CC‐Spaltungen bei ionisierten Carbonsäuren als Umkehrung der [1,4]‐Addition von Alkyl‐Radikalen an protonierte α,β‐ungesättigte Carbonsäuren. Zur Rolle von Enol‐Radikalkationen als reaktive Zwischenstufen bei Isomerisierungs‐/Dissoziationsprozessen in der Gasphase

Abstract

Th. Weirke, H. Halim und H. Schwarz Chem. Ber. 118, 495 -509 (1985) 495 C -C-Spaltungen bei ionisierten Carbonsauren als Umkehrung der [ 1,4]-Addition von Alkyl-Radikalen an protonierte a,P-ungesattigte CarbonsaurenMetastabile Molekul-Ionen der Pivalinsaure (5) und 2-Methylbuttersaure (8) verlieren unimolekular Methyl-Radikale, wobei aus 5 ausschlieRlich protonierte Crotonsaure (6) entsteht, wahrend 8 nur protonierte Methacrylsaure (7) liefert. Die Untersuchung von 13C-und D-markierten Isotopomeren von 5 und 8 belegt, zusammen mit MNDO-Rechnungen fur den Zerfall von 8, daR C -C-Bindungsbruch ausschlieRlich aus Enol-Radikalkationen der allgemeinen Struktur 1 erfolgt. 1st das Radikalzentrum nicht zur protonierten Carboxylgruppe benachbart, wie beispielsweise in 3, so ist der Dissoziation metastabiler Ionen immer eine Umlagerung vorgelagert, die letzten Endes ein Enol-Radikalkation erzeugt. Die zentrale Rolle der Enol-Radikalkationen, wie 1, 13 und 14, bei C -C-Spaltungen ist darauf zuriickzufiihren, da8 die Dissoziation als Umkehrung der [ 1,4]-Addition von (nucleophilen) Alkyl-Radikalen an protonierte u,b-unges&ttigte Carbonshren anzusehen ist.C-C Cleavage of Ionized Carboxylic Acids as Reversal of [1,4] Additions of Alkyl Radicals to Protonated a,P-Unsaturated Acids. -On the Role of Enol Cation Radicals as Reactive Intermediates in Isomerization/Dissociation Reactions in the Gas Phase Metastable molecular ions of pivalic acid (5) and 2-methylbutanoic acid (8) decompose unimolecularly via methyl loss, thereby specifically generating protonated crotonic acid (5 + 6) and protonated methacrylic acid (8 -+ 7), respectively. The investigation of 13C and 2H labelled isotopomers of 5 and 8 together with MNDO calculations of parts of the potential energy surface of 8 reveals that C -C cleavage requires the presence of enol cation radicals of the general structure 1 as reactive intermediates. Cation radicals of carboxylic acids in which the radical centre is not adjacent to the protonated carboxylic function, as for example in 3, do not serve as actual precursors for the C -C cleavage; instead, isomerization to 1 is required. The fact that en01 cation radicals, i.e. 1, 13 and 14, are of essential importance in the splitting of C -C bonds of ionized carboxylic acids is a direct consequence of the fact that these dissociations can be viewed as the reversal of [ t ,4] addition of (nucleophilic) alkyl radicals to protonated a$-unsaturated acids, which proceeds preferentially via enol cation radical intermediates.In mehreren Ubersichtsartikeln') wurde in den vergangenen Jahren iiberzeugend gezeigt, da8 der unimolekulare Zerfall von Radikalkationen in der Gasphase selbst dann nicht einstufig verlaufen mu8, wenn das zu eliminierende Radikal bereits als komplette strukturelle Gruppe vorhan-0 VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 1985 0009 -2940/85/0202 -0495 $02.50/0 Th. Weiske, H. Halim und H. Schwanden ist. Im Gegenteil, mehrstufig verlaufende Reaktionswege sind oftmals energetisch wesentlich gunstiger als der direk...

Electron ionization induced fragmentation of macrocyclic amines CnH2n−1NH2: Evidence for the rearrangement of aminocarbene radical cations and a comparison with long-chain esters

“…For example, the 1,2-migration of the [C(OHh] +. radical cation occurs in the dissociation of carboxylic acids [8][9][10][11][12], and the 1,2-migration of the [C(OH)(OCH 3 ) ] +. radical cation in the case of esters [13].…”

The structure and mechanism of formation of C5H9O+from ionized phytyl methyl ether

“…Such steps involve additional energy barriers and do not normally occur in simple radicals [10], even in the gas phase in cases where they are energetically favorable [11]. Nevertheless, there is clear evidence from related research on distonic ions isomeric with ionized carboxylic acids [12,13] and work on distonic ions derived from ionized alkenols [14] or ionized alkenyl methyl ethers [8,9] that slow 1,2-H shifts to the radical site do occur in such systems. Each of these 1,2-H shifts to a spin center is usually the key step in the mechanism for dissociation.…”

“…Thus, CID studies reveal that [M -CH 3]+ in the El spectrum of isoprenyl methyl ether has a different structure from the corresponding ion in the spectrum of prenyl methyl ether [9]. Moreover, metastable CH 2 =CHC(CH3)20CD;eliminates both CH; and CD;, whereas metastable centers in distonic ions that are isomeric with ionized carboxylic acids, even though 1,2-H shifts were documented for these radical cations [12,13].…”

“…Instead, it dissociates independently via another mechanism, which almost certainly involves two 1,2-H shifts followed by y-cleavage. The strong preference for expulsion of C I 6H 33 in the final step probably reflects generic trends in the relative rates for loss of various alkyl radicals from distonic ions (loss of CH; occurs much more slowly than elimination of C 2H s and larger species) [12,13]. Further work shows that alkyl radical loss from RI R 2 C = CHCH 20CHr radical cations generally involves ')I-cleavage (Bowen, R. D.; Clifford, P.; Gallagher, R. T., in preparation).…”