L-pentanes

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... 

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. 

For n = 1, this is the standard synthesis of [l.l.l]propellane 40a.20,21 Also for n = 3, this method makes [3.1.1]propellane 40c accessible with reasonable effort.22 Although [2.1.1]propellane 40b was detected by its IR spectrum at liquid nitrogen temperature,23 it could not be obtained as a stable compound via this route.22 The addition of halomethanes across the central bond by a radical chain mechanism is common to 40a and 40c. The addition of carbon tetrabromide to 40c afforded a 45% yield of 41.18 Likewise, a number of bicy-clo[l.l.l]pentane derivatives 42 were obtained by reaction of the corresponding halomethanes with 40a.17,24... 

A comparison between BS and closed-shell calculations was shown for diamagnetic [Fe(CN)2(L)] (L = pentane-2,4-dione-bisGS-alkylisothiosemicarbazonato)) (1) (91,92). Through the... 

Fig.11. Three redox states of ligand L (pentane-2,4-dione-bis(S-alkylisothiosemicarbazonato).

Shtarev et al.121 272 have measured 4J(F, F) proximate couplings, i.e., in a non-planar U configuration, in a series of bridge-fluorinated dimethyl bicylo[l.l.l]pentane-l,3-dicarboxylates, [31-a,b], where they obtained values of such couplings ranging from 48.9 to 97.9 Hz (in absolute value). They linearly correlated these couplings with the calculated d(F- F) distance... 

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...

As can be seen, the isolation of l-halobicyclo[l.l.l]pentanes offered an opportunity for further molecular manipulation. To illustrate this, the adduct generated from 3-iodo-4,5-dihy-drofuran-2(3//)-one and [1.1.IJpropellane was reacted with tributyltin hydride in the presence of the radical trapping agent methyl acrylate to afford the 1,3-disubstituted biey-clo[l.l.l]pentane 27 in a total yield of 39%.43... 

Further transformations of l-iodobicyclo[l.l.l]pentane derivatives can be achieved by lithia-tion with rerf-butyllithium. transmctalation with zinc(II) chloride and palladium-catalyzed coupling with aryl iodides or aryl bromides using (dppf)FMCl2 as a catalyst.47... 

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... 

However, the fact that dichlorocarbene reacts with methyl bicyclo[l. 1.0]butane-l-carboxylate (4) to generate exclusively pentadienes 5 and 6 in a total yield of 41 % renders this type of reaction impracticable for the synthesis of bicyclo[l.l.l]pentanes.3 Numerous examples have indeed shown that the reaction between bicyclo[1.1.0]butanes and carbenes are synthetically practical routes for pentadiene preparation4 9 and interesting evidence has been obtained to indicate that a concerted mechanism occurs.10-11... 

Although it has been reported that dihalocarbenes generated by mild methods add across the central bond of methyl 3-methylbicyclo[1.1.0]butane-l-carboxylate (7) to furnish methyl 2,2-di-halo-3-methylbicyclo[l.l,l]pentane-1-carboxylates 8 and 9 in low yields,12 when the reaction was repeated on 7 using a different dichlorocarbene source, NMR spectrometry confirmed that the product obtained is actually diene 10.13 The structure of 9 has, however, not been challenged. 

Despite the aforementioned potential complication that has more or less precluded the use of these procedures in the synthesis of bicyclo[l.l.1]pentanes, several bicyclo[l.l.l]pentanes have been realized by utilizing this approach and are summarized below.13 17... 

For example, methyl 3-phenylbicyclofl.l.0]butane-l-carboxylate (11, R1 = Ph R2 = C02Me) was converted to methyl 2,2-dichloro-3-phenylbicyclo[l.l.l]pentane-l-carboxylate (12, R1 = Ph R2 = C02Me) in 46% yield on reaction with dichlorocarbene generated by thermolysis of sodium trichloroacetate. 3,14... 

Methyl 3-Phenyl-2,2-dichlorobicyclo l.l.l pentane-l-carboxylate (12, R1 = Ph R2 = C02Mc) Typical Procedure 14... 

Special interest has been paid to the synthesis of bridged four-membered rings. The smallest system of this type, the bicyclo[1. l.l]pentane skeleton 6, could be prepared by photolysis of 3-diazobicyclo[2.1.1]hexan-2-one in methanol,43 however, the yield was very low ( 2%). 

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. 

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