α-Nitration of Ketones via Enol Silyl Ethers. Radical Cations as Reactive Intermediates in Thermal and Photochemical Processes

Abstract

Highly colored (red) solutions of various enol silyl ethers and tetranitromethane (TNM) are readily bleached to afford good yields of alpha-nitro ketones in the dark at room temperature or below. Spectral analysis show the red colors to be associated with the intermolecular 1:1 electron donor-acceptor (EDA) complexes between the enol silyl ether and TNM. The formation of similar vividly colored EDA complexes with other electron acceptors (such as chloranil, tetracyanobenzene, tetracyanoquinodimethane, etc.) readily establish enol silyl ethers to be excellent electron donors. The deliberate irradiation of the diagnostic (red) charge-transfer absorption bands of the EDA complexes of enol silyl ethers and TNM at -40 degrees C affords directly the same alpha-nitro ketones, under conditions in which the thermal reaction is too slow to compete. A common pathway is discussed in which the electron transfer from the enol silyl ether (ESE) to TNM results in the radical ion triad [ESE(*)(+), NO(2)(*), C(NO(2))(3)(-)]. A subsequent fast homolytic coupling of the cation radical of the enol silyl ether with NO(2)(*)() leads to the alpha-nitro ketones. The use of time-resolved spectroscopy and the disparate behavior of the isomeric enol silyl ethers of alpha- and beta-tetralones as well as of 2-methylcyclohexanone strongly support cation radicals (ESE(*)(+)) as the critical intermediate in thermal and photoinduced electron-transfer as described in Schemes 1 and 2, respectively.

Intimate contact of enolic species with silver sites benefits the SCR of NOx by ethanol over Ag/Al2O3

“…In organic synthesis, enols, and/or enolate anions were common intermediates in nitrosation reaction between aldehydes/ketones and the substrates containing NO group such as HONO, NO + , NO À , neutral NO, C(NO 2 ) 4 , and ½FeðCNÞ 5 NO À 2 to form the initial product of nitroso and nitro compounds [69][70][71]. This mechanism permitted us to hypothesize that similar reactions would occur during exposure of enolic species to NO on Ag/Al 2 O 3 , resulting in the formation of organo-nitroso compound.…”

Reaction of 1,1′‐Divinyl Ferrocene with One‐Electron Oxidants: Entry into Functionalised [4]Ferrocenophanes and Observation of an Isotope‐Dependent Chemoselectivity Effect

“…Although this result supported the intermediacy of an enolA C H T U N G T R E N N U N G (ate) en route to keto-alcohol 4, in an effort to obtain more conclusive evidence we converted the keto functionality in 17 into the corresponding silyl enol ether. [25] In this way, although hydrolysis of the TMS enol ether would likely compete under our reaction conditions, we would maximise the chances of forming significant quantities of keto-alcohol 4 by guaranteeing the presence of stoichiometric quantities of an enol functionality at the outset of the reaction, were oxidation of such an intermediate indeed a step along the reaction pathway. As expected, TMS enol ether 18, prepared by treatment of ketone 17 with Me 3 SiCl in the presence of NaI and Et 3 N, [25] proved to be hydrolytically labile and was, therefore, used directly without purification (see the Experimental Section).…”

“…[25] In this way, although hydrolysis of the TMS enol ether would likely compete under our reaction conditions, we would maximise the chances of forming significant quantities of keto-alcohol 4 by guaranteeing the presence of stoichiometric quantities of an enol functionality at the outset of the reaction, were oxidation of such an intermediate indeed a step along the reaction pathway. As expected, TMS enol ether 18, prepared by treatment of ketone 17 with Me 3 SiCl in the presence of NaI and Et 3 N, [25] proved to be hydrolytically labile and was, therefore, used directly without purification (see the Experimental Section). Thus addition of a prepared mixture of K 2 CO 3 (6 equiv) and K 3 [Fe(CN) 6 ] (6 equiv) in a 1:1 tBuOH/H 2 O solvent mixture to a solution of 18 led to rapid (5 min) consumption of starting material and the formation of keto-alcohol 4 along with hydrolysis product 17 (Scheme 9).…”

Synthetic Procedures for the Preparation of Nitroalkanes

“…Successively, a very efficient procedure was developed from acyclic silyl enol ethers 92 (derived from the corresponding ketones) by treatment with tetranitromethane (TNM) as mild nitrating agent (Scheme ) …”