Cyclopentadiene fulvene formation

Ketene acetals show a pattern of product formation very similiar to enamines79 Diphenyl-4,4-diacetyl triafulvene is converted to diacetylmethyl cyclopentadiene 529 by S,N-acetals, whilst diphenyl-4,4-dicyano triafulvene undergoes C—C-inser-tion to S,N- and N,N-acetals, e.g. 530/531, resulting in cross-conjugated systems 533/534 by analogy with enamines. Cyclic S,N-acetals 532, however, yield exclusively the bicyclic fulvenes 535 due to additional loss of methyl mercaptan. 

Niggli and Neuenschwander294 studied the reaction of fulvene (461) with cyclopen-tadiene. The main product fraction consisted of three 1 1 adducts, as illustrated in equation 138. Diels-Alder Adducts 462 and 463 resulted from attack of cyclopentadiene at the endocyclic and exocyclic double bonds of fulvene, respectively. The formation of 464 was rationalized by a [6 + 4] cycloaddition reaction followed by two [1,5] hydrogen shifts. It was stated that due to the absence of electron-donating and electron-withdrawing groups on both triene and diene, fulvene may have reacted via its HOMO as well as its LUMO. 

Scheme 10.5 Selective formation of Qq adducts with integral fulvene, cyclopentadienide and cyclopentadiene moieties.

The condensation of aldehyde 2 with cyclopentadiene yields the fulvene 20 which can be transformed to the respective cyclopentadienyl anion 21 by addition of a hydride anion (Scheme 1.5.8). The anion 21 opens up the possibility of the formation of sandwich and half-sandwich type of complexes with suitable metal fragments. 

The trithionium salts also condense with the weak C—H acids cyclopentadiene, indene, and fluorene, but again only when the latter are present as their sodium salts. As we have already pointed out, however, the reactions with such strong bases are rather unpredictable, and the desired 1,2-dithiafulvalenes (146) may be obtained (if at all) only in very low yields, together with a large number of by-products. The formation of some of these by-products is attributed to the fact that the 1,2-dithiafulvalenes, like the sesqui-fulvalenes, also react as fulvenes and add on strong bases.112... 

Small- and medium-ring conjugated dienes undergo smooth addition of dichlorocarbene to give the diadducts, often in high yield, particularly if the chloroform/base/phase-transfer catalyst method is applied (for the reaction of cyclopentadiene and fulvenes, see Houben-Weyl, Vol. El9b, p 1545 for the first example of the isolation of the stable monoadduct of dichlorocarbene to cyclopentadiene derivative, see ref 4). The reaction conditions, the type of catalyst and the solvent, as well as their relative amounts, must be considered if the monoadducts are the synthetic target, e.g. formation of monoadducts 24 and 26 and diadducts 25 and 27. ... 

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