Comprehensive Model for the Electronic Structures of 1,2,4-Cyclohexatriene and Related Compounds

Abstract

To gain a better understanding of the chemistry of cyclic allenes, in the present work six-membered monocyclic systems containing second- (B−O) and third-row elements (Al−S) of the periodic table of elements are studied in detail, regarding the energetic order of the electronic states and the planarization energies, which together determine their chemical behavior. The data obtained by high-level ab initio methods show that the properties of the compounds strongly depend on the heteroatoms and can be related to trends in the periodic table of elements. These trends in the series B to O and Al to S are rationalized by the degree of the interaction between the various fragments and the interplay between this interaction and the strain in the allene moiety. In addition, by applying the model to charged species, we reveal an unexpected link between the cyclic allenes and the phenyl anion. Furthermore, our computations answer open questions about the chemical properties of 1-aza-2,3-cyclohexadienes. Finally, we analyze how the various kinds of interaction influence the chemistry of this important class of intermediates.

Are Allenes with Zwitterionic Character Still Allenes? Of Course!

“…Until our communication appeared, Christl and Engels seemed to agree with our usage of nomenclature, since the title of one of their papers10 refers to compound 3 as “1‐azacyclohexa‐2,3,5‐triene”, implying allene 3 a , despite the fact that the ground state of this transient compound was already known to be that of 3‐dehydropyridinium ylide 3 b 11 In other work by Engels et al.,11c a scheme, which is entitled “Possible Electronic Structures of Cyclic Allenes”, shows allene, zwitterion, and diradical forms of a compound. Thus the Christl and Engels recognized that, regardless of the various possible ground states of an allene, it is still appropriate and most convenient to classify the compound as an allene.…”

Are Allenes with Zwitterionic Character Still Allenes? Of Course!

“…Until our communication appeared, Christl and Engels seemed to agree with our usage of nomenclature, since the title of one of their papers10 refers to compound 3 as “1‐azacyclohexa‐2,3,5‐triene”, implying allene 3 a , despite the fact that the ground state of this transient compound was already known to be that of 3‐dehydropyridinium ylide 3 b 11 In other work by Engels et al.,11c a scheme, which is entitled “Possible Electronic Structures of Cyclic Allenes”, shows allene, zwitterion, and diradical forms of a compound. Thus the Christl and Engels recognized that, regardless of the various possible ground states of an allene, it is still appropriate and most convenient to classify the compound as an allene.…”

Electronic Structural Trends in Divalent Carbon Compounds

“…Compound 16 has a π-population of 0.68 electrons and appears approximately in the middle of the range. Previous studies have shown that 16 is best represented as a dipolar compound − so, with deference to this description, our scale implies that only 9 – 13 , 15 , and 17 should be described formally as carbenes; the MIC structures 1 – 8 , 14 , and 16 are better described as having a negative charge at C d . The values for the NPA π-population at the C d center in 19 and at the “abnormal” and “normal” sites in 18 indicate that the divalent carbon atoms in anionic compounds are electronically quite similar to those in mesoionic compounds.…”

Quest for Even Higher Stabilized Foiled Carbenes

“…Cyclopropylidene 2 easily undergoes ring expansion to the energetically favored allene 1,2,4-cyclohexatriene ( 3 ) (Figure ). Experimentally, starting from 2 , it has already been shown that this cyclopropylidene-allene rearrangement to 3 is the main pathway in aprotic solvents . In water/dioxane, probably through a cationic pathway, the Skattebøl rearrangement to 1 occurs, leading to exo -bicyclo[2.1.1]hex-2-en-5-ol …”

Are Allenes with Zwitterionic Character Still Allenes? Of Course!

“…Until our communication appeared, Christl and Engels seemed to agree with our usage of nomenclature, since the title of one of their papers10 refers to compound 3 as “1‐azacyclohexa‐2,3,5‐triene”, implying allene 3 a , despite the fact that the ground state of this transient compound was already known to be that of 3‐dehydropyridinium ylide 3 b 11 In other work by Engels et al.,11c a scheme, which is entitled “Possible Electronic Structures of Cyclic Allenes”, shows allene, zwitterion, and diradical forms of a compound. Thus the Christl and Engels recognized that, regardless of the various possible ground states of an allene, it is still appropriate and most convenient to classify the compound as an allene.…”

Electronic Structural Trends in Divalent Carbon Compounds

“…Compound 16 has a π-population of 0.68 electrons and appears approximately in the middle of the range. Previous studies have shown that 16 is best represented as a dipolar compound − so, with deference to this description, our scale implies that only 9 – 13 , 15 , and 17 should be described formally as carbenes; the MIC structures 1 – 8 , 14 , and 16 are better described as having a negative charge at C d . The values for the NPA π-population at the C d center in 19 and at the “abnormal” and “normal” sites in 18 indicate that the divalent carbon atoms in anionic compounds are electronically quite similar to those in mesoionic compounds.…”

Quest for Even Higher Stabilized Foiled Carbenes

“…Cyclopropylidene 2 easily undergoes ring expansion to the energetically favored allene 1,2,4-cyclohexatriene ( 3 ) (Figure ). Experimentally, starting from 2 , it has already been shown that this cyclopropylidene-allene rearrangement to 3 is the main pathway in aprotic solvents . In water/dioxane, probably through a cationic pathway, the Skattebøl rearrangement to 1 occurs, leading to exo -bicyclo[2.1.1]hex-2-en-5-ol …”

Are Allenes with Zwitterionic Character Still Allenes? Of Course!

“…Until our communication appeared, Christl and Engels seemed to agree with our usage of nomenclature, since the title of one of their papers10 refers to compound 3 as “1‐azacyclohexa‐2,3,5‐triene”, implying allene 3 a , despite the fact that the ground state of this transient compound was already known to be that of 3‐dehydropyridinium ylide 3 b 11 In other work by Engels et al.,11c a scheme, which is entitled “Possible Electronic Structures of Cyclic Allenes”, shows allene, zwitterion, and diradical forms of a compound. Thus the Christl and Engels recognized that, regardless of the various possible ground states of an allene, it is still appropriate and most convenient to classify the compound as an allene.…”

Electronic Structural Trends in Divalent Carbon Compounds

“…Compound 16 has a π-population of 0.68 electrons and appears approximately in the middle of the range. Previous studies have shown that 16 is best represented as a dipolar compound − so, with deference to this description, our scale implies that only 9 – 13 , 15 , and 17 should be described formally as carbenes; the MIC structures 1 – 8 , 14 , and 16 are better described as having a negative charge at C d . The values for the NPA π-population at the C d center in 19 and at the “abnormal” and “normal” sites in 18 indicate that the divalent carbon atoms in anionic compounds are electronically quite similar to those in mesoionic compounds.…”

Quest for Even Higher Stabilized Foiled Carbenes

“…Cyclopropylidene 2 easily undergoes ring expansion to the energetically favored allene 1,2,4-cyclohexatriene ( 3 ) (Figure ). Experimentally, starting from 2 , it has already been shown that this cyclopropylidene-allene rearrangement to 3 is the main pathway in aprotic solvents . In water/dioxane, probably through a cationic pathway, the Skattebøl rearrangement to 1 occurs, leading to exo -bicyclo[2.1.1]hex-2-en-5-ol …”