Various two-dimensional (2D) carbon allotropes with nonalternant topologies, such as pentaheptites and phagraphene, have been proposed. Predictions indicate that these metastable carbon polymorphs, which contain odd-numbered rings, possess unusual (opto)electronic properties. However, none of these materials has been achieved experimentally due to synthetic challenges. In this work, by using on-surface synthesis, nanoribbons of the nonalternant graphene allotropes, phagraphene and tetrapenta-hepta(TPH)-graphene, have been obtained by dehydrogenative C−C coupling of 2,6-polyazulene chains. These chains were formed in a preceding reaction step via on-surface Ullmann coupling of 2,6-dibromoazulene. Low-temperature scanning probe microscopies with CO-functionalized tips and density functional theory calculations have been used to elucidate their structural properties. The proposed synthesis of nonalternant carbon nanoribbons from the fusion of synthetic line-defects may pave the way for large-area preparation of novel 2D carbon allotropes.
The electrophilic ipso-substitution of trimethylsilyl-substituted benzene derivatives into nitrosobenzene derivatives is reported. The optimization of the reaction conditions was performed for moderately electron-deficient, electron-rich, and sterically hindered starting materials by varying reaction time, temperature, and equivalents of NOBF. Also, a stable intermediate of the nitrosation reaction could be characterized by F NMR which can be assigned to a NO adduct with the nitrosobenzene derivative. This complex decomposes upon aqueous workup and liberates the desired nitrosobenzene derivative.
The
transposition of a homoallyl pinacol boronic ester was realized
by a highly reactive nickel-catalyst system comprising NiCl2(dppp), zinc powder, ZnI2, and Ph2PH. The in situ generated Z-crotyl pinacol boronic
esters were reacted with various aldehydes to form syn-homoallylic alcohols in high diastereoselectivities. The present
nickel-catalyzed reaction is complementary to the iridium-catalyzed
transposition reported by Murakami leading to the corresponding anti-homoallylic alcohols. Also, the multiple transposition
of pentenyl pinacol boronic ester was realized.
Herein,
detailed mechanistic investigations into formamide-catalyzed
nucleophilic substitution (S
N
) of alcohols
are reported. Alkoxyiminium chlorides and hexafluorophosphates were
synthesized and characterized as a key intermediate of the catalytic
cycle. The determination of reaction orders and control experiments
indicated that the nucleophilic attack of the formamide catalyst onto
the reagent BzCl is the rate-determining step. Linear free energy
relationship revealed a correlation between the quantified Lewis basicity
strength of formamides by means of 11B NMR spectroscopy
and their catalytic activity in S
N
-transformations.
The observed difference in catalytic ability was attributed to the
natural bond order charge, dipole moment, and Sterimol parameter B5. Importantly, this rationalization enables the prediction
of the capacity of formamides to promote S
N
-type transformations in general.
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