Carbon-oxygen bond cleavage reactions

Carbon-oxygen bond cleavages are particularly attractive in relation to development of environmentally benign halogen-free processes and are frequently 

Activation of Substrates with Polar Single Bonds 

Similar 1,3-acetato shift reaction had also been known by Pd(OAc)2/PPh3 system as shown in Eq. 3.6 [39], 

The first example of C-0 bond oxidative addition has been observed by use of a coordinatively unsaturated palladium(O) complex having basic bulky phosphine ligands, Pd(PCy3)2 at room temperature to give Pd(j7 -allyl)(PCy3)(OAc) accompanied by formation of aphosphonium salt, Cy3P(CH=CHMe)+(OAc) (Eq. 3.7) 

Stereochemistry of the C-0 bond cleavage has been initially elucidated indirectly by the following experiments. Catalytic C-C bond formation reaction of (Z)-3-acetoxy-5-carbomethoxycyclohexene with sodium dimethyl malonate 

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Okada, K., Hasegawa, E., and Mukai, T., Photosensitized carbon-oxygen bond cleavage reaction of epoxides by 2,4,6-triphenylpyryHum tetrafluoroborate salt, Chem. Lett., 305,1983. 

Use has been made of the bond cleavage processes initiated by an adjacent carbonyl function for the modification of steroidial ketols such as 18 [97], Reduction in ethanol eliminates the hydroxyl function and in the same reaction, the carbonyl function is reduced to a secondary alcohol. In compound 19 where there are several groups to act as electrophores, carbon-oxygen bond cleavage is initiated from the most easily reduced dienone function [98], Cleavage of the carbon-oxygen bond in an a-acetoxycarbonyl function is achievable in good yields from multifunctional compounds such as the sesquiterpene taxol [99]. 

Many classes of perfluoroethers such as acyclic [63], cyclic [64, 65], glymes [66,67], highly branched [63],macrocyclic [68-73], orthoformates [74-76] and poly-ethers [15, 77], have been succesfully prepared in high yield by direct fluorination techniques (Figs. 19 and 20). Carbon-oxygen bond cleavage is minimised in these processes by the addition of an HF scavenger, such as sodium fluoride, to the reaction mixture [14,45,46]. 

The amyl alcohols undergo the typical reactions of alcohols which are characterized by cleavage at either the oxygen—hydrogen or carbon—oxygen bonds. Important reactions include dehydration, eslenficalion, oxidation, animation, etherification, and condensation. 

An analogous reaction occurred with oxiranes which underwent mainly carbon-oxygen bond cleavage, with formation of a-hydroxyketones. In some instances byproducts arising from carbon-carbon bond cleavage were also formed in triphenyl-oxirane this mode dominated and the sole product was the hydroxyketone Ph2C(OH)COPh (80%) [54],... 

The replacement of chlorine atoms of N3P3C16 by fluoroalkoxy groups proceeds nongeminally (21). Phosphorus-oxygen and carbon-oxygen bond cleavage have been observed in the reactions of N3P3(OAr)6 with several nucleophiles (3). 

It is postulated that while iron catalysts may facilitate carbon-oxygen bond cleavage, as evidenced by the Increased reactivity of benzyl and aliphatic ethers, when phenoxy radicals are produced they are strongly adsorbed on the catalyst surface. They are not therefore readily available to propagate the radical chain reactions. 

Another type of activation of the hydroxy group of alcohols involves the introduction of functional groups in order to facilitate the carbon-oxygen bond cleavage in the subsequent nucleophilic displacement reactions, i.e. conversion of an alcohol into a reactive alkylating reagent. 

Cyclic alkyl aryl ethers lead also to functionalized organolithium compounds by reductive carbon-oxygen bond cleavage in arene-catalyzed lithiation process. Thus, the treatment of 2,3-dihydrobenzofuran (47) with an excess of lithium in the presence of a catalytic amount of DTBB in THF at 0°C gives the dianion (48) which after reaction with different carbonyl compounds and final hydrolysis with water leads to... 

As noted earlier (Section 4.10) primary carbocations are too high in energy to be intermediates in most chemical reactions. If primary alcohols don t form primary carbocations, then how do they undergo elimination A modification of our general mechanism for alcohol dehydration offers a reasonable explanation. For primary alcohols it is believed that a proton is lost from the alkyloxonium ion in the same step in which carbon-oxygen bond cleavage takes place. For example, the rate-determining step in the sulfuric acid-catalyzed dehydration of ethanol may be represented as ... 

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