Cyclopentadienate radical

One method of producing penta(methoxycarbonyl)cyclopentadiene radical is by irradiation of [SnBu3(OH2)2]+[C5(C02Me)5]" (reaction 36)58. The radical thus produced is remarkably stable, with the ESR signal not decreasing in intensity one hour after cessation of photolysis. 

Clavulones I and II are members of an unusual family of marine prostanoids from the coral Clavularia viridis which are biosynthesiied by a cationic (i.e. non-radical, non-endoperoxide) pathway. The total synthesis of clavulones I and II was accomplished from cyclopentadiene as SM goal. 

Active Figure 15.4 Generating the cyciopentadienyl cation, radical, and anion by removing a hydrogen from cyclopentadiene. Sign in afwww.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

In practice, both the cyciopentadienyl cation and the radical are highly reactive and difficult to prepare. Neither shows any sign of the stability expected for an aromatic system. The six-77-electron cyciopentadienyl anion, by contrast, is easily prepared and remarkably stable. In fact, cyclopentadiene is one of the most acidic hydrocarbons known, with p/C, = 16, a value comparable to that of water Cyclopentadiene is acidic because the anion formed by loss of H+ is so stable (Figure 15.5). 

The photopolymerization of this monomer with a mercury arc89,9°) produces small yields of low molecular-weight products. In the presence of oxygen an induction period is noted and the polymers contain an appreciable amount of peroxide units in the chains9 ). The photolysis of 2-vinylfuran was briefly described by Hiraoka92 cyclopentadiene and CO were reported as products. It is not certain if free radicals are involved in this photodecomposition, but presumably they are. 

The relative amounts of double bond addition, hydrogen abstraction and 13-scission observed are dependent on the reactivity and concentration of the particular monomer(s) employed and the reaction conditions. Higher reaction temperatures are reported to favor abstraction over addition in the reaction of t-butoxy radicals with AMS413 and cyclopentadiene 417 However, the opposite trend is seen with isobutylene.2 1 24... 

As a continuation of these studies, Bauld recently reported evidence of a stepwise mechanism in the cation-radical Diels-Alder reaction of phenyl vinyl sulfide with cyclopentadiene [34, 35] (Scheme 1.6). 

An analogous stepwise mechanism was also proposed by Wohrle [36] for the cation-radical-initiated cycloaddition of electron-rich allenes with pentamethyl-cyclopentadiene in the presence of tris (p-tolyl) aminium hexafluoroantimonate (TTA SbF6 ) (Equation 1.15). 

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

Whitesides TH, Shelly J (1975) Thermolysis and photolysis of (cyclopentadiene)iron tricarbonyl. Evidence for a radical mechanism involving iron(I). J Organomet Chem 92 215-226 Shackleton TA, Mackie SC, Fergusson SB, Johnston LJ, Baird MC (1990) The chemistry of (r -C5H5)Fe(CO)2H revisited. Organometallics 9 2248-2253... 

The relative reactivity of cyclopentadiene and ds-dichloroethylene toward triplet cyclopentadiene was found to be greater than 20 1 while that for cyclopentadiene and trans-dichloroethylene is less than 5 1. Thus the trans isomer is about four times more reactive toward the triplet cyclopentadiene than the cis isomer. An interesting temperature dependence of the product distribution of this reaction has been reported (Table 10.8). The data in Table 10.8 indicate that the relative amount of 1,4 addition [products (39) and (40)] is much more sensitive to temperature than 1,2 addition [products (35)—(38)], especially for the trans-olefin. The data also indicate that some rotation about the CHC1-CHC1 bond occurs in intermediate radicals derived from both cis- and trans-dichloroethylene. However, rotational equilibrium is not established at ring closure since the ratios of ds-dichlorocyclobutanes... 

Table 3 lists all polyenes whose radical cations have been investigated by one or other of the above-described techniques and some of the structures listed are shown below the table. Note that some nonconjugated dienes do not retain their structure upon ionization [e.g. semibullvalene 104 (equation 61) or the cyclopentadiene dimers 126 and 294 (equation 62)] but break a bond to form a bisallylic radical cation, a rather common tendency of radical cations that have this possibility. 

The addition of tosyl cyanide to cyclopentadiene leads to intermediate formation of radical 27, which then is trapped by tosyl cyanide by cyano group transfer. The trans-... 

Photolysis of dicyclopentadienyltin results in formation of the Cp- radical (again detected by ESR), along with the precipitation of some unidentified yellow solid54. In contrast, photolysis of dicyclopentadienyllead produces no Cp-, unless di-f-butyl peroxide or biacetyl are added to the reaction mixture. The trimethylstannylcyclopentadienyl radical was produced by photolysis of bis(trimethylstannyl)cyclopentadiene (reaction 35), and was detected using ESR spectroscopy57. 

Diels-Alder adduct from cyclopentadiene, 8 222t Diels-Alder reactions of, 25 488-489 economic aspects of, 25 507-509 electrophilic addition of, 25 490 in ene reactions, 25 490 esterification of, 25 491 free-radical reactions of, 25 491 from butadiene, 4 371 Grignard-type reactions of, 25 491 halogenation of, 15 491—492 health and safety factors related to, 25 510-511... 

The formation of the bridged product 191 was investigated using the cyclopentadiene system as a model. Thus, the salt of the tosylhydrazone 198 was prepared and thermolyzed in order to examine three possible variants of rearrangements (equation 62)75. Analysis of the reaction products 200-202 and their transformations [e.g. the pyrolysis of bicyclic triene 202 to cA-8,9-dihydroindene 203 (equation 63) rather than to product 200 or 201] allows one to conclude that the mechanism involves a transformation of carbene 188 into diradical 204 which can be the precursor of all the products observed (equation 64)75. An analogous conversion takes place via radical 205 in the case of carbene 199 (equation 65). 

High-temperature flow-reactor studies [60,61] on benzene oxidation revealed a sequence of intermediates that followed the order phenol, cyclopentadiene, vinyl acetylene, butadiene, ethene, and acetylene. Since the sampling techniques used in these experiments could not distinguish unstable species, the intermediates could have been radicals that reacted to form a stable compound, most likely by hydrogen addition in the sampling probe. The relative time order of the maximum concentrations, while not the only criterion for establishing a mechanism, has been helpful in the modeling of many oxidation systems [4,13]. 

Heptachlor is produced commercially by the free-radical chlorination of chlordene in benzene containing from 0.5% to 5.0% of fuller s earth. The reaction is run for up to 8 hours. The chlordene starting material is prepared by the Diels-Alder condensation of hexachlorocyclopentadiene with cyclopentadiene (Sittig 1980). Technical-grade heptachlor usually consists of 72% heptachlor and 28% impurities such as trans-chlordane, cis-chlordane, and nonachlor (HSDB 1990a). 

Observation of the list of borazine derivatives produced photochemically (Table 3) reveals that all of the reagents yielding photochemical products have an electronegative element (either 0,N,F,C1, or Br) which substitutes at the boron site. No B-C bonds have been formed photochemically, even when, as in the CH3Br and the HFA reactions, methyl or perfluoromethyl radicals are present Also, no B-C bonded compounds were formed when borazine was photolyzed with benzene, pyridine, cyclopentadiene, allene, ketene, acetylene, or acetone. 

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