Cyclopropene ring opening

The elimination is successful with a range of substituents at C-2 or C-3 of the cyclopropane, although in some cases the derived cyclopropenes ring-open even at room temprature to give vinylcarbenes, which may be trapped in inter- or intramolecular processes. It is successful when X or Y = Br even if R = H, but when X = Y = Cl and R = H, an alternative 1,2-elimination of HC1 occurs. By careful control of the quantity of reagent, it is possible to carry out the elimination in the presence of functional groups which are relatively reactive to such reagents (final two examples below)112-118,120,80. 

The alkyne is formally derived by a 1,4-dehydrochlorination with ring opening, although subsequent work has shown that the cyclopropene ring-opens even at ambient temperature to 3-chloro-3-methylbut-l-yne and so a 1,2-dehydrochlorination of this is an alternative. The carbene could be formed by several routes from the cyclopropene, and an early experiment with a 14C-labelled compound indicated that the label at C-l of the cyclopropane became C-l of the carbene ... 

The synthesis of cyclopropanes via cyclopropene ring opening with subsequent [1 +2] cycloaddition is restricted to thermolytically or photochemically initiated, and transition metal catalyzed reactions, and furthermore is only applicable to suitably substituted alkenes. 

Inasmuch as the cyclopropene ring opening is reversible it constitutes a carbene-carbene interconversion ... 

The heat of formation of triplet vinylcarbene is estimated as 97—99.3 kcal/mol (Table I). With an activation energy of ca. 35.2 kcal/mol, the transition state for cyclopropene ring opening lies at 100.3 kcal/mol, i.e. only 1—4 kcal/mol... 

A cyclopropene ring opening to vinylcarbene followed by intramolecular addition to tetramethylenebenzvalene was proposed to occur on the pathway to the major products (Scheme 11.67). 

Silyl and geimyl functionalised vinyl cyclopropenes ring-open thermally to give allene and alkyne derivatives with the former dominating, a-trimethylsilyl a-allenyl ketones couple to give enediynes related to the neocarzinostatin chromophore and also result fnnn Y-(t-BuMe2Si)allenylborane and acetylenic aldehydes. Allenylsilanes couple with AdQ and... 

The gold(I)-catalysed rearrangement of cyclopropenes has been explored by DFT calculations, which indicate that the product ratio is controlled by the initial formation of a gold carbene intermediate by substituent-directed cyclopropene ring opening. " ... 

The products from cyclopropene ring opening have the advantage of easy conversion into tricarbonyl(Ti -diene)iron complexes (34), its regioisomer and (35), by thermal loss of carbon monoxide, or photochemically without CO evolution (eqs 19 and 20). ... 

A thermal ring opening reaction of an imine-substituted cyclopropene led to a mixture of 2,3,4-trisubstituted and 3,4,5-trisubstituted pyrroles <06TL5793>. 

UV irradiation (A. > 305 nm) of cyclopropene 3a results in the ring-opening and formation of ketene 4a. Two reaction pathways for the 3a - 4a rearrange-... 

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. 

To direct a solvolytic ring opening, 2-methoxycyclopropyllithium (6) was developed as a chain extension conjunctive reagent. The failure of P-elimination to occur in 6 presumably derives from the high strain of cyclopropene and poor orbital overlap for elimination. The aldehyde adducts smoothly solvolyze to give p,Y unsaturated aldehydes (Eq. 22) 23) which are best initially isolated as their hemithioacetals. 

Matrix isolation photolytic studies on tetrazolo[l,5- ]pyridazine 25 have been reported by Hill and Platz <2003PCP1051> (Scheme 5) and formation of the l-cyano-3-diazopropene 27, triazacycloheptatetraene 28, and cyano-cyclopropene 29 was detected. Upon the absence of electron spin resonance absorptions at 7 K, the authors concluded that triplet nitrene was not formed but, instead, the resulting singlet nitrene rapidly underwent further ring openings. 

In addition to the ring opening of cyclopropenes noted above, vinylketene complexes 103 have been prepared by (1) ligand initiated carbonyl insertion of vinyl carbene complexes 104 and (2) benzoylation of ,/3-unsaturalcd acyl ferrates 105 (Scheme 20)114. X-ray diffraction analysis of these vinylketene complexes indicates that the structure may be best represented as a hybrid between an /j4-dicnc type complex (103) and an jj3-allyl r/1 acyl complex (106). The Fe-Cl distance (ca 1.92 A) is shorter than the Fe-C2, Fe-C3, or Fe-C4 distances (ca 2.1-2.2 A)113a-C. In addition, the C—C—O ketene array is not linear (bend angle ca 135°). 

Both sets of observations confirm that the ring-opening process is subject to both steric and electronic influences, the combination of which produces some subtle effects in product distribution. Ultimately, the regiochemistry of the product must be decided after a cyclopropene-Fe(CO)4 complex is formed, but probably during the cleavage of one of the C-C bonds in the cyclopropene ring. 

The most common method for the generation of the metal alkylidene species seems to be a-elimination from an intermediate dialkyl-metal species. This procedure gives the most active catalysts. Above we mentioned the addition of other carbene precursors, which leads to active catalysts. Other methods to generate the metal alkylidene species involve alkylidene transfer from phosphoranes [16] or ring-opening of cyclopropenes [17], In Chapter 16.4 we will describe a few compounds that are active by themselves as metathesis catalysts. 

One special case of cross metathesis is ring-opening cross metathesis. When strained, cyclic alkenes (but not cyclopropenes [818]) are treated with a catalytically active carbene complex in the presence of an alkene, no ROMP but only the formation of monomeric cross-metathesis product is observed [818,937], The reaction, which works best with terminal alkenes, must be interrupted when the strained cycloalkene is consumed, to avoid further equilibration. As illustrated by the examples in Table 3.22, high yields and regioselectivities can be achieved with this interesting methodology. 

The dichlorocarbene adduct 148 of 9-methoxyphenanthrene eliminates HCl instead of MeOH and forms a cyclopropene 152, Ring-opening produces the substituted vinylcarbene 154. The latter inserts intramolecularly into the methoxy group and, after elimination of HCl from 157, a phenanthrofuran 158 is obtained. The sequence is applicable to substituted furans and even to phenanthrocyclopen-tadienes. ... 

The synthesis of cycloproparenes resulting from formal fusion of a cyclopropene to furan and thiophene has been attempted with limited success. Reaction of the dichloro-oxabicyclohexane 180 (X = O) " afforded a cyclopropene 181 which ring-opened to a vinylcatbene 182, but the cycloproparene 183 was not produced. Similarly, the thia-analogue 180 (X = S) could not be converted to 184. The intermediate cyclopropenes and/or vinylcarbenes have been trapped. A cyclopropathiophene derivative 186 was generated, however, from 185. Although it was not isolable, it afforded a bis-adduct 187 when it was produced in the presence of isobenzofuran (45)." ... 

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