Hydroaminoalkylation of Allenes

Abstract

The first examples of early-transition-metal-catalyzed hydroaminoalkylation reactions of allenes are reported. Initial studies performed with secondary aminoallenes led to the identification of a suitable titanium catalyst and revealed that under the reaction conditions, the initially formed hydroaminoalkylation products undergo an unexpected titanium-catalyzed rearrangement to form the thermodynamically more stable allylamines. The assumption that this rearrangement involves a reactive allylic cation intermediate provides a simple explanation of the fact that no successful early-transition-metal-catalyzed hydroaminoalkylations of allenes have previously been reported. As a result of the generation of the corresponding cation, the titanium-catalyzed intermolecular hydroaminoalkylation of propa-1,2-diene unexpectedly gives an aminocyclopentane product formed by incorporation of two equivalents of propa-1,2-diene.

Intermolecular Hydroaminoalkylation of Propadiene

“… Previous results on intermolecular hydroaminoalkylation reactions of propadiene with N ‐methylaniline, [13] and targeted products II and III (this work).…”

“…Instead, the aminocyclopentane derivative I was formed from N ‐methylaniline and two equivalents of propadiene in low yields [13] . The formation of I is presumably based on the generation of an allylic cation intermediate from the initially formed allylamine product ( II , R 1 =Ph, R 2 =H) caused by the presence of the Lewis acidic titanium catalyst [13] …”

“…Scheme1.Previousresultso ni ntermolecularhydroaminoalkylation reactions of propadienewith N-methylaniline, [13] andtargeted products II and III (this work). molL À1 )w as chosen as the benchmark reactiona nd ac atalyst screening was performed (Scheme 2, reaction conditions: 140 8C, 30 min, 10 mol %c atalyst, scale:0 .1 mmol, control by GC analysis).…”

“…[12] Previous work has demonstratedt hat the titanium-catalyzed intermolecular hydroaminoalkylation of propadiene with Nmethylaniline (Scheme 1) did not lead to the formation of the envisaged homocoupling products, for example, II [branched (b)] or III [linear (l)].I nstead,t he aminocyclopentaned erivative I was formed from N-methylaniline and two equivalents of propadiene in low yields. [13] The formation of I is presumably based on the generation of an allylic cation intermediate from the initially formed allylamine product (II,R 1 = Ph, R 2 = H) caused by the presence of the Lewis acidic titaniumc atalyst. [13] As as tarting point, the reactionb etween N-methylaniline (1a)a nd propadiene (2,s ynthesized according to al iterature procedure [13,14] and used as as olution in toluene, c = 0.4…”

“…[13] The formation of I is presumably based on the generation of an allylic cation intermediate from the initially formed allylamine product (II,R 1 = Ph, R 2 = H) caused by the presence of the Lewis acidic titaniumc atalyst. [13] As as tarting point, the reactionb etween N-methylaniline (1a)a nd propadiene (2,s ynthesized according to al iterature procedure [13,14] and used as as olution in toluene, c = 0.4…”