Dienes fulvenes

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. 

Oxidation of thiophene with peracid under carefully controlled conditions gives a mixture of thiophene sulfoxide and 2-hydroxythiophene sulfoxide. These compounds are trapped by addition to benzoquinone to give ultimately naphthoquinone (225) and its 5-hydroxy derivative (226) (76ACS(B)353). The further oxidation of the sulfoxide yields the sulfone, which may function as a diene or dienophile in the Diels-Alder reaction (Scheme 88). An azulene synthesis involves the addition of 6-(A,A-dimethylamino)fulvene (227) to a thiophene sulfone (77TL639, 77JA4199). 

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). 

Dimethyl anthracene and diphenyl isobenzofuran form remarkably stable233 cyclopropanone derivatives (353/354), whilst with other diene components (butadiene, tetracyclone, and fulvene) the primarily formed Diels-Alder adducts either suffer ketalizing attack of the solvent (356 - 357, 359 - 358/360) or undergo irreversible changes such as decarbonylation to 362 or rearrangement to 355. 

In contrast to the usual (4 + 2) diene reactivity of fulvenes 3-amino fulvenes 539 in some cases are capable of expanding the five-membered ring to heptafulvenes 541 by addition of acetylene dicarboxylate in a (2 + 2) fashion299. ... 

FIGURE 11. Comparison of the PE spectra of (a) fulvene 175 and (b) homofulvene (spiro[2.4]hepta-4,6-diene) 74... 

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... 

Joining olefin metathesis on the very short list of exchange reactions involving carbon-carbon bonds, the Diels-Alder reaction was studied in 2005 by Lehn and colleagues [52]. As the authors note, most Diels-Alder reactions proceed only in the forward direction at room temperature, with retro Diels-Alder reactions typically requiring elevated temperatures. Careful tuning of the diene and dienophile, however, can alter this significantly. In particular, reactions of substituted fulvenes (32) with diethylcyanofumarate (33) were... 

In some cases it is the diene component from the retro reaction which is the desired product and extrusion of a volatile alkene such as ethylene is then ideal. Pyrolysis of the 1,4-oxathiin systems 21 proceeds in this way to give the a-oxothiones 22 which for R = Pr1, exists mainly as the enethiol tautomer 2323. Thermal extrusion of ethylene from 24 provides convenient access to the interesting fulvene 25 in quantitative yield24, and the corresponding reaction of 26 at 650 °C and 10-4 torr gives the cyclopentadienoben-zopyrene 28 in 95% yield, presumably by way of the intermediate 2725. 

The catalyst Col2(dppe)-Zn/ZnI2 catalyses the 6 + 2-cycloaddition of cyclohepta-triene with terminal alkynes to afford 7-alkylbicyclo[4.2.1]nona-2,4,7-trienes in good yields 239 The intermolecular 6 + 3-cycloaddition of fulvenes (213) with 3-oxidopy-rylium betaines (212) yields (214), which, after a 1,5-hydrogen shift, yields the 5,8-fused oxabridged cyclooctanoids (215). This methodology can be used for the preparation of fused cyclooctanoid natural products such as dactylol and precapnella-diene (Scheme 60).240... 

The electron transfer induced reaction of this diene system results in rapid [4 + 2]dimerization conversely, the dimer rapidly undergoes cycloreversion upon electron transfer. Both reactions result in strong CIDNP effects. The monomer polarization supports a radical cation with a spin density distribution like those of the butadiene or fulvene radical cations. The dimer polarization identifies a dimer radical cation with appreciable spin density only on two carbons of the dienophile fragment this species can only be the doubly linked radical cation D [135, 136], Significantly, a second dimer radical cation is implicated in a pulsed... 

In the first reaction, the dominant FO interaction occurs between the fulvene LUMO and the 1-aminobutadiene HOMO. It is easy to prove that the 6 + 4 reaction is favored the bond which forms most easily links Ca in the fulvene to C4 in the diene. 

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. 

Super Hydride is one of the most powerful nucleophilic reducing agents available, capable of reducing many functional groups. It is also highly selective. The exocyclic double bond in aryl-substituted fulvenes has an increased polarity, due to the inductive effects of their respective aryl groups. This increased polarity allows for selective nucleophilic attack at this double bond and not at the diene component of the fulvenes. Other examples of the nucleophilic addition of hydrides to substituted fulvenes (albeit with alkyl or unsubstituted phenyl group functionality)... 

Pyridinium-3-olates and pyrylium-3-olates react with a variety of monoenes, dienes, and trienes. Each of these systems react readily with dienophiles to yield cycloadducts of type 433. For Z = NMe, an electron-withdrawing X group is required in the dienophile, but with Z = 0 or A-(2-pyridyl) even unactivated alkenes react. 1-Phenylpyridi-nium-3-olate and benzyne give 434 dienes give adducts of type 435. Fulvenes behave as trienes to give adducts across the 2,6-positions 436. 2-Benzothiopyrylium-4-olate 437 gives a thermal dimer across the 1,3-positions. 

Nucleophiles attack C-6, since this is the site of highest LUMO coefficient. Similarly, nucleophilic 4 n addends should cycloadd across the 2 and 6 positions, with the more nucleophilic end of the 4 rr addend attacking C-6. By contrast, electrophiles attack C-2, the site of highest HOMO coefficient, and electrophilic 4 n addends can only add across C-2 and C-3, due to the node at C-6. The presence of the second highest occupied MO (SHOMO) of the fulvene complicates this simple picture, and substituents may drastically change the HOMO shape, as described below. Nevertheless, this picture does help explain, at least qualitatively, the varying periselec-tivity of diene and 1,3-dipole cycloadditions to fulvenes. 

We have described the cycloadditions of a variety of dienes, ranging from cyclo-pentadiene to cyclopentadienones with alkyl and aryl fulvenes80-82. In these cases, only the [4 + 2] cycloadducts across the 2 and 3 positions are observed. Similarly, 1,3-dipoles such as nitrones and nitrile oxides add in this fashion, as well. We discovered the first authentic [6 + 4] cycloaddition of a fulvene in 197083. The cycloadditions of tropone to fulvenes, which we originally suggested involved [6-fulvene + 4-tropone] cycloadditions, now appear to be [6-tropone + 4-fulvene] cycloadditions8... 

Additions occur more easily if a carbanion with resonance or inductive stabilization is formed in the addition. Thus, fulvenes are very reactive, vinylsilanes and highly fluorinated alkenes somewhat less so. Styrene, 1,3-dienes, and enynes are more reactive than isolated alkenes, and Grignard reagents may be used to initiate anionic polymerization of styrenes, dienes, and acryhc monomers. Strained alkenes such as norbomenes and cyclopropenes are also more reactive. Examples of additions facilitated by resonance or substitution are shown in Scheme 8. 

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. ... 

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... 

An important ancfllary study with signifrcant synthetic implications revealed that electron-rich dienes attached to fulvenes by a multicarbon chain also exhibited a high degree of preference for forming the corresponding [6 -i- 4] adducts. For example, when substituent X in compound (60) was either diethyl-amino or trimethylsiloxy, the [6 -I- 4] adduct (61) was the major product-type isolated. However, if no substituent was present on the diene portion of the molecule, a mixture consisting of [6 -i- 4], exocyclic [4 -I- 2] (62) and endocyclic [4 -I- 2] (63) cycloadducts was obtained. ... 

Tlie regioselectivity of the reaction of unsynunetrical diene addends to various fulvene systems has also been explored in some detail. Heating 2-ethyl-5-methylthiophene dioxide in the presence of 6-di-... 

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