Cyclopentadiene diphenylketene

Staudinger observed that the cycloaddition of ketenes with 1,3-dienes afforded cyclobutanones from a formal [2+2] cycloaddition [52] prior to the discovery of the Diels-Alder reaction. The 2+2 cycloadditions were classified into the symmetry-allowed 2+2 cycloaddition reactions [6, 7], It was quite momentous when Machiguchi and Yamabe reported that [4+2] cycloadducts are initial products in the reactions of diphenylketene with cyclic dienes such as cyclopentadiene (Scheme 11) [53, 54], The cyclobutanones arise by a [3, 3]-sigmatropic (Claisen) rearrangement of the initial products. 

Another example of cycloaddition selectivity was observed for the reaction of cyclopentadiene with diphenylketene (Equation (37)). Originally only products of [2+2] cycloaddition were observed and these reactions were considered as highly periselective. 

The metastable byproduct of [4+2] cycloaddition was detected when the reaction of cyclopentadiene with diphenylketene was (Equation (38)) was examined by low-temperature NMR experiment at — 30°C.67 The [4+2] cycloadduct was not observed at elevated temperature because easily isomerizes to [2+2] cycloaddition product via [3,3] sigmatropic (Claisen) rearrangement. This observation was supported by kinetic measurements NMR) and isolation of [4+2] cycloadduct.68 Mechanism proposed by Machiguchi and Yamabe was re-examined by Singleton.69... 

Figure 5 Intramolecular BC KIEs for the reaction of cyclopentadiene with diphenylketene at 25°C.

On the other hand diphenylketene does not give Diels-Alder products, but only 1,2-cycloadducts, on reaction with cyclopentadiene, piperylenes, iso-prene, and other dienes - . 

The cycloaddition of diphenylketene with cyclopentadiene is a classic reaction in ketene chemistry that has long been studied, but the interpretation of this... 

The use of ketene equivalents in 4- -2-cycloaddition reactions for organic synthesis has been reviewed. " a-Carbonyl ketenes behave as dienes in Diels-Alder reactions with 4-aryl-2-methyl-2,3-dihydro-l,5-benzothiadiazepines to yield regiospecific cycloadducts. The reactions of diphenylketene with cyclic ( -cw)-l,3-dienes such as cyclopentadiene and cyclohexadiene initially yield the Diels-Alder adducts, which are converted into the Staudinger cyclobutanones by [3,3]-sigmatropic rearrangementsl-Benzyl-l,3-diazabuta-l,3-dienes react with ketenes to produce the 4 - - 2-cycloadducts, substituted l-benzyl-4-(benzylidenimino)-4-phenylazetidin-2-ones, which rearrange into the more stable 5,6-dihydro-3//-pyrimidin-4-ones. The formal 4 - - 2-cycloaddition reaction of ketenimines has been used to synthesize benzoimidazo[l,2-Z ]isoquinolines. ... 

Interestingly, reaction of diphenylketene with spirocyclic azanorbomene 34, prepared by the method of Grieco et al. [1] from spirocyclopropyl-cyclopentadiene and benzyliminium ion in 62% yield, results in the formation of an unstable tricyclic intermediate 35, which upon workup gives allylic alcohols 36 in 51% yield (Scheme 2.3) [13]. A single cyclopentenone derivative... 

The initial reaction of diphenylketene with cyclopentadiene at low temperatures affords the [4+2] cycloadduct 131, which subsequently undergoes a [3,3] sigmatropic (Claisen) rearrangement to give the isolated Staudinger reaction product 132 . ... 

In the catalyzed thermal reaction of alkylarylketenes with cyclopentadiene [4+2] cycloadducts are obtained (see Section 4.1.4.2). In this reaction, the ketene undergoes the reaction across its C=0 bond initially, but the isolated cycloadducts derive from addition across the C=C bond. In contrast, olates, such 6-oxo-3,6-dihydro-l-pyrimidinium-4-olates 377, react with dimethyl- and diphenylketene across the C=0 bonds to give the stable... 

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