 – 100 atm. Branching fractions were then calculated at 650 K and 825 K, the peak temperatures at which cyclic ethers form in alkane oxidation. Isomer-specific reaction pathways of anti-ROO and syn-ROO and resulting impact on radical production were evident. Q̇OOH ring-opening reactions were significant as were rates of bi-cyclic ether formation common in alkyl radical oxidation. Detailed prescription of rates and reaction mechanisms describing cyclic ether consumption mechanisms are important to enable accurate modeling of reactions of ephemeral Q̇OOH radicals because of the direct, isomer-specific formation pathways. In addition, detailed cyclic ether mechanisms are required to reduce mechanism truncation error. The results herein provide insight on connections between cyclic ethers and chain-reaction pathways yielding ȮH, HOȮ, and other radicals, in addition to pathways leading to performic acid (HOOC(=O)H), the decomposition of which via O–O scission results in an exothermic, chain-branching step. 

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4. Challenges in the Highly Selective [3 + 1]-Cycloaddition of an Enoldiazoacetamide to Form a Donor–Acceptor Cis-Cyclobutenecarboxamide

   https://doi.org/10.3390/molecules26123520  

   Joyasawal, Sipak; Ma, Donghui; Doyle, Michael P. (June 2021, Molecules)

   A substituted donor–acceptor cyclobutenecarboxamide is synthesized with modest enantiocontrol through a chiral copper(I) complex catalyzed [3 + 1]-cycloaddition reaction of α-acyl diphenylsulfur ylides with 3-siloxy-2-diazo-3-butenamides. With a methyl substituent on the 4-position of the 3-butenamide, the cis-vicinal-3,4-disubstituted cyclobutenecarboxamide is formed with >20:1 diastereocontrol. Donor-acceptor 3-methyl-2-siloxycyclopropenecarboxamide is rapidly formed from the reactant enoldiazoamide and undergoes catalytic ring opening to give only the Z-γ-substituted metallo-enolcarbene. Elimination from 3-siloxy-2-diazo-3-pentenamide to form the conjugated 3-siloxy-2,4-pentadienamide is competitive but minimized at low temperature. 

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5. Bench‐Stable Dinuclear Mn(I) Catalysts in E ‐Selective Alkyne Semihydrogenation: A Mechanistic Investigation**

   https://doi.org/10.1002/chem.202201766  

   Abhyankar, Preshit C.; MacMillan, Samantha N.; Lacy, David C. (July 2022, Chemistry – A European Journal)

   Abstract Dinuclear manganese hydride complexes of the form [Mn₂(CO)₈(μ‐H)(μ‐PR₂)] (R=Ph,1; R=iPr,2) were used inE‐selective alkyne semi‐hydrogenation (E‐SASH) catalysis. Catalyst speciation studies revealed rich coordination chemistry and the complexes thus formed were isolated and in turn tested as catalysts; the results underscore the importance of dinuclearity in engendering the observedE‐selectivity and provide insights into the nature of the active catalyst. The insertion product obtained from treating2with (cyclopropylethynyl)benzene contains acis‐alkenyl bridging ligand with the cyclopropyl ring being intact. Treatment of this complex with H₂affords exclusivelytrans‐(2‐cyclopropylvinyl)benzene. These results, in addition to other control experiments, indicate a non‐radical mechanism forE‐SASH, which is highly unusual for Mn−H catalysts. The catalytically active species are virtually inactive towardscistotransalkene isomerization indicating that theE‐selective process is intrinsic and dinuclear complexes play a critical role. A reaction mechanism is proposed accounting for the observed reactivity which is fully consistent with a kinetic analysis of the rate limiting step and is further supported by DFT computations. 

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