π‐Participation in Diazoketone Hydrolysis II; Exo‐endo Cyclization Ratio in the Hydrolyses of 7‐syn‐ and 5‐endo‐Diazoacetyl‐2‐norbornene

Abstract

SummaryThe rate of the acid-catalysed hydrolysis of 7-syn-diazoacetyl-2-norbornene (la) is enhanced relative to that of the saturated analogue 5a by a factor of 835. In contrast to the behaviour of other primary diazoketones, the substitution step is no longer rate-determining (mechanism A -2), but so much accelerated that the preceding proton transfer becomes the slow step (mechanism A-SE2, demonstrated by a solvent isotope effect kH/kD = 1.76). Product analysis shows 100% cyclization; the product formation is explained in terms of brexyl and brendyl type carbenium ions (or ion pairs). -5-endo-Diazoacetyl-2-norbornene (3a) shows very slight acceleration, and forms only 27% cyclization products (identical to those formed from la). Thus, in spite of the absence of steric hindrance by hydrogen atoms, the exo-endo rate ratio for anchimeric assistance is 3 lo3.Acid-catalysed hydrolysis of primary diazoketones follows the A -2 mechanism, rapid preequilibrium protonation forming a diazonium ion, followed by ratedetermining substitution [2] by a nucleophile. As the diazonium group is very sensitive, a weak nucleophile, for instance H 2 0 [3], is sufficient.H' , rapid R-CO-CHN, FR-CO-CHZNZ H20 ,R-CO-CH,OH+ N2+ H+ rate-det.A double bond in 5,6-position relative to the diazomethyl group can compete with a weak external nucleophile and form cyclic products [l] [4] [5]. The neighbouring group participation can furthermore result in a rate enhancement; we found this particularly with methylated cyclic olefins: 4-diazoacetyl-1,2-dimethylcyclopent-1-ene is hydrolysed about ten times more rapidly than diazoacetylcyclopentane [5]. We now present the results of a study of the hydrolyses of compounds in which the diazoacetyl group is kept in a rigid framework; 7-syndiazoacetyl-2-norbornene (la) and its anti isomer (2a), 5-endo-diazoacetyl-2-norbornene (3a) and its exo isomer (4a) have been compared with the saturated analogues 7-diazoacetyl-and 2-endo-diazoacetyl-norbornanes (5 a, 6 a).

Investigation of the Transition-Metal- and Acid-Catalyzed Reactions of β-(N-Tosyl)amino Diazo Carbonyl Compounds

“…A possible pathway to account for the 1,2-phenyl migration is presented in Scheme . The initial protonation occurs on the negatively polarized diazo carbon . The aryl migration to the α-carbon may proceed with simultaneous extrusion of nitrogen gas to generate a cationic intermediate, 32 , which is deprotonated to give 1,2-phenyl migration product 11a .…”