Oxidative dimerization general reaction scheme

Asymmetric C-H insertion using chiral rhodium catalysts has proven rather elusive (Scheme 17.30). Dimeric complexes derived from functionalized amino acids 90 and 91 efficiently promote oxidative cychzation of suifamate 88, but the resulting asymmetric induction is modest at best ( 50% ee with 90). Reactions conducted using Doyle s asymmetric carboxamide systems 92 and 93 give disappointing product yields ( 5-10%) and negligible enantiomeric excesses. In general, the electron-rich carboxamide rhodium dimers are poor catalysts for C-H amination. Low turnover numbers with these systems are ascribed to catalyst oxidation under the reaction conditions. 

It is generally agreed that alkenyl hydroperoxides are primary products in the liquid-phase oxidation of olefins. Kamneva and Panfilova (8) believe the dimeric and trimeric dialkyl peroxides they obtained from the oxidation of cyclohexene at 35° to 40° to be secondary products resulting from cyclohexene hydroperoxide. But Van Sickle and co-workers (20) report that, The abstraction/addition ratio is nearly independent of temperature in oxidation of isobutylene and cycloheptene and of solvent changes in oxidations of cyclopentene, tetramethylethylene, and cyclooctene. They interpret these results to support a branching mechanism which gives rise to alkenyl hydroperoxide and polymeric dialkyl peroxide, both as primary oxidation products. This interpretation has been well accepted (7, 13). Brill s (4) and our results show that acyclic alkenyl hydroperoxides decompose extensively at temperatures above 100°C. to complicate the reaction kinetics and mechanistic interpretations. A simplified reaction scheme is outlined below. 

Phosphole oxides are generally unstable since they dimerize rapidly via Diels-Alder reactions. Sterically demanding aryl substituents at the P-atom can provide some stabilization of the phosphole oxide, but not enough to allow for their isolation <1996JOM7801, 1997JOM109>. Note that such sterically hindered phosphole oxides can also be trapped by A-phenylmaleimide to give [4-F2] cycloadducts (Scheme 23) <1997JOM109>. 

The general mechanism of anodic oxidation and the main products of dimerization are similar for various C-substituted anilines, but detailed steps for a given reactant and the products distribution depend strongly on the nature and position of the substituent as well as on the medium used. It becomes comprehensible when taking into account that substituents exert a strong influence on the acid-base properties of the neutral reactants as well as on the reactivity of the electrogenerated radical cations, and that protona-tion/deprotonation steps participate in reaction schemes. 

The ultimate loss of the HAS activity occurs by destruction of the heterocycle initiated thermally, photochemically, chemically or by high-energy radiation. An intramolecular H-abstraction from the p-carbon atom in thermolysis of 2,2,6,6-tetramethyl-4-oxo-piperidinyl-l-oxyl 133 via a general reaction (Eq. 8) was proposed as a pathway of thermal selfdestruction of the piperidine cycle [25] (Scheme 25). The respective hydroxylamine was isolated in the yield of 66.5%. The biradical intermediate 137 either dimerizes to nitroxide 138 or thermolyses via 139 to a nitrogen-free fragment 140 (phorone) and nitric oxide. 

Tetrathianes. (1) Oxidative dimerization of a,a-disuhstituted alkanedithioic acid dianions (Scheme 38) or 1,1-dithiols (Equation 17) - very limited examples and a-monosuhstituted alkanedithioic acids decompose (2) reductive or pyrolytic dimerization of gi OT-disulfenyl dichlorides (Equation 18) - only malonate-derived examples (3) reaction of a-chloro sulfenyl chlorides with sodium trithiocarhonate (Equation 19) - only malonate-derived examples (4) sodium thiophenoxide-catalyzed reaction of thioketones with elemental sulfur (5) reaction of benzo-furan-3(2//)-one with S2CI2" (6) UV irradiation of a CS2 solution of a diazirine (7) reaction of a 2,2,4-trisubstituted-1,3-dithietane with Oxone (Scheme 54)". Method (4) is the most convenient and general of these. 

Substituted biphenols can be prepared by oxidative dimerization of phenols under various conditions [20] (e.g., with air/oxygen [21] or by the effect of high temperatures) [22]. Because of the formation of several side products, the yields with phenols are generally only modest. Thus, in the reaction with aqueous NaOH, a 46% yield of the solid coupling product was isolated [21]. Only recently, improved results were reported by Jana and Tunge with KjFeJCNJg as a catalyst (Scheme 2.62) [23]. 

Cycloadditions involving nitrile oxides have received only scant attention in the literature yet this year has seen exploration of the generalized reaction leading to isoxazolines, and its use during the synthesis of a natural product. The method affords high yields under mild conditions and is stereospecific (Scheme 51). A serious limitation to the use of simple aliphatic nitrile-oxides would appear to be their facile dimerization to l,2,5-oxadiazole-2-oxides. 

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