Reversible coupling chemistry

As we have outlined above, bridging the gap between the small molecule world and the RNA world (or proto-RNA world) reqiures a means of selecting and coupling base pairs into an oligonucleotide. We propose two complementary solutions to the quandaries of selection and coupling for the synthesis of proto-RNAs, molecular midwives and reversible coupling chemistries, which are elaborated below. 

Thermodynamically Controlled Polymerization via Reversible Coupling Chemistries... 

Fremy s salt, 0N(S03)22 is the last of the stable free radicals in aqueous solution to be described in this review. Quantitative studies of its chemistry are rare, and the reader is referred to a recent paper by Balasubramanian and Gould for details (26). According to these workers the protonated radical has a pK less than 5.6. They cite a potential of —0.350 V for the 0N(S03)2270N(S03)23 couple in alkaline media. This potential was derived from potentiometric titrations of this electrochemically reversible couple by Aoyagui and Kato, who also noted that for H0N(S03)22 the pKt = 12.0 (20). These results have been confirmed in a voltammetric investigation (255). 

The close electrochemical relationship of the simple quinones, (2) and (3), with hydroquinone (1,4-benzenediol) (4) and catechol (1,2-benzenediol) (5), respectively, has proven useful in ways extending beyond their offering an attractive synthetic route. Photographic developers and dye syntheses often involve (4) or its derivatives (10). Biochemists have found much interest in the interaction of mercaptans and amino acids with various compounds related to (3). The reversible redox couple formed in many such examples and the frequendy observed quinonoid chemistry make it difficult to avoid a discussion of the aromatic reduction products of quinones (see Hydroquinone, resorcinol, and catechol). 

The ability of enzymes to achieve the selective esterification of one enantiomer of an alcohol over the other has been exploited by coupling this process with the in situ metal-catalysed racemisation of the unreactive enantiomer. Marr and co-workers have used the rhodium and iridium NHC complexes 44 and 45 to racemise the unreacted enantiomer of substrate 7 [17]. In combination with a lipase enzyme (Novozyme 435), excellent enantioselectivities were obtained in the acetylation of alcohol 7 to give the ester product 43 (Scheme 11.11). A related dynamic kinetic resolution has been reported by Corberdn and Peris [18]. hi their chemistry, the aldehyde 46 is readily racemised and the iridium NHC catalyst 35 catalyses the reversible reduction of aldehyde 46 to give an alcohol which is acylated by an enzyme to give the ester 47 in reasonable enantiomeric excess. 

The chemistry of silver(I) with crown thioethers has developed in the last years,1094,1095 but now numerous examples with a great variety of ligands have been reported. The first homoleptic silver compounds have been described with 1,4,7-trithiacyclononane ([9]aneS3), [Ag([9]aneS3)2]OTf (168), which is octahedral and shows a reversible oxidation assigned to the Ag1/Ag11 couple,1096,1097 and the trimeric species [Ag3([9]aneS3)3]3+ (169).1097 The complexes... 

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