Stereochemische Aspekte der Addition von Ketenen an Cyclopentadien

Abstract

Earlier work has shown that the cyclo-addition of a ketene to a conjugated diene is always (a) 2 + 2, (b) polarily directed, and (c) suprafacial with respect to the diene C=C. The adducts of ketenes and cyclopentadiene are thus always 7-substituted bicyclo [3.2.0] hept-2-ene-6-ones. New evidence is presented to show that unsymmetrically substituted ketenes add to cyclopentadiene in such a manner that the larger substituent has a greater tendency to take up the endoposition in the adduct. This is interpreted to mean that a ketene participates in such reactions antarafacially. Thus the ketene approaches cyclopentadiene (a) with its functional plane perpendicular to that of the ring, (b) with the carbonyl carbon over the middle of the ring, and (c) with the larger of the two substituents oriented preferentially away from the ring (transition state 11).This endo-specificity for the larger ketene substituent is demonstrated by the indicated endo/ exo ratios observed in the cyclo-adducts from ketenes with the following substituent pairs :C,H,/CH, = >95/<5, CH,/Cl = SO/ZO, CH,/CH=CH, = -65/35, C,H,/CH, = -60/40, n-C,H,/CH, = -60/40, CH,/Br = 56/44. These ratios enable a list to be compiled indicating the endo-specificity of the ketene substituents. The order closely parallels the space filling capacity as derived by other methods. The establishment of such ratios required reliable configurational assignments at carbon 7. These were derived by five methods based upon the following effects: (1) Both H-C7 and CH3-C7 cause nmr. signals at higher field in endo-position (compared with e m ) . (2) The CH,-C7 group in exo-position gives rise to a nuclear Overhauser effect with the vicinal H-C1, and in one case also with the trans-annular H-C5. (3) The nmr.-coupling constants of H-C7-exo (observed at H-C7) with H-C1 is always iarger than of H-C7-endo. (4) The coupling constant of H-C7-exo with H-C6(known to be exo-) of the LiAlH, reduction products of the cyclo-adducts (observed at H-C6) is always larger than that of H-C7-endo. (5) The nmr. signals of most protons in the cyclo-adducts are a t higher field in benzene than in chloroform solution; this "benzene shift" is larger for H-C7 or for CH,-C7 when in exo-than when in endo-position.Ketene (1) reagieren mit Olefinen (2) gewohnlich unter Bildung von Cyclobutanonen (3) [l] [a]. Die kinetischen Parameter und Losungsmitteleffekte zeigen [3], dass es sich -I -A-)=c=o .+ )={ + 4-4 1 2 3 dabei um eine Mehrzentren-Cycloaddition handelt . Zur Erklarung der Beschleunigung und der sehr wirksamen konstitutione118n Spezifitat bei Olefinen mit Donator-Substituenten (4) nimmt man einen polaren ubergangszustand (5) an [3]. Sorgfaltige x 1 4 5 Analysen der Cycloaddukte weisen auf eine ebenfalls sehr wirksame Stereospezifitat im Sinne einer syn-(= suprafacialen) Addition an das Olefin (6) [4] [5] hin. Besonders ---_ _---: f 6 interessant, und bis vor kurzem immer wieder zu Widerspruchen Anlass gebend, ist die offenbar stark bevorzugte ( 2 + 2)-Cycloaddition (7) auch in Fallen (8) wo eine (2 + ...

Acid‐Catalyzed Rearrangements of Vinylketene/Cyclopentadiene Adducts

“…As expected, the [2 + 21-cycloaddition of alkylvinylketenes 7 (A = alkyl) to cyclopentadiene sometimes gave mixtures of the two C(7)-stereoisomers 8 and 9 of the 7-alkyl-7-vinylbicyclo[3.2.0]hept-2-en-6-ones. In accordance with the well documented effect in ketene cycloadditions to cyclopentadiene [4] [9] [lo], the stereoisomer with the larger substituent in the endo-position (here 9b, 8c, 8d and 8e) was formed in preference. Scheme I shows the isomer ratio 8/9 and the yields of the cycloadducts 8 + 9.…”

Vinylketenes as Synthons for Bicyclo[4.2.1]nonadienones

“…substituents on the ketene function: increasing the size difference between these two substituents leads to a greater preponderance of the cycloadduct with the larger substituent in the 7endo-position, an effect which has been observed [24] [25] in other ketene/cyclopentadiene cycloadditions. The two stereoisomers of the cycloadducts 10 to 13 were not separated; the vinyl-endo-/vinyl-exo-(10/11 and 12/13) isomer distribution was determined by gas-liquid chromatography and verified by the relative intensities of the characteristic 'H-NMR.…”

“…The same effect can be used with the chlorovinylketene/fulvene cycloadducts 12/13 (A= Cl), even though their H-C(5) signals occur further downfield (by about 0.5 ppm) due to the additional exocyclic double bond at C(4). In the cycloadducts 12f/13f (A = CH,), the configuration at C(7) was derived from the larger chemical shift difference of the exo-methy1 group in CC14-and in C6D6-sohtion as compared to the endo-methyl group, a criterion previously employed for a series of 7,7-dialkyl substituted bicycloheptenones [24] including 10f/llf. Thus A6cclrCaDa is 0.19 ppm for 12f (exo-methyl) and 0.03 ppm for 13f (endo-methyl).…”

Gas Phase Thermal Reactions of exo-8-Cyclopropyl-bicyclo[4.2.0]oct-2-ene (1-exo)

“…Whereas the 1 H-NMR spectrum of 1-exo exhibits three signals between 0.0 and 0.9 ppm for the five cyclopropyl hydrogens, the ketone 3 shows five unique signals between 0.1 and 0.8 ppm, each integrating for one hydrogen. While the 13 C-NMR spectra of both 1-exo and 3 have three shielded carbon signals, the cyclopropyl methine in ketone 3 appears considerably more upfield at 6.5 ppm relative to the corresponding methine in 1-exo at 15.5 ppm due to the phenomenon of endo shielding [6,10].…”