Cyclopropylidenes, structure

Fig. 1.10 The cyclopropylidene and the allene from opening of the spiropentane moiety ofdime-thanospiro[2.2]octaplane. These are molecular mechanics structures. The cyclopropylidene structure shown here actually has the carbons of the three-membered ring and the two carbons of the double bond in the same positions as the corresponding carbons of the spiropentane moiety of dimethanospiro[2.2]octaplane, and so its function here is only heuristic (cf. Fig. 1.9)...

The formula of the smallest ring including a complete allene moiety is that of cyclo-propadiene (11) (Scheme 6.5). However, this structure does not seem to be an energy minimum at the C3H2 hypersurface [21]. Cyclopropylidene (12) was calculated to be the most stable species followed by open-chain isomers [22]. 

Since the reactive substructure of cA-2-(l,3-butadienyl)cyclopropylidene is contained in the bicyclic carbene 124, there is a possibility that a carbene-carbene rearrangement occurs together with 1,5-carbon migration. Analysis of the probable reaction pathways allows one to conclude that 1,5-C-migration (124 - 132) in the fixed c -l,3-butadienyl fragment of structure 124 is impossible. The 1,3-carbon migration (124 - 130) which takes place instead is mechanistically analogous to the vihylcyclopropylidene-cyclopentylidene... 

Occasionally the cyclopropylidene to allene isomerization cannot take place for structural reasons. If, for example, the expected allene would be very highly strained, as is the case for certain cyclic allenes, then the reaction is forced to follow an alternative path. A case in point is provided by 1-alkyl-7,7-dibromonorcaranes which undergo a carbene insertion reaction when treated with methyl lithium to provide bicyclobutanes rather than allene derivatives. 

It is interesting to note that the yield of 39 can be increased to 95 % in the presence of a Ni(0) catalyst (see Section I.3.I.2.).43 In unsymmetrically substituted buta-l,2,3-trienes, head-to-head dimerization takes place as well, as seen in the examples below.44 The intermediate cyclopropy-lidenecumulenes 41 are formed from vinylidene insertions into alkenes and reactions occur at the terminal unsaturated site of the cyclopropylidene group. Structures 42 were confirmed by X-ray crystallographic analysis and revised the original assignment of the head-to-tail structures for these derivatives.45... 

The rotanes are an unusual type of hydrocarbon constructed from conjugated spiroannulated cyclopropyl fragments. Structurally, rotanes 28 can be considered as cyclic oligomers of cyclopropylidene. 

Density functional theory computational studies have been used to determine die importance of secondary orbital interactions for the stability of transition-state structures for die 4 + 2-cycloaddition of furan with cyclopropene.175 Kinetic studies of die 2 + 4-cycloaddition of 2-cyclopropylidene acetates with furan and dimethylful-vene suggest a mechanism involving diradicals or zwitterions as intermediates.176 Cyclopropene, produced by die reaction of allyl chloride with sodium bis(bimediyl-silyl)amide, reacts with 1,3-diphenylisobenzofuran to produce both endo- and exo-Diels-Alder cycloadducts isolated for the first tune.177... 

Generation and reaction of a 1-(cyclopropylidene)alkylzinc synthesis of 1 -[2-(phenylmethyl)cyclopropylidene]pentanenitrile (Structure 39)21... 

This is the other possible cyclic isomer of C3H4 and may be considered as a cyclic carbene. Only the singlet state has been considered, although it is possible that an associated triplet state may be lower in energy. C v symmetry was assumed in the structure determination. The cyclopropylidene skeleton is rather similar to that of cyclopropane, from which... 

Concomitant formation of a C-H and a C-C bond to the same carbon atom in a cyclopropane ring can be achieved in a number of ways. The best established method involves treatment of 1,1-dibromocyclopropanes with a strong base, usually methyllithium or butyllithium, which generates cyclopropylidene intermediates that can undergo intramolecular C-H insertion if the substrate structure and the reaction conditions are right. These reactions have been discussed thoroughly elsewhere (Houben-Weyl, Vol. E19b). 

Organometallic reagents exhibit their expected typical reaction pattern towards a, S-unsaturated carbonyl compounds. Whereas methyllithium reacted cleanly at — 10°C with 3-cyclo-propylidenebutan-2-one (3) to give 3-cyclopropylidene-2-methylbutan-2-ol (4) in 80% yield via 1,2-addition, methylmagnesium iodide converted the ketone 3 to a 60 40 mixture of the 1,2- and 1,4-addition products 4 and 5 in 55% yield. The ratio depended on the substrate structure and was totally reversed when a phenyl substituent was present on the double bond. Soft methylcuprates only gave the 1,4-adducts 2l-o upon reaction with 1 in diethyl ether for 20 minutes at — 10°C to — 30 °C and subsequent warming to 20 °C (Table 1). 

The etiolate of cyclopropanecarboxylic acid and its derivatives is an important building block for organic synthesis.However, the strained methylenecyclopropane structure of the enolate renders this species not only difficult to prepare, but also unstable. A rather simple method for the preparation of substituted 2-cyclopropylidene-5,5-dimethyl-l,3-dioxanes 15 is the thermolysis of the corresponding 2-alkylidenecyclopropanone acetals 14 which rearrange at 150 °C to the hydrolytically unstable ketene acetals. 

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