Diols, dehydrogenative lactonization

Dehydrogenation of 1,4- or 1,5-diols to lactones. This Ru-catalyzed transfer dehydrogenation can be effected with high selectivity when the 2-position of 1 is... 

Because the conversion of primary diols into lactones is accomplished not only by oxidizing agents but also by catalytic dehydrogenation, it is likely that the reaction takes place stepwise via hemiacetal intermediates (equation 294) [355]. 

Zhao J, Hartwig JF. (2005). Acceptorless, neat, ruthenium-catalyzed dehydrogenative cycliza-tion of diols to lactones. Organometallics, 24, 2441-2446. 

Dehydrogenative lactonization of diols is an efficient way to various lactones (Scheme 22). ° The lactone formation is found to be catalyzed by a recoverable stable dicationic iridium complex with 6,6 -dihydroxy-2,2 -bipyridine ligands, and employs a variety of benzylic and aliphatic diols in aqueous media. In comparison with the esterification of hydroxyl acids, hydroacyloxylation of olefmic acids and Baeyer-ViUiger reaction of cyclic ketones, the dehydrogenative lactonization of diols proceeds without any oxidant hence, it is more environmentally benign and atom economical. 

Fujita K, Ito W, Yamaguchi R. Dehydrogenative lactonization of diols in aqueous media catalyzed by a water-soluble iridium complex bearing a functional bipyridine ligand. ChemCatChem. 2014 6 109-112. 

A similar conversion of a bis-primary diol into a lactone is achieved by biochemical dehydrogenation (equation 295) [1035]. 

Triphenylsilyl ethers are typically prepared by the reaction of the alcohol with triphenylsilyl chloride (mp 92-94 °C) and imidazole in DMF at room temperature. The dehydrogenative silylation of alcohols can be accomplished with as little as 2 mol% of the commercial Lewis acid tris(pentaf1uorophenyl)borane and a silane such as triphenylsilane or triethylsilane [Scheme 4.98]. Primary, secondary, tertiary and phenolic hydroxyls participate whereas alkenes, alkynes, alkyl halides, nitro compounds, methyl and benzyl ethers, esters and lactones are inert under the conditions. The stability of ether functions depends on the substrate. Thus, tetrahydrofurans appear to be inert whereas epoxides undergo ring cleavage. 1,2- and 1,3-Diols can also be converted to their silylene counterparts as illustrated by the conversion 983 98.4. Hindered silanes such as tri-... 

A collection of results obtained with the most effective catalyst systems is summarized in Figure 6.4. Noteworthy examples include the oxidation of a-aminoalcohols with no loss in enantiopurity and the oxidation of cis-allyhc alcohols without Z E isomerization. Stahl also demonstrated the chemoselective oxidation of primary diols to form lactones [22]. While ABNO provides efficient oxidation of symmetric diols, TEMPO discriminates between subtle steric differences in nonsymmetrical substituted diols. Cu/nitroxyl catalysts have also been applied to a variety of tandem reactions [23], perhaps the most noteworthy of which is the conversion of primary alcohols to nitriles via in situ condensation of ammonia with the aldehyde and subsequent dehydrogenation of the primary imine to the nitrile (Figure 6.5) [24]. 

RuH2(PPh3)4 is used as a dehydrogenation type oxidation catalyst for alcohols and aldehydes [66a,66b]. For example, the esterification of alcohol, lactone formation from diol, N-alkylation of amine with alcohol, and the condensation of aldehyde with alcohol. 

Second, 1,4-diols undergo intermolecular hydrogen transfer, giving lactones efficiently in acetone containing Cp Ru(P-N) catalysts the TOF of this reaction at 30 C exceeds 1,000 h (Fig. 21) [67]. The catalytic oxidative lactonization of diols is characterized by its unique chemo- and regioselectivity. The significant rate difference between primary and secondary alcohols in dehydrogenation, and the rate difference between 1,4-diols and 1,5- or 1,6-diols enable unique oxidative lactonization of triols. 

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