Angewandte Research Articles Chemie How to cite: Angew.Chem. Int. Ed. 2020, 59,18809–18815 Carbocations International Edition: doi.org/10.1002/anie.202009644 German Edition: doi.org/10.1002/ange.202009644 Planar Cyclopenten-4-yl Cations:Highly Delocalized p Aromatics Stabilized by Hyperconjugation Samuel Nees,Thomas Kupfer,Alexander Hofmann, and Holger Braunschweig* 1 B Abstract: Theoretical studies predicted the planar cyclopenten- being energetically favored by 18.8 kcalmolÀ over 1 (MP3/ 4-yl cation to be aclassical carbocation, and the highest-energy 6-31G**).[11–13] Thebishomoaromatic structure 1B itself is + 1 isomer of C5H7 .Hence,its existence has not been verified about 6–14 kcalmolÀ lower in energy (depending on the level experimentally so far.Wewere now able to isolate two stable of theory) than the classical planar structure 1C,making the derivatives of the cyclopenten-4-yl cation by reaction of bulky cyclopenten-4-yl cation (1C)the least favorable isomer.Early R alanes Cp AlBr2 with AlBr3.Elucidation of their (electronic) solvolysis studies are consistent with these findings,with structures by X-raydiffraction and quantum chemistry studies allylic 1A being the only observable isomer, notwithstanding revealed planar geometries and strong hyperconjugation the nature of the studied cyclopenteneprecursor.[14–18] Thus, interactions primarily from the C Al s bonds to the empty p attempts to generate isomer 1C,orits homoaromatic analog À orbital of the cationic sp2 carbon center.Aclose inspection of 1B,bysolvolysis of 4-Br/OTs-cyclopentene were unsuccessful. the molecular orbitals (MOs) and of the anisotropyofcurrent Instead allylic 1A was immediately formed by 1,2-hydride (induced) density (ACID), as well as the evaluation of various shift, indicating both the lability of 1B/1C and the thermody- aromaticity descriptors indicated distinct aromaticity for these namic stability of allylic cation 1A.[14,16, 18] This hydrogen cyclopenten-4-yl derivatives,which strongly contrasts the scrambling process has been used to estimate the activation classical description of this system. Here,strong delocalization free energy DG# for the hydrogen migration step connecting of p electrons spanning the whole carbocycle has been verified, stable 1A and 1B/1C by means of NMR line-broadening/spin- 1 [17] thus providing rare examples of p aromaticity involving saturation transfer experiments (18.0 0.9 kcalmolÀ ) and Æ saturated sp3 carbon atoms. low temperature 2HNMR spectroscopy (18.3 0.1 kcal 1 [18] Æ molÀ ). Theenergy difference DE between allylic cation Introduction 1A and its high-energy isomers 1B/1C has been approximated 1 1 to 6.7–8.6 kcalmolÀ and 12 kcalmolÀ by ion cyclotron Recent interest on cyclopentenyl cations 1 stems from measurements[11] and mass spectrometric studies,[19] respec- their intermediacyinimportant organic (Nazarov cyclization tively.Experimental studies also indicated that anchimeric –formation of cyclopentenones)[1–5] and industrial processes (methanol-to-hydrocarbon conversion).[6–9] Historically how- ever,itwas the presence of aC=Cdouble bond in close proximity to acationic sp2 carbon center, and hence the possibility of gaining “extra stabilization” by allylic or homoaromatic delocalization, that put this system into the focus of carbocation chemistry.[10] From atheoretical point of view,three isomeric forms of the cyclopentenyl cation 1 appear plausible,that is,allylic 1A,bishomoaromatic 1B, and planar 1C (Figure 1A). Semi-empirical and high-level ab initio studies established allylic 1A as the most stable isomer, [*] S. Nees, Dr.T.Kupfer,Dr. A. Hofmann, Prof. Dr.H.Braunschweig Institut fürAnorganische Chemie, Julius-Maximilians-Universität Figure 1. The cationic cyclopentenyl system.(A)Isomeric descriptions Würzburg of the cyclopentenyl cation:allylic 1A,bishomoaromatic 1B,and planar Am Hubland, 97074 Würzburg (Germany) C 1 1 .Reportedrelative energies (kcalmolÀ )calculated at the MP3/6- and 31G** level of theory are given in brackets.[12] (B)Regioselective Institute for Sustainable Chemistry & Catalysis with Boron, Julius- synthesis of cyclopenten-4-yl cations 3a and 3b by reaction of Cp- Maximilians-UniversitätWürzburg substituted alanes 2a and 2b with one equivalent of AlBr in benzene. Am Hubland, 97074 Würzburg (Germany) 3 Isolated yields are given in brackets. (C)Molecular structures of 3a E-mail:[email protected] (top) and 3b (bottom) in the solid state. Only one molecule of the Supportinginformation and the ORCID identification number(s) for asymmetric unit of 3a is shown. Most of the hydrogen atoms have the author(s) of this article can be found under https://doi.org/10. been omitted for clarity.Selected bond lengths (): 3a C1-C2 1.435(4), 1002/anie.202009644. C1-C5 1.424(4), C2-C3 1.477(4), C3-C4 1.347(4), C4-C5 1.479(4), Al1- 2020 The Authors. Published by Wiley-VCH GmbH. This is an C2 2.031(3), Al2-C5 2.038(3), Si1-C2 1.937(3), Si2-C5 1.943(3); 3b C1- open access article under the terms of the Creative Commons C2 1.427(4), C1-C5 1.426(4), C2-C3 1.492(4), C3-C4 1.376(4), C4-C5 Attribution License, which permits use, distribution and reproduc- 1.469(4), Al1-C2 2.033(3), Al2-C5 2.043(3), Si1-C2 1.955(3), Si2-C5 tion in any medium, provided the original work is properly cited. 1.939(3), Si3-C3 1.894(3). Angew.Chem. Int.Ed. 2020, 59,18809 –18815 2020 The Authors. Published by Wiley-VCH GmbH 18809 Angewandte Research Articles Chemie 2Si 2Si assistance to solvolysis by the C=Cdouble bond is absent in Thus,weattempted the syntheses of Cp AlBr2 (2a;Cp = B C 5 3Si 3Si 5 cyclopenten-4-yl systems (1 /1 ), instead reduced rates of h -1,3-(Me3Si)2-C5H3)and Cp AlBr2 (2b;Cp = h -1,2,4- solvolysis as compared to their saturated analogs were (Me3Si)3-C5H2)byreacting two equivalents of AlBr3 with [12,14] 2Si 3Si noted. either (Cp )2Mg or (Cp )2Mg in benzene solutions at room This is in stark contrast to the results obtained for temperature.Due to the high solubility of the magnesium structurally related 7-norbornenyl systems;the solvolysis salts in aromatic solvents,the reactions proceeded smoothly rates are enhanced by afactor of up to 1011 over those of the and quantitatively within 2–4 hours,and molecules 2a (64%) saturated 7-norbornyl analogs,affording carbocations of and 2b (83%) were isolated as off-white solids.The identity exceptional stability.[20] These 7-norbornenyl cations are of 2a and 2b was readily verified by NMR spectroscopy, dramatically stabilized by homoaromatic 3-center-2-electron elemental analysis,and in the case of 2b,X-ray diffraction bonding,[10] making them readily amenable for structural (Figure S16). Characteristic solution NMR parameters of 2a characterization even in the solid state.[21–24] Hence,solvolysis and 2b include asingle 27Al NMR resonance (2a 32 ppm; À studies argue against any degree of bishomoaromatic deloc- 2b 39 ppm), and 1HNMR resonances for the aromatic À alization in cyclopenten-4-yl cations (1B); thus aclassical protons (2a 6.75, 7.05 ppm; 2b 7.42 ppm) and the peripheral C description as 1 is usually preferred here.This view was Me3Si protons (2a 0.23 ppm; 2b 0.27, 0.38 ppm) with 3t [37] further supported by low values for the calculated delocaliza- chemical shifts similar to those of Cp AlBr2. tion indices (DI = 0.09) of the transannular C Cbonds in When the reactions were carried out with alarger excess C [25] À planar 1 . It should be noted, however,that adifferent of AlBr3 (approx. 4equiv.), however, we were surprised that theoretical approach recently indicated significant bisho- asecond equivalent of AlBr3 was consumed along with moaromatic delocalization in 1C.[26] Overall, it is not surprising aslight darkening of the pale-yellow benzene solutions. that stable derivatives of the cyclopentenyl cation are only 27Al NMR spectroscopy suggested gradual conversion of 2a known for allylic isomer 1A,including asmall number of and 2b into new aluminum-containing species with broad structurally characterized molecules.[27–33] By contrast, deriv- 27Al NMR signals at 102 ppm (3a, 3b). Thelarge low-field atives of isomers 1B and 1C are still absent in the literature; shifts of these signals are indicative of substantial changes only adianionic boron analog of bishomoaromatic 1B has around the aluminum centers.Full conversion of 2a and 2b been realized so far.[34] was achieved at room temperature within approximately Herein we report the experimental evaluation of the 12 hours.Inboth cases,wewere able to separate colorless electronic structure of cyclopenten-4-yl cations.Aspart of our cyclopenten-4-yl cations 3a (27%) and 3b (30%) in moder- ongoing studies on aluminum molecules with bulky Cp ate yields from the reaction mixtures.Wenote that the ligands,[35–39] we were now able to capture two stable synthesis of 3a and 3b can also be accomplished by reacting C derivatives of the elusive cyclopenten-4-yl cation 1 .Their isolated 2a and 2b with one equivalent of AlBr3,which high stability and the regioselectivity of their formation are notably simplifies purification of the cyclopenten-4-yl cations, readily explained by strong hyperconjugation from C Al and and allows for their large-scale isolation in yields of 42%(3a) À C Si s bonds into the empty porbital of the cationic sp2 and 52%(3b)(Figure 1B). Forconvenience,this method is À carbon. We have also examined the anisotropy of the current the preferred one.The formation of cationic cyclopenten-4-yl (induced) density (ACID)[40,41] and different aromaticity systems is clearly evident from the 1Hand 13CNMR spectra of indicators,which revealed that the cyclopenten-4-yl cation 3ain solution, which show signal patterns and chemical shifts should be considered ahighly delocalized p aromatic system consistent with such astructural motif.Accordingly,the rather than aclassical carbocation.