Piers' Borane-Induced Tetramerization of Arylacetylenes

Abstract

We herein report that the reaction of Piers' borane, i. e. HB(C₆ F₅ )₂ , with an excess of arylacetylenes at room temperature leads to tetramerization of the acetylene and the diastereoselective formation of boryl-substituted tetra-aryl-tetrahydropentalenes. The reaction mechanism was investigated by isotope labeling experiments and DFT computations. These investigations indicate that a series of 1,2-carboboration reactions form an octatetraene that undergoes an electrocyclization. Two skeletal rearrangements then presumably lead to the formation of the tetrahydropentalene core. Overall, this intricate and unprecedented transformation comprises five carbon-carbon bond formations in a single reaction.

Keywords: 1,2-carboboration; Lewis acids; alkynes; boranes; density functional theory.

Scheme 1

Formation of cis‐2‐boryl‐1,3,3a,5‐tetraphenyl‐tetrahydropentalene 1 a upon reaction of Piers’ borane with five equivalents phenylacetylene. The yield was determined by NMR with trimethoxybenzene as internal standard. The yield of the isolated product is given in parenthesis.

Figure 1

Molecular structure of cis‐2‐boryl‐1,3,3a,5‐tetraphenyl‐tetrahydropentalene 1 a derived from SCXRD (50 % probability ellipsoids, all hydrogens attached to phenyl rings are omitted and C₆F₅ rings are shown in stick representation for clarity). Selected bond lengths and angles: C1−C2: 1.533(2) Å, C2−C3 1.357(2), C3−C3a 1.542(2) Å, C3a−C4 1.513(2) Å, C4−C5: 1.338(3) Å, C5−C6: 1.509(3) Å, C2−C6a: 1.535(3) Å, C6a−C1: 1.540(2) Å.

Scheme 2

Tetramerization of aryl allenes by Piers’ borane (a) yields determined by ¹H NMR with trimethoxybenzene as internal standard. The yields have an estimated error of +/−5 % (see Supporting Information for details).

Scheme 3

Follow‐up reactivity of 1 a in a Suzuki‐Miyaura coupling, oxidations, and protodeborylation (yields of isolated products given). The insets show the molecular structure for 4 and 5 b derived from SCXRD analysis (all hydrogens attached to phenyl rings are omitted for clarity).

Scheme 4

a) Isotope labeling experiments that reveal the position of hydrogen from Piers’ borane and that of the initially hydroborated phenylacetylene in the final product. b) NMR experiment that disclosed the position of the non‐deuterated phenyl moiety of 6 in the final product 1 a‐ d ₁₅ . Deuterium incorporation in the indicated position in each case was at least 88 %, as determined by ¹H NMR.

Figure 2

Schematic PES of the formation of the cyclooctatriene 10 and barriers for the 1,2‐ and 1,1‐carboboration of phenylacetylene by 6 computed at PCM(DCM)‐revDSD‐PBEP86‐D4/def2‐QZVPP//PCM(DCM)‐PBh‐3c. The insets show the computed transition state structures of the 1,2‐carboboration leading to 7 and the 8π‐electrocyclization leading to cycloocatriene 10.

Scheme 5

Proposed mechanism for the formation of 1 a. The first equivalent alkenylborane, whose position was determined by isotope labeling experiments, is highlighted in red.