Fluorinated allenes reactions

Rearrangements arc common in reactions of sulfur tetrafluoride with acetylenic and allylic alcohols. Highly acidic acetylenic alcohols derived from hexafluoroacetone and other haloacetones react readily at ambient temperature to give fluorinated allenes 10 as the sole... 

The 19F NMR spectra of several highly fluorinated derivatives of 2//-pyran (81MI22201, 83JCS(P1)1239) and 4/f-pyran <83JCS(P1)1235> have been reported, as have those of some fluorinated 2,2-dimethyl-2//-chromenes (80JHC1377). The structures of some pyran-2-ones derived from the reaction between phenylacetylene and fluorinated allenes have been ascertained with the aid of their 19F spectra (77IZV2517). 

Indium-mediated allylation of trialkyl(difluoroacetyl)silane 70 in aqueous media gives homoallylic alcohol 71 exclusively (Scheme 60). Both water and THF are essential for the allylation reaction. It is worth noting that homoallylic alcohol 71 is formed exclusively under these reaction conditions. On the contrary, enol silyl ether 72 is a major product of the fluorinated acylsilanes reaction with other organometallic compounds than indium via a Brook rearrangement and defluorination. Indium-mediated allylsilylation of carbonyl compounds provides a facile route to 2-(hydroxyethyl)allylsilanes. The allene homologs are similarly prepared (Scheme 61).244,244a... 

Because the fluorine substituents both inductively and hyperconjugatively withdraw electron density from the C(2)-C(3) tt bond, the LUMO is located there, and Diels-Alder reactions take place exclusively with this bond [25] 1,1 -Difluoro allene and fluoroallene reaet readily with a large selection of cyclic and acyclic dienes, and acyclic dienes, [2+2] cycloadditions compete with the Diels-Alder processes As shown in the example in equation 79, a significantly different regiochemistry is observed for the [2+4] cycloaddition compared with the [2+2]... 

In the reaction of 1 with alkynes possessing electron-withdrawing substituents, the corresponding silacyclopropene derivatives 66 and 67 are formed, as described in Scheme 23.29 An unexpected pathway was observed in the reaction with the electron-poor hexafluorobutyne(2) the X-ray characterized heterocycle 68 was most likely obtained by nucleophilic attack of 1 at the triple bond. A subsequent shift of a fluorine atom from carbon to silicon creates an allene-type molecule which was stabilized by a [2 + 2] cycloaddition process involving a double bond from the pentamethylcyclopentadienyl unit, as described in Scheme 24.33... 

The a-selectivity for carbon radical addition to propadiene (la) is retained on substituting chlorine or fluorine for hydrogen in radicals of the type CX3 (X=F, Cl), no matter whether the reaction is conducted in the liquid or in the gas phase (Table 11.4) [14, 49-51]. /3-Selective addition to allenes becomes progressively more important for the CC13 radical with an increase in number of methyl substituents [14, 47]. For example, treatment of optically active (P)-(+)-2,4-dimethylpenta-2,3-diene [(P)-(lc)] with BrCCl3 affords a 59 41 mixture of a- and /3-monoadducts [47]. The a-addition product consists of a 20 80 mixture of E- and Z-stereoisomers, whereas the product of /3-addition exclusively exhibits the Z-configuration. The fraction of 2,4-dimethylpenta-2,3-diene (P)-(lc) that was recovered from this reaction mixture had completely retained its optical activity. These results indicate that the a-and the /3-CCl3 addition proceed under kinetic control. If one of the addition steps were reversible, at least partial racemization would inevitably have taken place. 

Propargylic mesylates such as fluorine-substituted derivative 265 react with PhZnCl in the presence of Pd(PPh3)4 (5 mol%) in THF at 0°C within 2 h to provide the anti-Si 2 product in excellent yield and complete transfer of the stereochemistry leading to the allene 266 (Scheme 78). Copper(I) catalyzed allylic substitutions with functionalized diorganozincs proceed with high 8 2 selectivity. Thus, the reaction of the chiral allylic phosphate 267 with 3-carbethoxypropylzinc iodide in the presence of CuCN 2LiCl (2 equivalents) furnishes the awf/-Sjv2 substitution product 268 in 68% yield. By the addition of w-BuLi (1.2 equivalents) and TMSCl (1.5 equivalents), the bicyclic enone 269 is obtained in 75% yield and 93% ee (Scheme 79) . 

The thermal reaction between two molecules of olefin to give cyclobutane derivatives (a 2 + 2 cycloaddition) can be carried out where the olefins are the same or different, but the reaction is not a general one for olefins.921 Dimerization of like olefins occurs with the following compounds F2C=CX2 (X = F or Cl) and certain other fluorinated alkenes (though not F2C=CH2), allenes (to give derivatives of 97),922 benzynes (to give biphenylene deriv-... 

The formation of four-membered rings through 2 + 2 cycloaddition is a well-established reaction and the most generally effective synthetic approach to cyclobutanes. Most olefins cannot be induced to undergo this reaction thermally, a finding that is readily rationalized by the forbidden nature of the 2s + 2s addition and the steric difficulties associated with the allowed 2s + 2a pathway. There are nevertheless exceptions. Olefins substituted by two or more fluorine atoms undergo thermal 2 + 2 additions under relatively mild conditions,16 as do ketenes and allenes. 

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. 

Cycloaddition Reactions of Fluorinated Alkenes with Dienes and Allenes... 

The favored reaction of fluorinated alkenes with dienes is normally [2 + 2] rather than [4 + 2] cycloaddition. There are, however, exceptions for example, 2-(A, A-diethylcarbamoyloxy)-l, 1-difluoroethene reacts with cyclopentadiene to give [4 + 2] cycloadducts as the major products, Fluoroalkenes also readily add to allenes to give cyclobutanes with exocyclic C = C bonds. Examples of such [2 + 2]-cycloaddition reactions are given in Table 7. 

Table 7. Fluorinated Cyclobutanes by [2 -f 2 -Cycloaddition Reactions of Fluoroalkenes with Dienes and Allenes...

Four-membered heterocydes are easily formed via [2+2] cycloaddition reac tions [63] These cycloaddition reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [77d] Via this route, p lactones are formed on addition of ketene derivatives to hexafluoroacetone [777, 77S] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, l,4-benzoxazin-2-ones, l,2,4-tnazin-5-ones, and l,2,4-tnazm-3,5-diones accelerates [2+2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluorinated oxetanes are formed thermally and photochemically [779, 720] The reaction of 577-1,2-azaphospholes with fluonnated ketones leads to [2+2] cycloadducts [727] (equation 27)... 

ALLENE (463-49-0) Extremely flammable gas. Violent reaction with oxidizers, fluorine. Incompatible with aluminum chloride, aluminum tetrahydroborate nitromethane, magnesium perchlorate, nitrosyl fluoride, ozone, peroxyformic acid. Forms explosive material with nitric oxide, copper(I) perchlorate. May accumulate static electrical charges, and cause ignition. 

Fluorination of allenes. Allene behaves differently toward fluorine in the presence or absence of dry NaF. The reaction of various allenes is explosion-prone and it must be carried out at low temperature. Cyanoallene requires even lower temperatures to avoid charring. 

Allenes appear to react in such a way that their central carbon atom is bound, in the most abundant product, to the central carbon atom of a second allene molecule, as in dimerisations -, to the terminal carbon of a vinyl derivative, or to the most fluorinated atom of a fluoro-halogenoethylene, as has been found in the reactions of propadiene with F2C=CFC1 and F2C=CCl2 . where 85% and 95% of the product, respectively, conformed to this orientation. 

Thus, the optimized carbometallation reaction of propargylic alcohols with aromatic Grignard reagents proceeds without an additional metal catalyst and leads to the formation of an alkenyl fluoride. The stereoselective defluorometallation proceeds to form predominantly the fluorinated (Z)-carbon-carbon double bond in the final allene 276. Two reasons were proposed by the authors for such a high level of stereoselectivity the defluorometallation is favored from the intermediate... 

Oxidative fluorination of olefins, allenes, and acetylenes are reported [11,12]. The reaction of OF2 with olefinic fluorocarbons is used for the synthesis of fluoropolymers [13]. 

Liu and coworkers have recently published the synthesis of (3) and (4), as well as several other fluorinated pyrrole analogues (not shown), by aminofluorination of allenes the synthetic approach is shown in Scheme 3 [15], This strategy takes advantage of a selective, silver-catalyzed intramolecular fluorination reaction, but the approach works best with substrates that possess electron-withdrawing R -groups on the 3-position. Because of this limitation, the yield of (3) is significantly higher than (4), 80 % versus 28 %, respectively. 4-Fluoro-pyrrole-2-carboxylic acid (5a, Fig. 2), synthesized from a fluorinated proline, was explored as a potential intermediate on the route to (3), but only extensive decomposition products were observed when (5a) was subject to flash pyrolysis [16]. 

Atomic fluorine addition to allene had at first been assumed to proceed entirely through attack at the central position, since only the product corresponding to initial F atom attack (and NFj termination) has been isolated from N2F4-allene photolysis e qperiments (, 46). No evidence was found for the presence of either CH2=C(NF2)CH2F or its expected isomerization product (CH2F)2C=NF (45). However, Rowland and coworkers (47, 48 ) found both terminal and central addition with relative yields in the approximate ratio of 2 1 (Table VI), indicatii a moderate selective preference toward reaction with the terminal end of the ff-bond. 

Fluorinated alkenes have been reported to give cyclobutane derivatives by thermal additions to other alkenes. Olefins in their ground electronic states have not been observed to give stereospecific cycloaddition one with another whereas a carbon-carbon double bond which is part of an allene or keten will undergo stereospecific concerted cycloadditions. The separate reactions of tetrafluoroethylene with cis- and trans-[l,2- H2]ethylene, which should not be subject to an adverse steric effect, have been studied to establish whether these reactions are stereospecific. Identical mixtures of products were obtained from both reactions, as expected if they proceed through the... 

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