No abstract

A nonphotochemical synthetic route to 3-hexahelicenol is reported. It involves a key [2+2+2] cycloisomerization of CH3O-substituted triyne that is readily available from 1-methoxy-3-methylbenzene and 1-bromo-2-(bromomethyl)naphthalene. Further functional group transformations led to 3-CO2CH3, 3-NH2, 3-PPh2, and 3-SC(O)N(CH3)2 substituted hexahelicenes.

Along with the recent progress in the development of advanced synthetic methods, the chemical community has witnessed an increasing interest in promising carbon-rich materials. Among them, helicenes are unique 3D aromatic systems that are inherently chiral and attractive for asymmetric catalysis, chiral recognition and material science. However, there have been only limited attempts at synthesizing long helicenes, which represent challenging targets. Here, we report on an organometallic approach to the derivatives of undecacyclic helicene, which is based on intramolecular [2 + 2 + 2] cycloisomerization of aromatic hexaynes under metal catalysis closing 6 new cycles of a helicene backbone in a single operation. The preparation of nonracemic compounds relied on racemate resolution or diastereoselective synthesis supported by quantum chemical (density functional theory) calculations. The fully aromatic [11]helicene was studied in detail including the measurement and theoretical calculation of its racemization barrier and its organization on the InSb (001) surface by STM. This research provides a strategy for the synthesis of long helical aromatics that inherently comprise 2 possible channels for charge transport: Along a π-conjugated pathway and across an intramolecularly π-π stacked aromatic scaffold.

Helicenes have attracted attention as unique inherently chiral three-dimensional aromatic compounds for several decades. Despite significant recent progress in the synthesis and applications of carbohelicenes and thiaheterohelicenes, [1] the potential of the aza analogues with a pyridine unit (pyridohelicenes) have not been explored. [2,3] There are only scattered examples of the preparation of pyridohelicenes, [4] but with no general synthetic methodology, since the photochemical approach can fail with pyridohelicenes [4b,c,e] while non-photochemical alternatives can be difficult to adapt to the synthesis of N-heteroaromatic compounds. The properties and chemical behavior of pyridohelicenes are practically unknown apart from their basicities [4e,g] and the self-assembly [5] of certain derivatives. Nevertheless, promising applications of pyridohelicenes in various branches of chemistry and material science might be envisaged and, therefore, further research in the field is required.Recently, we observed remarkably high proton affinities of pyridohelicene derivatives, as measured by mass spectrometric techniques.[6] In the gas phase, helically chiral 1-aza[6]helicene (2, Scheme 1), for example, exhibits a comparable proton affinity (1000 kJ mol À1 ) to 4-(dimethylamino)-pyridine (DMAP, 997 kJ mol À1 ). Herein, we report the practical syntheses of 1,14-diaza[5]helicene (1), [7] 1-aza[6]helicene (2), [8] and 2-aza[6]helicene (3).[9] Moreover, we have succeeded in resolving racemates of 2 and 3 into their enantiomers, assigning their absolute configuration, determining the energy barriers to racemization, and obtaining Xray structures of their corresponding silver complexes. The strategy for the preparation of 1-3 relies on a [2+2+2] cyclotrimerization of an aromatic triyne in the presence of a Co I catalyst to build the helical scaffold. We have already proven that such a methodology is robust and reliable for the preparation of helicenes and their analogues.[10] Thus, the straightforward synthesis of 1 started from the readily accessible bromopyridine 4, [11] which was treated with lithiated 1-(triisopropylsilyl)-1-propyne to yield 5 which contained an attached alkyne side arm (Scheme 2). Sonoga-