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. 2010 Sep 22;132(37):13064-71.
doi: 10.1021/ja105530q.

Mechanistic study of gold(I)-catalyzed intermolecular hydroamination of allenes

Z Jane Wang  1 , Diego Benitez, Ekaterina Tkatchouk, William A Goddard 3rd, F Dean Toste
Affiliations

Mechanistic study of gold(I)-catalyzed intermolecular hydroamination of allenes

Z Jane Wang et al. J Am Chem Soc. .

Abstract

The intermolecular hydroamination of allenes occurs readily with hydrazide nucleophiles, in the presence of 3-12% Ph(3)PAuNTf(2). Mechanistic studies have been conducted to establish the resting state of the gold catalyst, the kinetic order of the reaction, the effect of ligand electronics on the overall rate, and the reversibility of the last steps in the catalytic cycle. We have found the overall reaction to be first order in gold and allene and zero order in nucleophile. Our studies suggest that the rate-limiting transition state for the reaction does not involve the nucleophile and that the active catalyst is monomeric in gold(I). Computational studies support an "outersphere" mechanism and predict that a two-step, no intermediate mechanism may be operative. In accord with this mechanistic proposal, the reaction can be accelerated with the use of more electron-deficient phosphine ligands on the gold(I) catalyst.

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Figures

Figure 1
Figure 1
Reaction of 1 with methyl carbazate where □ = [ 1], △ = [2], and ○ = [1] + [2].
Figure 2
Figure 2
The hydroamination reaction was monitored by 31P NMR spectroscopy.
Figure 3
Figure 3
A plot of kobs versus concentration of methyl carbazate, where kobs was determined by the method of initial rates.
Figure 4
Figure 4
A representative plot of −ln[1(t)/10] versus time (h) for the reaction of 1 and methyl carbazate shows first order dependence in 1.
Figure 5
Figure 5
A plot of kobs versus [Ph3PAuNTf2], using 2.5 equiv methyl carbazate.
Figure 6
Figure 6
A Hammett plot of kobs for hydroamination of 1 in the presence of Ar3PAuNTf2.
Figure 7
Figure 7
Structural and natural atomic charge of selected complexes.
Scheme 1
Scheme 1
Outersphere and innersphere mechanisms proposed for the hydroamination of allenes with gold(I) catalysts.
Scheme 2
Scheme 2
Geometries of (S)-penta-2,3-diene and [Ph3PAu]+ as coordination, transition state, and allylic cation complexes.
Scheme 3
Scheme 3
Partial reaction coordinate diagram for Au(I) catalyzed hydroamination of (S)-penta-2,3-diene. Energy values are free energies (ΔG) for the lowest energy isomer at 298 K.
Scheme 4
Scheme 4
Proposed mechanism for hydroamination of 1 with Ph3PAuNTf2.
See this image and copyright information in PMC

References

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