Cyclopropanes Electronic effects

Scheme 10.12 gives some examples of enantioselective cyclopropanations. Entry 1 uses the W.s-/-butyloxazoline (BOX) catalyst. The catalytic cyclopropanation in Entry 2 achieves both stereo- and enantioselectivity. The electronic effect of the catalysts (see p. 926) directs the alkoxy-substituted ring trans to the ester substituent (87 13 ratio), and very high enantioselectivity was observed. Entry 3 also used the /-butyl -BOX catalyst. The product was used in an enantioselective synthesis of the alkaloid quebrachamine. Entry 4 is an example of enantioselective methylene transfer using the tartrate-derived dioxaborolane catalyst (see p. 920). Entry 5 used the Rh2[5(X)-MePY]4... 

Another aspect of the geometry of the bound olefin that has been barely studied is the orientation of substituents capable of x interactions with the olefin. The most studied x substituent is the cyano group, but its linearity precludes discussion of the nature of the interaction. The structures of two complexes of diphenylethylenes (IV and XVIII, Table I) have been determined. On the basis of electronic effects one would expect a phenyl ring either to be coplanar with the olefin double bond for better conjugation or to be perpendicular to the metal-olefin plane for greatest x overlap (in the cyclopropane model). The limited evidence favors the second orientation. However, structural studies of olefins with substituent groups such as -COH, -COOR, or -N02 would be useful for the further definition of the orientation of x substituents. 

Irradiation of the diazocyclohexadienone (108) in the presence of isoprene affords the spiro-adduct (109), whereas the thermally induced reaction gives only (110), which is probably formed via (109) by a vinylcyclopropane rearrangement cf. p. 111). Photolysis if diazomethyltrimethylsilane with frans-but-2-ene gives the trans-cyclopropane (111) (23%) and olefin (112) (61%), consistent with singlet carbene formation. With ethylene, only 17% of cyclopropyl trimethylsilane was obtained, along with 30% of (112). The steric hindrance in tetramethylethylene completely prevented cyclopropane formation, as did electronic effects in fluoro-olefins. No... 

The electronic effects of substituents on the structure of the cyclopropane ring continue to attract attention. The X-ray structure of 1,1,2,2-tetracyanocyclopropane shows the C-1 —C-2 bond to be lengthened (1.563 A), whereas the remote ring bonds in the cyclopropane derivatives (1), (2), (3), and (4) are shortened in accord with the Walsh orbital model. The microwave spectrum of (1) shows that the molecule adopts the bisected conformation depicted with the chlorine atom cis with respect to the C-1 proton. A bisected conformation is also observed in the dione (2) where the carbonyl groups are each cis with respect to the adjacent cyclopropane ring, but trans with respect to each other. The n.m.r. spectra of partially oriented chloro-, bromo-, and cy ano-cyclopropane provide some indirect evidence in support of the orbital theory... 

R=R =Ph, C02Me R-CMCj, Ph Rl=Me, H) and R2C=CH (R2 Pr, Bu, hexyl) has been examined . a new synthesis of trisubstituted alkenes by the Pd(OAc)2/PPh3 catalysed arylation of mono- and di-substituted acetylenes is observed in the presence of formic acid and a tertiary amine . Cyclopropanation of olefins is catalysed by a number of complexes of late transition metals and the electronic effects of the metal provide the stereochemical and regiochemical control . 

The electron effects of substituents on the structure of the cyclopropane ring have received considerable attention during the year. Microwave studies on 1-cyano- and 1,1-dicyano-cyclopropane have shown that the remote C-2—C-3 bonds are reduced in length (1.500 and 1.485 A, respectively) when compared with cyclopropane (1.510 A). These observations are in accord with the simple Walsh model for electron-accepting substituents. The prediction of a lengthened C-1—C-2 bond was borne out by 1-cyanocyclopropane (1.529 A), but this bond length was not determined for... 

A symmetry approach to the effect of temperature and substitution on Cope rearrangements has revealed that with increasing temperature the loss of symmetry can be considered as a collective variable which has a positive linear relationship with temperature. The results of an aromatic Cope rearrangement of a trans-l-aryl-2-ethenylcyclobutanecarbonitrile have been reported for the construction of the fused benzocyclooctene ring. The effects of gem-dimethyl substitution on the cyclopropane, alkene geometry, relative stereochemistry of the cyclopropane, and steric and electronic effects of functional groups on the thermal Cope rearrangement of divinylcyclopropanes have been reported (Scheme 10). " ... 

Fig. 4.6 Electronic effect on structure of cyclopropane and Dewar-Hoffmann semibullvalenes...

Asymmetric cyclopropanation was actively investigated in the last 10 years and an enormous number of reports were published. For example, proline-daived Rh2(5-DOSP)4 160 was used for asymmetric cyclopropanation. Asymmetric cyclopropanation of iV-Boc-pyrrole 161 and fitran 162 was carried out by Davies and coworkers (Scheme 1.76) [121]. Face selectivity was influenced by steric and electronic effects on the acceptor unit iV-Boc-pyrrole 161 underwent asymmetric double cyclopropanation to give chiral azatricycloheptane... 

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. 

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. 

Alumina-supported KF is an effective reagent for Michael addition of nitroalkanes to electron-deficient olefins. Subsequent cycloalkylations afford cyclopropanes.37 However, the reaction of a, 3-unsaturated ketones with nitroalkanes in the presence of KF-A1203 in acetonitrile gives 4,5-dihydrofuranes (Eq. 7.39).40... 

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