Acyl-oxygen bond

The more usual mechanism of the anionic ROP of lactones relies on the addition of alkoxides onto the ester group followed by the cleavage of the acyl-oxygen bond (Fig. 5). 

Fig. 5 Mechanism of anionic ROP based on acyl-oxygen bond cleavage...

Until the middle of the 1980s, it was accepted that the mechanism for the cationic ROP initiated by alkylating agents was based on the reaction of the cation with the endocyclic oxygen, followed by cleavage of the acyl-oxygen bond (Fig. 16). 

For coordination initiators, the metal coordinates with the carhonyl oxygen followed hy insertion of an alkoxy (or other anionic fragment depending on the initiator) into the acyl-oxygen bond. 

Since there are but two possibilities, acceptable evidence may be pos tive or negative. It is sufficient, for example, in order to establish acyl-oxygen fission, to show either that the acyl-oxygen bond is cleaved, or that the alkyl-oxygen bond is not, and both types of evidence have been used. Some of the evidence, for reactions involving alkyl-oxygen fission, has already been discussed in the sections dealing with the AA) 1 reaction (p. 87). 

Positive evidence that the acyl-oxygen bond is broken on ester hydrolysis and formation is available from experiments using H2,80. Hydrolysis in the enriched solvent results in a net incorporation of, sO into the carboxylic acid produced if acyl-oxygen cleavage occurs, viz-... 

Reactions involving acyl-oxygen bond formation... 

In (blactones, scission of either the acyl-oxygen bond or the alkyl-oxygen bond may take place leading to the formation of alkoxide- or carboxylate-grow-ing chains [ 115]. Methylene chloride end groups were observed in the ZnCl2-in-... 

Scheme 3. The reaction pathway for the ROP of a cyclic ester by anionic initiation. Ringopening of monomer by 1) acyl-oxygen bond cleavage and 2) alkyl-oxygen bond cleavage...

Recently, tin(II) butoxide was used in the polymerization of l-LA [88]. The initiation is fast and quantitative and no transesterification or back-biting reactions are observed. The reaction proceeds with acyl-oxygen bond scission with retention of the configuration, and can be used both in bulk and solution (THF, 20-80 C) polymerization. It is possible to control the molecular weight in the range of 103 to 106 with a MWD of 1.15-1.85. The polymerization is very fast, kp=5 x 10 1 mol-1 L s-1, with only the rare earth alkoxides being faster. 

Cleavage of alkyl—oxygen bond Cleavage of acyl—oxygen bond... 

The detailed study of the mechanism of polymerization initiated with various alkylating agents showed that, in the polymerization of 4-, 6-, and 7-membered lactones, reaction a, i.e., attack on endocyclic oxygen with following opening of acyl-oxygen bond, proceeds exclusively [184-187]. 

That a single solvent molecule clustered to a nucleophile can drastically change the reaction pathway has been demonstrated by studying the reaction of phenyl acetate with methoxide ion in the gas phase [671, 672]. Alkaline hydrolysis of esters in solution is known to proceed by attack of the nucleophile at the carbonyl carbon atom to form a tetrahedral intermediate, followed by cleavage of the acyl-oxygen bond (Bac2 mechanism) cf. Eq. (5-138a). 

Hydrolysis with H2 0 ArSOjH + R OH Cleavage of alkyl—oxygen bond R CO OH + ROH Cleavage of acyl—oxygen bond... 

Diethylaluminium dimethylamide and ethanethiolate, Et2AlX (X = NMe2 or SEt), react with diketen, via acyl-oxygen bond cleavage and a 1,3-hydrogen shift, to give the chelates (67 X = NMe2 or SEt).372... 

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