Tautomerization‐Mediated Molecular Switching Between Six‐ and Seven‐Membered Rings Stabilized by Hydrogen Bonding

Abstract

1,3,4,6-Tetraketones typically undergo keto-enol tautomerism forming bis-enols stabilized by intramolecular hydrogen bonding in two six-membered rings. However, 1,3,4,6-tetraketones derived from the terpene ketone camphor and norcamphor exist as isomers with two distinguishable modes of intramolecular hydrogen bonding, namely, the formation of six- or seven-membered rings. The structural requirements for this so far unknown behavior were investigated in detail by synthesis and comparison of structural analogues. Both isomers of such 1,3,4,6-tetraketones were fully characterized in solution and in the solid state. Intriguingly, they slowly interconvert in solution by means of tautomerism-rotation cascades, as was corroborated by DFT calculations. The influence of temperature and complexation with the transition metals Pd, Rh, and Ir on the interconversion process was investigated.

Conformational and Tautomeric Control by Supramolecular Approach in Ureido-N-iso-propyl,N’-4-(3-pyridin-2-one)pyrimidine

“…The most important is that the highest energy barrier to overcome in the mentioned path has a value of 143.2 kJ/mol and it is related to the proton shift. That kind of barrier should be taken into account for the explanation of previous results in tautomerizable switches [51]. More detailed discussion is collected in the Supplementary Materials.…”

(+)‐ and (−)‐Xanthostones A–D: Four Pairs of Enantiomeric Cinnamoyl‐β‐Triketone Derivatives from Xanthostemon chrysanthus

“…As shown in Figure 2, the 1 H -1 H COSY spectrum of (+)-1a suggested the presence of four isolated spin systems (HÀ C(8) to HÀ C(9), HÀ C (11) to HÀ C(15), H 2 À C(7') to H 2 À C(8'), HÀ C(10') to HÀ C(14')). In the HMBC spectrum, the correlations between HÀ C (16) and C(1)/C(2)/C(3), between HÀ C(18) and C(3)/C(5)/ C (17), between HÀ C(8) and C (10), as well as between HÀ C(9) and C(7)/C(11) indicated the presence of a 2,2,4-trimethylated-cinnamoyl-β-triketone unit (part 1 A, Figure 2). [14] Notably, the unusual downfield shifts of OHÀ C(7) (δ H 18.42), as well as the HMBC cross peaks between OHÀ C(7) and the five carbon signals at C(4)/ C(5)/C(6)/C(7)/C(8) indicated the keto-enol tautomeric equilibrium between C(7) and C(5).…”

“…[15] As shown in the HMBC experiment of (+)-1a, correlations between the enolic hydroxy signal at δ H 18.42 and C(4)/C(5)/C(6)/C(7)/C(8) were observed (Figure S34), indicated that the enolic hydroxy unit interconverted fast between C(5) and C(7) to form tautomers T1 ((E)À C(7)-enol form) and T2 (C(5)-enol form) through keto-enol tautomerism (Figure 4). [15][16][17] Moreover, the correlation signals between the enolic hydroxy proton and C(7)/C(8) are stronger than that with C(4)/C(5) (Figure S34), indicated that tautomer T1 predominated over tautomer T2. [15] Similar to (+)-1a, the minor isomer (+)-1b could be regarded as an equilibrium mixture of T3 (C(1)-enol form) and T4 ((Z)À C( 7)-enol form), and T4 was slightly more favorable (Figure S34).…”

Collaboration between Trinuclear Aluminum Complexes Bearing Bipyrazoles in the Ring-Opening Polymerization of ε-Caprolactone

“…Alkane- and arene-1,3,4,6-tetraketones were prepared using methods based on those described in the literature. , A series of bipyrazoles was synthesized through the condensation reaction of hydrazine hydrate and an alkane- or arene-1,3,4,6-tetraketone (2:1 ratio). D Bu 2 Al 3 Me 5 , D Ad 2 Al 3 Me 5 , D Pr 2 Al 3 Me 5 , D tolyl 2 Al 3 Me 5 , and D ph 2 Al 3 Me 5 were synthesized through the reaction of bipyrazole ligands and AlMe 3 (2:3 ratio).…”