John C. Sheldon

Does MeCO− undergo 1,2-hydride ion transfer to yield −CH2CHO in the gas-phase? The application of experiment and theory in concert

Abstract Ab initio calculations (6–311 ++ G) suggest that the conversion MeCO − → (CH 2 CHO) − could occur by a stepwise 1,2-hydride ion transfer. There is a calculated endothermic barrier of 236 kJ mol −1 for the first step of this reaction and (CH 2 CHO) − is 114 kJ mol − more negative in energy than MeCO − . Experimentally, MeCO − , upon collisional activation, prefers to fragment (for example, by loss of CO) rather than undergo the 1,2-hydride transfer. In the case of the process MeCO − → Me − + CO, the barrier is significantly less than that required for the 1,2-hydride transfer. Similarly, EtCO − prefers to fragment directly rather than rearrange by 1,2-hydride transfer to (MeCHCHO) − . The pivalyl anion Me 3 CCO − , which could, in principle, undergo 1,2-methyl anion or 1,3-proton transfer, also prefers to fragment directly upon collisional activation.

Carbanion rearrangements. The collision-induced losses of ketene from acetylacetone enolate anions

Abstract Deprotonation of acetylacetone yields two enolate ions, MeCO C HCOMe and C H2COCH2COMe, with the former predominating. On collisional activation, the ion C H2COCH2COMe fragments principally by competitive eliminations of ketene and acetone: minor fragmentations occur after conversion of the ion to MeCO C HCOMe. The ion MeCO C HCOMe eliminates H., CH3., CH4, H2O, CH2CO and MeCOMe on collisional activation. Loss of ketene is the major fragmentation: a combination of labelling studies and ab initio caculations suggest the pathway MeCO C HCOMe → MeCOCH2COCH2− → MeCOCH2− + CH2CO.

Elimination of Molecular Hydrogen and Methane from Collision-Activated Alkoxide Negative Ions in the gas Phase. Anab initioand Isotope Effect Study

Ab initio calculations indicate that the collisional induced losses of molecular hydrogen from the ethoxide negative ion and methane from the t- butoxide negative ion to be stepwise processes in which the key intermediates are [H-… MeCHO ] and [Me-…Me2CO] respectively. Deuterium kinetic isotope effects observed for these and other alkoxide negative ions are in accord with the operation of a stepwise reaction.

Collision-induced dissociations of aryl-substituted alkoxide ions. Losses of dihydrogen and rearrangement processes

The ion PhCH2O– undergoes competitive losses of H˙, H2, CH2O, and C6H6 upon collisional activation. The loss of H2 occurs mainly to form (C6H4)–CHO, and ab initio calculations suggest the reaction proceeds by the stepwise mechanism PhCH2O–→[H–(PhCHO)]→(C6H4)–CHO + H2. The losses of CH2O and C6H6 are accompanied (or preceded) by partial phenyl H–benzyl H interchange. The ion Ph(CH2)3O– undergoes many fragmentations including the losses of H2O and, CH2O and loss of H2. The loss of H2 occurs by both 1,2- and 1,3-eliminations. A number of minor fragmentations occur after partial interchange of phenyl hydrogens and hydrogens at position 2. The first example of a specific double proton transfer is noted, viz. Ph(CH2)3O–→ C6H7–+ CH2CH–CHO. Ions Ph(CH2)nO–(n= 2–5) all decompose to produce PhCH2– ions: when n= 3–5 it is proposed that the reactions may involve Smiles intermediates, i.e. reaction (a). [graphic omitted]

Nucleophilic attack at αβ-unsaturated carbonyl systems. The reactions of acrolein and methyl acrylate with CF3O–, [F–⋯ HOMe], RO–, and [RO–⋯ HOR]. An ab initio and ion cyclotron resonance study

Ab initio calculations suggest that the fluoride negative ion should react with acrolein to form a number of potentially stable [M + F–] ions. These include a number of ‘solvated’ ions together with those formed by conjugate addition, and addition to the carbonyl centre. The energies of many of these species relative to reactants are low and comparable (–130 to –180 kJ mol–1) and it is likely that they will be in equilibrium unless collisional deactivation removes the excessive energy of the system. Acrolein reacts with the fluoride ion donors CF3O– and [F–⋯ HOMe] to give detectable [M + F–] ions, and with MeO– and [MeO–⋯ HOMe] to yield [M + MeO–] species. There is no experimental evidence available to suggest structures for these species. The methoxide negative ion deprotonates acrolein and methyl acrylate to produce allenic ions [CH2CC(R)(O–)](not the corresponding species [CH2C–COR]) and these ions undergo a number of complex reactions with the neutral substrate. The CD3O– negative ion reacts with methyl acrylate [α-2H1](CH2CD–CO2Me) to yield both stable and decomposing [M + CD3O–] species. The decomposing adduct yields both [CH2CC(OMe)(O–)] and [CH2CC(OCD3)(O–)] by a reaction sequence which must involve the intermediacy of the tetrahedral species (CH2CH)(MeO)(CD3O)C–O–

The bis(methylene) metaphosphite, [P(CH2)2]–, and tris(methylene) metaphosphate, [P(CH2)3]–, families of anions. An experimental and ab initio study

Collisional activation of a number of phosphorus-containing anions yields families of negative ions based on bis(methylene) metaphosphite, [P(CH2)2]–, and tris(methylene) metaphosphate, [P(CH2)3]–, structures. Individual product ions produced include [P(CH2)2]–, [CH2PO]–, [CH2PS]–, [PO2]–, [POS]–, and [PS2]–, together with [P(CH2)3]–, [(CH2)2PO]–, [CH2PO2]–, [(CH2)2PS]–, [CH2P(O)(S)]–, and [CH2PS2]–. Ab initio calculations indicate that all bonds to phosphorus (in these ions) show appreciable double bond character, and that all methylene-containing phosphorus anions have coplanar atoms except [P(CH2)3]–, which shows out-of-plane twisting of the methylene groups to give an ion of D3 symmetry.

Elimination of dihydrogen from collision-activated alkoxide negative ions in the gas phase. An ab initio and isotope effect study

Ab initio calculations indicate the loss of H2 from the exoxide negative ion to be a stepwise process in which the key intermediate is the ‘long-lived’ species [H–⋯ HCH2CHO]; deuterium isotope effects which are observed for the collision-induced eliminations of H2 and HD from deuteriated ethoxide ions support the operation of a stepwise reaction.

The synthesis and structures of C5H– and C5–˙

The Ion C5H– is formed in the gas phase by the process –(CC)2CH2OMe → C5H–+ MeOH; ab initio calculations indicate a ground state triplet structure with a near linear carbon skeleton, but with the terminal H at a marked angle to the carbon backbone.

Are tetrahedral intermediates formed by addition of nucleophiles to organoboranes in the gas phase

Nucleophilic addition of CD3O– to Me2BOMe gives the same addition product as the corresponding reaction between Me2BOCD3 and MeO–, as evidenced by the identical collisional activation mass spectra of the products. This is interpreted in terms of exclusive formation of a boron product ion of tetrahedral geometry. The decompositions of the product involve loss of MeOH and CD3OH and the formation of MeO– and CD3O–. The major decompositions of (CD3)3B +–OCH2CH2XMe (X = O, S, or NMe2) are similar to those outlined above and may be explained by initial formation of (CD3)3 OCH2CH2XMe. However, there are some unusual fragmentations (e.g. loss of CH3D) which may occur through the alternative structure (CD3)3[graphic omitted](Me)CH2CH2O–. It is suggested that other ambident species may also react with Me3B to form several tetrahedral species, e.g. deprotonated methyl acetate could yield Me3CH2CO2Me, Me3OC(OMe)CH2, and Me3—[graphic omitted](Me)[graphic omitted]. The formation of the third structure is supported by the pronounced loss of ketene from this system.

Do carbanions undergo 1,2 hydrogen transfers in the gas phase?

Certain anions X––CH–CH2–Y and X–CH2–CH–Y interconvert under collisional activation conditions in the gas phase. Experiments designed to probe the mechanism(s) of particular interconversions suggest (i) that the ions MeO–CH2––CH–CN and MeO––CH–CH2–CN interconvert via 1,2 H transfer processes, and (ii) when X = Ph and Y = CO2Me or CN, deuterium labelling and fragmentation data indicate that equilibration of Ph––CHCH2Y and PhCH2––CHY occurs, at least in part, by proton transfer to and from the phenyl ring.