Direct functionalization of rudimentary and cheap starting materials to yield complex value added products is of great interest to synthetic chemists. Particularly, direct functionalization of cheap commodity molecules that have been traditionally considered inert to known reactions are of widespread interest. We have previously demonstrated the functionalization of benzylic C–H bonds via an allyl transfer reaction using various allyl-phthalimido-N-oxyl substrates. In this work, we demonstrate the extension of our mild, metal-free, and neutral allyl transfer methodology to the direct functionalization of ethers. The C–H bond in α position to the ether oxygen in various acyclic and cyclic ethers was functionalized in high yields demonstrating wide substrate scope for this transformation. Furthermore, selective mono-functionalization of symmetrical cyclic ethers and regioselective functionalization of unsymmetrical ethers was achieved, thus demonstrating further utility of this reaction. Finally, kinetic chain length measurements were performed, which provided valuable insights into the initial rates and efficiency of the chain process involving the phthalimido-N-oxyl (PINO) radical.

A second-generation approach to the synthesis of hydromorphone by oxidative dearomatization/Diels–Alder cycloaddition was investigated. Detailed analysis of the stereochemical outcome of the [4 + 2] cycloaddition was performed first on a truncated model system as well as on the material leading to ent-hydromorphone. The stereochemical assignments were made by NMR and X-ray methods. The second-generation synthesis of hydromorphone was completed in both enantiomeric series. Improvements in the dearomatization conditions were attained using hypervalent iodine reagents instead of Pb(OAc)4. Electrochemical methods of oxidative dearomatization were also investigated. New conditions enabling the rearomatization of ring A from the methoxyketal were developed, and a formal synthesis of the natural enantiomer of hydromorphone was completed. Experimental and spectral data are provided for all new compounds.

Previously, we reported allyl transfer reactions of allyl bromide and allyl phthalimido-N-oxyl substrates with hydrocarbons that result in CC bond formation. In both cases, efficient chain transfer processes along with high reaction yields were observed. Since PINO chemistry leads to an environmentally friendly method of hydrocarbon functionalization, additional studies were performed in order to improve the process. To expand the utility of this reaction, we carried out experiments to optimize reaction conditions and tested the effect of Lewis acids and low temperature initiators. Although allyl-PINO substrates reacted slightly slower than the bromides, the reactions were cleaner with little or no side products. The chain lengths for these reactions were compromised at lower temperatures, attributable to the high activation energy required for the hydrogen atom abstraction by PINO. The addition of a Lewis acid catalyst (AlCl3) improves the product yield and reaction rate, possibly due to the formation of a PINO/AlCl3 complex which lowers the activation energy for hydrogen abstraction step.Graphical abstract

A photochemical model study of benzophenone triplet (3BP) with the MAO-B substrate 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine [MPTP (1)] and two of it’s derivatives, 1-cyclopropyl-4-phenyl-1,2,3,6-tetrahydropyridine (2) and (±)-[trans-2-phenylcyclopropyl-4-phenyl-1,2,3,6-tetrahydropyridine (3) were performed. Literature precedent and calculations reported herein suggest that the barrier to ring opening for aminyl radical cations derived from N-cyclopropyl derivatives of tertiary amines (such as MPTP) will be low. The LFP results reported herein demonstrate that pathways for the reaction of 3BP with 1, 2, and 3 are very similar. In each instance, disappearance of 3BP is accompanied solely by appearance of bands corresponding to the diphenylhydroxylmethyl radical and neutral radical derived from MPTP and it’s two derivatives 2 and 3. These results suggest that the reaction between benzophenone triplet and tertiary aliphatic amines proceed via a simple hydrogen atom transfer reaction. Additionally these model examinations provide evidence that oxidations of N-cyclopropyl derivatives of MPTP catalyzed by MAO-B may not be consistent with a pure SET pathway.

The hydroxyl radical (HO•) is a highly reactive oxygen-centered radical whose bimolecular rate constants for reaction with organic compounds (hydrogen atom abstraction) approach the diffusion-controlled limit in aqueous solution. The results reported herein show that hydroxyl radical is considerably less reactive in dipolar, aprotic solvents such as acetonitrile. This diminished reactivity is explained on the basis of a polarized transition state for hydrogen abstraction, in which the oxygen of the hydroxyl radical becomes highly negative and can serve as a hydrogen bond acceptor. Because acetonitrile cannot participate as a hydrogen bond donor, the transition state cannot be stabilized by hydrogen bonding, and the reaction rate is lower; the opposite is true when water is the solvent. This hypothesis explains hydroxyl radical reactivity both in solution and in the gas phase and may be the basis for a “containment strategy” used by Nature when hydroxyl radical is produced endogenously.

Using direct and indirect electrochemical methods, the rate constant for ring opening of the radical cation generated from N-cyclopropyl-N-methylaniline was found to be 4.1 × 104 s−1.

Using direct and indirect electrochemical methods, the rate constant for ring opening of the radical cation generated from N-cyclopropyl-N-methylaniline was found to be 4.1 × 104 s−1.