Molybdenum(0) Dihapto-Coordination of Benzene and

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Molybdenum(0) Dihapto-Coordination of Benzene and Trifluorotoluene: The Stabilizing and Chemo-Directing Influence of a CF3 Group Jeffery T. Myers, Jacob A. Smith, Steven J. Dakermanji, Justin H. Wilde, Katy B. Wilson, Philip J. Shivokevich, and W. Dean Harman J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05009 • Publication Date (Web): 25 Jul 2017 Downloaded from http://pubs.acs.org on July 26, 2017

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Journal of the American Chemical Society

Molybdenum(0) Dihapto-Coordination of Benzene and Trifluorotoluene: The Stabilizing and Chemo-Directing Influence of a CF3 Group

Jeffery T. Myers, Jacob A. Smith, Steven J. Dakermanji, Justin H. Wilde, Katy B. Wilson, Philip J. Shivokevich, W. Dean Harman* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States

Abstract: The preparation of the complex TpMo(NO)(DMAP)(h2-PhCF3) (5) and TpMo(NO)(DMAP)(h2benzene) (3) are described. The CF3 group is found to stabilize the metal-arene bond strength in 5 by roughly 3 kcal/mol, compared to 3, allowing the large-scale synthesis and isolation of the trifluorotoluene analog (5, 37 g, 70%). When a benzene solution of 5 is allowed to stand, clean conversion to the benzene analog 3 occurs, and this complex may be precipitated from solution upon the addition of pentane, and isolated. The trifluorotoluene complex is shown to be a synthetic precursor to functionalized cyclohexadienes: In solution, it selectively protonates at the ortho position and the resulting h2-arenium species undergoes reactions with nucleophiles at the adjacent meta carbon. Thus, reactions of 5, triflic acid, and either N-methylpyrrole or 1-methoxy2-methyl-1-(trimethylsilyloxy)-1-propene result in 5-substituted-1,3-cyclohexadienes after removal of the metal. Introduction With the development of high throughput bioassays and small-molecule libraries to identify potential leads for new pharmaceuticals,1 new synthetic methods are sought that allow the rapid, stereoselective synthesis of novel molecular frameworks. This is especially true of methodologies that provide access to new regions of chemical space.2 In principle, benzenes are ideal precursors to functionalized cyclohexadienes and cyclohexanes. Benzenes are commercially available with a diverse range of substituents and substitution patterns and, significantly, feature a ring of adjacent unsaturated carbons, which provide multiple points for possible chemical elaboration. Reactions that utilize benzenes as synthons for more saturated molecules, such as the Birch reduction,3,4 photocycloaddition,5 and enzymatic oxidation,6 have become powerful tools for the synthetic chemist.7 The chemical reactivity of benzene may be profoundly altered by its coordination to a transition metal (Figure 1). For example, in complexes such as (h6-benzene)Cr(CO)3, [(h6+

+

benzene)Mn(CO)3] , [(h6-benzene)FeCp] , the arene ligand is activated toward nucleophilic substitution, nucleophilic addition, and side-chain activation, ultimately leading to the formation of substituted benzenes or cyclohexadienes.8 A complementary approach is the activation of aromatic molecules through dihapto-coordination.9-11 In these complexes, the metal-arene bond is stabilized primarily by interaction of a filled metal dπ orbital with a π* orbital of the aromatic ligand, activating h2-bound aromatic systems toward electrophilic rather than nucleophilic reactions.

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Nu:-

E+ electron flow

M

L L

η6-Arene

L

complex: undergoes nucleophilic addition

electron flow L L

M

L L

L η2-Arene complex: undergoes electrophilic addition

Figure 1. A comparison of hexahapto and dihapto coordination.

Whereas the vast majority of reports utilizing h6-benzene complexes in organic synthesis utilize first- and second-row transition metals, reports of dihapto-coordinated benzene complexes stable enough to undergo organic reactions have exclusively featured the heavy metals Os, Re, and W.10-12 The enhanced back-bonding interactions of these metals make them well suited to the formation of dihapto-coordinate complexes that are sufficiently stable to undergo ligand-centered reactions. Yet, the use of these third-row transition metals can carry significant environmental or economic disadvantages compared to their lighter counterparts. A recent report by Chirik et al. describes a molybdenum(II) h2-benzene complex stable enough to be spectroscopically characterized at room temperature,9 and the arene complex TpMo(NO)(MeIm)(h2-naphthalene) has been known for over a decade.13 More recent work finds that the 4-(dimethylamino)pyridine (DMAP) analog, TpMo(NO)(DMAP)(h2-naphthalene), (1) is equally stable in solution, and superior in effecting reactions of the h2-arene with electrophiles.14 Using the {TpMo(NO)(DMAP)} system, we hoped to develop a complex with a benzene analog suitable for exploration of organic reactions. Compound (1) is conveniently prepared from the sodium reduction of the air-stable precursor TpMo(NO)(DMAP)(I) (2).14 However, all attempts to prepare a benzene analog of 1 directly from this method have been unsuccessful, resulting in intractable products. Earlier observations of the d6 heavy-metal complexes [Os(NH3)5(h2benzene)]2+,15 TpRe(CO)(MeIm)(h2-benzene),16 and TpW(NO)(PMe3)(h2-benzene),17 as well as a meticulous kinetics investigation by Bengali et al. of Cp*Re(CO)2(h2-benzene),18 demonstrated that displacement of the arene by another ligand was dissociative, or weakly interchangedissociative, and we anticipated the same would hold true for the {TpMo(NO)(DMAP)} system. Thus, we posited the general mechanism shown in Scheme 1, where the heterogeneous reduction of the molybdenum(I) precursor, 2, leads to an h2-benzene complex 3 via the {TpMo(NO)(DMAP)(I)}- (Na+[2]-), and {TpMo(NO)(DMAP)}(SP), intermediates. However, benzene release (k-2) from 3 reforms the unstable square pyramidal intermediate, SP, which likely leads to decomposition of the {TpMo0(NO)(DMAP)} system. Apparently, this arene dissociation happens sufficiently fast that over the course of the reaction, decomposition prevents the net accumulation of the benzene complex 3. In comparison, the naphthalene complex 1, which is expected to be otherwise chemically and electrochemically similar to the benzene analog 3, has a longer half-life t1/2 = ~5 days at 25 °C (vide infra), and can be prepared and isolated in 54% yield. In other words, the rate of decomplexation for 1, [1]k-2, is much slower than the rate of the formation of 1, where the rate determining step is the heterogeneous reduction of 2 by sodium metal.


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N

Na 0 +

N Mo

Tp

NO I

N

4

Mo Tp

NO

k1

N

- I-

Mo

I

acetone k4

N

Tp

N

khet

N

NO O

Decomposition (unidentified paramagnetic materials)

NO

Tp

[2] -

2

Mo

Na +

N

SP k-2 k3

k 2 arene N

N

NO

Mo Tp

arene

1 (arene = naphthalene) 3 (arene = benzene)

Scheme 1. Formation of naphthalene (1), benzene (3), and acetone (4) complexes of {TpMo(NO)(DMAP)}. To better understand the mechanism of the arene ligand displacement, we explored “trapping” the 5-coordinate intermediate (SP) using acetone as the solvent to give the more stable acetone complex (4).14 Assuming a dissociative substitution mechanism is operative for the conversion of the naphthalene complex 1 to the acetone complex 4 (Scheme 1), we proceeded to collect rate data in order to estimate the free energy of activation for the naphthalene dissociation process. As others have shown,18,19 if the conversion of 1 to 4 passes through a 5-coordinate intermediate, held at steady state (SP in Scheme 1) then: d[4]/dt = kobs [1], where kobs = k-2k4[acetone]/{k2[arene] + k4[acetone]} If [acetone] >> [arene], while k2 is similar to k4, then the observed rate constant becomes approximately k-2 , as long as the decomposition pathway (k3) is inoperative. For the naphthalene complex 1 in acetone, we find that kobs = 1.1 ± 0.2 x 10-6 s-1 (25 ± 1 °C) which corresponds to a value of DG‡ = 25.6 ± 0.2 kcal/mol for arene dissociation. A similar calculation was carried out using available rate data for arene displacement reactions in acetone solution with several heavy-metal systems (Os, Re, W), and these findings are collected in Table 1. We also include available data for some first-row transition-metal complexes of dihapto-coordinated benzenes, obtained via photolysis experiments in other solvents. 20-23 A comparison of free energies of activation for {Os(NH3)5}2+, {TpRe(CO)(MeIm)}, and {TpW(NO)(PMe3)} in Table 1 reveal that between the benzene and naphthalene systems, DDG‡ averages ~ 7 kcal/mol. Thus, we predicted that the purported Mo-benzene analog 3 likely has a DG‡ close to 19 kcal/mol. If this were true, the isolation of 3 would be formidable at ambient temperature. However, a comparison of Os and W benzene complexes (Table 1) revealed that a single CF3 group could provide roughly 2-3 kcal/mol stabilization of the metal-benzene complex. Presumably, this increase in stability suggests that while both s-donation and π-backbonding are important contributions to the overall bond-strength, the latter is the dominant interaction for these π-basic metal systems. Hence, we set out to prepare the a,a,a-trifluorotoluene complex, TpMo(NO)(DMAP)(CF3Ph) (5) in the hopes that this compound would be stable enough to isolate and characterize.



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Table 1. Kinetic parameters for the displacement of benzene Compound

T (K)

DG*

(kcal/mol)

DS* (eu)c

DH*

Reference

(kcal/ mol)

2

a

b

-2.4

9.4

22

Cr(CO)5(η -PhF)

2

a

b

2.2

9.6

23

Cr(C6H6)(CO)2(η -benzene)

2

a

b

5.0

10.7

21

MnCp(CO)2(η -toluene)

a

b

6 .0

14.2

21

Cr(CO)5(η -benzene)

2

2

278

2

a

b

2

298

25.6

Os(NH3)5(η -benzene)

2

298

2

MoTp(NO)(DMAP)(η -benzene) MoTp(NO)(DMAP)(η -PhCF3)

~ 23

This work

12.1 ± 4.5 25.5 ± 1.4 This work ~ 29

This work

23.9

=12

~ 28

15

298

25.6

=12

~ 29

15

2

298

25.5

=12

~ 29

15

2

373

29.4

=12

~ 34

12

a

b

3.4

20.8

18

2

295

22.6

=12

~ 26

16

2

373

28.5

=12

~ 33

16

2

295

22.3

=12

~ 26

17

2

295

23.0

=12

~ 27

17

2

295

25.4

=12

~ 29

17

2

323

24.9

=12

~ 29

17

2

295

23.1

=12

~ 27

17

2

351

30.4

=12

~ 35

17

MoTp(NO)(DMAP)(η -naphth)

Os(NH3)5(η -PhCF3) Os(NH3)5(η -anisole) Os(NH3)5(η -naphth) 2

ReCp*(CO)2(η -benzene) ReTp(MeIm)(CO)(η -benzene) ReTp(MeIm)(CO)(η -naphth) WTp(PMe3)(NO)(η -benzene) WTp(PMe3)(NO)(η -anisole) WTp(PMe3)(NO)(η -PhCF3) WTp(PMe3)(NO)(η -PhCF3) WTp(PMe3)(NO)(η -DMB) WTp(PMe3)(NO)(η -naphth)

19.4 ± 0.5 =12

=12


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a. Range of temperatures; b. Temperature dependent. c. The entry “=12” indicates an assigned value, specifically for an acetone solution, based on experimental results from 5. DMB = 1,3-dimethoxybenzene; naphth = naphthalene.

Results and Discussion. Under a nitrogen atmosphere, the Mo(I) precursor 2 (50 g, 0.085 mol) was suspended in neat PhCF3 (100 mL) along with a sodium metal dispersion (24 g, 0.365 mol). After 24 h of vigorous stirring, the resulting mixture was decanted away from the sodium, diluted with ether, and methanol was added slowly (CAUTION! Hydrogen evolution)24 to both quench the sodium and precipitate the product 5. Spectroscopic (IR, 1H-, 13C-, 19F-NMR), electrochemical (CV), and combustion data of the isolated material are consistent with 5 being the desired trifluorotoluene complex shown Scheme 2. Proton NMR analysis shows that 5 is present in solution as a mixture of two isomers, which interconvert on the NMR timescale (600 MHz): broadened peaks are present at 3.70, 3.61, 3.30, 3.18 ppm, which upon reducing the temperature to -30 °C sharpen to reveal their fine structure. Coupling data are consistent with the presence of two coordination diastereomers,10 5p (proximal to DMAP) and 5d (distal to DMAP), in roughly equal amounts (1:1.2). At room temperature, evidence for spin-saturation-exchange in NOESY data reveal that the dominant dynamic process is an intrafacial (i.e., ring-walk) rather than interfacial (i.e., face-flip) isomerization, occurring on the seconds timescale. A 1H NMR experiment of a d6-acetone solution of 5 at 0 °C revealed the coupling constant between H4 and H5 to be 6.2 Hz. Given that the difference in resonances between H4 of 5d and H4 of 5p is 341 Hz at 0 °C (600 MHz), and that the approximate coalescence temperature is 50 °C, the approximate specific rate of isomerization was determined to be kiso = 750 s-1. This rate constant corresponds to a free energy of activation for the intrafacial isomerization of 14.7 ± 0.5 kcal/mol at 50 °C. This value is similar to that observed for [Os(NH3)5(h2-benzene)]2+, where DG‡ = 12.5 kcal/mol at 10 °C.15 (For 5, an upper limit of 60 °C and a lower limit of 40 °C were used to approximate an error for DG‡ of ± 0.5 kcal/mol). Remarkably, the PhCF3 complex 5 is stable at room temperature and undergoes substitution in neat acetone at a relatively slow rate (t1/2 = 39 minutes). Judging from 1H NMR data, the acetone complex 4 initially is formed as a 3:1 mixture of two coordination diastereomers (4p and 4d; Scheme 2; oxygen distal or proximal to the DMAP), but after 16 h, only isomer 4p is observed along with free PhCF3.14 Monitoring the rate of arene displacement for 5 in acetone-d6 over a temperature range of 25 - 45 °C, we determined using the Eyring equation that DS‡ = 12.1 ± 4.5 eu and DH‡ = 25.5 ± 1.4 kcal/mol. Given that the free energy of activation for the desired benzene complex 3 was likely only ~2-3 kcal lower than that for 5 (21.7 kcal/mol at 25 °C), we posited that 3 might be accessible via ligand displacement using 5 as the precursor, provided that the equilibrium shown in Eq 1 could be established: TpMo(DMAP)(CF3Ph) + benzene = TpMo(DMAP)(benzene) + CF3Ph

Keq

(1)

Taking 10:1 as an acceptable ratio of purity for the benzene complex (the ratio of 3 : 5 at equilibrium), and estimating DG° as ~DDG‡ = 3 kcal/mol (the predicted difference between metal-benzene and metal-trifluorotoluene bond strengths, based on heavy metal comparisons), then at equilibrium:



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DG° = -RTln[{[CF3Ph]/[PhH]}•(10)] = 3 kcal/mol, which would require the ratio [CF3Ph]/[PhH] to be ~ 1/1500 at 25 °C. Assuming this is true, for 1 mL of benzene (11.2M), 4.6 mg of 5 (MW = 609) would be required to produce 7.5 mM free CF3Ph. Following up with this prediction, when ~ 5 mg of the trifluorotoluene complex 5 is dissolved in 1 mL of benzene-d6, a single new set of Tp and DMAP peaks grows in over time (Figure 2), reaching a 95% conversion from 5 to 3 in approximately 2.5 h. These data suggest the clean formation of benzene complex 3 in solution. Following up on this encouraging observation, 250 mg of 5 was added to freshly dried benzene (15 mL) and allowed to stand 2.5 h, after which it was added to 40 mL of cold pentane. A golden precipitate formed and was collected (11%). An NMR tube was then charged with a sample of the material (3) and a -30 °C solution of acetone was added. The resulting spectrum, recorded at -35 °C revealed Tp and DMAP resonances similar to those seen during the exchange in benzene-d6. Unfortunately, the benzene peaks were severely broadened even at this low temperature. Over time these peaks were replaced with signals corresponding to the acetone complex 4 accompanied by concomitant growth of a signal for free benzene at 7.36 ppm. A kinetic analysis of a fresh sample of 3 in acetone-d6 at 5 °C reveals a free energy of activation of 19.4 ± 0.5 kcal/mol for 3, corresponding to an estimate of 23 kcal for DH‡, if DS‡ is taken as 12 eu. In a separate experiment, approximately 20 mg of the trifluorotoluene complex 5 was added to a 1:1 mixture of THF (used to solubilize electrolyte) and benzene in a CV cell. The Ep,a associated with 5 (-0.29 V) decreased and the concomitant growth of an Ep,a = -0.55 V occurred over an hour, consistent with the formation of the more electron-rich benzene complex 3. Addition of 1 mL of acetone to this solution resulted in the loss of the anodic wave at -0.55 V and the growth of a new anodic wave at +0.08 V, consistent with the formation of acetone complex 4 (see Scheme 2).

N

N Mo Tp

N

1. CF3Ph + Na0 2. MeOH

N

N

NO

Mo

N

NO

I

CF3

Tp CF3

5d

2

NO

Mo

Tp

5p

O

N

N

N Mo

O

N

N

NO

Mo Tp

Tp O

N

NO O

Mo

4p

4d

NO

Tp

2

3

Scheme 2. Synthesis of h -benzene complexes of Mo(0).


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blue

red

180 min

Tp

Tp

15 min Tp PhCF3

6.00

DMAP

DMAP Tp

PhCF3 + Tp

5.75

PhCF3

5.55 ppm

6

Figure 2. Solution of PhCF3 complex 5 in benzene-d at 25 °C, 15 min and 180 minutes after mixing.

According to published reports, the entropy of activation for benzene displacement is typically small and positive. Assuming for arene displacement in acetone that the value of DS‡ = 12 eu (determined for 5) holds true for other arene displacement reactions in acetone, we have calculated an estimate of DH‡ for other compounds reported in Table 1, whose rate data were obtained for an acetone solution. These values of DH‡ should be reasonable approximations for the corresponding heterolytic metal-arene bond dissociation energies (i.e., metal-arene bond strength).18 Previous studies of heavy-metal benzene complexes document a broad range of electrophilenucleophile tandem addition reactions that can occur to the benzene ligand, resulting in 1,4hexadienes such as 11 in Scheme 3.25 Two reagents were chosen to test the viability of 5 for this type of reaction, N-methylpyrrole and 1-methoxy-2-methyl-1-(trimethylsilyloxy)-1-propene (MTDA).26 These reactions represent two important classes of carbon-carbon bond forming reactions: Friedel-Crafts alkylation of aromatic heterocycles27,28 and addition of protected enolates.29 When a solution of the trifluorotoluene complex 5 was treated with triflic acid (60 °C) followed by MTDA, a single new complex was formed (6). Proton NMR, COSY, NOESY, IR and electrochemical data are consistent with 6 being a 1,3-cyclohexadiene complex. The protonation and nucleophilic addition reactions were both found to be regioselective, and the addition of the masked enolate was also stereoselective, with addition occurring anti to the metal. When iodine is used as the decomplexation agent of 6, the precursor 2 can be recovered in 89% and the organic diene 7 is isolated at 46%. A similar reaction sequence was successfully carried out with N-methylpyrrole as the nucleophile in place of MTDA. In this case, the diene 8 is formed as the final product (52%). Similar reaction sequences were attempted with the toluene analog of 5 by first allowing a solution of 5 to stand several hours in the presence of toluene. Then this solution was cooled to -60 °C and subsequently HOTf and MTDA were added. Unfortunately, these preliminary reactions were unsuccessful. The regiochemistry that leads to the formation of 7 and 8 is in stark contrast to that observed with the toluene complex TpRe(CO)(MeIm)(toluene), 25 (10) where the electrondonating properties of the methyl group support formation of the allyl complex 10H+ (Scheme 3). Subsequent nucleophilic addition is directed to the meta carbon, as is true for the



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trifluorotoluene analog 5, but in the case of allyl complex g-10H+, a 1,4-diene complex is the only product formed (11). N

N

N

N

Comparison

CH 3

Mo

NO

[Re] 10

NO

Mo

H+

CF 3

Tp

Tp 5d

H

5p

CF 3

H

CH 3

[Re] HOTf -60 °C (EtCN)

HOTf -60 °C (EtCN)

N

N

N

N

γ-10H + MTDA

H H

Mo

NO H H

Mo

H

CF 3

Tp

Tp α-5H+

H

β-5H+

CF 3

CH 3

NO O

H

OMe

N

11 (62%)

[Re] = TpRe(CO)(MeIm)

MTDA

N Mo

NO H H

CF 3

Tp γ-5H+

N CF 3

CF 3 H H

O

N

I2

Mo

7 (46%)

NO CF 3

Tp

OMe 6 (50%)

H H N CH 3

H H

O

9 (52%) (via I 2 oxidation of 8)

OMe

Scheme 3. The net 1,2-addition of an ester or a pyrrole to a,a,a-trifluorotoluene.

In order to better understand the remarkably high selectivity of arene protonation, and nucleophilic addition for the molybdenum complex 5, DFT calculations were carried out using the Gaussian 03 program suite. Relative energies were determined using a hybrid density functional B3LYP expressed in a hybrid basis. The basis incorporates the Los Alamos pseudopotential LANL2DZ and the associated basis functions for molybdenum and the 6-31G(d) basis for all other atoms. This combination has proved to be reliable for Os, Mo, Re, and W systems for relative (binding) energies, charge transfer, and preferred structures, especially in similar systems.30 Various isomers of the PhCF3 complex 5 were modeled. These calculations show that the two thermodynamically preferred orientations of TpMo(NO)(DMAP)(h2-PhCF3) are those depicted in Scheme 3, with 5d and 5p differing by 0.1 kcal/mol. A third isomer is also predicted to be energetically accessible, in which the metal binds C2 and C3 with (0.6 kcal/mol higher than 5d); however, no experimental evidence of this isomer was observed. Previous studies have determined that dihapto-coordinated (1,2-h2) dienes are highly nucleophilic at the d carbon (C4).31-33 This trend is upheld with the highly selective protonation of the PhCF3 complex 5, where protonation (leading to product) is directed exclusively to the ortho position, even though this site is adjacent to the electron-withdrawing CF3 group. The


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resulting build-up of positive charge in the “h2-arenium” complex34 (b-5H+) is farther from the CF3 group and distal to the DMAP ligand. Correspondingly, nucleophilic addition to the purported arenium intermediate occurs to the meta carbon adjacent to the site of protonation. This tandem addition results in a 1,3-diene complex (e.g., 6 in Scheme 3) with the CF3 group in hyperconjugation with the 1,3-diene, such that the withdrawing group enhances the metal backbonding interaction. Attempts to characterize the purported allyl intermediate b-5H+ at ambient temperature have been unsuccessful. However, all 28 of the possible enantiomeric pairs of isomers resulting from a ring-protonation of 5 were modeled (Supplemental material). These calculations predict that there are actually three low-energy isomers, a-, b-, and g-5H+ (see Scheme 3). These three h2-arenium intermediates place the most electron-deficient carbon (indicated by + charge) in the same position relative to the {TpMo(NO)(DMAP)} fragment and are separated by less than one kcal/mol. Yet, only the b-arenium (b-5H+) is predisposed to a nucleophilic addition reaction that would lead to a thermodynamically favorable 1,3-diene complex. Models of both 1,4- and 1,3-h2-cyclohexadiene complexes reveal that the conjugated system is 5.3 kcal/mol more stable (Supplemental material). The analogous reaction with TpRe(CO)(MeIm)(h2-toluene) (10, see Scheme 3) has a different outcome: owing to the stabilization of the g-10H+ isomer via hyperconjugation with the methyl group, h2-1,4-diene products such as 11 result, rather than a thermodynamically favored conjugated diene isomer. In Figure 3, selected bond lengths (Å) are included for the productive h2-arenium intermediate b-5H+ that show the highly distorted nature of the allyl-molybdenum binding. We note that the preferred orientation of the h2-arenium ligand is that which places the formal positive charge at the meta carbon, away from the DMAP ligand (Figure 2). This predicted geometry further supports the findings that the addition of a nucleophile such as MTDA and Nmethylpyrrole occurs with high regio- and stereoselectivity to the distal meta position of the trifluorotoluene, rendering a 1,3-diene product with the carbon substituent distal to the DMAP ligand. H

1.3 CF3 4 0 2.3 1.44 1.47 2.38 1.3 H 7 9 2.85 1.4 H 7

1.4

Mo

H H H

CF3

H

Mo H H H

CF3

H Mo

H

H

H H

Figure 3. DFT modeling of b-5H+, the second lowest energy isomer (of 28) for the arenium intermediate 5H+. Bond lengths (Å) indicate predisposition to forming 1,3-diene complex. This study reveals that the CF3 group promotes a highly selective tandem addition of an electrophile (proton) and carbon nucleophile to the ortho and meta carbons of trifluorotoluene,



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respectively. This is distinct from the reaction patterns observed with other types of substituted benzenes or with other π-basic metal complexes that form h2-benzene complexes. As mentioned earlier, toluene forms a complex with either osmium or rhenium ([M] = {[Os(NH3)5]2+} or {TpRe(CO)(MeIm)}) however, this reaction sequence leads exclusively to cis-3,6-disubstituted 1,4-cyclohexadienes (11, Scheme 3).25,35 In the case of the tungsten dearomatization agent {TpW(NO)(PMe3)}, attempts to carry out electrophilic addition reactions (e.g., protonation) with either benzene or alkylated benzenes have been unsuccessful, owing to the oxidative decomposition of the metal.25 The present study appears to be the first report describing the synthesis of an non-aromatic product from a metal-promoted addition to trifluorotoluene, regardless of hapticity. The complex Cr(CO)3(PhCF3) has been reported to undergo nucleophilic addition with a carbanion, which after oxidation yielded substituted arenes (33%) as a 3:7 mix of meta and para isomers, along with polyalkylated products.36 The same complex reacts with the nucleophile TMSCF3 at the ortho carbon, leading to the hexafluorinated xylene.37 Additionally, an example has been reported for a Ru2+ system in which a nucleophile adds para to the CF3 group,38 but no organics were isolated. Cr(CO)3(arene) complexes with other withdrawing groups can lead to 5,6-trans-disubstituted cyclohexadienes, with the nucleophile preferentially adding ortho to the withdrawing group.39 Finally, Ritter et al. have shown that oxygen nucleophiles add selectively to the ortho position of trifluorotoluene h6-coordinated to Ir(III).40 In pharmaceutical design, selective fluorination has become a strategy for optimizing drug delivery and binding specificity. There are predictable effects stemming from the electronegativity of this heteroatom, and fluorine substitution also increases the bioavailability and fat solubility for a compound. Many of the most popular medicines on the market, including Lipitor, Prevacid and Prozac, include at least one fluorine atom.41 Dienes with CF3 groups are of interest in Diels-Alder reactions42,43 and should be good candidates to participate in the inverseelectron-demand variant, but compounds such as 7 and 8 have not previously been reported for lack of any practical method to produce them. The closest examples are diendiols obtained from enzymatic oxidation of trifluorotoluenes.44 Conclusions. While {TpMo(NO)(DMAP)} is less π basic than its heavy-metal analogs, the addition of a single electron-withdrawing group (CF3) to benzene enhances the back-bonding interaction in the complex TpMo(NO)(DMAP)(PhCF3) to the point that this species can be isolated on a practical (37 g; 70%). The air-stable precursor TpMo(NO)(DMAP)(I) can been produced on a 170 g scale in 65% overall yield from Mo(CO)6, in air, and without chromatography.14 To our knowledge, the trifluorotoluene complex (5) represents the first example of an h2-benzene complex, not derived from a heavy-metal, that has been shown to undergo selective electrophile/nucleophile additions to yield a diene complexes. The advent of complexes such as 5 could provide chemists with access to a broad range of h2-coordinated benzene complexes of Mo, suitable as scaffolds for chemical elaboration into highly functionalized cyclohexadienes and cyclohexenes. The low cost, accessibility of the precursor, and efficient recyclability of {TpMo(NO)(DMAP)} compared to its heavy-metal analogs make these molybdenum complexes potentially valuable as new tools for organic synthesis. Experimental Section.


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General Methods. NMR spectra were obtained on a 600 or 800 MHz spectrometer. All chemical shifts are reported in ppm, and proton and carbon shifts are referenced to tetramethylsilane (TMS) utilizing residual 1H or 13C signals of the deuterated solvents as an internal standard. Fluorine chemical shifts are referenced to a solution of hexafluorobenzene in d6-acetone (-164.9 ppm relative to CFCl3) contained in a capillary tube. Coupling constants (J) are reported in hertz (Hz). Infrared spectra (IR) were recorded as a glaze on a spectrometer fitted with a horizontal attenuated total reflectance (HATR) accessory or on a diamond anvil ATR assembly. Electrochemical experiments were performed under a dinitrogen atmosphere. Cyclic voltammetry data were taken at ambient temperature (~25 °C) at 100 mV/s in a standard threeelectrode cell with a glassy carbon working electrode, N,N-dimethylacetamide (DMA) or acetonitrile (CH3CN) solvent (unless otherwise specified), and tetrabutylammonium hexafluorophosphate (TBAH) electrolyte (approximately 0.5 M). All potentials are reported versus NHE (normal hydrogen electrode) using cobaltocenium hexafluorophosphate (E1/2 = −0.78 V), ferrocene (E1/2 = +0.55 V), or decamethylferrocene (E1/2 = +0.04 V) as an internal standard. The peak-to-peak separation was less than 100 mV for all reversible couples. Unless otherwise noted, all synthetic reactions were performed in a glovebox under a dry nitrogen atmosphere. Deuterated solvents were used as received. Pyrazole (Pz) protons of the (trispyrazolyl) borate (Tp) ligand were uniquely assigned (e.g., “Pz3B”) using a combination of two-dimensional NMR data and (dimethylamino)pyridine−proton NOE interactions. When unambiguous assignments were not possible, Tp protons were labeled as “Pz3/5 or Pz4”. All J values for Pz protons are 2 (±0.2) Hz. Synthesis of TpMo(NO)(DMAP)(η2-benzene) (3). N

N

N

N

Mo

N B H

N

B A NO

N

N

A 4-dram vial was charged with 5 (0.201 g, 0.328 mmol) and benzene that had been dried over sodium (9.98 g, 128 mmol). This mixture was stirred at room temperature for 3 h and then slowly added to a -60 °C solution of pentane yielding a brown precipitate. This precipitate was then isolated on a 15 mL fine porosity fritted disc, washed with chilled pentane (3x10 mL) and desiccated to yield a golden-brown solid, 3 (0.072 g, 11.2%). CV (THF: Benzene 1:1, TBAH, 100 mV/s vs NHE): Ep,a= -0.55 V. IR, νBH= 2459 cm-1, νNO= 1563 cm-1. 1H NMR (d6-benzene, 25 oC, δ): 7.93 ppm (1H, d, Tp), 7.64 (1H, d, Tp), 7.55 (1H, d, Tp), 7.46 (1H, d, Tp), 7.31 (2H, m, DMAP-A), 7.08 (1H, d, Tp3,5), 7.07 (1H, d, Tp3,5), 6.03 (1H, t, Tp4), 5.88 (1H, t, Tp4), 5.69 (1H, t, Tp4), 5.66 (2H, m, DMAP-B) 2.06 (s, 6H, NMe). Synthesis of TpMo(NO)(DMAP)(h2-acetone) (4).



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N

N

N

N

Mo

N B H

N

N

B A NO O

A

B

N

To a 4-dram vial charged with a stir pea were added 5 (0.543 g, 0.894 mmol), acetone (0.500 g, 8.60 mmol) and THF (4 mL). This mixture was stirred at room temperature for 3 h and then slowly added to stirring pentane (30 mL). The resulting precipitate was then isolated on a 15 mL fine porosity fritted disc, washed with pentane (2 x 10 mL) and desiccated to yield a light grey solid, 2 (0.4015 g, 86.0 %). CV (DMAc) Ep,a = -0.28 V (NHE). IR: νBH = 2479 cm-1, νNO = 1574 cm-1. 1H NMR (d6-Acetone, δ): 8.07 (1H, d, Pz3A), 7.91 (1H, d, Pz5B), 7.88 (1H, d, Pz5A), 7.84 (1H, d, Pz5C), 7.62 (2H, m, DMAP-A), 7.57 (1H, d, Pz3B), 7.32 (1H, d, Pz3C), 6.57 (2H, m, DMAP-B), 6.35 (1H, t, Pz4A), 6.29 (1H, t, Pz4B), 6.15 (1H, t, Pz4B), 3.08 (6H, s, N-Me), 2.16 (3H, s, CH3A), 0.91 (3H, s, CH3B). 13C NMR (d6-Acetone, δ): 155.55 (DMAP-C), 151.89 (DMAP-A), 143.56 (Pz3A), 143.46 (Pz3B), 143.18 (Pz3C), 136.96 (Pz5), 136.16 (Pz5), 135.96 (Pz5), 107.19 (DMAP-B), 106.60 (Pz4B), 106.39 (Pz4A), 105.73 (Pz4B), 101.92 (CO), 39.19 (NMe), 31.49 (CH3B), 27.05 (CH3A).

Synthesis of TpMo(NO)(DMAP)(3,4-h2-a,a,a-trifluorotoluene) (5).

N

N

N

N

Mo

N B H

N

N

B A NO 3 4

N

N

A

2

1 CF 3

N

Mo

N

6 5

N

N

B H

N

B A NO 4 3

N

N

5

6

1 2

CF 3

B

24 g scale (13 g yield): To a 250 mL round bottom flask charged with a stir egg were added sodium dispersion (30-35% by weight, 12 g dispersion, 0.183 mol) and hexanes (200 mL). The grey mixture formed was stirred at 1150 RPM (note: reaction rate depends strongly on stir rate) for 18 h at which point the hexanes was decanted off. Then, PhCF3 (200 mL, 0.431 mol) and 2 (24 g, 0.0408 mol) were added to the remaining solid. The resulting green reaction mixture was stirred at 1150 RPM for 24 h. This dark red mixture was then filtered through a 350 mL medium porosity fritted disc and washed with Et2O (3 x 200 mL). The resulting precipitate, containing sodium flakes, was then transferred to a 500 mL beaker charged with a stir bar, suspended in Et2O (200 mL), and stirred rapidly. Next, the stirring was stopped and this mixture was decanted into a 500 mL Erlenmeyer flask taking care to prevent the sodium from being transferred. Et2O (200 mL) was added to the


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beaker containing the sodium and the red mixture was transferred to the aforementioned 500 mL Erlenmeyer flask. While purging the box with nitrogen, MeOH (50 mL) was added to the red mixture, yielding a red precipitate. This precipitate was then collected on a 350 mL fine porosity fritted disc, washed with H2O (100 mL) and Et2O (4 x 100 mL). The resulting red precipitate was desiccated for 2 h to yield the solid 5 (13.10 g, 65%). 50 g scale (37g yield): To a 500 mL round bottom flask charged with a 1” stir egg were added sodium dispersion (2535% by weight, 24 g dispersion, 0.365 mol) and hexanes (400 mL). The grey mixture was stirred at 900 RPM for 18 h, at which point the hexanes was decanted off. Then, PhCF3 (150 mL, 0.862 mol), Et2O (50 mL) and 2 (50 g, 0.0850 mol) were added to the remaining solid. The resulting green reaction mixture was stirred at 900 RPM (note: reaction rate depends strongly on stir rate) for 16 h. The stirring was then stopped, and the reaction mixture was allowed to settle for 3 min. The liquid layer was then slowly decanted into a 500 mL Erlenmeyer flask, with care taken not to disturb the solid layer containing the sodium metal. The solid in the round bottom was resuspended in Et2O (100 mL). The mixture allowed to settle for 3 min, and the liquid layer was then decanted slowly into the 500 mL Erlenmeyer flask. The combined liquid layers were stirred for 1 min, allowed to settle for 3 min, and then the liquid layer was decanted into a 1 L Erlenmeyer flask. The remaining solid was re-suspended in Et2O (100mL), allowed to settle for 3 min, and then the liquid was decanted slowly into the 1 L Erlenmeyer flask. Et2O was added to the combined liquid layers to give a total volume of 800 mL. While purging the box with nitrogen, MeOH (100 mL) was added dropwise to this vigorously stirring mixture. This precipitate was then collected on a 2 L medium porosity fritted disc and washed with Et2O (2 x 100 mL). The resulting orange precipitate was transferred to a 500 mL Erlenmeyer flask containing Et2O (250 mL) and this mixture was stirred for 1 h. This precipitate was then collected on a 350 mL fine porosity fritted disc, washed with Et2O (3 x 100 mL), and desiccated for 2 h to yield the solid 5 (37.18g, 70%). CV (DMAc) Ep,a = -0.28 V (NHE). IR: νBH = 2481 cm1 , νNO = 1586 cm-1. Two coordination diastereomers A:B = 1:1 1H NMR (d6-Acetone, δ, -30 °C): 8.17 (4H, buried broad s, DMAP-A for A and B), 8.11 (2H, overlapping doublets, Tp3,5), 8.02 (1H, d, Tp3,5), 8.01 (1H, d, Tp3,5), 7.95 (3H, m, Tp3,5), 7.84 (1H, d, Tp3,5), 7.58 (1H, d, Tp3,5), 7.57 (1H, d, Tp3,5), 7.46 (1H, d, J = 6.3, H2B), 7.12 (1H, dd, J = 9.1 & 6.2, H5A), 7.08 (1H, d, J = 5.8, H2A), 6.90 (1H, d, Tp3,5), 6.88 (1H, d, Tp3,5), 6.74 (4H, buried broad s, DMAP-B for A and B), 6.70 (1H, dd, J = 9.1 & 6.2, H5B), 6.41 (3H, m, Tp4), 6.37 (1H, t, Tp4), 6.29 (1H, dd, J = 9.1 & 1.5, H6A), 6.25 (1H, dd, J = 9.1 & 1.5, H6B), 6.15 (2H, m, Tp4), 3.71 (1H, dd, J = 8.5 & 6.2, H4B), 3.60 (1H, t, J = 7.3, H3A), 3.30 (1H, dd, J = 8.5 & 6.3, H4A), 3.14 (1H, t, J = 7.2, H3B), 3.09 (12H, s, NMe). 13C NMR (d6-Acetone, δ, -30 °C): 154.7 (DMAP-C), 150.2 (DMAP-C), 142.4 (Tp3,5), 142.3 (Tp3,5), 142.0 (Tp3,5), 141.5 (2C, Tp3,5), 137.5 (2C, Tp3,5), 136.9 (Tp3,5), 136.8 (Tp3,5), 135.9 (4C, C6A & C2B, Tp3,5), 135.8 (2C, C1B & C1A), 135.2 (2C, C2A & C5B), 127.3 (q, J = 261.0, CF3), 125.5 (q, J = 261.0, CF3), 111.7 (C5A), 111.4 (C6B), 108.0 (4C, DMAP-B), 106.9 (3C, Tp4), 106.4 (3C, Tp4), 78.7 (C4B), 76.2 (C4A), 75.4 (C3A), 72.9 (C3B), 30.07 (NMe). 19F NMR (d6-Acetone, d): -62.41. Calculated for C23H25BF3MoN9O: C, 45.49; H, 4.15; N, 20.76; F, 9.39. Found: C, 45.11; H, 4.34; N, 20.39; F, 9.05. Synthesis of TpMo(NO)(DMAP)(h2-methyl 2-methyl-2-(5-(trifluoromethyl) cyclohexa-2,4dien-1-yl)propanoate) (6).



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N

N

N

N

4

N

N

2

NO 3

Mo

N B H

B A

N

1 CF 3 5

7

H6 H 6' O

O

5 (500 mg, 0.80 mmol), CH3CH2CN (5 mL) and a stir pea, were added to a test tube and this orange mixture was cooled for 15 min at -60 °C. A -60 °C, 1M solution of HOTf in CH3CH2CN (2.0 mL, 2.0 mmol) was added to the reaction mixture and the resulting red solution was left standing at -60 °C. After 15 min, MTDA (1.0 mL, 4.9 mmol) was added to the reaction mixture and the resulting red solution was left stirring at -60 °C. After 18 h, a -60 °C solution of triethylamine (1.0 mL, 7.17 mmol) was added to the reaction mixture and the resulting brown solution was chromatographed through a 60 mL medium porosity fritted disc ¾ full with silica gel. The product was eluted with 1:1 Et2O:benzene (100 mL) as a yellow band, collected as a yellow solution, and evaporated in vacuo. The resulting yellow oil was then dissolved in DCM (1 mL) and the product was precipitated in stirring pentane (20 mL). The precipitate was collected on a 15 mL fine porosity fritted disc, washed with pentane (3 x 50 mL), and desiccated for 15 min yielding the light yellow solid 6 (299 mg, 50%). CV (DMAc) Ep,a = +0.32 V (NHE). IR: νBH = 2480 cm-1, νCO = 1721 cm-1, νNO = 1571 cm-1. H NMR (d6-Acetone, δ): 7.96 (2H, d, Pz5C and Pz3A), 7.92 (1H, d, Pz5A), 7.79 (1H, d, Pz5B), 7.78 (2H, bs, DMAP-A), 7.66 (1H, d, Pz3C), 7.09 (1H, d, Pz3B), 6.62 (3H, m, DMAP-B & H2), 6.38 (1H, t, Pz4A), 6.37 (1H, t, Pz4C), 6.13 (1H, t, Pz4B), 3.29 (1H, d, J = 9.8, H5), 3.24 (3H, s, OMe), 3.08 (6H, s, N-Me), 2.93 (1H, m, H6), 2.86 (1H, t, J = 7.9, H3), 2.11 (1H, d, J = 18.6, H6’), 1.89 (1H, dt, J = 9.8 & 1.8, H4), 1.23 (3H, s, Me), 1.04 (3H, s, Me). 13C NMR (d6-Acetone, d): 178.2 (CO), 155.3 (DMAP-C), 151.1 (DMAP-A), 142.6 (Pz3A), 142.1 (Pz3B), 141.6 (Pz3C), 137.5 (q, J = 6.9, C2), 137.4 (Pz5C), 137.2 (Pz5A), 135.8 (Pz5B), 126.8 (q, J = 264.0, CF3), 115.5 (1C, q, J = 29.0, C1), 108.2 (DMAP-B), 106.9 (Pz4), 106.4 (Pz4), 106.3 (Pz4), 65.3 (C4), 60.8 (C3), 51.2 (OMe), 51.1 (C7), 42.6 (C5), 39.2 (NMe), 22.9 (Me), 22.4 (C6), 22.3 (Me). 19F NMR (d6-Acetone, d): -65.38. EA: Calculated for C28H35BF3MoN9O3: C, 47.41; H, 4.97; N, 17.77. Found: C, 47.68; H, 5.22; N, 17.65. Synthesis of methyl 2-methyl-2-(5-(trifluoromethyl)cyclohexa-2,4-dien-1-yl)propanoate (7). 2

1 CF 3

3 4 O

6 5 7

O

To a 50 mL filter flask charged with a stir pea were added 6 (100 mg, 0.145 mmol), DCM (5 mL), and a 0.06 M solution of I2/Et2O (1.2 mL, 0.073 mmol) resulting in a green solution. The solution was stirred at room temperature for 5 min and then evaporated in vacuo to an oil. The oil was dissolved in DCM (1 mL), giving a solution that was then added to stirring pentane (25 mL), creating a green precipitation. The precipitate was collected on a 15 mL fine porosity fritted disc, washed with pentane (3 x 10 mL), and desiccated to yield 2 (73 mg, 89%). The filtrate was removed from the glovebox and evaporated in vacuo to a brown oil. The oil was dissolved in


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DCM (3 x 0.3 mL) and the resulting solution was dropwise added onto a 250 µ m silica preparatory plate. The product was eluted with 10% EtOAc:hexanes (200 mL), scraped off as a band at Rf: 0.41-0.62 and this silica gel was sonicated in EtOAc (20 mL) for 15 min. The silica was filtered off on a 15 mL medium porosity fritted disc and washed with DCM (3 x 2 mL). The colorless filtrate was then evaporated in vacuo and desiccated to yield the colorless oil 7 (16 mg, 46% yield). IR: νC-H sp2 = 2292 cm-1, νCO = 1731 cm-1. 1H NMR (d6-Acetone, δ): 6.36 (1H, m, H2), 6.04 (1H, m, H3), 5.83 (1H, dd, J = 9.6 & 3.9, H4), 3.69 (3H, s, OMe), 2.86 (1H, m, H6), 2.32 (1H, m, H5), 2.21 (1H, m, H6), 1.19 (3H, s, Me), 1.18 (3H, s, Me). 13C NMR (d6-Acetone, d): 177.4 (CO), 131.4 (C4), 125.6 (q, J = 30.9, C1), 124.8 (q, J = 6.5, C2), 124.0 (q, J = 270.0, CF3), 123.5 (C3), 52.2 (OMe), 45.7 (C7), 40.6 (C5), 22.4 (Me), 22.2 (Me), 21.2 (C6). 19F NMR (d6-Acetone, d): -68.98. EA: Calculated for C12H15F3O2: C, 58.06; H, 6.09. Found: C, 58.61; H, 6.22. Synthesis of TpMo(NO)(DMAP)(h2- 1-methyl-2-(5-(trifluoromethyl)cyclohexa-2,4-dien-1yl)-1H-pyrrole) (8). N

N

N

N

Mo

N B H

N N

N

B A

2

NO 3 4

1 CF 3 5

H6 H 6'

7 8

N 9

10

5 (500 mg, 0.80 mmol), CH3CH2CN (5 mL) and a stir pea, were added to a test tube and this orange mixture was cooled for 15 min at -60 °C. A -60 °C, 1M solution of HOTf in CH3CH2CN (2.0 mL, 2.0 mmol) was added to the reaction mixture and the resulting red solution was left standing at -60 °C. After 15 min, N-methylpyrrole (1.0 mL, 11.0 mmol) was added to the reaction mixture and the resulting red solution was left stirring at -60 °C. After 18 h, a -60 °C solution of triethylamine (1.0 mL, 7.17 mmol) was added to the reaction mixture and the resulting brown solution was chromatographed through a 60 mL medium porosity fritted disc ¾ full with silica gel. The product was eluted with Et2O (100 mL) as a yellow band, collected as a yellow solution, and evaporated in vacuo. The resulting yellow oil was then dissolved in DCM (2 mL) and the product was precipitated in stirring pentane (75 mL). The precipitate was collected on a 15 mL fine porosity fritted disc, washed with pentane (3 x 10 mL), and desiccated for 15 min yielding the light yellow solid 8 (205 mg, 37%). CV (DMAc) Ep,a = +0.35 V (NHE). IR: νBH = 2479 cm-1, νNO = 1574 cm-1. 1H NMR (d6-Acetone, δ): 8.12 (1H, d, Pz3A), 7.94 (1H, d, Pz5C), 7.93 (1H, d, Pz5A), 7.83 (2H, m, DMAP-A), 7.80 (1H, d, Pz5B), 7.61 (1H, d, Pz3C), 7.12 (1H, d, Pz3B), 6.81 (1H, m, H2), 6.63 (2H, m, DMAP-B), 6.40 (1H, t, Pz4A), 6.35 (1H, t, J = 2.0, H10), 6.33 (1H, t, Pz4C), 6.14 (1H, t, Pz4B), 6.00 (1H, m, H8), 5.77 (1H, t, J = 3.0, H9), 4.03 (1H, d, J = 8.2, H5), 3.34 (3H, s, NMe), 3.16 (1H, m, H6), 3.08 (6H, s, NMe), 2.85 (1H, t, J = 9.1, H3), 2.33 (1H, t, J = 9.1, H6’), 2.24 (1H, dt, J = 9.1 & 1.7, H4). 13C NMR (d6-Acetone, d): 155.3 (DMAP-C), 151.1 (DMAP-A), 142.8 (Pz3A), 142.2 (Pz3C), 141.6 (Pz3B), 141.3 (C7), 137.5 (q, J = 6.1, C2), 137.4 (Pz5C and Pz5A), 135.8 (Pz5B), 126.8 (q, J = 268.0, CF3), 120.7 (C10), 114.4 (q, J = 30.0, C1), 108.2 (DMAP-B), 106.9 (Pz4C), 106.8 (C9), 106.6 (Pz4A), 106.4



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Page 16 of 19

(Pz4B), 105.7 (C8), 71.8 (C4), 58.8 (C3), 39.2 (NMe), 33.4 (NMe), 32.9 (C5), 27.3 (C6). 19F NMR (d6-Acetone, d): -65.31. Synthesis of 1-methyl-2-(5-(trifluoromethyl)cyclohexa-2,4-dien-1-yl)-1H-pyrrole (9). 2

3 4

7

8

1 CF 3 5

6

N 9

10

To a 50 mL filter flask charged with a stir pea were added 8 (100 mg, 0.145 mmol), DCM (5 mL), and a 0.06 M solution of I2/Et2O (1.2 mL, 0.073 mmol) resulting in a green solution. The solution was stirred at room temperature for 5 min and then evaporated in vacuo to an oil. The oil was dissolved in DCM (1 mL), giving a solution that was then added to stirring hexanes (25 mL), creating a green precipitation. The precipitate was collected on a 15 mL fine porosity fritted disc, washed with hexanes (3 x 10 mL), and desiccated to yield 2 (76 mg, 89%). The filtrate was removed from the glovebox and evaporated in vacuo to a brown oil. The oil was dissolved in DCM (3 x 0.3 mL) and the resulting solution was dropwise added onto a 250 µm silica preparatory plate. The product was eluted with 10% EtOAc:hexanes (200 mL), scraped off as a band at Rf: 0.50-0.71 and this silica gel was sonicated in EtOAc (20 mL) for 15 min. The silica was filtered off on a 15 mL medium porosity fritted disc and washed with DCM (3 x 2 mL). The colorless filtrate was then evaporated in vacuo and desiccated to yield the colorless oil 9 (17 mg, 52% yield). 1H NMR (d6-Acetone, δ): 6.59 (1H, t, J = 2.3, H10), 6.49 (1H, m, H2), 6.10 (1H, m, H4), 6.08 (1H, 1, J = 3.1, H8), 6.07 (1H, m, H3), 6.00 (1H, m, H9), 3.78 (1H, m, H5), 3.68 (3H, s, NMe), 2.62 (1H, dd, J = 17.2, 8.9, H5), 2.49 (1H, m, H6). 13C NMR (d6-Acetone, d): 133.6 (C7), 133.4 (C4), 125.0 (q, J = 30.5, C1), 124.9 (q, J = 6.8, C2), 124.0 (q, J = 272.0, CF3), 122.6 (C10), 122.4 (C3), 107.1 (C8), 106.2 (C9), 33.9 (NMe), 31.5 (C5), 26.7 (C6). 19F NMR (d6Acetone, d): -68.81. EA: Calculated for C12H12F3N: C, 63.43; H, 5.32; N, 6.16. Found: C, 63.22; H, 5.36; N, 6.13.

ASSOCIATED CONTENT Supporting Information Figures of 1H and 13C NMR spectra for selected compounds. DFT calculations for various complexes, including arenium intermediates of 5H+. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail (W. Dean Harman): [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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The authors gratefully acknowledge the National Science Foundation (CHE-1152803).

References: (1) (2) (3) (4) (5)

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CF3

CF3 H

N

N

H

O N

N Mo Tp

NO

Mo

CF3

Tp

37g scale

OMe

NO

CF3 H H N CH3

TOC graphic


Molybdenum(0) Dihapto-Coordination of Benzene and

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