1.2-Propadienes, cycloaddition

In contrast to the limited success with vinyl sulfides as components of [2 + 2] cycloadditions, allenyl sulfides show wide applicability. As illustrated in Scheme 8.91, Lewis acid-catalyzed [2 + 2] cycloadditions of l-trimethylsilyl-l-methylthio-1,2-propadiene (333) with a variety of electron-deficient olefins 336 provide cycloadducts 337 with excellent regioselectivity but with moderate stereoselectivity [175c], Nara-saka and co-workers reported the first Lewis acid-catalyzed asymmetric [2 + 2] cycloaddition of C-l-substituted allenyl sulfides 319 with a,/3-unsaturated compounds 338 using a chiral TADDOL-titanium catalyst. The corresponding cycloadducts 339 were obtained with 88-98% ee, but a low level of trans/cis selectivity (Scheme 8.92) [169,175d[. 

Knoke and de Meijere [60] recently developed a highly flexible domino Heck-Diels-Alder reaction of a symmetrically substituted cumulene 125, which also involves cross-couplings of an allene at the central position. Both aryl and hetaryl halides react efficiently with l,3-dicyclopropyl-l,2-propadiene (125) and furnish 1,3,5-hexatriene derivatives 126 as intermediates, which are usually trapped by acceptor-substituted olefins in a subsequent cycloaddition, providing adducts 127a/b in moderate to good overall yields (Scheme 14.30). 

This pathway is electronically favorable, but the steric interference between the groups attached to the double bonds is likely to be severe. Such repulsions can be relieved if there are no groups sticking out sidewise at one end of the double bond, as with the central carbon of 1,2-propadiene, CH2=C=CH2, and ketene, CH2=C=0. These substances often undergo. [2 + 2] cycloadditions rather readily (Section 13-3D), and it is likely that these are concerted additions occurring by the Mobius route. 

Propadiene also appears to have the potential for much easier formation of a biradical than does ethene, as you will see if you work Exercise 21-27. Not all [2 + 2] cycloadditions proceed by biradical mechanisms, some clearly occur by stepwise reactions involving ionic intermediates (see Exercise 21-43). 

Introduction of an alkylthio group on the allene system increased the reactivity of the allene moiety in [2 + 2] cycloaddition reactions. It proved possible to conduct reactions of this allene at much lower temperatures. By adding Lewis acids, the reaction temperature could be decreased even more, as was illustrated by the Lewis acid catalyzed [2-1-2] cycioadditions of l-trimethylsilyl-l-methylthio-l,2-propadiene with a variety of electron-poor alkenes, including cyclic and non-cyclic enones, acrylates, methyl fumarate and acrylonitrile. When a chiral diol 21 based titanium catalyst was employed, the [2-1-2] cycloaddition reactions of /-acryloyl-l,3-oxazolidin-2-ones 17a and 17b with allenyl sulfides 18 yielded methylenecyclobutanes 19 and 20 with high optical purities (equation The highest yields were obtained with electron-poor allenophile 17b. 

The photochemical reactivity of metastable diacetylene with styrene has been studied. The triplet state of the diyne is involved and several reaction products are formed. Two of the major ones are the eneyne (57) and the ethynylstyrene (58). Irradiation of the tetrayne derivatives (59) in diethyl ether affords a mixture of the trans- and cis-derivatives. Cations can be produced by the photolysis of 4-methoxyphenylalkynes in 2,2,2-trifluorethanol." 1-0-Acetylaminophenyl-2-(pentamethyldisilanyl)ethyne is photochemically active on irradiation in benzene. An intramolecular cycloaddition takes place via a l-sila-l,2-propadiene intermediate. " Park and Baek" have reported the photo-... 

In the context of the ethynyl sulfones 6.30 Padwa s cycloadditions of phenylsulfonyl-substituted propa-1,2-dienes like 6.31 with diazomethane should be mentioned briefly (Padwa et al., 1993a, with many references on allenes used as dienophiles). The example in (6-22) demonstrates the regiospecificity of the cycloaddition to the more reactive electron-deficient 71 bond. The ready reactivity of cumulated double bonds is based on the relief of strain when the 1,2-propadiene... 

The a-allylpalladium intermediate 135a, which must be formed on coupling of l,3-dicyclopropyl-l,2-propadiene (134), with, for example, iodobenzene under palladium catalysis rapidly undergoes rearrangement to the homoaUylpalladium species 135b and subsequent P-hydride elimination to yield the 1,3,5-hexatriene 136 [152g,h]. This in turn undergoes [4 + 2] cycloaddition with an added dienophile to furnish the 3-(2-cyclopropyl-l-phenyl)cyclohexene derivatives 138 as a mixture of trans,trans- and ds,trows-diastereomers (Scheme 8.31). 

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