Michael B. Stringer

A combined field ionization kinetics, collision-induced, and stable isotopic labelling study of the ethene and ethyl (and subsequent) eliminations from the molecular ion of 2-ethylbutanoic acid

Abstract It is shown by field ionization kinetics that the ethene elimination from ionized 2-ethyl-butanoic acid is the most dominant channel at molecular ion lifetimes ⩽10 −9 s. This channel, however, becomes rapidly less important with respect to ethyl elimination at molecular ion lifetimes ⪢10 −9 s. Both eliminations occur without any detectable exchange between hydrogen or carbon atoms from different positions as shown by specific 2 H- and 13 C-labelling. The same observations are made for molecular ions decomposing in the metastable time frame of 10 −6 to 10 −5 s. On the basis of collision-induced dissociation experiments, it is demonstrated that ∼95% of the (MC 2 H 5 ) + ions have the structure of carbonyl oxygen-protonated crotonic acid which, in line with the 2 H- and 13 C-labelling, are formed by a successive, irreversible hydrogen shift from C-3 to the carbonyl oxygen and cleavage of the C-2C′-3 bond to eliminate ethyl. The remaining ∼5% of the (MC 2 H 5 ) + ions have the structure of carbonyl oxygen-protonated methacryclic acid. In line with the 2 H- and 13 C-labelling results, these ions are generated by a successive, irreversible hydrogen shift from C-3 to the carbonyl oxygen, migration of the C(OH) 2 group from C-2 to C-3, a hydrogen shift from C-3 to C-2, and eventual cleavage of the C-2C′-3 bond to eliminate ethyl. Further metastable decompositions of the (MC 2 H 5 ) + ions correspond to eliminations of molecules of water, C 2 H 2 O, and C 2 H 4 O. The water molecule contains the original hydroxylic hydrogen and one of the hydrogen atoms of C-3. The eliminated C 2 H 2 O molecule contains the C-1 and C-2 atoms, while the eliminated C 2 H 4 O molecule contains the C-3 and C-4 atoms. Combined with the obtained 2 H-labelling results, strong support, if not evidence, is provided for the intermediacy of ion/molecule complexes during the eliminations of C 2 H 2 O and C 2 H 4 O from the (MC 2 H 5 ) + ions.

Collision-induced dissociations of carboxylate negative ions from 2-ethylbutanoic, 2-methylpropanoic, and pivalic acids. An isotopic labelling study

Deprotonation of Et2CHCO2H yields Et2CHCO2–. On collisional activation this ion forms CO2–˙, CH2CH–, and MeCHCH–. In addition, elimination of H˙ and Et˙ yield Et(R)CCO2–˙(R = Et and H, respectively). The elimination of Et˙ is not a simple cleavage but occurs by loss of H˙ from a methyl group followed by loss of ethene. The carboxylate ion also rearranges to Et2CCO2H; this species decomposes to HO–, EtCCH2, and also eliminates the elements of C3H8 and CH4. All fragmentations have been studied using 2H and 13C labelling: for example it is proposed that loss of CH4 from Et2CCO2H occurs by a six-centre stepwise process in which the first step (formation of an incipient methyl anion) is rate determining. The collisional activation mass spectra of Et2CHCO2–, Me2CHCO2–, and Me3CCO2– are different, all showing characteristic decompositions. For example, all three ions eliminate methane; the mechanism is different in each case.

Carbanion rearrangements. Collision-induced dissociations of enolate ions derived from 3-ethylpentan-2-one

Reaction of HO– with MeCOCHEt2 produces two enolate ions, MeCOEt2 and –CH2COCHEt2. The primary carbanion competitively eliminates C2H4 and C4H8, and forms C2HO–. The elimination of C2H4 is a stepwise reaction proceeding through a six-membered transition state; the first step (deprotonation) is rate-determining. The loss of C4H8 is a rearrangement reaction –CH2COCHEt2 [graphic omitted] –CH2COMe + EtCHCH2. The tertiary carbanion competitively eliminates H2, CH4, and C3H8. The losses of CH4 and C3H8 are stepwise processes occurring through six- and five-membered transition states, respectively. A double isotope fractionation experiment (2H, 13C) shows that both steps of the CH4 elimination are rate-determining.

1-Benzazepines. Novel ring transformations occurring during the reactions of 5-methyl- and 5-phenyl-1,3-dihydro-2H-1-benzazepin-2-ones with phosphoryl chloride

Treatment of 8-substituted (H, Me, MeO) 5-methyl-1,3-dihydro-2H-1-benzazepin-2-ones with phosphoryl chloride under reflux produces two products. X-Ray structure determination of the products from the 8-methoxy derivative shows them to be 2,12-dimethoxy-5,9-dimethyl-7H-[1]benzazepino-[1′,2′ : 1,2]pyrrolo[5,4-b]quinoline (major product) and 5-chloromethyl-2,11-dimethoxy-5,8-di-methyl-5,6-dihydroquino[1′,2′ : 1,2]pyrrolo-[5,4-b]quinoline (minor product). The corresponding reaction of 8-methoxy-5-phenyl-1,3-dihydro-2H-1-benzazepin-2-one yields 2,12-dimethoxy-5,9-diphenyl-7H-[1]benzazepino[1′,2′: 1,2]pyrrolo[5,4-b]quinoline as the sole product.A mechanistic pathway is proposed in which the first step involves condensation of two molecules of the appropriate 2-chloro-5-methyl-1H-1-benzazepine or its 3H analogue.