Carbenes as Substrates:  Bimolecular Fragmentation of Alkoxychlorocarbenes

Abstract

Fragmentation reactions of n-butoxychlorocarbene (9), isobutoxychlorocarbene (10), and benzyloxychlorocarbene (2) were studied by product analysis and by laser flash photolysis (LFP). The carbenes were generated photochemically from 3-alkyl-3-chlorodiazirines in (1) MeCN solution; (2) 5.77 M pyridine in MeCN; (3) 0.504 M tetrabutylammonium chloride (Bu4NCl) in MeCN; or (4) 5.77 M pyridine + 0.504 M Bu4NCl in MeCN. In MeCN, 9 gave mainly HCl-capture product, n-butyl dichloromethyl ether (45.1%), carbene dimer (9.8%), and azine (14.6%). Fragmentation products 1-butene (14.4%), 2-butyl chloride (6.0%), and 1-butyl chloride (10.2%) were limited. With added pyridine, HCl was scavenged, the dichloromethyl ether was suppressed, and 1-butene (42.1%), 2-butyl chloride (7.3%), and 1-butyl chloride (24.1%) were dominant. With added Bu4NCl, 1-butyl chloride increased to 46.4%. With both pyridine and Bu4NCl, 1-butyl choride was 63% of the product, with 1-butene at 22.8%. A similar pattern was observed with carbene 10. Products in MeCN included isobutene (23%), 1- and 2-butene (8−9%), tert-butyl chloride (1.7%), 2-butyl chloride (5.8%), isobutyl chloride (0.4%), isobutyl dichloromethyl ether (53%), and carbene dimer (8%). The isobutyl chloride fragmentation product increased from 0.4% in MeCN, to 7.3% with pyridine, to 32% with Bu4NCl, and to 38% with both addends. With 2, benzyl chloride increased from 83% in MeCN to 91% with added pyridine and Bu4NCl. The increases in chloride displacement products are attributed to bimolecular attacks of chloride ions at the α-carbon atoms of the carbenes, particularly 9 and 10. LFP kinetic studies show that the rate constants for fragmentations of these carbenes increase linearly with the concentration of added Bu4NCl in pyridine−MeCN. Second-order rate constants (k 2) as a function of [Cl-] (M-1 s-1, 24 °C) for the fragmentations are 8.2 × 106 (9), 2.7 × 106 (10), and 2.2 × 106 (2). The decrease in k 2 as R in ROCCl changes from n-butyl to isobutyl to benzyl is in accord with a SN2-like mechanism for the carbene fragmentations in which the benzyl case involves the competitive incursion of SN1-like fragmentation.

The Fragmentation of Alkoxyhalocarbenes

“…29 Strikingly, the yields of primary (unrearranged) chlorides from the fragmentations of 26 and 27 are very sensitive to added chloride ion (as Bu 4 N + Cl -): with 0.5 M chloride in the pyridine-MeCN solvent, 1-chlorobutane from 26 increases to 63% (with 23% 1-butene), while i-butyl chloride from 27 increases to 38% (with 27% isobutene). 29 These results suggest a bimolecular ("S N 2") fragmentation of carbenes 26 and 27 with chloride ion (eq 11). A kinetic test of this idea is illustrated in Figure 1.…”

“…These k 2 values (M -1 s -1 ) are 8.2 × 10 6 , 2.7 × 10 6 , and 2.2 × 10 6 for 26, 27, and PhCH 2 OCCl, respectively. 29 Thus, k 2 is highest for the straight-chain primary n-BuOCCl and lowest for the benzyloxychlorocarbene, where "unimolecular" decomposition via ion pairs (7 or 8) will compete with external chloride attack. 6,29 The bimolecular fragmentation of 26 and 27 extends the analogy between the behavior of alkoxychlorocarbenes and alkyldiazonium ions.…”

“…26 We first examined the fragmentations of benzyloxychlorocarbene, neopentyloxychlorocarbene (24), and (1-adamantyl)methoxychlorocarbene (25). 26 Rate constants appear in Table 1, along with data for n-butoxychlorocarbene (26) and isobutoxychlorocarbene (27), 29 which are discussed below. The k frag ranges in the table reflect alternative analytical methods applied to the kinetic data.…”

Adventures in Reactive Intermediate Chemistry: A Perspective and Retrospective

“…The bridgehead cation/chloride ion pairs collapse to the bridgehead chlorides in DCE, whereas in DCE-MeOH, both bridgehead chlorides and methyl ethers form by return and solvolysis processes . The rates of these reactions are likely comparable to solvent/counterion equilibration of the ion pairs, and carbocation-trapping experiments with trimethoxybenzene ,, lead to estimates of ∼70 ps for the lifetimes of the cations in DCE …”

“…For example, ROCCl where R is primary, e.g., n-butyl, undergoes S N 2 decomposition with added chloride ion. 79 Cases in which R is secondary are particularly interesting because the mechanism of carbene fragmentation is solvent-sensitive: S N i in pentane, but with increasing intervention of ion pairs (S N 1) in polar solvents like acetonitrile. 76b Thus, ROCCl (R = cyclohexyl) decomposes in pentane to chlorocyclohexane and cyclohexene.…”