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Mechanistic Investigations Into the Cation Radical Newman-Kwart Rearrangement Cole L Cruz, and David A Nicewicz ACS Catal., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019
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Mechanistic Investigations Into the Cation Radical NewmanKwart Rearrangement Cole L. Cruz and David A. Nicewicz* Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, 27599-3290, United States
Supporting Information Placeholder ABSTRACT: Efforts to elucidate the governing factors in the cation radical Newman-Kwart Rearrangement are described. Through a combination of spectroscopic and kinetic analyses it has been shown that the reactive intermediate is a thione cation radical that has significant thiyl radical character. This intermediate undergoes similar chemistry, such as olefin stereomutation, that has been observed for thiyl radicals generated by other means. Moreover, kinetic studies demonstrate that the electronic dependence observed in the system is the result of ratelimiting intramolecular nucleophilic trapping of the thione cation radical. Importantly, the thiyl radical character of the reactive intermediate can be used to rationalize all reaction observations. This mechanistic model is in line with observed rate enhancement at higher dilutions that decreases the formation of an off-cycle intermediate. A thermochemical cycle was devised to predict which substrates will most easily undergo the cation radical mediated rearrangement.
INTRODUCTION First detailed in 1966 in separate reports from Newman1 and Kwart2, the Newman-Kwart Rearrangement (NKR) represents a simple synthesis of thiophenols from the corresponding phenols. A general synthetic sequence begins with formation of the O-aryl carbamothioate from a phenol and the commercially available carbamoyl chloride (Scheme 1). Heating this adduct to the appropriate temperature allows for the NKR to occur affording the S-aryl carbamothioate. Cleavage of the carbamothioate moiety by methanolysis or reduction finally affords the free thiol. While constituting three separate synthetic steps, this sequence has found significant use due to generally high yields of each step and an operationally simple procedure. NMe2
S O R
- or PdCatalysis
A) Thermal Newman-Kwart NMe2
S O
S
NaOMe - or LiAlH4
S
O
R
R
Mechanism Supported By: - First-Order Kinetics - Hammet Studies - Negative S‡ B) Cation-Radical Newman-Kwart
O
-
NMe2
S
NMe2
S
e-
R
O
R
Scheme 1. Typical reaction sequences utilizing the Newman-Kwart Rearrangement.
S
+e
R
NMe2
O -
R
Experimental Observations: - Higher Dilution Enhances Reactivity - Electron-Rich "R" Favored - Pendant Olefins Isomerize C) Characterization of Cation-Radical Intermediate NMe2
S O
NMe2
S O
NMe2
S O
R Arene-Centered
SH
NMe2
O
NMe2
S
R
NMe2
O R
The NKR requires significantly elevated temperatures most commonly achieved via batch heating. In general, arenes containing sufficiently electron-withdrawing functionality lower the requisite temperature, however substrates bearing electron-donating substitution require temperatures in excess of 300 oC. Modes of catalysis for the NKR have been particularly desirable due to the challenges posed by heating and risk of increased substrate degradation at these extreme temperatures. Developed alternatives include more efficient heating via microwave irradiation3,4 as well as a Pd-catalyzed system developed by Lloyd-Jones5.
Thione-Centered
Figure 1. Important mechanistic features of both A) classical Newman-Kwart Rearrangement and B) Cation radicalmediated Newman-Kwart Rearrangement. C) Two possible intermediates in the cation radical-mediated NewmanKwart Rearrangement.
The proposed mechanism for the NKR was based upon the structurally similar Schonberg rearrangement6.
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Thus, the NKR proceeds via an SNAr mechanism, with the thiocarbonyl moiety undergoing an ipso nucleophilic capture of the aromatic ring. The extreme temperatures required for NKR can be rationalized by the formation of the spirocyclic thiotane ring and disruption of aromaticity. The reduction in reaction temperature with substituents of increasing electron-withdrawing ability7 can be rationalized by stabilization of the Meisenheimer complex in the transition state. The accepted mechanism for the thermal NKR has been supported by a number of studies. Work by Pizzolato8 demonstrated that the kinetic profile follows first-order kinetics, supporting the mechanistic proposal of an intramolueclar rearrangement. An Arrhenius analysis of the rearrangement revealed a large enthalpy of activation (ΔH‡ = 37.3 kcal/mol) and a negative entropy of activation (ΔS‡ = -6.0 e.u). These thermochemical data support a high energy, highly ordered transition state, such as the proposed spirothiotane with disrupted aromaticity. Miyazaki undertook a Hammett analysis for a range of O-aryl thiocarbamate substrates and found a rho value of -1.97 vs. the σ- set. They suggested that this data supports a transition state in which there is a buildup of negative charge, in support of the proposed transition state bearing a Meisenheimer-type complex. Also reported were negative crossover experiments, reinforcing the unimolecularity of the rearrangement.9 Recently we reported a system to affect the NKR via photoredox catalysis10. This system featured a pyrylium photocatalyst11,12 and a reactivity profile complementary to the classical NKR (Scheme 2). We proposed that this reactivity resulted from activation of the O-aryl carbamothioate 1 by single electron transfer13 (SET) to furnish the corresponding cation radical 1+. Rearrangement from this intermediate to give the S-aryl carbamothioate cation radical 2+ skeleton followed by a second SET event furnishes the product (2). We initially posited that this SET occured from the arene moiety, however initial cyclic voltammetry (CV) experiments suggested instead that oxidation of the thione moiety is instead most likely. Consequently, we were interested in characterizing this intermediate and its reactivity.
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Despite our initial findings, other observations of the cation radical NKR system prompted further questions as to the operative mechanism. A critical component of the success of the cation radical NKR was substrate concentration. Higher dilution reactions resulted in faster reaction times, especially for less activated substrates, suggesting either substrate or product inhibition. We were also intrigued by the apparent electronic dependence on the success of the reaction. Arenes bearing electron-withdrawing substitution failed to undergo productive chemistry even at extreme dilutions. These groups could be tolerated, however, if an electron donating substituent was also present on the ring. Thus, we were hopeful that a method to predict successful reactivity could be devised to rationalize the observed substrate scope.
RESULTS AND DISCUSSION Nature of the Cation-Radical We were initially curious about the initial quenching of the excited photocatalyst by the substrate. We first proposed that the excited state catalyst would undergo SET from the substrate to afford an arene-centered cation radical. However, CV experiments offered evidence to the contrary.10 CVs of both an aryl and an alkyl carbamothioate gave traces with an irreversible oxidation peak around +1.2 V vs. SCE. Given this data, we proposed instead that this peak in the CV corresponded to a redox event primarily involving the thione moiety. Table 1. Results of pyrylium fluorescence quenching by aryl carbamothioates.
Thiocarbamate
KSV (M–1)
kq s–1)
(M–1
Ep/2 (V vs. SCE)
NMe2
S O
77.8
1.68×1010
+1.20
58.0
1.30×1010
+1.15
74.9
1.68×1010
+1.09
70.6
1.59×1010
+1.15
63.4
1.50×1010
+1.21
76.7
1.72×1010
+1.21
1a
MeO
NMe2
S O
1b
AcHN
NMe2
S O
1c
Ph
NMe2
S O
1d
Me
NMe2
S O
1e
H
NMe2
S O Cl
1f
Scheme 2. Initial mechanistic proposal for the cation radical Newman-Kwart Rearrangement.
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Indeed, this oxidation peak is present in all substrates from our initial report that we submitted to CV, regardless of the substituent present on the arene, including substrate (1f) that failed to undergo NKR in this system10 (Table 1). While the CV data would suggest that the oxidation primarily occurs from the thione moiety, we were curious if the trends in reactivity were reflected in the rates at which the substrates quench the photocatalyst. In order to gain insight into the kinetics of the presumed SET process, we turned to fluorescence spectroscopy, specifically Stern-Volmer analysis.14 A series of O-aryl carbamothioates were tested as potential quenchers of the pyrylium photocatalyst. All substrates tested were found to be effective quenchers of pyrylium fluorescence. The quenching constants, kq, for all O-aryl carbamothioate substrates tested were found to be within the same order of magnitude, near the diffusion limit in acetonitrile, though did vary by 15% depending on the identity of the R-group in the 4position. This variation, however, was not found to fit well with any linear free energy relationship (see SI).15,16 These findings suggest that the electronic identity of the aryl component of the carbamothioate does not have a significant impact in the quenching event between substrate and photocatalyst. Moreover, given that all quenching rates were found to be within a narrow, neardiffusion limited region, quenching of the photocatalyst is unlikely to be the limiting factor in predicting successful reactivity. Given this data we believe that quenching of the chromophore by the carbamothioate substrate occurs through an SET process and generates a cation-radical intermediate that undergoes rearrangement.17,18 The SET event occurs mainly from the thione moiety, with minimal involvement of the arene system, resulting in a cation radical species with charge and spin density localized in the thione moiety. We then became curious about how best to characterize this cation radical species and if its characterization could help explain some of the features of the cation radical NKR. Reactivity of Thione Cation-Radical In our initial communication, we reported that aryl carbamothioate 1h underwent NKR under photoredox conditions.10 We also noted that the olefin in 1h underwent isomerization during the course of the reaction. Substrate 1h was synthesized as a 90:10 Z:E mixture of isomers, however the NKR product was obtained as a 1:19 Z:E ratio. We were curious to discern the pathway through which this isomerization takes place during the reaction and if this pathway could offer evidence concerning the reactivity of the thione cation radical intermediate. Control experiments confirmed that no isomerization occurred during irradiation without the pyrylium chromophore, ruling out a direct sensitization mechanism of isomerization (see SI). Monitoring the olefin isomer ratio during the course of the reaction revealed that isomerization had occured prior to significant product formation and the geometric isomers reached equilibrium (i.e. 1:19 Z:E) after only ~10% conversion (Figure 2A). Thus, the isomerization process occurs after quenching the excited chromophore but
prior to the OS aryl migration event and is evidently much faster than the rearrangement process. Pyryliums have been shown to affect the isomerization of olefins via both electron transfer and energy transfer pathways.19–22 However, given the CV and fluorescence quenching data we conjectured that buildup of charge and spin density in, and weakening of, the olefin moiety to be unlikely. Moreover, irradiation of carbamate derivative 5 in the presence of 3 showed a small degree of isomerization, though never reached the ratio observed for the carbamothioate analog (Figure 2B). However, the carbamate substrate was observed to undergo complete isomerization when thiocarbamate 1f, which does not undergo rearrangement under the reaction conditions, was added to the solution. We reason that while SET or sensitization of a styrene may result, in some cases, in olefin isomerization, it is not the operative mechanism for isomerization observed for 5 or 1h. Furthermore, isomerization must be an intermolecular process and is dependent on the presence of an O-bound carbamothioate, suggesting a bimolecular reaction with the thione cation radical. A) O
Me
O
NMe2
S
S
3 (1 mol%) 1h +
[0.12 M] MeCN 450 nm LEDs Time
1h 90:10 Z:E
B)
O
Me 2h
Time = 2 min
99% 69:31 Z:E
0% N/A
Time = 30 min
86% 5:95 Z:E
14% 5:95 Z:E O
NMe2 O
Me
[0.12 M] MeCN 450 nm LEDs 20 h
83:17 Z:E
Me >95% recovered 71:29 Z:E
w/ 100 mol% 1f S
NMe2
NMe2 O
3 (1 mol%)
5
C)
NMe2
>95% recovered 5:95 Z:E O
O
NMe2 S
1a MeO
3 (1 mol%) +
[0.12 M] MeCN 450 nm LEDs 20 h
2a MeO 28% conversion (99% in absence of styrene) Me
Me OMe 3:1 Z : E
6
OMe
6
1:19 Z:E
Figure 2. Experiments investigating olefin isomerizations during cation radical NKR. All reactions run with 1.0 mol% of 3. A) Time-course profile of olefin isomerization during cation radical NKR. B) Isomerization of a carbamate under cation radical NKR conditions. C) Inhibition of NKR by added olefin.
Despite this, we were curious if 3 could assist generally in olefin isomerization as has been shown for stilbene.19,20 To probe this question, we subjected styrene derivative 6 to irradiation in the presence of 3 and found
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the isomer ratio essentially unchanged. However, when carbamothioate substrate 1a was added, isomerization of the styrene was again observed with no other styrenederived species having been formed (Figure 2C).23 The carbamothioate was also found to have rearranged to the S-aryl product in 28% conversion. The lack of significant olefin isomerization in both 5 and 6 when irradiated in the presence of 3 suggests that oxidation does not result in stereomutation for β-alkyl styrenes.24 The equilibration observed only in the presence of carbamothioates suggests the formation of a reactive intermediate by SET between the pyrylium and 1. We hypothesize that this species is thione-centered cation radical 1+, which in light of these results can best be described through the resonance structure shown25, with significant spin density on the sulfur atom (Figure 3A). Thiyl radicals are known to induce isomerization in lipids and have been shown to give thermodynamic ratios of olefins.26–28 We observe the same stereomutation when 5 is irradiated in the presence of phenyl disulfide (see SI). We have shown previously that irradiation of phenyl disulfide with blue LEDs results in homolytic cleavage of the disulfide bond, forming thiyl radicals.24 Consequently, the stereomutations observed in reactions with thiocarbamates can best be explained by the significant thiyl radical character of 1+. Similar intermediates have been proposed in oxidations of thioamides.29,30 Moreover, we posit that characterizing thione cation radical intermediates in this fashion may lead to its use in additional transformations and have implications in other systems such as RAFT-type photopolymerizations developed by Fors.31–33 A mechanism to rationalize the isomerization is shown in Figure 3B.28 Upon formation of 1+, radical addition from the sulfur center in a thiol-ene-type manifold to the styrene results in the formation of a benzylic radical species. Free rotation about the - and positions gives rise to the s-trans and s-cis conformers. Retro thiol-ene from either of these benzylic radicals reforms the styrene and 1+. The E-styrene is formed from elimination of the s-trans intermediate and vice versa for the Z-alkene isomer. The isomer distribution is reflective of the relative stabilities of the styrene isomers. A) Characterization of Cation-Radical NMe2
S
NMe2
S
O
O
R
B) Mechanism for Olefin Isomerization
+
Ar R
R Ar
Ar
R
+
OAr
R H NMe2
OAr
NMe2
S
OAr
H H S
s-cis
NMe2
S
H Ar S
NMe2 OAr
s-trans
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Figure 3. Reactivity of aryl carbamothioate radical. A) Likely electronic structure of carbamothioate radical. B) Mechanism for alkene isomerization pathway.
Concentration Dependence In our initial report we found that lower concentrations favored higher conversions for a number of substrates. We found that this concentration dependence was especially pronounced for arenes that lacked strong electron-donating groups. Moreover, we found that in all cases, faster conversions could be achieved at lower concentrations for a given substrate. For example, methoxy-substituted carbamothioate 1a underwent rearrangement at >95% conversion in 2.5 hrs when irradiated at a concentration of 0.25 M, but only 10 minutes when diluted to 0.06 M (Figure 4). Conversely, methyl-substituted 1d afforded trace conversion at 0.5 M, but could undergo 76% rearrangement in 24 hours at 0.06 M. In order to probe this concentration dependence, we performed a series of in-situ IR experiments. NMe2
S O
1a MeO
NMe2
O
3 [0.0025 M] [0.06 to 0.25 M] MeCN 450 nm LEDs
S
2a MeO
[1a] (M)
Φ
0.06
0.44
0.12
0.30
0.19
0.25
0.25
0.20
Figure 4. In-situ FT-IR traces of cation radical NKR rearrangement and quantum yields for conversion of 1a at different concentrations. Quantum yields were determined between 10-15% conversion of 1a.
Monitoring the conversion of 1a at a range of concentrations using in-situ IR revealed a dramatic difference in rates as a function of concentration. To rule out product inhibition, the reaction was monitored at [0.06 M] with varying amounts of 2a added at the beginning of the reaction (Figure 5). Overlap was good up to a full equivalent of product added. In accordance with the observed rate enhancement, the quantum yield of the rearrangement was also found to increase at higher dilutions (Figure 4). Rate enhancement upon dilution would suggest a mechanism whereby either substrate or product
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inhibited the transformation. As the reactions with added 2a show good overlap with the normal conditions, product inhibition can be ruled out. Catalyst deactivation may NMe2
S
O
1a MeO
[0.06 M] 2a
NMe2
O
3 [0.0025 M] MeCN 450 nm LEDs
S
2a MeO
[0.0 - 0.06 M]
6B). Given the thiyl-radical nature of the cation radical intermediate, it is possible that this intermediate takes the form of a disulfide-type adduct. The formation of sulfur-sulfur adducts between thiyl radicals and thiones has been shown in Barton-type deoxygenation systems.34 This reactivity would be consistent with that observed between O-aryl carbamothioates and alkenes under these oxidative conditions, however we do lack spectroscopic evidence for this structure. Regardless of the structure of this adduct, lower substrate concentrations would disfavor formation of this off-cycle intermediate and allow the rearrangement to proceed. A) Decreased Back Electron Transfer Ar
S
NMe2 O
Ar
Ar
O 3•
1+•
Back-Electron Transfer Favored At High Concentrations
play a role at late stages of the reaction, as time-adjusted overlays of the [0.12 M] and [0.25 M] data sets show poor overlap (see SI). However, at early time points a large divergence in rate is observed suggesting that there appears to be some degree of substrate inhibition. One possible mode of substrate inhibition would involve static quenching of the chromophore by the substrate. Steady-state fluorescence quenching experiments (see SI) showed a linear quenching profile, suggesting that dynamic quenching is the only operative mechanism of chromophore quenching (vide infra).14 Thus, the concentration dependence is instead the result of post-quenching interactions, which could be explained in two ways. First, lower concentrations of 1 would disfavor back electron transfer from the reduced photocatalyst to 1+ by decreasing the likelihood of recombination of the reduced pyrylium species and 1+. This would increase the lifetime of the reactive intermediate and allow it to undergo rearrangement (Figure 6A). However, this alone seems unlikely to account for the observed rate changes. Decreasing substrate concentration would impact both back-electron transfer and the initial PET. Given that the quenching fractions (Q) in this concentration range are less than unity (0.81 – 0.95), decreasing substrate concentration should not cause an increase in reaction efficiency. Instead, an alternative pathway must be operative at high substrate concentrations that returns the reactive intermediates to the beginning of the catalytic cycle (and thus wasting a photon and decreasing quantum yield). The formation of an off-cycle intermediate with neutral 1 and cation radical 1+, would account for substrate inhibition if this adduct can also undergo backelectron transfer to reform the starting materials (Figure
+
3
Figure 5. In-situ FTIR traces of cation radical NKR with added product at beginning of reaction.
Rearrangement Favored At Low Concentrations
O 1
3
•
NMe2 S
+
2+• B) Off-Cycle Intermediate S
NMe2
NMe2 O
1
Me2N
S
S
O
O 1+•
7
3•
Rearrangement
Chain Termination
3 O
NMe2
S
S
NMe2 O
2+•
2
1
Figure 6. Possible modes of substrate inhibition. A) Back electron transfer. B) Proposal for off-cycle intermediate formation.
An interesting corollary to the observed concentration dependence is that this trend was also seemingly coupled to the electronic nature of the arene. In our initial report, substrates bearing less activated arenes required more dilute conditions to achieve high conversions. For example, substrate 1d, bearing a lone methyl substituent, proceeded in only 95% (>95%) +1.40 V vs. SCE
S
S
OMe
23% (>95%) +1.76 V vs. SCE
NMe2
O
OMe
OHC
>95% (>95%) +1.42 V vs. SCE
NMe2
O MeO
NMe2
O
S
NMe2
O
S
S Cl
0% (15%) +1.75 V vs. SCE
0% (50%) +1.76 V vs. SCE
0% (0%) +1.87 V vs. SCE
- Borderline Substrates -
Figure 8. A) Thermodynamic cycle for the cation radical NKR. B) Scope of cation radical NKR with respect to S-aryl carbamothioate redox potential. Yields for substrates are for reactions performed at 0.5 M substrate concentration. Parenthetical yields refer to reactions run at lower concentrations (0.12 M or less).
A thermodynamic cycle based on known thermochemical parameters was constructed to predict
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when the cation radical rearrangement is most favored. These calculations offer a reliable prediction of substrate reactivity in this system from an easily obtained thermochemical value: the oxidation potential of the Saryl carbamothioate. This value can be obtained to a reasonable degree of accuracy via simple computational means. We posit that the results reported here will aid the incorporation of the cation radical NKR into standard use. The mechanistic trends allow for substrates to be more judiciously selected for this transformation. Moreover, a simple predictive tool has been devised to determine if a given substrate is suitable for this transformation possibly saving time and material when devising syntheses. More generally, the development of a thermochemical cycle in cation radical mediated transformations should aid in rationalizing reaction trends. We imagine that similar cycles may be constructed for other transformations to obtain valuable information about reaction efficiencies.
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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures, data and spectra along with further discussion of mechanism (PDF)
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AUTHOR INFORMATION Corresponding Author
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*
[email protected] Notes The authors declare no competing financial interest.
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ACKNOWLEDGMENT We are grateful for support from Eli Lilly (Eli Lilly Grantee Award to D.A.N.). This research made use of instrumentation (Hewlett–Packard 8453 Chemstation absorption spectrometer and Edinburgh FLS920 spectrometer) funded by the UNC EFRC: Center for Solar Fuels, an Energy Frontier Research Center supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DE-SC0001011). The authors thank Andrew Perkowski, Nathan Romero and Jeremy Griffin for useful discussion of results.
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