Zwitterion cycloadducts, formation

Non-enolizable imines such as 9-fluorene imines react with alkynylcarbene complexes to afford mixtures of mesoionic pyrrolium carbonyltungstates and dihydropyrrole derivatives [68] (Scheme 23). Although both compounds can be considered as [3C+2S] cycloadducts, formation of each of them follows a very different pathway. However, the first intermediate of the reaction is common for both compounds and supposes the conjugated addition of the imine to the alkynylcarbene complex to form a zwitterionic intermediate. A cyclisation... 

As noted above, formation of a furan [4 + 3]-cycloadduct during irradiation of a 4-pyrone was advanced as evidence for the zwitterionic intermediate. This process can be moderately efficient (equation 4)68, and can be envisioned as an approach to substituted cyclooctanoids. Besides the formation of three new carbon-carbon bonds, an additional attractive feature is the complete diastereoselectivity, arising from a compact [4 + 3]-cycloaddition transition state with approach from the face opposite the epoxide. However, the generality of the intermolecular reaction is limited, as competing [2 + 21-photodimerization, solvent trapping and rearrangement often predominate58. 

There is no published example of a cyclopropanation of the double bond in chlorocyclopropylideneacetate 1-Me with retention of the chlorine atom. Thus, attempted cyclopropanations under Simmons-Smith [37] or Corey [38] conditions failed [25]. The treatment of the highly reactive methylenecyclopropane derivative 1-Me with dimethoxycarbene generated by thermal decomposition of 2,2-dimethoxy-A -l,3,4-oxadiazoline 26 (1.5 equiv. of 26,PhH, 100 °C,24 h),gave a complex mixture of products (Scheme 7) [39], yet the normal cycloadduct 28 was not detected. The formation of compounds 29 - 33 was rationalized via the initially formed zwitterion 27, resulting from the Michael addition of the highly nucleophilic dimethoxycarbene to the C,C-double bond of 1-Me. The ring closure of 27 to the normal product 28 is probably reversible, and 27 can rearrange or add a second dimethoxycarbene moiety and a molecule of acetone to form 33. 

A similar scheme was investigated at the RHF/AM1 level for the reaction between methoxyketene (38) and methyl methylimidoformiate (58) [66]. The computational results showed that at the 3x3CI-HE/AMl level the rotation about the N1-C4 bond in zwitterionic intermediate (59) to form the intermediate (61) was favored over direct formation of the cycloadduct ck-60, thus resulting in the exclusive formation of trans-60, in good agreement with the experimental evidence on related imidates [67] (Scheme 14). 

Mattes and Farid [42] have argued that the formation of phenylsuccinic anhydride need not be regarded as an indication that the ortho cycloadduct is formed via a zwitterionic intermediate. They considered the possibility that charge-transfer excitation of the maleic anhydride-benzene complex leads to the anhydride radical anion and the benzene radical cation. Protonation of the radical anion followed by addition to the benzene radical cation can be considered as an alternative route to phenylsuccinic anhydride. 

Upon irradiation of a,a,a-trifluorotoluene and 1,3-dioxole in dioxane, two ortho cycloadducts and two meta cycloadducts are formed, next to small amounts of substitution products and some unidentified products. Mattay et al. [15] have proposed a mechanism in which an exciplex may be the common precursor of both types of adducts (Scheme 41). Formation of one bond within the exciplex gives rise to zwitterions in which the negative charge is optimally stabilized by the CF3 group. The formation of the second C—C bond then leads to the 1-substituted endo-and exo-ortho adducts. The meta adducts, which both have the exo configuration, may be formed either by simultaneous formation of two bonds in the exciplex or by an alternative bond formation in the zwitterion that also leads to the endo-ortho adduct. 

Scheme 41 Formation of ortho and meta cycloadducts from ot,ot,ot-trifluorotoluene and 1,3-dioxole via exciplex and zwitterions.

Rhodium (Il)-catalysed 3 + 2- and 3 + 4-cycloaddition of diazocarbonyl compounds (7) with conjugated dienes (e.g. 2,3-dimethylbuta-l,3-diene) provides a simple and rapid route to dihydrofurans (8) and dihydrooxepins (9) in high yields. A stepwise mechanism involving delocalized zwitterions has been proposed for the formation of the cycloadducts (Scheme 2).3... 

Besides the common oleftnic dipolarophiles, other unsaturated systems have been evaluated in cycloaddition reactions of zwitterionic TMM-Pd complexes, including polyenes and acetylenes. While acyclic electron-poor dienes generally gave mixtures of five- and seven-membered rings [48], a limited number of selective [3 + 4] and [3 + 6] cycloaddition reactions have been achieved with cyclic polyenic substrates as illustrated by formation of cycloadducts 41 and 42 from pyrone [49] and tropone [50], respectively (Scheme 16). On the other hand, activated alkynes have failed to produce the corresponding cyclopentene derivatives [51]. 

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