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. 2012 Sep 12;134(36):15154-62.
doi: 10.1021/ja3075924. Epub 2012 Sep 4.

The discovery of [Ni(NHC)RCN]2 species and their role as cycloaddition catalysts for the formation of pyridines

Ryan M Stolley  1 , Hung A Duong, David R Thomas, Janis Louie
Affiliations

The discovery of [Ni(NHC)RCN]2 species and their role as cycloaddition catalysts for the formation of pyridines

Ryan M Stolley et al. J Am Chem Soc. .

Abstract

The reaction of Ni(COD)(2), IPr, and nitrile affords dimeric [Ni(IPr)RCN](2) in high yields. X-ray analysis revealed these species display simultaneous η(1)- and η(2)-nitrile binding modes. These dimers are catalytically competent in the formation of pyridines from the cycloaddition of diynes and nitriles. Kinetic analysis showed the reaction to be first order in [Ni(IPr)RCN](2), zeroth order in added IPr, zeroth order in nitrile, and zeroth order in diyne. Extensive stoichiometric competition studies were performed, and selective incorporation of the exogenous, not dimer bound, nitrile was observed. Post cycloaddition, the dimeric state was found to be largely preserved. Nitrile and ligand exchange experiments were performed and found to be inoperative in the catalytic cycle. These observations suggest a mechanism whereby the catalyst is activated by partial dimer-opening followed by binding of exogenous nitrile and subsequent oxidative heterocoupling.

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Conflict of interest statement

Notes

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1
Ortep plot of 3a at the 30% probability level. Hydrogen atoms omitted for clarity. Pertinent bond lengths include: Ni(1)-C(28): 1.854 Å, Ni(1)-C(15): 1.8677 Å, Ni(1)-N(3): 1.9523 Å, Ni(1)-N(3)_3: 1.9951, N(3)- C(28): 1.226. Pertinant bond angles include: C(28)-Ni(1)-C(15): 120.29°, C(28)-Ni(1)-N(3): 37.37°, C(15)-Ni(1)-N(3)_3: 132.83°,N(3)-Ni(1)-N(3)_3: 95.37°, N(3)-C(28)-C(29): 134.6°.
Figure 2
Figure 2
Ortep plot of 3c. Hydrogen atoms are omitted for clarity. Pertinent bond lengths include: Ni(1)-C(28): 1.857 Å, Ni(1)-C(15): 1.895 Å, Ni(1)-N(3): 1.974 Å, Ni(1)-N(4): 1.982 Å, N(3)-C(28): 1.223 Å. Pertinant bond angles include: C(28)-Ni(1)-C(15): 119.48°, C(28)-Ni(1)- N(3): 37.08°, C(15)-Ni(1)-N(4): 109.50°, N(3)-Ni(1)-N(4): 93.93°, N(3)- C(28)-C(29): 136.1°.
Figure 3
Figure 3
Plots of [3a] vs. time, kobs vs. [IPr], and kobs vs- [MeCN] for the cycloaddition of 4 at 0 °C in C7D8
Scheme 1
Scheme 1
Divergent Pathways for Alkyne/Nitrile Cycloaddition
Scheme 2
Scheme 2
Observed regioselectivity in Ni/SIPr catalyzed cycloaddition
Scheme 3
Scheme 3
Possible Mechanistic Pathways for [Ni(IPr)RCN]2-Catalyzed Cycloadditions
Scheme 4
Scheme 4
Modified Pathway A′
Scheme 5
Scheme 5
Modified Pathway A″
Scheme 6
Scheme 6
Contradictory reactivities of 3ab required by Pathway D to account for observed product ratios
Scheme 7
Scheme 7
Proposed Mechanism of [Ni(IPr)RCN]2-Catalyzed Cycloaddition
See this image and copyright information in PMC

References

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