Peter C. H. Eichinger

A comparison of skeletal rearrangement reactions of even-electron anions in solution and in the gas phase

Abstract There is significant correspondence between certain skeletal rearrangement processes of close-shell organic anions in the condensed and gas phases. Examples of such correspondence include the acyloin, acyl oxyacetate, anionic oxy Cope, anionic Wolff, benzilic acid, Dieckmann, Lossen, Smiles and Wittig rearrangements. In contrast, there are some rearrangements observed in the condensed phase, which are either minor or do not occur at all in the gas phase. The Favorskii, Tiemann and Carroll rearrangements fall into this category. Finally, there are some gas phase rearrangements which have no condensed phase analogy: for example the negative ion pinacol/pinacolone and Beckmann rearrangements. These, and related processes are discussed in this Review.

Collision-induced “radical” losses from even-electron negative ions in the gas phase

Abstract Both radicals and (even-electron) neutrals are lost when even-electron organic negative ions are subjected ot collisional activation in the gas phase. Generally, radical loss is less pronounced and less diagnostic (of some structural feature) than the ubiquitous (even-electron) neutral loss. There are exceptions to this generalisation: these often fall into one of the following classifications, i.e. There is a further type of “radical” loss, which is a stepwise process involving loss of H and (R − H). This occurs in particular cases and only when R is alkyl(⩾Et). The reactions are diverse but they may all be rationalized as proceeding through five-membered ring intermediates. The loss of “C 2 H 5 ” from an alkylcarboxylate anion is a suitable example:

Characteristic negative ion fragmentations of deprotonated peptides containing post-translational modifications: mono-phosphorylated Ser, Thr and Tyr. A joint experimental and theoretical study.

Peptides and proteins may contain post-translationally modified phosphorylated amino acid residues, in particular phosphorylated serine (pSer), threonine (pThr) and tyrosine (pTyr). Following earlier work by Lehmann et al., the [M-H]- anions of peptides containing pSer and pThr functionality show loss of the elements of H3PO4. This process, illustrated for Ser (and using a model system), is CH3CONH-C(CH2OPO3H2)CONHCH(3) --> [CH3CONHC(==CH2)CONHCH3 (-OPO3H2)] (a) --> [CH3CONHC(==CH2)CONHCH3-H]- + H3PO4, a process endothermic by 83 kJ mol(-1) at the MP2/6-31++G(d,p)//HF/6-31++G(d,p) level of theory. In addition, intermediate (a) may decompose to yield CH3CONHC(==CH2)CONHCH3 + H2PO4 - in a process exothermic by 3 kJ mol(-1). The barrier to the transition state for these two processes is 49 kJ mol(-1). Characteristic cleavages of pSer and pThr are more energetically favourable than the negative ion backbone cleavages of peptides described previously. In contrast, loss of HPO3 from [M-H]- is characteristic of pTyr. The cleavage [NH2CH(CH2-C6H4-OPO3H-)CO2H] --> [NH2C(CH2-C6H4-O-)CO2H (HPO3)] (b) --> NH2CH(CH2-C6H4-O-)CO2H + HPO3 is endothermic by 318 kJ mol(-1) at the HF/6-31+G(d)//AM1 level of theory. In addition, intermediate (b) also yields NH2CH(CH2-C6H4-OH)CO2H + PO3 - (reaction endothermic by 137 kJ mol(-1)). The two negative ion cleavages of pTyr have a barrier to the transition state of 198 kJ mol(-1) (at the HF/6-31+G(d)//AM1 level of theory) comparable with those already reported for negative ion backbone cleavages.

The fallaxidin peptides from the skin secretion of the Eastern Dwarf Tree Frog Litoria fallax. Sequence determination by positive and negative ion electrospray mass spectrometry: antimicrobial activity and cDNA cloning of the fallaxidins

The glandular skin secretion of the Eastern Dwarf Tree Frog Litoria fallax contains nine peptides named fallaxidins. The sequences of these peptides were elucidated using a combination of positive and negative electrospray mass spectrometry together with Edman sequencing. Among these peptides are: (i) fallaxidins 1.1 and 2.1 which have the sequences YFPIPI-NH2 and FWPFM-NH2. The activities of these peptides are unknown, but it has been shown that they are not smooth muscle active, opioids or antimicrobially active, nor do they effect proliferation of lymphocytes; (ii) two weakly active antibiotics, fallaxidins 3.1 and 3.2 (e.g. fallaxidin 3.1, GLLDLAKHVIGIASKL-NH2), and a moderately active antibiotic fallaxidin 4.1 (GLLSFLPKVIGVIGHLIHPPS-OH). Fallaxidin 4.1 has an unusual sequence for an antibiotic, containing three Pro residues together with a C-terminal CO2H group. cDNA cloning has confirmed the identity of the nine isolated peptides from L. fallax, together with five additional peptides not detected in the peptide profile. The pre-regions of the nine preprofallaxidins are conserved and similar to those of the caerin peptides from L. caerulea and L. splendida, suggesting that the fallaxidin and caerin peptides, although significantly different in sequence, originated from a common ancestor gene.

A convenient route to alkylidenecyclopropanes from cyclopropyldiphenylphosphine oxide and carbonyl compounds

Abstract Alkylidenecyclopropanes were readily prepared by thermal decomposition of the potassium or lithium salt of the adduct formed from the treatment of cyclopropyldiphenylphosphine oxide with n -butyl lithium and a carbonyl compound.

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.

The gas-phase Wittig–oxy Cope rearrangement of deprotonated diallyl ether

Collisional activation of deprotonated diallyl ether is found to give major products formd by competitive losses of H2, C3H4O and C4H6. Products are rationalised in terms of the Wittig-oxy Cope rearrangement sequence CH2CH–H–O–allyl →(CH2CH)(CH2CH–CH2)CHO–→ CH2CH(CH2)2 CHCHO → products. The spectrum of CH2CH–CDOCD2CHCH2 shows the operation of both 1,2- and 1,4-Witting rearrangements, while that of CH2CH–CDO–allyl shows proton transfer (to form CH2CH–CHD–O–HCHCH2) to be a minor process.

A Theoretical Study of the Cyclization Processes of Energized CCCSi and CCCP

Calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31+G(d) level of theory have shown that cyclization of both the ground state triplet and the corresponding singlet state of CCCSi may rearrange to give cyclic isomers which upon ring opening may reform linear C(3)Si isomers in which the carbon atoms are scrambled. The cyclization processes are energetically favorable with barriers to the transition states from 13 to 16 kcal mol(-1). This should be contrasted with the analogous process of triplet CCCC to triplet rhombic C(4), which requires an excess energy of 25.8 kcal mol(-1). A similar cyclization of doublet CCCP requires 50.4 kcal mol(-1) of excess energy; this should be contrasted with the same process for CCCN, which requires 54.7 kcal mol(-1) to effect cyclization.

Anionic rearrangement in the gas phase. The collision-induced loss of carbon monoxide from deprotonated pyruvates and hydroxyacetates

Deprotonated pyruvates and hydroxyacetates, upon collisional activation, undergo loss of CO by anionic rearrangement through alkoxycarbonyl ion complexes, e.g. [graphic omitted] and [graphic omitted] Such processes have been substantiated by labelling (2H, 13C), and production studies.

Do deprotonated vinylcarbinols undergo [1,3] sigmatropic rearrangements in the gas phase?

Abstract [1,3] Sigmatropic rearrangements, e.g. R 1 R 2 C(O − )CHCH 2 → R 1 C(O − )CHCH 2 R 2 and/or R 2 C(O − )CHCH 2 R 1 , only occur under special circumstances in the gas phase. No rearrangements occurs when R 1 and R 2 are alkyl or phenyl; when either R group benzyl, minor rearrangement is observed. When the R groups comprise a cycloalkyl ring, [1,3] rearrangement occurs readily for small strained rings (e.g. cycloropyl and cyclobutyl) but not at all for cyclopentyl and cyclohexyl systems.