Cyclopropanones strained

Cyclopropanone is highly reactive because of its large amount of angle strain, but methylcyclopropenone, although even more strained than cyclopropanone. is nevertheless quite stable and can even be distilled. Explain, taking the polarity of the carbonyl group into account. 

Quantum mechanical/molecular mechanical study on the Favorskii rearrangement in aqueous media has been carried out.39 The results obtained by QM/MM methods show that, of the two accepted mechanisms for Favorskii rearrangement, the semibenzilic acid mechanism (a) is favored over the cyclopropanone mechanism (b) for the a-chlorocyclobutanone system (Scheme 6.2). However, the study of the ring-size effects reveals that the cyclopropanone mechanism is the energetically preferred reactive channel for the a-chlorocyclohexanone ring, probably due to the straining effects on bicycle cyclopropanone, an intermediate that does not appear on the semibenzilic acid pathway. These results provide new information on the key factors responsible for the behavior of reactant systems embedded in aqueous media. 

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... 

The isolation of cyclopropenones and their undoubtedly increased stability compared to the less-strained saturated cyclopropanones might well be attributed to the validity of the above symbolism of aromatic cyclopropenium contribution to the ground state of 7. It should nevertheless be clear, that the information available on the electronic structure of cyclopropenones demands certain refinements of this very useful qualitative concept. 

The results are as expected for 2-cyclohexenone and for 5-valeroactone. Apparently, trichloromethyl is not as effective an electron-withdrawing group as is trifluoromethyl, although the equilibrium for perchloroacetone is predicted to tilt toward the hydrate. Relief of strain in cyclopropanone is not enough to favor the hydrate. 

Besides the activation of the olefinic partner by a metal, the unfavorable thermodynamics associated with the addition of an enolate to a carbon—carbon multiple bond could be overwhelmed by using a strained alkene such as a cyclopropene derivative286. Indeed, Nakamura and workers demonstrated that the butylzinc enolate derived from A-methyl-5-valerolactam (447) smoothly reacted with the cyclopropenone ketal 78 and subsequent deuterolysis led to the -substituted cyclopropanone ketal 448, indicating that the carbometallation involved a syn addition process. Moreover, a high level of diastereoselectivity at the newly formed carbon—carbon bond was observed (de = 97%) (equation 191). The butylzinc enolates derived from other amides, lactams, esters and hydrazones also add successfully to the strained cyclopropenone ketal 78. Moreover, the cyclopropylzincs generated are stable and no rearrangements to the more stable zinc enolates occur after the addition. 

Consequently if the carbonyl group is on a small carbocyclic ring, there will be substantial angle strain and this will amount to about 120° — 60° = 60° of strain for cyclopropanone,... 

Thus the hemiketal from cyclopropanone will have 109.5° — 60° = 49.5°, and that from cyclobutanone 109.5° — 90° = 19.5° of strain at Cl. This change in the angle strain means that a sizable enhancement of both the reactivity and equilibrium constant for addition is expected. In practice, the strain elfect is so large that cyclopropanone reacts rapidly with methanol to give a stable hemiketal from which the ketone cannot be recovered. Cyclobutanone is less reactive than cyclopropanone but more reactive than cyclohexanone or cyclopentanone. 

Cyclopropanones deserve special comment, not because of their practical importance (they have no commercial value at this time), but because of their novel behavior and reactivity. No unambiguous synthesis of cyclopropanones was known prior to 1965, and the older textbooks usually contained statements such as cyclopropanones apparently cannot exist. However, they had been postulated as intermediates in various reactions (see, for example, the Favorskii rearrangement, Section 17-2C and Exercise 17-15), but until recently had defied isolation and identification. The problem is that the three-ring ketone is remarkably reactive, especially towards nucleophiles. Because of the associated relief of angle strain, nucleophiles readily add to the carbonyl group without the aid of a catalyst and give good yields of adducts from which the cyclopropanone is not easily recovered ... 

Butanedioic and pentanedioic acids take a different course. Rather than form the strained cyclic ketones, cyclopropanone and cyclobutanone, both acids form cyclic anhydrides that have five- and six-membered rings, respectively. 1,2-Benzenedicarboxylic (phthalic) and cis-, 4-butenedicar-boxylic (maleic) acids behave similarly ... 

Cyclopropanon.es are highly reactive organic systems containing a number of labile sites on a small carbon skeleton. They represent valuable substrates for studying theoretical aspects of the chemistry of small strained ring systems and are of special interest in synthesis because of the variety of transformations in which they take part. 

The high-frequency shifts in the CO and CH stretching vibrations of cyclopropanones are related to ring strain and occur to a lesser extent in cyclobutanone. Although normal coordinate analysis has been employed in predicting these shifts, the theoretical models do not appear to be completely reliable.6 ) An important factor causing the above shifts... 

The carbonyl group in cyclopropanone readily adds many types of nucleophiles, even at low temperature, e.g. water, amines, acids, Grignard reagents. The unusual tendency toward adduct formation extends to polymerization and is a consequence of the strain energy released by the sp2->-sp3 rehybridization of a carbon atom constrained by a three-membered ring. 

Reactions of cyclopropanones with nucleophiles frequently lead to ring enlargement reactions since the formation of four-membered rings from the reactive intermediates is accompanied by a considerable reduction in strain energy. Thus, 2 reacts with diazomethane to form cyclobutanone96>, with hydrazoic acid to form (3-lactam 76,89) and, under special conditions, with amines and hydroxyl amine derivatives to form N-sub-stituted (3-lactams 87> (Scheme 24). 

Cyclopropanone is a highly reactive ketone, presumably because of the extra angle strain introduced into the three-membered ring by the, vp2-hybridized carbonyl carbon. Cyclopropenone is much less reactive even though it has more angle strain. Offer an explanation for this experimental observation. 

The three-membered ring is, of course, flat. The others are not. Even the four-membered ring is slightly puckered, the five- and especially the six-membered rings more so, This is all discussed in Chapter IB, But you have already met the concept of ring strain in Chapter 6, where we used it to explain why cyclopropanones and cyclobutanones are readily hydrated. 

Translation of these results into compound I leads to structure X. Unraveling of the strained zwitterion XI derived from this would yield keto aldehyde XII, a structure that plays a central role in the various possible reaction mechanisms that branch off from the starting material I. Furthermore, under photo-lytic conditions, some alkenes react with carbonyl compounds to form four-membered cyclic ethers, namely, oxetanes, by way of a [2-1-2] cycloaddition reaction known as the Patemo-Buchi process. Such a reaction would be all that is necessary to convert XII into the bicyclic cyclopropanone XIII required for the Favorskii-type rearrangement (see Scheme 42.3). Splitting by methanol attack would directly yield compound II. 

Cyclopropanone gave a carbonyl absorption at 1825 cm at 20and in CH2CI2 solution bands were subsequently reported at 3045 cm" (C-H)and 1813 cm" The high frequency of the carbonyl stretch is testimony to the large strain inherent in this... 

The strain energy of methylenecyclopropane (2) exceeds" that of cyclopropane by about 13 kcal mol "S a value similar to the excess strain in 1,1-difluorocyclopropane. This is certainly a part of the propensity for ring-opening although the stability of trimethylene-methane (3) is undoubtedly the more important factor (cf. equation 3). Similarly, the ringopening facility of cyclopropanone (see Chapter 23 by Wasserman and coworkers) partially reflects additional ring strain. 

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