Polymerization cyclopropenes

Acetylenes may be regarded as the first members of the series their polymerization using olefin metathesis catalysts is described in Ch. 10. There is no recorded attempt to polymerize cyclopropene with metathesis catalysts the product would probably be cyclohexa-1,4-diene rather than polymer. 

Methylene cyclopropene (5), the simplest triafulvene, is predicted to be of very low stability. From different MO calculations5 it has been estimated to possess only minor resonance stabilization ranging to 1 j3. Its high index of free valency4 at the exocyclic carbon atom causes an extreme tendency to polymerize, a process favored additionally by release of strain. Thus it is not surprising that only one attempt to prepare this elusive C4H4-hydrocarbon can be found in the literature. Photolysis and flash vacuum pyrolysis of cis-l-methylene-cyclopropene-2,3-dicarboxylic anhydride (58), however, did not yield methylene cyclopropene, but only vinyl acetylene as its (formal) product of isomerization in addition to small amounts of acetylene and methyl acetylene65 ... 

Cyclopropene has rather exceptional properties compared to the other cycloalkenes. It is quite unstable and the liquid polymerizes spontaneously although slowly, even at —80°. This substance, unlike other alkenes, reacts... 

Over the last few years it has become clear that rhodium(II) acetate is more effective than the copper catalysts in generating cyclopropenes.12 126 As shown in Scheme 28,12S a range of functionality, including terminal alkynes, can be tolerated in the reaction with methyl diazoacetate. Reactions with phenyl-acetylene and ethoxyacetylene were unsuccessful, however, because the alkyne polymerized under the reaction conditions. 

Many chemists once believed that a cyclopropene could never be made because it would snap open (or polymerize) immediately from the large ring strain. Cyclopropene was eventually synthesized, however, and it can be stored in the cold. Cyclopropenes were still considered to be strange, highly unusual compounds. Natural-product chemists were surprised when they found that the kernel oil of Sterculia foelida, a tropical tree, contains sterculic acid, a carboxylic acid with a cyclopropene ring. 

Additions occur more easily if a carbanion with resonance or inductive stabilization is formed in the addition. Thus, fulvenes are very reactive, vinylsilanes and highly fluorinated alkenes somewhat less so. Styrene, 1,3-dienes, and enynes are more reactive than isolated alkenes, and Grignard reagents may be used to initiate anionic polymerization of styrenes, dienes, and acryhc monomers. Strained alkenes such as norbomenes and cyclopropenes are also more reactive. Examples of additions facilitated by resonance or substitution are shown in Scheme 8. 

Cyclopropenes are highly strained and thus very reactive molecules. Cyclopropene itself tends to polymerize even below 0 C and 1-methylcyclopropene dimerizes within minutes at room temperature via an ene reaction. In contrast, 3,3-dimethylcyclopropene does not undergo an ene reaction even at 100 C, but it readily oligomerizes under the influence of nickel(O) or cyclodimerizes to 3,3,6,6-tetramethyltricy-clo[3.1.0.0 ]hexane (80% yield) in the presence of palladium(O). 1,2-Diphenylcyclopropene is converted to 1,2,4,5-tetraphenylcyclohexa-1,4-diene in the presence of palladium(O) in high yield (equation 32). 4... 

However, the authors did not observe in their system cyclopropene or Q hydrocarbon, but since cyclopropene is unstable it may have polymerized to an unsaturated polymer. 

The polymerization of naturally occurring sterculic acid (2) in the presence of proton sources has been known for over 40 years and proceeds via acid-catalysed opening of the cyclopropene ring. The rearrangement of carboxylic acid 264a to lactone 265 is... 

An optically active diethylcyclopropcne was prepared from the corresponding optically active trimethylcyclopropylammonium salt cyclopropene (536) could not be prepared by pyrolysis of phenylcyclopropylammonium hydroxide (535) the volatile product polymerized at — (equation 140). A difunctional cyclopropenylcyclopropene was... 

Disubstituted cyclopropenes do not react in the above sence. 1,2-dimethyl-cyclopropene polymerizes at 0 °C in the presence of [(T)3-C3H5)PdCl]2 30). 1,2-Diphenylcyclopropene cyclodimerizes nearly quantitatively in the presence of Pd(dba)2 or Pd(r)5-C5H5)(r 3-C3H5) to yield 1,2,4,5-tetraphenyl-cyclohexa-1,4-diene 72) (see p. 96). 

Besides polymeric material, a mixture of three ene reaction products is obtained (18% combined yield) in the thermal reaction, which proceeds at 20°C within several minutes. Similar results are found when zeolites with a large pore diameter (5-13 A) are used as catalysts. After irreversible addition of the cyclopropene to the zeolite, only polymeric material is obtained. Zeolites with 3- or 4-A pores give rise to a product spectrum similar to that obtained with phosphane-free palladium(O) catalyst systems, i.e. 90-97% of the tricyclic dimers are formed from 1- or 3-methylcyclopropene at temperatures between — 35 and 35 C. The palladium(II) chloride catalyzed reaction almost quantitatively yields a 2 1 mixture of the isomeric 1,2- and l,4-dimethyl-evo-tricyclo[3.1.0.0 ]hexane (lA and IB) from 1-methylcyclopropene. The reaction is strongly exothermic and therefore must be carried out in dilute solutions with effective... 

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. ... 

The formation of cyclopropene (1) by a retro-Diels-Alder reaction occurred with low efficiency, and the product polymerized under the reaction conditions. -... 

If stored as a liquid, even at —78°, cyclopropene undergoes a fairly rapid polymerization reaction. However, in the gas phase, at temperatures above 325° (in a stream of helium), it isomerizes smoothly to yield methylacetylene. This is clearly analogous to the isomerization of cyclopropane to propylene. 

Disubstituted cyclopropenes do not react in the above sence. 1,2-dimethyl-cyclopropene polymerizes at 0 "C in the presence of [(T) -C3H5)PdCl]2 1,2-... 

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