Cyclopropanes additions

The cyclopropane cyclizations by elimination of triflinic acid (CFjSOjH) are readily effected by basic treatment of triflones (trifluoromethyl alkyl sulfones) with activated y-protons (equations 46 and 47). The cyclopropane diesters 45 are formed on treatment of 44 with potassium hydride in DMSO or sodium methoxide in methanol (equation 48). In contrast, the monoester 46 failed to give the desired cyclopropane . Addition of carbanions derived from fi, y-unsaturated phenyl sulfones to a, 8-unsaturated carboxylic esters and subsequent elimination of benzenesulfinate ion give cyclopropanes possessing the unsaturated side chain and the ester function in trans positions (equation 49) . ... 

In agreement with the results of experiments on pyrolysis of l,l-bis(trimethyl-silyl)cyclopropane, additional (2/2)CASPT2/6-31G calculations predict that a rapid 1,2-silyl shift will occur in 12a, forming 13a. However, if cis- and trans-l,l-bis(trimethylsilyl)-2,3-dimethylcyclopropanes (11b and 11c) were pyrolyzed, then, as shown in Figure 22.10, the stereochemistry of ring opening could, presumably, still be inferred from the stereochemistry of the double bonds in the expected rearrangement products (13b and 13c). 

To draw each product, add the carbene carbon from either side of the alkene, and keep all substituents in their original orientations. The cis methyl groups in c/s-2-butene become cis substituents in the cyclopropane. Addition from either side of the alkene yields the same compound—an achiral meso compound that contains two stereogenic centers. 

Overlap control by steric factors of phenyl substituents in cyclopropanes additionally containing a carbonyl group have been described by Zimmerman and coworkers in the liquid ammonia reduction of the isomeric diphenyl cyclopropyl ketones cis- and trans-137. 

With electron-rich cyclopropanes, addition of pyridine yields stable metallocyclic complexes, from which cyclopropanes could be regenerated upon treatment with aqueous potassium cyanide. This and the related reaction of cyclopropane with tetracarbonyl-dichlorodirhodium [Rh(CO)2Cl]2 which yields rhodacyclopentanone (equation 38) have become the precedents for a wide range of insertions and isomerizations of strained compounds ... 

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. 

Scheme 6.26. Auxiliary-based asymmetric cyclopropanations (addition of CH2 ) of a, 3-unsaturated aldehydes and ketones, (a) [98] (b) [99,100] (c) [101-104] (d) Proposed transition structures [104]. Only one zinc and the transfer methylene are shown other atoms associated with the Simmons-Smith reagent are deleted for clarity.

The order of activity for both cyclopropane addition and propane exchange is Rh > Pt > Pd, as found for ethane exchange (3), but the differences here reside almost entirely in the preexponential factors. Since the rates were measured only per unit weight of metal and not per unit surface area, this order may be without basic significance and will not be discussed further here. The dependence of the parameter 6 on temperature is shown... 

Table 22. Frontier molecular orbital energy gaps (eV) for acetylene, ethylene, and cyclopropane addition to benzohetrocycles computed by the AMI semiempirical method...

To determine the reliability of these computational approaches, we have computed activation barriers for these reactions at both AMI ab initio and B3LYP/6-31G(d) DFT theory levels (Table 30). Knowing that activation barriers for the addition of cyclopropene to furan computed at the B3LYP/6-31G(d)/AMl theory level were 18.7 and 18.4 kcal/mol for an endo and exo cycloaddition reaction (Table 8) and it is experimentally feasible, it becomes obvious that none of the cycloaddition reactions presented in Table 25 should be able to be accomplished experimentally. All activation barriers were around 40 kcal/mol or higher, with the exception of the cyclopropane addition to 1,2-oxazole (Table 31). The comparison... 

Figure 11. The AMI computed transition state structures cyclopropane addition to 4,4-dimethyl-[4H]-l,3,4-oxadiazole.

Table 47. Activation barriers for ethylene, acetylene, and cyclopropane addition to...

Table 48. The frontier molecular orbitals and frontier molecular orbital energy changes (eV) for the cyclopropane addition to cyclopentadiene and to 3,5-dichloro-...

To confirm these findings, we have computed activation barriers for the acetylene, ethylene and cyclopropane additions to 3,5-dichloro-[4H]-1,2-diazole. The computed activation barriers (Table 49) were even slightly lower than the activation barriers for same reaction with 4,4-dimethyl-[4H]-l,2-diazole as a diene (Table 47). This is a reasonable observation because the cycloaddition reaction is LUMO heterocycle (diene) controlled. The LUMO energy of 3,5-dichloro-[4fir -1,2-diazole was substantially lower than the LUMO energy of 4,4-dimethyl-[4/Tl-1,2-diazole. Subsequently, with a modest activation barrier of 20.1 kcal/mol, even poor dienophiles such as acetylene should be capable to react with 3,5-dichloro-[4i/]-1,2-diazole as a diene in the Diels-Alder reactions (Table 49). 

Some of the other reactions in which N,0-acetals have been used as chiral auxiliaries include conjugate addition reactions, cyclopropanation, " addition of nucleophiles to cyclic ketones and Diels-Alder reactions. ... 

The trans methyl groups in frans-2-butene become trans substituents in the cyclopropane. Addition from either side of the aikene yieids an equai amount of two enantiomers—a racemic mixture. 

In Summary Diazomethane is a useful synthetic intermediate as a methylene source for forming cyclopropanes from alkenes. Halogenated carbenes, which are formed by dehydro-halogenation of halomethanes, and the Simmons-Smith reagent, a carbenoid arising from the reaction of diiodomethane with zinc, also convert alkenes into cyclopropanes. Additions of carbenes to alkenes differ from other addition processes because a single carbon atom becomes bonded to both alkene carbons. 

We will assume the harmonic approximation for the stretching reactive modes for the isomerisation of cyclopropane. Additionally, we assume that the CCC angle at the transition state is intermediate (0 = 90°) between the value in the reactants (60°) and that in the products (120°), as illustrated in Scheme 8.VI. If such modes have a local character, with force constants % the effective force constant is just the result of a vectorial addition as shown in Figure 8.8. Generalising for three bonds, in order to encompass the CH bond perpendicular to the CC bonds, one obtains the following expression ... 

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