Carbonyl compounds, transfer hydrogenation

A biphasic system consisting of the ionic liquid [BMIM]PF6 and water was used for the epoxidation reactions of a, 3-unsaturated carbonyl compounds with hydrogen peroxide as an oxidant at room temperature 202). This biphasic catalytic system compared favorably with the traditional phase transfer catalysts. For example, under similar conditions (15°C and a substrate/NaOH ratio of five), the [BMIM]PF6/H20 biphasic system showed a mesityl oxide conversion of 100% with 98% selectivity to oc, 3-epoxyketone, whereas the phase-transfer catalyst with tet-rabutylammonium bromide in a CH2CI2/H2O biphasic system gave a conversion of only 5% with 85% selectivity. 

Table 3.5 Oxidation of alcohols to the corresponding carbonyl compounds with hydrogen peroxide catalysed by Mo(VI) or W(VI) complexes under phase-transfer conditions at 70°C ...

The reaction between the photoexcited carbonyl compound and an amine occurs with substantially greater facility than that with most other hydrogen donors. The rate constants for triplet quenching by amines show little dependence on the amine a-C-H bond strength. However, the ability of the amine to release an electron is important.- - This is in keeping with a mechanism of radical generation which involves initial electron (or charge) transfer from the amine to the photoexcited carbonyl compound. Loss of a proton from the resultant complex (exciplex) results in an a-aminoalkyl radical which initiates polymerization. The... 

The transfer hydrogenations of carbonyl compounds to alcohols catalysed by a variety of NHC complexes have been intensively studied. The strong bond... 

Scheme 2.4 Experimental evidence in support of the mechanism for the base-free transfer hydrogenation of carbonyl compounds catalysed by complex 43...

The catalytic hydrosi(ly)lations of other C=X functional groups (X = O, NR) constitute alternative routes to the reduction of aldehydes, ketones, imines and other carbonyl compounds (Scheme 2.9), circumventing the use of molecular hydrogen or occasionally harsh transfer hydrogenation conditions. 

Up to about 10 percent of crs-stilbene was obtained when trimethyl-dioxetane 129 was decomposed in the presence of trans-stilbene 182) the electronic excitation energy of the excited carbonyl compounds formed in the cleavage of 129 (see Section V.) was transferred to trans-stilbene, so effecting the photochemical trans-cis isomerization. When bis (2.4-dinitrophenyl) oxalate reacted with hydrogen peroxide (see Section V. C. in the presence of o-tolyl-propane-1.2-dione 130, 2-methyl-2-... 

Transfer hydrogenations of carbonyl compounds are often conducted using 2-propanol as the hydrogen donor. One advantage of this compound is that it can be used simultaneously as a solvent. A large excess of the hydrogen donor shifts the redox equilibrium towards the desired product (see also Section 20.3.1). 

Scheme 20.4 Possible pathways of hydrogen transfer during the racemizations of alcohols using the corresponding carbonyl compound and a hydrogen transfer catalyst.

The (3-metaloxy radical was first exploited for synthetic purposes in C—H and C—C bond-forming reactions by Nugent and RajanBabu through the use of titanocene(III) chloride as an electron-transfer reagent [5]. They established that the (3-titaniumoxy radicals formed after electron transfer can be reduced by hydrogen atom donors, e. g. 1,4-cy-clohexadiene or tert-butyl thiol, that they add to a,(3-unsaturated carbonyl compounds, and that they can react intramolecularly with olefins in 5-exo cyclizations. 

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