Cyclopentanone cyclobutanone

Predict the C=C stretching frequencies of the alkenes formed when cyclopentanone, cyclobutanone, and... 

Simple ketones absorb at 1710-1715 cm" simple aldehydes absorb at 1720-1725 cm. Aldehydes also have a characteristic absorption near 2710 cm for the aldehyde C—H bond. Cyclohexanones have carbonyl absorptions at the same position as simple acyclic ketones. However, decreased ring size results in shifts to higher wavenumber. Cyclopentanone, cyclobutanone, and cyclopropanone absorb at 1745, 1780, and 1850 cm , respectively. 

It s fortunate that even small variations in the position of the band can be interpreted in terms of the type of carbonyl group (Table 5.5). Anhydrides show two bands because the carbonyl vibrations are coupled, and they can vibrate in phase or out of phase (5.7, 5.8) these are called the symmetric and antisymmetric stretches. The carbonyl stretching frequency can also be used to establish ring size if the carbonyl carbon atom is part of the ring. Cyclohexanone shows an entirely normal stretching frequency of 1716 cm"h the values for cyclopentanone, cyclobutanone, and cyclopropanone are, respectively, 1748,1783, and 1850 cm b Similar differences are noted for lactones (cyclic esters) and lactams (cyclic amides). 

The photoreduction of cyclobutanone, cyclopentanone, and cyclohexanone by tri-n-butyl tin hydride was reported by Turro and McDaniel.<83c> Quantum yields for the formation of the corresponding alcohols were 0.01, 0.31, and 0.82, respectively. Although the results for cyclopentanone and cyclohexanone quenching were not clear-cut (deviations from linearity of the Stem-Volmer plots were noted at quencher concentrations >0.6 M), all three ketone photoreductions were quenched by 1,3-pentadiene, again indicating that triplets are involved in the photoreduction. 

From the reactions presented in this section one can conclude that cyclic acetal formation via addition to a carbene intermediate is a general reaction for type I cleavage of cyclobutanones, tricyclic compounds, and certain bridged bicyclics as minor products. No acetal has been isolated from photolyses of cyclopentanones or cyclohexanones except for the special case of an a-sila ketone previously discussed. 

Whereas lactone annulation invokes a relief of strain of the four membered ring by migration of the ring bond to an electron deficient oxygen, a similar migration to an election deficient carbon creates a cyclopentanone synthesis (Eq. 73). The release of approximately 84 kJ/mole (20 kcal/mole) provides a strong driving force. Thus, the 1,1-cyclobutanone annulation of ketones translates into a 1,1-cyclopentanone annulation. 

For example, an oxaspirohexane <52, readily available by condensing cyclobutanone 61 with dimethylsulfonium methylide, rapidly rearranges (isomerizes) to the cyclopentanone 63 upon exposure to a catalytic amount of lithium bromide55). The high diastereoselectivity of the initial cyclobutanone formation translates into a high diastereoselectivity for cyclopentanone annulation as this example of Eq. 74 demonstrates. 

The ring expansion of cyclobutane derivatives to other carbocycles remains to be one of the most powerful tool in synthetic organic chemistry. Cyclobutanones are exceptionally facile starting materials for the preparations of y-lactones as well as cyclopentanones. 

If the 3-position is a tertiary, rather than a quaternary, stereocenter, Rh(I)/Tol-BINAP effects an intriguing parallel kinetic resolution - thus, one enantiomer of the substrate selectively undergoes hydroacylation to generate a cyclobutanone, while the other enantiomer is transformed into a cyclopentanone (Eq. 22) [24]. This observation is quite interesting, given the limited number of examples of parallel kinetic resolutions, particularly catalytic processes that involve carbon-carbon bond formation, and catalytic methods for the construction of cyclobutanones. 

A similar example is the ring expansion of cyclobutanone via l-tris(methylsulfanyl)methyl-cyclobutanol (6) to 2,2-bis(methylsulfanyl)cyclopentanone (7).43 The use of Af,A-diisopropyl-ethylamine is unneccessary in this reaction as the hydroxy group has already been deprotonated with butyllithium. Further examples of this type of reaction can be found in refs 43 and 44. 

Table 4. Cyclopentanones by Pinacol-Type Rearrangement of Cyclobutanones with oc-Lithio Selenoxides...

Table 6. Cyclopentanones by Addition of Diazo Compounds to Substituted Cyclobutanones...

The chemical shift dependence of the carbonyl resonances on ring size in cycloalkanones is particularly remarkable In the series of cycloalkanones, cyclopentanone is found to have the largest carbonyl shift (219.6 ppm). The CO signals of cyclobutanone and cyclohexanone are both observed at higher field (x 209 ppm). The carbonyl carbons of cy-clooctanone and cyclononanone are much more deshielded than those of cyclohexanone, cycloheptanone, cyclodecanone and cycloundecanone. The carbonyl resonances of the twelve to seventeen membered ring ketones occur at S values similar to those of acyclic ones [282, 288]. 

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. 

Exercise 28-10 Write a mechanism for formation of cyclobutane from the photolysis of cyclopentanone, and ketene from the photolysis of cyclobutanone. 

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