Ruthenium-Catalyzed Oxidations with

The regeneration of the benzoquinone can also be achieved with dioxygen in the absence of the cobalt cocatalyst. Thus, Ishii and coworkers [41] showed that a combination of RuCl2(Ph3P)3, hydroquinone, and dioxygen, in PhCp3 as solvent, oxidized primary aliphatic, allylic, and benzylic alcohols to the corresponding aldehydes in quantitative yields (Eq. (5.2)). 

RuCl2(Ph3P)3 (10mol%) hydroquinone (10 mol%) K2CO3, PhCP3 

According to Ref. [40] (2002). Reaction conditions 1 mmol substrate, 1 mLtoluene, 100 °C, 1 atm air employing 0.5 mol% [(C4Pli4COHOCC4Pli4)(p-H)(CO)4Ru2], 20mol% 2,6-dimethoxy-l,4-benzoquinone, and 2 mol% bis(salicylideniminato-3-propyl)methylamino-cobalt(II) as catalysts. 

The results obtained in the oxidation of representative primary and secondary aliphatic alcohols and allylic and benzylic alcohols using this system are shown in Table 5.3. 

Although, in separate experiments, secondary alcohols are oxidized faster than primary ones, in competition experiments the Ru/TEMPO system displayed a preference for primary over secondary alcohols. This can be explained by assuming that initial complex formation between the alcohol and the ruthenium precedes the rate-limiting hydrogen transfer and determines substrate specificity that is, complex formation with a primary, not a secondary, alcohol is favored. 

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The conventional synthesis of trans-2,5-dialkyl phospholanes starting from a chiral 1,4-diol is shown in Scheme 24.1. Originally, these 1,4-diols were obtained via electrochemical Kolbe coupling of single enantiomer a-hydroxy adds [25], but this method proved to be commercially impracticable and has since been replaced by more viable biocatalytic routes [26]. Reaction of the chiral 1,4-diol with thionyl chloride followed by ruthenium-catalyzed oxidation with so-... 

Selective oxidation of alcohols. Primary alcohols are oxidized by this RuCL complex about 50 times as rapidly as secondary alcohols. Use of benzene as solvent is critical lor this high selectivity. Little or no reaction occurs in CH3CN, THF, or DMF. Most oxidants, if they show any selectivity, oxidize secondary alcohols more rapidly than primary ones. However, ruthenium-catalyzed oxidations with N-mcthylmorpholine N-nxide and oxidations with PCC4 proceed about three times as rapidly with primary alcohols as with secondary ones. 

Table 2 Ruthenium catalyzed oxidation with t-BuOOH. Activity of some ruthenium complexes ...

A similar ketene generation from terminal alkynes utilized ruthenium catalyzed oxidation with use of catalyst L3 gave intramolecular cyclobute-none formation (Scheme 4.12). Similar ketene generation in the presence of imines led to (d-lactams (Figure 4.9). 

Oxidative modification of peptides has been performed by ruthenium-catalyzed oxidation with peracetic acid. For example, the reaction of N,C-protected peptides containing glycine residues with peracetic acid in the presence of RUCI3 catalyst gives a-ketoamides derived from the selective oxidation at the C position of the glycine residue (81%, conv. 70%) (Eq. (7.67)) [116]. Direct conversion of N-acylproline to N-acylglutamate was achieved by Ru(TMP)Cl2 and 2,6-dichloropyridine N-oxide (Eq. (7.68)) [117]. 

Aerobic oxidation of P-lactams can be performed highly efficiently in the presence of acetaldehyde, an acid, and sodium acetate [119]. Typically, the RuCls-catalyzed oxidation of P-lactam 54 with molecular oxygen (1 atm) in the presence of acetaldehyde and sodium carboxylate gave the corresponding 4-acyloxy p-lactam 55 in 91% yields (de >99%) (Eq. (7.72)). This aerobic oxidation shows similar reactivity to the ruthenium-catalyzed oxidation with peracetic acid. 

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