Catechol esters

Figure 3.20. Scope of Rh/(5)-binap-catalyzed asymmetric 1,4-addition of alkenylboronic acid catechol esters to a,P-enones.

The catechol ester of 1-octenylboronic acid can be isolated by distillation, bp 112-120 °C/0.2 mmHg. 

A related compound, piperidinomethaneboronic acid, has been prepared and then was converted to its catecholate ester and methylated with methyliodide to give the betaine shown below 104>. 

The dimethylaminomethaneboronic acid was prepared as its catecholate ester but was not methylated 104h... 

The exceptional reactivity of N-hydroxysuccimide and 1-hydroxybenzotria-zole esters is due to the effect of the neighboring N-atom. Similar kind of activation can be recognized in esters of 2-hydroxypyridine and 2-mercaptopy-ridine. A close-by oxygen atom, such as in catechol esters... 

Chan and co-workers have demonstrated tiiat boronic esters can be used in place of boronic acids in both O- and N-arylations in the parent phenyl case (Scheme 5.26) [10]. In botii arylations, boronic esters 24-27 are even more efficient than the parent acid. However, catechol ester 28 and pinacolate 29 were less efficient, perhaps due to instability (possibility of catechol O-arylation) and steric hindrance respectively. The overall superior performance of the triphenylboroxine (30) is noteworthy since it is... 

Bromination of dibutyl vinylboronate at low temperature readily affords a stable di-bromo derivative [2, 3]. Similarly, ( )-alkenylboronic acids or their catechol esters yield the corresponding substituted compound by stereospecific trans addition of bromine. Treatment with base induced anti elimination of the boronic group and the bromine ion. The final result of this sequence was the replacement of boron by bromine with inversion of olefin geometry in excellent yields and 99% stereochemical purities (Scheme 9.1) [4]. 

Most of the examples of this class of reaction are found with catechol esters or with esters and amides of salicylic acids. Examples of intramolecular nucleophilic catalysis are also known [19] but these fall outside the scope of this chapter. 

Although catechol esters are not especially reactive towards imidazole they are reactive towards primary and secondary amines and this has led to their use in peptide synthesis. A mechanism involving intramolecular general-base catalysis as symbolized by 22 seems most likely [27]. 

The rates of the second-order reactions of 8-acetoxyquinoline with primary and secondary amines are enhanced [63-65] but by much smaller factors (1.5 to 10) and these reactions may also involve intramolecular catalysis. 8-Quinolyl esters, like catechol esters, have been used in peptide synthesis [63-65]. 

---