1-Methyl-3-(naphthalen-2-yl)cyclopentadiene

An asymmetric naphthyl-/methyl-substituted cyclopentadiene was synthesized and one isomer of five accessible through sigmatropic rearrangement was isolated and characterized by 1H NMR and X-ray diffraction. The title compound is envisioned as a π-bonded ‘antenna’ ligand to enhance lanthanide ion luminescence sensitization.

The title compound, C 16 H 14 , an asymmetric naphthyl-/methyl-substituted cyclopentadiene was synthesized and one isomer of five accessible through sigmatropic rearrangement was isolated and characterized by 1 H NMR and X-ray diffraction.The crystal packing features an intermolecular C-H� � �� interaction.

Structure description
Aryl-substituted cyclopentadienes, as functionalized cyclopentadienyl ligands, complexed to rare-earth metals have been poorly explored until recently.The title compound will expand organolanthanide chemistry and is envisioned as a �-bonded 'antenna' ligand to enhance the photoluminescence of lanthanide coordination compounds (Roitershtein et al., 2018).Similar ligands have been leveraged as effective light-harvesting �-coordinated ligands that serve as an alternative approach to traditional �-bonded antennae for lanthanide ion luminescence sensitization (Vinogradov et al., 2022).
The title compound was synthesized from the reaction between 2-lithium-naphthalene (made from 2-bromo-naphthalene) and 3-methyl-2-cyclopenten-1-one following syntheses similar to Rausch (Rausch et al., 2002) and Butts (Butts, 2002).The first step in this synthetic approach required very aggressive tert-butyl lithium to accomplish metalhalogen exchange to generate 2-lithium-naphthalene from 2-bromo-naphthalene. Since 3-methyl-2-cyclopenten-1-one has an enolizable proton, the naphthyl-lithium generated an unreactive enolate and naphthalene as side products, which necessitated recrystallization of the title compound to obtain pure material.The asymmetrically disubstituted product is thermally unstable with respect to dimerization, therefore product purification must be performed quickly at room temperature with recrystallization at À 30 � C. Five isomers are possible with mild heating through sigmatropic rearrangement (�G ‡ = 26 kcal mol À 1 ) (Bachrach, 1993), with one isomeric form isolated and studied by X-ray diffraction.In the crystal structure (Fig. 1), it is evident from bond distances that the title compound is a 1,3-disubstituted cyclopentadiene, with the methylene C-atom in the 5-position (C4) and naphthyl and methyl substituents in the 1and 3-positions, respectively.The bond distances between C1-C5 and C2-C3 are 1.364 (2) and 1.370 (3) A ˚, respectively, while bond distances between C1-C2, C3-C4, and C4-C5 are 1.452 (2), 1.498 (2), and 1.494 (2), respectively.There is no indication of the presence of any of the other isomers in the crystal analyzed.Fig. 2 shows a crystal packing diagram of the title compound with a canted view down along the b axis of the unit cell (Z = 8).Symmetry elements are included in the figure, with inversions (orange dots) and orthogonal screw axes (green lines with arrows).An intermolecular C-H� � �� interaction is also of note that seems to facilitate the observed packing, specifically between the proton of C13 from one molecule and the C2-C3 bond of another molecule (2.877 (3) A ˚).

Synthesis and crystallization
2-Bromonaphthalene (1.114 g, 5.379 mmol) was added to a 100 ml three-necked round-bottom flask containing a stir bar that was fitted with a gas inlet adapter, a 50 ml addition funnel, and a rubber septum; the apparatus was assembled in a glovebox under nitrogen.Dry tetrahydrofuran (THF, 15 ml) was added to dissolve the 2-bromonaphthalene and tert-butyllithium (7.394 ml of a 1.7 M solution in pentane, 12.57 mmol, 2.3 equiv) was added to the addition funnel.The apparatus was then carefully brought out of the box and the pale-yellow solution of 2-bromonaphthalene was cooled to À 78 � C in a dry ice-acetone bath with stirring under nitrogen provided by a Schlenk-line.tert-Butyllithium was added dropwise by the addition funnel to the THF solution with stirring at À 78 � C.After 15 min at À 78 � C, the reaction was placed in an ice bath and stirred for 1 h.Then, 3-methyl-2cyclopenten-1-one (dried over 4 A ˚sieves activated by heating to 100 � C for 48 h at 100 mT, 0.53 ml, 5.35 mmol) was added dropwise by syringe through the remaining rubber-stoppered neck of the three-necked round-bottom flask.The mixture was stirred for 1.5 h after which point an aqueous solution of NH 4 Cl (5 M, 2.4 ml, 12 mmol) was added dropwise and slowly by syringe.The reaction mixture was stirred for an additional 45 min while cooled in ice after which the volume was reduced under vacuum to �4 ml.The resulting semi-solid material was extracted with diethyl ether and using a separatory funnel, washed with distilled water, once with aqueous NaHCO 3 , and again with water.The organic layer was dried over MgSO 4 and then reduced under vacuum to a viscous oil.This material was immediately stored at À 30 � C to prevent dimerization.To crystallize the title compound, a concentrated 50:50 diethyl ether:hexane solution of the compound was allowed to sit at À 30 � C overnight.Yield 75% (4.01 mmol, 0.83 g).A translucent colorless block-shaped crystal with dimensions 0.12 � 0.07 � 0.05 mm 3 was chosen and mounted using a nylon loop for data collection.

Refinement
The crystal data, data collection and structure refinement details are summarized in Table 1.A number of reflections were omitted from a similar region of reciprocal space due to grazing of the incident beam by the tip of the steel shaft of the mounting pin.Beam graze was apparent from inspection of frame data.

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.All non-hydrogen atoms were refined anisotropically and all H atom positions were calculated geometrically and refined using a riding model.
Figure 2A crystal packing of the title compound.Hydrogen atoms are omitted to show symmetry elements.

Figure 1
Figure 1The molecular structure of the title compound showing atom labeling.Displacement ellipsoids are drawn at the 50% probability level.

Table 1
Experimental details.