The thermal decomposition of quaternary ammonium hydroxides. Part 5. The importance of conformational factors in β-eliminations from quaternary hydroxides derived from piperidines, morpholines, and decahydroquinolines

Abstract

Classical work on the thermal decomposition of the 1 -methylmethohydroxides of piperidine and C-methylpiperidines has been confirmed, although the elimination products from 2and 3-methylpiperidines contain small proportions of isomeric alkenes. The products of thermal decomposition of 3-oxa-6-azoniaspiro [5,5] undecane hydroxide show that @-elimination occurs several times faster in a morpholine ring than in a piperidine ring. The requirement for easy elimination in six-membered rings is the anti-coplanarity of the four centres Hp, Cb, C,. and N+.Strikingly, molecules which lack this requirement, such as the 1 -methylmethohydroxide of cis-2.6-dimethylmorpholine gave no elimination whatever on thermal decomposition. On the other hand, thermal decomposition of the 1 -methylmethohydroxide of trans-3.5-dimethylpiperidine gave ca. 50% p-elimination. Elimination of the P-hydrogens of an a-attached methyl group are strongly preferred to elimination of ring (3-hydrogens, as in the degradation of cis-2,6-dimethylpiperidine, but a P-attached oxygen atom allows the corresponding morpholine compound to undergo 16% elimination of a ring hydrogen. The 1 -methylmethohydroxides of substituted transand cis-decahydroquinolines (type 1 ) undergo elimination of a P-hydrogen in a 2-alkyl group, as expected from conformational considerations. However, a cis-molecule in which the type 2 conformation is dominant such as the 1 -methyl met ho h yd roxide of cis (4H ,4a H ) ,cis( 4a H ,8a H )deca hydro -2,2,4-trimet hy Iqu i no I i ne, undergoes a n appreciable proportion of elimination of the correctly oriented 8ax-H in the cyclohexane ring. In all cases reported, the direction of.elimination is readily explained inductive effects of @-attached substituents.IN a historic paper in 1950, Barton1 suggested that certain 1 ,Z-eliniination reactions from the cyclohexane rings in steroidal molecules required the trans-coplanarity of the four atomic centres involved. McKenna extended the concept to include the p-elimination reaction occurring during the thermal decomposition (Hofmann degradation) of the quaternary hydroxides of reduced heterocyclic bases. It now seems likely that concerted ionic 1,2-elimination reactions only occur with ease when the four participating centres are cop l a ~i a r . ~. ~The majority of substituted cyclohexanes probably undergo elimination reactions from a transition state with a chair-like conformation in which coplanarity necessarily involves anti-coplanarity. The consequence for cyclohexylammonium hydroxides is the preferential elimination of an axial p -h y d r ~g e n . ~ The consequence for the quaternary hydroxides of heterocyclic bases with six-membered rings is the preferential elimination of an equatorial p-hydrogen.The decompositions described in this paper were performed, as earlier (cf. ref. ti), by the thermal decomposition of syrupy quaternary hydroxides without added base. The reactions have been studied solely by product analysis: no attempt was made to investigate the mechanism of elimination, a su...

Poly(N,N-diallylazacycloalkane)s for Anion-Exchange Membranes Functionalized with N-Spirocyclic Quaternary Ammonium Cations

“…33 The low stability of PDAMorCl compared to PDAPipCl can be attributed the inductive effect of the oxygen atom in the ring of the former polyelectrolyte. 35 In some previous studies the alkaline stability was assessed in KOD/CD 3 OD/D 2 O solutions instead of KOD/D 2 O solutions, as in the present case. 11,12 In the former cases, CD 3 OD was added mainly to accelerate the degradation rates and to ensure complete solubility of the cationic polymers or model compounds (and their degradation products) throughout the experiments.…”

“…Lillocci et al have shown that the degradation rate of heterocyclic QAs is related to the ring strain and is thus significantly higher for the 5- and 7-membered rings than for the 6-membered one. − The same group also reported that azepane-based QAs degrade primarily via ring-opening β-elimination, while the pyrrolidinium QAs were predominantly cleaved by nucleophilic ring-opening substitution . The low stability of PDAMorCl compared to PDAPipCl can be attributed the inductive effect of the oxygen atom in the ring of the former polyelectrolyte …”

Facile, high-yielding preparation of pyrrolidinium, piperidinium, morpholinium and 2,3-dihydro-1H-isoindolinium salts and ionic liquids from secondary amines

“…This is slightly lower than the 88% yield 2). [47][48][49] The original synthetic methodology involves the addition of either pyrrolidine, piperidine, or oxazoline to a boiling solution of the appropriate a,u-dibromoalkane in aqueous NaOH for 12-24 h. Aer the work-up, yields of the desired bromide salts were in the range of 19-68%. In this work, yields of the corresponding NTf 2 salts ranged from $65% (for the morpholinium salts) and 80-89% (for the pyrrolidinium and piperidinium salts).…”

Stability of ionic liquids in Brønsted-basic media

“…Under these conditions, the share of substitution reduces even further, and elimination becomes the only substantial mechanism of decomposition. 82,101 Taking into account the stability of the ring structures in aqueous solutions, Marino and Kreuer designed a pair of highly stable, spiro-fused ammonium salts, which do not contain linear alkyl groups. 63 These compounds, shown in Fig.…”