[2+2] Cycloaddition Products of Tetradehydrodianthracene: Experimental and Theoretical Proof of Extraordinary Long CC Single Bonds

Abstract

An extremely long CC single bond of 1.713 Å is found in the [2+2] cycloadduct of ortho‐didehydrobenzene and tetradehydrodianthracene. The force constant of this central bond is reduced to only 30% of that of the CC bond in ethane. The corresponding CC stretching vibration (687 cm−1) in the Raman spectrum is red‐shifted 300 cm−1 relative to the bands of “normal” hydrocarbons.

Longest C–C Single Bond among Neutral Hydrocarbons with a Bond Length beyond 1.8 Å

“…The observed Raman shift for 10c is 587 cm À1 , which represents a large shift compared with that (993 cm À1 ) in ethane 1, 48 because the extremely elongated C-C bond has a much smaller force constant than the ordinary C-C bond ( Figure S13); the estimated force constants (108.3 N m À1 for 10c and 441.0 N m À1 for ethane 1) were obtained as second derivatives of the energy to the bond length by DFT calculations (M06-2X/6-31G*). 21 Figures 3A-3C also show other absorptions assigned as normal modes, including another stretching vibration and/or deformation vibration.…”

“…Although this standard bond length is observed in almost all compounds, 18 several attempts have been made to elongate the C-C bond to gain new insight into the covalent bond and understand its properties upon expansion (Figure 1). [19][20][21][22][23][24][25][26][27][28][29][30][31] For example, a longer C 1 -C 2 bond was predicted for hexaphenylethane (HPE) 2 as a result of large steric repulsion among its six phenyl groups. 32 However, the parent HPE 2 is not stable because of facile fission of the C 1 -C 2 bond to generate two triphenylmethyl radicals, which finally produces the a,p-dimer, the more stable isomer of HPE.…”

“…20 Although some polycyclic hydrocarbons have been reported to have much longer C-C bonds, 33,34 those values were later proven to be artifacts because of the coexistence of a bond-dissociated valence isomer. 35,36 The Herges 21 and Toda 22 groups synthesized 4 and 5, which can be regarded as stabilized HPE derivatives that do not undergo valence isomerization. They are stable compounds because C 1 -C 2 bond fission is suppressed because of sharing of two benzene rings of the HPE framework despite much greater d 1 values (1.713(2) and 1.734(5) Å , respectively) than in HPE 3.…”

Exceptionally Long C−C Single Bonds in Diamino‐o‐carborane as Induced by Negative Hyperconjugation

“…[1][2][3] Unusually large deviations from this standard bond length challenge bond theory.Z avitsas investigated the relationship between CÀCb ond lengths (r)a nd their bond dissociation energies (BDEs) for al arge range of strained and unstrained compounds,a nd reported al inear relationship between r and BDEs,f rom which am aximum bond length of 174.8 pm for au nsupported C À Cb ond was predicted (BDE = 0kJmol À1 ). [6][7][8] Fore xample,u ltra-long C À C bonds have been found in tetra-substituted 3,8-dihalobuta-[b]naphthalenes prepared by Toda, Herges,a nd co-workers, with CÀCd istances ranging from 168.6 pm to 173.4 pm [9][10][11] (Figure 1). [5] Larger deviations from the standard value have been found in compounds having extraordinary ring strain, extreme steric congestion, or electronic perturbations.…”

“…[5] Larger deviations from the standard value have been found in compounds having extraordinary ring strain, extreme steric congestion, or electronic perturbations. [6][7][8] Fore xample,u ltra-long C À C bonds have been found in tetra-substituted 3,8-dihalobuta-[b]naphthalenes prepared by Toda, Herges,a nd co-workers, with CÀCd istances ranging from 168.6 pm to 173.4 pm [9][10][11] (Figure 1). Recently,F okin, Schreiner, and co-workers reported the synthesis of hydrocarbons with extremely long CÀCbonds,stabilized by attractive London dispersion forces between two large diamondoid substituents [12][13][14] (Figure 1).…”