Preparation cyclopropylidenes from

In refluxing cyclooctene, exo-7-bromo-c t/f>-7-(trimethyistannyl)bicyclo[4.1.0]heptane (20) reacted via the corresponding cyclopropylidene intermediate to afford spiro[bicyclo[4.1.0]hep-tane-7,9 -bicyclo[6.1.0]nonane] (21) in 76% yield. However, this reaction is not preparatively interesting and not applicable to a-bromo-a-(trimethylstannyl)cyclopropanes in general, because the cyclopropylidenes from the latter gave ring-opened allenic products. ... 

Methyl 2-chloro-2-cyclopropyliden cetate (4) was readily prepared in two stejK from ethylene and tetrachlorocyclopropene [7], and reacted with 4-methylcycIohexa-l,3-dien-2-ol trimethylsilyl ether (62a) at 60 °C to give a complex mixture containing about equal amounts of both regioisomeric adducts 63a, 64a besides the tricyclic ketoester 65a after acidic workup (Schemes 12 and 13) [15]. Each regioisomer was a mixture of endo and exo-diastereomers. The trimethylcyclohexadiene 62b yielded, after 2 days at 100 °C and acidic work-up, the tricyclic ketoester 65b as the main product (Schemes 12 and 13) [15]. 

Commonly known methods for the preparation of bkyclopropylidenes of types 55-62 (Fig. 3) are the cyclopropanations of appropriately cyclopropanated al-lenes according to the Gaspar-Roth [60] or modified Simmons-Smith protocol [61] or addition of cyclopropylidene generated in situ from AT-nitroso-N-cyclo-propylurea [62,63]). Along these routes, the compounds 55 [63], 58-60 [33,64, 65], and 62 [33,64] were obtained in low to moderate yields (Scheme 11). 

Rotane (128) was also prepared by addition of cyclopropylidene liberated from in situ generated diazocyclopropane [62] onto 1 [65,105] (Scheme 27). This carbene adds also to bicyclopropylidenes 56 and 62 producing triangulanes 129 [33,65] (Scheme 27) and 67 [33,64,65] (Scheme 11). 

Novel preparative possibilities arise when cyclopropylidene intermediates are generated from functionalized dihalocyclopropanes. Thus Baird has shown that bicyclic ethers may be formed in high yield by an intramolecular insertion from cyclopropyl alkyl ethers [120],... 

Unprotected 2-acylcyclobutanones 21 are only stable in solution. They are obtained from the corresponding cyclopropylidene ketones 17 if the double bond is fully substituted. If not, they must be prepared from the corresponding cyclopropylidene alcohols 18 through a sequence of epoxidation, oxidation and rearrangement.53 54... 

Oxaspiropentanes have been obtained from the cyclopropylide 103, prepared by treatment of cyclopropyldiphenylsulfonium tetrafluoroborate 102 either with sodium methylsulfmyl carbanion in dimethoxyethane at —45 °C or with potassium hydroxide in dimethylsulfoxide at 25 °C. While the reaction of the ylide 103 with a,p-unsaturated carbonyl compounds has resulted in selective cyclopropylidene transfer to the a, 3-carbon-carbon double bond leading to spiropentanes, condensation of 103 with non-conjugated aldehydes and ketones led to oxaspiropentanes such as 104, which have been isolated in 59-100% yields, Eq. (30) 57). 

Several reagents have been used for the dehydrobromination of bromocyclopropanes to prepare substituted alkylidenecyclopropanes. For example, powdered potassium hydroxide and quartz sand were employed at 90 C and at low pressure in the preparation of (bromomethylene)cyclo-propane (15) from l-bromo-l-(bromomethyl)cyclopropane(14). Similar procedures resulted in l-(bromomethylene)-2-methylcyclopropane (17, as a 90% pure Z/E mixture) " and [bro-mo(cyclopropylidene)methyl]benzene (19). ... 

Adding appropriate nucleophiles with leaving group capabilities, such as phenylselenide (SePh) or dimethylamide (NMe2) gives access to precursors of a variety of modified cyclo-propylideneacetates bearing almost any substituent (X = OSiRj, N3, F, H, SMe, SPh). - ° The chiral methyl 2-cyclopropylidene-2-(phenylsulfinyl)acetate can be prepared from methyl 2-cyclopropylidene-2-(phenylsulfanyl)acetate under strictly anhydrous conditions. This bisac-ceptor-substituted methylenecyclopropane can be isolated, but it rapidly attracts water upon standing in air at room temperature. ... 

Although cyclopropylidenes have been generated by a number of independent methods which include cycloaddition of atomic carbon to alkenes, decarbonylation of cyclo-propylideneketenes, decarboxylation of oxaspiropentanes, from a preparative viewpoint, the a-elimination reaction of cyclopropane derivatives is most practical because of the mild reaction conditions and the availability of the starting materials. These methods are illustrated by the formation of allenes from atomic carbon/alkene reactions in Table 1 and in the preparation of 3/4, and from cyclopropanes in the preparation of 5/6. Examples showing allenes from decarbonylation of cyclopropylideneketenes are shown in the preparations of compounds 7/8, 9, 10, " and 11. ... 

Solvolysis can also occur from the diazonium salts derived from the intermediate diazocy-clopropanes. Ring opening of cyclopropyldiazonium salts accounts for the solvolysis products in Table 2, entries 8 and 9. Evidence for this is seen when sodium methoxide is replaced by the weaker base, sodium hydrogen carbonate, in which case only the methyl ethers derived from methanolysis were obtained. The a-elimination reaction of 1-cyclopropyl-1-nitrosourea with base likely involves diazocyclopropanes as intermediates which readily eliminate molecular nitrogen to form cyclopropylidenes. A more direct approach to diazocyclopropanes involves the Bamford-Stevens route via tosylhydrazones. Thus di-/er/-butylallene (19) was prepared from the tosylhydrazone of ( )-2,3-di-tert-butylcyclopropanone on treatment with base. ... 

The dibromocyclopropanes are frequently preferred to their less reactive dichlorides, although the latter are often prepared in higher yields from dichlorocarbene additions to alkenes. Di-fluorocyclopropanes are not useful precursors to cyclopropylidenes with reactions leading to alkylated cyclopropenes and alkynes. ... 

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