Selective Radical Reactions in Multiphase Systems: Phase-Transfer Halogenations of Alkanes (original) (raw)
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Aminoalkyl radicals as halogen-atom transfer agents for activation of alkyl and aryl halides
Science, 2020
Amines as a gateway to alkyl radicals In recent years, photoredox catalysis driven by blue light has often been used to oxidize carbon centers adjacent to nitrogen. Constantin et al. now show that these aminoalkyl radicals can, in turn, conveniently strip iodine atoms from a variety of alkyl carbons. The new alkyl radicals that result readily undergo deuteration and couplings such as alkylation, allylation, and olefination. The upshot is that simple amines can replace more hazardous conventional reagents such as trialkyltin compounds in the homolytic activation and functionalization of halocarbons. Science , this issue p. 1021
European Journal of Inorganic Chemistry, 2011
Transition-metal-catalyzed atom transfer radical reactions of halogen compounds to olefins constitute a versatile tool in organic synthesis within the area of CC bond forming transformations. The inter-or intramolecular versions, respectively atom transfer radical addition (ATRA) or cyclization (ATRC), lead to the atom economic, valuable synthesis of compounds that can be further functionalized. This contribution summarizes the recent developments in this area in terms of catalyst design as well as the applicability of this methodology in sequential, domino or tandem reactions.
ES Critical Reviews: Transformations of halogenated aliphatic compounds
Environmental Science & Technology, 1987
Bansformations of halwenated aliphatic cornpun& oxidation. r e d d o n , substitulion, and akhydrohalogerdon reactions occur abwtidly or in m i d i a l and mammalian systems otic and biotic chemistry of halogenated aliphatic compounds. Knowledge of cxmcqld tiamavo& for understand-abiotic aansformatloos ' canprovidea tiolls. Mast abiotic transformations are slow, but t&ey urn stiU be significaa withinthetimescalescommonly associated with groundwater m o v m. In contrast, biotic transformations typid y proceed much faster, provided that there are sufficient substrate and nutrients and a microbial poprlation that canldiate SUChtransformations. Recent studies, which describe transformations of halogenated aliphatic canpounds in mi&i and lIwnmal i a n s y m , are akodiscussedinthis systemingeneral. ing bidogicalhl mediated t r a n s f~marticle. These studies reveal broad patterns of transformalion in biological All three systems (abiotic, mammalian, and microbial) have similarities in r r-TABLE 1 Production, proposed maximum contaminant levels, ratings of comnlon halogenated aliphatic compounda Compound Trihalomethanes Vinyl chloride 1 .l-Dichlorcethylene trans-I ,2-Dichloroethylene Trichloroethylene Tetrachloroethylene 1,l-Dichloroethane 1.2-Dichloroethane 1 ,1, 1-Trichloroelhane 1 ,BDibromoelhane .Reference 4. 'Maximum mntaminenl level. Reference 5. <Carcinogenicity: 1 = chemical is carcinr^^^'^' chemical cannot be classified. bReference 6. reaction mechanism and transforma-I ences. For each, the transformation of halogenated aliphatic compounds can that q* ~~~~yydrohalogenation-elimina-lonizatbn potential-thi dil-fer (oxidations and reductions) and tion of HX to form an alkene, ence between the energy of ultra Dlhalo-elimination-reductive let radiation used to bombard a mc+ those that do not (substitutions and deh y~' % e M t i m)~ External elimination of two halide substi-ecule and the energy of the ejected transfer i s as the transfer Of tuents to form an alkene. electron. e1-m to and from some agent other Electwhi16e reacting specie Yonooxygenase-an enzyme than the that accepts an electron pair. that catalyzes reactions in which one Eiiiinatlon-a reaction in which atom of O2 appears in the product Processesin this article and terms that frequently appear = detwo groups, such as hydrogen and and the other in H a. f i in the side E-1-Of trans-Nudeophila-a reacting specie chlorine, are lost from adjacent car-f0nnatiOm are listed in Figure 1. COmbon atoms so that a double bond is that brings an electron pair. mon abbreviations for the various Sohrolysls-a reaction in which for&. halogenated aliphatic Compounds are Epoxldation-a reaction in which the solvent serves as the nulisted in Table 2. an epoxide is generated. cleophiie. Substitutiin-a reaction in wh ~ ~ Hydro@%noly&i-a reduction in substitution which a carbon-halogen bond is one substituent on a molecule is Halogenated aliphatic compounds ken and hydrogen replaces the placed by an0t'-undergo substitution and dehydmhalo-PERMISSUN m USE PICMAN WAS GIVEN BY A T A R I GAMES. tion products. They also have differbe divided into classes: re-Definitions of terms Coupling-a reaction in which halogensubstituent. Hydroxylation-addition alkyl or aryl groups connect tcdroxyl gmP. ' systems (Table 4) (22-29). Glutathione reacts with halogenafur-containing compounds. Under turic acids (28). Some chlorinated alkanes are transformed into alcohols in 724 Envimn. Sei. TBchnol., W. 21. No. 8. Igs7 strength may inrrease the L i l i o o d of creased stability of charged imermedicompounds more readily than is chloted aliphatic compounds to produce sulproper conditions, these compounds are further trrmsformed into mercap
Metal-free, selective alkane functionalizations
Advanced Synthesis & Catalysis, 2003
The present overview of alkane functionalization reactions presents comparisons between radical and metal-initiated (sometimes metal-catalyzed) methodologies. While metal-catalyzed processes are excellent approaches to this problem, metal-free alternatives are equally if not, at least from an environmental and cost perspective, more useful. This conclusion is supported by the fact that many so-called metal-catalyzed reactions also work without the metal present, and the large variety of metals showing the same product distributions emphasizes that the metal often just aids in the generation of the active species, i.e., the metal itself is not participating in the crucial CH activation step. Highly selective alkane functionalization reactions such as those derived from nitroxyl and related radicals as well as through radical reactions conducted in phase-transfer catalyzed systems are available but generally underutilized. These systems, in contrast to typical metal-catalyzed approaches, are also applicable to highly strained alkanes and offer the highest 3°/2° CH selectivities reported to date in a radical reaction. The article closes with representative experimental protocols for the PTC bromination of cubane as an example of the applicability of this method to strained hydrocarbons and the direct iodination of cyclohexane as well as adamantane as typical alkanes bearing secondary and tertiary CH bonds.
ES&T Critical Reviews: Transformations of halogenated aliphatic compounds
Environmental Science & Technology, 1987
Bansformations of halwenated aliphatic cornpun& oxidation. r e d d o n , substitulion, and akhydrohalogerdon reactions occur abwtidly or in m i d i a l and mammalian systems otic and biotic chemistry of halogenated aliphatic compounds. Knowledge of cxmcqld tiamavo& for understand-abiotic aansformatloos ' canprovidea tiolls. Mast abiotic transformations are slow, but t&ey urn stiU be significaa withinthetimescalescommonly associated with groundwater m o v m. In contrast, biotic transformations typid y proceed much faster, provided that there are sufficient substrate and nutrients and a microbial poprlation that canldiate SUChtransformations. Recent studies, which describe transformations of halogenated aliphatic canpounds in mi&i and lIwnmal i a n s y m , are akodiscussedinthis systemingeneral. ing bidogicalhl mediated t r a n s f~marticle. These studies reveal broad patterns of transformalion in biological All three systems (abiotic, mammalian, and microbial) have similarities in r r-TABLE 1 Production, proposed maximum contaminant levels, ratings of comnlon halogenated aliphatic compounda Compound Trihalomethanes Vinyl chloride 1 .l-Dichlorcethylene trans-I ,2-Dichloroethylene Trichloroethylene Tetrachloroethylene 1,l-Dichloroethane 1.2-Dichloroethane 1 ,1, 1-Trichloroelhane 1 ,BDibromoelhane .Reference 4. 'Maximum mntaminenl level. Reference 5. <Carcinogenicity: 1 = chemical is carcinr^^^'^' chemical cannot be classified. bReference 6. reaction mechanism and transforma-I ences. For each, the transformation of halogenated aliphatic compounds can that q* ~~~~yydrohalogenation-elimina-lonizatbn potential-thi dil-fer (oxidations and reductions) and tion of HX to form an alkene, ence between the energy of ultra Dlhalo-elimination-reductive let radiation used to bombard a mc+ those that do not (substitutions and deh y~' % e M t i m)~ External elimination of two halide substi-ecule and the energy of the ejected transfer i s as the transfer Of tuents to form an alkene. electron. e1-m to and from some agent other Electwhi16e reacting specie Yonooxygenase-an enzyme than the that accepts an electron pair. that catalyzes reactions in which one Eiiiinatlon-a reaction in which atom of O2 appears in the product Processesin this article and terms that frequently appear = detwo groups, such as hydrogen and and the other in H a. f i in the side E-1-Of trans-Nudeophila-a reacting specie chlorine, are lost from adjacent car-f0nnatiOm are listed in Figure 1. COmbon atoms so that a double bond is that brings an electron pair. mon abbreviations for the various Sohrolysls-a reaction in which for&. halogenated aliphatic Compounds are Epoxldation-a reaction in which the solvent serves as the nulisted in Table 2. an epoxide is generated. cleophiie. Substitutiin-a reaction in wh ~ ~ Hydro@%noly&i-a reduction in substitution which a carbon-halogen bond is one substituent on a molecule is Halogenated aliphatic compounds ken and hydrogen replaces the placed by an0t'-undergo substitution and dehydmhalo-PERMISSUN m USE PICMAN WAS GIVEN BY A T A R I GAMES. tion products. They also have differbe divided into classes: re-Definitions of terms Coupling-a reaction in which halogensubstituent. Hydroxylation-addition alkyl or aryl groups connect tcdroxyl gmP. ' systems (Table 4) (22-29). Glutathione reacts with halogenafur-containing compounds. Under turic acids (28). Some chlorinated alkanes are transformed into alcohols in 724 Envimn. Sei. TBchnol., W. 21. No. 8. Igs7 strength may inrrease the L i l i o o d of creased stability of charged imermedicompounds more readily than is chloted aliphatic compounds to produce sulproper conditions, these compounds are further trrmsformed into mercap
Catalytic, Asymmetric Halofunctionalization of Alkenes-A Critical Perspective
Angewandte Chemie International Edition, 2012
Despite the fact that halogenation of alkenes has been known for centuries, enantioselective variants of this reaction have only recently been developed. In the past three years, catalytic enantioselective versions of halofunctionalizations with the four common halogens have appeared and although important breakthroughs, they represent just the very beginnings of a nascent field. This Minireview provides a critical analysis of the challenges that accompany the development of general and highly enantioselective halofunctionalization reactions. Moreover, the focus herein, diverges from previous reviews of the field by identifying the various modes of catalysis and the different strategies implemented for asymmetric induction.
Introduction of Halogen Atoms into Organic Compounds Under Solvent- Free Reaction Conditions
Current Organic Chemistry, 2009
Avoiding volatile and toxic organic solvents during each particular phase of synthetic protocols should become important goal of chemical synthesis designers in academia and particularly in the industrial research community. Solvent-free reaction conditions (SFRC) are becoming a widely used experimental technique for the selective and efficient introduction of halogen atoms into organic compounds. The different approaches to the preparation of halosubstituted organic molecules using equimolar amount of the substrate and reagent under SFRC are reviewed and evaluated. Fluorination of various types of organic compounds under SFRC using fluorine gas, N-F reagents, and hydrogen or metal fluorides is compiled. Chloro-, bromo-or iodofunctionalisation under SFRC using molecular halogens, N-halosuccinimides, hydrogen or others halides is reviewed. Oxidative halogenations of comprehensive types of organic compounds under SFRC are evaluated.
Journal of the American Chemical Society, 1996
The radical allylation of a series of-alkoxy esters using allyltrimethylsilane in the presence of MgBr 2 ‚ OEt 2 is described. Under bidentate chelation-controlled conditions, allyltrimethylsilane rivals allyltributyltin in efficiency and is a superior reagent from ecological and practical perspectives. The reactions work with iodides and bromides as well as phenylselenides. The isolation of γ-phenylseleno intermediates indicates that the reaction proceeds by an atom transfer process. These reactions require initiation with Et 3 B and can be inhibited by galvinoxyl, m-and p-dinitrobenzene indicating that this atom transfer sequence involves the intermediacy of radicals. Asymmetric induction in free radical reactions is a topic of current interest. 1 Facial selectivity in radical reactions can be influenced by a stereogenic center adjacent to a radical center (1,2-induction) 2-4 and by the use of chiral auxiliaries. 5 A significant development in this field is the observation that complexation with a Lewis acid can be used to enhance levels of stereoselection or to reverse the facial bias of radical reactions. 6-9 Lewis acids have been shown to be compatible with reduction, allylation, and-addition processes. In this paper we report results we have obtained in chelation-controlled allylation reactions of R-halo-alkoxy esters using allyltrimethylsilane and present evidence for a radical atom transfer mechanism. To our knowledge, this represents the first example of an atom transfer reaction in which facial selectivity is controlled by Lewis acid complexation. 10,11 Results A general mechanism for a typical atom transfer reaction is shown in Scheme 1. In order to be successful, the reactivity of * Author to whom correspondence should be addressed (IRCM).
J Org Chem, 1985
The development of Phase Transfer Catalysis (PTC) represents a major step forward in the employment of many organic reactions and renders them very convenient and useful processes. These reactions involve the application of nucleophiles in general, anions and bases in particular, in reactions carried out in a water-organic solvent system. They can be performed both in the laboratory and on an industrial scale. The ease of application of PTC processes is the main reason for their increasing utilization in industry. An outstanding achievement of this technique is the employment of aqueous bases in reactions which traditionally would otherwise require a strong base in a nonaqueous medium. The classical procedures that require severe anhydrous conditions, expensive solvents and dangerous bases such as metal hydrides and organometallic reagents are now replaced by aqueous solutions of, e.g., sodium or potassium hydroxides (PTC/OH processes). In contrast to the extensive synthetic applications of PTC/OH systems, the detailed mechanisms of these processes have been the subject of a great deal of controversy and various mechanisms have been suggested. However, it would seem that our knowledge concerning the mechanistic aspects of such reactions has now reached the stage where it can be used to advantage in synthesis planning. A better understanding of the various factors which influence the reaction would undoubtedly help to optimize PTC/OH processes such as to enable higher yields in shorter reaction times at lower temperatures. The importance of, inter aha, the catalyst will be pointed out and it is highly recommended that such catalysts be always available in the laboratory, for the range of organic reactions that they can efficiently, conveniently and safely catalyze is vast indeed.