Stereoselective Propargylations with Transition-Metal-Stabilized Propargyl Cations

Abstract

Cationic propargylations have been known for quite some time but only the advent of transition metal stabilization has initiated their stereoselective applications. The introduction of nucleophilic additions to dicobalthexacarbonyl‐complexed propargyl cations, known as the Nicholas reaction, has led to widespread applications in the synthesis of complex molecules. Besides the stabilization by direct complexation of the triple bond, the complementary mode, i.e. transition metal complex substituents as propargyl cation stabilizing functional groups, is almost unknown. However, upon ionization, ferrocenyl‐ and (arene)chromiumcarbonyl‐substituted propargyl derivatives give the desired cationic species, and these cations can be regarded as quite reactive propargyl cations without simultaneous complexation of the triple bond, thus enabling highly diastereoselective nucleophilic additions with the potential of exploiting the ambident nature of propargyl cations.

Propargylic Cations Stabilized on Nickel−Molybdenum and Nickel−Tungsten Bonds

“…Their full characterization is described below. 32,33 and their 1 H, 13 C NMR and IR data (Tables 1-3) are consistent with the structures shown in Scheme 1. The molecules have no (C 1 ) symmetry, so an ABCD type pattern is observed for the C 5 H 4 Me aromatic protons.…”

“…Some puzzling features in the NMR spectra of 3b deserve comment. In contrast to the NMR data observed for 3a, the aromatic Cp′ resonances and a HC 2 CCMe 2 -(OMe) methyl resonance of 3b are broad at ambient temperature; 13 Reaction of the µ-Alkyne Complex 2a and 2b with HBF 4 . Formation of the Allenyl Cation 4a.…”

Propargylic Cations Stabilized on Nickel−Molybdenum and Nickel−Tungsten Bonds

“…Their full characterization is described below. 32,33 and their 1 H, 13 C NMR and IR data (Tables 1-3) are consistent with the structures shown in Scheme 1. The molecules have no (C 1 ) symmetry, so an ABCD type pattern is observed for the C 5 H 4 Me aromatic protons.…”

“…Some puzzling features in the NMR spectra of 3b deserve comment. In contrast to the NMR data observed for 3a, the aromatic Cp′ resonances and a HC 2 CCMe 2 -(OMe) methyl resonance of 3b are broad at ambient temperature; 13 Reaction of the µ-Alkyne Complex 2a and 2b with HBF 4 . Formation of the Allenyl Cation 4a.…”

Gold(I)‐Catalyzed Allylation and Tandem Allylation–Cycloisomerization of Stannylated Propargyl Acetates with Allylsilanes

“…Moreover, the reaction conditions tolerated unsaturated substituents, such as the o ‐(TIPS)‐ethynyl group, to afford 3ga and 4ga in 69% and 21% yield, respectively, and the alkyne remained intact. Although the substitution of 1,1‐disubstituted propargyl acetates with soft nucleophiles reportedly leads to the formation of allenic products such as 4 or 5 , [ 43–45 ] applying the method described herein to 1,1‐disubstituted propargyl acetate 1h afforded 3ha and 4ha in 71% and 21% yield, respectively. For a heteroaryl substituent, allylation of the 2‐thienyl substituted propargyl acetate 1i was successfully performed with a high yield.…”

Cationic indium catalysis as a powerful tool for generating α-alkyl propargyl cations for SN1 reactions

“…The propargylation of disubstituted benzenes p-xylene and p-dimethoxybenzene exclusively produced propargylated products 6c and 6d, respectively. Bulky aromatics, such as benzo[d] [1,3]dioxole and mesitylene, were tolerated. Heteroaromatic compounds, such as 2,5-dimethylthiophene, benzo[b]thiophene, benzo[b]furan, and 2-methylthiophene, were also good nucleophiles in this process; however, neither pyrroles nor indoles were suitable nucleophiles for this propargylation reaction.…”

Propargylic Cations Stabilized on Nickel−Molybdenum and Nickel−Tungsten Bonds

“…Their full characterization is described below. 32,33 and their 1 H, 13 C NMR and IR data (Tables 1-3) are consistent with the structures shown in Scheme 1. The molecules have no (C 1 ) symmetry, so an ABCD type pattern is observed for the C 5 H 4 Me aromatic protons.…”

“…Some puzzling features in the NMR spectra of 3b deserve comment. In contrast to the NMR data observed for 3a, the aromatic Cp′ resonances and a HC 2 CCMe 2 -(OMe) methyl resonance of 3b are broad at ambient temperature; 13 Reaction of the µ-Alkyne Complex 2a and 2b with HBF 4 . Formation of the Allenyl Cation 4a.…”

Gold(I)‐Catalyzed Allylation and Tandem Allylation–Cycloisomerization of Stannylated Propargyl Acetates with Allylsilanes

“…Moreover, the reaction conditions tolerated unsaturated substituents, such as the o ‐(TIPS)‐ethynyl group, to afford 3ga and 4ga in 69% and 21% yield, respectively, and the alkyne remained intact. Although the substitution of 1,1‐disubstituted propargyl acetates with soft nucleophiles reportedly leads to the formation of allenic products such as 4 or 5 , [ 43–45 ] applying the method described herein to 1,1‐disubstituted propargyl acetate 1h afforded 3ha and 4ha in 71% and 21% yield, respectively. For a heteroaryl substituent, allylation of the 2‐thienyl substituted propargyl acetate 1i was successfully performed with a high yield.…”

Cationic indium catalysis as a powerful tool for generating α-alkyl propargyl cations for SN1 reactions

“…The propargylation of disubstituted benzenes p-xylene and p-dimethoxybenzene exclusively produced propargylated products 6c and 6d, respectively. Bulky aromatics, such as benzo[d] [1,3]dioxole and mesitylene, were tolerated. Heteroaromatic compounds, such as 2,5-dimethylthiophene, benzo[b]thiophene, benzo[b]furan, and 2-methylthiophene, were also good nucleophiles in this process; however, neither pyrroles nor indoles were suitable nucleophiles for this propargylation reaction.…”

Propargylic Cations Stabilized on Nickel−Molybdenum and Nickel−Tungsten Bonds

“…Their full characterization is described below. 32,33 and their 1 H, 13 C NMR and IR data (Tables 1-3) are consistent with the structures shown in Scheme 1. The molecules have no (C 1 ) symmetry, so an ABCD type pattern is observed for the C 5 H 4 Me aromatic protons.…”

“…Some puzzling features in the NMR spectra of 3b deserve comment. In contrast to the NMR data observed for 3a, the aromatic Cp′ resonances and a HC 2 CCMe 2 -(OMe) methyl resonance of 3b are broad at ambient temperature; 13 Reaction of the µ-Alkyne Complex 2a and 2b with HBF 4 . Formation of the Allenyl Cation 4a.…”

Gold(I)‐Catalyzed Allylation and Tandem Allylation–Cycloisomerization of Stannylated Propargyl Acetates with Allylsilanes

“…Moreover, the reaction conditions tolerated unsaturated substituents, such as the o ‐(TIPS)‐ethynyl group, to afford 3ga and 4ga in 69% and 21% yield, respectively, and the alkyne remained intact. Although the substitution of 1,1‐disubstituted propargyl acetates with soft nucleophiles reportedly leads to the formation of allenic products such as 4 or 5 , [ 43–45 ] applying the method described herein to 1,1‐disubstituted propargyl acetate 1h afforded 3ha and 4ha in 71% and 21% yield, respectively. For a heteroaryl substituent, allylation of the 2‐thienyl substituted propargyl acetate 1i was successfully performed with a high yield.…”

Cationic indium catalysis as a powerful tool for generating α-alkyl propargyl cations for SN1 reactions

“…The propargylation of disubstituted benzenes p-xylene and p-dimethoxybenzene exclusively produced propargylated products 6c and 6d, respectively. Bulky aromatics, such as benzo[d] [1,3]dioxole and mesitylene, were tolerated. Heteroaromatic compounds, such as 2,5-dimethylthiophene, benzo[b]thiophene, benzo[b]furan, and 2-methylthiophene, were also good nucleophiles in this process; however, neither pyrroles nor indoles were suitable nucleophiles for this propargylation reaction.…”