Cyclopropenes Methylenecyclopropenes

One may see [2 + 2] cycloadditions of alkenes even when radical-stabilizing groups are lacking if the alkene is of relative high energy cyclopropenes, methylenecyclopropenes, bridgehead alkenes and rrun5-cycloalkenes exemplify the point. 

When subjected to strong bases, gem-dihalocyclopropanes undergo dehydro-halogenations, and cyclopropenes are formed. These are generally unstable under the reaction conditions and participate in further transformations. The most common of these processes is the isomerization of the newly formed double bond from the endo- to the exo-orientation, followed by a second dehydrohalogenation step. The methylenecyclopropenes thus generated are still not stable, and subsequently tend to rearrange to less strained systems. 

Oxidative activation of cyclopropenes is much less frequently encountered. The reactions of various platinum(O) complexes with the electron-deficient methylenecyclopropene 170 affords platinacyclobutene complexes, as reported nearly 30 years ago <1978ICA19>. More recent investigation has established that in the presence of two or more equivalents of the metal, bicyclic diplatinum complexes can be generated (Scheme 40) <1996JBS75>. 

The addition of dihalocarbenes to alkynes is again a rather inefficient process and usually leads, to the isolation of the cyclopropenone rather than the 3,3-dichlorocyclo-propene. In a rather unusual example, however, 2-butyne is reported to be converted to (67). This product is apparently derived by addition of dichlorocarbene to the corresponding methylenecyclopropene, derived in turn by elimination of HC1 from the primary adduct (68). The cyclopropene (67) does not appear to ring open to a vinylcarbene, but can be trapped in Diels-Alder reactions with cyclopentadiene 60). A related addition of dichlorocarbene to ethyl 2-butynoate also leads to a low yield of the 3,3-dichlorocyclopropene, which may be hydrolysed to the cyclopropenone 6l). 

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 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). 

Photolysis of matrix isolated diazo(2-furyl)methane led to the aldehyde (79) by stereospecific rearrangement of the carbene (80). " The corresponding 3-furyl compound gave the (5-Z)-methylenecyclopropene (81) by ring closure of the initially formed vinylcarbene to give cyclopropene (82) followed by ring opening of the furan. 

Knowledge of the energetics of (C-C3H2 )-CH 2 (15) gives information relevant to those of other cyclopropene derivatives. If one accepts the above experimentally determined heat of formation of C4H4, the theoretical energy difference of vinylacetylene and methyl-enecyclopropene, and an ill-defined experimentally derived heat of formation of vinylacetylene S the ionization potential of methylenecyclopropene is indirectly deduced to be 8.2 eV. This value is meaningfully compared to the 9.5 eV directly measured as the ionization potential of cyclopropenone. The derived 1.3 eV difference is comparable to the 1.1 eV difference for the antiaromatic cyclopentadienone (with a vertical value of 9.5 eV) and non-aromatic methylenecyclopentadiene (i.e. fulvene, with an IP of 8.4 eV). 

Because of the dipolar property and high strain, triafulvenes are susceptible to nucleophilic attack by bases at the cyclopropene moiety hence, the elimination method has limited utility except for the preparation of the parent compound methylenecyclopropene (1). 

Following sections of this chapter discuss in turn cyclopropenium salts, cyclopropenones and other cyclopropene derivatives such as methylenecyclopropenes, also called triafulvenes. 

Again, the main reactions correspond to formal insertion of the ring-opened cyclopropen-one (or -thione) into the C—N bond rather than into the C—C bond of the unsaturated nitrogen compounds. The comparable reactions of (431) with l-azirines and of enamines and ynamines with substituted methylenecyclopropenes have recently been reported. 

I 1.5. Assign the ionizations indicated in the PE spectra for cyclopropane, methylenecy-clopropane, cyclopropene, and methylenecyclopropene. 

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