Destabilized carbenium ions. Secondary and tertiary α-acetylbenzyl cations and α-benzoylbenzyl cations

The secondary α-acetylbenzyl and α-benzoylbenzyl cations, as well as their tertiary analogues, have been generated in a mass spectrometer by electron impact induced fragmentation of the corresponding α-bromoketones. These ions belong to the interesting family of destabilized α-acylcarbenium ions. While primary α-acylcarbenium ions appear to be unstable, the secondary and tertiaiy ions exhibit the usual behaviour of stable entities in a potential energy well. This can be attributed to a ‘push-pull’ substitution at the carbenium ion centre by an electron-releasing phenyl group and an electron-withdrawing acyl substituent. The characteristic unimolecular reaction of the metastaible secondary and tertiary α-acylbenzyl cations is the elimination of CO by a rearrangement reaction involving a 1,2-shift of a methyl group and a phenyl group, respectively. The loss of CO is accompanied by a very large kinetic energy release, which gives rise to broad and dish-topped peaks for this process in the mass-analysed ion kinetic energy spectra of the corresponding ions. This behaviour is attributed to the rigid critical configuration of a corner-protonatei cyclopropanone derivative and a bridged phenonium ion derivative, respectively, for this reaction. For the tertiary α-acetyl-α-methylbenzyl cations, it has been shown by deuterium labelling and by comparison of collisional activation spectra that these ions equilibrate prior to decomposition with their ‘protomer’ derivatives formed by proton migration from the α-methyl substituent to the carbonyl group and to the benzene ring.     

Destabilized carbenium ions: α-carbomethoxy-α,α-dimethyl-methyl cations

Tertiary α-carbomethoxy-α,α-dimethyl-methyl cations a have been generated by electron impact induced fragmentation from the appropriately α-substituted methyl isobutyrates 1–4. The destabilized carbenium ions a can be distinguished from their more stable isomers protonated methyl methacrylate c and protonated methyl crotonate d by MIKE and CA spectra. The loss of I^— and Br˙ from the molecular ions of 1 and 2, respectively, predominantly gives rise to the destabilized ions a, whereas loss of Cl˙ from [3]^{+ ˙} results in a mixture of ions a and c. The loss of CH₃˙ from [4]⁺˙ favours skeletal rearrangement leading to ions d. The characteristic reactions of the destabilized ions a are the loss of CO and elimination of methanol. The loss of CO is associated by a very large KER and non-statistical kinetic energy release (T₅₀ = 920 meV). Specific deuterium labelling experiments indicate that the α-carbomethoxy-α,α-dimethyl-methyl cations a rearrange via a 1,4-H shift into the carbonyl protonated methyl methacrylate c and eventually into the alkyl-O protonated methyl methacrylate before the loss of methanol. The hydrogen rearrangements exhibit a deuterium isotope effect indicating substantial energy barriers between the [C₅H₉O₂]⁺ isomers. Thus the destabilized carbenium ion a exists as a kinetically stable species within a potential energy well.     

Destabilized carbenium ions: α-imidoyl carbenium ions and the electron impact mass spectra of α-haloaldimines and α-haloketimines

A series of α-chloro- and α-bromoketimines compounds (1-9) with different substituents at the α-position and at the imino group has been investigated by electron impact mass spectrometry as possible precursors of the correspondingly substituted α-imidoyl carbenium ion, an important class of destabilized carbenium ions. The main fragmentation of the molecular ions of compounds, 1-9 in the ion source corresponds to an α-cleavage at the imino group; however, fragment ions are also formed by loss of the α-halo substituent. These fragment ions correspond at least formally to α-imidoyl carbenium ions. Their further reactions in dependence on the type of substituents at the imino group and at the α-C atom, were studied by mass-analysed ion kinetic energy and collisional activation mass spectrometry. The results agree with the initial formation of destabilized α-imidoyl carbenium ions but indicate an easy rearrangement of these ions in the presence of suitable alkyl substituents by 1,2- and 1,4-hydrogen shifts to more stable isomers.     

1,4-addition reactions to α,β unsaturated tertiary amides

Ketones, phenylacetonitrile, ethylmalonate, cyanoethyl acetate, nitromethane and nitroethane were found to add directely to α,β-unsaturated tertiary amides in the presence of CsF/Si(OMe)₄ to give 1,4-addition products in fair to good yields.     

Metallocenyl cations : III. Cymantrenyl carbenium ions containing one phosphine ligand

A number of carbinols, cyclopentadienylmanganese tricarbonyl derivatives of general formula (CO)₂LMnC₅H₄C(OH)RR′ (I) (L = PPh₃ or P(isoC₃H₇)₃, R and R′ = H, Me, Et, Ph) have been synthesised. In solution, in the presence of CF₃COOH they form stable secondary and tertiary carbenium ions, stabilised by the (CO)₂LMnC₅H₄ group. IR spectra and ¹H and ¹³C NMR spectra of the carbenium ions were recorded. Substitution of a tertiary phosphine for a carbonyl group sharply increased the stability of the α-cymantrenylcarbenium ions.     

Metallocenyl cations : IV. cymantrenyl carbenium ions containing two phosphine ligands

Cymantrene carbinols (I) with two phosphine ligands (PP) have been synthesized. Carbinols I (where PP is the chelate diphosphine Ph₂PCH₂CH₂PPh₂ or Ph₂PCH₂CH₂CH₂PPh₂) form the corresponding carbenium ions (II), stabilized by manganese, in the presence of CH₃COOH. The phosphorus atosm of the chelate diphosphine ligands become non-equivalent in the carbenium ions. IR spectra and ³P and ¹³C NMR spectra have been recorded, and the nature of the non-equivalence and of the structure of cymantrenyl carbenium ions are discussed. In the presence of CF₃COOH carbinols i (where PP = 2PMe₂(C₆H₄CH₃-p or 2PPH₂Me) are protonated at the metal atom.     

Relative stabilities of the methyltropylium and α-phenylethyl cations

Metastable decomposition of ethylbenzene molecular ions should yield [C₈H₉]⁺ ions of nearly threshold energies. Mass spectral data from collisionally activated dissociation of these ions show them to be mainly the methyltropylium (a) isomer, which is also that formed from 7-methylcycloheptatriene and isopropylbenzene. Combined with the threshold photoionization studies of McLoughlin, Morrison and Traeger, this establishes a as the most stable [C₈H₉]⁺ isomer. This is more stable than the α-phenylethyl isomer (b), which can be formed from α-bromoethylbenzene molecular ions; higher energy b ions appear to isomerize to a.     

α-Distonic ions as transient species in the reactions of metastable alkyl benzoate radical cations

The key intermediates to the fragmentation of metastable methyl and ethyl benzoate radical cations are α- and β-distonic isomers of the molecular ions. The α-distocic isomers are also formed by fragmentation of longer chain alkyl benzoates, but may not be long-lived, stable species. Rearrangement of the α-distonic ions prior to fragmentation can take place, but (re)formation of the benzoate molecular ions does not occur.     

Reaction of [α-(trimethylsilyl)benzyl]ferrocenes with α-ferrocenylbenzyl cations

Although methanolysis of [α-(trimethylsilyl)benzyl]ferrocene (I) and [p-methyl-α-(trimethylsilyl)benzyl]ferrocene (II) in the presence of anhydrous ferric chloride merely gave α-ferrocenylbenzyl methyl ether (III) and p-methyl-α-ferrocenylbenzyl methyl ether (IV), respectively, acid-catalyzed methanolysis of (I) and (II) in the presence of an equimolar amount of (III) or (IV) afforded 1,2-diferrocenyl-l,2-diarylethanes. It is suggested that one electron oxidation of [α-(trimethylsilyl)benzyl]ferrocene by α-ferrocenylbenzyl cation generated from α-ferrocenylbenzyl methyl ether, and subsequent methanolysis of the resulting substituted ferricenium ion may occur to give the two species of α-ferrocenylbenzyl radical, which in turn undergo an approximately statistical coupling.     

A novel synthesis of α-hydroxy- and α,α′-dihydroxyketone from α-iodo and α,α′-diiodo ketone using photoirradiation

A novel reaction of α-iodo ketone (α-iodocycloalkanone, α-iodo-β-alkoxy ester, and α-iodoacyclicketone) with irradiation under a high-pressure mercury lamp gave the corresponding α-hydroxyketone in good yields. In the case of α,α′-diiodo ketone, α,α′-dihydroxyketone which little has been reported until now was obtained. This reaction affords a new, clean and convenient synthetic method for α-hydroxy- and α,α′-dihydroxyketone.     

Microwave-assisted synthesis of α-hydroxy ketone and α-diketone and pyrazine derivatives from α-halo and α,α′-dibromo ketone

A novel reaction of α-halo ketone (α-bromo and α-chloro ketone) with irradiation under microwave gave the corresponding α-hydroxyketone and pyrazine derivative in good yields. In the case of α,α′-dibromo ketone, α-diketone was obtained. This reaction affords a new, clean and convenient synthetic method for α-hydroxyketone, α-diketone, α-chloro ketone and pyrazine derivative.     

Stereoselective Synthesis and Retentive Trapping of α‐Chiral Secondary Alkyllithiums Leading to Stereodefined α,β‐Dimethyl Carboxylic Esters

The treatment of α‐chiral secondary alkyl iodides with tBuLi at ?100 °C leads to the corresponding secondary alkyllithiums with high retention of configuration. Subsequent quenching with various electrophiles such as Bu₂S₂, DMF, MeOB(OR)₂, or Et₂CO provides the desired products with retention of configuration. Furthermore, a transmetalation with CuBr?P(OEt)₃ also allows retentive trapping with acid chlorides and ethylene oxide. The quenching of the resulting alkyllithiums with ClCO₂Et furnishes stereoselectively syn‐ and anti‐ethyl‐2,3‐dimethyl ester carboxylates (d.r.>94 %). Related esters bearing three adjacent stereo‐controlled centers (stereotriads) have also been prepared. This method has been applied to the synthesis of the ant pheromone (±)‐lasiol in 26 % overall yield (four steps) with d.r.=97:3 starting from commercially available cis‐2,3‐epoxybutane.