Propellane cation

Bridgeheads. The Sn2 mechanism is impossible at most bridgehead compounds (p. 392). Nucleophilic attack in [l.l.l]propellane has been reported, however. In general, a relatively large ring is required for an SnI reaction to take place (p. 396). " The SnI reactions have been claimed to occur for l-iodobicyclo[l.l.l]pentane via the bicyclo[l.l.l]pentyl cation, but this has been disputed and the bicyclo[1.1.0]butyl carbinyl cation was... 

The oxidation of 3,6-dehydrohomoadamantane (52) with NO+BF4, photo-excited tetracyanobenzene, and under anodic conditions has been found to involve a common radical cation intermediate. The study has shown that the activation of propellane cTc-c bonds with strong oxidizing electrophiles occurs by a sequence of single-electron transfer steps. These findings are supported by ab initio computations showing that the isomeric radical cations can equilibrate with low barriers and lead to a common product. ... 

Figure 11. Density difference plots for some hydrocarbon radical cations. The compounds are A cyclopropane, B bicydo[1.1.0]butane, C bicycio[l.l,l]penlane, and D [l.l.l]propellane (reproduced from ref 108 with the permission of the American Chemical Society).

These spectroscopic techniques tie the states of [1.1.1 ]propellane to those of its radical cation and radical anion and are usually interpreted as semiquantitative indicators of the nature of occupied and unoccupied molecular orbitals of the neutral species, respectively, through the use of Koopmans theorem. 

An unresolved puzzle remains22 the first two vibrational peaks in the first band in the photoelectron spectrum are separated only by 360 20 cm4, less than the lowest frequency vibration observed in neutral propellane (529 cm4), and very much less than its lowest totally symmetric vibration (908 cm 1). Yet, the authors calculations22 suggest that the lowest frequency totally symmetrical vibration of the radical cation will be at higher and not lower frequencies. The authors suggested that the vibrational structure may be due to vibronic mixing with the lowest excited state of the radical cation. 

Before proceeding to a discussion of the reactions of the central bond of [1.1.1] propellane, it will be useful to describe the properties of the first-formed intermediates, the bicyclo[l, l.l]pent-l-yl anion 16, radical 6, and cation 19. 

In some electrophilic reactions of 1 the bicyclic ring system is preserved. It has been postulated that in the reaction of l,3-diiodobicyclo[l.l.l]pentane (15b) with sodium methox-ide, [1.1.1 [propellane generated in situ reacts with iodine in the presence of pyridine67 132 or with MeOI,66 an I+ source also generated in situ, to give the 3-iodobicyclo[l.l.l]pent-l-yl cation 6267 90. This cation is subsequently trapped with a nucleophile such as MeOH, N3, pyridine or triethylamine to give 63 (Table 15)66 67132. 

Reaction of [1.1. l]propellane with Hg(OAc)2 also gives the methylenecyclobutyl cation which was trapped with the acetate anion to give 59 ... 

Despite these modifications, Ireland s original method of both vinyl phosphate formation and reduction remains the most popular. In studies on the cationic rearrangements of [4.3.2]propellanes, Smith et converted a tricyclic ketone, via the diethyl vinylphosphate derivative, into the corresponding alkene (Scheme 26). In a similar manner, Kamata et al prepared A - and A -steroids (Scheme 27) with excellent control of regiochemistry. As shown in this example, esters are susceptible to cleavage under the standard reduction conditions (Li, NHa, Bu OH, -35 C), while acetals survive. 

Solvolysis of a series of benzo[4.3.1]propellane p-nitrobenzoates substituted in the aromatic ring is interpreted in terms of a homonaphthalenium cation. ... 

This is mainly the work of Philip Warner who also (cf. Ref. 44) appears to have been introduced into the field of propellanes by Saul Winstein in work concerned with the protonation of l,6-methano[10]annulene, work in which they ruled out the presence of a propellane-type cation". ... 

In the full paper on [3.2.1]propellane several additional results were reported. Bromination in CH2CI2 at — 50°C afforded not only the dibromide 72. This was accompanied by a chloro-bromo derivative (73) indicating that a bromo-bridgehead radical or cation intermediate then either reacts with bromine or abstracts chlorine from the solvent. The Wiberg group also reported a new approach, from the bridgehead dibromide to the parent propellane as shown in the accompanying scheme. 

Figure 7.48 (a) The 4.6-net sheet in [d-Co(en)3][AI3P4Oj6] 3H20. The [3.3.3] propellane-like chiral motif (A configuration) is also shown, (b) The Co(en)33+ cation with A configuration occluded in the interlayer region. Reproduced with permission from [91]. Copyright (2003) Wiley-VCH... 

Oxidative decarboxylation of [n.2.2]propellane carboxylic acids lla-c with lead tetraacetate in benzene at 80 °C for 1 hour gave bicyclic acetates 12 and/or the tricyclic acetates 13, depending on the ring size. It is assumed that the formation of a cyclobutyl cation is responsible for this oxidative rearrangement. The product formation is apparently determined by the relative... 

C-1. The product distribution was found to depend upon the amount of water present in the system, more 6-(bromomethylene)cyclodecanone being obtained with less water present. This observation was explained in terms of capture of the carbonium ion formed from (56 = 2). There would seem to be no examples yet where an intramolecular shift process competes with ring-opening of a cyclopropyl cation. Solvolysis of 10,10-dibromo[4,3,l]propellane in acetic acid-sodium acetate-acetic anhydride gave products formed via the strained bicyclic acetate (56 n = 1, R = Ac). In the absence of acetic anhydride products derived from the corresponding alcohol (56 n = 1, R — H) were obtained as well. Solvolyses of 10,10-dibromo[4,3,l]propell-3-ene in acetic acid-sodium acetate, in the presence and absence of acetic anhydride, and in acetic acid-silver perchlorate, were also reported. Most of the products may have been formed by reaction pathways similar to those observed for (55). ... 

The NOs- radicals can also be generated by flash photolysis of CAN in acetonitrile (Del Giacco et al., 1993). These photochemically produced nitrate free radicals have been used to study the one-electron oxidation of methylbenzenes. Depending on the oxidation potential of the substrate, radical cations or benzyl-type neutral radicals were formed. Formation of radical cations was observed for most of the methylbenzene derivatives that were studied, whereas formation of the benzyl radical was observed with toluene and with ortho- and meta-xylene. This reaction is not useful for synthetic applications. The reactivity of propellane C-C bonds towards free nitrate radicals photochemically generated from ammonium hexanitrato-eerate(IV) in acetonitrile was studied from both theoretical and experimental points of view (Fokin et al., 2000). Baciocchi and coworkers reported that the photochemical reaction of CAN in acetonitrile with cyclohexene, 1-octene and styrene derivatives leads to the formation of 1,2-dinifrate adducts in high yields (scheme 65) (Baciocchi et al., 1988b). 

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