Cyclopropane dianion

Electron transfer to cyclopropane should lead to the cyclopropane radical anion which, in principle, can isomerize to the ring-opened trimethylene radical anion. Further reduction of the trimethylene radical anion should give a 1,3-dianion. A less likely two-electron transfer to cyclopropane could conceivably give the ring-opened 1,3-dianion via the corresponding cyclopropane dianion. 

The formation of the 1,3-dianions 180" from the corresponding cyclopropane dianions would correspond exactly to these reactions. If the dianions 180" were intermediates in the ET-catalyzed stereoisomerization of the cyclopropanes the reverse reaction—formation of the cyclopropanes 178 from the dianions 180"—should also take place. Preparation of the 1,3-dianion 180a"2K (Li ) from the cyclopropane cis-178a with Na/K alloy or with lithium at -78°C shows clearly that 180a" (Li" ) is stable at — 78°C (Scheme 19). 

It seemed unlikely that, instead of the radical anions 46 and 47, the corresponding l,3- dianions had been formed directly from their common cyclopropane dianion precursor. This, however, has not been rigorously excluded by means of these experiments. 

Attempts to exploit the reaction of the dianion with alkyl halides to produce a c/.v-dialkyl complex by using 1,2- or 1,3-dihaloalkanes did not indeed give this result. The reaction of Ru(Por) " with 1,2-dibromoethane was sucessful, but the resulting metallacyclopropane product is better formulated as a /r-complex of ethene, and will be discussed below in the section on alkenc and alkyne complexes. The corresponding reaction of the diiinion with 1,3-dichloropropane gave no evidence for a metallacyclobutane. but instead free cyclopropane was detected by GC analysis and the porphyrin product was Ru(TTP)(THF)2. ... 

The two-step reduction of tris(9-fluorenylidene)cyclopropane 2731 and of hexakis(tri-methylsilylethynyl)cyclopropane 5226a requires increasingly more negative potentials than in the cases listed in Table 1 this is, of course, a consequence of the presence of less electron-accepting substituents. Nevertheless, dianion 272 has been generated by reduction with sodium or lithium the lithium salt could be kept in THF solution for up to one year at 20 °C without detectable decomposition31. 

A total synthesis of the sesquiterpene ( )-illudin C 420 has been described. The tricyclic ring system of the natural product is readily quickly assembled from cyclopropane and cyclopentene precursors via a novel oxime dianion coupling reaction and a subsequent intramolecular nitrile oxide—olefin cycloaddition (463). 

Cyclopropanation of enones. The adduct (a) of 1 to cyclohexenone can be metallated at - 45° by sec-BuLi to give a dianion b, which when quenched at 0°... 

Fig. 3. 2-4. Radial (bottom) and tangential (top) bonding molecular orbitals for cyclopropane [12e] (left) and hexa-borane(6) dianion (right) illustrating the planar and three dimensional a aromaticity, respectively. Tangential orbitals of [B6H6]2 are triply degenerate.

Wade and co-workers used a similar strategy for the synthesis of fran5-l,2-dinitrospiro-pentane (10), which is prepared in 43 % yield by treating the dianion of l,l-few(nitromethyl)-cyclopropane (9) with iodine in DMSO the latter prepared by treating the corresponding diamine (8) with excess ozone while absorbed onto the surface of silica gel. 

Ullmann and Buncel attempted to deprotonate A -carene-2,5-dione (4) and the benzannelated derivative 5 in the hope of generating the possibly aromatic dianions 6 and 7. However, the dianions (and the monoanions) could not be isolated. On the grounds of deuterium exchange experiments, the strain energy of benzocyclopropene (1) was estimated to at least 45.5 kcal/mol higher than that of cyclopropane. 

The dianions of 1,2-dicarboxylates 13. prepared with LDA from the parent esters, react with dihalomethanes to form annulated cyclopropane derivatives 14101. 

Dianions of the above types may not fall into the category of homoenolate in a strictly formal sense. Nevertheless the amide dianion does show a behavior typical of the homoenolate. Thus, the reaction of the isotopically labeled stannylpropionate results in scrambling of the label probably via a cyclopropane intermediate Eq. (47) [44]. As the result of such an equilibration, isomerization of a-methyl and a-phenyl substituted propionate homoenolates may occur to give the thermodynamically more favorable isomers, respectively. 

The cyclic voltammograms show three reduction waves in the potential window between 0 and —1.3 V versus SCE (Fig. 6.20). The first process is fully reversible, while the second one is chemically irreversible since the dianion is subject to a bond breaking of the cyclopropane ring, known as the retro-Bingel reaction.75 The third... 

Baizer et al. (1972) extensively studied the electroreduction of carbon tetrachloride in the presence of electrophiles for synthetic purposes. Since then the formation of the dichloro-dianion (CCl ) or -carbene (Takita et al. 1990) was considered for the synthesis of cyclopropane derivatives. Recently, carbene formation was observed in the direct reduction of CC14 and DDT (1,1 -bis(p-chlorophenyl)-2,2,2-trichloroethane) on hemin-modified TiC>2 supports (Stromberg et al. 2006). 

Condensation of (-)-Dimenthyl Succinate Dianion with 1, u-Dihalides (-)-Dimenthyl Cyclopropane-(lS,2S)-1,2-dicarboxylate... 

In fact, only in some cases are real carbenoid species involved in these reactions. The reactions employing oxophilic metals such as Zn, Fe, Sm, and In were reported [9], and representative examples are given in Scheme 3. Such reactions exhibit obvious synthetic limitations, since only aromatic (unsaturated) carbonyl compounds can be used, and/or cyclopropanes of a very specific structure obtained. The Kulinkovich and related reactions, involving dianion equivalents (1, Scheme 1), represent the only synthetically useful deoxygenative process at present. 

Self-condensation of the dianion of cyclopropanecarboxylic acid (450) at 50 °C affords the electrophilic cyclopropane (451). Reaction of the dianion with other carboxylate salts. 

The stereoisomerization of the cyclopropanes 178 via dianions 180" (Scheme 18) is also rigorously excluded. A priori, this is not a totally unlikely pathway since Lagendijk and Szwarc have deduced from kinetics that it is the dianion that breaks the C-C bond in the reaction of electron sources with 1,2-di-a-naphthylethane to give a-naphthylmethyl anions. Similarly, Grovenstein and coworkers have recently found that bond cleavage in the reaction of 1,2-di-p-tolylethane with Cs/K/Na alloy occurs at the dianion stage. 

SCHEME 19. 1,3-Dianions are not intermediates in the ET-catalyzed isomerization of cyclopropanes... 

Cyclopropanation with dianion precursors a. Dianion precursors and a C(2) unit... 

Cyclopropanation of 209 by a formamide acetal represents a cyclopropane synthesis from a dianion and a C(l)unit . 210and 211 were formed ina6 4ratio (equation 51). No rearrangement of 210 and 211 could be detected on standing at 70°C in CeDg. 

Next, the most efficient route to (+)-7b (MGS0028), to date, via monoprotected fluo-rinated diol 43 and ketal 45 as key intermediates is discussed (see Schemes 3.6 and 3.7) [45]. Key intermediate 43 was synthesized from hydroxycyclopentenylacetate 39 in four steps (i) fluorination of the dianion of 39 (85% yield) to form 40, (ii) vanadium-mediated epoxidation of 40 to give epoxide 41 (93% yield), (iii) protection of the hydroxyl moiety of 41 to form TBS-ether 42 (95% yield), and (iv) cyclopropanation of 42 through ringopening of the epoxide moiety to afford key intermediate 43 (96% yield). It should be noted that the epoxidation of 40 proceeded with excellent stereoselectivity to give the... 

The cyclopropane stereoisomerization cis-78a trans-79a via the l,3-lidianiori" 81a (a two electron pathway) is also excluded. A priori, this is not a totally unlikely pathway. There are at least two reports in the literature dealing with a C—C bond fission at the dianion stage 63,64). The formation of the l,3- di-anion 81 a would exactly correspond to the observations reported in these publications (see Scheme 5). 

If 1,3- dianions of the typ 81 were intermediates in the ET-catalyzed reversible stereoisomerization of the cyclopropanes 78 and 79, the reverse reaction — formation of the cyclopropanes from such dianions — should also take place. Preparation of the dianion 81a from the cyclopropane cis-78a with Na/K or with Li at —78 °C shows clearly that 81 a is stable at this temperature — although cis/trans rearrangement of the cyclopropanes cis-78 and trans-79 takes place ... 

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