Vanadium (V) oxide mediated bromolactonization of alkenoic acids

Vanadium (V) oxide mediated bromolactonization of alkenoic acids

Tetrahedron xxx (2015) 1e8 Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet Vanadium (V) oxide m...

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Tetrahedron xxx (2015) 1e8

Contents lists available at ScienceDirect

Tetrahedron journal homepage: www.elsevier.com/locate/tet

Vanadium (V) oxide mediated bromolactonization of alkenoic acids McKenzie L. Campbell, Samuel A. Rackley, Lauren N. Giambalvo, Daniel C. Whitehead * Department of Chemistry, Clemson University, Clemson, SC 29634, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 March 2015 Received in revised form 9 April 2015 Accepted 10 April 2015 Available online xxx

A full account of our recently communicated method for the V2O5 mediated bromolactonization of alkenoic acids is presented. Here we describe the extensive evaluation of the metal oxide catalyst, terminal oxidant, halide source, solvent system, and reaction temperature that resulted in optimal conditions employing 0.05 equiv V2O5, 3 equiv urea-hydrogen peroxide complex (UHP), and 3 equiv of NH4Br in an acetone:H2O (6:1) solvent system at room temperature. Additionally, we have expanded the substrate scope of the reaction with the aim of evaluating the functional group tolerance of the transformation. Further, we present preliminary data of a related halogenation of b-diketones with our optimal conditions. Finally, we probed the role of urea in the transformation. Ó 2015 Elsevier Ltd. All rights reserved.

Keywords: Haloactonizaton Vanadium oxide Bromenium Bromine equivalent

1. Introduction Many groups have invested significant efforts in order to develop synthetically feasible and cost-effective methods to circumvent the use of elemental bromine in synthetic operations.1 The strategy involves the development of means to effect the in situ oxidation of less corrosive, volatile, and toxic bromide salts to synthetically competent bromenium (Brþ) equivalents via the incorporation of an exogenous oxidant. A number of terminal oxidants are effective including sodium perborate,2 ceric ammonium nitrate,3 sodium periodate,4 lead acetate,5 and SelectflourÒ.6 More recently, a superstoichiometric loading of trimethylsilyl triflate (TMSOTf) and an organic sulfoxide were used to promote the oxidation of sodium bromide.7 While effective, these methods rely on relatively expensive and environmentally unfriendly oxidants as compared to an ideal case of using either molecular oxygen or aqueous hydrogen peroxide as the terminal oxidant. The most commonly employed method for the in situ oxidation of bromide relies on treatment of ammonium or sodium bromide with the peroxysulfate salt, OxoneÒ.8 While generally effective, this method suffers from the considerable salt waste stream associated with unpurified OxoneÒ (2KHSO5$KHSO4$K2SO4). Further, OxoneÒ demonstrates a broad reactivity profile with a number of common organic functional groups,9 thus presenting the potential for detrimental side-reactions with functionalized substrates. Efforts aimed at employing more idealized co-oxidants such as hydrogen peroxide, oxygen, or air have met with some success. The use of

* Corresponding Whitehead).

author.

E-mail

address:

[email protected]

(D.C.

hydrogen peroxide as the terminal oxidant requires strongly acidic conditions (e.g., H2O2/H2SO410 and H2O2/HBr11) or chalcogen-based catalysts.12 Additionally, the aerobic oxidation of HBr can be mediated by catalytic loadings of sodium nitrite.13 From a biological perspective, several algae have evolved the ability to use the millimolar marine concentration of bromide to carry out electrophilic halogenation during the course of their biosynthesis of secondary metabolites. In these organisms, the critical oxidation of bromide to bromenium is promoted by haloperoxidase metalloenzymes harboring a vanadium (V) oxo active site and relies on hydrogen peroxide as the terminal oxidant.14 Inspired by this biological phenomenon and our on-going interest in vanadium oxide catalysis,15 we sought to develop a reliable method for the bromolactonization of alkenoic acids hinging on the in situ oxidation of bromide to a bromenium equivalent catalyzed by the cheap (w$0.25/g), abundant, and safe vanadium (V) oxide (i.e., V2O5). We targeted a bromolactonization protocol as our initial foray into this field based on its historical prominence as a convenient method for the efficient and stereoselective formation of small and medium sized heterocycles in organic synthesis.16 Indeed, halolactonizations have received substantial attention from the synthetic community in the past decade. Several groups have developed new catalysts that efficiently promote the bromolactonization of alkenoic acids employing N-bromosuccinimide as the terminal halogen source.17 Additionally, a number of asymmetric halolactonization methods have appeared.18 Due to the efforts of a number of groups, chloro,19 bromo,17d,20 and iodo21 lactonizations have all been rendered enantioselective. Our approach is unique among these efforts based on the strategy of in situ oxidation of bromide to bromenium (Brþ) by action of vanadium (V) oxide.

http://dx.doi.org/10.1016/j.tet.2015.04.029 0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.

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Vanadium (V) oxide has received some attention as a useful catalyst for several organic transformations, but it has not achieved the broad applicability observed for other transition metal oxides of osmium, ruthenium, chromium, and rhenium. We attribute this fact to the sparing solubility of the material in most organic solvents and the relatively limited substrate scope of many of the V2O5-mediated processes. Nonetheless, V2O5 has been used effectively as a catalyst for the oxidation of alcohols to ketones and aldehydes with various co-oxidants.22 In alcohol solvents, aldehydes and ketals can be oxidized directly to esters.23 Other miscellaneous oxidations mediated by V2O5 include the oxidation of dibenzylthiophene,24 the oxidative coupling of naphthols to form biaryls,25 the cyanation of tertiary anilines,26 the aromatization of 1,4dihydroxypyridines,27 and the oxidative conversion of the opium alkaloid thebaine to 14-hydroxycodeinone.28 Additionally, several transformations mediated by V2O5 hinging on the oxidation of bromide have been reported including the bromination of arenes,29 b-diketones and b-ketoesters,30 the deprotection of dithianes,31 the bromocyclization of 2hydroxychalchones,32 and the preparation of organoammonium perbromide salts.33 The major limitation of these initial efforts has been the relatively large catalyst loadings (i.e., w0.5 equiv of V2O5 relative to substrate) in some cases. Herein, we provide a full account of our recently communicated method34 for the V2O5-mediated bromolactonization of alkenoic acids mediated by the in situ oxidation of bromide to bromenium (Brþ). 2. Results and discussion 2.1. Exploratory experiments We began our investigation with a series of exploratory grounding experiments targeting the bromolactonization of 4phenylpentenoic acid 1 mediated by the V2O5 catalyzed oxidation Table 1 Initial exploratory experiments

V2O5 (eq.)

Solvent system

Co-ox. (eq.)a

Halide source (eq.)

Yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 14 16 17 18 19

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1

ACN:H2O (6:1) ACN:H2O (6:1) ACN:H2O (6:1) ACN:H2O (6:1) ACN:H2O (6:1) ACN:H2O (1:1) ACN:H2O (1:6) 30% aq H2O2 ACN:H2O (6:1) ACN:H2O (6:1) ACN:H2O (6:1) ACN:H2O (6:1) ACN:H2O (6:1) ACN:H2O (6:1) ACN:H2O (6:1) ACN:H2O (6:1) ACN:H2O (6:1) ACN:H2O (6:1) ACN:H2O (6:1)

H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 d H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2

NH4Br (15) NaBr (15) CsBr (15) LiBr (15) KBr (15) NH4Br (15) NH4Br(15) NH4Br (15) NH4Br (5) NaBr (5) NH4Br (5) NaBr (5) NH4Br (5) NH4Cl (5) NaCl (5) NaBr (5) NH4Br (5) CsBr (5) LiBr (5)

73 84 66 76 74 71 51 68 84 73 65 75 89c 36d 28d 59 54 42 40

b c d

2.2. Further optimization: solvent, co-oxidant, temperature Based on our initial screening efforts, we selected a combination of 0.2 equiv V2O5 in the presence of 5 equiv NH4Br as the halide source, in a 6:1:1 ratio of acetonitrile, water, and 30% aq hydrogen Table 2 Further optimization

Entry

a

of bromide (Table 1). We were encouraged by our ability to effect the desired bromolactonization of 1 in a reasonable 73% yield by means of the oxidation of ammonium bromide (15 equiv) catalyzed by 0.5 equiv of V2O5 with 30% aq H2O2 in an acetonitrile:water (6:1) solvent system (entry 1). A brief survey of other bromide salts, including sodium bromide, cesium bromide, lithium bromide, and potassium bromide returned lactone 2 in yields ranging from 66 to 84% (entries 2e5). Increasing the water portion of the solvent system from 6:1 (i.e., entry 1) to 1:1 (entry 6) and 1:6 (entry 7) resulted reduced yields of 71 and 51%, respectively. Conducting the reaction in the absence of solvent aside from 30% aqueous H2O2 returned 2 in a 68% yield (entry 8). Reducing the loading of halide salt from 15 equiv to 5 equiv resulted in comparable yields for ammonium bromide (84% yield, entry 9) and sodium bromide (73% yield, entry 10). Next, we attempted to reduce the catalyst loading to more reasonable levels. A 20 mol % loading of V2O5 resulted in comparable or reduced yields compared to 0.5 equiv loadings (see entries 11 and 12). The 20 mol % V2O5 loading returned an acceptable 89% yield of 2 when the reaction mixture was warmed to 65  C (entry 13). Two experiments with chlorides salts (i.e., NH4Cl and NaCl, entries 14e15) returned the corresponding chlorolactonization product (not shown) in poor yields ranging from 28 to 36% yield. Further reduction of the catalyst loading to 10 mol % V2O5 returned bromolactone 2 in poor yields ranging from 40 to 59% yield, regardless of the bromide source (entries 16e19). This initial panel of experiments confirmed our hypothesis that V2O5 could serve as a viable haloperoxidase-like catalyst for the bromolactonization of alkenoic acids. Subsequent efforts, detailed below, were geared toward optimizing the protocol to one of reasonable synthetic utility.

(aq) (aq) (aq) (aq) (aq) (aq) (aq) (aq) (aq) (aq) (aq) (aq) (aq) (aq) (aq) (aq) (aq) (aq)

H2O2 (aq) denotes a 30% aqueous solution of hydrogen peroxide. Yields are isolated yields after acid/base extraction. Reaction warmed to 65  C. Corresponding chlorolactone product was isolated.

Entry

V2O5 (eq.)

Solvent system

Co-ox. (eq.)a

Halide source (eq.)

Yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.05 0.01

ACN:H2O (6:1) EtOAc: H2O (6:1) PhMe: H2O (6:1) Acetone:H2O (6:1) Acetone:H2O (6:1) Acetone:H2O (6:1) Acetone:H2O (6:1) Acetone:H2O (6:1) Acetone:H2O (12:1) Acetone:H2O (30:1) Acetone:H2O (6:1) Acetone:H2O (6:1) Acetone:H2O (6:1) Acetone:H2O (6:1) Acetone:H2O (6:1) Acetone:H2O (6:1)

H2O2 (aq) H2O2 (aq) H2O2 (aq) H2O2 (aq) UHPc (5) UHP (5) UHP (3) UHP (5) UHP (5) UHP (5) UHP (3) UHP (3) UHP (2.5) UHP (2.5) UHP (3) UHP (3)

NH4Br (5) NH4Br (5) NH4Br (5) NH4Br (5) NH4Br (5) NH4Br (5) NH4Br (3) NH4Br (5) NH4Br (5) NH4Br (5) NH4Br (3) NaBr (3) NH4Br (2.5) NH4Br (2.5) NH4Br (3) NH4Br (3)

65 67 66 67 87 91d 92 96e 87 36 93 43 84 93d 90 12

a H2O2 (aq) denotes a 30% aqueous solution of hydrogen peroxide employed as a co-solvent in a 6:1:1 ratio with organic solvent and water, respectively. b Yields are isolated yields after acid/base extraction. c UHP¼urea-hydrogen peroxide complex. d Reaction warmed to 65  C. e Identical yield observed at rt and 65  C.

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peroxide (i.e., Table 2, entry 1; transcribed from Table 1, entry 11) as our starting point for a second round of optimization. We chose these conditions as a good starting point for subsequent optimization efforts, despite its disappointing 65% yield of 2, since it returned a clean sample of the product, free from vicinal dibrominated by-product at room temperature. Changing the organic component of the solvent system from acetonitrile to ethyl acetate, toluene, and acetone resulted in comparable yields of 2 ranging from 66 to 67% (Table 2, entries 2e4). A key breakthrough for our study was the observation of an increase in yield of bromolactone 2 when the co-oxidant was changed to the commercially available urea-hydrogen peroxide complex (UHP) in an acetone/water (6:1) solvent system. In the event, employing 5 equiv of UHP as the cooxidant returned 2 in an 87% yield with 0.2 equiv of catalyst (entry 5). Otherwise identical conditions at 65  C returned 2 in an improved yield of 91% (entry 6). Finally, reducing the loading of both UHP and NH4Br to 3 equiv each returned 2 in 92% in the presence of 20 mol % V2O5 (entry 7). After establishing UHP as the optimal co-oxidant, we embarked on a systematic screen of reaction conditions with the overall goal of reducing the loadings of the catalyst, co-oxidant, and terminal halide source to more reasonable levels. The loading of V2O5 could be reduced to 0.1 equiv while still returning the desired product 2 in excellent yield at both room temperature and 65  C (entry 8). Decreasing the aqueous component of the solvent system from 6:1 acetone/water to 12:1 and 30:1 resulted in a dramatic decline in the yield of 2 (entries 9 and 10). Reducing the loading of UHP and NH4Br from 5 equiv to 3 equiv in the presence of 0.1 equiv V2O5 returned 2 in a 93% yield (entry 11). Employing sodium bromide in lieu of ammonium bromide resulted in a significantly reduced 43% yield of the desired bromolactone product 2 (entry 12). A significant portion of the vicinal dibromination product resulting from trapping of the initial bromonium intermediate with bromide was observed. Further reduction of the loading of UHP and NH4Br to 2.5 equiv each resulted in a reduced yield of 84% (entry 13) unless the reaction was warmed to 65  C, whereby the yield rose again to 93% (entry 14). Nonetheless, we were able to lower the catalyst loading to 0.05 equiv while maintaining the co-oxidant and halide source loading at 3 equiv each and recover bromolactone 2 in only a slightly reduced 90% isolated yield (entry 15). Unfortunately, further lowering the catalyst loading to 1 mol % resulted in a very poor 12% isolated yield of 2 (entry 16).

Table 3 Screen of other transition metal oxides

3

2.3. Metal oxide screen We also elected to evaluate several other commercially available metal oxides to ensure that V2O5 was indeed the catalyst of choice for the desired transformation. In this series of experiments (Table 3), we screened various metal oxides in a 0.1 equiv catalyst loading in the presence of 3 equiv of urea-H2O2 and 3 equiv NH4Br in a 6:1 acetone/H2O solvent system at room temperature for 18 h. In the absence of metal oxide catalyst (entry 1), no desired bromolactone was isolated or detected by 1H NMR analysis, confirming that uncatalyzed oxidation of NH4Br by UHP alone is not operative in our system. Entry 2 reiterates the observed 93% yield of the target bromolactone in the presence of 0.1 equiv of V2O5. Next we evaluated other commercially available metal oxides to determine whether V2O5 was uniquely effective at promoting the in situ oxidation of bromide. Oxides of niobium, including niobium pentoxide and niobium dioxide returned negligible, but still spectroscopically detectable, amounts of the desired bromolactone, along with mostly recovered starting material (entries 3e4). Tungsten trioxide catalysis (entry 5) returned pristine starting material while use of tungsten dioxide resulted in the isolation of the desired lactone product in 32% yield (entry 6). Chromium trioxide catalysis also returned bromolactone 2 in a 29% isolated yield (entry 7). Oxides of tantalum including lithium tantalate (entry 8) and tantalum pentoxide (entry 9) returned trace product and recovered starting material, respectively. Interestingly, molybdenum trioxide and molybdenum dioxide returned the desired bromolactone 2 in acceptable yields of 78% (entries 10e11). This brief evaluation of other commercially available transition metal oxides confirmed V2O5 as the catalyst of choice for the desired transformation. Nonetheless, the corresponding catalytic activity of the molybdenum oxides is also noteworthy. 2.4. Optimal conditions Based on the extensive screening process described in the previous three sections, we arrived at our optimal reaction conditions for the desired transformation (Scheme 1).

Scheme 1. Optimal reaction conditions for V2O5 catalyzed bromolactonization.

Entry

Metal oxide

Yield (%)

1 2 3 4 5 6 7 8 9 10 11

No catalyst V2O5 Nb2O5 NbO2 WO3 WO2 CrO3 LiTaO3 Ta2O5 MoO3 MoO2

0 93 Trace Trace 0 32 29 Trace 0 78 78

Specifically, urea-hydrogen peroxide complex (3.0 equiv) and V2O5 (0.05 equiv) were dissolved in acetone/H2O (6:1) (0.08 M relative to substrate) and stirred at 0  C for 30 min. Ammonium bromide (3.0 equiv) was added and stirred for an additional 30 min. After addition of the substrate (1.0 equiv), the mixture was allowed to stir for an additional 15 min at 0  C before gradually warming to room temperature overnight. Notably, the desired bromolactone product could be isolated in acceptable purity after a simple acid/ base extraction protocol without recourse to column chromatographic purification. With these optimized conditions in hand, we set out to evaluate the substrate scope of the transformation. 2.5. Substrate scope Chart 1 depicts the substrate scope of the method. As mentioned previously, cyclization of 1 returned bromolactone 2 in 90% isolated yield. The method was effective for the cyclization of several related

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Chart 1. Substrate scope.

para-substituted 4-phenylpentenoic acid substrates including: pbromo (3, 83%), p-chloro (4, 85%), p-methoxy (5, 96%), p-methyl (6, 87%), and p-ethyl (7, 82%). The success of the p-ethyl substrate (returning lactone 7) is noteworthy given that we did not observe any bromination of the relatively activated 2 benzylic position, suggesting that bromine radicals may not be operative in the reaction. Next, we investigated the effect of extending the linker between the carboxylate nucleophile and the alkene. The corresponding d-lactone arising from cyclization of 5-phenyl-5-hexenoic acid was initially isolated in a disappointing 26% yield (8a). Increasing the catalyst loading to 0.1 equiv resulted in an improved yield of 50% (8b). In both cases, a significant amount of the undesired 5,6-dibrominated uncyclized product was isolated. Attempts to promote the desired intramolecular cyclization by diluting the reaction (i.e., in an effort to preclude the biomolecular dibromination pathway) failed to improve on the initially observed yields for this substrate. Nevertheless, the incorporation of a gemdimethyl substituent in the linker returned the analogous 3,3dimethyl d-lactone 9 in an excellent 97% yield, taking advantage of the well-known Thorpe-Ingold effect.35 Cyclization of 4pentenoic acid returned the corresponding unsubstituted g-lactone in yields ranging from 50 to 51% yield (10a and 10b). Further, the method provides facile access to benzolactones. For example, cyclization of 2-allylbenzoic acid provided benzolactone 11 in an

excellent 93% isolated yield. Finally, we investigated the cyclization of trans-styrylacetic acid. We were particularly interested in this transformation due to the predictable lability of the initially formed bromolactone product 12, which we surmised might rapidly eject HeBr to form the corresponding unsaturated butenolide. In the event, our optimal condition with 0.05 equiv of V2O5 proved impractically sluggish. Cyclization in the presence of 0.1 equiv V2O5, however, returned a 58% isolated yield of a 1:1 mixture of bromolactone 12 and the a,b-unsaturated lactone resulting from bromide elimination (entry 12a). Conducting the reaction in the presence of 3 equiv of p-toluenesulfonic acid precluded the formation of the elimination product, thus returning an acceptable 63% yield of the bromolactone product. Presumably, the additive sufficiently acidifies the reaction medium so as to prevent the elimination of HBr from the initially formed bromolactone. Unfortunately, several attempts to optimize the reaction conditions to favor the exclusive formation of the butenolide product failed. 2.6. Scaled experiments Next we evaluated the ability of the method to perform at larger scale (Scheme 2). A one gram portion (5.7 mmol) of 4phenylpentenoic acid 1 was cyclized in excellent yield employing either 0.1 or 0.05 equiv of V2O5. Specifically, bromolactone 2 was

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isolated in acceptable purity in a 95% or 90% yield respectively by means of a simple acid/base extraction without the need for chromatographic purification. Similarly, trans-styrylacetic acid 13 was converted to bromolactone 12 in a 66% yield on a 5.7 mmol scale, indicating that the modified protocol in the presence of the ptoluenesulfonic acid additive is also reasonably scalable.

5

Denmark17b groups have also independently detailed significant rate acceleration in halocyclization reactions in the presence of exogenous nucleophiles and Lewis bases. In the context of our methodology, we wondered whether urea might effect a similar activation by engaging the initially formed bromenium equivalent to form a complex like 18 (cf. Braddock’s intermediate 19). In order to probe any potential activating contribution from urea we conducted a set of parallel experiments whereby we employed 30% aqueous H2O2 as the terminal oxidant instead of urea-H2O2 complex. We compared the yield of this transformation in the presence or absence of 3 equiv of added urea (Scheme 4, Panel B). Lactonization of 1 under these modified conditions proceeded in a comparable yield with or without added urea (cf. 40% vs 41% yield respectively). These results suggest that urea does not play a catalytic role in our system similar to the Braddock methodology. 3. Conclusions

Scheme 2. Gram scale bromolactonization experiments with optimized conditions.

2.7. a-Halogenation of b-diketone compounds We surmised that our optimal conditions might also represent a convenient means to effect other useful transformations including the a-halogenation of activated methylene moieties. In order to probe this possibility we briefly investigated the a-bromination of two b-diketone substrates (Scheme 3). In the event, diketones 14 and 15 were mono-brominated in 92% (16) and 94% (17) yield, in the prescence of 0.05 equiv V2O5 using identical conditions to our optimized bromolactonization protocol. These results indicate that our optimal protocol for the in situ oxidation of bromide to bromenium may provide a convenient route for other related transformations.

We have presented a full account of the development of a novel method for the bromolactonization of alkenoic acids catalyzed by vanadium (V) oxide. The method hinges on the biologicallyinspired in situ oxidation of bromide (Br) to bromenium equivalent (Brþ). The method allows for facile access to bromolactone products in acceptable purity without recourse to column chromatography. Further, we have presented preliminary data that indicates that other transition metal oxides, most notably oxides of molybdenum, can promote similar reactivity. Preliminary investigation of our reaction conditions in the a-bromination of bdiketones suggests that the strategy described herein could be more broadly applicable to other related bromination reactions. Finally, we have probed the role of urea in the transformation. Current efforts in our laboratory are focused on further exploiting this unique method for the generation of reactive halogen species in organic synthesis. Of particular interest is the possibility of rendering the protocol asymmetric by means of the incorporation of appropriate chiral ligands for vanadium. Indeed, the use of vanadium oxides for asymmetric catalysis is an emerging area in organic synthesis.36 4. Experimental section 4.1. General information

Scheme 3. a-halogenation of b-diketone compounds.

2.8. Does urea play a role in the transformation? Finally, since our conditions employed the urea complex of hydrogen peroxide in lieu of aqueous H2O2, we wondered whether the significant concentration of urea (i.e., 3 equiv relative to substrate) might play an activating role in the reaction. We were intrigued by a report from Braddock and co-workers that highlighted the ability of electron-rich nitrogenous nucleophiles to accelerate the N-bromosuccinimide promoted bromolactonization of alkenoic acids.17d For instance, Braddock and co-workers disclosed that the bromolactonization of various alkenoic acids could be significantly accelerated in the presence of N,N,N0 ,N0 -tetramethylguanidine, evidently by means of the formation of an active species such as 19 (Scheme 4, Panel A). Similar rate enhancements were realized with other additives including amides like N,N-dimethylformamide and N,N-dimethylacetamide.17d Additionally, the Tang17a and

All reagents were purchased from commercial sources and used without purification. Alkenoic acid substrates were either purchased from commercial sources or prepared following an established protocol that included Wittig methylenation followed by saponification of the terminal ester to the carboxylic acid.19a 1H and 13 C NMR spectra were collected on 300 and 500 MHz NMR spectrometers using CDCl3 as solvent. Chemical shifts are reported in parts per million (ppm) and were referenced to the residual solvent peak. Coupling constants, J, are reported in hertz (Hz). All previously known products were characterized by 1H and 13C NMR and are in agreement with samples reported elsewhere. Compounds 3 and 7 are previously unknown compounds, and were characterized by means of 1H and 13C NMR, IR, and HRMS. 4.2. General procedure for bromolactonization and a-halogenation of b-diketones Urea-hydrogen peroxide complex (0.851 mmol, 80 mg, 3.0 equiv) and vanadium pentoxide (0.014 mmol, 3 mg, 0.05 equiv) were dissolved in 3.5 mL acetone/H2O (6:1) and stirred at 0  C for 30 min. To this ice-cold solution, ammonium bromide (0.851 mmol, 80 mg, 3.0 equiv) was added and stirred for an additional 30 min. After addition of the substrate (0.284 mmol, 1.0 equiv), the mixture

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Scheme 4. Probing the role of urea in the V2O5 catalyzed bromolactonization reaction.

was allowed to stir for an additional 15 min at 0  C before gradually warming to room temperature overnight. The reaction mixture was diluted with water (15 mL) and extracted with DCM (315 mL). The combined organics were washed with saturated aqueous sodium bicarbonate. The combined aqueous layers were back-extracted with DCM (15 mL). Finally, all organic extracts were combined (60 mL total volume), dried over anhydrous sodium sulfate, concentrated by rotary evaporation, and dried in vacuo. 4.3. 5-(Bromomethyl)-5-phenyloxolane-2-one, 217d Light yellow oil, 1H NMR (300 MHz, CDCl3): d 7.44e7.37 (m, 5H), 3.78e3.70 (dd, J¼10.2, 18.3 Hz, 2H), 2.88e2.79 (m, 2H), 2.62e2.52 (m, 2H); 13C NMR (90 MHz, CDCl3): d 175.4, 140.8, 128.9, 128.7, 124.9, 86.4, 41.0, 32.4, 29.1. 4.4. 5-(Bromomethyl)-5-(4-bromophenyl)dihydrofuran-2(3H)-one, 3 Light yellow oil; 1H NMR (300 MHz, CDCl3): d 7.57e7.52 (dt, J¼2.4, 4.5, 9.3, 11.1, 13.1 Hz, 2H), 7.32e7.28 (dt, J¼2.7, 4.5, 9.3, 11.4, 14.4 Hz, 2H), 3.73e3.64 (dd, J¼11.4, 13.5 Hz, 2H), 2.85e2.73 (m, 2H), 2.62e2.50 (m, 2H); 13C NMR (90 MHz, CDCl3): d 175.0, 139.8, 132.0, 126.8, 122.9, 86.0, 40.5, 32.4, 28.9; IR (DCM): 1784 cm1; HRMS (ESI-TOF, positive mode): C11H10O2Br2; Calculated (MþH): 332.9126; Found: 332.9138. 4.5. 5-(Bromomethyl)-5-(4-chlorophenyl)dihydrofuran-2(3H)-one, 420c Light yellow oil. 1H NMR (300 MHz, CDCl3): d 7.41e7.34 (m, 4H), 3.74e3.64 (dd, J¼11.4, 16.8 Hz, 2H), 2.86e2.73 (m, 2H), 2.63e2.50 (m, 2H); 13C NMR (90 MHz, CDCl3): d 175.1, 139.2, 134.7, 129.0, 126.4, 85.9, 40.6, 32.4, 28.9. 4.6. 5-(Bromomethyl)-5-(4-methoxyphenyl)dihydrofuran2(3H)-one, 520c Light yellow oil. 1H NMR (300 MHz, CDCl3): d 7.36e7.31 (dt, J¼3.0, 5.1, 9.9, 12.0, 15.0 Hz, 2H), 6.95e6.90 (dt, J¼3.0, 5.1, 9.9, 12.0, 15.0 Hz, 2H), 3.82 (s, 3H), 3.75e3.64 (dd, J¼11.1, 20.7 Hz, 2H),

2.86e2.70 (m, 2H), 2.61e2.48 (m, 2H);

13

C NMR (90 MHz, CDCl3):

d 175.4, 159.8, 132.6, 126.2, 114.2, 86.3, 55.3, 41.0, 32.2, 29.1.

4.7. 5-(Bromomethyl)-5-(4-methylphenyl)dihydrofuran2(3H)-one, 620c Light yellow oil. 1H NMR (300 MHz, CDCl3): d 7.31 (d, J¼8.4 Hz, 2H), 7.22 (d, J¼8.1 Hz, 2H), 3.76e3.67 (dd, J¼11.4, 17.4 Hz, 2H), 2.87e2.75 (m, 2H), 2.61e2.49 (m, 2H), 2.37 (s, 3H); 13C NMR (90 MHz, CDCl3): d 175.5, 138.6, 137.7, 129.5, 124.8, 86.4, 41.0, 32.3, 29.0, 21.0.

4.8. 5-(Bromomethyl)-5-(4-ethylphenyl)dihydrofuran-2(3H)one, 7 Light yellow oil. 1H NMR (300 MHz, CDCl3): d 7.36e7.24 (m, 4H), 3.78e3.67 (dd, J¼9.3 Hz, 17.4 Hz, 2H), 2.86e2.76 (m, 2H), 2.72e2.51 (m, 4H), 1.25 (t, J¼7.5 Hz, 3H); 13C NMR (90 MHz, CDCl3): d 175.6, 144.9, 137.9, 128.3, 124.9, 86.5, 41.1, 32.3, 29.1, 28.4, 15.4; IR (DCM): 1786 cm1; HRMS (ESI-TOF, positive mode): C13H15O2Br; Calculated (MþH): 283.0334; Found: 283.0328.

4.9. 6-(Bromomethyl)-6-phenyloxan-2-one, 817d Light yellow oil. 1H NMR (300 MHz, CDCl3): d 7.58e7.30 (m, 5H), 3.72e3.63 (dd, J¼11.1, 15.9 Hz, 1.65H), 2.85e2.32 (m, 3.77), 1.90e1.79 (m, 1H), 1.66e1.52 (m, 1H); 13C NMR (90 MHz, CDCl3): d 170.5, 140.2, 129.0, 128.5, 125.39, 85.1, 41.5, 30.0, 29.1, 16.2.

4.10. 6-(Bromomethyl)-4,4-dimethyl-6-phenyl)tetrahydro2H-pyran-2-one, 920h Light yellow oil. 1H NMR (300 MHz, CDCl3): d 7.48e7.29 (m, 5H), 3.62e3.58 (dd, J¼11.1, 18.0 Hz, 2H), 2.40 (s, 2H), 2.32e2.18 (dd, J¼16.5, 27.3 Hz, 2H), 1.09 (s, 3H), 0.75 (s, 3H); 13C NMR (90 MHz, CDCl3): d 170.9, 141.4, 128.9, 128.4, 125.2, 83.9, 43.6, 42.5, 31.6, 30.5, 28.4.

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4.11. 5-(Bromomethyl)oxolan-2-one, 1017d Light yellow oil. 1H NMR (300 MHz, CDCl3) d 4.80e4.72 (m, 1H), 3.61e3.51 (m, 2H), 2.74e2.58 (m, 2H), 2.54e2.40 (m, 1H), 2.19e2.07 (m, 1H); 13C NMR (90 MHz, CDCl3) d 176.1, 77.8, 34.0, 28.3, 26.2. 4.12. 3-(Bromomethyl)-3,4-dihydro-1H-2-benzopyran-1-one, 1117d Light yellow oil. 1H NMR (300 MHz, CDCl3): d 8.10 (dd, J¼0.9, 7.8 Hz, 1H), 7.58 (td, J¼1.5, 7.5 Hz, 1H), 7.40 (t, J¼7.5 Hz, 1H), 7.30 (d, J¼7.8 Hz, 1H), 4.79e4.70 (m, 1.0), 3.71e3.57 (m, 1H), 3.25e3.13 (m, 1.0); 13C NMR (90 MHz, CDCl3): d 164.3, 137.9, 134.2, 130.4, 128.0, 127.7, 124.5, 76.7, 32.5, 31.5.

9. 10. 11. 12.

4.13. 4-(Bromodihydro)-5-phenyl-2-(3H)-furanone, 1220h Light yellow oil. 1H NMR (300 MHz, CDCl3): d 7.48e7.36 (m, 5H), 5.68 (d, J¼5.1 Hz, 1H), 4.42e4.36 (ddd, J¼5.4, 6.3, 7.5 Hz, 1H), 3.30e3.21 (dd, J¼7.2, 18.0 Hz, 1H), 3.03e2.95 (dd, J¼6.3, 18.0 Hz, 1H); 13C NMR (90 MHz, CDCl3): d 173.0, 135.8, 129.3, 129.0, 125.4, 87.8, 45.5, 38.8. 4.14. 2-Bromo-1-phenyl-1,3-butanedione, 1637 Light yellow oil. 1H NMR (300 MHz, CDCl3): d 8.04 (d, J¼17.4 Hz, 2H), 7.65 (t, J¼9.3, 1H), 7.50 (t, J¼9.0 Hz, 2H), 5.64 (s, 1H), 2.47 (s, 3H); 13C NMR (90 MHz, CDCl3): d 198.2, 189.9, 134.5, 133.7, 129.3, 129.0, 52.9, 27.1.

13. 14.

15. 16. 17.

18.

4.15. 2-Bromo-1,3-diphenylpropan-1,3-dione, 1737 Light yellow oil. 1H NMR (300 MHz, CDCl3): d 8.01 (d, J¼9.0 Hz, 4H), 7.62 (tt, J¼3.0 Hz, 6.0 Hz, 2H), 7.48 (t, J¼15.0 Hz, 4H), 6.60 (s, 1H); 13C NMR (90 MHz, CDCl3): d 189.0, 134.3, 133.8, 129.1, 129.0, 52.6.

19.

Associated Content

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Supporting Information. Proton and carbon NMR spectra. Acknowledgements The authors gratefully acknowledge Clemson University for generous start-up funds. The authors wish to thank Rieke Metals, Inc. and Oakwood Chemical for their generous donation of a substrate precursor in support of this effort. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.tet.2015.04.029. References and notes 1. Eissen, M.; Lenoir, D. Chem.dEur. J. 2008, 32, 9830e9841. 2. Kabalka, G. W.; Yang, K.; Reddy, N. K.; Narayana, C. Synth. Commun. 1998, 28, 925e929. 3. Nair, V.; Panicker, S. P.; Augustine, A.; George, T. G.; Thomas, S.; Vairamani, M. Tetrahedron 2001, 57, 7417e747422. 4. Dewkar, G. K.; Narina, S. V.; Sudalai, A. Org. Lett. 2003, 5, 4501e4504. 5. Muathen, H. A. Synth. Commun. 2004, 34, 3545e3552. 6. Ye, C.; Shreeve, J. M. J. Org. Chem. 2004, 69, 8561e8563. 7. Klimczyk, S.; Huang, X.; Fares, C.; Maulide, N. Org. Biomol. Chem. 2012, 10, 4327e4329. 8. (a) Kennedy, R. J.; Stock, A. M. J. Org. Chem. 1960, 25, 1901e1906; (b) Dieter, R. K.; Nice, L. E.; Velu, S. E. Tetrahedron Lett. 1996, 37, 2377e2380; (c) Ross, S. A.; Burrows, C. J. Tetrahedron Lett. 1997, 38, 2805e2808; (d) Lee, K.-J.; Lin, K. W.;

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