Alkynes electron-withdrawing substituents

Thiirenes have been isolated in argon matrices at 8 K by photolysis of 1,2,3-thiadiazoles or vinyl trithiocarbonates (Scheme 85) <80PAC1623, 81JA486). They are highly reactive and decompose to thioketenes and alkynes. Electron-withdrawing substituents stabilize thiiranes somewhat, but no thiirenes stable at room temperature are known, unlike the relatively stable thiirene 1-oxides and thiirene 1,1-dioxides. 

The Diels-Alder cycloadclition reaction occurs most rapidly if the alkene component, or dienophile ("diene lover"), has an electron-withdrawing substituent group. Thus, ethylene itself reacts sluggishly, but propenal, ethyl propenoate, maleic anhydride, benzoquinone, propenenitrile, and similar compounds are highly reactive. Note also that alkynes, such as methyl propynoate, can act as Diels-Alder dienophiles. 

These reactions are found to be promoted by electron-donating substituents in the diene, and by electron-withdrawing substituents in the alkene, the dienophile. Reactions are normally poor with simple, unsubstituted alkenes thus butadiene (63) reacts with ethene only at 200° under pressure, and even then to the extent of but 18 %, compared with 100% yield with maleic anhydride (79) in benzene at 15°. Other common dienophiles include cyclohexadiene-l,4-dione (p-benzoquinone, 83), propenal (acrolein, 84), tetracyanoethene (85), benzyne (86, cf. p. 175), and also suitably substituted alkynes, e.g. diethyl butyne-l,4-dioate ( acetylenedicarboxylic ester , 87) ... 

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... 

After extensive screening of various aldehydes to optimize the reaction conditions, it was found that aromatic aldehydes were able to serve as a carbon monoxide source, in which the electronic nature of the aldehydes is responsible for their ability to transfer CO efficiently [24]. Consequently, aldehydes bearing electron-withdrawing substituents are more effective than those bearing electron-donating substituents, with pentafluoro-benzaldehyde providing optimal reactivity. Interestingly, for all substrates tested the reaction is void of any complications from hydroacylation of either the alkene or alkyne of the enyne. Iridium and ruthenium complexes, which are known to decarboxylate aldehydes and catalyze the PK reaction, demonstrated inferior efficiency as compared to... 

The [2,3- ]> [2,3- ]. and [i,Z-b] isomers of 2,3-disubstituted furopyridines can be obtained from alkynes and o-iodoacetoxypyridines by a palladium-catalyzed cross-coupling reaction followed by an electrophilic iodocyclization reaction or a palladium-catalyzed cyclization in the presence of CO, as illustrated in Scheme 32 for [2,3-3] and [3,2-3] derivatives. When the pyridine ring contains strongly electron-withdrawing substituents such as carboethoxy, the reaction proceeded smoothly with benzyloxyalkynylpyridines <20020L2409>. 

Products of monoaddition may be isolated with tetrabutylammonium bis(hydrogen fluoride) if electrophilic alkynes (one or two electron-withdrawing substituents) are used, e.g. formation of 3. In contrast to these results, 70% hydrogen fluoride/pyridine does not add hydrogen fluoride to dimethyl acetylenedicarboxylatc.77... 

Alkynes add nitrile oxides and diazoalkanes to give isoxazoles (60) and pyrazoles (61), respectively, in 1,3-dipolar cycloadditions. If an alkene is used instead of an alkyne the non-aromatic analogues (62 Z = NH, O) result (94AHC(60)26l) yields are best when the alkene contains an electron-withdrawing substituent. 

Typical examples of this reaction involve the heterocumulenes C02, CS2, COS, RNCO, RNCS and other related molecules such as S02, S03, RC=N and RNC, and activated alkynes bearing strongly electron-withdrawing substituents such as C02Me. These reactions find utility in the syntheses of homoleptic carbamates (equation 77),223 monothiocarbamates and dithiocarbamates from homoleptic amides though redox reactions can occur in some instances (equations 78102 and 79224). 

Several kinds of cycloaddition products have been observed in the reaction of 82 with various activated double and triple bond species. A simple 2 1 cycloaddition takes place in the reaction of 82 with alkynes possessing electron-withdrawing substituents (equation 75)199. In contrast, five-membered silaheterocycles are formed regio- and stereo-specifically in multi-step reactions with certain organic carbonyl196 (equation 76) and nitrile compounds (equation 77)197. The observed C—C bond formation, which generally takes place under mild reaction conditions, might be of synthetic value. 

Thiophene reacts with alkynes to give the corresponding benzene through a photochemical Diels-Alder reaction followed by extrusion of a sulfur atom (Scheme 9) (72TL1909 73CB674). Thienyl derivatives with electron withdrawing substituents allow adducts isolation (78CJC1970). 

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