Hydrogenation of cyclopropene

Cyclopropanes are now readily available and have become useful, through hydrogenolysis, for synthesis of compounds containing quaternary carbons, em-dialkyl, r-butyl, and angular-methyl substituents (779), compounds often available only with difficulty otherwise (.77,5i,55,750,756), Cyclopropanes can be formed in good yields by hydrogenation of cyclopropenes (26). 

On the other hand, the observation that hydrogenation of cyclopropene is much more exothermic than would be expected for a typical cycloalkene, reveals the effect of ring strain. 

The ds-hydrogenation of cyclopropenes is unexceptional in itself but when preceded by the addition of diazopropane to the yne component of an enyne (Section II.B) it provides a stereospecific synthesis of chrysanthemic acid esters (natural pesticides) in high yield. Reduction of unsymmetrical cyclopropenes with lithium aluminium... 

Table 1. Formylated Cyclopropanes 7 and Cyclopropanes 8 by Cobalt and Manganese Carbonyl Hydride Mediated Hydroformylation/Hydrogenation of Cyclopropenes ...

TE Nalesnik, JH Freudenberger, M Orchin. Radical hydroformylation and hydrogenation of cyclopropenes with HCo(CO)4 and HMn(CO)5. J Organomet Chem 236 95-100, 1982. 

These energies may also be obtained from the data in Table 2, but here one must correct for the very large enthalpy of hydrogenation of cyclopropene, as compared to ci5-2-butene. 

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

In the early syntheses of alkenyl alkylidene-mthenium catalysts, the first generation of Grubbs catalyst, it was observed that propargyl chloride could be a convenient source of the vinylcarbene initiator [53] with respect to the previous one arising from activation of cyclopropene [4] (Equation 8.3). In this synthesis the alkylidene hydrogen atom arises from the ruthenium hydride. 

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) ... 

The triplet states of cyclopropenes containing a heteroatom at the 2-position of a 3-substituent undergo hydrogen atom abstraction to produce diradicals, which close to produce bicyclo[3.1.0]hexanes, eg.206) ... 

Klett and Johnson have demonstrated the interrelationship between the photoreactions of allenes (93) and cyclopropenes. Their results indicate that irradiation of (93) leads to the carbenes (94) and (95) such carbenes have been proposed as the key intermediates in the photoreactions of cyclopropenes. Thus the irradiation of (93a) yields the indene (96) as the primary photo-pnoduct via the carbene (94) produced by phenyl migration. With aUene (93b) irradiation affords the carbene (95), by hydrogen migration, which then reacts to form photoproducts (97)—(99). For comparison, the irradiation of cyclo-propene (100) was carried out it yielded the allene (93b) and the two indenes (97) and (99). 

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. 

Donor-substituted halocyclopropanes are very versatile building blocks in organic synthesis. An early application which is demonstrated in equation 135 constitutes the overall transformation of an enol ether to a J -methyl a,j5-unsaturated ketone which in the case presented gives muscone after hydrogenation. A cyclopropene and allene are the proposed intermediates in this process. 

Chlorination of cyclopropenes is best effected under radical conditions due to the greater stability of the cyclopropyl radical over its cation equivalent. Thus chlorination of 135 is stereospecifically cis irrespective of the reaction conditions but the radical path is most efficient. Radical hydrogenation and hydroformylation of cyclopropenes is effected with HMn(CO)s and HCo(CO)4. Intermediate caged radical pairs are... 

The formation of propyne and allene by pyrolysis of cyclopropene arises from opposite [1,2]H shifts in diradicals 191 or 192 The substantially larger activation energy (by some 24.5 kJ mol" ) for formation of allene reflects differences in the transition state structures for the two processes. Thus, the propyne-forming reaction requires the migrating hydrogen atom to span a single bond (see 194), whilst in the allene-forming process a double bond is involved and a more strained situation ensues (see 195). The formation of but-2-yne from 3-methylcyclopropene is similarly rationalized but the... 

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