Ortho-cleavage pathway

Fig. 6.9. The ortho-cleavage pathway of benzene oxidation by dioxygenase enzymes.29...

Figure 3 Ortho-cleavage pathways for catechol and protocatechuic acid.

Figure 17 Stereochemical course of ortho-cleavage pathway enzymes.

The degradation of 2-chlorobenzoic acid by P. cepacia also involves removal of the chlorine substituents in the first step [121,122], but here the aromatic ring undergoes oxidation with elimination of CO,. Catechol, which is the first intermediate of this metaboHc pathway, is further degraded by an ortho-cleavage pathway. A metabolic pathway for the degradation of 3-chlorobenzoic acid by Pseudomonas sp. B13 is shown in Figure 4. Zaitsev and Baskunov [123] reported a similar scheme for the metabolism of 3-chlorobenzoic acid in Acinetobacter calcoaceticus. 

Zeyer, J., A. Wasserfallen, and K. N. Timmis. 1985. Microbial mineralization of ring-substituted anilines through an ortho-cleavage pathway. Appl. Environ. Microbiol. 50 447-453. 

As a result, pathways are available in many microoganisms for processing di-hydroxybenzene derivatives. Such continued breakdown proceeds via enzymatic pathways which open the ring between the two hydroxyl substituents (ortho cleavage) or adjacent to them (meta cleavage). The resulting chain compounds are... 

Example The molecular ion of l,2-bis(trimethylsiloxy)benzene, m/z 254, undergoes methyl loss by Si-C bond cleavage as typically observed for silanes (Fig. 6.48). Rearrangement of the [M-CHb] ion then yields [Si(Me)3], m/z 73 (base peak). This is not an ortho elimination with concomitant H transfer as defined in the strict sense, but the observed reaction is still specific for the orthoisomer. [190,203] In the spectra of the meta- and para-isomers the [Si(Me)3] ion is of lower abundance, the [M-CHb] ion representing the base peak in their spectra. Moreover, the m/z 73 ion is then generated directly from the molecular ion which is clearly different from the two-step pathway of the ortho-isomer. 

The 1,3-silyl shift in aryl disilanes is suppressed when the aromatic ring is ortho-substituted144. An attempted silylene synthesis from 1,3-dimesitylhexamethyltrisilane 259, however, led to low yields of silylene trapping products (ca 30% generation of Me2S ). The major pathway is the homolytic cleavage of the trisilane, followed by disproportionation of the radicals 260 and 261 to the silene 262 and the disilane 263 (equation 65). 

In 2001, Oi et al. [54] reported on the ruthenium(II) phosphine catalyzed re-gioselective arylation of 2-arylpyridines using aryl halides (Eq. 29). C-C bond formation occurs predominantly at the position ortho to the pyridyl group. The same catalyst system is also effective for the arylation of aromatic imines (Eq. 30) [55]. Although the reaction mechanism has not been elucidated, it was proposed that a tetravalent arylruthenium complex,for example,Ru(Ph)(Br)(Cl)2(I) ,reacts electrophilically with the arylimines. Therefore, C-H bond cleavage is believed to proceed via an electrophilic substitution pathway. 

What happens if both a meta- and a para- (or ortho-) hydroxyl group are present at the same aromatic nucleus, such as in the catechols and the hydroquinones The two reaction pathways are followed in competition, albeit with a slight preference for the benzylic cleavage. For example, the radical cations of the two isomeric long-chain catechols m,p-2S and o,m-28 give nearly the same El mass spectra with a ratio [C7H802 +]/[C7H702+]... 

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