Stereoselective Ring Opening of Electronically Excited Cyclohexa-2, 4-dienones: Cause and effect

Computational and Further Experimental Explorations of the Competing Cascades Following Claisen Rearrangements of Aryl Propargyl Ethers: Substituent Effects on Reactivity and Regioselectivity

We report a computational investigation of two reaction cascades occurring following the Claisen rearrangements of aryl propargyl ethers to the alternate ortho positions in unsymmetrical reactants. Our computations explain how substituents influence reactivity and regioselectivity. Rearrangement to the substituted ortho carbon leads to a tricyclo[3.2.1.0]octane core, while rearrangement to an unsubstituted ortho carbon leads to a benzopyran. Density functional theory with ωB97X-D indicates that these reactions involve rate-determining Claisen rearrangements followed by subsequent reaction cascades of the Claisen rearrangement products depending on the presence or absence of a substituent at the ortho carbon.

Machine learning prediction of UV-Vis spectra features of organic compounds related to photoreactive potential

Machine learning (ML) algorithms were explored for the classification of the UV-Vis absorption spectrum of organic molecules based on molecular descriptors and fingerprints generated from 2D chemical structures. Training and test data (~ 75 k molecules and associated UV-Vis data) were assembled from a database with lists of experimental absorption maxima. They were labeled with positive class (related to photoreactive potential) if an absorption maximum is reported in the range between 290 and 700 nm (UV/Vis) with molar extinction coefficient (MEC) above 1000 Lmol^-1 cm^-1, and as negative if no such a peak is in the list. Random forests were selected among several algorithms. The models were validated with two external test sets comprising 998 organic molecules, obtaining a global accuracy up to 0.89, sensitivity of 0.90 and specificity of 0.88. The ML output (UV-Vis spectrum class) was explored as a predictor of the 3T3 NRU phototoxicity in vitro assay for a set of 43 molecules. Comparable results were observed with the classification directly based on experimental UV-Vis data in the same format.

Photochemical rearrangement reactions of bicyclic molecules that contain a cyclopropane ring

Our theoretical observations indicate that for [2.1.0], [3.1.0], [4.1.0], and [5.1.0] molecules, the quantum yields of the tautomeric keto forms are larger than those of the corresponding enol isomers.

Thermal and photochemical rearrangement of bicyclo[3.1.0]hex-3-en-2-one to the ketonic tautomer of phenol. Computational evidence for the formation of a diradical rather than a zwitterionic intermediate

The ground state (S(0)) and lowest-energy triplet state (T(1)) potential energy surfaces (PESs) concerning the thermal and photochemical rearrangement of bicyclo[3.1.0]hex-3-en-2-one (8) to the ketonic tautomer of phenol (11) have been extensively explored using ab initio CASSCF and CASPT2 calculations with several basis sets. State T(1) is predicted to be a triplet pipi lying 66.5 kcal/mol above the energy of the S(0) state. On the S(0) PES, the rearrangement of 8 to 11 is predicted to occur via a two-step mechanism where the internal cyclopropane C-C bond is broken first through a high energy transition structure (TS1-S(0)()), leading to a singlet intermediate (10-S(0)()) lying 25.0 kcal/mol above the ground state of 8. Subsequently, this intermediate undergoes a 1,2-hydrogen shift to yield 11 by surmounting an energy barrier of only 2.7 kcal/mol at 0 K. The rate-determining step of the global rearrangement is the opening of the three-membered ring in 8, which involves an energy barrier of 41.2 kcal/mol at 0 K. This high energy barrier is consistent with the fact that the thermal rearrangement of umbellulone to thymol is carried out by heating at 280 degrees C. Regarding the photochemical rearangement, our results suggest that the most efficient route from the T(1) state of 8 to ground state 11 is the essentially barrierless cleavage of the internal cyclopropane C-C bond followed by radiationless decay to the S(0) state PES via intersystem crossing (ISC) at a crossing point (S(0)()/T(1)()-1) located at almost the same geometry as TS1-S(0)(), leading to the formation of 10-S(0)() and the subsequent low-barrier 1,2-hydrogen shift. The computed small spin-orbit coupling between the T(1) and S(0) PESs at S(0)()/T(1)()-1 (1.2 cm(-)(1)) suggests that the ISC between these PESs is the rate-determining step of the photochemical rearrangement 8 --> 11. Finally, computational evidence indicates that singlet intermediate 10-S(0)() should not be drawn as a zwitterion, but rather as a diradical having a polarized C=O bond.

Photochemistry of 3-substituted bicyclo[3.1.0]hex-3-en-2-ones. Regioselective synthesis of ortho-substituted phenols by Pauson-Khand reaction

[reaction: see text] 3-Substituted bicyclo[3.1.0]hex-3-en-2-ones 3, easily obtained by Pauson-Khand reaction between terminal alkynes and cyclopropene, have been quantitatively converted into ortho-substituted phenols 4 by irradiation with UV light (350 nm). The kinetics and mechanism of this photochemical process have been studied by means of FT-IR and semiempirical (AM1 3x3 CI) calculations.

A solvolytic C-C cleavage reaction of 6-acetoxycyclohexa-2,4-dienones: mechanistic implications for the intradiol catechol dioxygenases

6-Acetoxycyclohexa-2,4-dienones are found to undergo a rapid reaction in methanol/water under mildly basic conditions to give an acyclic ketoester as the major product for 6-phenyl and 6-methyl substrates. Reaction monitoring by UV spectroscopy indicates the formation of an unsaturated ketone reaction intermediate (lambda(max) 275 nm, R = Ph) and the transient appearance of a highly conjugated species. Reaction of the 6-phenyl substrate (4.95 x 10(-6) s(-1)) is 2-fold faster than the 6-methyl substrate (2.47 x 10(-6) s(-1)). The reaction rate is first order with respect to substrate concentration, and the final step in the reaction is pH-dependent. No cleavage was observed for a substrate lacking an acetyl substituent. A reaction mechanism for C-C cleavage is proposed involving a benzene oxide-oxepin interconversion. The possible relevance to the catalytic mechanism of the intradiol catechol dioxygenases is discussed.

Amination of Ketenes: Kinetic and Mechanistic Studies

The rate constants for reaction of PhMe(2)SiCH=C=O (6) with amines to form amides in CH(3)CN are best fitted with a mixed second- and third-order dependence on [amine], in stark contrast to previous studies of Ph(2)C=C=O and other reactive ketenes in which only a first-order dependence on [amine] was observed in H(2)O or in CH(3)CN. Derived third-order rate constants for 6 depend on the amine basicity, with a 1.7 x 10(7) greater reactivity for n-BuNH(2) compared to CF(3)CH(2)NH(2). These kinetic results are consistent with recently reported theoretical studies for reaction of CH(2)=C=O with NH(3). For 6 the relative reactivity k(n-BuNH(2))/k(H(2)O) is estimated to be 10(13) in CH(3)CN. The crowded ketene t-Bu(2)C=C=O (10) is enormously deactivated toward amination and reacts in neat n-BuNH(2) with rates 10(12) and 2 x 10(5) times slower than those for t-BuCH=C=O and t-BuC(i-Pr)=C=O (11), respectively. The observed rate constants for 11 also show a higher than first-order dependence on [n-BuNH(2)]. The absence of higher order kinetic terms in [amine] for more reactive ketenes is attributed to irreversibility of addition of an initial amine to the ketene, while with more stable ketenes the initial step is reversible and later steps involving additional amine molecules are kinetically significant. The general acid CF(3)CH(2)NH(3)(+) catalyses the addition of CF(3)CH(2)NH(2) to 6 in a process independent of [CF(3)CH(2)NH(2)]. The reactivity of 6 with n-BuNH(2) is 370 times greater in CH(3)CN compared to isooctane, a result attributed to the polar nature of the transition state and possible catalysis of the addition by CH(3)CN.