Fulvene, 6- cycloaddition reactions dienes

The study of the cycloaddition behavior of l,l-dichloro-2-neopentylsilene, C Si =CHCH2Bu (2) [3], reveals the high polarity of the Si=C bond and a strong electrophilicity. The [4+2] cycloaddition reactions with anthracene (3), cyclopentadiene (4) and fulvenes (5) proceed as expected surprising, however, the Diels-Alder reactions with dienes are of lower activity, like naphthalene (6) and furans (7). 

These phenomena can be illustrated by the cycloaddition reactions of fulvenes with electron-deficient a-pyrones. In general, the Diels-Alder reactions of electron-deficient dienes such as 458 with 6-alkyl substituted fulvenes favor addition across one of the endocyclic... 

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. 

As the focus of this chapter is on the synthetic utility of the rDA reaction, an overview of mechanism is beyond the scope of this review however, the subject has beoi reviewed previously. Structural and medium effects on the rate of the rDA reaction are of prime importance to their synthetic utility, and therefore warrant discussion here. A study of steric effects cm the rate of cycloreversicHi was the focus of early work by Bachmann and later by Vaughan. The effect of both diene and dioiophile substituticHi on Ae rate of the rDA reaction in anthracene cycloadducts has been reported in a study employing 45 different adducts. If both cycloaddition and cycloreversion processes are fast on the time scde of a given experiment, reversibility in the DA reaction is observed. Reversible cycloaddition reactions involving anthracenes, furans, fulvenes and cyclopentadienes are known. Herndon has shown that the well-known exception to the endo rule in tiie DA reaction of furan with maleic anhydride (equation 2) occurs not because exo addition is faster than endo addition (it is not), but because cycloreversion of the endo adduct is about 10 000 times faster than that of the exo adduct. ... 

Fulvenes, like their trpericyclic reactions. A reasonably well-defined reactivity profile of these systems has emerged as the result of extensive scrutiny of the cycloaddition behavior of the fulvene nucleus. To a large extent, fulvenes undergo concerted cycloadditions to dienes as either the 6ir or lit participant and the factors governing which of these reactivities is expressed in a particular circumstance has been elucidated employing fiontier molecular orbital considerations. ... 

There are also very many examples of fulvenes participating in cycloaddition reactions with other molecules, both as a diene and as a dienophile, roost commonly as the latter. Examples include the following [230-232] ... 

Reactions of simple fulvenes with dienophiles commonly take place at C(2) and C(5) and those with dienes at C(2) and C(3), i.e. the latter proceed by a [2 + 4] rather than a [6 + 4] cycloaddition. This, and other features of cycloaddition reactions involving fulvenes can be nicely explained in terms of frontier orbital theory [234,235]. In some reactions fulvenes may participate as 6ir rather than 27t components, for example with diazomethane ... 

The 4-1-2- and 8 + 2-cycloaddition reactions of hepta- and penta-fulvenes have been reviewed. The novel intramolecular 4 + 2-cycloaddition of the fulvene (164) yields 6-oxatricyclo[6.4.0.0 ]dodeca-2,l l-diene[4,5-a]naphtho-ennlo 9-carboxylic acid esters (165) (Scheme 63) ... 

Applying Paddon-Row s method to the second reaction, and taking into account all four FOs, suggests that the 6 + 4 cycloaddition is the most favorable reaction (having an interaction energy of 0.546/ ) followed by the 4 + 2 wherein the fulvene acts as a diene (interaction energy 0.359/ ) and finally 4 + 2 where the fulvene provides the di-enophile component (interaction energy 0.284/ ). Nonetheless, experiments prove that this last compound is the main product. 

The variations of periselectivity exhibited in the cycloadditions of fulvenes to dienes have been rationalized by rqjplication of frontier molecular orbital theory. The fulvene orbitals of interest in these reactions are depicted in Figure 2. - The controlling orbitals in the reaction of fulvene with an... 

Cycloadditions between substituted fulvenes and 1,3-dipolar reagents have been the subject of considerably less study than the corresponding diene-mediated chemistry, paralleling the situation seen in the tropone series. For the most part, however, the reactivity patterns formulated in the fulvene-diene studies are repeated in the analogous 1,3-dipole reactions. 

The reactions of fulvenes (373) also provide examples where the longest conjugated system available is not always the one involved in cycloadditions, but this time frontier orbital theory is rather successful in accounting for the experimental observations. The orbital energies and coefficients are illustrated in Fig. 4-71, where it can be seen that there is a node through C-l and C-6 in the HOMO. The result is that when a relatively unsubstituted fulvene might react either as a n6 or as a n2 component with an electron-deficient (low-energy LUMO) diene or dipole, it should react as a n2 component because... 

By contrast, if the important frontier orbital in a cycloaddition is the LUMO of the fulvene, and if the fulvene is to react as n2 or n6 component, it will now react as a ic6 component, because the largest coefficients are on C-2 and C-6. Thus with diazomethane, the LU (fulvene)/HO (diazomethane) interaction is probably closer in energy than the HO(fulvene)/LU(diazomethane) interaction (Fig. 4-72), and the adduct actually obtained (374)298 is the one expected from these considerations. Although reaction with a simple diene ought also to be dominated by the LU.(fulvene)/HO(diene) interaction (Fig. 4-72), nevertheless the observed294 product (376) from the reaction of dimethyl-fulvene (375) with cyclopentadiene has the fulvene acting as a n2 component rather than... 

The thermal and photochemical [4 + 2] cycloadditions of o-quinones with olefinic and acetylenic dienophiles have been extensively reviewed4,5,200 and include their 4tt heterodiene Diels-Alder reactions with olefins,201-204 vinyl ethers,205 enamines,206 selected dienes,207-209 dipheny-lketenimines,210 ketenes,209,210 fulvenes,211 and selected heterocycles including furan,207-209,212 benzofuran,209,212,215 indoles,213 azepines,214 and 1,2-diazepines.214 The tetrahalo-substituted o-quinones, tetrachloro- and tetrabromo-o-quinone, generally participate in heterodiene [4 + 2] cycloadditions at an increased rate over the unsubstituted systems and generally provide higher overall yields of the Diels-Alder products.4,5 With simple olefins, the dienophile geometry is maintained in the course of the thermal [4 4- 2] cycloadditions [Eq. (52)],203,204... 

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