Diels–Alder Reaction of Photochemically Generated (E)-Cyclohept-2-enones: Diene Scope, Reaction Pathway, and Synthetic Application

Abstract

Upon irradiation at λ = 350 nm, cyclohept-2-enone undergoes an isomerization to the strained (E)-isomer. The process was studied by XMS-CASPT2 calculations and found to proceed by two competitive reaction channels on either the singlet or the triplet hypersurface. (E)-Cyclohept-2-enone is a reactive dienophile in thermal [4 + 2] cycloaddition reactions with various dienes. Ten different dienes were probed, most of whichexcept for 1,3-cyclohexadieneunderwent a clean Diels−Alder reaction and gave the respective trans-fused six-membered rings in good yields (68− 98%). The reactions with furan were studied in detail, both experimentally and by DLPNO−CCSD(T) calculations. Two diastereoisomers were formed in a ratio of 63/35 with the exoproduct prevailing, and the configuration of both diastereoisomers was corroborated by single crystal X-ray crystallography. The outcome of the photoinduced Diels−Alder reaction matched both qualitatively and quantitatively the calculated reaction pathway. Apart from cyclohept-2-enone, five additional cyclic hept-2-enones and cyclooct-2-enone were employed in their (E)-form as dienophiles in the Diels−Alder reaction with 1,3-cyclopentadiene (80−98% yield). The method was eventually applied to a concise total synthesis of racemic trans-α-himachalene (four steps, 14% overall yield).

Time-resolved X-ray and XUV based spectroscopic methods for nonadiabatic processes in photochemistry

“…For the construction of pump-probe signals, the probepulse interaction is treated as a time-dependent perturbation to the photoexcited molecular Hamiltonian (eqn (12)). The resulting total Hamiltonian for the simulation of the pumpprobe spectrum takes the form,…”

“…where H ˆI is the Hamiltonian of the molecule prepared by the pump-pulse interaction (see eqn (12)), and H ˆ(m/a) X is the interaction Hamiltonian corresponding to the probe-pulse interaction, with m and a in the superscript representing either resonant or off-resonant nature of the probe process. Interaction Hamiltonians for resonant and off-resonant probes can be written as,…”

“…52 Based on high-level nonadiabatic dynamics simulations, the use of TXAS has been proposed to resolve competing CIs pathways in trans-1,3-butadiene. 72 Within our simulation protocol, the studied molecule is excited using a resonant pump-pulse (see eqn (12)) and the signal expression for TXAS can be derived using eqn (19), and reads, 73…”

“…The interaction of visible or ultraviolet (UV) photons with a molecular system is a fundamental step of various chemical, physical, and biological phenomena such as the primary event of vision, 1,2 photosynthesis, [3][4][5] vitamin D synthesis [6][7][8] and different types of photocatalysis. [9][10][11][12] Photoexcited molecules can undergo photochemical reactions and decay back to the ground state without the emission of photons, by means of nonadiabatic decay channels. If the spin state is not changed by this transition, the process is called internal conversion (IC) and is happening mainly in the vicinity of an avoided crossing or a conical intersection (CI).…”

Photochemical Ring Contraction of 5,5-Dialkylcyclopent-2-enones and in situ Trapping by Primary Amines

“…They can be directly excited at relatively long wavelengths (λ = 300–350 nm), and their reactive triplet states are populated rapidly by an efficient intersystem crossing (ISC). , For most enones, conformational relaxation leads to the lowest lying triplet state T 1 that is located at the double bond and is ππ* in character . The conformational restriction of the carbocyclic skeleton prohibits Z / E isomerization , and the long-lived triplet state is efficiently trapped by olefins in an inter- or intramolecular addition reaction. An initial C–C bond formation is followed by ISC and subsequent cyclobutane formation. , There are literally thousands of [2 + 2] photocycloaddition reactions known today, and the number of their applications in total synthesis continues to grow.…”