Deuterium exchange regioselective

The regioselectivity of the hydride addition step has been probed by searching for deuterium exchange into isomerized alkenes that have undergone partial reduction.15 The results suggest that Rh is electrophilic in the addition step and that the hydride transfer is nucleophilic. 

The hydrogen-deuterium exchange rates of indolizine and some methyl derivatives have been measured at 50 °C in D20/dioxane. It has been shown that the exchange rates reflect the regioselectivity of protonation. Kinetic data have been discussed using HMO and PPP calculations (71T4171). Some pKa values of indolizines and azaindolizines are given in Table 9. 

In a study of the synthesis of marine 2-aminoimidazole alkaloids, the aminal C-N bond of bicyclic 898 was cleaved regioselectively to form the allylic amine 899. Deuterium exchange results in H-1, H-2, and H-5, but not H-7 exchange, although the C(6)-C(7) double bond underwent isomerization 901. This is atributed to the assistance of the carbamoyl group that formed the oxazoline 900. Hydrolysis of the carbamate group of 899 under basic conditions affords either pyrrole 902 or pyridine 903 (Scheme 218) <2004OL3933>. 

A similar regioselective exchange pattern was found later in triflic acid. Furthermore, the deuterium distribution observed in isobutane recovered after short contact times with DF-SbFs at 0°C (in contrast to -78°C) showed a fast deuterium exchange at all C-H bonds. " Three isomeric carbonium ions in equilibrium are formed (Scheme 6.5) and the relative concentration of these prevalent reaction intermediates depends only on the relative basicity of the proton-accepting bonds in accord with the Olah o-basicity concept. 

In agreement with this mechanism, reduction with sodium borodeuteride in AcOH or AcOD results in the regioselective introduction of one or two deuterium atoms, respectively. In deuterated acetic acid, exchange of the N—H proton must be faster than reduction and hydride transfer to carbon (2.42). 

The introduction of hydrogen or deuterium atoms into thiophenes is possible via the treatment of thienyUithiums with alcohols, water, deuterated alcohols, or deuterium oxide (Scheme 30) [24, 173, 174]. The combination of this method with a prior halogen-metal exchange reaction is a powerful tool for the regioselective partial dehalogenation of halogenated thiophenes [20, 174—190]. 

Figure 3.37 Regioselectivities in exchange labeling of model compounds from deuterium gas catalyzed by [lr(H)2(acetone)2(PPb3)2]BF4...

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