Enantiomerically pure quaternary ammonium salts with a chiral alkyl chain N(CH₃)(n‐C₃H₇)₂(sec‐C₄H₉)I: Synthesis and physical studies

Abstract

A pair of enantiomerically pure quaternary ammonium salts with a chiral side chain, methyl-(R)-(1-methylpropyl)di(n-propyl)ammonium iodide 1 and methyl-(S)-(1-methylpropyl)di(n-propyl)ammonium iodide 2, and the related racemate, methyl-(rac)-(1-methylpropyl)di(n-propyl)ammonium iodide 3, were synthesized through a reductive alkylation procedure, starting from enantiomerically pure and, also, racemic forms of (rac)-(1-methylpropyl)amine. A spectroscopic chiroptical signature in solution was provided by the Raman optical activity spectra of compounds 1 and 2. The crystallographic structures of 1, 2, and 3 were examined by single crystal X-ray diffraction. 1 crystallizes in the tetragonal space group P4(3)2(1)2 (no. 96), a = b = 12.826 (2) A, c = 17.730 (2) A, V = 2916.9 (5) A(3), Z = 8, Flack coefficient 0.04 (2). 2 crystallizes in the tetragonal space group P4(1)2(1)2 (no. 92), a = b = 12.842 (1) A, c = 17.749 (2) A, V = 2927.0 (5) A(3), Z = 8, Flack coefficient 0.05 (2). The crystal structures and space groups for 1 and 2 are enantiomorphs and the crystallographic investigation confirmed the absolute configuration of the stereocenter in both compounds. 3 crystallizes in the monoclinic space group P2(1)/n(no. 14), a = 8.178 (1) A, b = 14.309 (2) A, c = 12.328 (2) A, beta = 96.811 (6) degrees, V = 1432.4 (2) A(3), Z = 4.

(S)‐(−)‐(2‐MeBu)N(Pr)2MeI Salt as Template in the Enantioselective Synthesis of the Enantiopure Two‐dimensional (S)‐(−)‐(2‐MeBu)N(Pr)2Me[ΛMnΔCr(C2O4)3] Ferromagnet

“…This compound was synthesised starting from the (S)‐2‐methylbutylamine according to Reference 16 . A 3 mL methanolic solution of propanal (0.72 g, 12.5 mmol) was added dropwise to an ice‐cooled solution of ( S )‐2‐methylbutylamine (1 g, 11.5 mmol) in methanol (3 mL).…”

“…This compound was synthesised starting from the (S)-2-methylbutylamine according to Reference 16. 16 A 3 mL methanolic solution of propanal (0.72 g, 12.5 mmol) was added dropwise to an ice-cooled solution of (S)-2-methylbutylamine (1 g, 11.5 mmol) in methanol (3 mL). After an hour of stirring at room temperature, the resulting (S)-(2-methylbutyl) propylimine was reduced with sodium borohydride (0,47 g, 12.5 mmol), which was added in small portions to the ice-cooled solution.…”

“…Because of the fast racemisation of K 3 Cr(C 2 O 4 ) 3 in solution, we chose to develop enantioselective templated‐assembly of these compounds starting from enantiopure chiral template cations. Taking in account the difficult resolution of chiral ammonium salts in which the chiral centre is the nitrogen atom itself, we decided to synthesise ammonium salts with a chiral carbon centre located on an alkyl chain on the nitrogen atom as is the case for ( S )‐NMe(Pr) 2 ( sec Bu)I and ( R )‐NMe(Pr) 2 ( sec Bu)I …”

“…Taking in account the difficult resolution of chiral ammonium salts in which the chiral centre is the nitrogen atom itself, we decided to synthesise ammonium salts with a chiral carbon centre located on an alkyl chain on the nitrogen atom as is the case for (S)-NMe(Pr) 2 (secBu)I and (R)-NMe(Pr) 2 (secBu)I. 16 In this way we have previously prepared optically active single crystals of opposite configurations [(S)-NMe(Pr) 2 (sec-Bu)][ΛMnΔCr(C 2 O 4 ) 3 ] and [(R)-NMe(Pr) 2 (sec-Bu)] [ΔMnΛCr(C 2 O 4 ) 3 ]. 8 In these compounds, the ammonium cations were disordered.…”

8.26 Spectroscopic Analysis: Raman Optical Activity

“…Georghe et al 16 studied the ROA spectra of enantiomerically pure N(CH 3 )(n-C 3 H 7 ) 2 (sec-C 4 H 9 ) þ I À in methanol. As expected, the spectra of the enantiomers are mirror images of each other.…”