Bicyclo pentane bond

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." ... 

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. 

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. 

The radical cation (19 " ) of the strained bicyclo[2.1.0]pentane also has a puckered conformation, supported by one strongly coupled (flagpole) proton (flsyn = 4.49 mT). Ab initio calculations indicate that the transannular bond retains some bonding and that the bridgehead carbons remain pyramidal. ... 

Cyclopropane derivatives, including spiropentanc, have proven to be virtually inert towards carbenes,1 For this reason, no literature report that describes cyclobutane synthesis from a C3 and a Cj building block by ring enlargement of cyclopropanes exists. However, due to the partial p character, as well as the increasing reactivity caused by its strain, the central bond of bicyclo[1.1.0]butane (l)2 has been found to react with carbenes.1 Photolysis of diazomethane in the presence of bicyclo[1.1.0]butane (1) at — 50 C provides a mixture of several compounds. The major fraction of the material (80%) was analyzed by means of NMR spectrometry and found to consist of penta-1,4-diene (2, 21%) and bicyclo[l.l.l]pentane (3, 1%), plus several other known compounds as well as some unidentified products.3 The mechanistic pathway for the formation of bicyclo[l.l.l]pentane (3) has not been addressed in detail, but it is believed that a diradical intermediate is involved, as shown below.3... 

Carben.es insert into a C — H bond of a cyclobutane.157 When there is a choice between a cyclopropane and a cyclobutane C — H bond, as in bicyclo[2.1. OJpentane, the insertion of phenylcarbe-ne takes place at a cyclobutane methylene position to give 2-benzylbicyclo[2.1.0]pentane (l).157... 

The procedure described above is an improved version of the one published by Kaszynski and Michl.4 [1.1.1]Propellane is a recently reviewed7 useful precursor for the synthesis of bicyclo[1.1.1]pentanes by radical addition across the central bond, followed by further transformations of the bridgehead substituents.4 8 Under suitable conditions, one can obtain mixtures of [njstaffanes [oligomeric bicyclo[1.11 [pentanes],... 

The addition of a hydrogen atom to la to yield the bicyclo[l. 1.1 ]pent-1 -yl radical (6) has been calculated (UHF/MP2 6-31G ) to be exothermic by only 47 kcalmol"1 19. The addition of the second hydrogen atom to the radical to form bicyclo[l.l.l]pentane was calculated (MP3 6-311G ) to be exothermic by 104.4 kcalmol"143 (equation 4). By this argument, the strength of the central bond in la is about 60 kcalmol"1319 and is comparable to that in cyclopropane. 

Addition across carbon—halogen bonds. Alkyl iodides48,62,65,109 "3,114 and also activated bromides48,65,107 "3,115 (e.g. methyl bromoacetate) react with la thermally62 or under UV irradiation to give 1,3-disubstituted bicyclo[l. 1. l]pentanes when the reaction is performed in diethyl ether, and the insertion of a single bicyclo[l. 1. l]pentane cage can be viewed as the standard reaction pattern. 

Three phosphonites (RO)2PR have also been found to add to la. The products contained the bicyclo[l.l.l]pentane cage inserted into the P—C bond and were isolated after oxidation to stable pentavalent phosphorus derivatives125. In a related process involving... 

Reaction with strained hydrocarbons. Bicyclo[2.1.0]pentane (2), when heated (160°) in the presence of an electron-deficient alkene, forms a number of monocyclic adducts.1 In contrast, in the presence of 1 the central bond of 2 is cleaved at 70u, and cycloadducts are formed with an electron-poor alkene in useful yields. A typical reaction is shown in equation (I).2... 

Bicyclo[ 1.1.1 ]pent-l-yl anion 16. The basicity and electrophilicity of 16 relative to ordinary tertiary alkyl anions are not easy to predict without a computation. On the one hand, the high percent of s character in the hybrid holding the lone pair on the bridgehead carbon atom suggests that this anion will have quite low basicity. On the other hand, the transannular interaction between the two bridgehead carbons can be expected to increase the basicity, due to a relief in transannular closed-shell repulsions when a lone pair is replaced by a bond at the bridgehead. Indeed, the computed (MP2/6-31G ) interbridgehead distance decreases from 1.974 A in the anion to 1.872 A in bicyclo[l.l.l] pentane ... 

Bicyclo[2.1.0]pentane (7 equation 3) readily undergoes a cycloaddition across carbon-carbon double bonds in the presence of nickel(O) complexes. The mode is formally analyzed as a thermally forbidden [2 + 2] process and is in striking contrast to that of the lower homolog, bicyclo[1.1.0]butane, which suffers cleavage of two o -bonds and affords formal allylcarbene addition products. When a solution of bi-cyclo[2.1.0]pentane (7) and [Ni(AN)2] in excess methyl acrylate is heated at 40 C for 36 h under a nitrogen atmosphere, the stereoisomeric cycloadducts exo- and c/ product methyl 3-(cyclopent-2-enyl)propionate (10a 22%). Reaction of (7) with acrylonitrile affords the corresponding adducts (8b), (9b) and (10b). 

---