Probing Hyperconjugative Aromaticity of Monosubstituted Cyclopentadienes

Xie, Qiong; Sun, Tingting; Orozco‐Ic, Mesías; Barroso, Jorge; Zhao, Yu; Merino, Gabriel; Zhu, Jun

doi:10.1002/ajoc.201800680

Cited by 21 publications

(13 citation statements)

References 66 publications

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“…The same phenomenon can be found in 1AuPMe 3 ‐pyrrolium (−25.9 ppm) and 2AuPMe 3 ‐pyrrolium (−26.6 ppm). This observation is consistent with recent work that monosubstitution on the sp 3 ‐hybridized carbon of cyclopentadiene by an electropositive substituent can lead to aromaticity . Furthermore, the slight energy difference between weakly antiaromatic 1F ‐pyrrolium (9.4 ppm) and nonaromatic 2F ‐pyrrolium (−1.8 ppm) indicates that aromaticity cannot be used to rationalize the thermodynamic stability of these isomers.…”

Section: Resultssupporting

confidence: 90%

Probing Reaction Mechanism of [1,5]‐Migration in Pyrrolium and Pyrrole Derivatives: Activation of a Stronger Bond in Electropositive Groups Becomes Easier

Xie

Zhao

Chen

et al. 2019

Chemistry An Asian Journal

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The [1,5]-migration reaction has attractedc onsiderable attention from experimentalists andt heoreticians for decades. Although it has been extensively investigated in various systems, studies on pyrrolium derivatives are underdeveloped. Herein, at heoretical study on the reaction mechanism of [1,5]-migrationi nb oth pyrroliuma nd pyrrole derivatives is presented. The results reveal lower activation barriers in [1,5]-migration of electropositive groups (AuPMe 3 and SnH 3 )i np yrrolium derivatives, although the bond dissociation energies of the AuÀNb ond (98.8 kcal mol À1 )a nd SnÀN bond (81.7 kcal mol À1 )a re larger than that of the NÀFb ond (57.6 kcal mol À1 ). The unexpectedly lower activation barriers (4.5 and 4.9 kcal mol À1 for AuPMe 3 and SnH 3 ,r espectively) for [1,5]-migration of electropositive groups,i nc omparison with the [1,5]-fluorine shift, can be attributed to aromaticity stabilizing the transition states, as revealed by significantly negative nucleus-independent chemical shift (NICS) values. Further studies indicate that charge distribution and frontier molecular orbitals also plays ome roles in [1,5]-migration of pyrrolium derivatives.[a] Q.

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Section: Resultssupporting

confidence: 90%

Probing Reaction Mechanism of [1,5]‐Migration in Pyrrolium and Pyrrole Derivatives: Activation of a Stronger Bond in Electropositive Groups Becomes Easier

Xie

Zhao

Chen

et al. 2019

Chemistry An Asian Journal

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“…We found a significant overlap of both shielding and deshielding regions, which change as the helix size does. Unexpectedly, we also noticed that in these structures, the contribution of core electrons is not only located close to the nuclei but it also exhibits a notable shielding cone and large negative values around the rings similar to those of the valence electrons, whereas in planar monocyclic rings, the core contribution is usually small and closely located in the vicinity of the atoms . Furthermore, the helical topology leads to the typical shielding cones obtained in planar rings to be reduced, owing to a deshielding region that grows along the helical axis as the number of turns increases.…”

Section: Introductionmentioning

confidence: 64%

“…Unexpectedly,w ea lso noticedt hat in these struc-tures, the contribution of core electrons is not only located close to the nuclei but it also exhibitsanotable shielding cone and large negative values aroundt he rings similar to those of the valence electrons,w hereas in planar monocyclic rings, the core contribution is usually small and closely located in the vicinity of the atoms. [19,20] Furthermore, the helicalt opology leads to the typical shielding cones obtained in planar rings to be reduced, owing to ad eshielding regiont hat grows along the helical axis as the number of turns increases. This implies that the most important contributionsofthe total magnetic response in the helical structures come from the s-a nd core electrons,a nd not from the p-ones as occurs in the case of PAHs.…”

Section: Introductionmentioning

confidence: 99%

Consequences of Curvature on Induced Magnetic Field: The Case of Helicenes

Orozco‐Ic¹,

Barroso²,

Charistos

et al. 2019

Chemistry A European J

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Helicenes consist of several fused rings twisted around an axis, forming a cylindrical helix, with π‐delocalized electrons in the non‐planar rings. Induced magnetic fields dissecting the orbital contributions of [6]‐, [7]‐, and [14]helicene are discussed. Computations show a deshielding cone produced by the π‐electrons along the helical axis. Unexpectedly, the response of the core electrons produces a shielding cone, which is cumulative and sensitive to the curvature of the systems owing to the overlap of the other ring responses. A warning is provided regarding the evaluation of the delocalization in curved systems in which the x‐ and y‐components of the induced magnetic field become relevant.

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“…Very recently, hyperconjugative aromaticity of monosubstituted cyclopentadienes (Figure 5) has been investigated. 14 The ISE, NICS(1) zz , and multicenter index (MCI) values of 3H-R, 3-R, and 3F-R demonstrate that monosubstitution of hydrogens in the CH 2 group by an electropositive substituent can also contribute "pseudo-2π electrons" by hyperconjugation, leading to the hyperconjugative aromaticity in the cyclopentadiene ring; whereas one electronegative substituent leads to nonaromaticity rather than antiaromaticity. When electropositive and electronegative substituents are considered simultaneously, an aromatic cyclopentadiene ring could be achieved (Table 3).…”

Section: ■ Introductionmentioning

confidence: 99%

Unconventional Aromaticity in Organometallics: The Power of Transition Metals

2019

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Conspectus Aromaticity, one of the most fundamental concepts in chemistry, has attracted considerable attention from both theoreticians and experimentalists. Much effort on aromaticity in organometallics has been devoted to metallabenzene and derivatives. In comparison, aromaticity in other organometallics is less developed. This Account describes how our group has performed quantum chemical calculations to examine aromaticity in recently synthesized novel organometallic complexes. By collaborations with experimentalists, we have extended several aromaticity concepts into organometallics to highlight the power of transition metals. In general, the transition metal could participate in delocalization either out of rings or in the rings. We examined the former by probing the possibility of transition metal substituents in hyperconjugative aromaticity, where the metal is out of the rings. Calculations on tetraaurated heteroaryl complexes reveal that incorporation of the aurated substituents at the nitrogen atom can convert nonaromaticity in the parent indolium into aromaticity in the aurated one due to hyperconjugation, thus extending the concept of hyperconjugative aromaticity to heterocycles with transition metal substituents. More importantly, further analysis indicates that the aurated substituents can perform better than traditional main-group substituents. Recently, we also probed the strongest aromatic cyclopentadiene and pyrrolium rings by hyperconjugation of transition metal substituents. Moreover, theoretical calculations suggest that one electropositive substituent is able to induce aromaticity; whereas one electronegative substituent prompts nonaromaticity rather than antiaromaticity. We also probed the possibility of Craig-type Möbius aromaticity in organometallic chemistry, where the position of the transition metals is in the rings. According to the electron count and topology, aromaticity can be classified as Hückel-type and Möbius-type. In comparison with numerous Hückel aromatics containing 4n+2 π-electrons, Möbius aromatics with 4n π-electrons, especially the Craig-type species, are particularly limited. We first examined aromaticity in osmapentalynes. Theoretical calculations reveal that incorporation of the osmium center not only reduces the ring strain of the parent pentalyne, but also converts Hückel antiaromaticity in the parent pentalyne into Craig-type Möbius aromaticity in metallapentalynes. Further studies show that the transition metal fragments can also make both 16e and 18e osmapentalenes aromatic, indicating that the Craig-type Möbius aromaticity in osmapentalyne is rooted in osmapentalenes. In addition, Möbius aromaticity is also possible in dimetalla[10]annulenes, where the lithium atoms are not spectator cations but play an important role due to their bonding interaction with the diene moieties. We then examined the possibility of σ-aromaticity in an unsaturated ring. Traditional π-aromaticity is used to describe the π-conjugation in fully unsaturated rings; whereas σ-aro...

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Probing Hyperconjugative Aromaticity of Monosubstituted Cyclopentadienes

Cited by 21 publications

References 66 publications

Probing Reaction Mechanism of [1,5]‐Migration in Pyrrolium and Pyrrole Derivatives: Activation of a Stronger Bond in Electropositive Groups Becomes Easier

Probing Reaction Mechanism of [1,5]‐Migration in Pyrrolium and Pyrrole Derivatives: Activation of a Stronger Bond in Electropositive Groups Becomes Easier

Consequences of Curvature on Induced Magnetic Field: The Case of Helicenes

Unconventional Aromaticity in Organometallics: The Power of Transition Metals

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