Carbenes as substrates: Bimolecular fragmentation of alkoxychlorocarbenes

Fragmentation reactions of n-butoxychlorocarbene (9), isobutoxychlorocarben e (10), and benzyloxychlorocarbene (2) were studied by product analysis and by loser flash photolysis (LFP). The carbenes were generated photochemical ly from 3-alkyl-3-chlorodiazirines in (1) MeCN solution; (2) 5.77 hi pyridi ne 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-capt ure product, n-butyl dichloromethyl ether (45.1%), carbene dimer (9.8%), an d 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. Wi th added Bu4NCl, 1-butyl chloride increased to 46.4%. With both pyridine an d Bu4NCl, I-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 is obutene (23%), 1- and 2-butene (8-9%), tert-butyl chloride (1.7%), 2-butyl chloride (5.8%), isobutyl chloride (0.4%), isobutyl dichloromethyl ether (5 3%), and carbene dimer (8%). The isobutyl chloride fragmentation product in creased from 0.4% in MeCN, to 7.3% with pyridine, to 32% with Bu4NCl, and t o 38% with both addends. With 2, benzyl chloride increased from 83% in MeCN to 91% with added pyridine and Bu4NCl. The increases in chloride displacem ent products are attributed to bimolecular attacks of chloride ions at the a-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-or der rate constants (k(2)) as a function of [Cl-] (M-1 s(-1), 24 degrees C) for the fragmentations are 8.2 x 10(6) (9), 2.7 x 10(6) (10), and 2.2 x 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 fragmen tations in which the benzyl case involves the competitive incursion of S(N) 1-like fragmentation.