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. 2013 Mar 15;15(6):1414-7.
doi: 10.1021/ol4004178. Epub 2013 Mar 5.

NHC-Cu-catalyzed protoboration of monosubstituted allenes. Ligand-controlled site selectivity, application to synthesis and mechanism

Fanke Meng  1 , Byunghyuck Jung, Fredrik Haeffner, Amir H Hoveyda
Affiliations

NHC-Cu-catalyzed protoboration of monosubstituted allenes. Ligand-controlled site selectivity, application to synthesis and mechanism

Fanke Meng et al. Org Lett. .

Abstract

Two types of NHC-Cu complexes catalyze protoborations of terminal allenes to afford valuable 1,1- or trisubstituted vinylboron species with high site selectivity and stereoselectivity. The scope of the method, application to natural product synthesis, and mechanistic basis for the observed selectivity trends are presented.

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Figures

Figure 1
Figure 1
Previous findings and questions addressed in this study; NHC = N-heterocyclic carbene; B(pin) = (pinacolato)boron.
Figure 2
Figure 2
Rationale for the site selectivity trends; Curtin–Hammett kinetics with smaller NHC ligand 4e.
Scheme 1
Scheme 1
Synthesis of 1,1-Disubstituted Vinylborons by NHC–Cu-Catalyzed Protoborations of Allenesa a >98% conv in all cases; same conditions as in Table 1. See the Supporting Information (SI) for details.
Scheme 2
Scheme 2
Site- and Z-Selective Synthesis of Trisubstituted Vinylboron Compoundsa a >98% conv (allene disappearance) and >98% Z in all cases; same conditions as in Table 1, except 1,4-dioxane used as solvent. See the SI for details.
Scheme 3
Scheme 3
Application to Stereoselective Synthesis of the C1–C10 Fragment of Macrolide Antibiotic Elansolid A
Scheme 4
Scheme 4
Effect of Site Selectivity on Efficiency
See this image and copyright information in PMC

References

    1. For applications of vinylboron compounds in C–C bond formation, see: Hall DG. Boronic Acids: Preparation and Applications in Organic Synthesis and Medicine. Wiley–VCH; Weimheim: 2005. . Tobisu M, Chatani N. Angew Chem Int Ed. 2009;48:3565.. For syntheses of cyclic and acyclic vinylborons by Pd-catalyzed cross-coupling reactions involving vinyl bromides and triflates, see: Takagi J, Takahashi K, Ishiyama T, Miyaura N. J Am Chem Soc. 2002;124:8001.. For a review on applications of vinyltrifluoroboron species, accessed via vinylborons, see: Molander GA, Ellis N. Acc Chem Res. 2007;40:275.

    1. Jang H, Zhugralin AR, Lee Y, Hoveyda AH. J Am Chem Soc. 2011;133:7859. - PubMed

    1. For synthesis of vinylboron compounds by Cu-catalyzed protoboration of alkynes (in addition to ref 2), see: Kim HR, Jung IG, Yoo K, Jang K, Lee ES, Yun J, Son SU. Chem Commun. 2010;46:758.. Semba K, Fujihara T, Terao J, Tsuji Y. Chem Eur J. 2012;18:4179.. Moure AL, Arrayás RG, Gárdenas DJ, Alonso I, Carretero JC. J Am Chem Soc. 2012;134:7219.. Park JK, Ondrusek BA, McQuade DT. Org Lett. 2012;14:4790.

    1. Jung B, Hoveyda AH. J Am Chem Soc. 2012;134:1490. - PMC - PubMed

    1. Subsequent to studies in ref. , a disclosure regarding site selective synthesis of vinylboron products by Cu-catalyzed protoboration of mono-substituted allenes appeared. However, catalysts were exclusively phosphine-based, substrates were almost all aryl-substituted, application to complex molecules synthesis was not demonstrated and detailed rationale for the origin of the observed selectivities was not provided. See: Yuan W, Ma S. Adv Synth Catal. 2012;354:1867.

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