1.2- Dimethylcyclopropene, structure

The structure of 65 was determined via a detailed mono- and bidimen-sional NMR study. In C NMR a signal at 20.3 ppm corresponds to the methyl groups and the carbon nucleus = C2 - H carbon appeared at 142.3 ppm. The gem-dimethylcyclopropene structure of 65 was consistent with an analysis of the HMBC spectrum. 

The cyclotrimerisation of dimethylcyclopropene is effected by Wilkinson s catalyst RhCl(PPh3)3 to provide a rhodacycloheptane derivative 111 which has been structurally characterised (Fig. 35). The metallacycle is cleaved upon treatment with carbon monoxide to provide both cyclohexane (reductive eliminations) and cycloheptanone (CO insertion) products (Scheme 34).120... 

Fig. 2. Structural data of cyclopropane M) and 3,3-dimethylcyclopropene 35> as established by electron diffraction and microwave spectroscopy, respectively...

Similar conversions of cyclopropenes can be achieved with other metal complexes. In the presence of trialkylphosphane-modified Pd(0) catalysts, 3,3-dimethylcyclopropene was quantitatively and stereoselectively cyclotrimerized to give 3,3,6,6,9,9-hexamethyl-c ,c/i,/ran -tetra-cyclo[6.1.0.0 0 ]nonane (14). The structure was derived from spectroscopic data and X-ray crystal analysis. The product undergoes thermal rearrangement to form 3,3,6,6,9,9-hexamethyl-trun. -tricyclo[6.1.0.0 ]non-4-ene (15). ° This method has been extended to other 3,3-disubstituted cyclopropenes. ... 

The terminal alkene that was previously suggested as a possible structure for A has five sets of non-equivalent hydrogens. The trisubstituted alkene that was suggested has four sets of non-equivalent hydrogens. Hence the only structure from part (a) that is consistent with the nmr spectrum is the tetrasubstituted alkene, 1,2-dimethylcyclopropene ... 

Conclusive evidence for palladacyclopentanes and palladacyclopentadienes as potentially stable intermediates has been put forward, and they have a close analogy to such metallacycles with other metals.f" They frequently are intermediates in metal-catalyzed or -mediated formal [2+2 + 2] cyclotrimerizations of alkynes and strained alkenes. It is plausible to assume that palladacyclopropanes or -cyclopropenes also play a role in such formal [2+2 + 2] processes catalyzed by palladium(O), for example, in the type of Pd-catalyzed cyclotrimerization of 3,3-dimethylcyclopropene reported by Binger et al. (see also Sect. IV.2.4), for which a palladacyclopentane as intermediate was already isolated. Moreover, dimethyl 3,3-dimethylcyclopropenedicarboxyIate 8 has recently been shown to form the tricyclic palladacyclopentane derivative 10, the structure... 

Further details have appeared concerning the polar 1,4-homoaddition of iV-chloro-sulphonylisocyanate with 1,4-bridged vinylcyclopropanes. The dispiro-system (304) is cleaved to a biradical which, in the presence of an appropriate styrene, affords the paracyclophane (305). Methylenecyclopropane and 3,3-dimethylcyclopropene afford the air-sensitive complexes (306) and (307), respectively, with 2,2 -bipyridyl-(cyclo-octatetraene)nickel, and 2f-ray structural analyses have been performed. The complexes can be easily decomposed, (306) to (308), and (307) to (309). 

MeC=CMe yields (138) of similar structure. Reaction of Cp H iCO)2 (M=Rh, Co) with 3,3-dimethylcyclopropene results in ring opening to give (139)... 

Complex (54) has been obtained by reaction of 3,3-dimethylcyclopropene with Fe2(CO)9 the analogous CpMn(CO)(4,4-dimethylbuta-l,3-dienone) may be obtained on reaction with CpMn(CO)2THF. A crystal structure determination of (54) reveals a square pyramidal configuration. Reaction of a-allylnaph-thalene with Fes(CO)i2 results in isomerization to yield (55) a similar isomerization occurs on reaction of o>-diallylbenzene to yield (56). A full paper on the reactions of the ligands (57) with iron carbonyls has appeared. ... 

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