Addition reaction with fluoroolefins

O Paleta, Ionic Addition Reactions of Halomethanes with Fluoroolefins... 

Because of thetr electron deficient nature, fluoroolefms are often nucleophihcally attacked by alcohols and alkoxides Ethers are commonly produced by these addition and addition-elimination reactions The wide availability of alcohols and fliioroolefins has established the generality of the nucleophilic addition reactions The mechanism of the addition reaction is generally believed to proceed by attack at a vinylic carbon to produce an intermediate fluorocarbanion as the rate-determining slow step The intermediate carbanion may react with a proton source to yield the saturated addition product Alternatively, the intermediate carbanion may, by elimination of P-halogen, lead to an unsaturated ether, often an enol or vinylic ether These addition and addition-elimination reactions have been previously reviewed [1, 2] The intermediate carbanions resulting from nucleophilic attack on fluoroolefins have also been trapped in situ with carbon dioxide, carbonates, and esters of fluorinated acids [3, 4, 5] (equations 1 and 2)... 

The addition of nucleophiles to cyclic fluoroolefins has been reviewed by Park et al. [2 ]. The reaction with alcohols proceeds by addition-elimination to yield the cyclic vinylic ether, as illustrated by tlie reaction of l,2-dichloro-3,3-di-fluorocyclopropene Further reaction results in cyclopropane ring opening at the bond opposite the difluoromethylene carbon to give preferentially the methyl and ortho esters of (Z)-3-chloro-2-fluoroacrylic acid and a small amount of dimethyl malonate [29] (equation 8). 

Fluoride ion produced from the nucleophilic addition-elimination reactions of fluoroolefins can cataly7e isomerizations and rearrangements The reaction of per fluoro-3-methyl-l-butene with dimethylamine gives as products 1-/V,/Vdimeth-ylamino-1,1,2,2,4,4,4-heptafluoro-3-trifluoromethylbutane, N,W-dimetliyl-2,2,4,4,4-pentafluoro 3 trifluoromethylbutyramide, and approximately 3% of an unidentified olefin [10] The butylamide results from hydrolysis of the observed tertiary amine, and thus they share a common intermediate, l-Al,A -dimethylamino-l,l 24 44-hexafluoro-3-trifluoromethyl-2-butene, the product from the initial addition-elimination reaction (equation 4) The expected product from simple addition was not found... 

The last comprehensive review on electrophilic reactions of fluoroolefins was published in 1969 [6], Since then, several reviews and papers dealing with different aspects of this chemistry, such as alkylation and alkenylation reactions [7], addition of halogen fluorosulfates [8], trifluoromethanesulfonates [9] and halogen fluorides [10] to fluoroolefins have been published. Additional information on the reactions involving carbocations could be found in two recent review articles [11,12] some data on the subject are scattered in several books and journals [13-19]. 

Fluorinated carbocations play an important role as intermediates in electrophilic reactions of fluoroolefins and other unsaturated compounds. For example, F-allyl cation 1 was proposed as a reactive intermediate in reactions of HFP with fluoroolefins catalyzed by Lewis acids [7]. The difference in stability of the corresponding allylic cations was suggested as the explanation for regio-specific electrophilic conjugated addition to CF2=CC1CF=CF2 [11]. Allylic polyfluorinated carbocations were proposed as intermediates in the reactions of terminal allenes with HF [53] and BF3 [54], ring-opening reactions of cyclopropanes [55], Carbocations are also an important part of the classic mechanism of electrophilic addition to olefins (see Eq. 2). This section deals with the questions of existence and stability of poly- and perfluorinated carbocations. 

In contrast to reaction of CF3S020X (see Eq. 100), addition of C10S02F or BrOS02F to chlorotrifluoroethylene leads to formation of two regioisomers. Reaction of bromine tris (fluorosulfate) with fluoroolefins was reported to pro-... 

Nitration of fluoroolefins can be achieved by several methods. Widely studied thermal reaction of N204 with fluoroolefins has a radical mechanism, although the low temperature reaction of nitrogen dioxide with polyfluorinated vinyl ethers proceeds as electrophilic addition of nitrosonium nitrate NO+ N02 across the C=C bond [6] ... 

Introduction of nitro and fluorosulfonoxy groups can be carried out by reaction of ionic 02N0S02F with fluoroolefins [8,116-118]. Nitronium fluorosulfa-te is not as active as halogen fluorosulfates addition to CH2=CF2 and CFH=CF2 proceeds only at ambient temperature [117]. Reaction with TFE and chloro-fluoroethylene proceeds at a noticeable rate at temperature above 50 °C, while addition to 2-H-F-propene only at temperatures higher than 120 °C [117] ... 

Although this work probably does not cover all the details of the developments in electrophilic chemistry of fluoroolefins, it has nevertheless attempted to provide a strategic overview in this area, hoping that chemists working on related subjects should be able to find answers to basic questions associated with reaction conditions, types of electrophiles, reactivity of olefins and orientation of addition. We also wish to acknowledge the tremendous work accomplished in the last 30 years in the area of electrophilic reactions of fluoroolefins that has made this review possible. 

A low-temperature addition reaction occurs between SF5OCl and symmetric fluoroolefins to form pentafluorosulfanylalkyl ethers in nearly quantitative yields (78,90-92). It is found that with unsymmet-rical olefins the chlorine atom of the hypochlorite most often bonds to the olefinic carbon atom with higher electron density. The reactions of SF6OCl and SF5OF with fluorinated ethylenes are used to prepare new SF50-substituted fluorocarbons in 44-77% yield (93). 

The reactions of difluorosilylene with olefins and fluoroolefins [e.g., Eqs. (46) and (47)] appear to proceed by quite different pathways. Similarly, the reaction with benzene resulted in addition products (71, 107) [Eq. (49)], whereas reaction with fluorobenzene led to insertion products (107) [Eq. (50)]. 

A fluormated enol ether formed by the reaction of sodium ethoxide with chlorotnfluoroethylene is much less reactive than the starting fluoroolefin To replace the second fluorine atom, it is necessary to reflux the reaction mixture. The nucleophilic substitution proceeds by the addition-elimination mechanism [30] (equation 26). 

Phenylmercury halides give the corresponding phenylmercury derivative, CgH5HgC(CF3)3 (61%) [156], and perfluorocyclobutene gives the corresponding cyclobutyl derivative [156], Mechanistically, the reaction could be interpreted as formation of the fluorocarbanion via nucleophilic addition of fluoride ion to the fluoroolefin followed by capture of the intermediate fluorocarbanion by the mercury salt [156]. The regiochemistry of the reaction is consistent with this mechanism [156] (equation 119). 

Today microemulsions are used in catalysis, preparation of submicron particles, solar energy conversion, extraction of minerals and protein, detergency and lubrication [58]. Most studies in the field of basic research have dealt with the physical chemistry of the systems themselves and only recently have microemulsions been used as a reaction medium in organic synthesis. The reactions investigated to date include nucleophilic substitution and additions [59], oxidations [59-61], alkylation [62], synthesis of trialkylamines [63], coupling of aryl halides [64], nitration of phenols [65], photoamidation of fluoroolefins [66] and some Diels-Alder reactions. 

The addition of various Kolbe radicals generated from acetic acid, monochloro-acetic acid, trichloroacetic acid, oxalic acid, methyl adipate and methyl glutarate to acceptors such as ethylene, propylene, fluoroolefins and dimethyl maleate is reported in ref. [213]. Also the influence of reaction conditions (current density, olefin-type, olefin concentration) on the product yield and product ratios is individually discussed therein. The mechanism of the addition to ethylene is deduced from the results of adsorption and rotating ring disc studies. The findings demonstrate that the Kolbe radicals react in the surface layer with adsorbed ethylene [229]. In the oxidation of acetate in the presence of 1-octene at platinum and graphite anodes, products that originate from intermediate radicals and cations are observed [230]. 

Miller et al. [9] hypothesized rules on the regioselectivity of addition from the study of the base-catalyzed addition of alcohols to chlorotnfluoroethylene. Attack occurs al the vinylic carbon with most fluorines. Thus, isomers of dichloro-hexafluorobutene react with methanol and phenol to give the corresponding saturated and vinylic ethers The nucleophiles exclusively attack position 3 of 1,1-dichloro-l,2,3,4,4,4-hexafluoro-2-butene and position 1 of 4,4-dichloro-l,l,2,3,3,4-hexafluoro-1-butene [/0]. In l,l-dichloro-2,3,3,4,4,4-hexafluoro-l-butene, attack on position 2 is favored [1I (equation 5) Terminal fluoroolefins are almost invariably attacked at the difluoromethylene group, as illustrated by the reaction of sodium methoxide with perfluoro-l-heptene in methanol [12 (equation 6). 

The reactivity and orientation of nucleophilic attack on fluoroolefins are determined by the stability of the possible carbamon intermediates Fluoroolefins react regioselectively with nucleophiles so as to maximize the number of fluorines (3 to the electron-rich carbon in the transition state The reactivities increase in the order CF2=CF2 < CF2=CFCF3 CF2=C(CF3)2 and CF2=CF2 < CF2=CFC1 < CF2=CFBr, and nucleophdes attack exclusively at the CF2= end of these olefins [129,141] The regiochemistry of nucleophilic attack normally is predictable, but the product distribution arising from addition and addition-elimination pathways depends upon the olefin, nucleophile, and reaction conditions (equations 13-15) The various factors that control product distributions have been reviewed [142,143 144]... 

The synthesis of chlorine (I) and bromine (I) trifluoromethanesulfonates (triflates) was reported by DesMarteau. Stability and reactivity of these materials are similar to those of perfluoroalkyl hypohalites. Both compounds readily react at low temperature with a variety of fluoroolefins. Based on NMR analysis of the products of adding CF3S020X to pure cis- or trans-isomers of 1, 2-difluoroethylene, it was concluded that the reaction proceeds as syn-addition [35]. This statement was later criticized [18], since the assignment of stereoisomers was found to be incorrect. According to [18], addition of CF3S020X to haloolefins, as well as reactions of ClF, BrF and IF proceed as anti-addition via cyclic halonium cationic intermediates. 

The first publication on addition of BrOS02F to several fluoroolefins, which appeared in 1966 [103], was followed by a communication on the reaction of chlorine and bromine fluorosulfate with HFP [97]. This initiated the appearance of a number of papers on the use of halogen fluorosulfates in fluoroorganic synthesis. Most of these publications came from Professor Fokin s group in the former Soviet Union. 

Addition of halogen fluorosulfates XOS02F (X=C1, Br, I) to fluoroolefins is considered an electrophilic reaction [8]. However, the question of whether this process is concerted or the reaction proceeds via an independent carbocationic intermediate (Eqs. 2 and 3) is still open. Formation of carboxylic acid esters as byproducts in the reaction of HFP with C10S02F, which was carried out in trifluoroacetic or heptafluorobutyric acids as solvents, could not be a solid proof of conjugate addition, since formation of esters may be a result of addition of C10C(0)Rf to olefin. These materials are known to be formed in the reaction of ClOS02F with fluorinated carboxylic acids, even at low temperature [99]. 

Even if in 1955 Haszeldine [237] or in 1957 Hauptschein et al. [114,273] started to show a certain livingness of the radical telomerisation of CTFE with C1CF2CFC1I, the up to date research has attracted many academic or industrial chemists towards such a fascinating area. In addition, Dear and Gilbert [102] or Sharp et al. [84,103] performed the telomerisation of various fluoroolefins with disulfides but they did not examine the living character of this reaction. 

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