Cyclopropane radical anion

Electron transfer to another molecule of starting cyclopropane gives the product and another cyclopropane radical anion to continue the chain. 

This chapter deals with some of the reactive intermediates of cyclopropanes—radicals, anion radicals and anions. It is hoped that the reader will appreciate that the cyclopropane ring, because of its unique bonding, affords one with a tool to study the mechanism of a variety of reactions. The mechanism of many of these reactions will be discussed in some detail in this chapter. It should also be noted that whenever possible stereochemistry has been used as a mechanistic probe. Pertinent literature has been reviewed through most of 1985. 

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 preparation of the cyclopropane radical anion was published in 1963 " However, in 1966 it was reported that upon failing to repeat the earlier results there is no adequate basis for further discussion of the species previously observed Cyclopropanes with... 

Another stable cyclopropane radical anion was possibly observed by Papa " when he reacted the cyclopropane 130 with nucleophiles such as potassium iodide, potassium cyanide and triethylamine. 

Other reversible ET-catalyzed stereoisomerizations of cyclopropanes have been observed with cjs-l-methyl-2-phenyl-, r-1-phenyl-l-methyl-c-2-methyl- and optically active l-methyl-2,2-diphenylcyclopropane (190, 191 and ( + )-(/ )-49, respectively) . Experimental evidence for the existence of intermediate cyclopropane radical anions like CIS- or trans-llH" (Scheme 18) has not been found in the course of these investigations. 

Stereoisomerization cis-78a and trans-79a accept an electron to give the diphenyl-cyclopropane radical anions cis-78a and trans-79a which rearrange reversibly into the trimethylene radical anion 80 a. The stereoisomerization takes place at the trimethylene radical anion stage. The reversible formation of 80a in THF is noteworthy because the trimethylene radical anions 46 and 47 have been formed irreversibly, however in this case in NH3, see Scheme 3, page 12. 

Biradicals have also been encountered as intermediates in the Mg reduction of ketones to pinacols (p. 218) and, as radical anions, in the acyloin condensation of esters (p. 218). The thermolysis of cyclopropane (131) to propene (132) at 500° is also believed to involve... 

In qualitative terms, the rearrangement reaction is considerably more efficient for the oxime acetate 107b than for the oxime ether 107a. As a result, the photochemical reactivity of the oxime acetates 109 and 110 was probed. Irradiation of 109 for 3 hr, under the same conditions used for 107, affords the cyclopropane 111 (25%) as a 1 2 mixture of Z.E isomers. Likewise, DCA-sensitized irradiation of 110 for 1 hr yields the cyclopropane derivative 112 (16%) and the dihydroisoxazole 113 (18%). It is unclear at this point how 113 arises in the SET-sensitized reaction of 110. However, this cyclization process is similar to that observed in our studies of the DCA-sensitized reaction of the 7,8-unsaturated oximes 114, which affords the 5,6-dihydro-4//-l,2-oxazines 115 [68]. A possible mechanism to justify the formation of 113 could involve intramolecular electrophilic addition to the alkene unit in 116 of the oxygen from the oxime localized radical-cation, followed by transfer of an acyl cation to any of the radical-anions present in the reaction medium. 

The above results lead to the conclusion that rearrangement of 124 to the cyclopropane 125, occurring in the reactions sensitized by DMA, must take place via radical-anion intermediates. Considering that the C—double bond in compounds 101, 117, 118, and 119 should be a better electron acceptor than the diphenylvinyl unit in 124, it is logical to assume that the rearrangement of the 1-azadienes also takes place via the same types of radical-ion intermediates. 

Almost all reactions of alkylidenecycloproparenes lead to opening of the cyclopropane ring. A notable exception to this is the reversible electrochemical reduction of237 and 240 which leads to the stablfe radical anions 396 and 397, with half-wave potentials of -2.32 and -1.93 V, respectively, and their oxidation to the quasi-stable radical cations 398 and 399 ( i/2(ox) = +0.68 and +0.81). The cations may be further oxidized to the corresponding very short-lived dications. In contrast, the photoelectron spectra of 237 and 240 reveal practically identical first-oxidation potentials of both compounds, which indicates that the difference in half-wave potentials for oxidation (in condensed phase) of 237 and 240 does not exist in the gas phase. This has been attributed to structure-specific solvation energies in the radical cations 398 and 399. °... 

In addition to nucleophilic capture of alkene or cyclopropane radical cations (see above) radicals may be generated by cleavage of C—X bonds, particularly C—Si bonds. Such cleavage is often assisted by a nucleophile. Because the radical is generated near the radical anion, to which it couples, the resulting C—C bond formation may be considered a reaction of a modified radical (ion) pair. 

We have mentioned that the structural parameters of C2H4 bridged compounds can vary over a wide range. Whereas most examples reported do not have metal-metal bonds, there is one conspicuous exception. Theopold and Bergman succeeded in synthesizing the propane-1,3-d iyl cobalt derivative 125 from the radical anion [(t) ,-C5H5)Co(/z-CO)12 and 1,3-dibromopropane (98, 295) in 40 5 yield. This compound is best described as a dimetallacyclopentane, and its chemistry (thermolysis and reaction with CO and phosphines Scheme 34) supports this view. Formation of cyclopropane (100°C or I2/25°C) is probably the most remarkable feature of this cyclic system. Simple C—C bond formation has never been observed before in ligand-induced or thermal reactions of either mono- or binuclear cyclopentadienylcobalt complexes. The architectural details of... 

Within the area of SET-promoted di-7t-methane reactions, recent studies have shown that irradiation of 1-aza-1,4-dienes, such as 32, and the 1,4-diene 34, using AW-dimethylaniline (DMA) as electron-donor sensitizer, leads to production of the corresponding cyclopropane derivatives 33 and 35 resulting from 1-ADPM and DPM rearrangements, respectively, in reactions that take place via radical-anion intermediates (Sch. 11) [25]. 

The reaction mechanism involved the cyclisation of ketyl radical anion 3 on to the methylenecyclopropane moiety in a 5-exo-trig manner. Ring opening of cyclopropane intermediate 4 gave rise to the cyclohexyl radical 5, which then cyclised in a 5-exo-dig fashion to form the second ring (Scheme 6.2).5,6... 

Tazuke reported the carboxylation of the radical anions of aromatic hydrocarbons that are generated by photoinduced electron- transfer from the tertiary amines to the excited singlet aromatic hydrocarbons (Scheme 36) [119]. Toki and his coworkers reported the photofixation of COj with styrene using tertiary amines as electron donors [120]. Tomioka reported the photoaddition of tertiary amines to electrophilic cyclopropanes [115]. 

The details of bonding and spectroscopic properties of cyclopropane derivatives have recently been reviewed.In addition, the properties and energetics of cyclopropyl cations, anions, radicals, anion radicals and cation radicals have been amply reviewed, and comparisons have been made with their corresponding alkenic counterparts. 

I) by oxidation with m-chloropcrbcnzoic acid, with 2 equivalents of lithium in liquid ammonia gives the alcohol (3) in 90% yield. In the same way, the epoxide (4) is converted into (5) in 95 % yield. In both cases (2) and (4) the cyclopropyl bond common to ihe two phenyl substiluents is ruptured with marked kinetic preference to give a radical anion, which is attacked by the proximate C-0 bond to form a new cyclopropane ring. The cyclopropane ring of (3) and of (5) is presumably protected from further reduction by dectrostatic factors. 

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