Reversibly bounded directing group

In contrast to the methodologies previously described, the use of a reversibly bounded directing group has the advantage of needing the requisite auxiliary only in catalytic quantities. Moreover, tedious modification/demodification procedures of substrate and product can be avoided. The precondition for the success of this elaborated approach is that three catalytic processes have to operate in a parallel and timed manner. Main contributions in this respect came from the groups of Breit and Tan within a short period of fascinating competition. 

Scheme 4.115 Hydroformylation of 2,5-dicyclohexadienyl-1-carbinols with the assistance of a reversibly bound directing group.

In nature, aminotransferases participate in a number of metabolic pathways [4[. They catalyze the transfer of an amino group originating from an amino acid donor to a 2-ketoacid acceptor by a simple mechanism. First, an amino group from the donor is transferred to the cofactor pyridoxal phosphate with formation of a 2-keto add and an enzyme-bound pyridoxamine phosphate intermediate. Second, this intermediate transfers the amino group to the 2-keto add acceptor. The readion is reversible, shows ping-pong kinetics, and has been used industrially in the production ofamino acids [69]. It can be driven in one direction by the appropriate choice of conditions (e.g. substrate concentration). Some of the aminotransferases accept simple amines instead of amino acids as amine donors, and highly enantioselective cases have been reported [70]. 

The reversibility of DNA cross-linking by the QMPs illustrated in Fig. 9.4 has not been examined directly but is presumably controlled as before by the leaving group strength of the DNA nucleophiles and the electronic characteristics of the QM intermediates. Unless both possible QMs form simultaneously, the cross-linking agents still remain bound to DNA at one site as the other is transiently released during... 

Yu et al. prepared 48, bound to an Au(lll) surface by a thiol on the biphenyl end, then deprotected on the bipyrimidine end to expose a thiol to bind an Au nanoparticle. Rectification was measured by scanning tunneling spectroscopy (STS), and it could be reversed by the addition of acid, which protonated the pyrimidine rings, converting them from donor to acceptor [121]. Yu s group has also inverted the attachment of a rectifier, as shown with 49 and 50, and confirmed that the rectification direction reversed [122]. This was recently verified by STM-BJ measurements [123]. 

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