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 (Bu 4 NCl) in MeCN; or (4) 5.77 M pyridine + 0.504 M Bu 4 NCl 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 Bu 4 NCl, 1-butyl chloride increased to 46.4%. With both pyridine and Bu 4 NCl, 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 Bu 4 NCl, and to 38% with both addends. With 2, benzyl chloride increased from 83% in MeCN to 91% with added pyridine and Bu 4 NCl. 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 Bu 4 NCl 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 × 10 6 ( 9 ), 2.7 × 10 6 ( 10 ), and 2.2 × 10 6 ( 2 ). The decrease in k 2 as R in ROCCl changes from n -butyl to isobutyl to benzyl is in accord with a S N 2-like mechanism for the carbene fragmentations in which the benzyl case involves the competitive incursion of S N 1-like fragmentation.