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. 2020 Nov 28;12(5):1745-1749.
doi: 10.1039/d0sc05755j.

Tri-insertion with dearomatization of terminal arylalkynes using a carborane based frustrated Lewis pair template

Jian Zhang  1 , Zuowei Xie  1
Affiliations

Tri-insertion with dearomatization of terminal arylalkynes using a carborane based frustrated Lewis pair template

Jian Zhang et al. Chem Sci. .

Abstract

Intramolecular vicinal Frustrated Lewis Pairs (FLPs) have played a significant role in the activation of small molecules, and their stabilities and reactivities are found to strongly depend on the nature of the bridging units. This work reports a new carborane based FLP, 1-PPh2-2-BPh2-1,2-C2B10H10 (2), which reacts with an equimolar amount of p-R2NC6H4C[triple bond, length as m-dash]CH (R = Me, Et, Ph) at room temperature to give C[triple bond, length as m-dash]C triple bond addition products 1,2-[PPh2C(R2NC6H4)[double bond, length as m-dash]CHBPh2]-1,2-C2B10H10 (3) in high yields. Compounds 3 react further with two equiv. of p-R2NC6H4C[triple bond, length as m-dash]CH (R = Me, Et) at 60-70 °C to give unprecedented stereoselective tri-insertion products, 3,3a,6,6a-tetrahydronaphtho[1,8a-b]borole tricycles (4), in which one of the aryl rings from arylacetylene moieties has been dearomatized with the formation of four stereocenters including one quaternary carbon center. It is noted that the phosphine unit functions as a catalyst during the reactions. After trapping and structural characterization of a key intermediate, a reaction mechanism is proposed, involving sequential alkyne insertion and 1,2-boryl migration.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Scheme 1
Scheme 1. Reaction types of alkynes with FLP templates: (a) reported reaction types, (b) tri-insertion reaction described in this work.
Scheme 2
Scheme 2. Intramolecular vicinal frustrated Lewis pairs.
Scheme 3
Scheme 3. Insertion of terminal alkynes into the FLP template.
Fig. 1
Fig. 1. Molecular structure of 3-Me. All H atoms are omitted for clarity; thermal ellipsoids are drawn at the 50% probability level. Key bond distances (Å) and angles (°): B(13)–C(41) 1.632(4), B(13)–C(2) 1.691(4), C(41)–C(42) 1.338(3), P(1)–C(42) 1.787(3), P(1)–C(1) 1.844(3), C(1)–C(2) 1.690(4), C(2)–B(13)–C(41) 108.7(2), B(13)–C(41)–C(42) 133.9(2), C(41)–C(42)–P(1) 116.9(2), and C(42)–P(1)–C(1) 107.2(1).
Fig. 2
Fig. 2. Molecular structure of 4-Me3. All H atoms are omitted, two phenyls and one Me2N group are shown in the wireframe format for clarity; thermal ellipsoids are drawn at the 50% probability level. Key bond distances (Å) and angles (°): B(13)–C(61) 1.495(6), B(13)–C(52) 1.586(6), B(13)–C(2) 1.618(6), C(61)–B(13)–C(52) 108.0(4), C(61)–B(13)–C(2) 125.1(4), and C(52)–B(13)–C(2) 126.3(4).
Fig. 3
Fig. 3. Molecular structure of 4-EtMe2. All H atoms are omitted, and two phenyl groups are shown in the wireframe format for clarity; thermal ellipsoids are drawn at the 50% probability level. Key bond distances (Å) and angles (°): B(13)–C(61) 1.513(5), B(13)–C(52) 1.603(6), B(13)–C(2) 1.609(6), C(61)–B(13)–C(52) 106.4(3), C(61)–B(13)–C(2) 125.5(4), and C(52)–B(13)–C(2) 127.7(3).
Scheme 4
Scheme 4. Reaction of 3-Ph with formaldehyde.
Fig. 4
Fig. 4. Molecular structure of 5-Ph. All H atoms are omitted for clarity; thermal ellipsoids are drawn at the 50% probability level. Key bond distances (Å) and angles (°): P(1)–C(70) 1.817(4), P(1)–C(1) 1.844(4), B(13)–O(1) 1.487(5), B(13)–C(2) 1.732(5), B(13)–C(42) 1.637(6), C(41)–C(42) 1.326(5), C(70)–O(1) 1.389(5), C(70)–P(1)–C(1) 102.9(2), and O(1)–B(13)–C(2) 106.7(3).
Scheme 5
Scheme 5. Proposed mechanism for the formation of 4.
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