In Search of Wasserman’s Catenane

We repeat the earliest claimed [2]catenane synthesis, reported by Wasserman over 60 years ago, in order to ascertain whether or not a nontemplate, statistical synthesis by acyloin macrocyclization does indeed form mechanically interlocked rings. The lack of direct experimental evidence for Wasserman’s catenane has led to it being described as a “prophetic compound”, a technical term used in patents for claimed molecules that have not yet been synthesized. Contemporary synthetic methods were used to reconstruct Wasserman’s deuterium-labeled macrocycle and other building blocks on the 10–100 g reaction scale necessary to generate, in principle, ∼1 mg of catenane. Modern spectrometric and spectroscopic tools and chemical techniques (including tandem mass spectrometry, deuterium nuclear magnetic resonance (NMR) spectroscopy, and fluorescent tag labeling) were brought to bear in an effort to detect, isolate, and prove the structure of a putative [2]catenane consisting of a 34-membered cyclic hydrocarbon mechanically linked with a 34-membered cyclic α-hydroxyketone.


General information
All reagents and solvents were purchased from commercial sources and used without further purification. Anhydrous THF, CH 2 Cl 2 , CH 3 CN and toluene were obtained by passing the solvent (HPLC grade) through an activated alumina column on a Phoenix SDS solvent drying system (JC Meyer Solvent Systems, CA, USA). All reactions were performed using flame-dried glassware under an atmosphere of N 2 , unless stated otherwise. Column chromatography was carried out using Aldrich Si 60 (particle size 40-63μm) as the stationary phase, while TLC was performed on precoated silica gel plates (0.2 mm thick, 60 F 254 , Macherey-Nagel, Germany) and visualized using Ceric Ammonium Molybdate (CAM) stain. Size-exclusion chromatography was carried out under gravity using a neutral, porous styrene divinylbenzene resin (1% crosslinked linked, Bio-Rad, Bio-Beads, S-X1) as stationary phase and CHCl 3 as an eluent. 1 H NMR spectra were recorded on a Bruker Avance III instrument with an Oxford AS600 magnet equipped with a cryoprobe [5mm CPDCH 13C-1H/D] (600 MHz) at 298 K. Chemical shifts are reported in parts per million (ppm) from high to low frequency using the residual solvent peak as the internal reference (CDCl 3 = 7.26 ppm). All 1 H resonances are reported to the nearest 0.01 ppm. The multiplicity of 1 H signals are indicated as: s = singlet; d = doublet; t = triplet; q = quartet; p = quintet; m = multiplet; br = broad; or combinations of thereof.
Coupling constants (J) are quoted in Hz and reported to the nearest 0.1 Hz. Where appropriate, averages of the signals from peaks displaying multiplicity were used to calculate the value of the coupling constant. 13 C NMR spectra were recorded on the same spectrometer at 298 K with the central resonance of the solvent peak as the internal reference (CDCl 3 = 77.16 ppm). All 13 C resonances are reported to the nearest 0.1 ppm in general, or to 0.01 ppm to aid in the differentiation of close but resolved signals. 2 H NMR spectra were recorded on a Bruker AVIII HD 500 equipped with a prodigy BBO 5 mm probe, with the central resonance of the solvent peak as the internal reference (CHCl 3 = 7.26 ppm). All 2 H resonances are reported to the nearest 0.01 ppm. DEPT, COSY, HSQC and HMBC experiments were used to aid structural determination and spectral assignment. Low resolution ESI mass spectrometry was performed with a Thermo Scientific LCQ Fleet Ion Trap Mass Spectrometer or an Agilent Technologies 1200 LC system with either an Agilent 6130 single quadrupole MS detector or an Advion Expression CMS L single quadrupole MS detector.
High resolution (ESI, APCI) mass spectrometry was carried out by the Department of Chemistry, University of Manchester. Infrared spectra were recorded neat on a Bruker Alpha II Platinum ATR.
Scheme S4. Synthesis of tagged acyloins S3 and S4 Scheme S5. Synthesis of a [2]catenane by statistical threading of alkyl diester through a deuterated macrocycle during an acyloin condensation Scheme S6. Control experiment to show that filtration with petroleum ether as an eluent is effective at removing the excess deuterated macrocycle from the mixture prior to oxidation with alkaline H 2 O 2 .

Experimental procedures and data
Ethyl 11-bromoundecanoate (S1) 11-bromo undecanoic acid (10.0 g, 37.7 mmol) was dissolved in EtOH (200 mL) and conc. H 2 SO 4 (10 mL) was added. The mixture was stirred at room temperature for 20 hours and the reaction was quenched with sat. aq. NaHCO 3 . The product was extracted with CH 2 Cl 2 (3 × 100 mL). The combined organic extracts were washed with brine, dried over anhydrous MgSO 4 , filtered and the solvent was evaporated under reduced pressure. S1 was obtained as a colorless oil (10.94 g, 37.5 mmol, 99%), which was sufficiently pure to be used without further purification. Experimental data consistent with literature. [ (11-ethoxy-11-oxoundecyl)triphenylphosphonium bromide (7) In a flame-dried flask, ethyl 11-bromoundecanoate S1 ( For larger scale synthesis of 6 (up to 50 g) a modified version of a previously reported procedure was used. [4] To a solution of 1,12-dodecanediol (51. a We found that the yield of the reaction was strongly dependent on the quality of the sodium dispersion formed. It is vital that the sodium gets finely dispersed before addition of the diester and this is best achieved by mechanical stirring. For the scale-up of the reaction, it was decided to use pre-dispersed Na in toluene. b The solubility of the diester 2 in cold toluene is poor, so regular heating of the addition funnel is required. Method 2. In a flame-dried flask equipped with a condenser and a 500 mL heated addition funnel, anhydrous toluene (100 mL) was added (see Fig. S1 for the reaction set-up). To this, 30% sodium dispersion in toluene (6 mL, ~1.8 g sodium, 78 mmol) was added and the reaction flask was brought to 115 °C under vigorous stirring. Diester 2 (7.0 g, 11.8 mmol) and anhydrous toluene (150 mL) were transferred to the addition funnel, which was heated to 40 °C to allow for the solvation of the poorly soluble diester. TMSCl (26.5 mL, 22.8 g, 210 mmol) was added to the diester solution and the resulting mixture was added dropwise to the main reaction flask. The addition was completed over 48 hours, during which time the reaction flask was kept at 115 °C and the addition funnel was kept at 40 °C. After the addition was complete, the mixture was stirred for further 5 hours and then cooled to room temperature. MeOH (10 mL) was added dropwise to quench the excess sodium, followed by the addition of aq. 1M HCl (20 mL). The suspension was filtered through a pad of celite, and the filtrate was evaporated under reduced pressure to give a yellow grease. The grease was redissolved in a mixture of THF (150 mL) and aq. 1M HCl (20 mL), and the solution was stirred at room temperature for 1 hour. The product was extracted with CHCl 3 (3 × 200 mL) and the combined organic phases were washed with sat. aq. NaHCO 3 , brine and dried over anhydrous MgSO 4 . The solvent was removed under reduced pressure and the crude product obtained was purified by flash column chromatography (SiO 2 , petroleum ether/EtOAc, 9:1) to yield 4 (4.1 g, 8.1 mmol, 67%) as a colorless solid.
Note: Spectroscopic data was identical to that obtained via Method 1. Figure S1. Acyloin reaction apparatus set-up with heated addition funnel.
The dry loaded SiO 2 was transferred onto a SiO 2 column and eluted with petroleum ether until all of 3/10 was reisolated (6.9 g). Subsequent elution with petroleum ether/EtOAc (1:1) was used to remove the remaining 4 from the column. Analysis by 2 H NMR (Fig. S7, a) indicated that no deuterium containing species were present in the mixture. The polar fraction obtained was dissolved in a mixture of CHCl 3 /MeOH (1:1, 10 mL), and NaOH (390 mg, 9.8 mmol) in H 2 O (2 mL) was added, followed by dropwise addition of 30% H 2 O 2 solution (10 mL). The reaction mixture was stirred at room temperature for 24 hours. Analysis by TLC (SiO 2 , petroleum ether/EtOAc, 9:1) showed complete consumption of the acyloin and no presence of the deuterated macrocycle in the reaction mixture ( Fig. S7, b). The same procedure was used for the oxidation of pure acyloin 4 which was not pre-mixed with deuterated macrocycle (Fig. S7, c) and gave identical results.

Attempt to form the original Wasserman catenane (1) with subsequent tagging of the crude product mixture with 4-nitrobenzoyl chloride
Scheme S10. Acyloin condensation in the presence of a large excess of deuterated cyclotetratriacontane/ene and subsequent tagging of the crude product mixture with 4-nitrobenzoyl chloride.
In a flame-dried flask, finely cut sodium pieces (1.0 g, 43.5 mmol) were added to a mixture of dry A second fraction was isolated from the column chromatography as a mixture of different compounds (1 mg, < 1%). Analysis by 1D/2D 1 H/ 13 C NMR (see Fig. S9 and Section 9, Spectra S21-S25) indicated the presence of at least four compounds, the structures of the major species are shown in Fig. S8. Figure S8. Product mixture isolated from acyloin condensation of diester 2. Figure S9. Partial 1 H NMR spectra (600 MHz, 298 K, CDCl 3 ) of (i) product mixture obtained from the tagging reaction; (ii) isolated tagged-acyloin S3; (iii) mixture of minor products S5, S10a, S11 and S12.
i Analytical data matches that obtained in Section 7.3.                              10. X-ray data Data collection: X-ray diffraction data were collected for compounds 3 and 4 on a dual source Rigaku FR-X rotating anode at 100 K with Cu-Kα (1.54184 Å) radiation, equipped with a Hypix000HE detector and Oxford cryosystem. All data were collected using CrysAlisPro software.

A B C
Crystal structure determination and refinements: X-ray data were processed and reduced using CrysAlisPro. Absorption correction was performed using empirical methods (SCALE3 ABSPACK) based upon symmetry-equivalent reflections combined with measurements at different azimuthal angles. The crystal structure was solved and refined against all F 2 values using the SHELX and Olex2 suite of programmes 7.8 All atoms were refined anisotropically. Hydrogen atoms were placed in calculated positions and refined using idealized geometries and assigned fixed isotropic displacement parameters. The location of deuterium atoms in compound 3 couldn't be determined crystallographically. Therefore, the deuterium atoms were placed in atoms C1-3 with 50 % occupancy, in order to correct the formula of compound 3. Data for crystal structure 4 was found to be twinned.