Cyclopropanone formation

The above mechanism would suggest that cyclopropanone formation is synchronous with an internal SN2-type displacement of halogen, with inversion. An alternate pathway assumes loss of halide ion before cyclopropanone formation to give a mesomeric zwitterion or a no bond form of a cyclopropanone which subsequently collapses to the cyclopropanone ... 

Reaction of the spirocyclopropane (470) with formic or acetic acid results in ringopening with simultaneous cyclopropanone formation, followed by addition of the acid... 

A related condensation concerns the reaction of 3-benzyl-l-mesyloxybut-3-en-2-one (38) with lithium dimethylcuprate in tetrahydrofuran/furan which undergoes Michael addition to the enone followed by cyclopropanone formation, the intermediate cyclopropanone 39 being trapped by the enolate derived from the starting material by reduction. The resulting cyclopropanol 40 was isolated as a 5 3 mixture of diastereomers (8% yield), together with an aldol-type product 41. 

The other diastereoisomer gave the isomeric acid, again illustrating that inversion accompanies cyclopropanone formation . ... 

Arguing that the geometry of the enolate tt orbital is not suitable for effective SN2-type overlap with the cr orbital of the a carbon atom, a number of authors have proposed that cyclopropanone formation is preceded by halide ion loss . An intermediate zwitterion (255),... 

No difference in rates between chlorides and bromides is observed. The reaction is over 200 times faster than that of 8, but the p value according to Hammett studies is 1.4. The RDS of the reaction is now enolate formation. Presumably cyclopropanone formation occurs by the same mechanism as before, but it is faster relative to the initial deprotonation. Side-product formation (14) is more prevalent here, as a function of several variables, including base concentration. 

The above discussion shows that cyclopropanone formation is a consequence of the halogen orientation and the nature of the solvent. Therefore the reaction mechanisms can be predicted and verified by conformational analysis of the stereochemistry of the rearranged products experimentally obtained. These ideas were at the origin of a great deal of research using cyclic a-haloketones where the halogen stereochemistry was known. The results are discussed in the following sections. 

Kinetic studies do not allow one to differentiate between the two mechanisms of cyclopropanone formation depending on the halogen stereochemistry. A difference in behavior can be observed, however, between equatorial brominated and chlorinated derivatives. Loftfield s process (Scheme 1), according to which the first step is reversible, is verified in the case of a-bromocyclohexanones. So in 17 and 22 (X = Br) hydrogen-deuterium... 

The following conclusions arise from a comparison of the mechanisms explaining the cyclopropanone formation in the cyclohexane and open-chain series. 

In open-chain compounds, where rearrangement products are more easily obtained, 2-bromo-2,4-dimethyl-3-pentanone gives the a-alkoxyketone as a major product even when the reaction is carried out under aprotic conditions.Consequently two factors seem to be decisive in inhibiting cyclopropanone formation and yielding the a-alkoxyketone even in an aprotic solvent. The first is the degree of substitution, which can play a role in open-chain or cyclic compounds. The second operates only in cyclic systems and is ring strain which can be calculated by force field techniques. This kind of strain can be due to valence angle deformations or to transannular nonbonded interactions or bonded interactions. ... 

Another example shows that under typical Favorskii rearrangement conditions, substituents stabilize the zwitterion and inhibit cyclopropanone formation 1-bromo- and 3-bromo-l,l,3-triphenylpropanones do not yield esters. In these cases the equilibrium between the cyclopropanone and the zwitterion is totally displaced in favor of the latter. (See Scheme 9.)... 

Three different areas can be defined on this diagram. In area I, the cyclopropanone is more stable than the zwitterion. Theoretically—and this is generally experimentally observed—one should only obtain rearrangement products. However this area corresponds to open-chain a-haloketones, and it is not possible to exclude cyclopropanone formation by a direct intramolecular S 2 reaction, or to rule out substitution processes involving Sy 2 mechanisms. 

Finally, in polycyclic strained derivatives of the cage type, the semibenzilic mechanism will be preferred because of the equatorial halogen and the fairly high ring strain, which actually prohibits cyclopropanone formation. In most of... 

Cyclopropanone Polymerization. Triethylamine is an efficient initiator for the polymerization of cyclopropanone. This initiator caused polymerization to start almost immediately as evidenced by the rapid increase in temperature and the formation of a precipitate within 2-3 minutes. From the data in Table 1 there does not appear to be any correlation between the amount of initiator added and the molecular weight of the resultant polymer. One possible explanation for this is that the polymer was synthesized under heterogeneous conditions thus limiting the access of monomer to growing polymer chains. 

The formation of a cyclopropanone derivative (originally determined by the isolation of degradation products from this unstable species) stimulated considerable interest in this reaction. Tetramethylcyclopropanone, however, cannot be isolated from the reaction mixture under normal photolysis conditions even with the use of an inert solvent. That it is indeed formed as an initial product of a-cleavage results from various trapping experiments in which chemical agents present in the reaction mixture were used to produce stable derivatives of the cyclopropanone [see equation (4.65)]. 

Using low-temperature techniques such as those described in the previous section, Haller and Srinivasanmb) obtained direct spectroscopic evidence for the formation of the cyclopropanone. At 4°K infrared bands corresponding to carbon monoxide, dimethylketene, tetramethylethylene, and a compound... 

Ethenylcyclopropyl tosylates 131 and 2-cyclopropylideneethyl acetates 133, readily available from the cyclopropanone hemiacetals 130, undergo the re-gioselective Pd(0)-catalyzed nucleophilic substitution via the unsymmetrical 1,1-dimethylene-jr-allyl complexes. For example, reduction with sodium formate affords a useful route from 131 to the strained methylenecyclopropane derivatives 132. The regioselective attack of the hydride is caused by the sterically... 

Hydrogenation of di-n-propyl cyclopropenone with Pd/C catalyst, however, gave rise to 2-propyl-2-hexenal (287) as a major product according to attack of H2 at the cyclopropenone CVC3 bond43. A cyclopropanone could not be detected spectroscopically in any case. The formation of diphenylcyclopropanol 283 reported for... 

A possible mechanism for the observed transformation includes the sequence outlined in Scheme 2.327 (i) propargyl (A) - allene (B) tautomerization, (ii) 8jt-cyclization (C), (iii) N-0 cleavage (diradical D), (iv) diradical recombination (cyclopropanone derivative E), and (v) one or two step cyclizations of the azadienyl cyclopropanone into azepinone F. The occurrence of cyclopropanones (type E), as intermediates, is supported by the formation, in some cases, of isoindoles (type I) (789) as minor products (Scheme 2.327) (139, 850, 851). 

The transition metal-catalyzed cyclopropanation of alkenes is one of the most efficient methods for the preparation of cyclopropanes. In 1959 Dull and Abend reported [617] their finding that treatment of ketene diethylacetal with diazomethane in the presence of catalytic amounts of copper(I) bromide leads to the formation of cyclopropanone diethylacetal. The same year Wittig described the cyclopropanation of cyclohexene with diazomethane and zinc(II) iodide [494]. Since then many variations and improvements of this reaction have been reported. Today a large number of transition metal complexes are known which react with diazoalkanes or other carbene precursors to yield intermediates capable of cyclopropanating olefins (Figure 3.32). However, from the commonly used catalysts of this type (rhodium(II) or palladium(II) carboxylates, copper salts) no carbene complexes have yet been identified spectroscopically. 

Ketenes rarely produce [3+ 2]-cycloaddition products with diazo compounds. The reaction possibilities are complex, and nitrogen-free products are often obtained (5). Formation of a cyclopropanone represents one possibihty. Along these lines, the synthesis of (Z)-2,3-bis(trialkylsilyl)cyclopropanones and (Z)-2-trialkylsilyl-3-(triethylgermyl)cyclopropanones from diazo(trialkylsilyl)methanes and appropriate silyl- or germylketenes has been reported (256,257). It was found that subsequent reaction of the cyclopropanone with the diazoalkane was not a problem, in contrast to the reaction of diazomethane with the same ketenes. The high cycloaddition reactivity of diazomethylenephosphoranes also extends to heterocumulenes. The compound R2P(C1)=C=N2 (R = N(/-Pr)2) reacts with CS2, PhNCO and PhNCS to give the corresponding 1,2,3-triazole derivative (60). 

The most common reaction involving this type of cycloaddition is the reaction of ketenes with diazoalkanes (Houben-Weyl, Vol. 4/4, pp 406-408) which proceed via cyclopropanone intermediates. This type of reaction finds limited use due to nonregioselective formation of substituted cyclobutanones as mixtures. 

Although the spectral properties of cyclopropanones and the easy formation of hydrates and hemiketals are inconsistent with the dipolar form, some reactions of cyclopropanones do indicate that the ring carbons are much more electrophilic than in other cyclic or acyclic ketones. For example, nucleophilic ring opening often occurs easily ... 

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