Design of neutral organic superacids using fulvene derivatives with di-enol substituent

Abstract

A new class of superacids was designed using enolic derivatives of fulvene. After deprotonation, bond rearrangement leads in a stable conjugate base with an aromatic cyclopentadienyl ring and a carboxyl group. The gas phase enthalpies ( ΔHacid°) of the deprotonation, as an index of acidity, were calculated employing the B3LYP method and 6‐311++G(d,p) and aug‐cc‐PVDZ basis sets. The acidity of these compounds without any electron withdrawing groups was more than H2SO4 in gas phase. The acidity increased by substituting electron withdrawing groups (F, CN, and O) into the molecules so that we could achieve a cyano derivative of fulvene with ΔHacid° = 250 kcal/mol.

Superacidity of P(OH)3 and SO(OH)2 derivatives of cyclopentadiene and vinylcyclopentadiene in the gas phase: A computational DFT analysis

“…The choice of this computational setup was prompted by its demonstrated success in evaluating gas-phase acidities of a large number of compounds. [4,[14][15][16][17][18][19][20][21][26][27][28][29]35,36] Still, to further gauge our results, the acidity parameters for the parent H 2 SO 4 and H 3 PO 4 were also calculated by the composite G3B3 method. The out-of-plane component of the nuclear-independent chemical shift (NICS zz ) [37] was used for comparing the aromaticities of acids and their conjugate bases, calculated at the ring surface, NICS(0), and 1Å above it, NICS(1).…”

“…[25] Fulvene derivatives with one and two enolic groups have already been considered as superacids. [26][27][28][29] The calculated ΔH acid of the simplest fulvene derivative with CN substituent, HC 6 (CN) 4 OH, is 277.4 kcal mol −1 . [28] The acidity of such compounds is enhanced by the inclusion of a Lewis acid, −(CH 2 ) 3 BX 2 , to interact with the negatively charged center and stabilize the conjugate base.…”

A density functional theory study on the superacidity of sulfuric, fluorosulfuric, and triflic acid derivatives with two cyclopentadiene rings: ion pairs formation in the gas phase

“…In other words, system 2 does not benefit from the charge delocalization as much as 1 and 3 do. The observed increase in the acidity of 3 of 64.1 kcal mol –1 relative to 1 demonstrates the exceptional acidifying effect of the cyano substituents and their high electron‐withdrawing efficiency, in agreement with earlier reports in the literature …”

“…The observed increase in the acidity of 3 of 64.1 kcal mol -1 relative to 1 demonstrates the exceptional acidifying effect of the cyano substituents and their high electronwithdrawing efficiency, in agreement with earlier reports in the literature. [21][22][23][24][25][26][27]33] Substitution of both oxygen atoms of the thionyl S═O group by two cyclopentadiene rings (isomer 1a) successfully increases the acidity. However, this structure is not stable, instead, the tautomer 1f with two C (sp 3 )H 2 groups in the rings is by as much as 46.5 kcal mol -1 more stable and, therefore, less acidic by the same amount.…”

Design of new organic superacids with fused and isolated pyrrole and cyclopentadiene rings and assessment of effect of –BX2(X = H, F, Cl, CN) substituents on the acidity enhancement: A DFT analysis

“…[15][16][17][18] Many types of proposed organic Brønsted superacids benefit from charge delocalization and induced aromaticity in the cyclopentadienyl rings. [19][20][21][22][23][24][25] After deprotonation, the negative charge is transferred to a cyclopentadiene ring via bond arrangement to form an aromatic system according to Hückel rule of 4n + 2 π-electron. Although in these acids the negatively charged conjugated base is stabilized via charge delocalization, for more acidity enhancement, inclusion of strong EWDs such as -CN are necessary.…”