Reaction mechanisms cyclopropanation

This type of cyclopropanation reaction catalyzed by a gold(I) complex produced cyclopropylmethyl carbene complex 321, which is reactive toward external alkenes or nucleophiles. The reaction depended on the ligand of the gold complex as well as the substituted patterns of enyne compounds. Echavarren and coworkers reported a cyclopropanation reaction mechanism. The cyclopropane gold complex intermediates 322 and 323 were trapped by external alkenes to give cyclopropanes 324 and 325, respectively (Scheme 1.157) [227]. 

From a historical perspective it is interesting to note that the Nozaki experiment was, in fact, a mechanistic probe to establish the intermediacy of a copper carbe-noid complex rather than an attempt to make enantiopure compounds for synthetic purposes. To achieve synthetically useful selectivities would require an extensive exploration of metals, ligands and reaction conditions along with a deeper understanding of the reaction mechanism. Modern methods for asymmetric cyclopropanation now encompass the use of countless metal complexes [2], but for the most part, the importance of diazoacetates as the carbenoid precursors still dominates the design of new catalytic systems. Highly effective catalysts developed in... 

It is thus anticipated that compressive stress inhibits while tensile stress promotes chemical processes which necessitate a rehybridization of the carbon atom from the sp3 to the sp2 state, regardless of the reaction mechanism. This tendency has been verified for model ring-compounds during the hydrogen abstraction reactions by ozone and methyl radicals the abstraction rate increases from cyclopropane (c3) to cyclononane (c9), then decreases afterwards in the order anticipated from Es [79]. The following relationship was derived for this type of reactions ... 

These reactions serve as a link in understanding selectivity differences between inter- and intramolecular cyclopropanation reactions, and they have been useful in defining the mechanism of addition as a function of catalyst [50,69,70]. 

Fig. 30. Mechanism for C-C activation of propene. Decay of the allyl hydride complex may proceed via migration of the metal-bound H atom to the /3-carbon atom in the allyl moiety (i.e. reverse /3-H migration), leading to formation of the same metallacyclobutane complex implicated in the Y + cyclopropane reaction. The dynamically most favorable decay pathway is to YCH2 + C2H4.

While direct photolysis of (46) gave (48), the sensitized photolysis of (46) with acetophenone gave only the cyclopropane product (>93%). These authors favor a stepwise oxa-di-w-methane reaction mechanism (8.69) inasmuch as a concerted reaction should produce the product without a change in multiplicity. That is, the product would have to be formed in the triplet state and not enough energy is available for the product to be formed in an excited state ... 

Intramolecular oxonium ylide formation is assumed to initialize the copper-catalyzed transformation of a, (3-epoxy diazomethyl ketones 341 to olefins 342 in the presence of an alcohol 333 . The reaction may be described as an intramolecular oxygen transfer from the epoxide ring to the carbenoid carbon atom, yielding a p,y-unsaturated a-ketoaldehyde which is then acetalized. A detailed reaction mechanism has been proposed. In some cases, the oxonium-ylide pathway gives rise to additional products when the reaction is catalyzed by copper powder. If, on the other hand, diazoketones of type 341 are heated in the presence of olefins (e.g. styrene, cyclohexene, cyclopen-tene, but not isopropenyl acetate or 2,3-dimethyl-2-butene) and palladium(II) acetate, intermolecular cyclopropanation rather than oxonium ylide derived chemistry takes place 334 ). 

Based on these mechanisms and ligand structures, various transition-state models to explain the stereochemistry of asymmetric cyclopropanation reactions have been proposed. For details, see the reviews17- 1 and the references cited for Figure 12. 

The cyclopropane acetals [10] and [11] also hydrolyze in aqueous sulfuric acid, but the reaction mechanisms for the two are not the same. Reaction of [10] involves pre-equilibrium oxygen protonation followed by rate-determining A1 ring opening, whereas that of [11] involves carbon protonation concurrent with... 

It was demonstrated (83) that the reaction of dinitrostyrenes (28) with aryl diazo compounds RR CN2 afford nitronates (24 g) in good yields. These products contain the nitro group at the C-4 atom in the trans position with respect to the substituent at C-5 (if R =H). Since the reaction mechanism remains unknown, the direct formation of cyclic nitronates (24 g) from pyrazolines A without the intermediate formation of cyclopropanes also cannot be ruled out. 

The mesityl diimine 88d was as effective a ligand in the aziridination as the 2,6-dichlorophenyl diimine 88a ( 65% ee vs 66% ee) (61). The bound face of the styrene undergoes aziridination (in contrast with Fu s selective crystallization of the wrong face of styrene in his copper-catalyzed cyclopropanation reaction, cf. Section II.A.8). Unfortunately, the potential racemization of 118 (by the mechanism... 

Of Lowry s "speculations" on the mechanisms of cyclopropane reactions in 1923, Ingold said, they "pass quite beyond anything we have experimentally demonstrated." Quoted in Seddon (1972 41). 

Experimental evidence and numerous theoretical investigations indicate that metathesis and cyclopropanation proceed by different reaction mechanisms, each requiring a characteristic electronic configuration of the group L M. 

For the sake of simplicity, carbenium ions, carbonium ions or protonated cyclopropane rings were used as reaction intermediates, omitting the anionic zeolite framework in the illustrahon of the reaction mechanisms for the reactions discussed here. Furthermore, it is conceivable that many such reachon paths involve alkoxide intermediates, instead of carbenium and carbonium ions. 

Unusual fragmentation reactions for thietanium salts have been observed. Their analysis may reveal more information about the influence of d-orbitals in the reaction mechanisms of organosulfur compounds. Alkylation of certain thietanes leads to 5-methylthietanonium salts. The thietanium salt 193, which is formed from 2,4-dimethylthietane and (CH3)30" BF4, breaks up when treated with n-butyllithium into a reactive biradical and its resulting cyclopropane and a thioether. 

Extremely slow class (4) reactions were observed for scavenging of (a) cyclohexane hole by cyclopropane [60] and (b) cyclohexane and decalins holes by O2 [75]. H atom transfer from the hole to O2 and H2 transfer from cyclopropane to the hole were suggested as the possible reaction mechanisms. 

A mechanism proposed for the skeletal rearrangement of enynes involved the presence of gold carbenes [161]. This proposed mechanism was supported by the capture of intermediate gold carbenoids trapped by reactive alkenes in intermolecu-lar cyclopropanation reactions [162]. 

The initial products may be considered to be excited cyclopropane derivatives formed by addition to the double bond and excited olefins formed by insertion into CH bonds. (The detailed reaction mechanisms are discussed in Sec. V.) The isomerization products are expected to be similar to those found in the thermal isomerizations of the corresponding cyclopropane derivative or olefins, the excitation energy being at least 80-85 kcal. in the former case and 85-90 kcal. in the latter (taking AHf° (CH2) 80-85 kcal.). The excitation energy is increased by any excess energy of methylene. 

Among the transition-metal catalysts that have been used, only those of Pd(II) are productive with diazomethane, which may be the result in cyclopropanation reactions [7,9,21] of a mechanism whereby the Pd-coordinated alkene undergoes electrophilic addition to diazomethane rather than by a metal carbene transformation in any case, asymmetric induction does not occur by using Pd(II) complexes of chiral bis-oxazolines [22],... 

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... 

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