The chemistry of adducts from acetylenes and quinaldines

This thesis is divided into two parts. The first part concerns the chemistry of products obtained from substituted 2-methylquinolines and dimethyl acetylenedicarboxylate, and the second part describes an attempted X-ray crystallographic structure determination of one of these products, the structure of which could not be elucidated by chemical methods.
In Part 1, Chapter 1, the previously known types of reactions of nitrogen-containing heterocycles with acetylenecarboxylic acids and their esters are reviewed. The investigations reported here concern new reactions between substituted 2-methylquinolines and dimethyl acetylenedicarboxylate; also, some previously described reactions were reinvestigated. These reactions usually gave complex mixtures which were resolved chromatographically A number of separate types of product were formed, and these types are considered in separate chapters.
In Chapter 2 the benzo[c]quinolizines and azepines such as (1) and (2), and the isomeric azepine (3), obtained from the reactions of substituted 2-methylquinolines with dimethyl acetylenedicarboxylate are discussed.
The u.v., n.m.r., i.r. and mass spectra of these compounds are entirely analogous to those of known benzo[c]quinolizines and azepines. New azepines (4), (5) and (6) were obtained from 2-ethylquinoline and (7) from 2-benzylquinoline; their structures were deduced from the u.v., n.m.r. and mass spectra.
The isolation of these compounds with a methyl- or a phenyl-group at position 10 or 11 has necessitated a re-appraisal of one mechanism of azepine-formation, and a new scheme involving a spiro intermediate is proposed (Scheme 1).
The 4a-isopropyl-4aH-benzo[c]quinolizine (8) is the sole product of the reaction of 2-isopropylquinoline with dimethyl acetylenedicarboxylate; this undergoes a smooth photo-isomerisation to the 1-isopropyl-1H-benzo[c]quinolizine (9).
Chapter 3 discusses the "red" adducts formed in the reactions of 2-methyl-, 2,8-dimethyl- and 2,6,8-trimethylquinoline with dimethyl acetylenedicarboxylate. Two red adducts are formed in each of these reactions, and it is proposed that these are geometric isomers of the structures (10), (11) and (12) respectively.
The n.m.r. spectra of these compounds show the 6- and 7-protons as AX quartets in the range 2.8andndash;3.5andtau;; the 11- and 12-protons and the 17-protons appear as AX quartets and singlets respectively in the range 4.4andndash;6.2andtau;. The chemical shifts of the 11-, 12- and 17- protons permit a simple classification into two groups, "first" and "second" red adducts. Shielding of the 11-ester methyl groups of the first red adducts, but not of their isomers, by the benzo-ring causes these methyl resonances to occur upfield from the other ester-methyl resonances. The n.m.r. spectra in trifluoroacetic acid solution are discussed. The red adducts readily eliminate the elements of dimethyl fumarate (maleate) in the mass spectrometer.
The first red adducts from 2-methyl- and 2,8-dimethylquinoline are most conveniently prepared pure by using methanol as the reaction solvent, as the first red, but not the second red, adducts crystallise from the reaction mixture. Bromination of the first red adduct from 2-methylquinoline gives the quinoline (13), which is readily debrominated to give the quinoline (14). Compound (14) is also obtained directly by treatment of the first red adduct from 2-methylquinoline with zinc in glacial acetic acid. Formation of compounds (13) and (14) involves the elimination of fumarate (maleate) from the red-adduct molecules, as also occurs in the mass spectrometer. This facile elimination, when considered in conjunction with the shielding effects observed in the n.m.r. spectra, provides good evidence in favour of a red-adduct structure involving a saturated-CHE.CHE-grouping attached to the nitrogen atom.
Hydrogenation of the first red adducts from 2-methyl- and 2,8-dimethyl-quinoline in glacial acetic acid solution using Adams' catalyst gives the hexahydro-derivatives (15) and (16) respectively; the tetrahydro-derivative (17) is obtained from the 2-methylquinoline adduct using 10% palladium-on-charcoal. The spectral properties of the derivatives (13)andndash;(17) are discussed.
A mechanism to account for the formation of the red adducts is proposed. Alternative structures for the red adducts, and the reasons for rejecting them, are discussed. The surprising observation that the benzo[c]quinolizines, azepines and other adducts (Chapter 4) formed when these 2-methylquinolines are reacted with dimethyl acetylenedicarboxylate in acetonitrile solution, are not formed when methanol is used as the reaction solvent, is noted.
A red compound isolated in very low yield from the reaction of 2,4-dimethylquinoline with dimethyl acetylenedicarboxylate in methanol is assigned the structure (18) on spectral evidence.
Chapter Four deals with other adducts isolated from the reactions of substituted 2-methylquinolines with dimethyl acetylenedicarboxylate. Analogues of the known adduct (19) are obtained from 2,8-dimethyl-, 2,6,8-trimethyl- and 2,4,6,8-tetramethyl-quinoline (compounds (20)andndash;(22) respectively).
An adduct assigned the structure (23) on the basis of the n.m.r. and u.v. spectra is obtained in very low yield from 2,4,6,8-tetramethylquinoline.
The adduct (24) is also obtained from 2,4,6,8-tetramethylquinoline, although the alternative structure (25), and corresponding structures for the degradation products, are not excluded. Catalytic reduction of this adduct using Adams' catalyst gives the phenols (26) and (27) by hydrogenolysis; the ether (28) is readily formed from the phenol (27) by methylation with ethereal diazomethane. Mixtures obtained by treating adduct (24) with both bromine and zinc in glacial acetic acid are discussed. Mechanisms accounting for the formation of both structures (24) and (25) are presented; it is not possible to differentiate between these structures on the basis of the present work.
Yellow adducts isolated from the reactions of 2,8-dimethyl-, 2,6,8-trimethyl- and 2,4,6,8-tetramethyl-quinoline with dimethyl acetylenedicarboxylate are tentatively assigned the structures (29)andndash;(31) respectively. A computer-simulated n.m.r. spectrum is in good agreement with the observed AMX pattern of the -CH2-CHE- grouping of adduct (29) in the range 5.4andndash;7.9andtau;. Irradiation of a methanolic solution of the adduct (31) with ultra-violet light yields a photoisomer.
Part 2, Chapter 5, concerns the "blue" adducts formed in the reactions of 2-methyl-, 2,8-dimethyl-, 2,6,8-trimethyl- and 2-ethyl-quinoline with dimethyl acetylenedicarboxylate, the latter two of which were first isolated in the present investigation. The 2-methylquinoline blue adduct has a molecular weight of 679, corresponding to a composition of 1 mole quinoline + 4 moles ester andminus; 1 mole methanol. Treatment of this adduct with bromine in glacial acetic acid yields a dark green isomer, but an attempted reduction with zinc in glacial acetic acid gave only polymeric ester. As the information obtainable from these reactions, and from the spectral data of the blue adducts, was very limited, an attempt was made to determine the structure of the 2-methylquinoline blue adduct by X-ray crystallography. This attempt failed, the most likely reason being that the total atomic contents of the unit cell were too great for the available computer programme to handle.
The n.m.r., u.v. and mass spectra of the blue adducts are discussed. The n.m.r. spectra suggest that the 2-methyl- and 2,8-dimethyl-quinoline adducts may not be analogues; the mass spectrum of the latter suggests that the molecular weight could be 707, and not 693 as stated by Gagan. The extremely broad absorption bands in the visible spectra of the adducts appear to be charge-transfer bands.
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Advisor: Acheson, R
Hands, A