(1R,2S,4r)-1,2,4-Triphenylcyclopentane-1,2-diol and (1R,2S,4r)-4-(2-methoxyphenyl)-1,2-diphenylcyclopentane-1,2-diol: application as initiators for ring-opening polymerization of ∊-caprolactone

Achiral (1R,2S,4r)-1,2,4-triphenylcyclopentane-1,2-diol and (1R,2S,4r)-4-(2-methoxyphenyl)-1,2-diphenylcyclopentane-1,2-diol form one-dimensional chains via O—H⋯O hydrogen bonding in their crystals. The diols may serve as precatalyst activators for ring-opening polymerization of cyclic esters.

1,2-Diphenyl-4-arylcyclopentane-1,2-diols can be readily synthesized by the reductive cyclization of 1,5-diphenyl-3arylpentane-1,5-diones with zinc in an acetic acid medium ( Fig. 1; aryl = Ph, 2-MeOC 6 H 4 ). The corresponding diones are formed by condensation of acetophenone with benzaldehyde/ 2-methoxybenzaldehyde under basic conditions (Hirsch & Bailey, 1978;Minyaev et al., 2015). The presence of only one isomer (see x2) has been detected by 1 H NMR studies in the samples of all isolated crystalline diols from repeated syntheses. However, examination of the reaction mixtures has allowed us to suppose that another minor isomer of (I) may sometimes be present (up to 20%), but it does not crystallize under the conditions used here.
In the case of the ratio [diol]/[Mg(BHT) 2 ] = 1:1 (entries 1 and 3, Table 1), the polymerization degree (the number of polymerized monomer units, P n ) found by 1 H NMR spectro-scopy and by size-exclusion chromatography (SEC) are very close to the calculated value (P n calcd. = 100). However, when the ratio [diol]/[Mg(BHT) 2 ] = 1:2, and two chains are growing at one diol, the P n values (entries 2 and 4) are somewhat higher than expected (P n calcd. = 50), which might be explained by a longer reaction time of the second [Mg(BHT) 2 (THF) 2 ] molecule with the same initiator molecule with respect to the time of polymer-chain propagation. This is also supported by larger polydispersity index (Ð) values (compare entries 2 and 4 with entries 1 and 3), pointing to unequal growth of the two chains.

General remarks
The starting compounds 1,3,5-triphenylpentane-1,5-dione and 3-(2-methoxyphenyl)-1,5-diphenylpentane-1,5-dione were obtained in high yields by the previously described procedure (Hirsch & Bailey, 1978) with certain minor modi-fications (Minyaev et al., 2015) to decrease formation of side products. They were recrystallized from hot ethanol or isopropanol followed by vacuum drying. The complex Mg(BHT) 2 (THF) 2 was prepared as described earlier (Nifant'ev et al., 2017). All polymerization tests and the synthesis of Mg(BHT) 2 (THF) 2 were performed under a purified argon atmosphere in a dry box in absolute solvent media. Tetrahydrofuran was pre-dried over NaOH and distilled from potassium/benzophenone ketyl. Hexane was distilled from an Na/K alloy. Toluene was distilled from sodium/benzophenone ketyl. "-Caprolactone ("-CL) was distilled from CaH 2 under reduced pressure of argon. CDCl 3 (Cambridge Isotope Laboratories, Inc., D 99.8%) was used as purchased. The NMR spectra were recorded on Bruker AV400 and AV600 spectrometers at 300 K; chemical shifts are reported in ppm relative to the solvent residual peak. The SEC analysis of polymer samples was performed at 323 K using an Agilent PL-GPC 220 gel permeation chromatograph equipped with a PLgel column, with DMF as eluent (1 ml min À1 ) and poly(ethylene oxide) standards.
A small portion of (I) was dissolved in a warm mixture of THF/hexane (1:10 v/v) to provide a saturated solution. Single crystals formed in two weeks.
Single crystals of (II), suitable for X-ray diffraction analysis, were grown from a THF/hexane mixture (1:10 v/v) over two weeks.

Polymerization procedure
In a typical polymerization experiment, a solution of 0.1 mmol of a diol [33 mg of (I) or 36 mg of (II)] in 1 ml of THF was added to a stirred solution of Mg(BHT) 2 (THF) 2 (0.1 mmol, 61 mg or 0.2 mmol, 121 mg) in 1 ml of THF. The resulting solution was stirred for 20 min. A solution of "-CL (1.14 g, 10 mmol) in 1 ml of THF was then added at once to the formed catalyst solution. The solution was stirred for 30 min and then a sample was taken to determine conversion of the monomer by 1 H NMR spectroscopy. A 100% conversion was established in all cases based on the absence of a resonance signal at 4.22 ppm ("-CL) and the presence of a signal at 4.05 ppm (PCL), both corresponding to the -CH 2 O(CO)-fragment. The remaining viscous solution was poured into methanol (50 ml) containing a drop of acetic acid. The resulting precipitate was separated by centrifugation, washed with methanol (3 Â 25 ml) and hexane (2 Â 10 ml) and dried under vacuum. Polymer samples were further studied by SEC and 1 H NMR analysis. The degree of polymerization was determined by integration of a PCL terminal group signal at 3.63 ppm (-CH 2 -CH 2 -OH).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The positions of all hydrogen atoms in (I) and the hydroxy H atoms in (II) were found from the difference maps. These H atoms were refined independently with individual isotropic displacement parameters. The other H atoms in (II) were positioned geometrically (C-H = 0.95 Å for aromatic, 0.98 Å for methyl, 0.99 Å for methylene and 1.00 Å for methine H atoms) and refined as riding atoms with relative isotropic displacement parameters U iso (H)= 1.5U eq (C) for methyl H atoms and 1.2U eq (C) otherwise. A rotating group model was applied for methyl groups. For (II), reflections 110 and 221 were affected by the beam stop and were omitted from the refinement. The extinction correction in SHELXL was used for (II) (Sheldrick, 2015). Computer programs: APEX3 and SAINT (Bruker, 2018), SHELXS and SHELXTL (Sheldrick, 2008), SHELXL2017 (Sheldrick, 2015), publCIF (Westrip, 2010) and Mercury (Macrae et al.,2006).

(1R,2S,4r)-1,2,4-Triphenylcyclopentane-1,2-diol (I)
where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.40 e Å −3 Δρ min = −0.23 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.