Aryl dioxygenases

Table I. Aryl dioxygenase expressing strains used to study the dioxygenation of aryl silanes.

Sulfoxidations are not restricted to MOs but can also be carried out by dioxygenases. For example. Pseudomonas mutant strain UV4 producing a toluene dioxygenase (TOO) and Pseudomonas NCIMB 8859 expressing a naphthalene dioxygenase (NDO) were used to oxidize aryl sulfides to antipodal chiral sulfoxides [203]. 

In contrast to 2,3-dioxygenases, the related ipso/ortho oxygenation of aryl carbox-ylates has received considerable less attention and has hardly been utilized by the synthetic community, so far. Biooxidation of benzoic acid and P-naphthalene carboxylate provide access to corresponding 1,2-dihydroxylated dihydroaryl compounds in excellent stereoselectivity (Scheme 9.35), analogous to TDO- and NDO-mediated ortho/meta oxygenations. Whole-cell-mediated biotransformations were performed with mutant strains of Rahtonia and Pseudomonas and enable access to preparative quantities in >5 gl titers [261,262]. 

After the first discovery of the asymmetric sulfoxidation by Kobayashi et al. [226], it could be shown that a large number of aryl alkyl sulfides are oxygenated with enantiomeric excesses higher than 98% [227-229]. Other peroxidases also catalyze this reaction. Interestingly, the plant peroxidase HRP [230] yields the (S)-sulfoxide, whereas mammalian myeloperoxidase [223] and lactoperoxidase [231] catalyze the formation of the R-enantiomers. The stereospecific sulfoxidation of aryl alkyl sulfides by purified toluene dioxygenase (TDO) from P. putida was also studied in this context [232] and showed that sulfoxidation yielded the (S)-sulfoxides in 60-70% yield, whereas CPO under the same conditions yielded 98% (R)-sulfoxides (Scheme 2.15). CPO is thus again an exception from the rule in that it produces R-enantiomeric sulfoxides, besides its bacterial origin [227]. The reason for this behavior lies in the... 

Various other heme-peroxidases were found to catalyze the enantioselective sulfoxidation of alkyl aryl sulfides. These included horseradish peroxidase (HRP),34,35 cytochrome c peroxidase (CcP),36 microsome peroxidase (MP),37 lac-toperoxidase (LPO),38 and dioxygenase.39 However, their turnover numbers (TON) and enantioselectivities were much lower than those observed with CPO (Table 4). 

Aryl-Hydroxylation, This is occasionally observed as the initial transformation of aromatic pesticides. The vast majority of aromatic pesticide degradation products are susceptible to aryl-hydroxylation, representing either cometaboHsm or the initial step in mineralization (17). Numerous genera of bacteria and fungi possess the monooxygenases and dioxygenases responsible for hydroxylation of aromatic products. Examples of aromatic products susceptible to aryl-hydroxylation include 2,4-dichlorophenol [120-83-2] (from 2,4-D) (eq. 3), 4-nitrophenol (from parathion) (eq. 4), 3,4-dichloroaniHne [95-76-1] (from propanil), and 3,6-dichlorosaHcyHc acid [3401-80-7] (from dicamba). 

Nishinaga further demonstrated that Con(salpr) [salpr = bis(3-salicylidineamino-propyl)amine] effected the oxygenation of 4-aryl-2,6-di-tert-butylphenols to yield a-ketohydroperoxycobalt(III) complexes 66Sh The free hydroperoxides were obtained by treatment with silica gel. These hydroperoxides are analogous to the peroxide intermediate proposed in the dioxygenase mechanism 33K... 

It is now well-established that some enzyme families, including various peroxidases and laccases, catalyze the polymerization of vinyl monomers and other redox active species such as phenol-type structures. Vinyl polymerization by these redox catalysts has recently been reviewed 93). These catalysts have been used to prepare polyanilines 94) and polyphenols 95,96). A few examples of related research are included in this book. For example. Smith et al (57) described a novel reaction catalyzed by horseradish peroxidase (HRP). In the presence of HRP and oxygen, D-glucuronic acid was polymerized to a high molecular weight (60,000) polyether. However, the authors have not yet illucidated the polyether structure. Two other oxidative biotransformations were discussed above i) the sono-enzymatic polymerization of catechol via laccase 31), and ii) the oxidation of aryl silanes via aromatic dioxygenases 30). 

We sought to examine the enzymatic dioxygenation of aryl silanes using a number of different aromatic dioxygenases in order to determine if such transformations were possible and to define the substrate-specificity profile. We were also motivated by the rich chemistry of silicon-based materials, which includes the hydrosilylation of alkenes and ketones, the addition of electrophiles to vinyl and allyl silanes, and palladium catalyzed cross-coupling of vinyl silanes with aryl halides (13). As a result, silyl functional cw-diols have potential as chiral intermediates for drug development, as polymer precursors/modifiers and as elements in non-linear optical materials. 

Table II. Successful conversions of aryl silanes to c -dihydrodiols using toluene dioxygenase (TDO). 

The dioxygenation was performed by the addition of the neat silanes to a 14L fermentor containing E. coli JM109 (pDTG601) expressing toluene dioxygenase. The tolerance of the cells to possible toxic effects of the aryl silanes was assessed by online monitoring of the respiratory quotient (RQ), an... 

The aryl silane library was also screened for their ability to function as the sole carbon source for Ralstonia eutropha A5, a wild type strain expressing a biphenyl dioxygenase (BPO) enzyme. A number of silanes were observed to support growth, including diphenylsilanes and trialkoxysilanes. Overall the study indicated the feasibility of the enzymatic conversion of arylsilanes to a novel series of silane cw-dihydrodiols and catechols. Such compounds may find application as chiral polymer precursors, intermediates for natural product synthesis and other usefol materials. 

Figure 1, DHS as a chemical building block. Boxed structures are of current industrial value. Catalysts (a) DHS dehydratase (aroZ) (b) reflux (c) catechol-O-methyltransferase (comt) (d) aryl-aldef de dehydrogenase (e) PCA decarboxylase (aroY) (f) catechol 1,2-dioxygenase (catA) (g) Pt/C, H2 (h) p-hydroxybenzoate hydroxylase (pobA ) (i)Cu(OAc) 2, HOAc.

Figure 4. The known anionic elimination of substituents like X=C1, SO3, O-Alkyl or 0-Aryl through (a) monooxygenases or (b) dioxygenases give rise to the release of nitrite in the case of nitroaromatics (X=N02). Hydrogen or COO in the or /io-position were eliminated as cations or CO2.

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