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40.1.1.3.1.2.6 Variation 6: Catalysis by Rhodium Complexes

DOI: 10.1055/sos-SD-040-00148

Doye, S.Science of Synthesis, (200940270.

On the basis of some initial studies on the oxidative amination of vinylarenes,[‌157‌] the first rhodium-catalyzed anti-Markovnikov hydroamination of styrene was achieved in 1999.[‌158‌] Unfortunately, addition reactions performed with piperidine and several N-arylpiperazines, in the presence of the cationic catalyst bis(cyclooctadiene)rhodium(I) tetrafluoroborate (2.5mol%) and triphenylphosphine (5mol%), give the desired 2-phenylethanamine products in very low yields (7%). The corresponding 2-phenylethenamine product, formed by oxidative amination, is the major product. Better results are obtained with slightly activated 2- or 4-vinylpyridine, and in some cases the hydroamination product is obtained as the major product. Scheme 41 shows the distribution of products 143145 obtained from 2-vinylpyridine (4 equiv) and various cyclic secondary amines.[‌159‌] Interestingly, the desired addition product 144 is formed exclusively by reaction of 2-vinylpyridine with piperidine when bis(cyclooctadiene)rhodium(I) tetrafluoroborate is used without triphenylphosphine. Note, however, that the use of piperazine leads to the formation of additional dialkylated piperazine derivatives, which are not shown in Scheme 41.

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Me8 M8 Meee (°M) Meee (e) ee Meeee (%) Mee
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8-MM8M8 M 88 88 88 [‌888‌]
8,8-M8M8M8 M 88 88 88 [‌888‌]
Me Me 88 88 88:8 88 [‌888‌]
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