Cyclopropanes catalysis

Catalytic, enantioselective cyclopropanation enjoys the unique distinction of being the first example of asymmetric catalysis with a transition metal complex. The landmark 1966 report by Nozaki et al. [1] of decomposition of ethyl diazoacetate 3 with a chiral copper (II) salicylamine complex 1 (Scheme 3.1) in the presence of styrene gave birth to a field of endeavor which still today represents one of the major enterprises in chemistry. In view of the enormous growth in the field of asymmetric catalysis over the past four decades, it is somewhat ironic that significant advances in cyclopropanation have only emerged in the past ten years. 

Keywords N,N-Containing ligands Asymmetric catalysis Cyclopropanation Diels-Alder reaction Nucleophilic allylic substitution... 

Apart from the cyclopropanation reaction, only one example has been published of the application of ionic liquids as reaction media for enantio-selective catalysis with bis(oxazoline) ligands. In this case, the complex 6b-ZnCl2 was used as a catalyst for the Diels-Alder reaction between cyclopen-tadiene and N-crotonyloxazolidin-2-one in dibutyUmidazoUiun tetrafluorob-orate (Scheme 9) [48]. Compared with the same process in CH2CI2, the reaction was faster and both the endofexo selectivity and the enantioselectivity in the endo product were excellent. However, experiments aimed at recovering the catalysts were not carried out. 

Organometallic catalysis insertion and ring opening mechanism of methylene-cyclopropane over Cp2LaH and Cp2LuH catalysts... 

Recently, Y. Yamamoto reported a palladium-catalyzed hydroalkoxylation of methylene cyclopropanes (Scheme 6-25) [105]. Curiously, the catalysis proceeds under very specific conditions, i.e. only a 1 2 mixture of [Pd(PPh3)4] and P(o-tolyl)3 leads to an active system. Other combinations using Pd(0 or II) precursors with P(o-tolyl)3 or l,3-bis(diphenylphosphino)propane, the use of [Pd(PPh3)4] without P(o-tolyl)3 or with other phosphine ligands were all inefficient for the hydroalkoxylation. The authors assumed a mechanism in which oxidative addition of the alcohol to a Pd(0) center yields a hydrido(alkoxo) complex which is subsequently involved in hydropal-ladation of methylenecyclopropane. 

Allyloxysilyl)diazoacetic esters 6 and the homologous butenyloxy derivative 9 undergo intramolecular cyclopropanation to form 7 and 10, respectively, when decomposed photochemically or by transition metal catalysis. The thermal reaction of 9 produces the same result, whereas the l-oxa-2-sila-3-cyclopentene 8 is formed from 6, presumably via a pyrazoline intermediate. 

The thermal [1] or photochemical [5] isomerization of N-silylated allylamine in the presence of Fe(CO)5 provides the corresponding N-silylated enamines 7a and 7b. Z-enamine 7b does not react in any of the examined cycloadditions. The cyclopropanation of E-enamine 7a with methyl diazoacetate under copper(I) catalysis provides the donor-acceptor-substituted cyclopropane 9 [1], which can be converted in good yield into the interesting dipeptide 10 [6]. 

As an explanation, if was suggested that the degree of charge development in the transition state determines the preferred site of cyclopropanation A transition state with little charge development should prefer the endocyclic double bond (Pd catalysis), whereas one with much charge development should favor the exocyclic bond (Rh catalysis). 

Products of a so-called vinylogous Wolff rearrangement (see Sect. 9) rather than products of intramolecular cyclopropanation are generally obtained from P,y-unsaturated diazoketones I93), the formation of tricyclo[2,1.0.02 5]pentan-3-ones from 2-diazo-l-(cyclopropene-3-yl)-l-ethanones being a notable exception (see Table 10 and reference 12)). The use of Cu(OTf), does not change this situation for diazoketone 185 in the presence of an alcoholl93). With Cu(OTf)2 in nitromethane, on the other hand, A3-hydrinden-2-one 186 is formed 160). As 186 also results from the BF3 Et20-catalyzed reaction in similar yield, proton catalysis in the Cu(OTf)2-catalyzed reaction cannot be excluded, but electrophilic attack of the metal carbene on the double bond (Scheme 26) is also possible. That Rh2(OAc)4 is less efficient for the production of 186, would support the latter explanation, as the rhodium carbenes rank as less electrophilic than copper carbenes. 

Furans and some of its derivatives have been cyclopropanated with the ketocarbenoids derived from ethyl diazoacetate and copper catalysts. The 2-oxabicyclo[3.1.0]hex-3-enes thus formed are easily ring-opened to 1,4-diacylbutadienes thermally, thermo-catalytically or by proton catalysis 14,136). The method has been put to good use by Rh2(OAc)4-catalyzed cyclopropanation of furan with diazoketones 275 to bicyclic products 276. Even at room temperature, they undergo electrocyclic ring-opening and cis, trans-dienes 277a are obtained with fair selectivity 257,258). These compounds served as starting materials in the total syntheses 257 259) of some HETE s (mono-... 

Special interest attaches to the cyclic aliphatic hydrocarbons. Cyclopropane can be converted to oligomers by cationic catalysis [75, 76], and these appear to be essentially linear but whether they are really different from the polypropenes formed under the same conditions from propene is not yet settled. The initiation most probably involves formation of a non-classical cyclopropyl ion [77], as in alkylations with cyclopropane [78],... 

Carbenes are both reactive intermediates and ligands in catalysis. They occur as intermediates in the alkene metathesis reaction (Chapter 16) and the cyclopropanation of alkenes. As intermediates they carry hydrogen and carbon substituents and belong therefore to the class of Schrock carbenes. As ligands they contain nitrogen substituents and are clearly Fischer carbenes. They have received a great deal of attention in the last decade as ligands in catalytic metal complexes [58], but the structural motive was already explored in the early seventies [59],... 

Sulfonium ylides R2S=CR 2 [672,673] and metallated sulfones [674-676] can cyclopropanate simple alkenes upon catalysis with copper and nickel complexes (Table 3.6). Because of the increased nucleophilicity and basicity of these ylides, compared with diazoalkanes, these reagents are prone to numerous side-reactions,... 

As mentioned in Sections 3.1.6 and 4.1.3, cyclopropenes can also be suitable starting materials for the generation of carbene complexes. Cyclopropenone di-methylacetal [678] and 3-alkyl- or 3-aryl-disubstituted cyclopropenes [679] have been shown to react, upon catalysis by Ni(COD)2, with acceptor-substituted olefins to yield the products of formal, non-concerted vinylcarbene [2-1-1] cycloaddition (Table 3.6). It has been proposed that nucleophilic nickel carbene complexes are formed as intermediates. Similarly, bicyclo[1.1.0]butane also reacts with Ni(COD)2 to yield a nucleophilic homoallylcarbene nickel complex [680]. This intermediate is capable of cyclopropanating electron-poor alkenes (Table 3.6). 

Weitkamp, J. (1980) New evidence for a protonated cyclopropane mechanism in catalytic isomerization of n-alkane, in Studies in Surface Science and Catalysis, vol. 17 (eds... 

Dihalocarbenes generated under phase transfer catalysis add to vinylic tellurides to give the corresponding gem-dihaloarylteUuro cyclopropanes. ... 

Finally, one example of trityl salt analogues in the phase-transfer catalysis is presented. The highly stable triazatriangulenium cations 62 [161, 162] were jnst recently introduced to the phase-transfer chemistry [163], Persistent to strongly basic and nncleophilic conditions, these salts revealed efficient catalytic activity in addition reactions (Scheme 64). Modification of the alkyl side chains on nitrogen allowed matching the fair hydro/lipophilicity with the optimised conditions in the alkylation, epoxidation, aziridination and cyclopropanation reactions. The results are comparable to those of tetrabutylammonium salts and in some cases showed even a better outcome. 

Keywords Absolute configuration. Amino acids, Bicyclopropylidene, Coupling reactions. Cycloadditions, Cyclopropanation, Cyclopropanes, Organolithium derivatives. Palladium catalysis. Radical reactions. Small ring polycycles, Spiro compounds. Strain energy. Sulfides... 

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