Captodative alkene

Photocycloadditions with Captodative Alkenes Dietrich Ddpp... 

Alkenes substituted with an electron-withdrawing (Z) and an electron-donating group (X) will be less reactive than the unsubstituted alkene if the substituents are vicinal (Z-C=C-X push-pull alkenes), but will generally become highly reactive when these two substituents are bound to the same carbon atom. The latter type of alkene (ZXC=CR2 1,1-captodative alkenes) readily dimerize to yield cyclobutanes and can react with 1,3-dienes to yield products of [2 + 2] and/or [2 + 4] cycloaddi-... 

Diels-Alder reactions of (Z)-A -substituted-4-methylene-5-propylidene-2-oxazoli-dinone dienes with methyl vinyl ketone, methyl propiolate, and captodative alkenes yield the highest regio- and stereo-selectivities in mixtures of H20 and MeOH or under BF3.Et20 catalysis.198 The asymmetric Diels-Alder reaction of cyclopentadiene and 3-acryloyl-2-oxazolidinone is catalysed by a new Cu(II) catalyst containing a chiral sterically congested roofed (2-diphenylphosphino)phenylthiazoline ligand (169).199... 

Symmetric 3 m -excited 1,2-diarylethanediones (35) undergo highly regio- and stereoselective head to head additions to various captodative-substituted alkenes (2-aminopropenenitriles 36) forming oxetanes 37 in moderate to good yield (Sch. 9) [36]. 

Only few reports deal with a para photocycloaddition as the major reaction path. Recently, however, several cinnamide derivatives like 21 were efficiently transformed into the corresponding para adducts 22 (Sch. 6) [37]. Yields higher than 90% could be achieved. The para photocycloaddition is also observed with naphthalene derivatives like 1-acetylnaphthalene 23 and captodative enamino nitriles as 24 [38]. Other captodative substituted alkenes [39] as well as the fluorinated uracil derivative 26 [40] are transformed in the same way. Especially in the cases of 21 and 23, the... 

Dopp D. Photocycloaddition with captodative alkenes. In Ramamurthy Y, Schanze KS, eds. Molecular and Supramolecular Photochemistry. Vol. 6. New York Marcel Dekker, Inc., 2000 101-148. 

Some mechanistic aspects of the above cascade reaction deserve comment. Thus, after the intermolecular addition of the nucleophilic acyl radical to the alkene, the electrophilic radical adduct A, instead of undergoing reduction, reacts intramolecularly at the indole 3-position (formally a 5-endo cyclization) to give a new stabilized captodative radical B, which is oxidized to the fully aromatic system. (For a discussion of this oxidative step, see Section 1.5.)... 

Fluorine and hydrogen appear comparable as radical-stabilizing groups, for both 1,1-difluoroethylene and trifluoroethylene react with butadiene to give mixtures of [2 + 2] regioisomers, as well as some [2 + 4] products. " Captodative substituted alkenes, not surprisingly, are well suited to [2 + 2] cycloadditions. ... 

Photoelectron, UV and NMR spectroscopy, as well as MNDO calculations all predict a reduced HOMO-LUMO gap for alkenes with captodative substitution, and an enhanced reactivity of the p carbon [43]. This explains their radicophilic behavior and their high reactivity in cycloaddition processes, the diradicaloid transition states of which are stabilized when the mechanism is asynchronous cf. Sec. 3.3.7). 

Captodative alkenes 67 can be dialkylated, for example, by addition of iso-butyronitrile radical derived from thermal decomposition of AIBN under the same conditions as those which lead to polymerization of other acrylic alkenes. For example, a-morpholino-acrylonitrile (67, c = CN, d = N(CH2CH2)20) leads to 69, in 71% yield (Scheme 12) [4a]. With a-/-butylthio-acrylonitrile (67, c = CN, d = SC(CHj)3), the same process leads to 70 in 88% yield [7]. The adduct radical 68 is highly stabilized, and is in equilibrium with dimer 70. The reaction is quite general, and has been applied to other captodative alkenes (c = CN, COR, CO2R and d = NR2, OR, SR) together with various sorts of radical partners, derived from alkanes, alcohols, thiols, thioethers, amines, amides, ketones, aldehydes, acetals and thioacetals [44, 45]. 

Nucleophilic radical 79, derived from bromide 78, is expected to add intra-molecularly to the eleetrophilic double bond to yield a-acyl radical 80, but it can also add to the radicophilic alkene group to yield captodative species 81 (Scheme... 

Several captodative alkenes have been found to dimerize spontaneously and quantitatively at room temperature via reversible formation of a few-captodative 1,4 diradical (Scheme 20). This occurs, for example, with a-alkylthioacrylonitriles 109 [57], with a-alkylselenoacrylonitriles 110 [58], with protoanemonin 111 [59], and with eyanopyrrolin 112 [60]. 

Eschenmoser et al. used the reverse thermal fragmentation reaction of cyclobutane 113 to generate captodative alkene 114 [61]. 

Captodative alkenes also react well with other alkenes in [2-I-2] cycloaddition reactions. For example, trifluorochloroethylene 115 reacts more efficiently with a-alkythioacrylonitrile 116 than with acrylonitrile [58]. 

Allenes also react with captodative alkenes to form 3-methylene cyclobutanes regioselectively. For example, 116 reacts with allene to give 117 upon heating at 140°C for two days [62]. Captodative allenes can even react at room temperature with captodative olefins as an illustration, a-methoxycyanoallene 118 adds to 116 to form 119 in 41% yield [62]. 

The bis-alkenes 125a-b undergo intramolecular [2+2] cycloaddition, the efficiency of which is dependent upon the magnitude of the captodative effect (Scheme 21) [64]. With 125a, 4 days at 140 °C are required for full conversion, against 5 min at 140 °C (or 3 days at 40 °C) for captodative 125b. 

Captodative alkenes are also good dipolarophiles [65] and dienophiles [66]. The corresponding [3+2] and [4+2] cycloaddition reactions seem to be concerted, as usual, so that diradicals are not directly involved as such. However, the mechanism may be rather asynchronous, with the transition state having pronounced di-radicaloid character, and thereby being stabilized by the captodative effect [67]. This can explain the higher dienophilic character of a-methylthioacrylonitrile 127 compared to acrylonitrile, despite the unfavorable steric effect (Scheme 22) [68]. 

Heterocyclization. Captodative radica a-chloroacetic esters by treatment with CuC tramolecularly by an alkene (e.g., a 3-butei Formation of five-membered ring hetcrocy 3-chloromethyltetrahydrofuran-2-carbox>U mixture of cis and trans isomers (ratio 64 3 lidine and piperidine derivatives. ... 

The consequence of the captodative effect on radical additions to alkenes is a strong directing p effect for addition of the radical to the center that gives the especially stabilized radical. The captodative effect has been used in a variety of synthetic schemes, many of which are given in the reference below. 

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