Electron cyclopropanes

Substituted cyclopropane systems also undergo nucleophilic addition of suitable solvents (MeOH). For example, the photoinduced ET reaction of 1,2-dimethyl-3-phenylcyclopropane (112, R = Me) with p-dicyanobenzene formed a ring-opened ether by anti-Markovnikov addition. The reaction occurs with essentially complete inversion of configuration at carbon, suggesting a nucleophilic cleavage of a one-electron cyclopropane bond, generating 113. The retention of chirality confirms that the stereochemistry of the parent molecule is unperturbed in the radical cation 112 " ". 

Since the mechanism involves electron transfer to a heteroatom rather than carbon, vinyl, and aryl derivatives can be easily reduced under these conditions. Many functional groups can be reduced, depending on their ability to accept an electron. Cyclopropane rings are opened under these conditions. The bridging cyclopropane moiety in 498, for example, was opened with lithium and ammonia to give 499 in 85% yield, as... 

The ZT+ form cannot easily relax through blcyclo[3.1. 0]hexene formation. This would Imply a dlsrotatory (2 n-electrons) cyclopropane ring closure resulting In strongly strained products with trans connections of the five- and slx-membered rings... 

MgATP. The numbers in parenthesis represent the number of electrons required for the reaction shown. is cyclopropene A cyclopropane. 

Tertiary bismuthines appear to have a number of uses in synthetic organic chemistry (32), eg, they promote the formation of 1,1,2-trisubstituted cyclopropanes by the iateraction of electron-deficient olefins and dialkyl dibromomalonates (100). They have also been employed for the preparation of thin films (qv) of superconducting bismuth strontium calcium copper oxide (101), as cocatalysts for the polymerization of alkynes (102), as inhibitors of the flammabihty of epoxy resins (103), and for a number of other industrial purposes. 

Later there was an attempt by ab initio calculation to fit the electron structure of diazirine into the Walsh model of cyclopropane (69MI50800). According to these SCF-LCAO-MO calculations three MOs add to the description of the lone electron pairs, all of which also contribute to some extent to ring bonding. As to strain, 7r-character and conjugative effect, the term pseudo-rr-character was used. 

The stabilization of chloromethoxycarbene (234) was intensively studied. It is formed from diazirine (233) in a first order reaction with fi/2 = 34h at 20 C. It reacts either as a nucleophile, adding to electron poor alkenes like acrylonitrile with cyclopropanation, or as an electrophile, giving diphenylcyclopropenone with the electron rich diphenylacetylene. In the absence of reaction partners (234) decomposes to carbon monoxide and methyl chloride (78TL1931, 1935). 

Photochemically produced chloromethoxycarbene revealed ambiphilicity. Relative rates of cyclopropanation showed that electron donating as well as electron attracting substituents favor the reaction (Scheme 2) (79JA4736). 

Small shift values for CH or CHr protons may indicate cyclopropane units. Proton shifts distinguish between alkyne CH (generally Sh = 2.5 - 3.2), alkene CH (generally 4, = 4.5-6) and aro-matic/heteroaromatic CH (Sh = 6 - 9.5), and also between rr-electron-rich (pyrrole, fiiran, thiophene, 4/ = d - 7) and Tt-electron-deficient heteroaromatic compounds (pyridine, Sh= 7.5 - 9.5). 

In contrast to H shifts, C shifts cannot in general be used to distinguish between aromatic and heteroaromatic compounds on the one hand and alkenes on the other (Table 2.2). Cyclopropane carbon atoms stand out, however, by showing particularly small shifts in both the C and the H NMR spectra. By analogy with their proton resonances, the C chemical shifts of k electron-deficient heteroaromatics (pyridine type) are larger than those of k electron-rieh heteroaromatic rings (pyrrole type). 

It is believed that this process involves migration through a pentacoordinate protonated cyclopropane in which an alkyl group acts as a bridge in an electron-deficient carbocation structure. The cyclohexyl- methylcyclopentyl rearrangement is postulated to occur by rearrangement between two such structures. 

Strong sp -sp a bonds are not possible for cyclopropane, because the 60° bond angles of the ring do not permit the orbitals to be properly aligned for effective overlap (Figure 3.10). The less effective overlap that does occur leads to what chemists refer to as bent bonds. The electron density in the carbon-carbon bonds of cyclopropane does not lie along the internuclear- axis but is distr-ibuted along an arc between the two carbon atoms. The r-ing bonds of cyclopropane are weaker than other carbon-carbon a bonds. 

There are three main criteria for design of this catalytic system. First, the additive must accelerate the cyclopropanation at a rate which is significantly greater than the background. If the additive is to be used in substoichiometric quantities, then the ratio of catalyzed to uncatalyzed rates must be greater than 50 1 for practical levels of enantio-induction. Second, the additive must create well defined complexes which provide an effective asymmetric environment to distinguish the enantiotopic faces of the alkene. The ability to easily modulate the steric and electronic nature of the additive is an obvious prerequisite. Third, the additive must not bind the adduct or the product too strongly to interfere with turnover. 

Fig. 3.11 Transition structure proposal for cyclopropanation with electronically differentiated ligands...

Alttmina-supported KF is an effecdve reagent for Michael addidon of nitroalkanes to electron-deficient olefins. Subsequent cycloalkyladons afford cyclopropanes.However, the reacdo n of a,fi-ttnsantrated ketones v/ithnitroalkanesin the presence of KF-A1,0 in acetonitrile gives 4,5-dihydrofliranes fEq. 7.39. ... 

Yet another kind of alkene addition is the reaction of a carbene with an alkene to yield a cyclopropane. A carbene, R2C , is a neutral molecule containing a divalent carbon with only six electrons in its valence shell. It is therefore highly reactive and is generated only as a reaction intermediate, rather than as an isolable molecule. Because they re electron-deficient, carbenes behave as electrophiles and react with nucieophiiic C=C bonds. The reaction occurs in a single step without intermediates. 

In the interaction of the local 2pv orbitals, two more bonding molecular orbitals are formed against one less bonding. In all previous cases the opposite occurred. This is due to the negative overlap between adjacent 2py orbitals—whether, by convention, all positive lobes point in the clockwise direction, or whether all positive lobes point in the anticlockwise direction. The two bonding 2pv combinations in fact fall below the two antibonding (hybrid 2s, 2px) combinations. The former each have two electrons while the latter are empty. The six electrons of the three C—C bonds are nicely accounted for. The method creates simultaneously the acc and or c molecular orbitals of cyclopropane (note that the latter three lie relatively close in energy). 

Finally, an ingenious synthetic sequence by Trost, Cossy and Burks201 includes a unique desulphonylation reaction that involves an electron-transfer process. The synthetic sequence uses 1, l-bis(phenylsulphonyl)cyclopropane as a source of three carbon atoms, since this species is readily alkylated even by weakly nucleophilic species. Given an appropriate structure for the nucleophile, Trost found that desulphonylation with lithium phenanthrenide in an aprotic solvent allowed for an efficient intramolecular trapping of the resultant carbanion (equation 88). This desulphonylation process occurs under very mild conditions and in high yields it will undoubtedly attract further interest. 

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