Bicyclo l-

The above assumes that C protonation is not excluded for steric reasons. Thus N protonation takes place with derivatives of dehydroquinuclidine and the alkaloids neostrychnine and trimethylconkurchine (( ). N protonation was also believed to occur in the case of 2-N-hexamethyleneimino-bicyclo[l,2,2]-2-heptene, which was believed to give the nortricyclene derivative (6) on protonation with perchloric acid. Later work, however, showed the salt to be the results of C protonation (15) and to have structure 7. 

In a modified procedure the free carboxylic acid is treated with a mixture of mercuric oxide and bromine in carbon tetrachloride the otherwise necessary purification of the silver salt is thereby avoided. This procedure has been used in the first synthesis of [1.1.1 ]propellane 10. Bicyclo[l.l.l]pentane-l,3-dicarboxylic acid 8 has been converted to the dibromide 9 by the modified Hunsdiecker reaction. Treatment of 9 with t-butyllithium then resulted in a debromination and formation of the central carbon-carbon bond thus generating the propellane 10." ... 

Zur Herstellung von Bicyclo[l.l.l]pentan wird von 1-Brom-l-brommethyl-cyclobutan ausgegangen4 ... 

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... 

It is well known that bicyclo[l. 1. OJbutanes are easily converted into the corresponding 1,3-butadienes under extremely mild conditions by treatment with various transition metal catalysts35. In fact, treatment of naphtho[l,8]tricyclo[4.1.0.02 73hep)tene (43)... 

Summing up these results, it has been shown that the carbene bridgehead olefin-carbene rearrangement is also observed when diazoalkanes are the precursors for the generation of the bicyclo[l.l.l]pent-l-ylcarbenes of type 52. 

Sodium, with l-bromo-3-chloro-cyclobutane to give bicyclo [l.l.O]butane, 51, 55 Sodium amalgam, 50, 50, 51 Sodium amide, with 2,4-pentane-dione and diphenyliodonium chloride to give l-phenyl-2, 4-pentanedione, 51, 128 Sodium azide, 50, 107 with mixed carboxylic-carbonic anhydrides, 51, 49 Sodium borohydride, reduction of erythro-3-methanesulfony-loxy-2-butyl cyclobutanecar-boxylate, 51, 12 reduction of 2-(1-phenylcyclo-pentyl)-4,4,6-trimethyl-5,6-dihydro-1,3(4H)-oxazine to 2-(1-phenylcyclopentyl)-4,4, 6-trimethyltetrahydro-l,3-oxazine, 51, 25 Sodium cyanoborohydride, used... 

One of the most fascinating transformations of free carbenes, generated for instance by photolysis of diazoalkanes or by a-elimination, is their insertion into aliphatic C-H bonds. This ability of carbenes is not only of theoretical interest, but also a unique tool for the synthesis of highly strained compounds such as, e.g., bicyclo[l. 1.0]butanes. 

The conversion of l-bromo-3-chlorocyclobutane to bicyclo-[l.l.OJbutane is described in Organic Syntheses.n... 

Bicyclo-[l, 2, 2]-heptane-2,3-diol Cyclopentane-1,3-dicarboxylic acid B [33]... 

Homocyclic Systems. Cyclobutanediyls exhibit spin states that are very close in energy. The triplet state is preferred by only l.Vkcal mol it can be observed by EPR spectroscopy because the ring closure to a bicyclo[l. 1. Ojbutane is spin-forbidden. Singlet cyclobutanediyls are predicted as very short-lived transition states for the ring inversion of bicyclo[1.1.0]butanes (AF 50kcal mol ). Quantum chemical calculations predict that the heavier group 14... 

Furthermore, this approach has been applied to the synthesis of substituted bicyclobutanes. For example, tetra-substituted dibromocyclopropanes carrying one or more aromatic substituents provide the diphenyl-bicyclo[l. 1. OJbutanes shown when treated with methyllithium. 

The ultraviolet spectra of saturated hydrocarbons are usually not very informative, and are dominated by Rydberg transitions.101 They correspond to the formation of a radical cation with the ejected electron being captured in a diffuse, atomlike orbital. Bicyclo[l.1. OJbutane is the most extensively studied molecule of this group, and the Rydberg nature of the excited... 

Figure 10. Density difference plots for some excited states of bicyclo[l. 1. Ojbutane in each case the molecule is drawn to scale (reproduced from ref 102 with the permission of the American Chemical Society).

As can be seen, both routes lead to the same product. Nevertheless, in compounds where the conformational flexibility of the bicyclobutane frameworks is restricted, 1,3-addition is found to be favored via the diequatorial mode.12,13 This aspect is illustrated in a reaction in which iodine reacted with tricyclo[4.1.0.02,7]heptane in carbon tetrachloride to give 6,7-diiodobicy-clo[3.1. l]heptane (9) in 55% yield.13 In support of this stereochemistry, the majority of results obtained from the dculeration of tricyclo[4.1.0.02,7]heptane suggested that the attack is from the equatorial position.14 Theoretical studies also support the notion that the equatorial approach of an electrophile to the bridgehead of bicyclo[l.1.0Jbutane is thermodynamically favored.14,15... 

Bicyclo[l. 1.0]butane-l-carbonitrile (1 g. 13 mmol) was dissolved in a 0.2 M solution of either NaOMc in MeOH or i-PrONa in i-PrOH (25 mL) and the mixture was left overnight at 50 "C. The alcohol was evaporated and the crude product separated by preparative GC yield 35 % two isomers (ratio not reported). 

In the absence of solvent, only syn stereoselectivity and not stereospecificity was observed for the nucleophilic addition of secondary amines such as dimethylamine, diethylamine, piperidine and morpholine to bicyclo[1.1.0]butane-l-carbonitrile. However, similar addition to 3-methylbicy-clo[l. 1.0]butane-l-carbonitrile was more stereoselective.22 However, the addition of sodium methancthiolate to 3-methylbicyclo[1.1.0]butane-l-carbonitrile, methyl bicyclo[l.l.0]butane-1-carboxylate, methyl 3-mcthylbicyclo[1.1.0]butane-l-carboxylate and methyl tricyclo[4.1.0.02 7]-heptane-l-carboxylate also afforded mixtures of cis- and p-rms-cyclobutancs in > 50% yield, with methyl bicyclo[1.1.0]butane-l-carboxylate showing greater anti stereoselectivity.23,24... 

Exhaustive examples have demonstrated that 1,3-disubstituted bicyclo[l.l.l]pentanes can be conveniently prepared by radical addition to [l.l.l]propellane.39-43,47 Of particular interest is the peroxide-catalyzed [( -BuO)2, hv] addition of acetaldehyde to [1.1, l]propellane, which gives the 2 1 adduct l-acetyl-3-(1-hydroxyethyl)bicyclo[l,l.l]pentane (25) in a transformation pre-... 

Table 11. 1,3-Disubstituted Bicyclo[l.l.l]pentanes by Radical Addition to [l.l.l]Propellane...

---