Intramolecular 1,2-alkyl shifts in unsymmetric dialkoxycarbenes studied by very low vapour pressure (VLVP) pyroiysis – mass spectrometry

Abstract

Methoxy-(2,2,2-trifluoroethoxy)carbene radical cations, CH,O-C-OCH,CF,'+, I", are cleanly generated by the dissociative electron ionization (EI) of 2-methoxy-5,5-dimethyl-2-(2,2,2-trifluoroethoxy)-~-1 ,3,4-oxadiazoline I. Neutralization-reionization (NR) mass spectrometry of the neutral carbene 1, generated by one-electron reduction of I'+, shows no recovery ion signal and thus 1 is not a viable species within the ks time scale of the experiment. Very low vapour pressure (VLVP) pyrolysis -mass spectrometry of I in conjunction with (multiple) collision experiments shows that 1 completely isomerizes, via a 1,2-trifluoroethyl shift, into methyl 3,3,3-trifluoropropionate, CF,CH,C(=O)OCH,, l a . This technique was also used to study the related dialkoxycarbenes C,H,O-C-OCH,CF,, 2, CH,O-C-0C2H,, 3, and CH,O-C-OCH(CH,),, 4, generated from the corresponding 2,2-dialkoxy-5,5-dimethyl-A3-1 ,3,4-oxadiazolines. The pyrolytically generated carbene 2 behaves analogously to 1 and completely isomerizes to ethyl 3,3,3-trifluoropropionate, 2a. The neutral carbenes 3 and 4 undergo only a partial isomerization via 1,2-alkyl shifts in which the ethyl and isopropyl groups show a slightly greater migratory aptitude, respectively, than the methyl group. The differences in migratory aptitude are explained in terms of a transition state model similar to that of a 1,2-H shift in carbenes, with development of negative charge in the migrating group. The greater migratory aptitude of CF,CH,, as compared to CH, and CH,CH2, is attributed to the stabilization of negative charge in the transition state by strongly electron-withdrawing p-fluorines whereas the differences in migratory aptitude between the alkyl groups in 3 and 4 are largely due to the greater polarizability of isopropyl and ethyl groups, as compared to the methyl group.Key words: dialkoxycarbenes, pyrolysis, tandem mass spectrometry.Resume : On peut gCnCrer proprement les cations radicaux mtthoxy-(2.2.2-trifluoroCthoxy)carbknes, CH30-C-0CH2CF,'+, 1'+, en procCdant ? I une ionisation Clectronique (IE) dissociative de la 2-mCthoxy-5,5-dim~thyl-2-(2,2,2-trifluoroCthoxy)-~3-1 ,3,4-oxadiazoline, I. La spectroscopie par neutralisation-rkionisation (NR) du carbkne neutre, 1, gCnCrC par une rCduction un electron du 1". ne prksente pas de signal correspondant ii de la rCcupCration d'ion et le composC 1 n'est donc pas une espkce approprike ii I'tchelle de temps, ks, de I'expCrience: La spectromCtrie de masse du composC I avec pyrolyse ii trks basse tension de vapeur (VLVP), rCalisCe de concert avec des expkriences de (multiples) collisions ont montrC que le composC 1 s'isomkrise complktement, par le biais d'un dCplacement-1,2 d'un groupe trifluoroCthyle, en un 3,3,3-trifluoropropionate de mCthyle, CF,CH,C(=O)OCH,, la. On a aussi utilisk cette technique pour Ctudier des dialkoxycarbknes apparentis : C2HjO-C-OCH,CF,, 2, CH,O-C-OC,H,, 3, et CH,O-C-OCH(CH,),, 4, gCnCrCs a parti des 2,2-dialko~~-5,5-dirnCth~l-~~-1,3,4-oxadiazolines correspondantes. Le carbkne 2 prCparC d'une f a~o n py...

Δ3-1,3,4-Oxadiazolines.† Versatile sources of reactive intermediates

“…Thus, ethoxy(trifluoroethoxy)carbene rearranges by migration of the trifluoroethyl group exclusively, suggesting that negative charge accumulates in the migrating group at the transition state. 19 The rearrangement may be mechanistically analogous to 1,2-H-migration in carbenes. 59,60 Alkoxy-and dialkoxycarbenes can fragment to radical pairs 2,61-65 but the reaction has usually been run under severe conditions.…”

Theoretical Study of Dithioformic Acid, Dithiohydroxy Carbene and Their Radical Cations: Unimolecular and Assisted Rearrangements

“…Conversely, the H-loss is the most facile fragmentation in ionized DTHC. It seems that DTHC 2 could be detected and characterized in a neutralization−reionization mass spectrometric (NRMS) experiment. , We note that cyclic dithiocarbenes have been evoked as intermediates in a number of cases …”

“…Although the dithiohydroxy carbene HS−C−SH (referred to hereafter as DTHC), which is a high-energy isomer connected to DTFA by a formal 1,2-H shift, has been shown to lie in a relatively deep potential well, participation of DTHC in the unimolecular chemistry of DTFA has not been evoked. Nevertheless, recent detection of dihydroxy carbene HO−C−OH, and hydroxy thiohydroxy carbene HO−C−SH, that are higher energy isomers of formic acid and monothioformic acid, respectively, suggest that a certain involvement of the carbene intermediates in the acid reactivity is probable. Therefore we have considered in the present study the missing link, described on the [CH 2 S 2 ] potential energy surface, between the reactivity of both DTFA and DTHC isomers.…”

Pericyclic Reactions

“…Singlet carbenes have a low-lying (approximately) empty 2p orbital and so, like carbocations, can undergo 1,2 hydride or alkyl migrations to the carbene carbon. Dimethoxy carbenes yield esters by this mechanism [160]. A stable crystalline carbene with two N substituents has been prepared and characterized [161]:…”