Cyclopropanation of allenes

Scheme 7. Cyclopropanation of allenes as a synthetic approach to substituted bicyclopropyl-idenes [48-50,52,53]...

We note that the cyclopropanation of ketene to form cyclopropanone, isoelectronically and otherwise formally related to the cyclopropanation of allene to form methylenecyclopropane, has an altogether different enthalpy of reaction. This value is (63 4) kJmoT favoring ketene and is credible in terms of considerable resonance stabilization of the type CH2=C=0 CH2—C=0 and of a 3-atom n system that includes the double bond and the appropriate, antisymmetric (i.e. difference) combination of the oxygen lone pairs. Further documentation of the stabilization of ketene is seen by comparing the energetics of the hydrogenation reaction... 

Alkylidenecyclopropyl ketones 2, accessible by the Rh-catalysed cyclopropanation of allenes with a-diazo ketones, afford 4//-pyrans through a Pd-catalysed cycloisomerisation. In one instance, where the substrate lacked a hydrogen atom on the alkylidene moiety, a 2//-pyran was obtained <04JA9645>. 

IV Pd-CATALYZED REACTIONS INVOLVING CARBOPALLADATION TABLE 7. Cyclopropanation of allenes with fliazomethane... 

Some remarks concerning the scope of the cobalt chelate catalysts 207 seem appropriate. Terminal double bonds in conjugation with vinyl, aryl and alkoxy-carbonyl groups are cyclopropanated selectively. No such reaction occurs with alkyl-substituted and cyclic olefins, cyclic and sterically hindered acyclic 1,3-dienes, vinyl ethers, allenes and phenylacetylene95). The cyclopropanation of electron-poor alkenes such as acrylonitrile and ethyl acrylate (optical yield in the presence of 207a r 33%) with ethyl diazoacetate deserve notice, as these components usually... 

The cyclopropanation of a-allenic alcohols 616 gave methylenecyclop-ropanes 617a, b and spiropentanes 618a, b in different proportions depending on the carbenoid reagent used (Scheme 88) [163,4b]. 

The discovery of carbene and carbenoid additions to olefins was the major breakthrough that initiated the tapping of this structural resource for synthetic purposes. Even so, designed applications of cyclopropane chemistry in total syntheses remain limited. Most revolve around electrophilic type reactions such as acid induced ring opening or solvolysis of cyclopropyl carbinyl alcohol derivatives. One notable application apart from these electrophilic reactions is the excellent synthesis of allenes from dibromocyclopropanes 2). 

Another feature of carbenoid-type reactivity is the cyclopropanation (reaction c). Again, this reaction does not only take place in vinylidene but also in alkyl carbenoids . On the other hand, the intramolecular shift of a /3-aryl, cyclopropyl or hydrogen substituent, known as the Fritsch-Buttenberg-Wiechell rearrangement, is a typical reaction of a-lithiated vinyl halides (reaction d) . A particular carbenoid-like reaction occurring in a-halo-a-lithiocyclopropanes is the formation of allenes and simultaneous liberation of the corresponding lithium halide (equation 3). ... 

The reaction has been extended to the chemo- and enantioselective cyclopropanation of polyenes (equations 84 and 85) and allenic alcohols that afforded spiropentane derivatives . 

Butadiene. The reaction of methylene with butadiene was studied by Frey44 under experimental conditions similar to those in the case of allene, except that lower pressures were required to avoid butadiene polymerization. Products formed by attack of methylene on the C—H bonds were cis and vinyl-cyclopropane resulting from addition of CH2 to the carbon-carbon double bond underwent collisional deactivation or isomerization to cyclopentene and C dienes, with the exception of isoprene. 

If cyclopropane may be thermochemically compared with ethylene, then spiropentane (7) may be thermochemically compared with methylenecyclopropane (24), and methylenecyclopropane (24) with allene (3). Said differently, if there were a constant change of enthalpy offormation on cyclopropanation of a double bond into a three-membered ring, then the difference of the enthalpies of formation of cyclopropane and ethylene would equal the differences found between methylenecyclopropane and allene, and between spiropentane and methylenecyclopropane. The previous section suggests we try to keep... 

Finally, a key method for the preparation of allenes is the dehalogenation of cyclopropane geminal-dibromides using methyllithium. This procedure has been reviewed previously341 and a recent series of papers concerning the synthesis of branched triangu-lanes have used this methodology (equation 39)343,343. 

For R = H and Me, the derived values are [321.3 ( >1.9)] and [323.3 ( >1.4)] klmoF , respectively. A value of [326 ( > 4)] kJmoF for AHf(g, 1,2,3-butatriene) is thus credible. What is found for the cyclopropanation enthalpies of butatriene There are seemingly no relevant data for either of its monocyclopropanation products, dimethylenecyclopropane (25a) or vinylidenecyclopropane (25b). The two dicyclopropanation products have comparable enthalpies of formation dicyclopropylidene (26), 286.6 (1) and 324.3 (g), and meth-ylenespiropentane (27), 287.0 (1) and 320.9 (g), respectively". The 2-6 kJ moF decrease in enthalpies of formation for gaseous dicyclopropanated products is not particularly in accord with the 3 kJ moF increase per alkyl substituent of cyclopropanation of simple olefins. However, in that the allene —> methylenecyclopropane —> spiropentane (3 23 7) enthalpy of formation changes are still enigmatic, and error bars are absent for the dicyclopropanated products, we do not fret. But we eagerly await more thermochemical data. 

Palladium-catalyzed methylene transfer from diazomethane has proved effective for the cyclopropanation of 1-alkenylboronic acid esters allylic alcohols and amines 1-oxy-l,3-butadienes and allenes " Readily accessible iron complex (CO)2FeCH2S Me2 BF4 35 undergoes direct reaction with a range of alkenes to give cyclopropanes (equation 67) The salt is sensitive to steric effects and the reaction proceeds... 

The parameters which are necessary to calculate molar rotations [0]d of cyclopropanes I (in chloroform, methanol, or ethanol) are given in Table 1. If one differentiates substituents bonded via a carbon atom to the cyclopropane skeleton and substituents bonded via a heteroatom, the A-parameters (which describe helix optical activity) exhibit two correlations with the parameters A (R) which are used to calculate the molar rotations of allenes XVII (Figure 2). This means that the helix contribution 5 (equation 3) to the... 

FIGURE 2. Correlations between the parameters to calculate helical optical rotations of allenes, Ay (R), and cyclopropanes, A(R)... 

The cyclopropanation of alkenes with dihalocarbenes, CX2 or CX X, except for CF2, can be efficiently executed by the PTC procedures (CHX3/strong aqueous base/catalyst), which are well-documented in books and reviews . The dichloro- and dibromo-carbene additions by the PTC procedure have successfully been applied for many alkenes, conjugated polyolefins and allenes. Satisfactory results are also reported for the reactions of sterically hindered olefins as well as electronically deactivated olefins, which frequently... 

The synthesis of alkylidene and allylidene cyclopropanes reported in this section takes advantage of the availability 77 78,81 a-82) of l-(l-silyl) cyclopropyl carbinols from a-lithio cyclopropylsilanes and carbonyl compounds. It, however, suffers from the sometimes modest yields obtained when ketones are involved (Schemes 21 a, 47) in the Peterson olefination reaction 77,78,81a) (Schemes 21, 48). This reaction seems much more difficult to achieve than when straight-chain analogs are involved and resembles the cases of allenes 1211 and chlorocyclopropenes120) reported by Chan. For example, thionyl chloride alone is not suitable for that purpose 77,136) but further addition of tetra-n-butylammonium fluoride (20 °C, 15 hrs) leads to the formation of undecylidene cyclopropane77,136 in 46% yield from the corresponding l-(l-silyl)cyclopropyl... 

Highly substituted alkylidenecyclopropanes 130, 131 and 133, 134 were made accessible by a regioselective hydroxy-directed cyclopropanation of a-allenic alcohols 129 and 132 using sa-marium/diiodomethane. ... 

The cyclopropanation of gaseous alkenes, butadiene, and allene (see Section 1.2.1.2.4.2.6.3.3., Table 11, entry 1) by diazoacetic esters can be achieved by passing a vapor-gas mixture of the alkene and the diazo compound at atmospheric pressure through a tubular continuous flow reactor which contains a copper catalyst (ca. 10%) deposited on pumice. In this manner, alkyl cyclopropanecarboxylates were obtained in yields of up to 50% with cop-per(II) sulfate (typical reaction temperature 65-110"C, contact time 3.6 s) or copper(II) oxide (85-200°C, 5s) as catalysts. 

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. ... 

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