Bromine-Radical-Mediated Bromoallylation of C–C Unsaturated Bonds: A Facile Access to 1,4-, 1,5-, 1,6-, and 1,7-Dienes and Related Compounds

Abstract

The radical bromoallylation of alkynes, allenes, and vinylidene cyclopropanes proceeds efficiently in the presence of a radical initiator to give 2-bromo-substituted 1,4-, 1,5-, and 1,6-diene derivatives, respectively. Three-component reactions comprised of allenes, electron-deficient alkenes, and allyl bromides give 1,7-dienes in good yields. The bromoallylation of an arylalkene can override β-scission of the bromine radical from β-bromoalkyl radicals to give 5-bromoalkenes, whilst the bromoallylation of vinylcyclopropanes is accompanied by 5-exo ring closure to give 1-(bromomethyl)-2-vinylcyclopentane derivatives in good yields. All of the products contain a reactive vinyl bromide moiety, which can be readily functionalized by Pd-catalyzed cross-coupling and radical cascade reactions.

1 Introduction

2 Synthesis of 1,4-Dienes by Bromoallylation of Acetylenes

3 Synthesis of 1,5-Dienes by Bromoallylation of Allenes

4 Synthesis of 1,6-Dienes by Bromoallylation of Methylenecyclopropanes

5 Synthesis of 1,7-Dienes by Bromoallylation of Allenes and Electron-Deficient Alkenes

6 Bromoallylation of Arylalkenes and Vinylcyclopropanes

7 Conclusion

Biographical Sketches

Shuhei Sumino received his Ph.D. from Osaka Prefecture University in 2016 under the supervision of Professor Ilhyong Ryu. He then became a Research Scientist at the Osaka Research Institute of Industrial Science and Technology (ORIST). Currently, he is also a visiting scientist in Prof. Ryu’s team at Osaka Metropolitan University (OMU). His present research interests include new synthetic methodologies based on radicals and photocatalysis.

Ilhyong Ryu received his Ph.D. from Osaka University in 1978. After serving as a JSPS Postdoctoral Fellow and a Research Associate, he was appointed as an Assistant Professor at Osaka University in 1988, and was promoted to Associate Professor in 1995. In addition, he worked with Professor Howard Alper at the University of Ottawa as a visiting scientist (1991–1992). In 2000, he moved to Osaka Prefecture University as a Full Professor. At present, he serves as a Specially Appointed Professor at Osaka Metropolitan University (OMU) and the Chair Professor of National Yang Ming Chiao Tung University (NYCU) in Taiwan.

Introduction

Bromine radicals can be generated conveniently by homolysis of molecular bromine and they exhibit two distinct types of reactivity towards organic molecules (Scheme [1]).[1] First, bromine radicals abstract a hydrogen atom from a C(sp³)–H bond to form alkyl radicals (Scheme [1], type 1). The traditional Wohl–Ziegler bromination using NBS as a source of bromine radicals has been widely utilized to synthesize benzylic and allylic bromides via C–H bond cleavage with bromine radicals followed by an S_H2 reaction with molecular bromine.[2] [3] Recent efforts have focused on controlling the production of bromine radicals using photoflow chemistry[4] and photoredox catalysis.[5,6] In the second type of reaction, bromine radicals reversibly add to C–C double or triple bonds to generate renewed radicals bearing β-bromine atoms (Scheme [1], type 2). The most well-known transformation in this category is the anti-Markovnikov addition of HBr to an alkene or an alkyne.[7] Such potential radical additions using A and B have not been associated extensively with subsequent C–C bond-forming reactions since they would be expected to compete with the rapid backward β-scission in which A is more elusive than B due to its weaker C(sp³)–Br bond. On the other hand, it is known that allylic bromides can serve as carbon-radical acceptors. In 1949, Kharasch and co-workers reported on the radical reactions of bromotrichloromethane with allyl bromide to give allyltrichloromethane under photoirradiation conditions.[8] The proposed mechanism for this reaction involved a non-chain radical mechanism caused by homolysis of the carbon–bromine bond of BrCCl₃. In 1999, Tanko and Sadeghipour reported on bromine-radical-mediated C–H allylation reactions that involve radical chain reactions, wherein the type 1 reaction was sequenced by addition to allyl bromides, followed by β-scission to give allylated alkanes (Scheme [2], eq 1).[9] A new type 1 methodology has been extended to the C–H allylation of aldehydes via acyl radicals (Scheme [2], eq 2)[10] and the site-selective C–H allylation of the methine C–H bonds of isooctane and to a related C–H alkenylation using alkenyl bromides.[11] It is noteworthy that recent work from Murakami and Ishida capitalized on the type 1 propensity for both toluenes and aldehydes to undergo cross-coupling reactions between benzyl radicals and acyl radicals by way of nickel catalysis, in which the bromine radical is regenerated from HBr with the liberation of dihydrogen (Scheme [2], eq 3).[12]

We hypothesized that the successful merger of a type 2 reaction and an S_H2′-type addition to allylic bromides would lead to a new class of cross-coupling reaction between unsaturated hydrocarbons and allylic bromides. To our delight, as illustrated in Scheme [3], the methodology provided facile access to 1,4-, 1,5-, 1,6-, and even 1,7-dienes based on synthetic control of the elusive radical intermediate B.[13] [14] [15] [16] A major advantage of this methodology is that bromine is incorporated into the product structure, where it can function as an additional reactive site for synthetically useful C–C bond-forming reactions, such as Pd-catalyzed cross-coupling and radical cascade reactions. On the other hand, the application of bromoallylation to alkenes is particularly difficult since radical A is less stable than radical B due to the weaker C–Br bond. However, we found some examples of the successful bromoallylation of C–C double bonds that involved the use of a substituent at the α-position, which has a stabilizing effect on radical A.[17] In this Account, we report on our efforts concerning the free-radical-mediated bromoallylation of unsaturated compounds.[18]

Synthesis of 1,4-Dienes by Bromoallylation of Acetylenes

The 1,4-diene unit is an important motif that is frequently found in biologically active compounds and natural products. Alkenyl radicals are classified as nucleophilic radicals,[19] [20] and we examined two types of allylic bromides: the simplest and a type that is substituted at C2 by an electron-withdrawing group (Scheme [4]).[13] Photoirradiation of a benzene solution of phenylacetylene and a two-fold excess of 2-ethoxycarbonyl-substituted allyl bromide delivered the expected 1,4-diene in 63% yield. The use of a 4-fold-excess and a longer reaction time (12 h) improved the yield to 87%. However, the reaction of phenylacetylene with a simple allyl bromide was quite sluggish. Consequently, we found that the use of a large excess (5 mL to 1 mmol of allyl bromide, 58 equiv) gave the 1-bromo-1,4-diene in 80% yield (E/Z = 23/77). Using either photoinitiation (conditions A: irradiation with a Xe lamp using a solar simulator (SOLARBOX 1500,[21] 350 W/m², 12 h)) or thermal initiation (conditions B: V-65 (2,2′-azobis(2,4-dimethylvaleronitrile)), 20 mol%, 60 °C, 6 h), the reaction of arylacetylenes with allyl bromides proceeded well to give the corresponding 1,4-dienes in good yields. The reaction of an internal alkyne, 1-phenylpropyne, proceeded regioselectively to give the bromoallylated product in 57% yield, in which bromine was incorporated into the methyl-substituted carbon. 1-Octyne also participated in the allylation reaction. The bromoallylation of arylacetylenes with 2-ethoxycarbonyl-1-propenyl bromide under photoirradiation conditions also proceeded smoothly to give good yields of the corresponding 1-bromo-2-aryl-4-ethoxycarbonyl-1,4-butadiene derivatives. The reaction of cyclopropyl acetylene gave the desired diene in 85% yield in which the cyclopropyl group was retained in the product.

In the photoinitiated reaction of 1-octyne with 2-ethoxycarbonyl-substitited allyl bromide, we isolated a small amount of a 1,5-hexadiene (Scheme [5a]). The addition of 10 mol% of 2,2,6,6-tetramethylpiperidine 1-oxyl free radical (TEMPO) to the reaction suppressed the bromoallylation and allylated TEMPO was detected. These results lend support to the initiation mechanism by photoinduced homolysis of allyl bromides. The thermal initiation process follows the thermal decomposition of diazo-type initiators, such as V-65, and the subsequent S_H2′ reaction with allyl bromides to a generate bromine radical (Scheme [5b]).

The radical chain mechanism is illustrated in Scheme [6]. Irrespective of whether thermal initiation or photoirradiation is used, bromine radicals are formed, and these then add to the alkyne terminus to give bromine-containing vinyl radicals. These subsequently react with allyl bromide to produce radical intermediates, which then undergo β-fission to give 1-bromo-1,4-dienes with the liberation of a bromine radical to sustain the radical chain.

Since all of the above products contain a vinyl bromide functionality, further chemical transformations to prepare functionalized 1,4-dienes are clearly feasible. Indeed, the Pd-catalyzed formic acid reduction, the Sonogashira reaction and the Suzuki–Miyaura coupling of the products proceeded smoothly to give the corresponding vinyl-substituted compounds, respectively (Figure [1], top compounds). Pd-catalyzed carbonylation also proceeded effectively (Figure [1], bottom compounds).

Synthesis of 1,5-Dienes by Bromoallylation of Allenes

The bromine-radical-mediated addition of allyl bromides to allenes proceeds regioselectivity to give excellent yields of 2-bromo-substituted 1,5-dienes (Scheme [7]).[14]

Thus, when a mixture of 1,1-dihexylallene (1 mmol), methyl 2-(bromomethyl)acrylate (2 mmol), and AIBN (2,2′-azobisisobutyronitrile) (30 mol%) was stirred for 6 hours at 80 °C, the expected 2-bromo-substituted diene was obtained in 84% yield. The bromoallylation of mono-substituted allenes also worked well. For example, the reaction of vinylidene cyclohexane with 2-(bromomethyl)acrylonitrile gave the corresponding bromoallylated product in 88% yield. As observed for the bromoallylation of alkynes, the reaction using simple allyl bromide required a large excess in order to proceed. The E-configured products were generally obtained predominantly or exclusively, and functional groups, such as OTIPS, NPhth, and CO₂Et, were tolerated. The bromoallylation of a cyclic allene gave the corresponding product in 87% yield.

A proposed reaction mechanism for the radical bromoallylation of allenes is shown in Scheme [8]. It is known that in the free-radical addition of HBr to allenes, bromine radicals selectively add to the allene central carbon to give 2-bromo-substituted allyl radicals.[22] [23] Thus, bromine radicals, generated by a radical initiation process, regioselectivity add to the allene central carbon to form allyl radicals. Next, addition to allyl bromide takes place in an S_H2′ manner to produce the desired 2-bromo-substituted 1,5-dienes and regenerate the bromine radical, thereby enabling the radical chain reaction.

The obtained 1,5-dienes could then be converted into substituted alkenes and keto esters via the subsequent treatment of the vinyl-bromine bonds under Pd-catalysis. Figure [2] shows examples of a Suzuki–Miyaura reaction and a Pd-catalyzed cyclocarbonylation with incorporation of two molecules of CO.

In summary, the regioselective radical bromoallylations of allenes with allyl bromides proceed smoothly to deliver to a wide variety of 5-bromo-substituted 1,5-dienes in high yields, which are somewhat difficult to access by the other methods.

Synthesis of 1,6-Dienes by Bromoallylation of Methylenecyclopropanes

Alkylidenecyclopropanes are useful building blocks in organic synthesis.[24] We speculated that 1,6-dienes might be obtained by the reactions of alkylidenecyclopropanes with allylic bromides, in the presence of a radical initiator, via bromine-radical-triggered cyclopropylcarbinyl radical ring opening[25] to give 2-bromo-substituted 1,6-dienes.[15]

The reaction of (1-butylpentylidene)cyclopropane and ethyl 2-(bromomethyl)acrylate in the presence of V-70 (2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile)) gave ethyl 6-bromo-7-butyl-2-methylene-6-undecenoate in a poor yield due to a side reaction of the vinylidene cyclopropane with hydrogen bromide. We circumvented this issue by adding anhydrous trisodium phosphate as a HBr scavenger, which resulted in a substantially improved yield of the 1,6-diene (74%) (Scheme [9]). Maintaining the reaction temperature at 40 °C or lower to avoid the isomerization of vinylidene cyclopropane is critical for obtaining good yields of the 1,6-dienes.

Scheme [9] demonstrates the general scope of the present 1,6-diene synthesis that proceeds via cyclopropane ring opening. The reaction of 4-cyclopropylidenetetrahydropyran with 2-ethoxycarbonyl-substituted allyl bromide yielded the corresponding 1,6-diene in 87% yield. The bromoallylation of sterically hindered adamantylidene-cyclopropane also proceeded smoothly to give the desired 1,6-diene in 97% yield. Other allylic bromides, including 2-bromomethylacrylonitrile, 2-bromomethylstyrene, and allyl bromide, also participated in the synthesis of 1,6-dienes. While in most cases 1.8 molar equivalents of allylic bromides were used, in the case of less reactive 2-phenyl-substituted allyl bromide we used three equivalents. With a simple allyl bromide, the use of a large excess (ca. 60 equiv) compensated for the lack of reactivity towards nucleophilic alkyl radicals. In the cases of mono-substituted alkylidenecyclopropanes, the dienes were obtained as E/Z mixtures favoring Z isomers. Phenyl-functionalized methylenecyclopropane gave 2-bromo 4-phenyl-1,6-dienes as the sole products, originating from the stable benzylic radical.

Homoallylic radicals, arising from the cyclopropylcarbinyl radical ring opening, can trap CO to form acyl radicals.[26] [27] The three-component coupling reaction comprised of alkylidenecyclopropanes, CO, and allylic bromides worked well to give good yields of 2-bromo-substituted 1,7-dien-5-ones (Scheme [10]). For example, the treatment of a benzene solution of (1-butylpentylidene)cyclopropane, ethyl 2-(bromomethyl)acrylate, anhydrous trisodium phosphate, and V-65 with CO (80 atm) at 60 °C for 6 hours resulted in the production of ethyl 7-bromo-8-butyl-2-methylene-4-oxo-7-dodecenoate in 72% yield after purification by silica gel column chromatography.

A proposed mechanism for this reaction is shown in Scheme [11]. A bromine radical is initially generated from the allylic bromide through a radical initiation process. The bromine radical then adds to the central carbon of the alkylidenecyclopropane to give a cyclopropylcarbinyl radical, which readily undergoes ring opening leading to the formation of a homoallyl radical.[28] The homoallyl radical then adds to the allylic bromide to give the 2-bromo-substituted 1,6-diene along with the liberation of a bromine radical, thus sustaining the radical chain cycle. The mechanism for the carbonylative bromoallylation of alkylidenecyclopropanes is also shown in Scheme [11] (blue cycle). The resulting homoallyl radical adds to CO to form an acyl radical, which then adds to the allyl bromide to give a 2-bromo-substituted 1,7-dien-5-one and a bromine radical.

Both types of products contain a vinyl bromide moiety and are thus amenable to further transformations. For example, tributyltin hydride mediated 6-endo and 7-endo cyclization were carried out, which proceeded smoothly to give alkylidene-substituted cyclohexane and cycloheptanone products, respectively (Figure [3]).

Synthesis of 1,7-Dienes by Bromoallylation of Allenes and Electron-Deficient Alkenes

So far, we have described the radical bromoallylations of alkynes, allenes, and alkylidenecyclopropanes, successfully leading to the synthesis of bromo-substituted 1,4-, 1,5-, and 1,6-dienes, respectively. In the former section, the three-component reaction of alkylidenecyclopropanes, CO, and allylic bromides, gave 1,7-dien-5-ones. To extend this radical-mediated bromoallylation strategy to the synthesis of 1,7-dienes, we focused on another three-component reaction of allenes, electron-deficient alkenes, and allyl bromides.[16] Polarity matching is a key factor in the design of multicomponent radical reactions.[29] In the presence of electron-deficient alkenes and simple allyl bromides or methallyl bromides, key allyl radicals arising from bromine radicals and allenes would be expected to gravitate to the electron-deficient alkenes rather than allyl bromides. The resulting radicals are electrophilic and now match allylic bromides rather than electron-deficient alkenes, leading to the desired reaction via a given sequence. To our knowledge, a tin-radical-mediated three-component process is known for the synthesis of 1,7-dienes, which employs allyl iodide, benzylidenemalononitrile, and allyltributyltin.[30]

The results for the synthesis of 1,7-dienes are illustrated in Scheme [12]. A benzene solution containing 3-butyl-1,2-heptadiene, acrylonitrile (1.8 equiv), methallyl bromide (1.8 equiv) and AIBN (20 mol%) was heated at 80 °C for 4 hours. After silica gel chromatography, 5-bromo-6-butyl-2-(2-methallyl)-5-decenenitrile was obtained in 79% yield. Ethyl acrylate, acrolein, methyl vinyl ketone, diethyl 2-methylenemalonate, and benzylidenemalononitrile were all reactive towards allyl radicals, giving the corresponding 2-bromo-1,7-dienes in good to excellent yields. Other allyl bromides such as 2-bromomethylstyrene, 3-bromo-2-chloropropene, 2,3-dibromopropene, and simple allyl bromide also participated in this sequence. Reaction of a 1,1,3-trisubstituted allene also proceeded well to give the desired 1,7-diene in 60% yield. The reaction of 1,2-cyclononadiene produced the corresponding product in 80% yield. We then speculated that if an alkylidenecyclopropane were used as a substrate, a similar three-component coupling reaction would give a 2-bromo-1,8-diene. Indeed, when a solution of 6-undecanylidene-cyclopropane, acrylonitrile and methallyl bromide in the presence of Na₃PO₄ and V-70 was heated at 40 °C for 6 hours, the expected 1,8-diene was obtained in 44% yield. The modest yield in this case is due to the competing formation of a methylenecyclopentane, which is formed by the 5-exo-cyclization of a radical arising from the addition of a homoallyl radical to acrylonitrile.

A polarity-matched radical chain mechanism, which is illustrated in Scheme [13], can account for the present three-component coupling reaction. A bromine radical attacks the central carbon of the allene to give an allyl radical.[22] [23] The thus formed allyl radical, having nucleophilic character, then reacts with the electron-deficient alkene, leading to formation of an electrophilic radical. This radical then preferentially reacts with the allyl bromide to produce an intermediate radical, which, followed by β-scission, gives the 2-bromo-1,7-diene, with the bromine radical being regenerated and thus sustaining the chain reaction.

Further derivatization of the trialkyl-substituted bromoalkene moiety of the 1,7-diene was examined (Figure [4]). A Suzuki–Miyaura cross-coupling reaction with 4-methoxyphenylboronic acid proceeded efficiently to give a diene (96% yield) having a tetrasubstituted C–C double bond.

Bromoallylation of Arylalkenes and Vinylcyclopropanes

Bromoallylation of Arylalkenes

In the traditional radical hydrobromination of alkenes with HBr,[7] the reaction involves the reversible addition of a bromine radical at the less hindered site of an alkene to form β-bromoalkyl radicals, which abstract hydrogen from HBr to give anti-Markovnikov products with the liberation of a bromine radical. For the bromoallylation of alkenes to occur, β-bromoalkyl radicals should have a sufficient lifetime to allow them to be trapped by allylic bromides against a backward β-fragmentation reaction (Scheme [14]). In this regard, aryl-substituted alkenes, such as styrene, appeared to be candidates for allowing this bromoallylation to occur, since the key radical retaining bromine would be stabilized by β-conjugation with the benzene ring.[17]

Treatment of styrene with ethyl 2-(bromomethyl)acrylate using AIBN as a radical initiator at 80 °C for 1 hour resulted in the formation of the envisaged bromoallylation product in 83% yield (Scheme [15]). Using similar reaction conditions, the bromoallylation of a variety of aryl alkenes took place to give good yields of 4-aryl-5-bromo-1-pentene derivatives. The bromoallylation of an enol ester and an enamide also proceeded smoothly. For example, the reaction of vinyl benzoate with ethyl 2-(bromomethyl)acrylate gave the desired bromoallylation product in 78% yield. Similarly, the reaction of N-vinylphthalimide gave the desired bromoallylation product in 71% yield.

Since these products contain a bromoalkyl chain, they are amenable to cyclizative radical carbonylation and ionic amination, leading to the formation of six-membered ring compounds (Figure [5]). The tributyltin hydride mediated cyclizative radical carbonylation with CO proceeded to give 3-phenyl-5-ethoxycarbonyl-cyclohexanone in 58% yield via the formation of an acyl radical and subsequent 6-endo acyl radical cyclization. Similarly, the reaction with n-butylamine in the presence of triethylamine gave 3-phenyl-5-ethoxycarbonyl-piperidine in 71% yield.

Bromoallylation of Vinylcyclopropanes

A sequence merging two well-established rapid kinetic events in radical chemistry, cyclopropylcarbinyl radical ring opening and 5-exo-radical cyclization, allows the bromoallylation of vinylcyclopropanes, competing with the β-scission from the initial β-bromoalkyl radicals. Scheme [16] outlines the concept of formal [3+2] cycloaddition using vinylcyclopropanes and allyl bromides.[18] Cyclopropylcarbinyl radicals, arising from the vinyl cyclopropane and a bromine radical, undergo rapid ring opening to give a bromine-containing homoallyl radical. This radical then adds to the allylic bromide to give a double-bromine-containing 5-alkenyl radical, which can then undergo 5-exo-cyclization onto the allyl bromide moiety. The resulting five-membered ring radical liberates a bromine radical via β-scission to give a 1-vinyl-2-bromomethylcyclopentane product.[31]

According to our concept, the reaction of 2-arylvinylcyclopropanes, bearing a substituent at the para-position of the phenyl ring, with allyl bromides proceeded efficiently to give the corresponding vinylcyclopentanes in good to high yields (Scheme [17]). While the [3+2] cycloaddition reaction of the meta-CF₃-substituted substrate gave the desired product in 72% yield, interestingly, the ortho-CF₃-substituted substrate gave a 1:1 mixture of the cyclized product and an uncyclized 1,6-diene. In a separate experiment, exposing the linear product to the same reaction conditions gave the cyclized product, albeit the reaction was sluggish. As shown in Scheme [18] summarizing the reaction mechanism, due to steric congestion between the o-CF₃ group and the bromomethyl group, the cyclized radical is inhibited from entering the resonance system with the aromatic ring, thus allowing the formation of the uncyclized product via β-scission.

The [3+2] cycloaddition products bear a 4-hexenyl bromide substructure, which allows for further transformations. For example, on treatment under the standard radical conditions of Bu₃SnH/AIBN, the 5-endo radical cyclization of a cis isomer proceeded smoothly to give 4-phenylbicyclo[3.3.0]octan-carbonitrile in 78% yield (Scheme [19]). The overall transformation represents a radical-mediated [3+2] cycloaddition, which has not been extensively reported with the exception of thiyl-radical-promoted reactions.[32]

Conclusion

Radical reactions of allylic bromides are highlighted in this account, with a special emphasis on the potential for the synthesis of dienes by cross-coupling reactions between allylic bromides and unsaturated C–C bonds, such as alkynes, allenes, and vinylidene cyclopropanes. Coupled with radical initiation, irrespective of whether the reaction is thermally or photoinduced, a series of 1,4-, 1,5-, and 1,6-dienes can be conveniently synthesized. Using three-component radical cascades with electron-deficient alkenes or carbon monoxide as guests, 1,7-dienes and 1,7-dien-4-ones were also synthesized. All these reactions are due to the ability of bromine radicals to readily undergo radical addition/elimination onto unsaturated C–C bonds. The products obtained by these radical processes naturally contain an incorporated vinyl bromide moiety, which is amenable for use in Pd-catalyzed cross-coupling reactions, radical carbonylation, and radical cyclization, to name but a few. We have also described successful examples of the bromoallylation of alkenes, which was thought to be difficult due to rapid reverse reactions from elusive β-bromoalkyl radicals. Either thermodynamic stabilization or rapid kinetic conversion has made these types of C–C bond-forming processes possible. Thus far, heteroallylic compounds, such as allyltins and allylsulfones, are by far the most well-performing unimolecular chain-transfer (UMCT) reagents.[33] All of the results discussed here, however, allow us to say with confidence that allylic bromides also qualify as excellent UMCT reagents.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

We wish to thank Dr. Takashi Kippo and Kanako Hamaoka for their seminal experimental contributions through their hard work on this challenging topic.

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For reviews, see:
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• 33b Curran DP, Xu J, Lazzarini E. J. Chem. Soc., Perkin Trans. 1 1995; 3049
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Corresponding Author

Publication History

Received: 07 November 2022

Accepted after revision: 15 December 2022

Accepted Manuscript online:
15 December 2022

Article published online:
13 January 2023

© 2022. Thieme. All rights reserved

Georg Thieme Verlag KG
Rüdigerstraße 14, 70469 Stuttgart, Germany

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For radical hydrobromination of allenes, see:
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For reviews on radical carbonylation, see:
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• 29b Fusano A, Ryu I. In Science of Synthesis: Multicomponent Reactions 2 . Muller TJ. J. Georg Thieme Verlag; Germany: 2014: 409
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