Communication pubs.acs.org/Organometallics
Reactivity of a Phosphaalkyne with Pentaarylboroles Jonathan H. Barnard,† Sam Yruegas,† Shannon A. Couchman,‡ David J. D. Wilson,‡ Jason L. Dutton,‡ and Caleb D. Martin*,† †
Department of Chemistry and Biochemistry, Baylor University, One Bear Place #97348, Waco, Texas 76798-7343, United States Department of Chemistry and Physics, La Trobe Institute for Molecular Science, La Trobe University, Melbourne 3086, Australia
‡
S Supporting Information *
ABSTRACT: The reactions of pentaarylboroles with 1-adamantylphosphaalkyne led to the formation of unique 1-phospha-6-boratricyclo-hept-3enes. A rational mechanism for this transformation is proposed and supported by DFT calculations.
oroles are five-membered, four-π-electron antiaromatic heterocycles possessing a rich palette of reactivity due to their high Lewis acidity and unusual electronic structure.1 In the reported reactions of boroles with triply bonded species, Diels−Alder and coordination modes of reactivity have been observed.2 Acetonitrile and carbon monoxide form Lewis acid/ base adducts with pentaphenylborole 1, which can subsequently undergo 1,1- or 1,2-insertions to give ring-expanded products.2a,b The nonpolar alkyne diphenylacetylene reacts with 1 via a [4 + 2] cycloaddition to yield a boranorbornadiene that rearranges to furnish a borepin.2d,e Interestingly, the analogous reaction with the perfluorinated variant of 1 resulted in both the Diels−Alder and coordination/ring expansion pathways.2c The latter was favored, resulting in a 1,1-insertion to produce an unsaturated six-membered BC5 heterocycle. Diels−Alder and ring expansion reactivity are not exclusive to triply bonded substrates and have also been observed for various other unsaturated organic compounds.2b,3 Intrigued by the diverse chemistry of boroles with triply bonded species and the potential to generate new P−B heterocycles, we were drawn to phosphaalkynes. Phosphaalkynes are the organophosphorus congeners of nitriles, featuring a PC triple bond that is polarized δ+PCδ−. Phosphaalkynes have well-established coordination chemistry with transition metals but also readily undergo a variety of cycloaddition reactions.4 In this context, we herein examine the reactivity of boroles with 1-adamantylphosphaalkyne. The 1:1 stoichiometric reaction of boroles 1 and 1Ph with 1adamantylphosphaalkyne in CDCl3 led to the rapid loss of the deep blue color, indicative of the consumption of the borole (Scheme 1). Acquiring 31P{1H} NMR spectra after 5 min showed no evidence of the phosphaalkyne and clean conversion to sharp singlets shifted upfield at −130.7 and −130.4 ppm, respectively (cf. −68 ppm for 1-adamantylphosphaalkyne). 1H NMR spectroscopy showed aromatic and aliphatic resonances integrating in 25:15 and 29:15 ratios, respectively, for the reactions of 1 and 1Ph, consistent with single products, and
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© XXXX American Chemical Society
Scheme 1. Synthesis of 2 and 2Ph with the Proposed Mechanism for 2 (Ad = Adamantyl)a
a
M06-2X/def2-TZVPP relative free energies are represented in parentheses in kJ/mol.
broad resonances at 30 ppm were observed by 11B{1H} NMR spectroscopy for both reactions. Larger scale reactions in CH2Cl2 allowed the isolation of the products in good yields (77% and 67%, respectively) as air-stable white solids. X-ray diffraction studies revealed the identities as 1-phospha-6boratricyclo-hept-3-enes 2 and 2Ph (Figure 1 for 2Ph; see the Supporting Information for 2). The presence of five phenyl groups on the boron and the four carbon atoms in 2 led to disorder of the boron and phosphorus atoms, preventing an accurate analysis of the metrical parameters beyond the atomic connectivity. Thus, only the biphenyl derivative 2Ph, which lacks disorder, is discussed. The chemistry of phosphaalkynes might be expected to mimic their lighter congeners; however, in contrast to nitriles, Received: February 12, 2016
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DOI: 10.1021/acs.organomet.6b00123 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
perturbation analysis of donor−acceptor NBO interactions indicates that the planar geometry and shortened B1−C4 bond can be traced to some donation from the occupied P−C σbond NBOs into the empty p orbital NBO on the boron atom, which may be considered a hyperconjugative interaction. Overall the calculations are consistent with the Lewis structure depicted for products 2 and 2Ph (Scheme 1), with the caveat that the minor hyperconjugative interactions are difficult to depict in Lewis structures. In summary, pentaarylboroles react with 1-adamantyl phosphaalkyne to give unprecedented 1-phospha-6-boratricyclohept-3-enes. Theoretical calculations support a mechanism proceeding through a Diels−Alder adduct for this reaction. These results demonstrate the synthetic versatility of boroles, particularly in the preparation of new phosphaboraheterocycles, and suggest new avenues for heteroatom-containing boracycles.
Figure 1. (a) Molecular structure of 2Ph. Thermal ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. (b) HOMO of 2.
the HOMO does not consist primarily of a lone pair on the pnictogen and is associated with the triple bond of the phosphaalkyne.5 Hence, a mechanism proceeding by coordination of the phosphorus is not expected and DFT calculations on the formation of 2 support this hypothesis. The reaction is initiated by interaction of the phosphaalkyne carbon atom with the boron center (TS1), consistent with the δ+PC δ− polarization of the phosphaalkyne triple bond. The formation of Diels−Alder adduct intermediate INT1 has a computed barrier of 20.5 kJ/mol and is thermodynamically favored by 66.2 kJ/mol. From INT1, the adamantyl-bound carbon inserts into the nucleophilic C−B bond of the five-membered ring remaining from the borole, concomitant with attack from the P−C double bond onto the ring and a shift of the C−C double bond, giving product 2. This second step is rate determining with a calculated barrier of 59.6 kJ/mol from INT1. This barrier is consistent with the short reaction time. The overall transformation is calculated to be thermodynamically favorable, with a ΔG value of −101.8 kJ/mol. In 2Ph, the phosphorus atom occupies an apical position of the 1-phospha-6-boratricyclohept-3-ene framework 1.3684(10) Å above the C3−C4−B1−C5 plane, the C3 and C5 atoms of which are linked by the C1−C2 unit (C1−C2 = 1.3341(18) Å). The three C−P bond lengths range between 1.8701(10) and 1.9672(18) Å, elongated in comparison to typical C−P single bonds (cf. 1.83 Å in P(o-Tol)3).6 The distance between B1 and P1 of 2.1468(17) Å is considerably longer than typical B−P single bonds (cf. 1.93−2.00 Å). 7 The C4−B1 bond (1.5423(19) Å) is slightly shorter than a typical single bond (cf. l.55−1.64 Å),8 and C4 is distorted from an idealized tetrahedral geometry. Both C2−C3 and C1−C5 distances (1.513(2) and 1.5274(18) Å, respectively) are consistent with single bonds. The bonding in 2 was analyzed by computational means. The key structural component is the area between C3, C4, C5, and the B and P atoms. Both molecular orbital (MO) and natural bond orbital (NBO) analysis indicate no appreciable orbital interaction between the B and P atoms (see the Supporting Information), despite a relatively short separation between the two atoms of 2.128 Å (Wiberg bond index (WBI) = 0.304), consistent with a very weak or nonbonding B−P interaction that is insufficient to be labeled a single bond. For comparison, Mes2B(H)−P(H)Mes2 has a B−P single bond of 2.00 Å7b and a calculated WBI of 0.887. The three calculated P−C bonds are all readily identified as σ interactions and single bonds. Although the short B1−C4 bond distance (1.541 Å) in 2 and planar environments about B1 and C4 could suggest the presence of a B−C multiple bond, there is no identifiable MO or NBO associated with a C−B double bond. Second-order
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00123. Experimental, crystallographic, and computational details (PDF) Crystallographic data (CIF) Cartesian coordinates for the calculated structures (XYZ)
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AUTHOR INFORMATION
Corresponding Author
*E-mail for C.D.M.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the Welch Foundation (AA 1846), NCI-NF and Intersect, and the ARC (J.L.D., DE130100186) for funding. REFERENCES
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