Cycloalkenes cyclopropanation

The smallest cycloalkanes and cis-cycloalkenes, cyclopropane (7) and cyclopropene (2) are isolable, although very reactive, compounds. 

The present procedure for ring expansion has also been applied to l,2-bis(trimethylsilyloxy)bicyclo[n.l.0]alkanes, which are prepared by cyclopropanation of l,2-bis(silyloxy)cycloalkenes. The latter are readily available from acyloin condensations in the presence of chlorotrimethylsilane. " This reaction provides a new route to cyclic 1,3-diketones and macrocyclic compounds containing two 1,3-diketone units in the ring. 

ZJE isomerization of the alkene bond in A -enecarboxyIic acids occurs during Kolbe electrolysis because the intermediate radical reacts reversibly with this function to form a cyclopropane [92], This process leads to a partial loss of stereochemistry in the synthesis of long chain alkenes [93]. However, it does not present stereochemical problems during the synthesis of cycloalkenes such as chaul-moogric acid [94]. 

Photolysis of 3-diazoindole 59a in cycloalkenes, via the addition of carbene 65 to the cycloolefine, gave rise to cyclopropane adduct 66, which can rearrange to indolenines 68 or 69 and then to indoles 70 (66LA17). 

Accompanying this trend is a monotonic decrease in the internal angle at the adjacent position (P), from 119.6° in tetralin to 113.0° in cyclopropabenzene (Figure 16). The bond lengths at the ring fusion are demonstrably shorter in cyclopropabenzenes. This has caused some discussion of bond fixation in cyclopropabenzene however, parallel effects are seen for cyclopropane and cyclo-propene among cycloalkanes and cycloalkenes, respectively. Thus, the shorter bonds in cyclopropabenzene are expected or rationalized by simple rehybridization independent of any bond fixation. 

These interactions may usefully be described as an acid-base type interaction, in which the cyclopropane ring acts as a base (electron donor) and the proton or cationic center acts as the acid (electron acceptor). One of the factors that controls the basicity of a hydrocarbon is the energy of the highest occupied molecular orbital (HOMO).60 The 6-31 G HOMO energies of some cycloalkanes and cycloalkenes are given in Table 4.61... 

Further cycloadditions used to prepare cycloalkenes on insoluble supports include the cyclopropanation of resin-bound alkynes and of polystyrene [165] (Figure 5.18). The latter reaction has been used to introduce tags onto polystyrene beads, which enable the recognition of a certain bead in compound libraries produced using the mix-and-split method (Section 1.2 [165-167]). The structure of polystyrene tagged in this way has not, however, been rigorously determined. 

A significant lack of uniformity is seen for the formal cyclopropanation of n-carbon cycloalkenes to form bicyclo[(n-2). 1.0]alkanes via the formal reaction shown in equation 5. 

With respect to the complications of method B, it is remarkable that generation of carbene 3a according to method C in the presence of c -but-2-ene or various cycloalkenes gave the corresponding (trimethylsilyl)cyclopropanes in yields up to 30%73,74. Similarly,... 

Transfer of carbene 3a from 19 to a variety of alkenes and cycloalkenes has been achieved under catalysis by copper(I) chloride74,79 - 82. However, with the exception of cyclohexene69,70 (72% yield), only moderate yields could be obtained. In all cases, the cyclopropanation was stereospecific with respect to the double bond configuration of the alkene and gave the sterically less crowded cyclopropane diastereomer in excess. As in the photochemical cyclopropanation, the formal carbene dimer trans-1,2-bis(trimethylsilyl)ethene is often formed as the major by-product. Cyclopropanation of fraws-but-2-ene with 19 with copper(II) chloride as catalyst was found to be even less... 

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