Dibenzo[a,e]pentalenophanes: Bending a Non‐Alternant Hydrocarbon

Abstract

In cyclophanes, an aromatic moiety is incorporated into a (strained) cyclic structure. Of particular interest as model systems for bent carbon nanostructures are those containing polycyclic aromatic hydrocarbons. Dibenzo[a,e]pentalene (DBP) is a non-alternant polycyclic hydrocarbon with small band gap and tunable optoelectronic properties. However, changing these properties by bending of the DBP structure has yet to be investigated. Herein, we report the synthesis, optoelectronic, and structural properties of (2,7)dibenzo[a,e]pentalenophanes with four different bridge sizes and bending angles of the DBP unit, accompanied by (TD)DFT calculations. The last, strain-inducing dehydration reaction was accomplished by using Burgess' reagent. The HOMO and LUMO levels and the magnetic shielding of protons pointing inside the cyclophane cavity grew stepwise with increasing ring strain. Single-crystal X-ray structures of the smallest three derivatives revealed a near semi-circle and a bend angle of the DBP unit of almost 88° for the smallest derivative. We demonstrated the synthetic versatility of our approach by varying the substituents at the DBP unit, allowing for further tuning of optoelectronic properties. The synthetic strategy presented herein may pave the way for the synthesis of conjugated DBP nanorings.

Enantioselective Synthesis of Polycyclic Aromatic Hydrocarbon (PAH)‐Based Planar Chiral Bent Cyclophanes by Rhodium‐Catalyzed [2+2+2] Cycloaddition

“…The undesired strain‐relieving carbocation rearrangement might proceed in the oxidation of 20 e with DDQ . Thus, Esser's dehydrative aromatization method using the Burgess reagent [methyl N ‐(triethylammoniumsulfonyl)carbamate], in which no carbocations are formed, is advantageous for the aromatization of highly bent structures . As an alternative aromatization method for the highly bent dibenzo‐fused cyclohexadiene, the Diels–Alder reaction of 20 g with N ‐phenylmaleimide afforded the corresponding phenanthrene‐based cyclophane 23 as a single diastereomer (Scheme ).…”

“…For example, in 2012, Bodwell reported the racemic synthesis of pyrene‐based chiral bent cyclophanes via the McMurry coupling followed by the VID (valence isomerization dehydrogenation) reaction, although the product yield was low (12 %) (Scheme b, middle) . In 2018, Esser reported the racemic synthesis of dibenzopentalene‐based chiral bent cyclophanes via the Suzuki–Miyaura cross‐coupling followed by arylation and dehydration reactions with improved product yields of 14–36 % (Scheme b, bottom) . However, in these examples, the optical resolution of racemates by chiral HPLC columns was required to obtain enantiopure cyclophanes …”

Enantioselective Synthesis of Distorted π‐Extended Chiral Triptycenes Consisting of Three Distinct Aromatic Rings by Rhodium‐Catalyzed [2+2+2] Cycloaddition

“…The enantioselective [2+2+2] cycloaddition of biphenyl‐linked internal 1,7‐diyne 1 a with 1,2‐dihydronaphthalene ( 2 a ) proceeded at 40 °C in the presence of a cationic rhodium(I)/( R )‐segphos complex (20 mol %) to give the desired chiral cyclohexadiene (+)‐ 3 aa in good yield with high ee value of 88 % (entry 1). Although the Diels–Alder reaction of (+)‐ 3 aa with benzyne did not proceed, that with 1,4‐naphthoquinone followed by reductive aromatization using LiAlH 4 and the Burgess reagent proceeded to give the desired triptycene precursor (+)‐ 4 aa in 30 % yield with complete diastereoselectivity. But unfortunately, the treatment of (+)‐ 4 aa with DDQ did not give the desired chiral triptycene (+)‐ 5 aa , instead, an unidentified complex mixture of products was generated…”

“…[6g] Because of this large distortion, there are concerns about the stability of 5.H erein, we have established that the appropriate introductiono fs ubstituents into the biphenyllinked 1,7-diyne 1 and 1,2-dihydronaphthalene (2a)r ealizes this concept.Scheme 2d isplays the enantioselective synthesis of chiral triptycene 5.T he enantioselective [2+ +2+ +2] cycloaddition of biphenyl-linked internal 1,7-diyne 1a with 1,2-dihydronaphthalene (2a)p roceeded at 40 8Ci nt he presence of ac ationic rho-Scheme1.Enantioselective synthesis of chiral triptycenes.[a] Y.Communication dium(I)/(R)-segphos complex (20 mol %) to give the desired chiral cyclohexadiene (+ +)-3aa in good yield with high ee value of 88 %( entry 1). Althought he Diels-Alder reactiono f( + +)-3aa with benzyne did not proceed, that with 1,4-naphthoquinone followed by reductive aromatization using LiAlH 4 andt he Burgess reagent [17] proceeded to give the desired triptycene precursor (+ +)-4aa in 30 %y ield with complete diastereoselectivity. But unfortunately,t he treatment of (+ +)-4aa with DDQ didn ot give the desired chiral triptycene (+ +)-5aa,i nstead, an unidentified complex mixture of products was generated.…”

Cations and Anions of Dibenzo[a,e]pentalene and Reduction of a Dibenzo[a,e]pentalenophane

“…[33] We were able to obtain its anion radical 1C À À ,c ation radical 1C + + and dication 1 2 + + and investigatedt heir structural and electronic properties by X-ray crystallography,E PR spectroscopy and DFT calculations. With the synthesis of (2,7)dibenzo[a,e]pentalenophanes [33] (DBP-phanes) and DBP-based nanohoops [34][35][36] we recently showed that the DBP unit can be bent [37] without strongly alteringi ts optoelectronic properties. Hence, we herein performed ac hemical reduction of DBP-phane 2 (Figure 1C)t oi nvestigate the influence of bending.…”