Crystal structures of (1E,4E)-1,5-bis(5-bromothiophen-2-yl)-2,4-dimethylpenta-1,4-dien-3-one and (E)-4-(5-bromothiophen-2-yl)-1,3-diphenylbut-3-en-2-one

Both title structures present non-classical intermolecular C—H⋯O, C—Br⋯π, C—H⋯π and π–π interactions which form three-dimensional supramolecular architectures by means of different linkages in their crystal structures.


Chemical context
Claisen-Schmidt reaction (Claisen & Claparede, 1881;Schmidt, 1881) is the condensation of aromatic aldehydes (or between ketones and aldehydes lacking -hydrogen with aliphatic or mixed alkyl aryl ketones in the presence of a relatively strong base to form ,-unsaturated ketones. This reaction is of tremendous value in synthetic organic chemistry (Wayne & Adkins 1940;Marvel & King, 1944) and is frequently encountered as a key step in several elegant total synthesis protocols. Claisen-Schmidt condensation can also be catalysed by acid. The first step is a condensation of an aldol type; enols or enolates are involved as intermediates in this reaction. This reaction involves the nucleophilic addition of enol or an enolate ion derived from methyl ketone to the carbonyl carbon of the aromatic aldehyde. Dehydration of the hydroxylketone to form the conjugated unsaturated carbonyl compound occurs spontaneously (see Scheme 1) (Stiles et al., 1959). Cycloalkanones like cyclohexanone, cycloheptanone readily undergo Claisen-Schmidt condensation (Nithya et al., 2014). In addition to cycloalkanones we attempted open-chain alkanones.
The asymmetric unit of (II) comprises one molecule of 5-bromothiophene-2-carbaldehyde with one molecule of dibenzylacetone. The two phenyl rings of the dibenzylacetone subtend a dihedral angle of 53.09 (18) . One of the phenyl rings (C15-C20) of the dibenzylacetone and the thiophene ring are normal to one another, forming a dihedral angle of 89.96 (16) .

Synthesis and crystallization
The title compounds were prepared by adapting a reported procedure (Alkskas et al., 2013). Title compound (I) was prepared by adding a mixture of pentan-3-one (0.50 g, 1.2 mmol) and 5-bromothiophene-2-carbaldehyde (2.2 g, 2.4 mmol) in methanol (25 mL) and potassium hydroxide pellets (0.2 g, 2.4 mmol) was also added. The reaction mixture was stirred at room temperature overnight whilst a paleyellow product separated out. The crude product was washed several times with cold ethanol (1 mL). Good quality single crystals suitable for X-ray analysis were obtained by recrys- Title compound (II) was prepared by mixing dibenzylketone (1 g, 4.7 mmol) and 5-bromothiophene-2-carbaldehyde (1.8 g, 9.5 mmol) in methanol (25 mL) and potassium hydroxide pellets (0.6 g, 9.5 mmol) were also added. The reaction mixture was stirred at room temperature overnight whilst a yellow product separated out. The crude product was washed several times with cold ethanol (1 mL The molecular chain in (I), formed along the b axis through C-BrÁ Á Á interactions.

Figure 4
The two molecular chains in (I), stacked byinteractions to form a three-dimensional supramolecular architecture.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. In both compounds, all H atoms on C were placed in calculated positions, guided by difference Fourier maps, with C-H bond distances of 0.93-0.97 Å . H atoms were assigned as U iso (H) = 1.2U eq (carrier) or 1.5U eq (methyl C). Four reflections were omitted owing to bad agreement for compound (I).