Isobutene cyclopropanation

The catalytic asymmetric cyclopropanation of an alkene, a reaction which was studied as early as 1966 by Nozaki and Noyori,63 is used in a commercial synthesis of ethyl (+)-(lS)-2,2-dimethylcyclo-propanecarboxylate (18) by the Sumitomo Chemical Company (see Scheme 5).64 In Aratani s Sumitomo Process, ethyl diazoacetate is decomposed in the presence of isobutene (16) and a catalytic amount of the dimeric chiral copper complex 17. Compound 18, produced in 92 % ee, is a key intermediate in Merck s commercial synthesis of cilastatin (19). The latter compound is a reversible... 

Investigation of Ethane, Propane, Isobutane, Neopentane, Cyclopropane, Cyclopentane, Cyclohexane, Allene, Ethylene, Isobutene, Tetramethylethylene, Mesitylene, and Hexamethylbenzene. Revised Values of Covalent Radii (by Linus Pauling and L. O. Brockway)... 

However, in order to avoid the epoxide moiety in 4-67, which might serve as a locus of nondiscrimimating cell toxicity, these authors focused their activity on the cyclopropane-analogue 4-70 by heating a mixture of 4-68 and 4-69, which led to 4-70 with extrusion of isobutene in 75% yield (Scheme 4.15). 

In addition, 18-19% of isobutene and chloroacetylene formed via fragmentation. Photolysis of the diazirine in up to 9 M trimethylethylene in pentane led to a sharp decrease in 27 and 28 (to 32% and 8.5%), along with 40% of cyclopropanes formed via the capture of 19. However, the yield of isobutene and chloroacetylene was unchanged, indicating that these products did not stem from the carbene, but arose directly by fragmentation of its excited diazirine precursor.45... 

Although the reaction of a titanium carbene complex with an olefin generally affords the olefin metathesis product, in certain cases the intermediate titanacyclobutane may decompose through reductive elimination to give a cyclopropane. A small amount of the cyclopropane derivative is produced by the reaction of titanocene-methylidene with isobutene or ethene in the presence of triethylamine or THF [8], In order to accelerate the reductive elimination from titanacyclobutane to form the cyclopropane, oxidation with iodine is required (Scheme 14.21) [36], The stereochemistry obtained indicates that this reaction proceeds through the formation of y-iodoalkyltitanium species 46 and 47. A subsequent intramolecular SN2 reaction produces the cyclopropane. 

The reaction of CH2 with cyclopropane1617 46177 gives excited methyl-cyclopropane which is deactivated or undergoes structural isomerization to butene-1, butene-2 (m and trans), and isobutene. The lifetime of methylcyclopropane wa6 found to depend on the CH2 source. 

Enantioselective Cyclopropanation of Alkenes. Cationic Cu complexes of methylenebis(oxazolines) such as (1), which have been developed by Evans and co-workers, are remarkably efficient catalysts for the cyclopropanation of terminal alkenes with diazoacetates. The reaction of styrene with ethyl diazoacetate in the presence of 1 mol % of catalyst, generated in situ from Copper(I) Trifluoromethanesulfonate and ligand (1), affords the (rans -2-phenylcyclopropanecarboxylate in good yield and with 99% ee (eq 3). As with other catalysts, only moderate transicis selectivity is observed. Higher transicis selectivities can be obtained with more bulky esters such as 2,6-di-r-butyl-4-methylphenyl or dicyclohexylmethyl diazoacetate (94 6 and 95 5, respectively). The efficiency of this catalyst system is illustrated by the cyclopropanation of isobutene, which has been carried out on a 0.3 molar scale using 0.1 mol % of catalyst derived firom the (R,R)-enantiomer of ligand (1) (eq 4). The remarkable selectivity of >99% ee exceeds that of Aratani s catalyst which is used in this reaction on an industrial scale. 

The aminocyclopropane derivative 374 and the propellane 372 were obtained, each in 26 % yield, by photocycloaddition of 370 and ethylene in acetone . Diradicals 371 and 373 were suggested as intermediates in the photoreaction (equation 90). The use of isobutene or 2,3-dimethyl-2-butene instead of ethylene gave no cyclopropane product . 

MAPP gas from Dow Chemical Company, Midland, Mich. Gas chromatography showed 8.1% propene, 20.2% propane, 28.9% propyne, 29.7% allene, 1.3% cyclopropane, 2% isobutane, and 9.7 % of 1-butene and isobutene. 

Fig. 6.3. Gas chromatogram of C,-C4 hydrocarbons on alumina-modified squalane. Peaks 1 = methane 2 = ethylene 3 = ethane 4 = acetylene 5 = propylene 6 = propane 7 = propyne 8 = propadiene 9 = cyclopropane 10 = 1-butene 11 = isobutene 12 = frans-2-butene 13 = c/s-2-butene 14 = n-butane 15 = isobutane 16 = 1,3-butadiene 17 = cyclobutane. From ref. 103.

Carhon-carbon bond distances. The electron diffraction investigation of ethane, propane, isobutane, neopentane, cyclopropane, cyclopentane, cyclohexane, allene, ethylene, isobutene, tetramethylethylene, mesitylene, and hexamethylbenzene. Revised values of covalent radii. J. Am. Chem. Soc. 59 (1937) 1223—1236. (Linus Pauling and L. O. Brockway). SP 60 The electron diffraction study of digermane and trigermane. J. Am. Chem. Soc. 60 (1938) 1605—1607. (Linus Pauling, A. W. Laubengayer, and J. L. Hoard). 

Scheme 6.5 A volatile alkene like isobutene filling the role of a pericyclic leaving group in the synthesis of cyclopropane-derivatized radicicol.

Complex 1 was found to activate a wide variety of hydrocarbons, including propane, pentane, cyclohexane, cyclopentane, methane, mesitylene, isobutene, and r-butylethylene [8, 9]. For linear hydrocarbons, a kinetic preference was observed for the exclusive activation of the C-H bonds of the terminal methyl groups. The activation of secondary C-H bonds was only observed when no other primary C-H bonds were available (e.g., cyclohexane, cyclopentane, cyclopropane [10]). With mesitylene, both aromatic and benzylic C-H bonds were cleaved. These observations were interpreted in terms of initial coordination of the hydrocarbon C-H bond to the 16-electron rhodium fragment, followed by rapid migration along the chain to... 

A different type of ring contractions, involving dihydrofuranes 749, occurs under excitation by hght, where a stabihzed radical is formed which recombines to the cyclopropane 150 [237] Reaction scheme 95). Here it is the transition from the 5-membered ring to the 3-membered ring. Now it is a question of how to prepare dihydrofurans. One proposal uses the aluminum-catalyzed addition of an enol upon double bonds, as shown in Reaction scheme 96. The needed precursor is the product 152 of a Claisen-rearrangement based on the chloral-isobutene adduct 126 [238]. 

The hydrocarbons acetone, benzene, butene, cw-butene, cyclohexane, cyclopentane, cyclopropane, ethylene, isobutene, isooctane, methylcyclohexane, propylcyclohexane, neopentane, propyne, franj-hutene, and toluene. 

By comparison, thermolysis of l-diazo-2-methylpropane gave only methyl-cyclopropane (33%) and isobutene (67%). Many other carbenes were generated via thermolysis and/or photolysis of the corresponding... 

Carbene Chemistry. Isobutene reacts readily with car-benes and metallocarbenes to form substituted cyclopropanes (eqs 25-27). Reaction of Ethyl Diazoacetate with isobutene in the presence of a chiral copper catalyst affords the cyclopropane in high optical purity (eq 28). Isobutene has also been used to form titanocyclobutanes that are more stable forms of the Tebbe reagent (see L-Chlorobis(cyclopentadienyl)(dimethylaluminum)- L-methylenetitanium) (eq 29). ... 

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