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Light-Mediated Reductive Debromination of Unactivated Alkyl and Aryl Bromides James John Devery, III, John Duy Nguyen, Chunhui Dai, and Corey R. J. Stephenson ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01914 • Publication Date (Web): 14 Jul 2016 Downloaded from http://pubs.acs.org on July 20, 2016

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Light-Mediated Reductive Debromination of Unactivated Alkyl and Aryl Bromides. James J. Devery, III,a John D. Nguyen,a Chunhui Dai,b Corey R. J. Stephenson.a,* a

Department of Chemistry, University of Michigan, 930 N University Drive, Ann Arbor, MI 48109.

b

Department of Chemistry, Boston University, 590 Commonwealth Avenue, Boston, MA 02215.

ABSTRACT Cleavage of carbon-halogen bonds via either single electron reduction or atom transfer is a powerful transformation in the construction of complex molecules. In particular, mild, selective hydrodehalogenations provide an excellent follow-up to the application of halogen atoms as directing groups or the utilization of atom transfer radical addition (ATRA) chemistry for the production of hydrocarbons. Here we combine the mechanistic properties of photoredox catalysis and silane-mediated atom transfer chemistry to accomplish the hydrodebromination of carbon-bromide bonds. The resulting method is performed under visible light irradiation in an open vessel and is capable of the efficient reduction of a variety of unactivated alkyl and aryl substrates.

Keywords: photoredox, hydrodehalogenation, silane, visible light, radical

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INTRODUCTION Visible light-mediated photoredox catalysis has advanced to productively exploit the latent reactivity of carbon-halogen bonds in the development of new, radical-based synthetic methods.1 To date, transformations that utilize C−X bonds bearing iodine, bromine, chlorine, and fluorine through photocatalysis have each been reported.2 Based on these initial findings, a panoply of new transformations have come about, applying both organic and transition metal photosensitizers in the use of visible light as an energy source.3 Chemists have utilized activated C−X bonds incorporated in α-halo carbonyls,1 benzyl halides,2a polyhalomethanes,4 electrondeficient aryl halides,2b,5-6 and iodonium salts7 to develop new reactions. In the case of unactivated halides, a variety of reductive methods have arisen that proceed via reduction of unactivated alkyl and aryl iodides.5-6,8-11 Formally, these transformations yield discrete hydrodeiodinated products as well as products resulting from radical cyclization. However, the corresponding reduction of unactivated alkyl and aryl bromides remains a greater challenge.12-13 Synthetic chemists utilize bromine as a directing group in many contexts, including electrophilic aromatic substitutions, Diels-Alder reactions14 and the Bartoli indole synthesis,15 which stand to benefit from a mild, efficient approach for cleaving carbon-bromide bonds. Traditional methods for the reduction of carbon-bromide bonds proceed through metal-halogen

Figure 1. Fragmentation mechanism and standard reduction potentials of halobenzenes.22a


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exchange,16 hydride reduction,17 atom transfer,18 or single electron reductions.19 However, these methods possess their own limitations: extreme basicity; undesired side reactions; functional group intolerance; toxic,20 explosive,21 or pyrophoric reagents; or stoichiometric loadings of metals. Organocatalytic Photoredox Methods i

O i Pr

Pr O

i Pr

N

N

N

O

O i Pr

S

PDI5

PTH6 Activated aryl bromide examples.

No alkyl bromide examples.

Transition Metal-Catalyzed Photoredox Methods t PF6Bu N Ir N

t

Bu

N N t Bu

Ph Ph P

Ph P Ph

Au Ph P Ph

Au P Ph Ph

Requires large amounts of electron source/H-atom donor.

N Pt N C CC6F5 [Pt(C^N^N)X]13

Requires ultra-violet light and 10 mol% gold loading.

TMS TMS Si H TMS TTMSS, (1)24-25

Bu

2Cl-

[Au2( -dppm)2]Cl212

[Ir(ppy)2(dtbbpy)]PF6, (2)9

t

Only alkyl bromide examples.

Silane-based Methods Only activated alkyl bromide examples. Two activated aryl bromide cyclization examples.

This work [Ir(ppy)2(dtbbpy)]PF6 (0.5 mol%) DIPEA (2 equiv) TTMSS (2 equiv) R-Br MeCN, open to air, rt, blue LED R = aryl/alkyl

R-H

Visible light LEDs Low catalyst loading Unactivated alkyl bromides Unactivated aryl bromides

Figure 2. Previous examples of light-mediated hydrodebromination methods.


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The reduction of aryl carbon-bromide bonds presents a different challenge than aryl carboniodide bonds.22 The addition of an electron to an aryl iodide can result in a purely dissociative, concerted bond cleavage (Figure 1).22a However, reduction of aryl bromides and chlorides occurs via the formation of a radical anion, which then undergoes rate-determining fragmentation. This multistep process results in a thermodynamically more difficult initial electron transfer. Photoredox methods have been developed in an attempt to address this problem with consecutive photoelectron transfer processes,5 long reaction times,6 large excesses of electron/H-atom source,9 and UVA light12 for a limited array of substrates (Figure 2). Herein, we describe a new, general, photoredox-facilitated method for the hydrodebromination of unactivated alkyl and aryl bromides. RESULTS AND DISCUSSION In approaching the challenge of developing a general method for the hydrodebromination of alkyl and aryl bromides, we began with examination of the conditions utilized by Lee et al. at 60 °C.9 Indeed, these conditions provided efficient conversions of primary alkyl bromides (Scheme 1a). However, some aryl bromides proved resistant to these conditions (Scheme 1b). We next Scheme 1. Light-mediated hydrodebromination at elevated temperature.

chose to explore photoredox-facilitated atom transfer4 as a means of supplementing single electron transfer. Tris(trimethylsilyl)silane (TTMSS, 1) is both a potent hydrogen atom donor


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and a competent halogen-atom transfer agent, representing an alternative to tin hydrides.20,23 However, methods employing this compound often rely upon the use of traditional radical initiators like AIBN. Alternatively, Jørgensen and coworkers have employed light and oxygen as an initiator for this process to reduce electron-deficient aryl bromides.24 Similarly, Paixão et al. have shown that two aryl bromides can participate in radical cyclizations with modest yields.25 Recognizing that [Ir(ppy)2(dtbbpy)]PF6 (2) and an amine were capable of hydrodebrominating CBr bonds of select substrates based on the findings of Lee and Kim9 for aryl bromides and our result for alkyl bromides (Scheme 1a), as well as the fact that TTMSS was capable of Br atom transfer under light-mediated conditions,24-25 we posited that the efficiency of 2 and a reductive quencher could be augmented by the presence of TTMSS acting as both a hydrogen atom source and atom transfer agent. Optimization of reaction conditions via an initial screen indicated that the combination of reductive quenching conditions for 2 (0.5 mol%), using two equivalents diisopropylethylamine (DIPEA), with two equivalents of TTMSS, at room temperature, and open to air provided the best substrate conversion by 1H NMR.26 The combination of the DIPEA-facilitated quenching cycle of 2 with the atom transfer capability of TTMSS were required for complete consumption of the substrate. Importantly, rigorous degassing via sparging or freeze-pump-thaw, the absence of 2, and the absence of light resulted in incomplete conversion. Taken together, these results provide important mechanistic details about this process. Complex 2 is capable of forming product in the absence of TTMSS with a large excess of DIPEA.9 Silane TTMSS facilitates the formation of product in the absence of photocatalyst. Exhaustive degassing inhibits reactivity, suggesting that oxygen is important for the success of this transformation. Both findings are


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consistent with Jørgensen and coworkers’ results.24 It is important to note that only partial conversion is observed under an oxygen atmosphere. Table 1. Hydrodebromination of Aryl Bromides.a

Entry Substrate

Product

Isolated Yield (%)

1

99b

2

94

3

82

4

84

5

95

6

83

7

91

a

Reactions performed at 0.5 mmol scale for 10 h. bCalculated by UPLC (PDA).


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With optimized conditions in hand, we examined the scope of this transformation. Beginning with aryl bromides, we found that these conditions were capable of reductive debromination of unactivated arenes (Table 1). These conditions are generally applicable to electron-rich arenes as well as heteroarenes. These conditions are tolerant of benzyl, tosyl, tetrahydropyranyl, and acetyl protecting groups. Formation of the aryl radical and subsequent cyclization occur preferentially over hydrosilylation of the pendant olefin (Entry 3, Table 1), consistent with the observations of Paixão et al.25 We next turned our attention to alkyl bromides (Table 2). These conditions facilitate the Table 2. Hydrodebromination of alkyl bromides.a Isolated Yield (%)

Entry

1

99b

4

91

2

85

5

89

3

84

6

92

89

9

83

85

10

88

Entry

Substrate

Br

7 Ph

Product

Br

Substrate

Product

Isolated Yield (%)

Ph

8

a

Reactions performed at 0.5 mmol scale, at room temperature with 2.2 equiv of DIPEA, 2.2 equiv TTMSS, and 0.5 mol% 2 for 10 h unless otherwise stated. bCalculated by UPLC (PDA).


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reductive debromination of both primary and secondary alkyl bromides. Analogous to the aryl system, the corresponding alkyl radical cyclization proceeds efficiently, favoring the desired reactivity over hydrosilylation (Entry 3, Table 2). As with aryl substrates, conditions displayed tolerance for Ts, Cbz, and Boc protecting groups. Visible light-mediated atom transfer radical addition products4 are reduced (Entries 5 and 6, Table 2), showing that photoredox can be used to produce alkanes in two steps. Finally, these conditions can remove both bromine atoms of 1,1dibromocyclopropanes (Entries 7-10, Table 2), providing efficient conversions for aryl-, diaryl-, and alkyl-substituted cyclopropanes. Traditionally, methods for hydrodebromination of these structures remove a single halogen atom; whereas, tin-, metalation-, or electrochemical-based methods are required to remove the second bromine.27 Indeed, this transformation proceeds to full conversion in the presence of four equivalents of TTMSS. Monitoring of reaction progress of the dihydrodebromination provides insight into this process. Using 3 as a model, the transformation begins by selective conversion of the starting material to a single intermediate in the first 30 min of the reaction. After complete consumption of the dibromocyclopropane, the intermediate is then consumed and converted into the alkane. The first step of this process is identical in the presence and absence of 2 under LED irradiation, forming a discrete intermediate in the first 30 minutes. Performing a reaction in the absence of catalyst with this time constraint results in the isolation of monodebrominated cyclopropane 4 in 83% yield (Scheme 2). Scheme 2. Silane-mediated monodebromination of 3.


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Importantly, 4 does not efficiently convert to the corresponding cyclopropane in the absence of 2. Due to the disparate mechanistic possibilities that exist in the literature,9,24-25 we attempted to gain insight into the reaction mechanism through examination of the kinetic behavior of this transformation by monitoring the hydrodebromination of 4-bromoacetophenone (5) by UPLC.26 We, also, examined the kinetic roles of the other components in this reaction, and probed if the mechanism varied for alkyl, activated aryl, and unactivated aryl bromides. Indeed, different mechanistic properties were observed for 5, bromoethylbenzene (6), and 4-bromophenol benzyl ether (7). Table 3 displays the rate orders observed for the different components present in the Table 3. Summary of Rate Orders26

DIPEA

0.28±0.06

0

-0.9±0.1

2

0.1±0.1

0.5±0.1

-0.28±0.06

2.2±0.1

1.92±0.01

TTMSS 0

reaction mixture. Substrate decay varied between each system. Substrates 5 and 6 decayed exponentially, while 7 decayed with constant slope, in keeping with zero order kinetics. Despite low or zero orders, the presence of both DIPEA and 2 are required for full conversion. Finally, 6 and 7 both displayed second order behavior with respect to TTMSS (Figure 3), while the rate of conversion of 5 displayed a negligible increase in rate.26 These collective data imply that a different dominant mechanism exists for each substrate. The second order in silane for 6 and 7, in concert with the orders for DIPEA and 2, implies a dominant, silane-mediated process for these substrates.


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a)

-5.5

b)

-11

-6.0

-12

ln(kobs)

-6.5

ln(kobs)

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-7.0

-13

-7.5

-14 -8.0 -8.5 -2.6

-2.4

-2.2

-2.0

-1.8

-1.6

-1.4

-15 -3.0

ln[TTMSS]

-2.5

-2.0

-1.5

ln[TTMSS]

Figure 3. ln(kobs) vs. ln[TTMSS] for hydrodebromination of 6 with slope of 2.2±0.1 (a) and 7 with slope of 1.92 ± 0.01 (b).26 The success of our optimized system, the results from kinetic analysis, as well as the partial successes in our initial screen provide significant insight into the complexity that exists in this process. First, a traditional reductive quenching cycle is capable of, at minimum, partial hydrodebromination (Figure 4a). Promotion of 2 to its excited state (2*) by visible light, followed by reductive quenching by DIPEA (8) results in the formation of Ir(II) (9) and amine radical cation 10. Single electron transfer to the substrate (11) regenerates 2 and forms carboncentered radical 12 via generation of a radical anion and unimolecular fragmentation in electronaccepting systems, and direct formation of 12 in alkyl systems. Radical 12 can then abstract a hydrogen atom from 10, yielding hydrodebrominated product.9 Alternatively, 12 can abstract a hydrogen atom from TTMSS, forming 13 and silyl radical 15. The need for oxygen suggests that 2* can also be directed through an oxidative quenching cycle, resulting in the formation of superoxide (16) and Ir(IV) (17) (Figure 4b). DIPEA (8) is capable of donating an electron to 17, yielding amine radical cation 10 and turning over the catalyst. A proton transfer cascade from 10


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Figure 4. Proposed mechanistic pathways. to 8 to 16 forms peroxyl radical 20, which can accept a hydrogen atom from TTMSS to form hydrogen peroxide and 15. Finally, 15 is capable of abstracting a bromine atom from 11 to form 12, which can result in product via hydrogen atom abstraction from TTMSS (Figure 4c). The differences in rate orders observed for 5, 6, and 7 suggest that the resting state and ratedetermining/turnover-limiting steps vary from substrate to substrate. However, based on the proposed pathways, it is possible that hydrogen atom transfer to both 12 and 20 accounts for the observed second order dependence with respect to [TTMSS]. CONCLUSIONS We have developed a mild hydrodebromination method, combining visible light mediated photoredox catalysis and silane-mediated atom transfer that is performed open to air. We have


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provided a mechanistic rationale to the design of reaction conditions that are applicable to both unactivated alkyl and aryl bromides. Product forming hydrogen atom transfer is slower than the disclosed radical cyclizations, rendering this method amenable to radical cyclization cascades after initial carbon-bromide bond cleavage. We have proposed an intricate mechanism that is consistent with our initial synthetic and kinetic observations. Importantly, our observation of 2nd order dependency in TTMSS for hydrodebromination is consistent with the silyl radicalactivated cross coupling reported by MacMillan and coworkers, a method requiring 1 equiv TTMSS to yield the cross-coupled product.28 We are currently investigating this mechanism in greater detail to validate our proposal for the manner in which oxygen participation occurs. ASSOCIATED CONTENT Supporting Information. Experimental details, kinetic data, and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT The authors thank Dr. Joel Beatty, Dr. James Douglas, Prof. Robert Flowers, Dr. Mitchell Keylor, Dr. Bryan Matsuura, and Dr. Niki Patel for their helpful discussions and suggestions. Financial support from the NSF (CHE-1440118), the Alfred P. Sloan Foundation, the Camille and Henry Dreyfus Foundation, and the University of Michigan is gratefully acknowledged. REFERENCES (1)

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