Cyclopropenes vinyl hydrogen

Compare the chemical shift of cyclopropene vinyl hydrogens (7-0, Wiberg and Nist, 1961) with normal alkenes ( 6 0). 

From the electronic populations on the vinylic hydrogens, the acidity of vinylic C—H was estimated to be higher in cyclopropenone than in cyclopropene (0.684 e/ 0.776 e). This agrees with kinetic measurements of the H-D-exchange at n-propyl cyclopropenone23 which showed an acidity of the vinylic C—H even higher than that of the acetylenic C—H in the reference compound propargyl alcohol. 

A review of infrared (and Raman) spectra of cyclopropenes has been presented (see also Ref. 316). In cyclopropene itself a weak symmetrical stretch (Aj) involving the vinyl hydrogens occurs at 3076 cm" and it was proposed that the C-H stretching vibrations that involve the methylene carbons of cyclopropene should approximate to those of cyclopropane. 

The addition of phenyllithium to cyclopropene occurred with 99% stereoselectivity to give cw-l-lithio-2-phenylcyclopropane, albeit in 3 /o yield. With 3-methylcyclopropene, the reaction was more efficient and, after protonation of the lithiocyclopropane, 2-methyl-1-phenyl-cyclopropane was obtained in 44.5% yield. The product is about 94% in the trans form thus attack of the organolithium occurs on the opposite face to that bearing the larger substituent at C3. When the cyclopropene is 3,3-disubstituted, e.g. 17, both faces are sufficiently hindered to slow the addition process and metalation of a vinylic hydrogen predominates. ... 

Vinylic hydrogen atoms in strained ring systems have an increased acidity [65]. Cyclopropenes can be smoothly metallated with alkyllithium [66] (in the case of the unsubstituted hydrocarbon, addition across the double bond occurs), lithium dialkylamides in THF [9], or with alkali amides in liquid ammonia [16]. Whereas reaction of propene with strong bases results in the exclusive formation of the allylic derivative, the kinetically preferred process with 1-methylcyclopropene is vinylic deprotonation ... 

The metal ion may be partially covalently bonded to the anion [199], Ab-initio calculations on the substitution of the hydrogen atoms of cyclopropene by lithium indicate that substitution of the vinylic hydrogen is energetically favoured compared to substitution of a methylene hydrogen because of the development of so-called antiaromatic character in the latter process [201]. 

When one deprotonates propene, it is the methyl hydrogens that are the most acidic. Deprotonation creates the resonance stabilized allylic anion. When the analogous reaction is attempted with cyclopropene, a vinylic hydrogen is the one removed. Deprotonation of the CH2 group in cyclopropene (Eq. 2.19) would create an antiaromatic anion, an undesirable effect, and this reversal in acidities provided early support for the notion of destabilization due to antiaromaticity. 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

Reaction of cyclopropenes with bases such as alkoxide or amide ions often leads to a methylenecyclopropene by removal of an allylic hydrogen and reprotonation 6 9-71) though other reactions such as nucleophilic addition (see Section 5) or metallation at a vinylic position (see Section 2) may compete. Thus the ester (209) is isomerised by KOH to (210), and under more vigorous conditions to (211)144) ... 

Addition of a rhodium carbenoid to an alkyne leads to a cyclopropene derivative. In an intramolecular context, the fused cyclopropene moiety is unstable and undergoes ring opening to generate a rhodium vinyl carbenoid entity, which can then undergo cyclopropanation or cyclopropena-tion, carbon hydrogen insertion, and ylide generation. This is illustrated... 

Organolithium reagents undergo addition to the 7t-bond of cyclopropenes, but the reaction competes with lithium-hydrogen exchange at the vinylic position and rearrangement to a methylenecyclopropane by removal of an allylic exocyclic hydrogen. 

A stepwise 1,7-vinyl shift was proposed to account for the reaction after carbene addition to make the cyclopropene. Deuterium labeling studies were consistent with this pathway. Interestingly, upon heating to 145°C the phenyl-substituted 5.2.0 tetraene isomerized to 2-phenylindene in what appears to be a cyclization of the cycloheptatriene moiety followed by opening of the bicyclo[2.1.0]pentene and a 1,5-hydrogen shift (Scheme 10.9). 

The CBS-QB3, MCG3, RRKM/master equation, and transition-state theory (TST) calculations were made for a detailed analysis of cyclopropene ozonolysis, treating all possible conformers of all intermediates and transition structures (TSs). The TST rate constant indicated that approximately 90% of the reaction proceeded through the endo-TS. It was predicted that approximately one-third of activated syn Criegee intermediates (Cls) would cyclize to dioxiranes despite the fact that the barriers to dioxirane formation were higher than the barriers to the 1,4-hydrogen shift that would lead to vinyl hydroperoxides and "OH. This helped to reduce the predicted OH yield for cyclopropene ozonolysis to 44%. It was also predicted that approximately 20% of either the endo-primary ozonide (PO) or its syn Cl derivatives would isomerize to the exo-PO or anti Cls. ... 

The different conformers of vinylcyclopropane will have different long-range n.m.r. coupling constants between the vinyl and the cyclopropyl hydrogens. This has been the subject of a semi-empirical theoretical study. There have also been reports of a theoretical study of the conformations of cyclopropene monohydrate and of MO calculations on carbo-cations which contain three- and four-membered rings. ... 

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