Predicting reactivity and stereoselectivity in the Nazarov reaction: a combined computational and experimental study

The Nazarov reaction of pentadienyl cations generated by protonation of either dienones or alkoxytrienes has been examined in detail both experimentally and by DFT calculations. In particular, calculations at the B3LYP/6‐311G** level of theory accurately predicted, and accounted for, the outcome of the Brønsted acid catalyzed electrocyclization of 4π‐electron systems in which one of the double bonds involved in the process was embedded in N‐ and S‐heterocyclic rings. Calculations showed that both heteroatoms are capable of accelerating the ring closure by stabilizing the partial positive charge which develops at C‐6 (C‐2) in the transition state, with S‐heterocyclic derivatives being more reactive than the corresponding N‐containing compounds. In general, pentadienyl cations generated by protonation of alkoxytrienes were expected to react faster than those obtained by protonation of the corresponding dienones, as the latter were stabilized by a hydrogen bond. The presence of a substituent on the heterocyclic ring significantly affects the stereoselectivity (torquoselectivity) only in the case of the N‐heterocyclic derivatives, in which a 2‐alkyl group is axially oriented, providing the cis‐2,5‐disubstituted isomer only. Instead, with substituted S‐heterocyclic compounds, the anticipated torquoselectivity was very low and, in fact, a 3:1 diastereomeric mixture between the trans and cis products was experimentally found after ring closure. For this study, the synthesis of the appropriate N‐ and S‐containing dienones and alkoxytrienes was realized to evaluate the predictivity power of the DFT computations, which was very good in all of the cases examined, both in terms of reactivity and stereoselectivity. The consistency observed between computational and experimental results, therefore, shows the usefulness of DFT calculations at the B3LYP/6‐311G** level of theory as a robust instrument for the prediction of reactivity and stereoselectivity in the Nazarov electrocyclic reaction.     

Density functional studies on the Nazarov reaction involving cyclic systems

Density functional theory (DFT) has been used to define the energy profiles of the Nazarov reaction involving cyclic systems. The calculations were carried out at the B3LYP/6-311G** level of theory and the solvent (dichloromethane) contribution was estimated by using the recently developed SM5.43R solvation model. DFT calculations were first carried out to determine the energy profiles associated with the electrocyclization reactions of 3-hydroxy- and 3-ethoxypentadienyl cations in which one of the double bonds is embedded in O-heterocyclic and carbocyclic systems. In particular, the effects on the reaction rate of modifications to the substrate, as well as the presence of the heteroatom in the cycle, have been investigated. The torquoselectivity of the electrocyclization reaction was then explored with substituted O-heterocycles to understand the factors that control the stereochemical outcome of the process that preferentially provides 2,5-trans-disubstituted products. These DFT-based results rationally explain most of the experimental observations related to the Nazarov reaction of the substrates herein investigated and could be useful in the rational interpretation, and likely in the prediction, of the outcome of Nazarov reactions involving other cyclic systems.     

Electrocyclic Reactions of Siloles: A Combined Experimental and Theoretical Study

The reaction of 4‐chloro‐1,2‐dimethyl‐4‐supersilylsila‐1‐cyclopentene ( 2 a ) with Li[NiPr₂] at ?78 °C results in the formation of the formal 1,4‐addition product of the silacyclopentadiene derivative 3,4‐dimethyl‐1‐supersilylsila‐1,3‐cyclopentadiene ( 4 a ) with 2,3‐dimethyl‐4‐supersilylsila‐1,3‐cyclopentadiene ( 5 a ). In addition the respective adducts of the Diels–Alder reactions of 4 a + 4 a and 4 a + 5 a were obtained. Compound 4 a , which displays an s‐cis‐silacyclopentadiene configuration, reacts with cyclohexene to form the racemate of the [4+2] cycloadduct of 4 a and cyclohexene ( 9 ). In the reaction between 4 a and 2,3‐dimethylbutadiene, however, 4 a acted as silene as well as silacyclopentadiene to yield the [2+4] and [4+2] cycloadducts 10 and 11 , respectively. The constitutions of 9 , 10 , and 11 were confirmed by NMR spectroscopy and their crystal structures were determined. Reaction of 4‐chloro‐1,2‐dimethyl‐4‐tert‐butyl‐4‐silacyclopent‐1‐ene ( 2 c ) with KC₈ yielded the corresponding disilane ( 12 ), which was characterized by X‐ray crystal structure analysis (triclinic, P$\bar 1$ ). DFT calculations are used to unveil the mechanistic scenario underlying the observed reactivity.     

A density functional theory study on the electrocyclization of 1,2,4,6-heptatetraene analogues: converting a pericyclic to a pseudopericyclic reaction

A comprehensive B3LYP/6-31+G* study on the electrocyclization of 1,2,4,6-heptatetraene analogues was conducted. Starting from the cyclization of (2Z)-2,4,5-hexatrienal, a pericyclic disrotatory process favored by the assistance of a electron lone pair, we incorporated small modifications in its molecular structure to obtain a truly pseudopericyclic process. To this purpose electronegative atoms (fluorine and nitrogen) were added to give a more electrophilic character on the carbon atom which is attacked by the electron lone pair of the oxygen atom. The complete pathway for each reaction was determined, and changes in magnetic properties were monitored with a view to estimating the aromatization associated with each process. This information, together with the energetic and structural results, allowed us to classify the reactions as pseudopericyclic or pericyclic. Among all studied reactions only one was a truly pseudopericyclic process and another was a borderline case. The features of this unequivocally pseudopericyclic case were analyzed in depth.     

Criteria for the elucidation of the pseudopericyclic character of the cyclization of (Z)-1,2,4,6-heptatetraene and its heterosubstituted analogues: magnetic properties and natural bond orbital analysis

The electrocyclization of heterosubstituted derivatives of (Z)-1,2,4,6-heptatetraene, (2Z)-2,4,5-hexatrien-1-imine and (2Z)-2,4,5-hexatrienal exhibit some features which suggest a pseudopericyclic mechanism. In order to examine this, a comprehensive study including the determination of magnetic properties to estimate aromaticity and an NBO analysis throughout the reaction path was conducted. The cyclization of 5oxo-2,4-pentadienal, a process of unequivocal pseudopericyclic nature, was studied for comparison. The results suggest that, although the lone electron pair on the heteroatom in the heptatetraene derivatives seemingly plays a crucial role in the reaction mechanism, it does not suffice to deprive the reaction from the essential features of a pericyclic disrotatory electrocyclization.     

Harnessing the Lewis Acidity of HFIP through its Cooperation with a Calcium(II) Salt: Application to the Aza‐Piancatelli Reaction

A method to extend the scope of the aza‐Piancatelli reaction between 2‐furylcarbinols and anilines is depicted. We found that 1,1,1,3,3,3‐hexafluoro‐2‐propanol (HFIP) is the solvent of choice for this transformation, as it outcompetes the usual solvents in terms of rate and yield. Side reactions and other issues raised by the title reaction are prevented, thereby providing an avenue to complex molecules that were previously inaccessible. Lewis acidity studies and computations were carried out to unveil the role of HFIP. Based on these results, we propose that HFIP is, in fact, acting as a Lewis acid and that its acidity can be enhanced when combined with a calcium(II) salt.     

Modular Construction of Fluoroarenes from a New Difluorinated Building Block by Cross‐Coupling/Electrocyclisation/Dehydrofluorination Reactions

Palladium‐catalysed coupling reactions based on a novel and easy‐to‐synthesise difluorinated organotrifluoroborate were used to assemble precursors to 6π‐electrocyclisations of three different types. Electrocyclisations took place at temperatures between 90 and 240 °C, depending on the central component of the π‐system; nonaromatic trienes were most reactive, but even systems that required the temporary dearomatisation of two arenyl subunits underwent electrocyclisation, albeit at elevated temperatures. Photochemical conditions were effective for these more demanding reactions. The package of methods delivered a structurally diverse set of fluorinated arenes, spanning a 20 kcal mol^?1 range of reactivity, by a flexible route.     

Synthesis of Sultones from Chlorosulfates by a Complex Cascade Reaction Occurring under Mild Thermal Conditions

Chlorosulfate derivatives are interesting reagents that have been traditionally used to get other sulfur-containing compounds by formal nucleophilic substitution of the chlorine atom. This work describes a different mode of reactivity of alkyne-containing chlorosulfates to get sultones, the sulfur analogues of lactones. The complex skeletal rearrangement observed in this transformation is comparable to those intricate processes promoted or catalyzed by organometallic compounds. However, the reaction here described does not require any reagent or additive, being just a thermal process. Computational calculations support a mechanism based on a series of cascade reactions where almost every step is counterintuitive. Some of these steps include original processes related to classical reactions, thus adding complementary views to traditional organic chemistry.     

Theoretical studies on the influence of metallic cations on ring opening of propylene oxide catalyzed by metal-salen complexes

We studied the ring opening of propylene oxide (PO) by salen-M coordinated OH⁻ group [M = Al(III), Sc(III), Cr(III), Mn(III), Fe(III), Co(II), Co(III), Ni(II), Cu(II), Zn(II), Ru(III) and Rh(III)]. The results show that the ring-opening energy barriers for M(II) complexes are much lower than those with M(III) complexes in the gas phase, and the barriers correlate linearly with the negative charges on the OH⁻ group and the Fukui function condensed on the OH⁻ group. The nucleophilicity ordering in the gas phase can be rationalized by the ratio of formal positive charges/radius of M cations. Solvent effect greatly increases the barriers of M(II) complexes but slightly changes the results of M(III) ones, making the barriers similar. Analysis indicates that the reaction heats are linearly proportional to the reverse reaction barriers. The relationships established here can be used to estimate the ring-opening barriers and to screen epoxide ring-opening catalysts.     

Coarctate versus pericyclic reactivity in naphthalene-fused azo-ene-ynes: synthesis of benzocinnolines and benzoisoindazoles

The cyclization reactions of naphthalene-fused azo-ene-yne compounds are explored both computationally and experimentally. Calculations reveal that naphtho-fusion to an azo-ene-yne scaffold does not significantly alter the transition state energies compared to the benzene-based systems; however, fusing the naphthalene in an angular fashion leads to lower energy intermediates due to the creation of arenes possessing greater aromaticity. Experimentally, the cyclization of the angular systems yields not only the expected monomeric benzocinnolines and benzoisoindazoles, but also several dimeric structures, including one that readily isomerizes in the presence of light and/or trace acid.