2-( Bicyclo [ 4 . 2 . 0 ] octa-1 , 3 , 5-trien-3-yl )-adamantan-2-ol

A new adamantan-2-ol with a 2-bicyclo[4.2.0]octa-1,3,5-trien-3-yl substituent in the position 2 was synthesized via two stage synthesis starting from benzocyclobutene and adamatan-2-one. The structure of the title compound was determined using 1H-and 13C-NMR, HRMS and XRD.


Introduction
Adamantanes are used in various fields of science such as material chemistry and medicine. Their inclusion into polymer structure gives materials with excellent properties. For example, adamatan-1-yl acrylate, methacrylate, and 1-vinyl-adamantane-based polymers and polymers containing pendant diamondoid moieties (adamantane and diamantane) are soluble, colorless polymers with an unusual combination of moderately high refractive index (1.48-1.60) and low optical dispersion [1]. These properties make the polymers valuable as optical plastics for lenses, viewfinders, data storage media, light-diffusing elements, etc.
Adamantyl is a non-polar substituent, which should contribute to the improvement of dielectric properties in polymers. This is a very useful characteristic for insulation materials. The development of microelectronics requires high performance interlevel dielectric materials with an extremely low dielectric constant and loss factor.
Adamantane and benzocyclobutene (BCB)-based polymer materials have attracted significant attention because of their low-dielectric constant, low loss factor, and excellent high-temperature performance. An example of such material is AdaDBDVS [4]. AdaDBDVS has Tg = 350 • C, T d5% = 449 • C and dielectric constant (K) = 2.78.
The combination of benzocyclobutene and adamantane in a monomer or polymer seems attractive because a benzocyclobutene ring opens when heated above 200 • C and gives an active isomer (o-xylylene) that provides crosslinking of the polymer. BCB crosslinked polymers have high values of glass transition temperatures (that is relatively low for linear, non-crosslinked adamantane polymers) and high thermal stability [5][6][7].

Results and Discussion
Synthesis of the title compound is presented in Scheme 1 and includes a three-stage process starting from adamantane (1) and benzocycobutene (3). Adamantan-2-one (2) was obtained from 1 by oxidation by hot sulfuric acid for 25 h [8]. 4-Brombenzocyclobutene (4) was obtained from 3 by bromination in water solution [9][10][11]. A reaction of adamantan-2-one (2) with a Grignard reagent synthesized from 4 gave 5 with 70% yield. All compounds were characterized by 1 H, 13 C NMR, and high-resolution mass spectrometry (HRMS). Structure of 5 was also confirmed by the X-ray diffraction method (XRD) ( Figure 1A,B). The NMR data of intermediates and title product are presented in the Supplementary Materials.
The bond lengths and bond angles in 5 have typical values for this class of compounds [12] (Figure 1). According to XRD, 5 crystallizes with two independent molecules (Z' = 2) in the non-centrosymmetric (chiral) space group P21. Taking into account that two independent molecules in 5 are characterized by the opposite configuration of the C(1) asymmetric center, we can conclude that 5 is a rather rare example of quasi-racemate [13].
The conformations of two independent molecules (A and B) in 5 are almost identical, thus leading to the presence of the non-crystallographic center of symmetry. The main difference between the two enantiomers in 5 is the slight variation of HO-C-C torsion angles. This variation is the consequence of different roles of OH-groups in the supramolecular organization in 5. All compounds were characterized by 1 H, 13 C NMR, and high-resolution mass spectrometry (HRMS). Structure of 5 was also confirmed by the X-ray diffraction method (XRD) ( Figure 1A,B). The NMR data of intermediates and title product are presented in the Supplementary Materials.
The bond lengths and bond angles in 5 have typical values for this class of compounds [12] (Figure 1). According to XRD, 5 crystallizes with two independent molecules (Z' = 2) in the non-centrosymmetric (chiral) space group P2 1 . Taking into account that two independent molecules in 5 are characterized by the opposite configuration of the C(1) asymmetric center, we can conclude that 5 is a rather rare example of quasi-racemate [13].
The conformations of two independent molecules (A and B) in 5 are almost identical, thus leading to the presence of the non-crystallographic center of symmetry. The main difference between the two enantiomers in 5 is the slight variation of HO-C-C torsion angles. This variation is the consequence of different roles of OH-groups in the supramolecular organization in 5. The OH group of one of the independent molecules (A) participate in the formation of OH . . .

Materials and Methods
NMR spectra were recorded on a Bruker AM-300 or a Bruker Avance 600 spectrometer (Bruker Corporation, Billerica, MA, USA) in CDCl3. Mass spectra were obtained on a Varian MAT CH-6 instrument (Varian, Inc, Palo Alto, CA, USA) using a direct inlet system; the ionization energy was 70 eV; the acceleration voltage was 1.75 kV. The reaction mixtures were analyzed and the purity of all products was checked by TLC on Merck Silica gel 60 F254 UV-254 plates. (5) 4-Bromobenzocyclobutene (1.22 g, 6.7 mmol) was added dropwise to Mg (160 mg, 6.7 mmol) and a crystal of I2 in THF (30 mL) under Ar atmosphere at a temperature of 25-40 °C. Magnesium was completely dissolved after 1 h of stirring. Adamantan-2-one (1.0 g, 6.7 mmol) in THF (7 mL) was added to the resulting Grignard reagent and the mixture was stirred overnight. Water (20 mL) was added to the reaction mixture that was then extracted with ethyl acetate (3 × 20 mL), dried and evaporated. The resulting crude product 5 was purified by flash chromatography to give 5 (1.23 g, 73%) as colorless crystals.

Materials and Methods
NMR spectra were recorded on a Bruker AM-300 or a Bruker Avance 600 spectrometer (Bruker Corporation, Billerica, MA, USA) in CDCl 3 . Mass spectra were obtained on a Varian MAT CH-6 instrument (Varian, Inc, Palo Alto, CA, USA) using a direct inlet system; the ionization energy was 70 eV; the acceleration voltage was 1.75 kV. The reaction mixtures were analyzed and the purity of all products was checked by TLC on Merck Silica gel 60 F254 UV-254 plates. (5) 4-Bromobenzocyclobutene (1.22 g, 6.7 mmol) was added dropwise to Mg (160 mg, 6.7 mmol) and a crystal of I 2 in THF (30 mL) under Ar atmosphere at a temperature of 25-40 • C. Magnesium was completely dissolved after 1 h of stirring. Adamantan-2-one (1.0 g, 6.7 mmol) in THF (7 mL) was added to the resulting Grignard reagent and the mixture was stirred overnight. Water (20 mL) was added to the reaction mixture that was then extracted with ethyl acetate (3 × 20 mL), dried and evaporated. The resulting crude product 5 was purified by flash chromatography to give 5 (1.23 g, 73%) as colorless crystals. and 7273 independent reflections (R int = 0.0607) were used in the further refinement. The structure was solved by the direct method and refined by the full-matrix least-squares technique against F 2 in the anisotropic-isotropic approximation. C-H hydrogen atoms in 5 were placed in calculated positions and were refined in the "riding" model with U(H) iso = 1.2U eq of their parent atoms. The hydrogen atoms of the OH groups were located from the Fourier density synthesis and refined in an isotropic approximation. The refinement converged to wR 2 = 0.1368 and goodness-of-fit (GOF) = 0.998 for all independent reflections (R 1 = 0.0605 was calculated against F for 5073 observed reflections with I > 2σ (I)). The maximum and minimum values of difference density were 0.262 and −0.253 e Å −3 . All calculations were performed using SHELXTL-2017. Cambridge Crystallographic Data Centre contains the supplementary crystallographic data for this paper No. CCDC 1943491. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk).