Mesoscale Ordering and Charge-Transport of Crystalline Spiro-OMeTAD Organic Semiconductors

The mesoscale ordering and charge-transport of crystalline spiro-OMeTAD, a hole-transporting material extensively used in perosvkite and dye-sensitized solar cell applications, were explored using molecular dynamics and hole mobility calculations. Morphologies were evaluated through conformational changes, nematic order and paracrystallinity at various temperatures. Charge transport is predicted with electronic structure methods employing a hopping mechanism. Our calculations show that along with strong fluorene backbone packing, phenylenes in the methoxyphenyl–amine substituents of spiro-OMeTAD are an integral part of the material performance. Backbone and substituent paracrystallinity predictions showed highly ordered crystalline phase. The methoxyphenyl substituents have multiple conformations in the unit-cell scale, but interphenylene electronic-coupling remain nearly constant. A thermal increase in positional disorder results in a systematic increase in energetic disorder and a decrease in hole mobility. The predicted crystalline hole mobility is approximately two-orders of magnitude higher than the experimental thin-film measurements, indicating that the performance of spiro-OMeTADs can be improved significantly by exploiting crystallinity

Terminal Substituent Effects on the Reactivity, Thermodynamics, and Stereoselectivity of the 8π–6π Electrocyclization Cascades of 1,3,5,7-Tetraenes

M06-2X/6-31+G­(d,p) computations are reported for the 8π–6π electrocyclization cascades of 1,3,5,7-tetraenes. The rate-determining step for these cascades is typically the second (6π) ring closure. According to experiment and theory, un- and monosubstituted tetraenes readily undergo 8π electrocyclic ring closure to form 1,3,5-cyclooctatrienes; however, the 6π electrocyclizations of these cyclooctatriene intermediates are slow and reversible, and mixtures of monocyclic and bicyclic products are formed. Computations indicate that di- and trisubstituted tetraenes undergo facile but less exergonic 8π electrocyclization due to a steric clash that destabilizes the 1,3,5-cyclooctatriene intermediates. Relief of this steric clash ensures the subsequent 6π ring closures of these intermediates are both kinetically facile and thermodynamically favorable, and only the bicyclic products are observed for the cascade reactions of naturally occurring tri- and tetrasubstituted tetraenes (in agreement with computations). The 6π electrocyclization step of these cascade electrocyclizations is also potentially diastereoselective, and di- and trisubstituted tetraenes often undergo cascade reactions with high diastereoselectivities. The <i>exo</i> mode of ring closure is favored for these 6π electrocyclizations due to a steric interaction that destabilizes the <i>endo</i> transition state. Thus, theory explains both the recalcitrance of the unsubstituted 1,3,5,7-octatetraene and 1-substituted tetraenes toward formation of the bicyclo[4.2.0]­octa-2,4-diene products, as well as the ease and the stereoselectivity with which terminal di- and trisubstituted tetraenes are known to react biosynthetically

Computational Analysis of Cyclophane-Based Bisthiourea-Catalyzed Henry Reactions

The Henry reaction between benzaldehyde and nitromethane catalyzed by a cyclophane-based bisthiourea has been studied with density functional theory [M06-2X-D3/def2-TZVPP/IEFPCM//TPSS-D2/6-31G­(d)/IEFPCM]. The results of our study reveal that the transformation involves the reaction of a thiourea–nitronate complex with the uncoordinated aldehyde. On the basis of our calculations, the formation of the major stereoisomer is kinetically preferred. Employing smaller model systems, we show that the observed stereoselectivity arises primarily from differences in hydrogen bonding in diastereomeric transition states

Origins of Stereoselectivity in Mannich Reactions Catalyzed by Chiral Vicinal Diamines

The origins of the enantio- and diastereoselectivities in the Mannich reactions between aldehydes and ketimines catalyzed by chiral vicinal diamines have been determined by density functional theory calculations and distortion–interaction analysis. Computational results indicate a strong energetic preference for hydrogen-bonded nine-membered cyclic transition states. The favored transition states involve eight heavy atoms in the crown (chair–chair) conformation using the nomenclature of the analogous cyclic hydrocarbons. Energetic discrimination in the chirality-imparting step arises from pseudogauche-butane-type interactions in the disfavored transition states, as well as steric clashes between the <i>N</i>-Boc protecting group and the ammonium <i>N</i>-substituents

Origins of Stereoselectivity in Intramolecular Aldol Reactions Catalyzed by Cinchona Amines

The intramolecular aldol condensation of 4-substituted heptane-2,6-diones leads to chiral cyclohexenones. The origins of the enantioselectivities of this reaction, disclosed by List et al. using a cinchona alkaloid-derived primary amine (cinchona amine) organocatalyst, have been determined with dispersion-corrected density functional theory (DFT). The stereocontrol hinges on the chair preference of the substrate–enamine intermediate and the conformational preferences of a hydrogen-bonded nine-membered aldol transition state containing eight heavy atoms. The conformations of the hydrogen-bonded ring in the various stereoisomeric transition structures have been analyzed in detail and shown to closely resemble the conformers of cyclooctane. A model of stereoselectivity is proposed for the cinchona amine catalysis of this reaction. The inclusion of Grimme’s dispersion corrections in the DFT calculations (B3LYP-D3­(BJ)) substantially improves the agreement of the computed energetics and experiment, attesting to the importance of dispersion effects in stereoselectivity

Diels–Alder Reactions of Allene with Benzene and Butadiene: Concerted, Stepwise, and Ambimodal Transition States

Multiconfigurational complete active space methods (CASSCF and CASPT2) have been used to investigate the (4 + 2) cycloadditions of allene with butadiene and with benzene. Both concerted and stepwise radical pathways were examined to determine the mechanism of the Diels–Alder reactions with an allene dienophile. Reaction with butadiene occurs via a single ambimodal transition state that can lead to either the concerted or stepwise trajectories along the potential energy surface, while reaction with benzene involves two separate transition states and favors the concerted mechanism relative to the stepwise mechanism via a diradical intermediate

How Cinchona Alkaloid-Derived Primary Amines Control Asymmetric Electrophilic Fluorination of Cyclic Ketones

The origin of selectivity in the α-fluorination of cyclic ketones catalyzed by cinchona alkaloid-derived primary amines is determined with density functional calculations. The chair preference of a seven-membered ring at the fluorine transfer transition state is key in determining the sense and level of enantiofacial selectivity

A Theoretical Study of Cyclohexyne Addition to Carbonyl–C_α Bonds: Allowed and Forbidden Electrocyclic and Nonpericyclic Ring-Openings of Strained Cyclobutenes

The mechanism of cyclohexyne insertion into a C­(O)–C_α bond of cyclic ketones, explored experimentally by the Carreira group, has been investigated using density functional theory. B3LYP and M06–2X calculations were performed in both gas phase and THF (CPCM, UAKS radii). The reaction proceeds through a stepwise [2 + 2] cycloaddition of cyclohexyne to the enolate, followed by three disparate ring-opening possibilities of the cyclobutene alkoxide to give the product: (1) thermally allowed conrotatory electrocyclic ring-opening, (2) thermally forbidden disrotatory electrocyclic ring-opening, or (3) nonpericyclic C–C bond cleavage. Our computational results for the model alkoxide and potassium alkoxide systems show that the thermally allowed electrocyclic ring-opening pathway is favored by less than 1 kcal/mol. In more complex systems containing a potassium alkoxide (<b>e</b>–<b>f</b>), the barrier of the allowed conrotatory ring-opening is disfavored by 4–8 kcal/mol. This suggests that the thermodynamically more stable disrotatory product can be formed directly through a “forbidden” pathway. Analysis of geometrical parameters and atomic charges throughout the ring-opening pathways provides evidence for a nonpericyclic C–C bond cleavage, rather than a thermally forbidden disrotatory ring-opening. A true forbidden disrotatory ring-opening transition structure was computed for the cyclobutene alcohol; however, it was 19 kcal/mol higher in energy than the allowed conrotatory transition structure. An alternate mechanism in which the disrotatory product forms via isomerization of the conrotatory product was also explored for the alkoxide and potassium alkoxide systems

Hyperconjugative, Secondary Orbital, Electrostatic, and Steric Effects on the Reactivities and <i>Endo</i> and <i>Exo</i> Stereoselectivities of Cyclopropene Diels–Alder Reactions

The factors controlling the reactivities and stereoselectivities in the Diels–Alder reactions of substituted cyclopropenes with butadiene were explored with M06-2X density functional theory. Differences in reactivities result from differences in the hyperconjugative aromaticities and antiaromaticities of the cyclopropenes. When the 3-substituent is a σ-donor, the ground state is destabilized, and the reactivity is enhanced. Acceptors have the opposite effect. Electrostatic, secondary orbital, and steric effects are all found to influence stereoselectivities

Alkene Distortion Energies and Torsional Effects Control Reactivities, and Stereoselectivities of Azide Cycloadditions to Norbornene and Substituted Norbornenes

The transition structures for 1,3-dipolar cycloadditions of phenyl azide to norbornene derivatives were located with quantum mechanical methods. Calculations were carried out with M06-2X/6-311G­(d,p) and SCS-MP2/6-311G­(d,p)//M06-2X/6-311G­(d,p) methods. The calculated activation barriers strongly correlate with transition state distortion energies (Δ<i>E</i>_d^⧧) but not with the reaction energies. Strain-promoted reactions are accelerated because it is easy to distort the strained reactants to a pyramidalized transition state geometry; a correlation of cycloaddition rates with substrate distortion was found for the bicyclic and tricyclic alkenes studied here. The stereoselectivities of reactions of norbornene derivatives are controlled primarily by torsional effects that also influence alkene pyramidalization. These reactions are distortion-accelerated