Cyclopropenes allenes

Welter, W., Hartmann, A., and Regitz, M., Carbenes. Part 18. Isomerization reactions of phosphoryl-vinyl-carbenes to phosphorylated cyclopropenes, allenes, acetylenes, indenes, and 1,3-butadienes, Chem. Ber, 111, 3068, 1978. 

The irradiation of the thiophene in gas phase yields ethylene, allene, methyl-acetylene, carbon disulfide, and vinylacetylene. No Dewar thiophene or cyclo-propene derivatives were isolated (69CJC2965). The irradiation in liquid phase gave the Dewar thiophene which can be trapped as a Diels-Alder adduct with furan (85JA723). The Dewar thiophene and cyclopropene-3-thiocarbaldehyde can be obtained by irradiation in argon matrices at 10 K (86JA1691). 

Triple-bond compounds react with carbenes to give cyclopropenes, except that in the case of acetylene itself, the cyclopropenes first formed cannot be isolated because they rearrange to allenes. Cyclopropenones (p. 58) are obtained by hydrolysis of dihalocyclopropenes. ... 

The reactive compound chlorosulfonyl isocyanate (ClSOaNCO, forms P-lactams even with unactivated alkenes, as well as with imines, allenes, conjugated dienes, and cyclopropenes. With microwave irradiation, alkyl isocyanates also... 

Allylic cations (180) were also generated by LFP of allenes (174) in TFE.86 Deuterium labels revealed that the cations 180 originate predominantly from vinylcarbenes (177), which are formed from 174 by way of a 1,2-H shift. Protonation at the central carbon of the photoexcited allenes87 is a minor reaction path with 174a,b,d. Vinylcarbenes are also known to arise in photolyses of cyclopropenes, 175 — 177.85bi88 However, LFP of 175 in protic media proved to be rather inefficient in generating allylic cations, presumably due to low quantum yields. 

The two-step process, depicted by path b, involves initial addition of the carbene carbon to an adjacent it bond to form bicyclo[4.1,0]hepta-2,4,6-triene (2a). This process has precedent in the analogous rearrangement of vinylcar-bene to cyclopropene (Scheme 6),lc18 and is supported by Gaspar s work on 1-cyclohexenylcarbene.17 In the second step of the mechanism in Scheme 5, subsequent six-electron electrocyclic ring opening of 2a yields the cyclic allene 3a. 

Reaction of Me3GeCl with a substituted cyclopropene in the presence of lithium diisopropylamide (LDA) yields different products depending on the order of addition of the reagents.97 Addition of LDA to a mixture of the reactants gives the dimetallated cyclopropene (Equation (76)). Dilithiation of the cyclopropane followed by addition of Me3GeCl gives the allene (Equation (77)). 

The photolysis of the furan derivatives 78 yielded the butadienals 79 as the main products [123], Further isomerizations leading to allenic esters used the radiation of a cyclopropene-1 -carboxylic acid ester [124] or applied flash vacuum pyrolysis to 3 -ethoxy cyclobut- 2-en-l-one[125]. 

The addition to a carbon-carbon triple bond results in the formation of cyclo-propene products, and with diazoacetates the catalyst of choice for intermolecular addition is the dirhodium(II) carboxamidate 13 (e.g., Eq. 26). The reactions are general, except for phenylacetylene whose cyclopropene product undergoes [2 + 2]-cycloaddition, and selectivities are high. However, high selectivities have not been reported for reactions with allenes. 

Gem-Dihalocydopropanes belong to the most readily available cyclopropane derivatives known today. They have been shown to be extremely valuable starting materials for the preparation of cyclopropanes and cyclopropenes, they may be converted to bicyclobutane derivatives and spiropentanes, can lead to allenes and the higher cumulenes, cyclopentenes and cyclopentadienes, and many other classes of compounds, both hydrocarbon systems and derivatives with valuable functional groups. The article summarizes the preparative developments in the area of gem-dihalocyclopropane chemistry during the last decade. 

To summarize, gwi-dihalocyclopropanes may serve as starting materials for the preparation of cyclopropane and cyclopropene derivatives, they can lead to compounds with bicyclobutane and spiropentane structures, provide allenes and... 

The reactive compound chlorosulfonyl isocyanate796 CIS02NC0 forms 3-lactams even with unactivated alkenes,797 as well as with allenes,798 conjugated dienes,799 and cyclopropenes.8 l, OS V, 673 65, 135, 140. 

Jacox and Milligan66 studied the reaction of methylene with acetylene by photolysis of dilute (50 1) suspensions of CH2N2 and HC = CH in an argon matrix at 4°K. Product analysis by infrared spectroscopy indicated allene as the major product. Cyclopropene and methylacetylene were not formed in detectable amounts. 

Both methylacetylene and allene, in the ratio 1.6 1, were found by Terao and Shida130 in the gas phase photolysis of ketene-acetylene mixture. The product ratio was independent of pressure and wavelength of photolysis. Again, cyclopropene was not a product. ... 

The foregoing mechanism for addition of methylene to the double bond is substantiated by the observation of Jacox and Milligan69 that allene but not cyclopropene is produced upon reaction of CH2 with acetylene in an argon matrix at 4°K. It is quite reasonable to expect in this case that the rate of ring closure would be slow compared to the rate of hydrogen migration. 

One of the major areas of study in the photochemistry of heterocycles is the photorearrangement of five-membered heteroaromatic systems. Various mechanisms have been proposed to account for these transformations.123 A ring contraction-ring expansion pathway, via an intermediate cyclopropene, appears to be involved in the rearrangement of derivatives of furan. The isolation of an intermediate (140) of this type together with an allene (141) has been effected on irradiation of 2,4-di-tert-butylfuran (142), as shown in Scheme 9.124 Similar transformations have been reported in perfluorotri-... 

Treatment of the oxime 63 with methyl lithium at -20° gave 2,3-dimethyl-1-hydroxy pyrrole (65) in 50% yield, along with 20% of the allene oxime (64), which is the product isolated from this type of reaction when the cyclopropene contains additional alkyl groups (80TL2893). 

In a similar manner, 3,3-disubstituted l,2-bis(trimethylsilyl)cyclopropenes rearrange to l,l-bis(trimethylsilyl)allenes, most likely by 1,2-silyl shift of primarily formed (1-silylvinyl)silylcarbenes. According to ab initio calculations, this reaction pathway is energetically more favorable than those including a 2,2-disilylcyclopropylidene or a 2,3-disilylpropylidene90b. 

In contrast to carbon chemistry, the three-membered cyclic C2FLtSi isomers are relatively stable. Thus, silacyclopropylidene 608 and 2-silacyclopropene 609 are more stable than 604 by 10.9 and 13.8 kcalmol-1, respectively28513, while cyclopropene 610 is by 22.4 kcalmol-1 less stable than allene 611 and more stable by 62.0 kcalmol-1 than singlet cyclopropylidene 612286. 

Silyl-substituted cyclopropenes have been observed to rearrange photochemically giving high yields of allenes. A typical example is shown in equation 2 where the silylcyclo-propene 3 apparently ring-opened to the carbene 4 which, following 1,2-silyl migration, led to the allene 513. 

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