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The oxidation of benzylic alcohols is an important transformation in modern organic synthesis. A plethora of photoredox protocols have been developed to achieve the aerobic oxidation of alcohols into carbonyls. Recently, several groups described that ultraviolet (UV) or purple light can initiate the aerobic oxidation of benzylic alcohols in the absence of an external catalyst, and depicted different mechanisms involving the photoinduction of •O2− as a critical reactive oxygen species (ROS). However, based on comprehensive mechanistic investigations, including control experiments, radical quenching experiments, EPR studies, UV–vis spectroscopy, kinetics studies, and density functional theory calculations (DFT), we elucidate here that HOO•, which is released via the H2O2 elimination of α-hydroxyl peroxyl radicals [ArCR(OH)OO•], serves as the real chain carrier for the autocatalytic photooxidation of benzylic alcohols. The mechanistic ambiguities depicted in the precedent literature are clarified, in terms of the crucial ROS and its evolution, the rate-limiting step, and the primary radical cascade. This work highlights the necessity of stricter mechanistic analyses on UV-driven oxidative reactions that involve aldehydes’ (or ketones) generation.

In the title compound, C22H16Br2O2, which has approximate non-crystallographic inversion symmetry, the dihedral angles between the central ring and the pendant rings are 89.1 (4) and 82.4 (3)°; the dihedral angle between the pendant rings is 12.1 (4)°. In the crystal, the packing is influenced by van der Waals forces and no aromatic π–π stacking is observed.

In the title compound, C21H17N2P, the dihedral angles between the 1,5-naphthyridine ring system (r.m.s. deviation = 0.005 Å) and the phenyl rings are 89.18 (8) and 77.39 (8)°. The phenyl rings are almost perpendicular, making a dihedral angle of 88.12 (8)°. The only possible intermolecular interaction is a very weak aromatic π–π stacking interaction [centroid–centroid separation = 3.898 (2) Å].

The fluorene fragment of the title compound, C22H25N, is essentially planar, with an r.m.s deviation of the five-membered ring of 0.005 (2) Å. The dihedral angle between this ring and the outer benzene rings are 1.5 (2) and 0.7 (2)° while that between the benzene rings is 2.1 (2)°. The cyano group makes an angle of 0.3 (2)° with the attached benzene ring.

In the crystal structure of the title compound, C5H8N2O, molecules are linked by weak C—H...O hydrogen bonds, forming a three-dimensional network.

The complete molecule of the title compound, C16H22N2O6, is generated by crystallographic inversion symmetry. The conformation of the N—C—O—C fragment of the side chain is approximately gauche [torsion angle = −74.84 (17)°]. In the crystal, weak C—H...O interactions link the molecules.

In the title compound, C13H10N2O4, the nitro N atom deviates by 0.031 (2) Å from the plane of the benzene ring to which it is attached. The aromatic rings are oriented at a dihedral angle of 50.6 (1)°. An intramolecular N—H...O hydrogen bond occurs. In the crystal, inversion dimers are formed by pairs of O—H...O interactions.

There are two independent molecules, A and B, in the asymmetric unit of the title compound, C6H8N2O2S. In the crystal, pairs of intermolecular S...O contacts [3.286 (1) Å] link the B molecules into inversion dimers.

In the crystal structure of the title compound, C24H31NO3, molecules are connected via C—H...O intermolecular hydrogen bonds, forming dimers. The benzene rings are oriented at a dihedral angle of 29.8 (1)°.

In the crystal structure of the title compound, C13H9ClO3, the molecules form classical O—H...O hydrogen-bonded carboxylic acid dimers. These dimers are linked by C—H...π interactions into a three-dimensional network. The benzene rings are oriented at a dihedral angle of 77.8 (1)°.

In the title compound, C27H23N3O·H2O, the benzimidazole ring system has an r.m.s. deviation of 0.0071 Å and makes dihedral angles of 34.51 (2), 55.22 (3) and 41.05 (5)° with the central and N-bonded phenyl rings, respectively. In the crystal, the water molecular is connected to the organic molecule by intermolecular O—H...N hydrogen bonds. Weak intermolecular C—H...O hydrogen bonds also occur.

The asymmetric unit of the title compound, C8H8Br2O2, contains one half-molecule, the complete molecule being generated by inversion symmetry.

In the title compound, C10H13NO, the N atom and the methyl group are almost coplanar with the benzene ring to which they are bonded [deviations of 0.131 (1) and 0.038 (1) Å, respectively, from the ring plane]. In the crystal structure, intermolecular C—H...O hydrogen bonds form a three-dimensional network. Molecules are stacked parallel to the b-axis direction.

In the title salt, C5H14NO+·C2F3O2−, the cation and anion are linked by N—H...O and O—H...N hydrogen bonds, generating a three-dimensional network.

The molecular structure of the title compound, C9H10N6, exhibits four cyanomethyl groups around a central N—CH2—N unit. In the crystal structure, molecules are connected via intermolecular C—H...N hydrogen bonds, forming a three-dimensional network.

In the title compound, C16H10S2, the S atoms are almost coplanar with the benzene rings to which they are bonded [deviations of 0.092 (1) and 0.022 (1) Å from their respective ring planes]. The benzene rings enclose a dihedral angle of 79.17 (3)°. An intramolecular C—H...S hydrogen bond results in the formation of a five-membered ring. In the crystal structure, molecules are stacked parallel to the a axis direction. π–π interactions between benzene rings are present, with a face-to-face stacking distance of 3.622 (10) Å.

The asymmetric unit of the title compound, C14H11NO2·0.33H2O, contains three 3-hydroxy-3-phenylisoindolin-1-one (HPIO) molecules and one water molecule. The three independent HPIO molecules differ in the orientations of hydroxy and phenyl groups substituted at the 3-position with respect to the planar [r.m.s. deviations of 0.0173, 0.0170 and 0.0102 Å] dihydroisoindolin-1-one ring system. In the crystal structure, molecules are linked into a three-dimensional network by O—H...O, N—H...O and C—H...O hydrogen bonds.

In the title compound, C16H17ClN2O5, the dihedral angle between the aromatic rings is 77.36 (4)°. An intramolecular C—H...O interaction results in the formation of a planar [r.m.s. deviation = 0.103 (2) Å] five-membered ring, which is oriented at a dihedral angle of 4.84 (4)° with respect to the adjacent benzene ring. In the crystal structure, weak intermolecular C—H...π interactions are found.

In the molecule of the title compound, C11H12ClN3O3, the five membered ring adopts an envelope conformation. In the crystal structure, intermolecular N—H...O hydrogen bonds link the molecules into centrosymmetric dimers.

In the zwitterionic title compound (systematic name: 4-{[amino(inimio)methyl]amino}benzenesulfonate), C7H9N3O3S, the dihedral angle between the plane of the guanidine grouping and the benzene ring system is 44.87 (7)°. The crystal packing is stabilized by intermolecular N—H...O hydrogen bonds involving all the potential donors.

In the molecule of the title compound, C11H13BrO, the adjacent C atoms are almost coplanar with the aromatic ring [maximum deviation 0.035 (3) Å]. In the crystal structure, weak intermolecular C—H...O interactions link the molecules into chains along the b axis. A very weak C—H...π interaction is also present.

In the molecule of the title compound, C16H15FN2O2, the benzene rings are oriented at a dihedral angle of 88.61 (3)°. The five-membered ring adopts an envelope conformation. Intramolecular C—H...N hydrogen bonds result in the formation of two planar five-membered rings. In the crystal structure, intermolecular N—H...O and C—H...F hydrogen bonds link the molecules, forming R22(8) and R22(18) ring motifs. Weak C—H...π interactions may further stabilize the structure.

The title compound, C20H25NO4, was synthesized by a Mitsunobu reaction of sinomenine [(9S,13R,14R)-7,8-didehydro-4-hydroxy-3,7-dimethoxy-17-methylmorphinan-6-one] with methanol. The chiral centers were unchanged during the reaction. Intramolecular C—H...O hydrogen bonds result in the formation of six-membered rings.

In the molecule of the title compound, C11H12O3, the dioxole ring adopts an envelope conformation. In the crystal structure, weak intermolecular C—H...O hydrogen bonds link the molecules into chains.

The title compound, C11H9BrClN3O2, is an intermediate in the synthesis of Rynaxypyre, a new insecticidal anthranilic diamide used as a potent and selective ryanodine receptor activator. The dihedral angle between the aromatic ring planes is 78.7 (2)°.

In the title compound, C6H5IO3S·C4H9NO·H2O, N,N-dimethylacetamide and 4-iodobenzenesulfonic acidmolecules are linked by an intramolecular C—H...O hydrogen bond. In the crystal structure, intermolecular O—H...O, O—H...I and C—H...O hydrogen bonds link the molecules.

The title compound, C15H12N2O, is a derivative of 4-(aminomethyl)benzonitrile, an important pestcide intermediate. In the crystal structure, molecules are linked via intermolecular N—H...O hydrogen bonds, forming infinite chains.

The asymmetric unit of the title compound, C22H16N2O4·2CH4O, contains one half-molecule and a methanol solvent molecule. The aromatic ring is oriented at a dihedral angle of 82.91 (3)° with respect to the planar indole ring systems. In the crystal structure, intermolecular O—H...O and N—H...O hydrogen bonds link the molecules into chains along the b axis.

In the molecule of the title compound, C16H13F3N2O·H2O, the two benzene rings are oriented at a dihedral angle of 82.55 (3)° and the pyrazole ring adopts an envelope conformation. In the crystal structure, intermolecular C—H...F hydrogen bonds link the molecules into chains.

In the molecule of the title compound, C15H13FN2O, the phenyl and fluorophenyl rings are oriented at a dihedral angle of 77.92 (3)°. The pyrazolidine ring adopts an envelope conformation. An intramolecular C—H...N hydrogen bond results in the formation of a five-membered ring adopting an envelope conformation. In the crystal structure, intermolecular N—H...O and C—H...O hydrogen bonds link the molecules. There are C—H...π contacts between between aromatic H atoms and the phenyl and fluorophenyl rings. A π–π contact between phenyl rings [centroid–centroid distance = 3.926 (1) Å] is also observed.

The asymmetric unit of the title compound, C8H4Br2O4·2H2O, contains one half-molecule of 2,5-dibromoterephthalic acid (DBTA) and one water molecule. The DBTA molecule is centrosymmetric. In the crystal structure, intermolecular O—H...O hydrogen bonds link the molecules, forming a three-dimensional framework.

In the organic molecule of the title compound, C19H25O2P·0.5H2O, the benzene rings are oriented at a dihedral angle of 54.04 (3)°. Intramolecular C—H...O hydrogen bonds result in the formation of two five-membered planar rings, which are oriented with respect to the adjacent benzene rings at dihedral angles of 2.66 (3) and 2.79 (3)°. In the crystal structure, intermolecular O—H...O hydrogen bonds link the molecules. The water oxygen atom lies on a twofold rotation axis.

The dihydrofuran ring of the title compound, C9H9NO3, adopts an envelope conformation. The nitro group is twisted slightly away from the attached benzene ring [dihedral angle = 21.9 (1)°].

In the title compound, C11H14O5S, there is an intramolecular C—H...O hydrogen bond, for which the C—C—S—O torsion angle involving the acceptor and donor atoms is 2.4 (4)°. The dihedral angle between the benzene ring and the methoxycarbonyl plane is 52.7 (4)°. In the crystal structure, molecules are linked via intermolecular C—H...O hydrogen bonds, forming a molecular chain along the b axis.

In the molecule of the title compound, C19H22N2O2, the pyrazolidinone ring has an envelope conformation, with the C atom attached to the 4-methoxyphenyl ring displaced by 0.354 (3) Å from the plane of the other ring atoms. The 4-isopropylphenyl ring is oriented with respect to the 4-methoxyphenyl ring at a dihedral angle of 88.94 (3)°. Intramolecular C—H...N hydrogen bonds result in the formation of two planar five-membered rings, which are oriented with respect to the adjacent 4-isopropylphenyl and 4-methoxyphenyl rings at dihedral angles of 4.05 (3) and 0.50 (3)°, respectively. In the crystal structure, intermolecular N—H...O hydrogen bonds link the molecules into centrosymmetric dimers.

In the title compound, C15H12O3·H2O, the two rings are oriented at a dihedral angle of 69.12 (3)°. In the crystal structure, intermolecular O—H...O hydrogen bonds link the molecules into a three-dimensional framework.

Molecules of the title compound, C24H28O12, are linked by intermolecular C—H...O hydrogen bonds. Bond lengths and angles are normal.

In the molecule of the title compound, C7H8IN, the methyl C, I and N atoms lie in the benzene ring plane. In the crystal structure, intermolecular N—H...N hydrogen bonds link the molecules in a stacked arrangement along the a axis.

The asymmetric unit of the title compound, C20H18N2O22+·2Cl−, is composed of one-half of the 2,5-dibenzoylbenzene-1,4-diaminium dication, located on a centre of inversion, and one Cl− ion. The dihedral angle between the central benzene ring and the benzoyl phenyl ring is 53.3 (2)°. In the crystal structure, ions are linked to form a two-dimensional network parallel to the (10overline{1}) plane by N—H...Cl hydrogen bonds.

In the molecule of the title compound, C9H11NO2, the methyl C atom bonded to the ring and the N atom lie in the benzene ring plane. An intramolecular O—H...O hydrogen bond results in the formation of a five-membered planar ring, which is oriented at a dihedral angle of 81.37 (3)° with respect to the benzene ring. In the crystal structure, intermolecular O—H...O hydrogen bonds link the molecules stacked along the b axis. There are also π–π interactions between benzene rings with a face-to-face stacking distance of 3.434 Å.

The asymmetric unit of the title compound, C8H10BrN, contains two independent molecules. The Br, N and methyl group C atoms lie in the benzene ring planes. In the crystal structure, N—H...N hydrogen bonds link the molecules.

The title molecule, C16H16Br2O8, lies on a crystallographic twofold axis. Weak intramolecular C—H...O hydrogen bonds may, in part, control the conformation of the molecule. In the crystal structure, molecules are connected into a two-dimensional network via weak intermolecular C—H...O hydrogen bonds.

In the molecule of the title compound, C8H6BrI, the H atoms of methyl groups are disordered; site-occupation factors were fixed at 0.50. The non-H atoms all lie on a crystallographic mirror plane. Weak intramolecular C—H...Br hydrogen bonds result in the formation of two non-planar five-membered rings.

The asymmetric unit of the title compound, C8H8BrI, contains three independent molecules. In each molecule, the Br, I and C atoms of the methyl groups lie in the benzene ring plane. Intramolecular C—H...I hydrogen bonds result in the formation of three planar five-membered rings, which are nearly coplanar with the adjacent rings.