Ring opening of cyclopropene

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°). 

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. 

The ring opening of cyclopropenes to species which behave like vinylcarbenes has long been established in a range of systems the reverse reaction of ring closure of... 

The ring opening of cyclopropenes may also be induced by metal salts or complxes. 3,3-Dimethylcyclopropene is converted to adducts (192) by treatment with Ni(COD)2 in the presence of electron poor alkenes with diethylmaleate the reaction proceeds with predominant retention of stereochemistry4,135. Other 3,3-disubstituted compounds are converted to adducts in good yield by reaction with (EtO)3P CuCl in the presence of alkenes at —40 to 20 °C. In the absence of a trap, a triene (193) is isolated103 ... 

Literature dealing with the electrophilic ring-opening of cyclopropenes is sparse in relation to that for the saturated analogs. The earliest review by Closs and that by Halton and Banwell (this volume) are noteworthy. The reader is referred to these reviews for more comprehensive coverage. 

A [6.1.0]-bicyclic ring fused to an aromatic ring was constructed by diastereos-elective, intramolecular, cyclopropanation of cyclopropene-enes (Scheme 2.101) [154]. The key (vinylcarbene)rhodium intermediate was generated by ring opening of cyclopropene with Rh2(OAc)4. 

Parrain and coworkers described an efficient cascade involving the ring opening of cyclopropene ketal of type 71 followed by CM with a variety of alkenes in the presence of catalyst [Ru]-I for the synthesis of protected divinyl ketones [42]. This sequence was employed by Kozmin and coworkers for the preparation of the linear center portion of bistramide A, a marine metabolite isolated from Lissodinum bistratum which exhibits potent cytotoxicity against several cell lines [43], as well as... 

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. 

A similar investigation of the base-catalysed ring opening of 3,4-diphenylcyclobut-3-ene-l,2-diones (77) to give (Z)-2-oxo-3,4-diphcnylbut-3-cnoatcs (78) has been carried out in aqueous DMSO.108 The evidence points towards a rapid, reversible addition of hydroxide to one carbonyl, followed by a benzilic acid-type rearrangement to give a cyclopropene intermediate (79), which ring opens. 

The interesting zwitterionic compound (39) with the cationic component a butadien-2-yl cation was obtained by reaction of l,4-di(t-butyl)butadiyne with 2 mol of di(r-butyl)aluminium hydride, with the structure being established by X-ray analysis.80 The reactions of the l,2-diferrocenyl-3-(methylthio)cyclopropenylium ion with carbanions derived from active methylene compounds were investigated.81 Products were derived by ring opening of the cyclopropene ring after the initial carbanion addition. The bis(ferrocenylethynyl)phenylmethylium cation (Fc-C=C-)2C+Ph (Fc = ferrocenyl) was prepared 82 This cation proved to be much less stable than its bis-ethenyl analogue (Fc-CH=CH-)2C+Ph. 

The most strained cycloolefins, which are substituted cyclopropenes, e.g. 3,3-dimethylcyclopropene or 3-methyl-3-ethylcyclopropene, appeared to be polymerised readily to respective substituted poly(l,2-cyclopropene)s in the presence of Pd-based catalysts containing very bulky non-labile ligands. Such catalysts are characterised by reduced activity in order to prevent ring opening of the cyclopropene monomer [23],... 

Ring opening of 1,3- and 1,2-disubstituted cyclopropenes has also been examined. The ester (194) rearranges on heating to 98 °C in the presence of copper to give furan (195) the less substituted cyclopropene single bond appears to be cleaved to produce a carbene-metal derivative, which cyclises to the ester group 136). A similar photochemical transformation of a cyclopropene-3-ester to a fiiran has already been described 115). 

Reaction of cyclopropenes with cyclopropenium ions leads to a three carbon ring expansion to aromatic derivatives, presumably through ring opening of an initially formed cyclopropenyl-cyclopropylcation 276) ... 

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. 

The reaction proceeds through a vinylketene (54) formed via a vinylcarbene (53) the product of ring-opening of the cyclopropene ring. Then, the ketene undergoes intramolecular cycloaddition in benzene or is trapped by methanol (equation 35) °. 

---