Crystal structure and halogen–hydrogen bonding of a Delépine reaction intermediate

The reaction of 1,5-dibromopentane with urotropine results in crystals of the title molecular salt, 5-bromourotropinium bromide, crystallizing in space group P21/n. The packing is directed mainly by H⋯H van der Waals interactions and C—H⋯Br hydrogen bonds, as revealed by Hirshfeld surface analysis.

The Delé pine reaction is a classic synthetic route to produce primary amines (Delé pine, 1895(Delé pine, , 1897. Alkyl or aryl halides are reacted with hmta to form a quaternary ammonium salt, followed by acid hydrolysis to give a primary amine. A major advantage of this reaction over other routes is that the formation of the quaternary urotropinium cation prevents further alkylation and high yields are possible (Galat & Elion, 1939). We recently made an attempt to find a cost-effective route to synthesize 1,5-diaminopentane (cadaverine) from 1,5-dibromopentane. On an industrial scale, this is produced by bacterial decarboxylation of lysine (Ma et al. 2017;Wang et al., 2018). Attempts to react 1,5-dibromopentane with hmta in the presence of NaI in ethanol (modified from Galat & Elion, 1939) led to the crystallization of a monosubstituted product, 5-bromourotropinium bromide, C 11 H 22 BrN 4 + ÁBr À (1), the structure and supramolecular features of which are presented here.

Structural commentary
Compound 1 crystallizes in the centrosymmetric monoclinic space group P2 1 /n. The asymmetric unit of 1 (Fig. 1) contains one C 11 H 22 BrN 4 + N-(5-bromopentyl)urotropinium cation and one bromide anion. The pentyl chain is in the all-trans configuration, unlike its hexyl relative (Reddy et al., 1994), which displays an anticlinal configuration between C4 and C6 of the hexyl chain (torsion angle = 133 ).
The overall packing (Fig. 3) is similar to the hexyl compound.

Figure 3
View of the crystal packing of compound 1 looking down the a axis.

Figure 1
Asymmetric unit of compound 1. Hydrogen atom labels are omitted for clarity. Displacement ellipsoids are at the 50% probability level.

Figure 4
Hirshfeld surface of the 5-bromopentylurotropinium cation in compound 1 with the bromide anion shown. Surface plotted for d norm in the range À0.1755 to 1.3045 a.u.

Database survey
Surprisingly few discrete alkylurotropinium salts have been submitted to the Cambridge Structural Database (version 5.41, May 2020 update 2, Groom et al., 2016), given that the salts are reported to crystallize in most cases. There are 48 in total, 17 of which are halide or polyiodide salts. In the remainder, the alkylurotropinium exists as a counter-ion to complex anions or is a bridging species in a coordination polymer. Of the 17, only six are bromide salts (refcode BUXZEZ, Qingchuan et al. 1983;CAQVUO, Aniol et al., 2017;CEXLOG, Mak, 1984;GINHAN, Betz & Klü fers, 2007;YOYWEO and YOYWIS, Reddy et al., 1994).
A close relative to compound 1 is 6-bromohexylurotropinium bromide, C 12 H 24 BrN 4 + ÁBr À (YOYWIS; Reddy et al., 1994), which is isostructural, also crystallizing in space group P2 1 /n. As mentioned above, this displays an anticlinal torsion angle in the alkyl chain, but presents very similar HÁ Á ÁBr À interactions and overall packing. For the purposes of comparison, the partial structure CIF in the CSD was completed in OLEX2 to add in the hydrogen atoms present in the original publication, and Hirshfeld surface analysis also undertaken. A potential difficulty in this structure is the presence of a possible disorder in the hexyl chain (atom C10 has a markedly larger U eq value than its neighbours, plus hydrogen atoms on C10 come into closer than van der Waals contact with hydrogen atoms on neighbouring C10 atoms in the crystal). However, the interactions between hydrogen and bromide account for a similar percentage of the overall Hirshfeld surface (16.6% in YOYWIS versus 18.4% in compound 1).
Direct comparisons with BUXZEZ (Yang et al., 1983) are difficult because of the disorder around the allyl group while the remaining compounds have other significant intermolecular interactions such as hydrogen bonds formed to bromide by a water molecule (CEXLOG, GINHAN) or from a carboxylic acid (YOYWEO). Similar interactions to compound 1 can be seen where chloride is the halide anion: BIDBIZ (Shao et al., 1982) shows hydrogen bonds from the benzylurotropinium cation to the chloride anion, accounting for 12.2% of the interaction surface. Polyiodide compounds appear not to show C-HÁ Á ÁI interactions in the same manner as the above bromide and chloride compounds, but a methylurotropinium monoiodide compound (VOBCIY; Ribá r et al., 1991) displays similar interactions to 1 with HÁ Á ÁI hydrogen bonds forming 15.5% of the Hirshfeld surface.

1-(5-Bromopentyl)-3,5,7-triaza-1-azoniatricyclo[3.3.1.1 3,7 ]decane bromide
Crystal 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.