Cyclopropanes reaction number

Pyridine-based N-containing ligands have been tested in order to extend the scope of the copper-catalyzed cyclopropanation reaction of olefins. Chelucci et al. [33] have carefully examined and reviewed [34] the efficiency of a number of chiral pyridine derivatives as bidentate Hgands (mainly 2,2 -bipyridines, 2,2 6, 2 -terpyridines, phenanthrolines and aminopyridine) in the copper-catalyzed cyclopropanation of styrene by ethyl diazoacetate. The corresponding copper complexes proved to be only moderately active and enantios-elective (ee up to 32% for a C2-symmetric bipyridine). The same authors prepared other chiral ligands with nitrogen donors such as 2,2 -bipyridines 21, 5,6-dihydro-1,10-phenanthrolines 22, and 1,10-phenanthrolines 23 (see Scheme 14) [35]. 

Aratani et al. (21) subsequently found that the use of chiral menthyl diazoacetate esters led to higher trans/cis ratios and improved facial selectivity. A number of bulky diazoesters provided high enantioselectivity in the cyclopropanation reaction, but trans selectivity was highest with /-menthyl esters, Eq. 6. It seems clear from these and subsequent studies that the menthyl group is used because of its bulk and ready availability. The chirality present in the ester has a negligible effect on facial selectivity in the cyclopropanation reaction. Slow addition of diazoester is required (7 h at ambient temperature) for high yields presumably to suppress the formation of fumarate byproducts. 

With respect to the large number of unsaturated diazo and diazocarbonyl compounds that have recently been used for intramolecular transition metal catalyzed cyclopropanation reactions (6-8), it is remarkable that 1,3-dipolar cycloadditions with retention of the azo moiety have only been occasionally observed. This finding is probably due to the fact that these [3+2]-cycloaddition reactions require thermal activation while the catalytic reactions are carried out at ambient temperature. A7-AUyl carboxamides appear to be rather amenable to intramolecular cycloaddition. Compounds 254—256 (Scheme 8.61) cyclize intra-molecularly even at room temperature. The faster reaction of 254c (310) and diethoxyphosphoryl-substituted diazoamides 255 (311) as compared with diazoacetamides 254a (312) (xy2 25 h at 22 °C) and 254b (310), points to a LUMO (dipole) — HOMO(dipolarophile) controlled process. The A -pyrazolines expected... 

This chapter presents an up-to-date account of the redox properties of the pristine fullerenes and a large number of their derivatives as revealed by electrochemical studies in solution. The description here is as exhaustive as possible, although not completely comprehensive due to the large number of reports on the subject that have appeared over the years. A section on electrosynthesis of fullerene derivatives is included, with special emphasis on the retro-cyclopropanation reaction, a reaction that has led to the formation of novel derivatives as well as... 

At this point, it is important to indicate that a very large number of C-bridged cyclopropanated fullerene derivatives undergo irreversible reduction processes leading to the removal of the addend and recovery of the pristine parent fullerene. The process has been advantageously used in electrosynthetic procedures, and thus a separate section covering the electrochemically induced retro-cyclopropanation reaction is presented later in this chapter (see Sect. 6.1.5.2). A number of other C-bridged cyclopropanated derivatives will be discussed there. 

Recent editions of Organic Reaction Mechanisms have highlighted a number of carbene and nitrene CH-insertion reactions. This field has now been reviewed with a focus on enantioselective reactions catalysed typically by dirhodium species.5 The use of C2-symmetric box ligands in asymmetric cyclopropanation reactions has been discussed in the context of a wider review of these ligands as a source of asymmetry.6... 

These cyclopropane vs olefin enthalpy of formation differences do not increase monotonically with the number of substituents. Worse yet, differences are found to be of differing signs. That is, there is no obvious pattern for all of the enthalpies of the cyclopropanation reaction 3. Neglecting any enthalpic contribution from the CH2 or cyclopropanation reagent and considering only un-, mono- and di-substituted olefins and cyclopropanes, we find the enthalpy of reaction 3 is rather coarsely equal to (3 2) kJ moT per alkyl substituent We will now accept this 3 kJ moF per alkyl substituent for reaction 3, where we admit the absence of justification and motivation other than arithmetic and convenience. We are forced to tolerate discrepancies of a few kJmoT per substituent differences between our correlations/models and experiment of several kJmoT must be considered as acceptable for our analysis. ... 

In summary, the method described here provides an efficient and convenient route to a variety of a-diazo ketones including unsaturated derivatives that were not previously available by diazo transfer. a-Diazo ketones serve as key intermediates in a number of important synthetic methods including the Arndt-Eistert homologation, the photo-Woltf ring contraction strategy, and the carbenoid-mediated cyclopropanation reaction. We anticipate that improved access to a-diazo ketones will serve to enhance the utility of these valuable synthetic strategies. 

Cyclopropanation reactions of nonheteroatom-stabilized carbenes have also been developed. The most versatile are the cationic iron carbenes that cyclopropanate alkenes with high stereospecificity under very mild reaction conditions. The cyclopropanation reagents are available from a number of iron complexes, for example, (9-alkylation of cyclopentadienyl dicarbonyliron alkyl or acyl complexes using Meerwein salts affords cationic Fischer carbenes. Cationic iron carbene intermediates can also be prepared by reaction of CpFe(CO)2 with aldehydes followed by treatment with TMS-chloride. Chiral intermolecular cyclopropanation using a chiral iron carbene having a complexed chromium tricarbonyl unit is observed (Scheme 61). 

In a large number of carbene and carbenoid addition reactions to alkenes the thermodynamically less favored syn-isomers are formed 63). The finding that in the above cyclopropanation reaction the anti-isomer is the only product strongly indicates that the intermediates are organonickel species rather than carbenes or carbenoids. Introduction of alkyl groups in the 3-position of the electron-deficient alkene hampers the codimerization and favors isomerization and/or cyclodimerization of the cyclopropenes. Thus, with methyl crotylate and 3,3-diphenylcyclopropene only 16% of the corresponding vinylcyclopropane derivative has been obtained. 2,2-Dimethyl acrylate does not react at all with 3,3-dimethylcyclopropene to afford frons-chrysanthemic add methyl ester. This is in accordance with chemical expectations 69) since in most cases the tendency of alkenes to coordinate to Ni(0) decreases in the order un-, mono-< di- < tri- < tetrasubstituted olefines. 

Under the conditions of homogeneous catalysis, decomposition temperatures are normally significantly lower than with the heterogeneous catalysts mentioned above, and cyclopropane yields in general are higher. However, catalysts of type 2 must first be converted into the active form [presumably a copper(I) monochelate] by brief heating or by in situ reduction (see Table 10). Another soluble catalyst, copper(I) triflate, even decomposes diazoacetic esters and diazomalonic esters at temperatures below 0 °C and sterically more encumbered diazocarbonyl compounds (e.g. a-diazo-a-trialkylsilyl acetic esters " ) still at room temperature, and has shown its effectiveness in a number of cyclopropanation reactions. Since copper(I) triflate is... 

Cyclopropanes exhibit similar modes of reactivity. [2Dipolar additions with electron-deficient alkenes and electron-donor-substituted cyclopropanes, additions of electron-rich alkenes to electron-deficient cyclopropanes, a number of radicaloid reactions and intramolecular photochemical cycloadditions are known, which may be described by the general scheme H-2 3. 

To conclude, for the cyclopropanation reaction, the difficulties consist in the achievement of high enantioselectivity close to a good stereoselectivity. To be useful in organic synthesis, the last requirement is preponderant and depends on the nature and the number of the substituents of the cyclopropane ring. It is the reason why, most of the catalytic systems used in heterogeneous supported catalysis cyclopropanation are formed by bis oxazolines complexes which have shown a excellent efficiency in non supported conditions. 

The nature of ligand substituent on effectiveness and diastereoselectivity of this reaction was also explored [70]. A number of catalysts of type [Ru2(CO)4(BL)2], where BL are bridging 2-pyridonate ligands (Scheme 35) were s3mthesized (53-61). The 6-halop3Tidin-2-olato complexes exist in head-to-head (HH) as weU as head-to-taU (HT) arrangement. These were employed for the cyclopropanation reaction of MDA with a variety of olefins (Scheme 36) [70]. It was found that 6-bromopyridonate complexes are better than their chloro counterpart, and in some cases, even superior to acetate/carbonyl complexes [200]. X-ray crystal structures... 

A number of chiral acetal derivatives have also proved effective in asymmetric cyclopropanation reactions, with auxiliaries based on tartaric acid proving to be partieularly usefiil. In the case of cyclic a,P unsaturated compounds, di-O-benylthreitol derivatives (see 51) imdergo efficient and diastereoselective Simmons-Smith reactions to give the cyclopropanated products SS. ... 

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