Gas phase synthesis of [4]-helicene

A synthetic route to racemic helicenes via a vinylacetylene mediated gas phase chemistry involving elementary reactions with aryl radicals is presented. In contrast to traditional synthetic routes involving solution chemistry and ionic reaction intermediates, the gas phase synthesis involves a targeted ring annulation involving free radical intermediates. Exploiting the simplest helicene as a benchmark, we show that the gas phase reaction of the 4-phenanthrenyl radical ([C14H9]•) with vinylacetylene (C4H4) yields [4]-helicene (C18H12) along with atomic hydrogen via a low-barrier mechanism through a resonance-stabilized free radical intermediate (C18H13). This pathway may represent a versatile mechanism to build up even more complex polycyclic aromatic hydrocarbons such as [5]- and [6]-helicene via stepwise ring annulation through bimolecular gas phase reactions in circumstellar envelopes of carbon-rich stars, whereas secondary reactions involving hydrogen atom assisted isomerization of thermodynamically less stable isomers of [4]-helicene might be important in combustion flames as well.


Supplementary Note 1: Rice-Ramsperger-Kassel-Marcus Master Equation (RRKM-ME) calculations of temperature-and pressure-dependent rate constants in the 4-phenanthrenyl plus vinylacetylene system
Phenomenological temperature-and pressure-dependent rate constants for the primary C14H9 + C4H4 and secondary C18H12 + H reactions were computed by solving the one-dimensional master equation 6 employing the MESS package. 7 Here, rate constants k(T) for individual reaction steps were calculated within RRKM (unimolecular reactions) or transition state theory (TST, bimolecular reactions) generally utilizing the Rigid-Rotor, Harmonic-Oscillator (RRHO) model for the calculations of densities of states and partition functions for molecular complexes and the number of states for transition states. Collisional energy transfer rates in the master equation were expressed using the "exponential down" model, 8 with the temperature dependence of the range parameter α for the deactivating wing of the energy transfer function expressed as α(T) = α300(T/300 K) n , with n = 0.62 and α300 = 424 cm -1 obtained earlier from classical trajectories calculations for the naphthyl radical (C10H7) plus argon system shown to be representative for acetylene addition reactions to C8H5 and C8H7 and also for vinylacetylene addition to C6H5 in argon or nitrogen bath gases. 9 We used collision parameters derived in the literature for similar systems; the Lennard-Jones parameters ε and σ for C18H13 intermediates were taken to be equal to those for pyrene put forward by Wang and Frenklach 10 and those for N2 bath gas were taken from the works of Vishnyakov and co-workers. 11,12 The calculated rate constants for various reaction channels were then fitted to modified Arrhenius expressions k = A*T  *exp(-Ea/RT) or to a sum of two modified Arrhenius equations k = A1* 1 *exp(-Ea 1 /RT) + A2* 2 *exp(-Ea 2 /RT), which are presented in the subsequent Section.
The RRKM-ME calculations of the rate constants have shown very little dependence on pressure at 0.01-0.1 atm and temperatures above 1000 K, i.e., under the conditions prevalent in the microreactor, and hence, single rate expressions were used in the subsequent simulations of the gas flow and chemical kinetics in the microreactor. According to the calculated rate constants, at the highest temperature in the microreactor, 1400 K, the branching ratios of the p1:p2:p3:p4 are 1:169:6:51.
This means that if only the primary C14H9 + C4H4 reaction occurs under these isothermal conditions, [4]-helicene p1 is a minor product and 4-((E)-but-1-en-3-yn-1-yl)phenanthrene p3 and 4-(but-3-en-1yn-1-yl)phenanthrene p4 should have been observed experimentally. However, as will be seen in the next section, the conditions in the microreactor are not isothermal and the residence time of 100-180 s in the reactive zone where the pyrolysis of 4-bromophenanthrene and hence the primary C14H9 + C4H4 reaction can occur allows for secondary reactions to also take place. For instance, p3 can add a hydrogen atom to the terminal carbon of the side chain to form [14], which then undergoes trans-cis S7 conformational change to [15] followed by hydrogen migration from an aromatic ring to the side chain producing [10], ring closure to [11], and finally hydrogen loss forming [4]-helicene p1. The highest barrier on this pathway is as low as 8 kJ mol -1 for the initial hydrogen addition step, and the calculated rate constant is high, 1.210 -10 cm 3 molecule -1 s -1 at 1400 K. The reaction of p4 with hydrogen predominantly produces the initial reactants and p2 via the p4 + H →

Supplementary Note 2. Modeling of the gas flow and kinetics of the 4-phenanthrenylvinylacetylene system
In the modeling of the gas flow, we employed the COMSOL Multiphysics package and used the same axial symmetrical design model of the microreactor as described in the previous work on the phenylvinylacetylene system. 13 Specifications of the main details of the microreactor model, equations for the electric current, heat transfer, Navier-Stocks equation to describe gas motion in the silicon carbide tube, and mass transfer equations as well as physical properties of the materials exploited in the microreactor, boundary conditions for the heat transfer equations, for the Navier-Stocks equation, and for mass transfer equations -except for molar fractions of various gases at the inlet to the silicon carbide tube -were also identical to those described in the previous publication. 13 A gas mixture of C4H4 and He along with C14H9Br was introduced as input gas at a temperature of T = 323.0 ± 0.5 K upstream of the choke orifice at an inlet pressure p = 300 Torr upstream of the choke orifice. Since the vapor pressure of 4-bromophenanthrene is not known, its exact molar fraction in the molecular beam cannot be exactly evaluated and hence the simulations were carried out at various molar fractions of the constituents, as will be discussed below. The maximum temperature is 1400 ± 10 K at the silicon carbide microreactor surface. Diffusion coefficients for chemical species in helium were estimated using their diffusion coefficients in air which were taken from the handbook. 14 The list of chemical species involved and rate constants of all the reactions (the kinetic package) used are presented in Supplementary Tables 1 and 2. We implied the following kinetic mechanism, which included thermal unimolecular decomposition of C14H9Br and C4H4, various product channels of the 4-phenanthrenyl (C14H9) + vinylacetylene (C4H4) reaction and H-assisted isomerization among its primary C18H12 products, as well as H-assisted decomposition of C4H4 and reactions of C14H9 with acetylene (C2H2) and H atoms. The rate constants were taken either from the RRKM-ME calculations described above or from the literature.
In summary, these studies propose a p1:p2:p3:p4 branching ratio of 1:169:6:51 at 1400 K indicating that [4]-helicene is only a minor product of the primary reaction of the 4-phenanthrenyl radical . This seems to be in contrast to our experimental findings.
However, as described above p3 can be converted to p1 and p4 can be converted to p2 via secondary, hydrogen-assisted isomerization through facile pathways (Supplementary Figure 4). The simulations results show that within the error bars of the present RRKM-ME calculations and initial molar fractions of 4-bromophenanthrene and vinylacetylene in the molecular beam, we can achieve the p1:p2:p3:p4 branching ratios to be 1:10:0.25:0.9, i.e., the yield of p1 to exceed those of p3 and p4.
We should note at this point that the exact evaluation of the product branching ratios is not possible due to the fact that the absolute ionization cross sections are not known for any of the considered C18H12 isomers. For instance, if the absolute ionization cross section of p1 significantly exceeds those of p3 and p4, the contribution of the latter two isomers to the experimental PIE curve is masked, whereas that of [4]-helicene is enhanced. In the meantime, the simulations of the gas flow/kinetics clearly indicate that the three-ring-side-chain isomers p3 and p4 yield more stable four-   Table 3) and also varied the initial concentrations of 4-bromophenanthrene and vinylacetylene.
The resulting branching ratios are shown in Supplementary Table 4. Branching ratios consistent with experiment can be obtained with initial 1% or 5% of C14H9Br and 5% of C4H4 in the molecular beam and even more so with 5% of C14H9Br and 10% of C4H4. In the latter case, the calculated yield of p1 exceeds those of p3 and p4. A direct comparison between the calculations and experiment is complicated by the fact that the absolute ionization cross sections are not available for any of the considered C18H12 isomers. If the absolute ionization cross section of p1 significantly exceeds those of p3 and p4, the contribution of the latter two isomers to the experimental PIE curve is masked and can reside within the experimental error limits of the ion counts, whereas that of [4]-helicene is enhanced. Nevertheless, our simulations clearly indicate that the three-ring-side-chain isomers p3 and p4 yield to four-ring PAHs [4]-helicene p1 and 4-vinylpyrene p2 via H-assisted isomerization.

General information
1 H (400 MHz) and 13 C (100.6 MHz) NMR spectra were recorded at ambient temperature in solution of CDCl3. Reaction progress was monitored by TLC on Merck Kieselgel 60-F254 sheets with product detection by 254 nm light. Products were purified by column chromatography using Merck Kiselgel 60 (230-400 mesh). Reagent grade chemicals were used and solvents were dried by reflux and distillation from CaH2 under N2 unless otherwise specified.

(E)-4-(2-iodovinyl)phenanthrene; B.
Step a. The 4-ethynylphenanthrene (101.1 mg, 0.5 mmol) and catecholborane (53.2 µL, 60 mg, 0.5 mmol) were placed in a flame-dried flask under N2 at ambient temperature and the reaction mixture were stirred for 60 min at 70 o C. Then H2O/EtOAc (1:1; 10 mL) were added and stirring was continued for 30 min at 25 o C to effect the hydrolysis of boronic ester. The reaction mixture was extracted with EtOAc, organic layer separated and the aqueous layer was back extracted with EtOAc twice. The combined organic layer was dried (Na2SO4) and evaporated. The residue was column chromatographed (20-40% EtOAc in hexane) to give A (45 mg, 37%) as a gummy solid, which was directly used for the next step.
Step b. The boronic acid A (45 mg, 0.18 mmol) was dissolved in 5 mL Et2O in a 25 mL flask and cooled to 0 o C. Then aqueous NaOH (180 µL, 3 N, 0.54 mmol) was added dropwise followed by elemental iodine (54.8 mg, 0.22 mmol) dissolved in 5 mL Et2O, while stirring at 0 o C. The reaction mixture was stirred for 30 min at 0 o C. The excess I2 was destroyed by addition of few drops of aqueous Na2S2O3 solution. The reaction mixture was extracted with Et2O and the organic layer was separated and the aqueous layer was back extracted with Et2O twice. The combined organic layer was dried (Na2SO4) and evaporated. The residue was column chromatographed (n-hexane) to give B (30 mg, 50%) as a white powder: 1