[Pt(PPh3)4]-Catalyzed Selective Diboration of Symmetrical and Unsymmetrical 1,3-Diynes

Abstract

A straightforward, efficient, and selective method for the preparation of novel boryl-functionalized enynes or dienes via [Pt(PPh₃)₄]-catalyzed diboration of a broad spectrum of symmetrical and unsymmetrical 1,3-diynes was developed. The catalytic cycle of diboration was proposed on the basis of low-temperature ³¹P NMR studies. An alternative isolation method via product condensation on a cold finger was developed, which, in contrast to previous literature reports, eliminates the need for the additional transformation of rapidly decomposing enynyl pinacol boronates to more stable silica-based column chromatography derivatives during the separation step. To prove the efficiency of this simple catalytic protocol, bisboryl-functionalized enynes were synthesized in a gram scale and tested as useful building blocks in advanced organic transformations, such as hydrosilylation and Suzuki and sila-Sonogashira couplings. The presence of silyl, boryl, as well as other functions like halogen or alkoxy in their structures builds a new class of multifunctionalized enynes that might be modified in various chemical reactions.

Scheme 1. TM-Catalyzed Diboration of 1,3-Diynes and 1,3,5-Triyne

Scheme 2. Scope of Products Obtained in the [Pt(PPh₃)₄]-Catalyzed Diboration or Tetraboration of Symmetrical 1,3-Diynes 2a–l. Standard Reaction Conditions: [Pt(PPh₃)₄]:[1]:[2a–l] = 0.01:1:1, Toluene (3a–f, 3k–l) or THF (3g–j) (0.125 M), 80 °C, 18 h, Ar. Isolated Yields Are Presented

110 °C, 48 h. [Pt]:[1]:[2] = 0.01:1:7. [Pt]:[1]:[2] = 0.01:3:1. Product yield based on GC–MS analysis.

Scheme 3. Scope of Products Obtained in the [Pt(PPh₃)₄]-Catalyzed Diboration of Unsymmetrical 1,3-Diynes 2m–z. Reaction Conditions: [Pt(PPh₃)₄]:[1]:[2m–z] = 0.01:1:1, Toluene (0.125 M), 80 °C, 18 h, Ar. Isolated Yields Are Presented

Isolated as the reaction mixture.

Scheme 4. Proposed Catalytic Cycle for 1,3-Diynes Diboration with B₂pin₂ Catalyzed by [Pt(PPh₃)₄]

Scheme 5. Application of 3a as a Building Block in Catalytic Transformations

Reaction conditions: for (i) and (ii): Pd(PPh₃)₄ (5 mol %), THF, 3 M Cs₂CO₃, 60 °C, 18 h, argon, for (i) iodobenzene 2.4 equiv, for (ii) (E)-styryl iodides 1.2 equiv (iii):[Et₃SiH]:[3a]:[Pt₂(dvs)₃] = [1.2]:[1]:[10^–3 per Pt atom], toluene (1 M), 100 °C, 24 h. (iv): [4-iodotoluene]:[3a]:[CuI]:[Pd(PPh₃)₄] = [1.1]:[1]:[0.5]:[0.05], DMF, 80 °C, 18 h.