Potential Carbon–Fluorine Reductive Elimination from Pincer

Oct 2, 2014 - Potential Carbon–Fluorine Reductive Elimination from Pincer-Supported Rh(III) and Dominating Side Reactions: Theoretical and Experimen...
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Potential Carbon−Fluorine Reductive Elimination from PincerSupported Rh(III) and Dominating Side Reactions: Theoretical and Experimental Examination Samuel D. Timpa, Jia Zhou, Nattamai Bhuvanesh, and Oleg V. Ozerov* Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77842, United States S Supporting Information *

ABSTRACT: This paper explores the potential for C−F reductive elimination from Rh(III) pincer complexes. A DFT computational study indicated that concerted C−F reductive elimination from (POCOP)Rh(CHCH2)(F) (3) (where POCOP is κ3P,C,P-2,6(iPr2PO)2C6H3, and aryl/bis(phosphinite) pincer ligand) possesses an experimentally plausible activation barrier of ΔG⧧ = 28.3 kcal/mol. This barrier is considerably lower than that calculated (35.7 kcal/mol) for the analogous C−F reductive elimination from (POCOP)Rh(Ph)(F) (1). The difference is ascribed to the need for a partial rotation of a phenyl or vinyl group in the transition state, with the phenyl being more encumbered by the steric bulk of the supporting pincer ligand. DFT calculations did not analyze the full range of possible side reactions, which have proven to be dominant. The attempted synthesis of 1 was unsuccessful because of competing C−C reductive elimination at the stage of the preparation of the (POCOP)Rh(CHCH2)(I) precursor. DFT calculations predicted C−C reductive elimination to be facile but markedly unfavorable in the monomeric unit because of inherent strain in the product. That strain is apparently relieved in dimerization that takes place with opening up of the pincer to become bridging between two Rh centers. The opening up of the pincer, and thus dimerization and C−C reductive elimination, was prevented by the use of the tBuPOCOP ligand (κ3P,C,P-2,6(iPr2PO)2-3,5-But2C6H3), which allowed isolation of (tBuPOCOP)Rh(CHCH2)(I) (11). Compound 11 was converted to (tBuPOCOP)Rh(CHCH2)(OTf) (12) via reaction with AgOTf. However, treatment of 11 with AgF or of 12 with CsF failed to produce (tBuPOCOP)Rh(CHCH2)(F), resulting instead in multiple products containing P−F bonds. On the other hand, 12 was cleanly converted to (tBuPOCOP)Rh(CHCH2)(OBut) (13) via reaction with NaOBut and then to (tBuPOCOP)Rh(CHCH2)(OC6H4F-p) (14) by treatment of 13 with p-fluorophenol. Neither 13 nor 14 gave any evidence of C−O reductive elimination. Instead, thermolysis of either 13 or 14 resulted in dehydroalkoxylation to the bimetallic divinylacetylene complex (tBuPOCOP)Rh(CH2CHCCCHCH2)Rh(tBuPOCOP) (15) as the major product.

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INTRODUCTION The C−F functional group is prevalent in a number of pharmaceuticals,1 agrochemicals,2 imaging agents,3 and new high-performance materials.4 However, well-evolved methods for C−F bond formation are still uncommon. Many of the more traditional methods for forming C−F bonds are limited in scope, especially for aromatic and C(sp2)−F bonds in general. Direct oxidative fluorination with F25 or CuF26 involves rather harsh conditions. Nucleophilic aromatic substitution with fluoride is applicable only to rather specific substrates with strongly electron withdrawing groups.7 The Balz−Schiemann fluorodediazonization8 and fluoridation of diaryliodonium salts9 require introduction of a nontrivial functionality early in the synthesis, which reduces the scope of these processes. Transition-metal catalysis offers the promise of C−F bond formation under milder conditions and with a broader scope of substrates.10−12 Concerted C−F reductive elimination (RE) is the common C−F bond-making step in most of the various transition-metal-based approaches to the synthesis of C(sp2)−F bonds. It has been pointed out that C−F RE is intrinsically the most difficult concerted C−X RE process, because of the low polarizability of fluorine and also because of the relative facility of side reactions for fluoride complexes.10−12 RE generally © 2014 American Chemical Society

becomes more thermodynamically favorable and kinetically accessible for metal centers in higher oxidation states. C−F RE has been implicated for a number of complexes of group 10 metals (Ni, Pd, Pt) in the +4 oxidation state,13−16 as well as proposed for Ag(II)/Ag(III)17 and Cu(III)18 complexes in Agand Cu-based catalysis. However, these high-oxidation-state complexes can only be reached with the use of strong oxidants and typically cannot be accessed via oxidative addition of C−X bonds under mild conditions, except perhaps with the Cu(I)/ Cu(III) couple.19,20 This has been exploited in electrophilic fluorination catalysis,21 where the high-valent metal fluoride is obtained by oxidation with an F2-derived “F+” reagent, such as XeF2 or one of the N−F22 reagents. The aryl group for C−F coupling is delivered in the “nucleophilic” capacity, either by directed C−H activation or via an arylboron or aryltin reagent. In a hybrid version of electrophilic fluorination, high-valent metal fluorides have been accessed via a combination of a fluoride-free strong oxidant and nucleophilic fluoride.23 Electrophilic fluorination appears especially viable for the synthesis of high-value C−F-containing products, including Received: August 29, 2014 Published: October 2, 2014 6210

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late-stage synthesis modification24 and 18F-labeled radiopharmaceuticals.25−27 Catalytic nucleophilic fluorination10 is conceived to mimic the well-established Pd(0)/Pd(II) chemistry of carbon−carbon and carbon−heteroatom coupling (Scheme 1).28,29 This

it involves different d electron configurations (d6/d8 for Rh vs d8/d10 for Pd) at the metal and different coordination geometries. OA of aryl halides requires a three-coordinate (pincer)RhI intermediate and forms a five-coordinate (pincer)RhIII product. RE of C−F would then have to take place from the corresponding five-coordinate Rh(III) aryl/fluoride complex. Five-coordinate (pincer)RhIII complexes are readily isolable and do not show propensity for dimerization,38−41 in contrast to the difficulty in accessing truly three-coordinate Pd(II) complexes.42 In addition, OA/RE processes at Rh do not require changes in the disposition of the donor atoms of the pincer. These considerations held some promise for Rhbased C−F coupling catalysis and encouraged us to explore whether C−F RE may be accessible with Rh. Presently, we report the results of our initial foray into this problem, which uncovered significant competing reactions.

Scheme 1. Mechanism for Nucleophilic C−F Bond Formation via the Pd(0)/Pd(II) Cycle and the Proposed Analogous Rh(I)/Rh(III) Cycle

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RESULTS AND DISCUSSION Theoretical Analysis. We selected (POCOP)Rh(Ph)(F) (1) and (POCOP)Rh(CHCH2)(F) (3) for our theoretical analysis of the kinetic and thermodynamic accessibility of C−F reductive coupling, as shown in Scheme 2 and Figure 1. The approach utilizes simple fluoride salts (e.g., CsF, AgF) as sources of nucleophilic fluoride and aryl halides or sulfonates as formal carbon electrophiles. Aryl halide oxidative addition (OA) to Pd(0) forms a PdII−C bond, and transmetalation with a fluoride source establishes a Pd(II) aryl/fluoride complex for C−F reductive elimination. Execution of C(sp2)-F coupling via a nucleophilic pathway with Pd has encountered many obstacles. Pioneering work by Grushin30 and co-workers produced a series of PR3-substituted LnPd(Ar)(F) compounds (L = PR3); however, none of these complexes underwent successful Ar-F RE. Theoretical analysis by Yandulov31 and more recently by Saeys32 identified two general issues with LnPd(Ar)(F): (1) the necessity of access to three-coordinate LPd(Ar)(F) for reasonable C−F RE activation barriers and (2) the competitiveness of side reactions such as C−C and P−F coupling, because of the relatively high barrier for C−F RE. More recently, Buchwald and co-workers have demonstrated the utility of their bulky phosphinobiaryl ligands, such as BrettPhos and RockPhos, with Pd to form Ar-F bonds both stoichiometrically and catalytically.33 The success of these ligands for challenging RE processes, including C−N and C−O coupling, is attributed to their large and enveloping steric profile, which facilitates pseudo-monocoordinate Pd(0) as well as pseudo-three-coordinate Pd(II).34 These ligands can also be broadly tuned by altering the phosphine substituents to achieve the desired steric and electronic environment at the metal center.35,36 However, the significant cost of the phosphinobiaryl ligands that support C−F coupling and the relatively low turnover numbers reduce the practicality of this method at present. Our group has been pursuing chemistry relevant to catalysis of aryl halide coupling reactions with Rh complexes of pincertype37 ligands. In addition to demonstrating the viability of the requisite C(sp2)−X OA/RE elementary reactions,38 we have recently reported that C−C,39 C−N,40 and C−S41 coupling of aryl halides can be carried out catalytically using simple aryl/ bis(phosphinite) POCOP pincer ligands with Rh. The putative Rh(I)/Rh(III) catalytic cycle (Scheme 1) for C−F coupling is functionally analogous to the Pd(0)/Pd(II) cycle, even though

Scheme 2. Calculated ΔG and ΔG⧧ Values for C−F Reductive Coupling from (POCOP)Rh(Ph)(F) (1) and (POCOP)Rh(CHCH2)(F) (3) at the M06/SDD/6311G(d,p) Level of Theory

products were optimized as the F-bound adducts of fluorobenzene and fluoroethylene with (POCOP)Rh (2 and 4a, respectively) in order to avoid the steric and electronic differences between fluorobenzene and fluoroethylene when binding via their π systems. Binding to the π system of fluoroethylene (4b) was calculated to be 18.3 kcal/mol more favorable than binding via the fluorine (Scheme 2), but as it occurs following the transition state, it should only affect the thermodynamics and not the barrier for C−F reductive coupling. In a putative catalytic reaction, any C−F coupled product would most likely dissociate and be replaced by OA of 6211

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Figure 1. Potential energy surface of C(vinyl)−F reductive elimination and C(vinyl)−C(aryl) reductive elimination with (POCOP)Rh at the M06/ SDD/6-311G(d,p) level of theory.

the substrate organohalide (halide = Br, I, OTf) to the unsaturated (POCOP)RhI center.43 The thermodynamics of the formation of 2 and 4a by C−F reductive coupling were calculated to be nearly the same (ΔGrxn values of 4.3 and 3.6 kcal/mol, respectively), suggestive of similar electronic factors in vinyl− and phenyl−fluoride bond formation. However, the reaction barrier was considerably lower for the vinyl−fluoride coupling (ΔG⧧ = 28.3 kcal/mol) than for the phenyl−fluoride coupling (ΔG⧧ = 35.7 kcal/mol). The disparity in reaction barriers for concerted reductive coupling reactions of phenyl vs vinyl has been previously studied in the (POCOP)Ir system, where it was attributed to the smaller steric profile of the vinyl group.44 For concerted reductive coupling, the aryl (or vinyl) group has to “face” its coupling partner (in our work fluoride) in the transition state. However, in pincer systems similar to (POCOP)Rh, the bulky phosphines force a perpendicular, “side-on” orientation of the aryl (or vinyl) group (Scheme 3). Therefore, the aryl or vinyl group must rotate about the metal−carbon bond to attain the “face-on” arrangement in the transition state. This rotation should be more facile for the smaller vinyl group. Notably, this is not a concern in the concerted Ar-X bond formation from

L2Pd(Ar)(X) or LPd(Ar)(X), where the bulky L ligands encourage the “face-on” arrangement expected for the transition state. We have also examined the potential for C−C reductive coupling involving the Rh-bound carbon of the POCOP ligand. Not surprisingly, calculations revealed the barrier for aryl−vinyl C−C coupling from 3 is much lower than that for C−F coupling (TS3-5, Figure 1). However, the product formed via C−C reductive coupling (5)45 is 6.1 kcal/mol less favorable than 4a, is 23 kcal/mol less favorable than 4b, and is considerably unfavorable (by 9.7 kcal/mol) with respect to 3. C−C reductive elimination from (pincer)RhIII species is typically quite favorable, but in this case, the constraint of the chelate ligand makes for a product (5) with a high degree of congestion and strain. Bearing this in mind, we decided to explore the experimental potential of C−F reductive coupling from (POCOP)Rh(CHCH2)(F). Attempted Synthesis of (POCOP)Rh(CHCH2)(X) Compounds. We previously described oxidative addition of aryl halides to (PNP)Rh(SiPr2)38,43 and believed an analogous reaction between CH2CHI and (POCOP)Rh(SiPr2) could provide a synthetically useful precursor toward the synthesis of 3. The reaction of (POCOP)Rh(SiPr2) (6) with vinyl iodide resulted in a reaction upon mixing at room temperature. By analogy with (POCOP)Rh(Ar)(I),39 (POCOP)Rh(CHCH2)(I) would be expected to display a doublet resonance in the 31 1 P{ H} NMR spectrum with JRh−P ≈ 120 Hz; however, such a resonance was not in evidence. The reaction instead resulted in a mixture of compounds with one dominant product (ca. 60%). This compound was isolated in a pure form in 52% yield from the reaction mixture as an orange crystalline solid and identified as 7 on the basis of the solution NMR data and an X-ray structural study on a single crystal (Scheme 4). 7 displayed two sets of doublets of doublets in the 31P{1H} NMR spectrum at 200.5 ppm (JRhP = 140 Hz, JPP = 473 Hz) and 163.7 ppm (JRhP = 128 Hz, JPP = 473 Hz). The solid-state structure showed a bimetallic complex containing two square-planar Rh(I) centers, ostensibly arising from 2 by way of C(aryl)−C(vinyl) reductive elimination to form 5 and subsequent dimerization of the latter. The C−C bond lengths (1.420(6) and 1.412(6) Å) of the bound olefins are typical for an η2-olefin complex of Rh(I).46 The key difference between the computationally evaluated 5 and its dimer 7 is that the two phosphinite donors of the same

Scheme 3. Illustration of the Rotation about the Rh−C(sp2) Bond Necessary in the Transition State vs the Ground State of 1 or 3a

a

Overlayed stick representations of the calculated transition states for the Ph−F (TS1-2) and vinyl−F (TS3-4) coupling are shown at the bottom. 6212

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straightforward than that of the corresponding (POCOP)Rh(H)(Cl).39 9 formed in nearly quantitative yield by the reaction of (tBuPOCOP)H (8) with [(cod)RhCl]2 after 2 h at 100 °C and displayed a characteristic hydride resonance appearing as a doublet of triplets (JRhH = 44 Hz, JPH = 13 Hz) at −25.19 ppm in the 1H NMR spectrum. There was no indication of the POCOP-bridged side product that plagued the reaction of (POCOP)H with [(cod)RhCl]2.39 Notably, that side product must require the POCOP ligand to contort in a fashion similar to that observed in 7. (tBuPOCOP)Rh(SiPr2) (10) was formed by reaction of 9 with NaOtBu in the presence of SiPr2. Treatment of 10 with vinyl iodide at room temperature resulted in an immediate color change to red and 95% conversion to (tBuPOCOP)Rh(CHCH2)(I) (11)a product analogous to P2. 11 was isolated as a red solid and displayed a doublet at 177.4 ppm with JRhP = 122 Hz in the 31P{1H} NMR spectrum. The reaction consistently produced ca. 5% of a side product at 204 ppm (JRhP = 155 Hz) in the 31P{1H} NMR spectrum, which would be consistent with formation of an Rh(I) olefin adduct. For example, the 1-hexene adduct of (tBuPOCOP)Rh displays a doublet at 197.3 ppm with JRhP = 160 Hz. We initially thought the impurity could be an equilibrium between the isomeric η2iodoethylene adduct and 11. However, the reaction of (tBuPOCOP)Rh(CHCH2)(OTf) (12), which is formed by treatment of 11 with AgOTf, with Me3SiI resulted in conversion to 11 with no observation of the 5% impurity. The vinyl iodide was used as received from the distributor with reported 95% purity, with no additional purification. Examination of the 1H NMR spectrum vinyl iodide showed a trace of divinyl ether, which could be the olefin source for the 5% impurity. The reaction of 12 with CsF in C6D6 after 24 h at room temperature resulted in complete conversion of 12 to a mixture of products, with no indication of the desired (tBuPOCOP)Rh(CHCH2)(F) product. Analysis of the reaction mixture by 19 F NMR spectroscopy indicated P−F bond formation, as evidenced by the presence of 19F NMR resonances displaying very strong coupling to 31P (JPF = 480−640 Hz), but no signal attributable to a Rh−F bond.47 Similarly, treatment of 11 with AgF produced no evidence of Rh−F and NMR resonances with

Scheme 4. Reactivity of Vinyl Iodide with (POCOP)Rh(SiPr2) (5) and ORTEP Drawing (50% Probability Ellipsoids) of 7 Showing Selected Atom Labelinga

a

Hydrogen atoms are omitted for clarity, with the exception of the vinylic protons on C1, C2, C27, and C28. Selected bond distances (Å) and angles (deg) for 7: P1−Rh1, 2.359(1); P2−Rh1, 2.259(1); Rh1− C1, 2.140(4); Rh1−C2, 2.123(4); C1−C2, 1.420(6); P1−Rh1−P2, 168.47(4); I1−Rh1−C1, 171.2(1); P2−O2−C4, 119.0(3); C8−O3− P3, 132.1(2), C1−C2−C3, 123.4(3).

POCOP ligand are bound to two different Rh atoms. This rearrangement relieves the strain inherent in 5 and leads to geometrically “normal”, square-planar Rh(I) product. From the X-ray structure, it is clear that, in order for 7 to form, the phosphinites must be able to rotate about the C−O bond. We surmised that, if this movement could be restricted, it should prevent formation of 7 and preclude formation of a stable Rh(I) product after C(aryl)−C(vinyl) reductive elimination. Synthesis of (tBuPOCOP)Rh Complexes. We envisioned that the analogous tert-butyl-substituted tBuPOCOP ligand would provide just such a restriction and investigated the synthesis of (tBuPOCOP)Rh(CHCH2)(I) (11, Scheme 5). Synthesis of (tBuPOCOP)Rh(H)(Cl) (9) proved much more

Scheme 5. Reactivity of (tBuPOCOP)Rh in the Direction of a Rh Vinyl Fluoride Complex

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large JPF coupling constants. P−F bond formation has been observed previously with phosphine-substituted rhodium fluoride complexes, including the fluoride analogue of Wilkinson’s catalyst.48 In order to probe for potential side reaction pathways in a related system, we turned to the analogous vinyl/alkoxide complexes. C−O reductive elimination is also regarded as quite challenging, largely for the same reasons as for C−F, if to a lesser degree.49 In this vein, we prepared two (tBuPOCOP)Rh(CHCH2)(OR) (R = tBu, p-C6H4F) complexes (Scheme 6).

The major product was isolated from the reaction mixture from the thermolysis of 14 and analyzed by 1H NMR and 31P NMR spectroscopy. The 1H NMR spectrum displayed no downfield vinylic proton resonances, indicating the absence of an η1-vinyl substituent. The spectrum also showed one aromatic resonance, corresponding to the C−H on the POCOP backbone, which indicated that the OC6H4F group was not present in the major product from the thermolysis of 14. The product displayed two doublets in the 31P{1H} NMR spectrum (197.6 ppm, JRhP = 158 Hz; 197.5 ppm, JRhP = 158 Hz), which we interpreted as an ABX pattern with outer bands too small to observe due to the close proximity of the chemical shifts of the inequivalent 31P nuclei. The identity of the major product was determined to be the bridging divinylacetylene complex 15 by X-ray analysis of a single crystal grown from a saturated toluene solution at room temperature (Figure 2). The exact mechanism of how 15 forms is unclear. However, the composition of 15 suggests a stoichiometry whereby it arose from two (tBuPOCOP)Rh fragments combined with three C2 units, with loss of three ROH molecules and one (tBuPOCOP)Rh fragment. This is consistent with the ca. two-thirds yield of 15. In addition, when thermolysis of 14 was carried out in the presence of SiPr2, a ca. 60:40 mixture (by 31P NMR intensity) of 15 and 10 was observed, and p-FC6H4OH was the only product detectable by 19 F NMR spectroscopy.

Scheme 6. Reactivity of (tBuPOCOP)Rh(CHCH2)(OR) Complexes

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CONCLUSION

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EXPERIMENTAL SECTION

The (POCOP)Rh and (tBuPOCOP)Rh systems were unsuccessful for C−F bond formation. Both reactions suffered from dominating lower energy side processes, a problem that has plagued many metal systems. The (POCOP)Rh system underwent C−C RE with the aryl backbone of the pincer ligand. Installation of tert-butyl groups on the central aryl ring of the POCOP system obviated C−C coupling as the side reaction, but the (tBuPOCOP)Rh system underwent P−F bond formation upon attempts to make (tBuPOCOP)Rh(CHCH2)(F). (tBuPOCOP)Rh(CHCH2)(OR) complexes were also examined. C−O reductive elimination was not observed, and instead a divinylacetylene complex was formed.

(tBuPOCOP)Rh(CHCH2)(OtBu) (13) was synthesized by reacting 12 with 1.2 equiv of NaOtBu in toluene at room temperature and isolated in 49% yield. (tBuPOCOP)Rh(CHCH2)(OC6H4F) (14) was isolated in 68% yield from the reaction of 13 with 1 equiv of p-FC6H4OH. 13 and 14 displayed doublets at 165.9 ppm (JRhP = 130 Hz) and 169.2 ppm in 31P NMR spectroscopy, respectively. Independent thermolyses of 13 and 14 at 110 °C in toluene resulted in mixtures of products; however, both reactions formed the same major product as ca. 70% of the 31P content of the mixture.50 Neither reaction displayed resonances consistent with formation of the free or coordinated C−O reductive elimination product by 1H NMR or 31P NMR spectroscopy.

General Considerations. Unless otherwise specified, all manipulations were performed under an argon atmosphere using standard

Figure 2. ORTEP drawings (50% probability ellipsoids) of 13 (left) and 15 (right) showing selected atom labeling. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg) for 13: Rh1−P1, 2.286(1); Rh1−P2, 2.308(1); Rh1−C1, 1.993(3); Rh1−O3, 2.053(2); C1− C2, 1.321(4); O3−C3, 1.419(3); P1−Rh1−P2, 151.19(3); C7−Rh1−O3, 159.69(9); Rh1−C1−C2, 135.6(2); Rh1−O3−C3, 134.7(2). Selected bond distances (Å) and angles for 15: Rh1−P1, 2.261(2); Rh1−P2, 2.242(2); Rh1−C1, 2.175(7); Rh1−C2, 2.206(8), C1−C2, 1.39(1); C2−C3, 1.42(1); C3−C3′, 1.23(1); P1−Rh1−P2, 156.72(7), C16−Rh1−C1, 159.1(3); C16−Rh1−C2, 163.9(3); C1−C2−C3, 123.4(8); C2−C3−C3′, 176.7(9). 6214

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solution of toluene layered with pentane at −35 °C. 13P{1H} NMR (C6D6): δ 184.4 (d, 122 Hz). 1H NMR (C6D6, Figure 5-5 in the Supporting Information): δ 7.22 (s, 1H, POCOP), 2.50 (m, 2H, CHMe2), 2.10 (m, 2H, CHMe2), 1.47 (s, 18H, tBu), 1.29 (q, 9.0 Hz, 6H, CHMe2), 1.16 (q, 9.0 Hz, 6H, CHMe2), 1.07 (m, 12H, CHMe2), −25.19 (dt, JRhH = 44 Hz, JPH = 13 Hz, 1H, Rh-H). 13C{1H} NMR (C6D6): 162.3 (t, JPC = 6.4 Hz), 130.5 (dt, JRhC = 30 Hz, JPC = 5.0 Hz), 127.6 (t, JPC = 5.1 Hz), 122.0 (s), 34.7 (s, CMe3), 30.4 (s, CMe3), 30.0 (t, JPC = 11 Hz, CHMe2), 28.5 (td, JPC = 14 Hz, JRhC = 2.0 Hz, CHMe2), 17.6 (s, CHMe2), 17.4 (t, JPC = 2.4 Hz, CHMe2), 17.1 (t, JPC = 4.4 Hz, CHMe 2 ), 16.8 (s, CHMe 2 ). Anal. Calcd for C26H48ClO2P2Rh: C, 52.66; H, 8.16. Found: C, 52.64; H, 8.20. Synthesis of (tBuPOCOP)Rh(SiPr2) (10). 9 (1.0 g, 1.7 mmol) was placed in a Schlenk flask and dissolved in toluene. NaOtBu (300 mg, 3.4 mmol) and SiPr2 (500 μL, 3.4 mmol) were added to the solution, and the reaction mixture was stirred for 1 h at room temperature. This resulted in an immediate color change to red-brown. The volatiles were removed by vacuum. The product was extracted with pentane and passed through a pad of Celite. The volatiles were removed by vacuum to give clean 10 as a brown-orange solid (1.1 g, 91%). 31P{1H} NMR (C6D6): δ 183.1 (d, 173 Hz). 1H NMR (C6D6, Figure 5-6 in the Supporting Information): δ 7.24 (s, 1H, POCOP), 2.72 (m, 2H, CHMe2), 2.20 (m, 2H, CHMe2), 1.65 (s, 18H, tBu), 1.27 (m, 36H, CHMe2 and S(iPr)2). 13C{1H} NMR (C6D6): δ 162.8 (t, JPC = 8.5 Hz), 144.2 (dt, JRhC = 33 Hz, JPC = 2.5 Hz), 124.4 (t, JPC = 6.0 Hz), 119.9 (s), 41.0 (t, JPC = 3.0 Hz, SCHMe2), 34.7 (s, CMe3), 30.8 (s, CMe3), 30.7 (td, JPC = 10 Hz, JRhC = 3.0 Hz, CHMe2), 24.2 (s, SCHMe2), 18.7 (t, JPC = 3.5 Hz, CHMe2), 17.8 (s, CHMe2). Anal. Calcd for C32H61O2P2RhS: C, 56.96; H, 9.11. Found: C, 56.94; H, 9.08. Synthesis of (tBuPOCOP)Rh(CHCH2)(I) (11). 10 (500 mg, 0.74 mmol) was placed in a Schlenk flask and dissolved in toluene. Vinyl iodide (165 μL, 2.2 mmol) was added to the solution, resulting in an immediate color change to red. The reaction mixture was stirred for 20 min at room temperature. The solution was passed through a pad of Celite, and the volatiles were removed by vacuum to give a red solid (489 mg, 93%). The red solid was 95% clean by 31P{1H} NMR spectroscopy, with a minor impurity at 204 ppm (d, 155 Hz). The identity of the impurity is still unknown. 31P{1H} NMR (C6D6): δ 177.4 (d, 122 Hz); 1H NMR (C6D6, Figure 5-7 in the Supporting Information): δ 7.30 (s, 1H, POCOP), 6.78 (d, 11 Hz, 1H, CHCH2), 3.88 (s, 1H, CHCH2), 3.79 (d, 14 Hz, 1H, CHCH2), 2.65 (m, 4H, CHMe2), 1.51 (s, 18H, tBu), 1.41 (q, 9.0 Hz, 6H, CHMe2), 1.31 (q, 9.0 Hz, 6H, CHMe2), 0.96 (q, 9.0 Hz, 6H, CHMe2). 13C{1H} NMR: δ 160.6 (t, JPC = 5.9 Hz), 145.6 (d, JRhC = 35 Hz, CHCH2), 142.4 (dt, JRhC = 34 Hz, JPC = 10 Hz), 128.4 (s), 122.1 (d, JRhC = 6.0 Hz, CHCH2), 119.8 (t, JPC = 4.4 Hz), 34.9 (s, CMe3), 31.6 (t, JPC = 11 Hz, CHMe2), 30.4 (s, CMe3), 28.6 (td, JPC = 13 Hz, JRhC = 2.0 Hz, CHMe2), 18.3 (s, CHMe2), 18.1 (s, CHMe2), 17.8 (s, CHMe2), 16.8 (s, CHMe2). Synthesis of (tBuPOCOP)Rh(CHCH2)(OTf) (12). 11 (101 mg, 0.15 mmol) was placed in a Schlenk flask and dissolved in toluene. AgOTf (46 mg, 0.18 mmol) was added to the sample, resulting in immediate precipitation and a color change to brown. The sample was stirred at room temperature for 60 min. The volatiles were removed by vacuum. The product was extracted with pentane and passed through a pad of Celite. The volatiles were removed to give 11 as a brown-orange solid. 31 1 P{ H} NMR (C6D6): δ 173.0 (d, 126 Hz); 1H NMR (C6D6, Figure 5-8 in the Supporting Information): δ 7.25 (s, 1H, POCOP), 6.80 (d, 13 Hz, 1H, CHCH2), 4.13 (m, 1H, CHCH2), 3.94 (d, 13 Hz, 1H, CHCH2), 2.66 (m, 2H, CHMe2), 2.56 (m, 2H, CHMe2), 1.43 (s, 18H, t Bu), 1.21 (m, 12H, CHMe2), 1.12 (q, 9.0 Hz, 6H, CHMe2), 0.92 (q, 9.0 Hz, 6H, CHMe2); 19F NMR (C6D6): δ-77.7 (s). 13C{1H} NMR (C6D6): δ 162.0 (t, JPC = 5.4 Hz), 140.2 (dt, JRhC = 37 Hz, JPC = 9.6 Hz, CHCH2), 133.3 (dt, JRhC = 40 Hz, JPC = 5.1 Hz), 123.4 (s), 122.1 (s, CHCH2), 118.1 (t, JPC = 4.0 Hz), 34.8 (s, CMe3), 30.9 (t, JPC = 10 Hz, CHMe2), 30.3 (s, CMe3), 28.3 (td, JPC = 13 Hz, JRhC = 2.4 Hz, CHMe2), 18.5 (t, JPC = 3.0 Hz, CHMe2), 18.1 (s, CHMe2), 16.7 (t, JPC = 2.2 Hz, CHMe 2 ), 16.1 (s, CHMe 2 ). Anal. Calcd for C29H50F3O5P2RhS: C, 47.54; H, 6.88. Found: C, 47.60; H, 6.77.

Schlenk line or glovebox techniques. Toluene, THF, pentane, and isooctane were dried and deoxygenated (by purging) using a solvent purification system and stored over molecular sieves in an Ar-filled glovebox. C6D6 and hexanes were dried over and distilled from NaK/ Ph2CO/18-crown-6 and stored over molecular sieves in an Ar-filled glovebox. Fluorobenzene was dried with and then distilled or vacuumtransferred from CaH2. All other chemicals were used as received from commercial vendors. NMR spectra were recorded on a Varian NMRS 500 (1H NMR, 499.686 MHz; 13C NMR, 125.659 MHz, ;31P NMR, 202.298 MHz; 19F NMR, 470.111 MHz) spectrometer. For 1H and 13 C NMR spectra, the residual solvent peak was used as an internal reference. 31P NMR spectra were referenced externally using 85% H3PO4 at δ 0 ppm. 19F NMR spectra were referenced externally using 1.0 M CF3CO2H in CDCl3 at −78.5 ppm. Elemental analyses were performed by CALI Laboratories, Inc. (Parsippany, NJ). Computational Details. All computations were carried out with the Gaussian09 program.51 All of the geometries were fully optimized in toluene solvent via the PCM model52 by the M0653 functional. The Stuttgart basis set (SDD) and the associated effective core potential (ECP) was used for Rh atoms, and an all-electron 6-311G(d,p) basis set was used for the other atoms. The harmonic vibrational frequency calculations were performed to ensure that either a minimum or firstorder saddle point was obtained. The energies reported here are Gibbs free energies with solvent effect corrections at 298.15 K and 1 atm unless noted otherwise. Reaction of 6 with CH2CHI. 6 (110 mg, 0.20 mmol) was placed in a J. Young tube and dissolved in C6D6. Vinyl iodide (15 μL, 0.21 mmol) was added to the solution, resulting in the immediate precipitation of a large amount of orange solid. Analysis of the solution after 24 h at room temperature by 31P{1H} NMR spectroscopy indicates no remaining (POCOP)Rh(SiPr2) and several products. The major product (60%) appears as two sets of doublets of doublets (200.5 ppm, dd, JRhP = 140 Hz, JPP = 473 Hz; 163.7 ppm, dd, JRhP = 128 Hz, JPP = 473 Hz). There is no indication of the desired product, (POCOP)Rh(CHCH2)(I), by 31P{1H} NMR, which would be expected to display a doublet with Rh−P coupling around 120 Hz. The sample was transferred to a Schlenk flask, and the solvent was decanted off from the orange solid. The solid was washed with pentane and dried under vacuum. Analysis of the solid by 31P{1H} NMR identified the precipitate as the major product 7. X-ray-quality crystals of the solid were grown from a concentrated solution of the solid in toluene at room temperature. 31P{1H} NMR (CD2Cl2): δ 200.2 (dd, JRhP = 140 Hz, JPP = 468 Hz), 163.0 (dd, JRhP = 128 Hz, JPP = 468 Hz. 1 H NMR (CD2Cl2, Figure 5-3 in the Supporting Information): δ 6.99 (t, 7.5 Hz, 2H, POCOP), 6.77 (d, 9.0 Hz, 2H, POCOP), 6.63 (d, 9.0 Hz, 2H, POCOP), 5.41 (m, 2H, CHCH2), 3.33 (m, 2H, CHCH2), 3.22 (m, 2H, CHMe2), 3.04 (m, 2H, CHMe2), 2.85 (m, 2H, CHCH2), 2.39 (m, 2H, CHMe2), 1.68 (dd, 15 Hz, 7.5 Hz, 6H, CHMe2), 1.59 (m, 12H, CHMe2), 1.45 (m, 12H, CHMe2), 1.18 (dd, 10 Hz, 7.0 Hz, 6H, CHMe2), 1.08 (dd, 11 Hz, 7.5 Hz, 6H, CHMe2), 1.00 (dd, 16 Hz, 7.0 Hz, CHMe2). Anal. Calcd for C40H68I2O4P4Rh2: C, 40.15; H, 5.73. Found: C, 39.99; H, 5.59. Synthesis of (tBuPOCOP)H (8). 4,6-Di-tert-butylresocinol (1.07 g, 4.8 mmol) was placed in a reaction vial and dissolved in toluene. Et3N (2.0 mL, 14.5 mmol) and ClPiPr2 (1.51 mL, 10.1 mmol) were added to the solution, and the reaction mixture was heated at 110 °C for 24 h. The mixture was passed through a pad of Celite, and the volatiles were removed by vacuum, leaving an oily white solid. The solid was dissolved in a minimum of pentane and left at −35 °C. The product crashed out of solution as a crystalline white solid (1.0 g, 46%). 31 1 P{ H} NMR (C6D6): δ 139.0 (s). 1H NMR (C6D6, Figure 5-4 in the Supporting Information): 8.52 (t, 7.0 Hz, 1H, Ar-H), 7.46 (s, 1H, ArH), 1.83 (m, 4H, CHMe2), 1.54 (s, 18H, tBu), 1.14 (dd, 7.5 Hz, 3.5 Hz, 12H, CHMe2), 1.02 (dd, 7.5 Hz, 7.5 Hz, 12H, CHMe2). Synthesis of (tBuPOCOP)Rh(H)(Cl) (9). 8 (1.5 g, 3.3 mmol) was placed in a reaction vial and dissolved in toluene. [(cod)RhCl]2 (0.81 g, 1.6 mmol) was added to the solution, and the mixture was stirred at 100 °C for 2 h. The solution was passed through a pad of Celite. The volatiles were removed by vacuum to give clean 9 as an orange solid (1.9 g, 95%). The product can be recrystallized from a saturated 6215

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Article

Treatment of 11 with AgF. (tBuPOCOP)Rh(CHCH2)(I) (11; 27 mg, 0.04 mmol) was combined in a Schlenk flask with AgF (25 mg, 0.20 mmol) and dissolved in THF. The reaction mixture was stirred at room temperature overnight, resulting in a brown suspension. Analysis of the reaction mixture by 31P NMR spectroscopy showed a complex mixture of products. The 19F NMR spectrum showed several signals with large coupling (J = 640 Hz, J = 853 Hz, J = 982 Hz), consistent with P−F bond formation. Treatment of 12 with CsF. 12 (33 mg, 0.045 mmol) was placed in a Schlenk flask and dissolved in C6D6. CsF (14 mg, 0.090 mmol) was added to the flask, and the reaction mixture was stirred at room temperature. After 24 h at room temperature, the mixture showed complete conversion to a mixture of new products by 31P{1H} NMR. Analysis of the 19F NMR spectrum indicated the presence of P−F bond formation (J = 639 Hz and J = 480 Hz), but there were no signals indicating the formation of a Rh−F bond. Synthesis of (tBuPOCOP)Rh(CHCH2)(OtBu) (13). 12 (240 mg, 0.327 mmmol) was combined with NaOtBu (38 mg, 0.39 mmol) in a flask and dissolved in toluene. The reaction mixture was stirred at room temperature for 60 min with no noticeable color change. The volatiles were removed by vacuum. The orange solid was extracted with pentane and passed through a pad of Celite. The volatiles were removed to yield an orange solid (154 mg, 72%). The solid was recrystallized from pentane at −35 °C to yield a crystalline orange solid (105 mg, 49%) and X-ray-quality crystals. Complex 13 displays limited stability at ambient temperature, decomposing over a few hours in C6D6 solution. This prevented us from collecting a reliable 13 C NMR spectrum and elemental analysis data. 31P{1H} NMR (C6D6): 165.9 (d, JRh−P = 130 Hz). 1H NMR (C6D6, Figure 5-9 in the Supporting Information): 7.49 (m, 1H, CHCH2), 7.22(s, 1H, POCOP), 4.54 (t, 5.5 Hz, 1H, CHCH2), 3.93 (d, 14 Hz, 1H, CHCH2), 2.58 (m, 2H, CHMe2), 2.36 (m, 2H, CHMe2), 1.57 (s, 27H, t Bu and OtBu), 1.32 (q, 8.0 Hz, 6H, CHMe2), 1.25 (m, 12H, CHMe2), 1.09 (q, 8.0 Hz, 6H, CHMe2). Thermolysis of 13. 13 (23 mg, 0.035 mmol) was placed in a J. Young tube and dissolved in C6D6. The sample was heated in a 110 °C oil bath and monitored by 31P{1H} NMR and 1H NMR spectroscopy. After 30 min at 110 °C, the reaction mixture showed complete conversion from the starting material to three new compounds, with a major compound making up 70% of the reaction mixture. The 1H NMR spectrum no longer displayed any downfield vinyl resonances, indicating there were no longer any η1 vinyl complexes present in the mixture. Additional heating at 110 °C for 24 h resulted in the presence of a new compound at 204.4 ppm (d, J = 154.3 Hz, 5%) in the 31 1 P{ H} NMR spectrum; however, the fraction of the major product was unchanged (70%). The reaction was passed through a pad of Celite, and the volatiles were removed by vacuum. The brown-orange solid was washed with pentane, leaving a bright orange solid. Analysis of the solid by 31P{1H} NMR and 1H NMR showed only the presence of the major product, identified as 15. The 31P{1H} NMR spectrum displays the complex as two doublets with the same Rh−P coupling constant (J = 157.7 Hz). The identity of the side products could not be determined. Synthesis of (tBuPOCOP)Rh(CHCH2)(OC6H4F) (14). 13 (43 mg, 0.065 mmol) was placed in a Schlenk flask and dissolved in toluene. pFC6H4OH (7.4 mg, 0.065 mmol) was added to the sample, and the reaction mixture was stirred for 30 min at room temperature. The reaction mixture was passed through a pad of Celite, and the volatiles were removed by vacuum to give a red solid. The solid was recrystallized from pentane at −35 °C to give a red crystalline product (31 mg, 68%). 31P{1H} NMR (C6D6): δ 169.2 (d, J = 128.2 Hz); 1H NMR (C6D6, Figure 5-10 in the Supporting Information): δ 7.29 (d, 13 Hz, 1H, CHCH2), 7.24 (s, POCOP), 7.02 (t, 9.5 Hz, 2H, Ar), 6.71 (m, 2H, Ar), 4.45 (t, 6.0 Hz, 1H, CHCH2), 4.02 (d, 13 Hz, 1H, CHCH2), 2.48 (m, 2H, CHMe2), 2.22 (m, 2H, CHMe2), 1.51 (s, tBu), 1.14 (q, 7.5 Hz, 6H, CHMe2), 1.07 (q, 7.5 Hz, 6H, CHMe2), 1.03 (q, 7.5 Hz, 6H, CHMe2), 0.99 (q, 7.5 Hz, 6H, CHMe2). 19F NMR (C6D6): δ −133.4 (s). 13C{1H} NMR (C6D6): δ 161.4 (t, JPC = 6.0 Hz), 154.4 (d, JFC = 230 Hz, CF), 140.2 (dt, JRhC = 36 Hz, JPC = 11 Hz), 138.9 (d, JRhC = 33 Hz), 121.8 (s), 119.0 (s), 118.9 (s), 118.6 (t, JPC = 4.6 Hz),

115.5 (s), 115.4 (s), 34.8 (s, CMe3), 30.8 (t, JPC = 9.5 Hz, CHMe2), 30.4 (s, CMe3) 28.4 (td, JPc = 15 Hz, JRhC = 2.5 Hz, CHMe2), 18.6 (s, CHMe2), 17.1 (t, JPC = 3.8 Hz, CHMe2), 16.7 (t, JPC = 2.0 Hz, CHMe2), 16.2 (s). Anal. Calcd for C34H54FO3P2Rh: C, 58.79; H, 7.84. Found: C, 58.76; H, 7.79. Thermolysis of 14. 14 (14 mg, 0.020 mmol) was placed in a J. Young tube and dissolved in C6D6. The sample was heated in a 110 °C oil bath and monitored by 31P{1H} NMR and 1H NMR spectroscopy. Analysis of the 31P{1H} NMR spectrum after 60 min indicated 30% conversion to two major products (197.5 ppm, d, J = 157.7 Hz, 17%; 183.9 ppm, d, J = 205.4 Hz, 15%). After 24 h at 110 °C, the reaction mixture displayed complete conversion from the starting material and the presence of four products by 31P{1H} NMR spectroscopy. The major product (197.5 ppm, d, J = 157.7 Hz, 70%) is the same major product from the thermolysis of 13, identified as 15. The identity of the side products could not be determined. The product mixture was washed with pentane, resulting in the isolation of a small amount of pure 15 as an orange solid. X-ray-quality crystals of 15 were grown from a saturated toluene solution of pure 15 at room temperature. 1H and 31P{1H} NMR data for 15 are as follows. 31P{1H} NMR (C6D6): δ 197.5 (d, 158 Hz), 197.6 ppm (d, 158 Hz). 1H NMR (C6D6): δ 7.31 (s, 2H, POCOP), 4.39 (m, 2H, vinyl), 3.52 (m, 2H, vinyl), 2.59 (t, 5 Hz, 2H, vinyl), 2.38 (m, 4H, CHMe2), 2.23 (m, 4H, CHMe2), 1.62 (s, 36H, tBu), 1.37 (m, 12H, CHMe2), 1.30 (m, 12H, CHMe2), 1.14 (m, 12H, CHMe2), 1.04 (m, 12H, CHMe2). Thermolysis of 14 in the Presence of SiPr2. 14 (25 mg, 0.039 mmol) and SiPr2 (6 μL, 0.039 mmol) were combined in a J. Young tube and dissolved in C6D6. A sealed capillary containing a solution of PPh3 in C6D6 was also added to the J. Young tube to act as an integration standard. Analysis of the sample by 31P{1H} NMR spectroscopy at room temperature showed only the presence of 14 and the signal for PPh3. After the sample was heated overnight in a 110 °C oil bath, the 31P{1H} NMR spectrum showed complete conversion of 14 to a mixture of 15 (60%) and 10 (40%). The 19F NMR spectrum showed only the presence of p-FC6H4OH.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, and CIF and mol files giving select details of experimental procedures, graphical NMR spectra, details of Xray structural determinations, and a calculated structure. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for O.V.O.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for the support of this work by the U.S. National Science Foundation (Grants CHE-0944634 and CHE1300299 to O.V.O.) and by the Welch Foundation (Grant A1717 to O.V.O.).

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REFERENCES

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