The kinetics of 18 different donor-acceptor cyclopropanes in the (3+2)-cycloaddition reaction with aldehyde were studied by insitu NMR spectroscopy. Increasing the electron density of the donor residue accelerates the reaction to a factor up to 50 compared to the unsubstituted one whereas electron withdrawing substituents slow down the reaction 660 times. This behavior is in agreement with the Hammett substituent parameter σ. The obtained rate constants form the (3+2)-cycloaddition correlated well with respective data from additionally studied (3+n)-cycloadditions with nitrone (n = 3) and isobenzofurane (n = 4). A comparison of the kinetic data with bond lengths in the cyclopropane (obtained by X-ray and computations), and 1 H and 13 C NMR shifts revealed no correlation. However, computed relaxed force constants of D-A cyclopropanes proved to be a good indicator for the reactivity of the three-membered-ring.
Reactions of 2-arylcyclopropane dicarboxylates with naphthoquinones are reported. The key feature was the use of catalytic amounts of SnCl , which acts as both an electron donor and a Lewis acid. By an in situ umpolung of naphthoquinone the formerly electrophilic species is converted into a nucleophile that is able to trigger the ring-opening of the three-membered ring with formation of a new C-C bond. Treatment of these products with base under oxidative conditions resulted-through loss of methyl formate-in cyclopentannulated products with fully conjugated π systems exhibiting intensive absorptions in the visible range.
Novel dyes based on extended fulvene motifs are reported. The carbon skeleton was generated by ac atalyzed additiono fd onor-acceptor cyclopropanes to naphthoquinone. The hydroxy group at the central ring of the tricyclic fulvenem otif was converted into the triflate, which reacted efficientlyw ith aw ide range of nucleophiles, resulting in substitution and thereby providingn ew derivatives. The synthetic versatility allowed us to investigate the absorption, electrochemical, and UV/Vis-NIR spectroelectrochemical properties of these dyes as af unctiono ft he substituents. The dyes were shown to participate in reductivee lectrochemistry,t he reversibility of which can be improved by appropriate selection of the substituents. Additionally,f irst signs of NIR electrochromism are presented, openingn ew avenues for the futurei nvestigations of such dyes.Supporting Information and the ORCID identification number(s) for the author(s) of this articlecan be found under: https://doi.
The reactivity of 3,3′‐linked bispyrroles, easily accessible by ring‐enlargement reactions of donor–acceptor cyclopropanes, towards various carbon electrophiles was investigated. A biscyclopropanated furan, equipped with imines as acceptor units, was converted by rearrangement and subsequent elimination of water to 3,3′‐linked bispyrroles. Thereby the nucleophilic 2‐position of the pyrroles stayed unsubstituted, thus permitting a further reaction with electrophiles. The heterocycles were treated with oxalyl chloride to form a new six‐membered ring between the pyrrolic subunits. The 1,2‐diketone moiety in these species was used as a starting point to extend the π‐system via a condensation reaction with aromatic 1,2‐diamines. The reaction of 3,3′‐linked bispyrroles with the phosgene surrogate triphosgene led to a seven‐membered anhydride. The use of the Vilsmeier reagent led to a formylation of the bispyrroles. Furthermore, a direct oxidative coupling between two bispyrroles resulted in a previously unknown linkage (3,3′:2′,2′′:3′′,3′′′) between four pyrrole subunits.
The challenging metal‐catalyzed asymmetric synthesis of highly functional quaternary carbon centers using decarboxylative C(sp3)−C(sp3) bond formation reactions is reported. The key substrate, a vinyl cyclic carbonate, is activated to provide concomitantly both the requisite nucleophile (by formal umpolung) and electrophile reaction partner preceding the asymmetric cross‐coupling process. A wide screening of reaction conditions, additives and catalyst precursors afforded a protocol that gave access to a series of compounds featuring densely functionalized, elusive quaternary carbon stereocenters in appreciable yield and with enantiomeric ratios (er's) of up to 90 : 10.
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