Bicyclo pentanes

Figure 2-37. Two different fundamental sets of rings for bicyclo[2.1.0]pentane.

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." ... 

Adamantan2 In einem 100-m/-Kolben mit Magnetriihrer legt man unter Stickstoff 25 ml Pentan und 4,1m/ (20mMol) 9-Butyl-9-bora-bicyclo[3.3.1]nonan vor. BeiO° werden 20 mMol Butyl-lithium in Hexan ( 14,4m/) zugegcben. Dann riihrt man 1 Stde. bei 20°, tropft 4,3 g (20 mMol) 1 -Brom-adamantan bei 0° zu und riihrt 12—15 Stdn. bei 20°. Dann wird oxidicrt. Man trennt die organ. Phase ab und filtriert durch eine Saule mit 40 g Alumi-niumoxid. Man eluiert mit 60 ml Hexan und zicht das Losungsmittel ab Ausbeute 2,43 g (89% d.Th.) F 212-215° (Subl.). 

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... 

The Diels-Alder reaction of isopyrazoles 365 with MTAD gives azoalkanes 366. Direct as well as triplet-sensitized (benzophenone) photolysis of these compounds leads to the corresponding housanes (bicyclo[2.1.0]pentanes) 367. Under acidic conditions, the housanes rearrange to the corresponding bicyclic products 368 <1995JOC308,... 

Claisen rearrangement. As for the mechanism, the reaction begins with intramolecular cyclopropanation the resulting bicyclo[2.1.0]pentan-2-one then undergoes fragmentation to a p,y-unsaturated ketene which finally is trapped by the added alcohol to afford a p,y-unsaturated ester (Scheme 41). The intermediates could be observed in selected cases. 

Dicyano-substituted triafulvenes react with enamines to produce exclusively the cross-conjugated dicyanomethylene compounds 519, whose formation can be rationalized by a methylene bicyclo(2,l,0)pentane intermediate 51879 296 Since cyclanone enamines 520 and other cyclic enamines 522 react analogously, this C-C-insertion 237) of the triafulvene ring skeleton into the enamine C=C bond represents a versatile ring expansion mode (C + C3), which makes accessible a series of unsaturated medium-ring compounds (521/523) that are otherwise difficult to synthesize. 

The validity of equation (12) has been checked for several families of alkyl halides for which D and E /x- are known (Ref. 32, see particularly figure 6 therein). It was thus found that for v = 0.1 V s the constant is equal to 0.3 eV at 20°C (expressing D in eV and the potentials in V). Equation (12) was then applied to the approximate determination of unknown BDEs in several series of compounds undergoing dissociative electron transfer, namely, TV-halosultams,32 sulfonium cations,33 vicinal dihalides,34 1,3-dihaloadamantes, 1,4-dihalo-bicyclo[2.2.2]octanes, and l,3-dihalobicyclo[l.l.l]pentanes.35 In the latter case, the mutual influence of the two halogens could be rationalized thanks to the conversion of the peak potential data to bond dissociation energies. 

Successive addition of methyllithium and methyl iodide to pentastannapropellane in pentane gives the bicyclo compound 5291 in two steps. 

C2S2-C3S 2,4-Dithia-bicyclo[ 1.1.1 ]pentane 2, 836 C3N-C3N 1-Azabicyclo[2.2.0]hexane As... 

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... 

Photochemical Synthesis of Bicyclo[1.1.1]pentane-1,3-dicarboxylic Acid. 

In solution, an initial photoequilibrium is established between the Z- and -isomers, while the rearrangement products 117 and 118 are formed along with traces of cyclohexadiene (CHD) over much longer irradiation times (equation 46). In solution, the major products are 3-vinylcyclobutene (117) and bicyclo[3.1.0]hex-2-ene (118) Z-l,2,4-hexatriene (119), which is a major product in the gas phase176,211, is formed in relatively low yields. The quantum yields for ,Z-photoisomerization of Z- and -l,3,5-hexatriene in pentane solution (265 nm excitation) are /, r = 0.034 and E—Z = 0.016, respectively188. 

The numerous transformations of cyclooctatetraene 189 and its derivatives include three types of structural changes, viz. ring inversion, bond shift and valence isomerizations (for reviews, see References 83-85). One of the major transformations is the interconversion of the cyclooctatetraene and bicyclo[4.2.0]octa-2,4,7-triene. However, the rearrangement of cyclooctatetraene into the semibullvalene system is little known. For example, the thermolysis of l,2,3,4-tetra(trifluoromethyl)cyclooctatetraene 221 in pentane solution at 170-180 °C for 6 days gave three isomers which were separated by preparative GLC. They were identified as l,2,7,8-tetrakis(trifluoromethyl)bicyclo[4.2.0]octa-2,4,7-triene 222 and tetrakis(trifluoromethyl)semibullvalenes 223 and 224 (equation 71)86. It was shown that a thermal equilibrium exists between the precursor 221 and its bond-shift isomer 225 which undergoes a rapid cyclization to form the triene 222. The cyclooctatetraenes 221 and 225 are in equilibrium with diene 223, followed by irreversible rearrangement to the most stable isomer 224 (equation 72)86. 

If all the pentane is removed before pyrolysis, the bicyclo [2.1.0]pentane shows no impurities on vapor phase chromatography with a 20% Dow 710 on 50/60 U Anaprep column. Analysis by n.m.r. also revealed the absence of any traces of cyclo-pentene in the spectrum consisting of three complex multiplets at 0.3-0.8, 1.1-1.7, and 1.9-2.4 p.p.m. (downfield from internal tetramethylsilane reference). 

The procedure described is a modification of that developed by Diels and Criegee. Bicyclo[2.1.0]pentane has been prepared by the pyrolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene, the photolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene, the pyrolysis of N-phenyl-2-oxo-3-azabicyclo[2.2.1]heptane, and the addition of methylene to cyclobutene. ... 

I he procedure described is suitable for the preparation of bicyclo 2.1,0]pentane on a large scale. The product is obtained free of im )urities and the general method is relatively safe. The starting materials are readily available. The hydrolysis of the diester is very reproducible, a feature that was not true of the literature... 

The relative stabilities of singlet and triplet electronic states of three different oxyallyl systems and the closed form of cyclopropanone, bicyclo[1.1.0]butanone, and bicyclo[2.1.0]pentan-5-one have been smdied by density-functional calculations (B3LYP/6-31G ). The results of these calculations are in good agreement with predictions of calculations based on multi-determinant methods. 

The reaction of 1,3-disubstituted bicyclo[2.1.0]pentanes with tris(4-bromophenyl)ammoniumyl hexachloroantimonate (the latter in catalytic amounts) leads to the corresponding cyclopentene after 1,2-hydrogen or 1,2-alkyl migration in the intermediary 1,3-cation-radicals (Adam and Sahin 1994 Scheme 7.47). 

A derivative of a fused three- and four-membered ring 5 (aza-l-bicyclo-[2.1.0]pentane) was obtained from Ala after alkylation with methyl a-bromoacrylate (85T2707). 

---