The Fragmentation of Alkoxyhalocarbenes

Abstract

Alkoxyhalocarbenes stand at an intersection of carbene, carbocation, elimination, and substitution chemistry. Their fates are decided by several fundamental mechanisms, and we can anticipate significant contributions to mechanistic organic chemistry from an understanding of their behavior. In the 1950s, Hine 1 and Skell 2 reported that dihalocarbenes reacted with alkoxides to form alkoxyhalocarbenes, 1, and Skell suggested that, when sec-or tert-alcohols were used, the derived carbenes fragmented to alkyl cations with the loss of CO and X -(eq 1, X ) Cl or Br). Noting the close resemblance between CO and N 2 , Skell offered an analogy between reaction 1 and the decomposition of alkyldiazonium ions (RN 2 + f R + + N 2 ).

The Fragmentation of Alkoxychlorocarbenes in Hydrocarbon Solvents and in Low Temperature Argon Matrixes

“…Unsurprisingly, the benzyl substituent leads to a substantial stabilization of the products of both homolytic and heterolytic ROCCl fragmentation. In fact, the latter process is even predicted to be slightly exothermic in acetonitrile (in DCjE it is calculated to be endothermic by 6.8 kcal/mol), thus supporting the idea that the formation of ions is likely an important process in polar solvents, although the pyridine ylide carbene-trapping experiments indicate that heterolytic cleavage cannot be activationless, even in acetonitrile.

9 Energies of PhCH 2 OCCl and products of rearrangement and fragmentation by different methods relative to the trans -carbene.

10 Structures of PhCH 2 OCCl and resulting species.

…”

“…5 In polar solvents, ionic fragmentation is fast enough such that trapping of the carbene by MeOH is not competitive; no carbene-methanol products are observed. 1,3,6 However, we would not expect ionic fragmentation in nonpolar solvents. Indeed, in 0.9 M MeOH in pentane (as opposed to MeOH in MeCN), up to 40% of 1 is trapped by the methanol affording benzyl alcohol, presumably by the sequence in eq 2, where traces of water and/or HCl hydrolyze the initially formed chloroacetal (3).…”

“…Stereochemical retention during the fragmentation of chiral α-deuterio- 1 , as well as the persistence of 43% of benzyl chloride even when the reaction occurred in methanol, led to a mechanistic formulation of the fragmentation via ion pair 2 . , The absolute rate constant of the fragmentation, measured by laser flash photolysis (LFP) using either UV , or time-resolved infrared (TRIR) monitoring, gave k frag ∼ 4.5 × 10 5 s -1 (MeCN) and ∼ 1.8 × 10 5 s -1 (DCE) …”

“…In polar solvents, ionic fragmentation is fast enough such that trapping of the carbene by MeOH is not competitive; no carbene−methanol products are observed. ,, …”

Dihalodiazirines and the photochemical generation of dihalocarbenes: new light on old problems

“…Among these diazirine‐generated species, several ‘key’ carbenes stand out, including ArCF, generated from fluorodiazirines prepared without the use of elemental fluorine;27 PhCN 3 , the unstable azidophenylcarbene, the decomposition of which is of mechanistic interest;12, 13, 15, 27 MeOCCl, the first well‐characterized ambiphilic carbene;28 ROCCl (ROCBr), alkoxyhalocarbenes which fragment to carbocations or ion pairs;29–31 MeCOMe, a carbene with substantial nucleophilic reactivity;32 MeOCOMe, the quintessential nucleophilic carbene;33 and ArOCOAr, whose generation from the diazirine engenders the formation of aryloxy radicals 34…”