Butynes. Diels-Alder with

Alkynes substituted with one or two trifluoromethyl groups are also highly reactive dienophiles [9] Indeed, hexafluoro-2-butyne is used increasingly as a definitive acetylenic dienophile in "difficult Diels-Alder reactions. It was used, for example, to prepare novel inside-outside bicycloalkanes via its reaction with cir,trnns -l,3-undecadiene [74] (equation 67) and to do a tandem Diels-Alder reaction with a l,l-bis(pyrrole)methane [75] (equation 68) Indeed, its reactions with pyrrole derivatives and furan have been used in the syntheses of 3,4-bis(tri-fluoromethyl)pyrrole [76, 77] (equation 69) and ],4-bis(trifluoromethyl)benzene-2,3-oxide [78] (equation 70), respectively. 

Arsenin, antimonin and bismin react readily with dienophiles in the Diels-Alder reaction, and the reaction with hexafluoro-2-butyne and other acetylenic dienophiles has been used to trap these heterocycles as barrelenes (115), in particular the unstable bismin (equation 21). The reactivity increases with increasing size of the heteroatom and this may be related to decreased aromatic character of the heterobenzenes with increasing size of the heteroatom. In fact, bismin and antimonin are so reactive that at low temperatures (< -10 °C and < - 50 °C respectively) both exist as Diels-Alder dimers (116 equation 20). 

Attempts to isolate the silole monomer 2 or to trap it by reaction with maleic anhydride or perfluoro-2-butyne have failed. This is surprising given the successful trapping of 1-methylsilole5 and 1,1-dimethylsilole10. The authors conclude that silole 2 is not very reactive in Diels-Alder cycloadditions, except toward self-reaction19. 

The addition of dihalocarbenes to alkynes is again a rather inefficient process and usually leads, to the isolation of the cyclopropenone rather than the 3,3-dichlorocyclo-propene. In a rather unusual example, however, 2-butyne is reported to be converted to (67). This product is apparently derived by addition of dichlorocarbene to the corresponding methylenecyclopropene, derived in turn by elimination of HC1 from the primary adduct (68). The cyclopropene (67) does not appear to ring open to a vinylcarbene, but can be trapped in Diels-Alder reactions with cyclopentadiene 60). A related addition of dichlorocarbene to ethyl 2-butynoate also leads to a low yield of the 3,3-dichlorocyclopropene, which may be hydrolysed to the cyclopropenone 6l). 

Five months later, Corey and Lee reported the enantioselective Diels-Alder reaction between cyclopentadiene and a,/ -acetylenic aldehydes catalyzed by a chiral cationic oxazaborinane catalyst [33]. Although initial studies of the Diels-Alder reaction between cyclopentadiene and 2-butyn-l-al or 2-octyn-l-al with 20 mol % catalyst at -94 °C to -78 °C revealed only 3-5 % conversion to product over a 24-h period, replacement of the /3-alkyl substituent on the aldehyde component by RsSi or RsSn groups resulted in much faster Diels-Alder addition (Eq. 33). The greater yield with 3-tributylstannyl-2-propyn-l-al, compared with the 3-silyl analogs, results from the rate of reaction with the former. In each instance, good enantioselectivity (80-87 % ee) was obtained. 

The recent development of dienes incorporating latent has greatly expanded the utility of Diels Alder approaches of complex molecules. Butadiene substituted by silicon at is available from l,4-dichloro-2-butyne by H2PtCl2catalyzed reaction with Et3SiH followed by dehydrohalogenation (Zn). The product reacts readily with dlenophiles. ... 

Hetero-Diels-Alder reaction. 3,4-Bis(trimethylsilyl)-5,6-dihydro-2A/-pyrans are accessible from a catalyzed cycloaddition under very mild conditions. The diene is prepared in 44-47% yield from the reaction of l,4-ditosyloxy-2-butyne with trimethylsilylmethylmagnesium chloride. 

Metal complexes can occasionally give good results as catalysts in Diels-Alder reactions, as for butadiene and isoprene dimerisations with Fe(CO)2(NO)2 and butadiene addition to 2-butyne in the presence of Fe (cyclooctatetraenela . However, these catalysts usually promote formation of several products in non-specific manner, through a two-step mechan-ism - , not dissimilar to most photochemical processes (see following section). 

The syntheses of peifluorinated epoxides by oxidation of perfluorodUiydrofurans with sodium hypochlorite and Diels-Alder reaction of 2,5-dialkyl-3,4-bis(triiluor-omethyl)furan with hexafluoro-2-butyne should me mentioned among the reactions proceeding with participation of endocyclic C=C bonds. 

The twisted alkene present in acetoxydithiocin (176 R = Ac) was suggested to be a possible cause of its relative unreactivity as a diene in Diels-Alder cycloaddition reactions. No reaction is seen with maleic anhydride, tetracyanoethylene, or hexafluoro-2-butyne at 45 °C higher temperatures were precluded by the thermal instability of (176 R = Ac) <78T363i>. iV-Phenyltriazolinedione reacted with (176 R = Ac), but afforded no identifiable products. 

The Diels-Alder adduct formed by treating furan with hexafluorobut-2-yne has been subjected to a retro-Diels-Alder reaction acetylene is eliminated and the reaction provides a route to 3,4-bis(trifluoromethyl)furan. The corresponding cycloadduct (154) formed from l,l-dimethyl-2,5-diphenyl-sllacyclopentadiene and the butyne was decomposed in an attempt to generate dimethylsilylene by a symmetry-allowed cheletropic fragmentation (Scheme 50) 836 evidence for silylene formation could be obtained unless the decomposition was effected photochemically or in refluxing cumene, under which conditions the addition of tolan enabled dimethylsilylene to be trapped... 

Acetone-sensitized photoreaction of 137 with butyne-2 gave products of mono-248 and bis [2h-2] cycloaddition 249 [149], Oppositely, 3,4-bis(trifluoromethyl) furans 250 participate in the Diels-Alder reaction. They reacted with hexafluorobut-2-yne 84 giving 7-oxanorbomadiene 251 [72, 73, 77, 150], Treatment of 251 with TiCl4/LiAlH4/Et3N system led to the formation of tetrakis(trifluoromethyl)benzene 252 [77], Irradiation of 251 produced mixture of cyclopentadienes 253 and 254 as well as oxepin 255 in variable ratios depending on the solvent [150]. 

Hexafluoro-2-butyne also gives Diels-Alder adducts with benzene, toluene, and... 

The rearrangement of 2-phenyl-3-butyn-2-ol to 3-phenybut-2-en-l-al was examined computationally. The carbocation PhMeC+-C=CH is considered an intermediate or, under some conditions, a transition state. The stepwise Diels-Alder reaction of 1-trimethylsiloxy-1,3-butadiene and 4,6-dinitrobenzofuroxan was studied by computational methods. A stepwise mechanism with a persistent intermediate is supported. However, this intermediate is suggested to be a heterocyclic adduct and not the zwitterionic allyl cation/furoxan anion adduct proposed in the previous studies. 

---