Reversible Intramolecular Cycloaddition of Phosphaalkene to an

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Reversible Intramolecular Cycloaddition of Phosphaalkene to an Arene Ring Liu Leo Liu, Jiliang Zhou, Levy L. Cao, Youngsuk Kim, and Douglas W. Stephan J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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Journal of the American Chemical Society

Reversible Intramolecular Cycloaddition of Phosphaalkene to an Arene Ring Liu Leo Liua, Jiliang Zhoua, Levy L. Caoa, Youngsuk Kima,b and Douglas W. Stephana* a Department b Department

of Chemistry, University of Toronto, 80 St. George St, Toronto, ON, M5S 3H6, Canada of Chemistry, Pohang University of Science and Technology, Pohang, 37673, Republic of Korea

Supporting Information Placeholder ABSTRACT: The phosphepinium cation 1 (R = adamantyl, Ad) is deprotonated by base generating a phosphaalkene that undergoes cycloaddition to the N-bound aromatic ring affording the 2-phosphabicyclo[2.2.2]octa-5,7-diene 2. The analogous deprotonation reaction of the phosphepinium cation 3 (R = tert-butyl, tBu) affords a reversible equilibrium between the phosphaalkene 4 and the corresponding cycloaddition product 5. This latter observation represents the first reversible cycloaddition of a main group multiply bonded species with an arene ring. The bicyclic species 2 was also shown to be oxidized or alkylated in reactions with S8 and MeOTf, affording 6 and 7 respectively. This finding and its implications for related cycloadditions of other main group multiply bonded species are considered computationally.

cycloaddition with benzene (Figure 1c).6 The group of Xie disclosed the generation of 1,3-dehydro-o-carborane and ensuing [4 + 2] cycloaddition reactions with a series of benzene derivatives (Figure 1d).7 (b) Sakurai (a)

Si Si

[4+2] h

The chemistry of heavier main group multiply bonded species has garnered increasing attention over the past few decades.3 Nonetheless, cycloadditions involving main group element-containing multiply bonded species and benzene derivatives have rarely been encountered. In 1991, Sakurai and co-workers demonstrated an irradiation-induced [4 + 2] cycloaddition of tetramethyldisilene with benzene at 10 K affording 7,7,8,8-tetramethyl-7,8-disilabicyclo-[2.2.2]octa-2,5diene (Figure 1b).4 Power et al. then described a [4 + 2] cycloaddition of a putative dialumene with toluene in 2003,5 while in 2013 Tokitoh et al. reported the analogous

Si Si

10 K, h

(c) Power, Tokitoh [3+2] + C6H6 h

Ar

[2+2] h

Al toluene Ar or C6H6 Ar Al Al Al r.t.

Ar

(d) Xie H

Cycloaddition reactions provide versatile and efficient strategies for the construction of structurally sophisticated cyclic molecules in a single step. Such reactions are prompted by readily accessible stimuli or inexpensive catalysts and tolerate a wide range of functional groups.1 As such, they have been widely applied in synthetic chemistry.1 Aromatic rings contain both alkene and diene subunits and thus are ideal candidates for cycloadditions from a topological point of view. However, arene rings are rarely involved in cycloadditions as the destruction of the conjugated aromatic systems is kinetically and thermodynamically disfavored.2 Nonetheless, literature reports have described selective photochemical activation as an avenue to overcome the kinetic barrier to cycloadditions of benzene derivatives and to prevent the thermal back-reaction.2 In this fashion, benzene derivatives in the excited-state are known to effect cycloaddition of olefin to give [2 + 2], [3 + 2] and [4 + 2] photo-cycloaddition products (Figure 1a).2

C 6H 6

C 6H 6 r.t.

H

(e) Present work iPr N

P

-H iPr

CH3

H C H

iPr

iPr N

H iPr P C H

N iPr

P

Figure 1. (a) Modes of photocycloaddition of arenes with olefins. (b) [4+2] cycloaddition of Me2Si=SiMe2 with benzene. (c) [4+2] Cycloaddition of ArAl=AlAr with toluene or benzene. (d) [4+2] Cycloaddition of 1,3-dehydro-o-carborane with benzene. (e) Present work. Phosphaalkenes, the P=C analogues of olefins, contain a conventional σ/π double bond between the phosphorus and carbon atoms, and generally mimic the chemical behaviour of olefins.8 That being said, there are no known examples of the cycloadditions of phosphaalkenes to aromatic rings. We recently reported the formation of the phosphepinium cations 1 and 3 (Scheme 1) via a ring expansion reaction.9 Herein, we demonstrate a reversible intramolecular [4 + 2] cycloaddition of a phosphaalkene moiety with the proximal 2,6diisopropylbenzene (dipp) ring (Figure 1e). This reaction represents the first example of a reversible cycloaddition of a main group multiply bonded species with arenes. This finding and its implications for analogous reactions of other main group multiply bonded species are considered. Generally, alkene-alkene [2 + 2] cycloadditions are orbitalsymmetry-forbidden in the ground state, whereas alkenediene [4 + 2] cycloadditions are orbital-symmetry-allowed.2, 10

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The π-bond strength in HP=CH2 is calculated to be 43 kcal/mol, which is much lower than that of the C=C π-bond of ethene (65 kcal/mol).11 Thus phosphaalkenes should be more susceptible to cycloaddition. This prompted us to target the [4 + 2] reactions of P=C double bonds with arenes. To gain insight, we initially performed DFT calculations on the model cycloadditions of benzene with CH2=CH2 and HP=CH2 (SMDM06-2X/Def2-TZVP//M06-2X/Def2-SVP) (Figure 2). As expected, the activation of benzene with CH2=CH2 was found to be energy-demanding (free energy barrier: 49.7 kcal/mol) leading to the unstable bicyclo[2.2.2]octa-2,5-diene (free energy of 20.1 kcal/mol) with an unfavorable enthalpy change of 5.8 kcal/mol. In contrast, the corresponding reaction with HP=CH2 was computed to be kinetically accessible with a significantly lower free energy barrier (32.5 kcal/mol). While the thermodynamics are unfavorable by 10.4 kcal/mol, the enthalpy change is -3.8 kcal/mol. These findings suggest the P=C double bond addition to an arene is possible if the entropy could be reduced.

E

32.5 (19.0) 20.1 (5.8)

E

10.4 (-3.8)

0.0 (0.0)

Figure 2. DFT predicted free energy profile for the [4 + 2] cycloadditions of benzene with CH2=CH2 or HP=CH2. The enthalpies are given in parentheses. Energies are given in kcal/mol. (a) OTf N

Et

P

KHMDS, r.t., C 6H 6

iPr

iPr CH3 - KOTf, - HMDS

Ad Et 1

N iPr Et

Et

H C H P Ad 2

(b) OTf

iPr N

Et

P

iPr

CH3

tBu Et 3

KHMDS or IPr r.t., C6H6 - KOTf, - HMDS or - [IPrH][OTf]

(a)

(b)

C(14)

C(14) C(28)

C(17)

N(1)

C(17)

C(34)

P(1)

N(1) P(1) C(2)

C(1)

C(2) C(1)

Figure 3. POV-ray depiction of the X-ray structures of 2 (a) and 5 (b). C black, N blue, P orange. Hydrogen atoms are omitted for clarity.

49.7 (36.1)

iPr

diffraction. X-ray data revealed 2 to be a 2phosphabicyclo[2.2.2]octa-5,7-diene formed from the apparent addition of a transient phosphaalkene across the arene substituent on N. Atoms P(1) and C(34) form bonds to C(17) and C(14), respectively, to make the latter two carbon atoms bridgehead centers (Figure 3a). The C(1)-C(2), C(1)P(1), C(17)-P(1), C(34)-P(1) and C(14)-C(34) bond lengths are 1.379(3), 1.878(2), 1.895(2), 1.875(2) and 1.577(3) Å, respectively.

E

E = CH2, in red PH, in blue

+

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iPr

iPr N Et

Et

iPr H P C H tBu 4

N iPr Et

Et 5

H C H P tBu

Scheme 1. (a) Synthesis of 2. Ad = admantyl. (b) Generation of the phosphaalkene 4 and the ensuing reversible intramolecular [4+2] cycloaddition affording 5. We speculated that the protic character of hydrogen of the PMe group of 1 and 3 (Scheme 1) is enhanced due to the cationic character and thus should be susceptible to reaction with base affording either λ5 or λ3 phosphaalkenes. Indeed, the reaction of 1 with potassium bis(trimethylsilyl)amide (KHMDS) in benzene at room temperature (20 min) gave a colorless crystalline solid 2 in 66 % yield (Scheme 1a). The 31P NMR signal of 2 appears as a doublet of multiplets at 3.9 ppm (JPH = 26 Hz), which collapses into a singlet upon proton decoupling. Storing a concentrated pentane solution of 2 at -20 °C overnight afforded colorless single crystals suitable for X-ray

Interestingly, when the less bulky phosphepinium cation 3 was used, the corresponding deprotonation with KHMDS or IPr (1,3-diisopropyl-1H-imidazol-3-ium-2-ide) initially afforded a new species 4, concurrent with the complete consumption of 3 after 10 min at room temperature (Scheme 1b) as indicated by a diagnostic triplet at 303.1 ppm with the P-H coupling constant of 31 Hz in the 31P NMR spectrum. In addition, the 1H NMR spectrum revealed two signals of doublet of doublets at 6.92 (JPH = 28 Hz, JHH = 5 Hz) and 6.88 ppm (JPH = 33 Hz, JHH = 5 Hz), integrating for one proton each. These signals are similar to those observed for Mes*P=CH2 (Mes* = 2,4,6-tBu3-C6H2) (31P NMR: 289.5 ppm, JPH = 31 Hz; 1H NMR: 7.02 ppm, dd, 2JPH = 33 Hz, 2JHH = 5 Hz, 6.73 ppm, dd, 2JPH = 28 Hz, 2JHH = 5 Hz).12 The observed 31P chemical shift is well matched to that computed for the proposed vinyl-phosphaalkene 4 (305.3 ppm) at the SMD-GIAO-B97D/Def2TZVP//M06-2X/Def2SVP level of theory.9, 13 Nevertheless, 4 proved to be kinetically unstable in solution above -20 °C and smoothly converted into compound 5 (δ(31P) = 7.6 ppm, JPH = 27 Hz) even in the dark. Remarkably, the establishment of an equilibrium mixture of 4 (9 %) and 5 (91 %) was observed when 4 was left standing in ClC6H5 at room temperature within 6 h. This equilibrium mixture can also be achieved upon dissolving the crystalline 5 in ClC6H5 within 6 h at room temperature. The equilibrium did not change after 3 days at room temperature (Figure S27 in the SI). Variable temperature NMR studies reveal not only that the position of the equilibrium is temperature dependent, but that reaction mixtures are slow to achieve equilibrium at low temperature. For instance, at -5 °C samples of freshly generated 4 in ClC6H5 took about 30 h to establish an equilibrium with 5 in a ratio of 5 (4) : 95 (5). A van't Hoff analysis of the equilibrium over the temperature range of 268-298 K shows the transformation of 4 to 5 is a thermodynamically downhill reaction with enthalpy and free energy changes of -2.9 and -1.4 kcal mol-1, respectively (Figure S26 in the SI). In the solid state (Figure 3b), the structural parameters of 5 are comparable to those of 2. It is noteworthy that 2 and 5 represent the first examples of crystallographically


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Journal of the American Chemical Society characterized 2-phosphabicyclo[2.2.2]octa-5,7-dienes.14 The species 2 and 5 can be stored in the solid state at ambient temperature under a nitrogen atmosphere over a week without noticeable decomposition. In addition, a mixture of 4 and 5 in ClC6H5 was inert towards excess cyclohexa-1,3-diene or benzophenone for 3 days at room temperature. In the presence of S8 or MeOTf, the phosphorus center of 2 was oxidized or methylated with the bicyclic skeleton remaining intact to afford species 6 (99 %) or 7 (99 %), respectively (Figure 4), and showcasing the potential utility of such bicyclic cycloaddition products for elaboration. C(16) C(34) iPr N iPr Et

H C H P S

C(13) N(1)

S(1) P(1)

C(2) C(1)

Ad Et 6

Four IBOs of the stationary points of 4, TS4 and 5 were identified and shown to involve the π-bonds of the phosphaalkene and the dipp ring (Figure 6a). Along the intrinsic reaction coordinate,18 these IBOs continuously convert from the electronic structure of 4 to that of 5, displaying the electron flow from one of the delocalized ring πorbitals through the phosphaalkene unit to the newly formed C-C σ-bonding orbital, as well as the redistribution of the ring π-electrons to afford the diene framework. Concurrent with this is the distortion of the six-membered C6 ring. This concerted process was also evidenced by the selected NLMOs of TS4 (Figure 6b). The formations of the P-C and C-C σ-bonds proceed at the same time with the contribution of highly delocalized electrons in the p-orbitals of the ring carbons and the phosphaalkene moiety, which can stabilize TS4 and facilitate the process accordingly. iPr

Et

C(16)

3 (0.0)

C(34)

MeOTf, r.t.

Et 7

Et

KHMDS + C6H6

2

H iPr C H OTf P Me N iPr

P

N

1/8 S8, r.t.

Et

the experimental observations that the formation of 2 is rapid and irreversible.

C(13) P(1)

iPr TMS C H N H H TMS tBu TS1 (14.3)

P

N

Et

HMDS

K(C6H6)OTf

N

tBu Et TS2 (6.7)

Et IN2 (-0.9)

(-6.2)

C(35)

iPr

iPr CH2

IN1

P

N

C(1)

Et

Et

P

N iPr CH2

tBu

P

iPr CH2

tBu Et TS3 (2.1)

iPr N iPr Et

iPr

iPr

N(1) C(2)

iPr

Et

Et

Et

CH2 P tBu

TS4 (-6.4)

iPr CH2

tBu

4 (-27.0)

5 (-30.1)

Figure 5. Free energy profile for formation of 5. Energies given in kcal/mol.

Ad

Figure 4. Synthesis of 6 and 7, and POV-ray depictions of the Xray structures of 6 and 7. C black, N blue, P orange. S yellow. Hydrogen atoms and the triflate anion are omitted for clarity. To better understanding the nature of the reversible binding event and the pathway leading to 5, a series of DFT calculations (SMD-M06-2X/Def2-TZVP//M06-2X/Def2-SVP), including intrinsic bond orbital (IBO)15 and natural localized molecular orbital (NLMO)16 analyses (See the SI for details), were carried out (Figure 5). At the outset, a salt metathesis of 3 with KHMDS occurs. Concurrent with this is the deprotonation process through TS1 (14.3 kcal/mol) in which the proton at the methyl group migrates to the [N(TMS)2]- anion, resulting in the formation of IN1 (-6.2 kcal/mol), which features a λ5 phosphorus center, as well as HMDS. Subsequently, the phosphorus atom in the seven-membered ring is extruded via TS2 (6.7 kcal/mol) to give the kinetically unstable 7phosphanorcaradiene IN2 (-0.9 kcal/mol).13b, 17 IN2 undergoes facile concerted P-C bond cleavage to afford the phosphaalkene 4 (-27.0 kcal/mol). Note that the phosphaalkene moiety in the optimized structure of 4 is very close to the proximal dipp group (Figure 6a), thus promoting the ensuing [4+2] cycloaddition. This cycloaddition is identified as a concerted process with an activation barrier of 20.6 kcal/mol (4→TS4), and the free energy of the product 5 (-30.1 kcal/mol) is slightly lower than that of 4 (-27.0 kcal/mol), consistent with the van't Hoff analysis (ΔG = -1.4 kcal/mol). In addition, with the bulky Ad substituent, the corresponding phosphaalkene 4’ undergoes similar cycloaddition with lower activation barrier of 18.9 kcal/mol compared to that of 4→TS4 (Figure S28 in the SI). The free energy change of 4’→2 is -4.3 kcal/mol, in line with

(a)

4

TS4

5

(b)

Figure 6. (a) Selected IBOs of species 4, TS4 and 5 with enclosing 60% of the orbital electron’s density. (b) Selected NLMOs of TS4 (isovalue = 0.04). It is noteworthy that the additional linkages (i.e., C-N and C-P bonds) in 4 and 5 play a crucial role in the cycloaddition reactivity of the phosphaalkene moiety toward the dipp ring, as well as the remarkable stabilization of the non-aromatic bicyclic skeleton. Without either of these linkages, the departure of the phosphaalkene moiety is thermodynamically and kinetically favorable via a [4 + 2] cyclo-reversion with an activation barrier of less than 21 kcal/mol (Figure S28 in the SI). It is also interesting to note that such cycloadditions may



be extended to other main group multiple bonds, as computations involving replacement of the P=C unit of 4 with As=C, Si=C, Ge=C, P=Si or P=Ge bond infer that the analogous cycloadditions would be spontaneous but likely irreversible (Figures S30-S34 in the SI). In summary, deprotonation of phosphepinium cations generates phosphaalkenes that undergo cycloadditons to the N-bound aromatic rings affording the 2phosphabicyclo[2.2.2]octa-5,7-dienes. The bicyclic product 2 was further derivatized by oxidization or alkylation to give the species 6 and 7, respectively. In the case of this reaction with 3 the cycloaddition is reversible and establishes an equilibrium between the phosphaalkene 4 and the corresponding 2phosphabicyclo[2.2.2]octa-5,7-diene 5. This represents the first reversible cycloaddition of a P=C multiple bond species to an arene, while DFT predictions suggest that the corresponding reactions of As=C, Si=C, Ge=C, P=Si or P=Ge bonds should also be possible. These predictions are the subject of ongoing experimental work.

ASSOCIATED CONTENT Supporting Information Synthetic, spectroscopic, crystallographic and computational data (pdf) Structure data (cif). The Supporting Information is available free of charge on the ACS Publications website at DOI: XX

AUTHOR INFORMATION Corresponding Author

[email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Financial support from NSERC of Canada is gratefully acknowldged. D.W.S. is grateful for the support of of Canada Research Chair program. L. L. C. is grateful for award of NSERC scholarship.

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iPr

iPr N

iPr E

E'

Page 6 of 6

E' N iPr

E

E = P, E' = CH2

this work

E = As, E' = CH2 E = SiH, E' = CH2 E = GeH, E' = CH2 Achievable ! E = P, E' = SiH2 E = P, E' = GeH2

DFT predictions

TOC


6

Reversible Intramolecular Cycloaddition of Phosphaalkene to an

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