4-tert-Butyl-2-[2-(1,3,3-trimethylindolin-2-ylidene)ethylidene]cyclohexanone

The title molecule, C23H31NO, has two alternative cyclohexanone configurations at the 4-position in a ratio of 0.663 (3):0.337 (3). The plane of the five-membered planar ring in the indolin-2-ylidene subtends an angle of 2.19 (7)° with its fused aromatic ring, an angle of 16.24 (8)° with the plane of the major cyclohexanone configuration and an angle of 8.54 (15)° with the bridging planar ethylidene C atoms. These last atoms subtend an angle of 8.37 (16)° with the mean plane through the major cyclohexanone configuration. The molecules pack approximately parallel to the (01) plane via C—H⋯π and C—H⋯O interactions.

The title molecule, C 23 H 31 NO, has two alternative cyclohexanone configurations at the 4-position in a ratio of 0.663 (3):0.337 (3). The plane of the five-membered planar ring in the indolin-2-ylidene subtends an angle of 2.19 (7) with its fused aromatic ring, an angle of 16.24 (8) with the plane of the major cyclohexanone configuration and an angle of 8.54 (15) with the bridging planar ethylidene C atoms. These last atoms subtend an angle of 8.37 (16) with the mean plane through the major cyclohexanone configuration. The molecules pack approximately parallel to the (101) plane via C-HÁ Á Á and C-HÁ Á ÁO interactions.
Cg1 is the centroid of the C1-C6 ring.

Comment
Organic nonlinear optical (NLO) materials show much promise due to their potential application in areas such as optical power limiting, optical data storage and two-photon fluorescence imaging (Ma et al., 2002;Parthenopoulos & Rentzepis, 1989;Denk et al., 1990). Such compounds are typically push-pull conjugated systems that can be modified by altering either the donor, acceptor or conjugated interconnect moieties. However these modifications can involve trade-offs insofar as improvements to the nonlinear optical properties typically result in compounds that are more complex to prepare, have lower stabilities and higher optical losses. Conjugated ketones are useful intermediates for increasing the chain length and/or substituting different donors and acceptors onto a basic chromophore backbone. This is because conjugated ketones are quite reactive species and are able to undergo a range of carbon-carbon double bond forming reactions including the Wittig reaction, Knoevenagel condensation and Peterson olefination. With this in mind, and in line with our ongoing work on the development of novel organic NLO compounds, we sought to prepare the title compound 3 using the method outlined in Fig.   1. This compound is a useful synthon for the preparation of a range of chromophore systems as it contains an electron-rich indoline donor unit and a conjugated ketone onto which a range of acceptors can be coupled. The title molecule 3 is conveniently prepared in excellent yield by the condensation of 4-tert-butyl-2-hydroxymethylenecyclohexanone 1 with Fisher's base 2. Compound 1 was prepared from 4-tert-butylcyclohexanone using the general procedure reported by Ainsworth (1963).
Compound REFCODES below are from the CSD (Version 5.32, with Feb. 2011 updates;Allen, 2002). In the title compound 3 (Fig. 2), the cyclohexanone ring exists in two configurations, S (C18a) and R (C18b), in the ratio a:b of 0.663 (3):0.337 (3). This model made chemical sense, was stable in refinement, with insignificant difference Fourier residual density. The data supported refinement in the centrosymmetric space group P2 1 /n even though there were 57 weak reflections (with intensities between values between 0.08 (2) and 0.87 (7)) that violated the n glide absence condition. Refinement in P2 1 did not improve the fit significantly as would be expected with such weak contributing data, and gave some very large correlations between thermal and positional parameters of the n glide related molecules.
The molecules are held in the lattice by weak C-H···π interactions (Table 1) over cell inversion centres and C-H···O hydrogen bonds, the latter forming C(10) motifs (Bernstein et al., 1995), Fig. 3.

Experimental
To a stirred solution of Fisher's base 2 (0.865 g, 5 mmole) in methanol was added compound 1 (0.91 g, 5 mmole). The mixture was refluxed for 2 h by which time its colour had changed from deep red to brown. The solvent was removed at reduced pressure and the residue purified by crystallization in ethanol, giving the title compound 3 as a yellow solid (1.5 g, 88% yield). X-ray quality crystals were grown by slow evaporation from methanol. m.p.: 450 K.

Refinement
A total of 15 outlier reflections (ΔF 2 /σ(F 2 )>4.5) were removed from the refinment using OMIT. There were 57 systematic absence violations involving weak reflections as discussed in the Comment section. The cyclohexanone ring was disordered across two configuraions (see Fig. 1); each was refined with common occupancy factors giving a final ratio a:b of 0.663 (3):0.337 (3). The bond lengths between C16 and C19 to their respective disorder atoms (C17 and C18, a and b) were restrained to be the same using SADI. The bond distances between C20b and each of the bound methyl C atoms were similarly restrained.