Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Fluorinated allenes

Fluorinated allenes are especially reactive in cycloadditions because of their highly strained double bonds [118, 119] 1,1-Difluoro- and 1-fluoroallene readily undergo both [2+2] and [4+2] cycloadditions [118 124] (equations 50-52) Exten sive studies of stereochemistry and regioselectivity show that cyclobutane forma-... [Pg.784]

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... [Pg.330]

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). [Pg.593]

Treatment of fluorohalo compounds with organometallic reagents (RLi, RMgX) allows the dehalofluorination to be carried out under mild conditions (see Table 9), e.g. to form highly reactive fluorinated allenes. [Pg.374]

Most of the new chemistry of fluorinated allenes which appeared during the period covered by the Report has concerned perfluoro-(2,4-dimethyl-penta-2,3-diene). Although this allene was reported previously to react with primary alcohols only in conjunction with a basic catalyst, it is now claimed to be attacked by ally alcohol in neutral ethereal solution. The initial product was detected spectroscopically but not isolated because it decomposes quite rapidly (tj = 1 h at 50 °Q by a Claisen rearrangement (see p. 64). ... [Pg.94]

Hung MH. Fluorinated allenes and alkynes. Tetrahedron Lett. 1990 31 3703-3706. [Pg.802]

The 1,3-dipolar cycloadditions offluonnatedallenes provide a rich and varied chemistry Allenes, such as 1,1-difluoroallene and fluoroallene, that have fluorine substitution on only one of their two cumulated double bonds are very reactive toward 1,3-dipoles Such activation derives from the electron attracting inductive and hyperconjugative effects of the allylic fluorine substituent(s) that give nse to a considerable lowering of the energy of the LUMO of the C(2)-C(3) n bond [27]... [Pg.803]

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]... [Pg.824]

The hexafluoroargentate(III) anion was first obtained as the CS2K salt by Hoppe and Homann [26), who fluorinated a 2 1 1 mixture of CsCl, KC1, and AgNC>3 at 300 °C. A moment of 2.6 B.M. was reported for the product, and the electronic spectrum was studied by Allen and Warren (9), using the diffuse reflectance technique. [Pg.117]

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... [Pg.24]

Scheme 4.34 Asymmetric synthesis of fluorine-containing allenes. Scheme 4.34 Asymmetric synthesis of fluorine-containing allenes.
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. [Pg.710]

In the case of the fluorine-substituted allene 85 with Ag(I) catalyst, only the product of a cyclization/elimination process, the furan 86, was produced (Scheme 15.20)... [Pg.888]

Ip. Begue, D. Bonnet-Delpon, F. Benayoud, T.T. Tidwell, R.A. Cox, A. Allen, Comparison of the formation energy of fluorinated alkoxycarbenium ions, Gazz. Chim. Ital. 125 (1995) 399 02. [Pg.621]

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) . [Pg.338]

Perfluorination of unsaturated hydrocarbons such as alkenes, allenes (Fig. 7) [53] and aromatics (Fig. 8) [54,55] is also possible since the total energy released upon fluorine addition to a carbon-carbon double bond (typically between 251.4-292.9 kjmol-1) is not sufficient to break carbon-carbon single bonds. [Pg.8]

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-... [Pg.855]

A shift from allenes to acetylenic products formed from acetylenic alcohols in which trifluoro-methyl groups are replaced by difluoromethyl groups can be explained in terms of lowering the positive character of the terminal acetylenic carbon atom, thus retarding the nucleophilic attack of fluorine at this position.56... [Pg.331]

Dehydrofluorination of the fluorinated alkene 18 gives a mixture of diene 19 and allene 20. [Pg.99]

The values presented in Table 2 depict the variation of C = C re-bond strength with increasing fluorination and are consistent with the differences in reactivity associated with differing degrees of fluorination.3 Fluorination also destabilizes allenes and acetylenes.20 22 Similarly, per-fluoroalkyl groups destabilize C = C bonds. Note, however, that perfluoroalkyl groups can lend kinetic stabilization to strained molecules.3... [Pg.294]

Fluorine-Substituted Carbocations Allen, A D, Tidwell, TT Adv Carbocatwn Chem /, 1-44 173... [Pg.20]

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. [Pg.629]


See other pages where Fluorinated allenes is mentioned: [Pg.825]    [Pg.825]    [Pg.443]    [Pg.1]    [Pg.10]    [Pg.257]    [Pg.1077]    [Pg.123]    [Pg.110]    [Pg.151]    [Pg.156]    [Pg.828]    [Pg.865]    [Pg.432]    [Pg.85]    [Pg.170]    [Pg.343]    [Pg.545]    [Pg.125]    [Pg.422]    [Pg.443]    [Pg.803]    [Pg.460]   


SEARCH



© 2024 chempedia.info