Propellanes reactivity

The propellanes are highly reactive substances which readily undergo reactions involving rupture of the central bond. It has been suggested that the poh erization of propellanes occurs by a dissociation of the central bond ... 

Somewhat surprisingly perhaps, it has been found that [l.l.l]propellane is considerably less reactive than [2.2.1]propellane. Use the theoretically calculated enthalpy data below to estimate the bond dissociation energy of the central bond in each of the three propellanes shown. How might this explain the relative reactivity of the [1-1.1]- and [2.2. Ijpropellanes ... 

The strained hydrocarbon [1,1,1] propellane is of special interest because of the thermodynamic and kinetic ease of addition of free radicals (R ) to it. The resulting R-substituted [ 1.1.1]pent-1-yl radicals (Eq. 3, Scheme 26) have attracted attention because of their highly pyramidal structure and consequent potentially increased reactivity. R-substituted [1.1.1]pent-1-yl radicals have a propensity to bond to three-coordinate phosphorus that is greater than that of a primary alkyl radical and similar to that of phenyl radicals. They can add irreversibly to phosphines or alkylphosphinites to afford new alkylphosphonites or alkylphosphonates via radical chain processes (Scheme 26) [63]. The high propensity of a R-substituted [1.1.1] pent-1-yl radical to react with three-coordinate phosphorus molecules reflects its highly pyramidal structure, which is accompanied by the increased s-character of its SOMO orbital and the strength of the P-C bond in the intermediate phosphoranyl radical. 

The reaction of EtsSiH with [l.l.l]propellane under photolytical decomposition of di-tert-butyl peroxide afforded products 17 and 18 in 1 3 ratio (Reaction 5.15) [36]. A rate constant of 6.0 x 10 M s at 19 °C for the addition EtsSi radical to [l.l.l]propellane was determined by laser flash photolysis [37]. Thus, it would appear that [l.l.l]propellane is slightly more reactive toward attack by EtsSi radicals than is styrene, and significantly more reactive than 1-hexene (cf. Table 5.1). 

Although the bonding is unusual, the central bond in A [l.l.l]propellane has 0.8 times the electron density at the center of the C—C bond in butane. There is a significant amount of electron density near the bridgehead carbons, and this presumably is responsible for the high reactivity of the compound toward electrophiles... 

There often is a correlation between the strain relief in a reaction and the rate of thermolysis, but other factors may also be of importance. The Cg propellanes 16 and 17 have quite different reactivities. Whereas 16 undergoes thermal cleavage at 360 17 undergoes cleavage at 25 °C. ° Aside from the difference in... 

Employment of the less sterically hindered yttrocene catalyst [(Cp )2YMe]2 or the more reactive zwitterionic zirconocene catalyst Cp 2ZrMe(/x-Me)B(C6E5)3 allowed cascade cyclization/hydrosilylation of trienes that possessed one or more 1,1-disubstituted alkene. As examples, reaction of 2-(3-butenyl)-l,6-hexadiene and phenylsilane catalyzed by [(Gp )2YMe]2 gave silylated spirocycle 74 in 88% yield. Likewise, the reaction of the dialkenyl alkylidene cyclopentane 75 gave silylated propellane 76 in good yield (Equations (50) and (51)). 

The central bond of [l.l.l]propellanes (1) is the center of their reactivity, and in many ways, it is useful to think of it as somewhat akin to the n bond in an alkene. The strengths of the two are comparable, and both are susceptible to electrophilic and radical attack. The main difference is that the central bond in la is apparently somewhat susceptible to nucleophilic attack as well, whereas the n bond in unsubstituted ethylene is not. In both cases, introduction of electron-withdrawing groups enhances reactivity towards nucleophiles. 

The versatile functionality pattern of bicyclic MO-acetal-y-lactams (found in conjunction with their ir-face-selective alkylation) can also be applied to Diels-Alder reactions of the corresponding alkenic methoxycarbonyl-activated derivative (427) (Scheme 102). ° Noncatalyzed addition of 2,3-dimethylbu-tadiene to dienophile (427) (60 C, 8 h) proceeded exclusively from the ir-face opposite the isopropyl substituent. The reactivities of the latent immonium and carbonyl groups in adduct (428) were exploit during transformation into [l,3,4)propellane (434). 

Within the framework of reactions of various propellane substrates with very reactive dienophiles, N-methyl- and iV-phenyl-triazolinediones, l,6-methano[10]annulene itself and many bridge- and ring-substituted derivatives were subjected to such Diels-Alder reactions. All of the products are (more complex) [4.4.1]propellane derivatives. In general it is clear that the rates of reaction are very much slower than those of various tetraenic. 

The propellanes differ greatly in their reactivity with acetic acid. The relative rates of cleavage are in the following order [3.2.1]propellane > [4.2.1]propellane > bicyclo[2.1.0]pentane > [3.3.1]propellane. In addition to the cleavage reactions shown in Table 9 te-tracyclo[4.2.1.1. 0 ]decane reacts at its quaternary carbon atoms with dimethyl acetylenedicarboxylate to give mono- and bisadducts. ... 

A less reactive bicyclo[3.2.0]hept-l(7)-ene substructure has been found in 164 (149). This compound was prepared from propellane 163 by photolysis and subsequent reduction. In contrast to 163, irradiation of the parent tricyclic ketone 165 in ethanol gives the saturated ketone 167. The formation of 167 is taken as evidence for the intermediate formation of the ketone 166. When 168 was treated with CsF in the presence of 1,3-diphenylisobenzofuran, an adduct was isolated, which was assigned the structure of the expected [2 + 4]cycloaddition product of 169 (150). 

Centropolyindanes constitute a complete family of arylaliphatic polycyclic hydrocarbons containing several indane units. Mutual fusion of the five-membered rings leads to three-dimensional, carbon-rich molecular frameworks bearing a central carbon atom, such as benzo-annelated [3.3.3]propellanes, triquinacenes, and [5.5.5.6]- and [5.5.5.5]fenestranes. In this review, the structural concept of centropolyindanes is contrasted to other fused indane hydrocarbons. Besides the syntheses of the parent centropolyindanes and recently described related indane hydrocarbons, the preparation of a large variety of bridgehead and arene substituted centropolyindanes is presented including strained, heterocyclic, and centrohexacyclic derivatives. In appropriate cases, the particular reactivity and some structural features of these unusual, sterically rigid polycyclic compounds are pointed out. 

Surprisingly, [l.l.ljpropellane is somewhat more stable to thermal decomposition than the next larger propellane, [2.1.1]propellane, indicating a reversal in the trend of increased reactivity with increased strain. To understand this observation, it is important to recognized that the energy of both the reactant and intermediate influence the rate of unimolecular reactions that lead to decomposition. In the case of propellanes, homolytic rupture of the central bond is expected to be the initial step in decomposition. This bond rupture is very endothermic for [l.l.ljpropellane. Because relatively less strain is released in the case of [l.l.ljpropellane than in the [2.1.1J- and [2.2.1J-homologs, [l.l.ljpropellane is kinetically most stable. ... 

Both radicals and electrophiles react at the bridgehead bond of [l.l.l]propellane. The reactivity toward radicals is comparable to that of alkenes, with rates in the range of 10 to 10 M s depending on the particular radical. For example, [l.l.l]propellane reacts with thiophenol at room temperature. ... 

Miscellaneous Reactions of Phosphines.- Theoretical treatments have appeared of a number of strained heterocyclic phosphine systems, notably phospha[3]radiaIene (148), triphospha[l, 1, l]propellane (149), and the tetraphosphacubane system (150). The reactivity of tetrakis-r-butyltetraphosphacubane has been studied in superacid media, with respect to protonation, alkylation, and alkynylation. Stable monophosphonium ions have been characterised. ... 

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