1,4-Didehydronaphthalin

Effects of the Distance between Radical Sites on the Reactivities of Aromatic Biradicals

Coupling of the radical sites in isomeric benzynes is known to hinder their radical reactivity. In order to determine how far apart the radical sites must be for them not to interact, the gas-phase reactivity of several isomeric protonated (iso)quinoline- and acridine-based biradicals was examined. All the (iso)quinolinium-based biradicals were found to react slower than the related monoradicals with similar vertical electron affinities (i.e., similar polar effects). In sharp contrast, the acridinium-based biradicals, most with the radical sites farther apart than in the (iso)quinolinium-based systems, showed greater reactivities than the relevant monoradicals with similar vertical electron affinities. The greater distances between the two radical sites in these biradicals lead to very little or no spin-spin coupling, and no suppression of radical reactivity was observed. Therefore, the radical sites can still interact if they are located on adjacent benzene rings and only after being separated further than that does no coupling occur. The most reactive radical site of each biradical was experimentally determined to be the one predicted to be more reactive based on the monoradical reactivity data. Therefore, the calculated vertical electron affinities of relevant monoradicals can be used to predict which radical site is most reactive in the biradicals.

1,3-Chlorine Shift to a Vinyl Cation: A Combined Experimental and Theoretical Investigation of the E-Selective Gold(I)-Catalyzed Dimerization of Chloroacetylenes

Metal-catalyzed dimerization reactions of terminal acetylenes are well known in the literature. However, only a few examples of the dimerization of halogen-substituted acetylenes are described. The products of the latter metal-catalyzed dimerization are the branched head-to-tail enynes. The formation of the corresponding linear head-to-head enynes has not been reported yet. Herein, we demonstrate by means of quantum chemical methods and experiments that the head-to-head dimerization of chloroarylacetylenes can be achieved via mono gold catalysis. Under the optimized conditions, a clean and complete conversion of the starting materials is observed and the dimeric products are obtained up to 75% NMR yield. A mechanistic investigation of the dimerization reaction reveals that the branched head-to-tail vinyl cation is energetically more stable than the corresponding linear head-to-head cation. However, the latter can rearrange by an unusual 1,3-chlorine shift, resulting in the highly stereoselective formation of the trans product, which corresponds to the gold complex of the head-to-head E-enyne. The activation barrier for this rearrangement is extremely low (ca. 2 kcal/mol). As the mono gold-catalyzed dimerization can be conducted in a preparative scale, this simple synthesis of trans-1,2-dichloroenynes makes the gold(I)-catalyzed head-to-head dimerization of chloroarylacetylenes an attractive method en route to more complex conjugated enyne systems and their congeners.

Effect of Extended Benzannelation Orientation on Bergman and Related Cyclizations of Isomeric Quinoxalenediynes

A combined computational and experimental study was conducted to examine the effect of extended benzannelation orientation on C¹-C⁵ and C¹-C⁶ cyclization of acyclic quinoxalenediynes. Calculations (mPW1PW91/cc-pVTZ//mPW1PW91/6-31G(d,p)) on terminal and phenylethynyl-substituted 5,6-diethynylquinoxaline and 6,7-diethynylquinoxaline showed C¹-C⁶ Bergman cyclization as the favored thermodynamic reaction pathway, with larger C¹-C⁶ preference for the angular quinoxalenediynes due to gain of a new aromatic sextet. Kinetic studies, as a function of 1,4-cyclohexadiene concentration, revealed retro-Bergman ring opening predominates over hydrogen atom abstraction (k_-1 > k₂) for 6,7-diethynylquinoxaline while 5,6-diethynylquinoxaline undergoes irreversible Bergman cyclization indicative of a large retro-Bergman ring opening barrier (k₂ > k_-1). The effect of extended linear versus angular benzannelation on reaction pathway shows in the contrasting photocyclizations of phenylethynyl derivatives. While angular 5,6-diethynylquinoxalines gave exclusive C¹-C⁶ photocyclization, linear 6,7-diethynylquinoxaline afforded C¹-C⁵ fulvene products. Computed singlet-triplet gaps and biradical stabilization energies indicated weak interaction between the nitrogen lone pair and proximal radical center in angular 5,6-diethynylquinoxalines. The overall data indicates extended angular benzannelation effectively renders Bergman cyclization irreversible due to favorable aromatic stabilization energy, while extended linear benzannelation results in increased retro-Bergman ring opening, allowing C¹-C⁵ cyclization to become a competitive reaction channel.

Au(I)-Catalyzed Dimerization of Two Alkyne Units-Interplay between Butadienyl and Cyclopropenylmethyl Cation: Model Studies and Trapping Experiments

In recent years, Au(I)-catalyzed reactions proved to be a valuable tool for the synthesis of substituted cycles by cycloaromatization and cycloisomerization starting from alkynes. Despite the myriad of Au(I)-catalyzed reactions of alkynes, the mono Au(I)-catalyzed pendant to the radical dimerization of nonconjugated alkyne units has not been investigated by quantum chemical calculations. Herein, by means of quantum chemical calculations, we describe the mono Au(I)-catalyzed dimerization of two alkyne units as well as the transannular ring closure reaction of a nonconjugated diyne. We found that depending on the system and the method used either the corresponding cyclopropenylmethyl cation or the butadienyl cation represents the stable intermediate. This circumstance could be explained by different stabilizing effects. Moreover, the calculation reveals a dramatic (>10¹²-fold) acceleration of the Au(I)-catalyzed reaction compared to that of the noncatalyzed radical variant. Trapping experiments with a substituted 1,6-cyclodecadiyne using benzene as a solvent at room temperature as well as studies with deuterated solvents confirm the calculations. In this context, we also demonstrate that trapping of the cationic intermediate with benzene does not proceed via a Friedel-Crafts-type reaction.

Polar Effects Control the Gas-phase Reactivity of Charged para-Benzyne Analogs

The gas-phase reactivity of charged para-benzynes is entirely unexplored as they and/or their precursors tend to undergo ring-opening upon their generation. We report here a gas-phase reactivity study of two such benzynes, the 2,5-didehydropyridinium and 5,8-didehydroisoquinolinium cations, generated in a modified dual-linear quadrupole ion trap (DLQIT) mass spectrometer. Both biradicals were found to form diagnostic products with organic molecules, indicating the presence of two radical sites. As opposed to earlier predictions that the singlet-triplet (S-T) splitting controls the radical reactivity of such species, the 2,5-didehydropyridinium cation reacts much faster in spite of its larger S-T splitting. Calculated vertical electron affinities of the radical sites of the para-benzynes, a parameter related to the polarity of the transition states of their reactions, appears to be the most important reactivity controlling factor.

Experimental and computational studies on the formation of three para-benzyne analogues in the gas phase

Experimental and computational studies on the formation of three gaseous, positively-charged para-benzyne analogues in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer are reported. The structures of the cations were examined by isolating them and allowing them to react with various neutral reagents whose reactions with aromatic carbon-centered σ-type mono- and biradicals are well understood. Cleavage of two iodine-carbon bonds in N-deuterated 1,4-diiodoisoquinolinium cation by collision-activated dissociation (CAD) produced a long-lived cation that showed nonradical reactivity, which was unexpected for a para-benzyne. However, the reactivity closely resembles that of an isomeric enediyne, N-deuterated 2-ethynylbenzonitrilium cation. A theoretical study on possible rearrangement reactions occurring during CAD revealed that the cation formed upon the first iodine atom loss undergoes ring-opening before the second iodine atom loss to form an enediyne instead of a para-benzyne. Similar results were obtained for the 5,8-didehydroisoquinolinium cation and the 2,5-didehydropyridinium cation. The findings for the 5,8-didehydroisoquinolinium cation are in contradiction with an earlier report on this cation. The cation described in the literature was regenerated by using the literature method and demonstrated to be the isomeric 5,7-didehydro-isoquinolinium cation and not the expected 5,8-isomer.

Syntheses, thermal reactivities, and computational studies of aryl-fused quinoxalenediynes: effect of extended benzannelation on Bergman cyclization energetics

A series of [b]-fused 6,7-diethynylquinoxaline derivatives have been synthesized through an imine condensation strategy to examine the effect of extended benzannelation on the thermal reactivity of enediynes. Absorption and emission spectra of the highly conjugated quinoxalenediynes were red-shifted approximately 100-200 nm relative to those of 1,2-diethynylbenzene. Strong exotherms indicative of enediyne cyclization were observed by differential scanning calorimetry, while solution cyclizations in the presence of 1,4-cyclohexadiene confirmed C(1)-C(6) Bergman cyclization. To provide further insight into Bergman cyclization energetics, computational studies were performed to compare changes in the cyclization enthalpy barrier, reaction enthalpy, and barrier of retro-Bergman ring-opening. Extension of benzannelation from 1,2-diethynylbenzene to either 2,3-diethynylnaphthalene or the 6,7-diethynylquinoxalines had a minimal effect on the cyclization barrier. In comparison, the enthalpies of cyclization were increased upon linearly extended benzannelation, which resulted in reduced barriers to retro-Bergman ring-opening. In addition, the orientation of extended benzannelation was found to have a significant effect on the cyclization endothermicity. In particular, 5,6-diethynylquinoxaline exhibited a 6.9 kcal/mol decrease in cyclization enthalpy compared to 6,7-diethynylquinoxaline due to increased aromatic stabilization energy in the respective angularly versus linearly fused azaacene cyclized products.

Matrix Isolation and Electronic Structure of Di- and Tridehydrobenzenes

Within the past four decades, matrix isolation spectroscopy has emerged as the method of choice for obtaining direct structural information on benzynes and related dehydroaromatics. In combination with quantum chemical computations, detailed insight into the structure and reactivity of di-, tri-, and tetradehydrobenzenes has been obtained. This Review focuses on rather recent developments in aryne chemistry with a special emphasis on the matrix isolation of tridehydrobenzenes and related systems.

Photochemical Triggering of the Bergman and Myers - Saito Cyclizations

Two strategies for the photochemical generation of reactive enediyne compounds and their subsequent cycloaromatization to p-benzyne or α,3-didehydrotoluene derivatives are discussed in this account. The first method employs a photo-Wolff reaction of stable 11- or 12-membered ring precursor enediynes containing the 2-diazo-1,3-diketone moiety. Irradiation of these compounds results in ring contraction and the formation of two isomeric enediynes possessing an enolized β-ketoester fragment. One of the isomers undergoes the conventional Bergman cyclization, whereas the other isomerizes into the enyne-allene tautomer, which rapidly cyclizes via a Myers–Saito mechanism. The second strategy consists of replacing the triple bond in a cyclic enediyne or enyne-allene structure with a cyclopropenone group, rendering them thermally stable. Photolysis of cyclopropenones results in efficient decarbonylation and the regeneration of a triple bond, restoring the enediyne π-system. The generation of reactive enediynes by non-resonant two-photon excitation using wavelengths within a ‘phototherapeutic window’ was also demonstrated. Photogenerated enediynes show significant nuclease activity, efficiently inducing single-strand dDNA cleavage.

Efficient and accurate characterization of the Bergman cyclization for several enediynes including an expanded substructure of esperamicin A1

Incorporation of enediynes into anticancer drugs remains an intriguing yet elusive strategy for the design of therapeutically active agents. Density functional theory was used to locate reactants, products, and transition states along the Bergman cyclization pathways connecting enediynes to reactive para-biradicals. Sum method correction to low-level calculations confirmed B3LYP/6-31G(d,p) as the method of choice in investigating enediynes. Herein described as MI:Sum, calculated reaction enthalpies differed from experiment by an average of 2.1 kcal x mol(-1) (mean unsigned error). A combination of strain energy released across the reaction coordinate and the critical intramolecular distance between reacting diynes explains reactivity differences. Where experimental and calculated barrier heights are in disagreement, higher level multireference treatment of the enediynes confirms lower level estimates. Previous work concerning the chemically reactive fragment of esperamcin, MTC, is expanded to our model system MTC2.

Gas-phase reactivity of aromatic sigma,sigma-biradicals toward dinucleoside phosphates

In order to improve the understanding of the interactions of aromatic sigma,sigma-biradicals with DNA, the reactivity of three isomeric sigma,sigma-biradicals toward four dinucleoside phosphates was studied in a mass spectrometer. The dinucleoside phosphates were evaporated into the mass spectrometer by using laser-induced acoustic desorption (LIAD). The results demonstrate that the structure of the sigma,sigma-biradical and the base sequence of the dinucleoside phosphate can have a major influence on these reactions.

Quantum chemical characterization of the vertical electron affinities of didehydroquinolinium and didehydroisoquinolinium cations

Vertical electron affinities (EA) are predicted for the lowest energy singlet states of the 21 didehydroquinolinium cation isomers and the 21 didehydroisoquinolinium cation isomers, as well as the doublet states of the seven dehydroquinolinium cation isomers, the seven dehydroisoquinolinium cation isomers, the seven N-methyldehydroquinolinium cations, and the seven N-methyldehydroisoquinolinium cations, by using density functional theory. For the monoradicals, the calculated EA of the radical site depends only on the distance from the (formally charged) nitrogen atom, and is reduced by 0.14-0.24 eV when the NH+ group is replaced with an NCH3+ group. Nearly all of the calculated EAs for the ortho biradicals are lower (by 0.04-0.72 eV) than those for either of the corresponding monoradicals. For the meta biradicals, the calculated EAs lie either between the EAs of the corresponding monoradicals or higher (by 0.07-0.58 eV), and they are extremely sensitive to the separation (distance) between the two dehydrocarbon atoms. For the biradicals that do not have either an ortho or meta relationship the calculated EAs are all higher (by 0.02-1.93 eV) than those for either of the corresponding monoradicals. The EAs are examined to gain insight into the nature of inductive/field and resonance effects that influence the electrophilicity of the radical site(s), which is a major factor controlling the reactivity of these types of (bi)radicals.

Quantum Chemical Characterization of the Structures, Thermochemical Properties, and Singlet−Triplet Splittings of Didehydroquinolinium and Didehydroisoquinolinium Ions

Structural and energetic properties are predicted for the 21 didehydroquinolinium ion isomers and 21 didehydroisoquinolinium ion isomers in their lowest-energy singlet and triplet states by using density functional and multireference second-order perturbation theories. Singlet-triplet splittings and biradical stabilization energies are examined to gain insight into the degree of interaction between the biradical centers, with comparison being made to analogous didehydronaphthalenes and didehydropyridines.

Gas-Phase Reactivity of Charged π-Type Biradicals

Four pi,pi-biradicals, 2,6-dimethylenepyridinium and the novel isomers N-(3-methylenephenyl)-3-methylenepyridinium, N-phenyl-3,5-dimethylenepyridinium, and N-(3,5-dimethylenephenyl)pyridinium ions, were generated and structurally characterized in a Fourier transform ion cyclotron resonance mass spectrometer. Their gas-phase reactivity toward various reagents was compared to that of the corresponding monoradicals, 2-methylenepyridinium, N-phenyl-3-methylenepyridinium, and N-(3-methylenephenyl)pyridinium ions. The biradicals reactivity was found to reflect their predicted multiplicity. The 2,6-dimethylenepyridinium ion, the only biradical in this study predicted to have a closed-shell singlet ground state, reacts significantly faster than the other biradicals, which are predicted to have triplet ground states. In fact, this biradical reacts at a higher rate than the analogous monoradical, which suggests that to avoid the costly uncoupling of its unpaired electrons, the biradical favors ionic mechanisms over barriered radical pathways. In contrast, the second-order reaction rate constants of the isomeric biradicals with triplet ground states are well approximated by those of the analogous monoradicals, although the final reaction products are sometimes different. This difference arises from rapid radical-radical recombination of the initial monoradical reaction products. The overall reactivity toward the hydrogen-atom donors benzeneselenol and tributylgermanium hydride is significantly greater for the radicals with the charged site in the same ring system as the radical site. This finding indicates that polar effects play an important role in controlling the reactivity of pi,pi-biradicals, just as has been demonstrated for sigma,sigma-biradicals.

Electronic Effects on the Bergman Cyclisation of Enediynes. A Review

The thermal cyclisation of enediynes to benzene-1,4-diyl diradicals (Bergman cyclisation) is affected by geometrical and electronic conditions. While the effect of ring strain or conformational constrains on the cyclisation temperature has been investigated in detail, electronic contributions have been less studied. Often geometrical and electronic contributions cannot be clearly distinguished. In most cases metal ion chelation does involve both. In this review we have summarised clear-cut observations of electronic substituents effects on the thermal enediyne reactivity. The effects of substituents in the vinylic and terminal alkyne position, the influence of benzo-fusion and hetarene fusion, as well as the changes induced by heteroatoms in the enediyne skeleton, are within the scope of this review. With the exception of more complex heterocyclic hetarene-fused enediynes the experimental data of electronic substituent effects on the thermal Bergman cyclisation of enediynes follow theoretical predictions. A review with 57 references.

Stability and three-dimensional aromaticity of closo-NB(n-1)H n azaboranes, n = 5-12

Computations on all the possible positional isomers of the closo-azaboranes NB(n)()(-)(1)H(n)() (n = 5-12) reveal substantial differences in the relative energies. Data at the B3LYP/6-311+G level of density functional theory (DFT) agree well with expectations based on the topological charge stabilization, with the qualitative connectivity preferences of Williams, and with the Jemmis-Schleyer six interstitial electron rules. The energetic relationship involving each of the most stable positional isomers, 1-NB(4)H(5), NB(5)H(6), 2-NB(6)H(7), 1-NB(7)H(8), 4-NB(8)H(9), 1-NB(9)H(10), 2-NB(10)H(11), NB(11)H(12), was based on the energies (DeltaH) of the model reaction: NBH(2) + (n-1)BH(increment) --> NB(n)()H(n)()(+1) (n = 4-11). This evaluation shows that the stabilities of closo-azaboranes NB(n)()(-)(1)H(n)() (n = 5-12) increase with increasing cluster size from 5 to 12 vertexes. The "three-dimensional aromaticity" of these closo-azaboranes NB(n)()(-)(1)H(n)() (n = 5-12) is demonstrated by their the nucleus-independent chemical shifts (NICS) and their magnetic susceptibilities (chi), which match one another well. However, there is no direct relationship between these magnetic properties and the relative stabilities of the positional isomers of each cluster. As expected, other energy contributions such as topological charge stabilization and connectivity can be equally important.

Butalene and related compounds: aromatic or antiaromatic?

Density functional theory (DFT) has been used to study the first three members of the condensed cyclobutadienoid series, butalene (3), bicyclobutadienylene (12), and dicyclobutenobutalene (20). The first is planar and is judged "aromatic" by comparisons with suitable models using both energetic and magnetic criteria. The second is nonplanar, and not aromatic, but not so antiaromatic as cyclobutadiene (11). The third is slightly more antiaromatic and best viewed as a butalene fused to two cyclobutadiene rings; its properties are the sum of aromatic and antiaromatic components, like benzocyclobutadiene. Ring-opening transition states for both 3 and 12 have been located, and these are conrotatorily twisted. The ring-opening barrier for 12 is more than twice that for 3. Ring-opening of 20 involves ring inversion as the only barrier.

The cyclization of parent and cyclic hexa-1,3-dien-5-ynes--a combined theoretical and experimental study

The thermal cycloisomerization of both parent and benzannelated hexa-1,3-dien-5-yne, as well as of carbocyclic 1,3-dien-5-ynes (ring size 7-14), was investigated by using pure density functional theory (DFT) of Becke, Lee, Yang, and Parr (BLYP) in connection with the 6-31G* basis set and the Brueckner doubles coupled-cluster approach [BCCD(T)] with the cc-pVDZ basis set for the parent system. The initial cyclization product is the allenic cyclohexa-1,2,4-triene (isobenzene), while the respective biradical is the transition structure for the enantiomerization of the two allenes. Two consecutive [1,2]-H shifts further transform isobenzene to benzene. For the benzannelated system, the energetics are quite similar and the reaction path is the same with one exception: the intermediate biradical is not a transition state but a minimum which is energetically below isonaphthalene. The cyclization of the carbocyclic 1,3-dien-5-ynes, which follows the same reaction path as the parent system, clearly depends on the ring size. Like the cyclic enediynes, the dienynes were found to cyclize to products with reduced ring strain. This is not possible for the 7- and 8-membered dienynes, as their cyclization products are also highly strained. For 9- to 11-membered carbocycles, all intermediates, transition states, and products lie energetically below the parent system; this indicates a reduced cyclization temperature. All other rings (12- to 14-membered) have higher barriers. Exploratory kinetic experiments on the recently prepared 10- to 14-membered 1,3-dien-5-ynes rings show this tendency, and 10- and 11-membered rings indeed cyclize at lower temperatures.

Electronic control of the Bergman cyclization: the remarkable role of vinyl substitution

We report an ab initio study of the effect of vinyl substitution on the cycloaromatization of 3-ene-1,5-diynes (the Bergman cyclization). The majority of the calculations were conducted by using the BLYP version of Density Functional Theory, and higher level Brueckner orbital calculations were used for a few key compounds. In all, 46 enediynes, 44 cyclization transition states, 39 singlet p-benzynes, and 28 related triplet p-benzynes were studied, including simple vinyl-substituted and annulated examples. The data indicate that strongly electron-withdrawing groups increase the cyclization barrier, while sigma-donating groups decrease it; pi conjugation, especially donation, has little effect. Most annulations, including those involving heteroaromatic rings, lower the barrier slightly (6 MR) or raise it slightly (5 MR). Larger effects are seen for smaller rings or charged rings. Some previously observed apparent rate inhibitions are seen to be due to reversibility or forward reactivity of the intermediate p-benzynes, which are thereby inhibited from the H abstraction step that completes cycloaromatization. H abstraction reactivity, as judged from the p-benzyne singlet-triplet energy gap and from isodesmic equations, is also examined. Unexpected behavior is predicted for some heteroaromatic systems. Finally, we anticipate how these results may be applied to the design of prodrug candidates for subsequent biological application.