Methylcyclopropane derivative

In the ring contraction of 2-bromocyclobutanone, the nature of the solvent does not seem important thus, traM-2-bromo-3-methylcydobutanone underwent stereospecific ring contraction to form tran -2-methylcyclopropane derivatives 23 exclusively, either using ammonia, aqueous sodium carbonate (basic medium) or water (acidic medium). ... 

Unlike the well-known chemistry of the vinylcyclopropane-cyclopentene rearrangement, there is no general method for the rearrangement of alkynyl-cyclopropane to cyclopentene derivatives. One specific example is the pyrolysis of l-ethynyl-2-methylcyclopropane to methylenecyclopentene and other compounds [5]. At 530°C, l-ethynyl-2-methylcyclopropane (1) undergoes a [1,5]-hydrogen shift to give hexa-l,2,5-triene (2), which further isomerizes to methy-lenecyclopentenes 3 and 4 in 38 and 29% yield, respectively (Scheme 1). 

The enthalpies of formation of liquid methylcyclopropane and cyclobutane are quite close, —1.7 0.6 and 3.7 0.5 kJmol . How do the enthalpies of formation of their corresponding Grignards compare The enthalpy of formation from Table 3 for cyclobutyl MgBr is —230 kJ mol. The enthalpy difference between the cychc C4H7MgBr isomers is thus ca 11 kJ mol which is not too different from that for the hydrocarbon counterparts, especially considering the uncertainties of the estimates used in this derivation. 

The first nonintroductory section of the text starts with the observation made early in this century that cyclopropanes have significant olefinic character. That is, corresponding ethylene and cyclopropane derivatives have significant similarities. There are literature comparisons of the thermochemistry of direct counterparts such as the parent species (1 and 2, X = H), propene and methylcyclopropane with X = Me, and of methyl acrylate and methyl cyclopropanecarboxylate 8 with X = COOMe. But the chemistry of substituted eth-ylenes is far richer than just that of vinyl compounds. One can retrieve enthalpy of formation data for cumulenes ( cumulenated olefins) such as allene (3) and both cis- and trans-2,3,4-hexatriene (4a and 4b), and for conjugated olefins such as 1,3-butadiene (5) and both (Z)- and (E)-1,3,5-hexatriene (6a and 6b). For the cyclopropane chemist it is natural to... 

Use of chiral cyclopropylethynyllithium derivatives permits the elegant selective synthesis of labeled chiral vinylcyclopropanes, for stereochemical studies of the thermal vinyl cyclopropane-cyclopentene rearrangement237. Thus, reductive elimination of (1 S,trans)-(2,2-dibromoethenyl)-l-methylcyclopropane with BuLi in pentane, followed by hydrolysis of the lithium acetylide, afforded (15,/ram)-2-ethynylmethylcyclopropane (equation 157). 

In these substances,as in tne 1,2,3 derivatives, it ie interesting to compare two cyclopropanes differing only by tne substitution of a carboxyl group by methyl two such compounds are tne 1,1,2,3 tetra acid and methylcyclopropane 2,2,3 triacid ... 

The reaction of either cis- or trans-1 with potassium /-butoxide in tetrahydrofuran at 25 °C leads to a /-butyl ether (2), apparently arising by attack of /-butoxide ion on an intermediate l,4-di-/-butylmethylenecyclopropene. If the reaction is carried out at low temperature and the volatile materials are distilled directly into a cold trap, the cyclopropene can be trapped, albeit in low yield (10 %), by added cyclopentadiene or detected directly by low-temperature NMR23. In a related example, a l,l-dihalo-2-bromo-3-methylcyclopropane (2a) leads to products which are also apparently derived through an intermediate 1 -chloro-3-methylenecyclopropene which undergoes nucleophilic addition (See Ref. 80). 

We start with their simplest systems and we first consider the parent hydrocarbons benzene and cyclopropane themselves. This enthalpy of formation difference (5i9(Ph, cypr H) is (29.3 0.9) kJ mol Turning now to the simple alkyl derivatives, and 1 and 11, X = Me in particular, we recall that there are no direct experimental data for the enthalpy of formation of gaseous methylcyclopropane but only for the corresponding liquid. Accepting the archival enthalpy of vaporization value from Kolesov and Kozina " is equivalent to accepting their value for the enthalpy of formation of methylcyclopropane,.namely 24.3 kJ mo We so deduce a value for (5i9(Ph, Cypr Me) of [26.1 ( >0.9)] kJ mol some 3 kJ moT different fom that for the parent hydrocarbons. We are not particularly bothered by this 3 kJ moT discrepancy—we recall in footnote 23 a 6 kJmoT spread of values suggested for the enthalpy of vaporization of methylcyclopropane. It is conceptually simplest, and procedurally most precise, to use the identical approach to compare ethylbenzene and ethylcyclopropane, for there are no enthalpy of vaporization measurements for the latter 3MR species. Encouragingly, consistency of results is obtained —the value of i9(Ph, Cypr Et) equal to 26.5 kJmoT is nearly identical to that of i9(Ph, Cypr Me). 

The pyrolysis of chlorodiazirines appears to parallel that of unsubstituted diazi-rines. For the 3-chloro-3-methyl, 3-chloro-3-ethyl, 3-chloro-3-isopropyl and 3-chloro-3-r-butyI derivatives the decompositions are kinetically of the first order with A factors close to 10 and energies of activation of about 31 kcal.mole . The intermediacy of carbenes is postulated by analogy with the photolysis of 3-chloro-3-methyl-diazirine in the presence of ethylene where the major product has been tentatively identified as 1-chloro-l-methylcyclopropane. 

The nucleophilicities of cyclopropenone acetals", bromoketene acetals and an oxaphosphorane derivative, 26, are high enough to enable reaction with some Michael acceptors. Thus, cyclopropaneacetic esters (e.g. equation 88), cyclopropanecarboxylic esters, and 1-acetyl-1-methylcyclopropanes have been generated, respectively, from these precursors. 

In hindered or strained systems the yields are more variable. Thus, the yield of methylene product from 1-chloro-l-methyl-2,2-diphenylcyclopropane was only 39%. ° In a series of methylenecyclopropanes containing a bicyclic system with aryl bridgehead substituents, the yields in some cases were very poor (Table 1). In the absence of such substituents, however, the yields are generally very good. Thus, a 78% yield of pure 7-methylenebicyclo[4.1.0]heptane (13, n = 2) and a 85% yield of pure c/5-9-methylenebicyclo[6.1.0]nonane (13, n = 6) were obtained from the 1-chloro-l-methylcyclopropanes 12 derived from cyclohexene and (Z)-cyclo-octene, respectively. ... 

When a side chain larger than methyl is not present, diene products are not formed. Thus, for example, 1,1 -dichloro-2,3-dimethylcyclopropane does not give bis(methylene)cyclopropane but rather products derived from double dehydrochlorination followed by addition of base (see Section 5.2.2.1.3.). l,l-Dichloro-2-methylcyclopropane gives only polymeric material, presumably due to the fact that the cyclopropene intermediate has no avenue of escape for the double bond from the three-membered ring. ... 

Photochlorination of cyclopropane gave chlorocyclopropane (la) and 1,3-dichloro-propane. The latter was the major product at low temperature. Photochlorination with tert-h Ay hypochlorite gave mostly chlorocyclopropane (la). Methylcyclopropane reacted with chlorine to give predominantly l-chloro-2-methylcyclopropane (lb), but small amounts of acyclic products such as 2-chlorobutane, 1,3-dichlorobutane, and 1,3-dichloro-2-methyl-propane were also obtained. With ferr-butyl hypochlorite 4-chlorobut-l-ene was isolated as the only acyclic product. Photochlorination of 1,1-dimethylcyclopropane in trichloro-fluoromethane atO°C gave the chloromethylcyclopropane derivative 2 in 67% yield after immediate workup. 

Isotope labeling experiments and EPR spectroscopic studies have shown that the cyclopropylmethyl radical is a discrete chemical species with a finite lifetime = 7 x 10 s at 25°C in methylcyclopropane solution). Unlike the corresponding cyclopropylmethyl cation, 1 has no nonclassical or fluxional characteristics, and it does not rearrange to cyclobutane derivatives. Its rate of formation from diazenes, peroxides, and other precursors is slightly greater than that of model primary acyclic radicals, which indicates that it has a small thermodynamic stabilization. EPR spectroscopic studies have shown that rotation of the methylene group carrying the unpaired electron is not free and that the preferred conformation is bisected rather than perpendicular. 

This consequence is directly verifiable in a variety of experiments. The most significant of these are the chemical activation studies (8-13) reported in the 1960s and 1970s. The principle idea of these studies is to prepare a highly excited molecule by two or more different chemical means and then to compare the rates and branching ratios of unimolecular reactions derived from these different preparations. This is illustrated by a comparison of the unimolecular reactions of methylcyclopropane following preparation from two reactions ... 

Besides electronic structure and bonding, other molecular features of cyclopropane derivatives, such as conformation, have been theoretically scrutinized. The barrier to rotation about the bond joining methyl to cyclopropyl in methylcyclopropane has been calculated to be 1.82 kcal mol" using the INDO approximation. When methyl is replaced by amino- or phosphino-groups, there is more than the single minimum -in the rotation energy profile but, for both compounds, (1) is the most stable conformation. It is calculated... 

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