Communication pubs.acs.org/Organometallics
Blocking Intramolecular Cycloadditions between CC Triple Bonds and Electrophilic Phosphinidene Complexes: Generation of Intermediates Able To React with Arenes Xu Zhao,† Zongming Lu,† Qiuyan Wang,† Donghui Wei,† Yunpeng Lu,‡ Zheng Duan,*,† and Francois Mathey*,†,‡ †
College of Chemistry and Molecular Engineering, International Phosphorus Laboratory, Joint Research Laboratory for Functional Organophosphorus Materials of Henan Province, Zhengzhou University, Zhengzhou 450001, People’s Republic of China ‡ Division of Chemistry & Biological Chemistry, Nanyang Technological University, 21 Nanyang Link, Singapore 637371 S Supporting Information *
ABSTRACT: Electrophilic terminal phosphinidene complexes [R1P-W(CO)5] bearing 2-alkynylphenyl R1 substituents undergo a spontaneous cyclization to give intermediate fivemembered species which are able to perform an electrophilic substitution reaction with toluene and mesitylene. An intramolecular version of this reaction is possible with appropriate R substituents on the CC triple bond.
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nterrupted cycloadditions nicely supplement classical cycloadditions as a powerful tool for the synthesis of cycles and heterocycles.1 To the best of our knowledge, this technique has never been used in the case of the already vast array of phosphinidene cycloadditions with unsaturated substrates.2 Among these reactions, the cycloaddition between electrophilic terminal phosphinidene complexes [RP-M(CO)5] (M = Cr, Mo, W) and alkynes yielding phosphirene complexes3 is so characteristic that it is now used as a probe for the formation of these transient monovalent phosphorus species.4 It was thus tempting to investigate what would happen if the normal [1 + 2] cycloaddition was forbidden by geometrical constraints. One possibility was to include both the phosphinidene and the C C triple bond in the same molecule. We have synthesized some
Figure 1. X-ray crystal structure of 4a. Main bond lengths (Å) and angles (deg): P1−W1 2.4951(10), P1−C6 1.805(3), P1−C13 1.821(4), C6−C11 1.399(5), C11−C12 1.484(4), C12−C13 1.352(5), C13−C14 1.492(5), C12−C20 1.494(5); C6−P1−C13 91.61(16), C6−P1−W1 119.35(12), C13−P1−W1 123.63(12).
of 4a is immediately visible on the 31P NMR spectrum: δ31P(4a) −31.0 (CDCl3), 1JPH = 337 Hz. However, the most striking
appropriate precursors using a previously described methodology5 (eq 1). The thermolysis of 3a in toluene gave a mixture from which the major product 4a could be isolated (eq 2). The P−H bond © XXXX American Chemical Society
Received: August 17, 2016
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DOI: 10.1021/acs.organomet.6b00642 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
Figure 4. NBO charges of intermediate 7. Figure 2. Computed structure of cyclized intermediate 7. Main bond lengths (Å) and angles (deg): P1−W2 2.4652, P1−C13 1.8160, P1− C24 1.8308, C13−C14 1.4091, C14−C23 1.4425, C23−C24 1.3341; C13−P1−C24 94.56, P1−C24−C23 99.59, C24−C23−C14 123.65, C23−C14−C13 107.90, C14−C13−P1 107.78; C13−C14−C23−C24 25.40.
Figure 3. HOMO (left) and LUMO (right) (Kohn−Sham) of 7. B
DOI: 10.1021/acs.organomet.6b00642 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
highly localized at P and C23 (Figure 4), confirming their electrophilic character. On this basis, it is logical to propose that 4a results from a classical electrophilic substitution reaction of C23 at the para position of toluene. With appropriate R substituents at the CC triple bond, it is possible to shift from the intermolecular to an intramolecular C−H activation, as shown in eqs 5 and 6). The structure of 8g was unambiguously established by X-ray crystal structure analysis (Figure 5) The molecule is not strictly planar. The phenantrene and the benzo ring make an angle of 12.26°. This intramolecular version of the C−H activation nicely complements the previously reported synthesis of annelated phospholes from phosphinidenes.5 We plan to study more in depth the chemistry of species such as 7.
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Figure 5. X-ray crystal structure of 8g. Main bond lengths (Å) and angles (deg): P1−W1 2.5081(11), P1−C12 1.812(4), P1−C21 1.807(4), C12−C13 1.373(5), C13−C20 1.496(5), C20−C21 1.406(5); C12−P1−C21 90.67(19), P1−C12−C13 111.9(3), C12− C13−C20 112.8(4), C13−C20−C21 112.7(3), C20−C21−P1 111.1(3).
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00642. Crystallographic data (CIF) NMR spectra of compounds 2−4, 6, 8, and 9, X-ray crystal structure analyses of compounds 4a and 8g, and NBO data for 7 (PDF) Crystallographic data (CIF) Carteisan coordinates of calculated structures (XYZ)
observation is that 4a incorporates a formal toluene unit. The structure of 4a was unambiguously established by X-ray crystal structure analysis (Figure 1) When the thermolysis is run in mesitylene, the reaction is much cleaner and gives a single product in satisfactory yield (eq 3). We were intrigued by this quite original reaction and decided to investigate some of its aspects by DFT calculations at the B3PW91/6-31+G(d,p)-Lanl2dz (W) level.6 It must be stressed here that a theoretical study of a biradicaloid M(CO)5 complex has revealed a satisfactory agreement between the DFT and higher level CASSCF methods.7 The overall reaction combines a cyclization and a C−H insertion. Since most of the cycloadditions of electrophilic terminal phosphinidene complexes with alkynes are run in boiling toluene, it is well established that these phosphinidene complexes are unable to activate the C−H bonds of toluene. We have checked that the electronic structure of phosphinidene 5 is unexceptional. The first step is thus, necessarily, the cyclization. Our investigations led us to discover a cyclized isomer of 5 corresponding to a genuine local minimum (0 negative frequency). The structure of this isomer 7 is shown in Figure 2. The striking point is that 7 lies only 0.73 kcal mol−1 higher in energy (ZPE included) than 5. The five-membered ring is strained, as shown by the value of the (P)C−C−C(benzo) angle of 123.65°, and distorted, as shown by the torsion angle (P)C−C−C−C(P) of 24.4°. Phosphorus is almost planar (∑(angles at P) = 348.5°), suggesting substantial delocalization of the P lone pair onto the ring. The dicoordinate carbon C23 displays an occupied p nonbonding orbital (HOMO, Figure 3). In order to get a more adequate picture of the electronic structure of 7, we carried out a NBO analysis whose results are shown in Figure 4. The species is not a diradical.8 It is clear that intermediate 7 displays two electrophilic centers, one at P (NBO charge +1.02) and one at C23 (NBO charge +0.134), but phosphorus is hindered by the complexing group whereas carbon is naked. A high concentration of negative charge is on tungsten (−2.01). The electronic density built at P upon attack by the CC triple bond is massively transferred onto W. The LUMO is also
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for Z.D.:
[email protected]. *E-mail for F.M.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (21272218), Research Fund for the Doctoral Program of Higher Education (20134101110004), and Zhengzhou Science and Technology Department (131PYSGZ204) of China.
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REFERENCES
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DOI: 10.1021/acs.organomet.6b00642 Organometallics XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.organomet.6b00642 Organometallics XXXX, XXX, XXX−XXX
