Dendralene

For a discussion, see Phelan, N.F. Orchin, M. J. Chem. Educ., 1968, 45, 633. Compound 9 is the simplest of a family of cross-conjugated alkenes, called dendralenes. For a review of these compounds, see Hopf H. Angew. Chem., Int. Ed. Engl., 1984,23,948. Traetteberg, M. Hopf, H. Acta Chem. Scand. B, 1994, 48, 989. 

Fig. 5. Insertion of buta-l,3-diynediyl fragments (a) into radialenes [108] produces carbon-rich expanded radialenes and (b) into dendralenes [109] produces expanded dendralenes...

In spite of the continued interest in cross-conjugated polyenes ( dendralenes ), of which more than 100 are known307, surprisingly few of these have been investigated by PE or radical ion spectroscopy. ... 

Scheme 27. Domino Heck-Diels-Alder reaction and formation of dendralenes by cycloisomerization of enediynes...

One of the ways to generate the tetramethylenethane-type diradical 350, an important reference compound in connection with non-Kekule hydrocarbons [147], consists in the thermal isomerization of hydrocarbon 34 at -100 °C [43, 148], Under the reaction conditions, the six-membered ring of 350 ruptures to yield [4]dendralene (3,4-bismethylene-1,5 -hexadiene) (351). 

Dendralenes From a Neglected Class of Polyenes to Versatile Starting Materials in Organic Synthesis... 

By connecting double and single bonds, formally five classes of hydrocarbons can be constructed which differ considerably from one another not only chemically and physically but also in terms of their practical significance [1] the linear polyenes 1, the annulenes 2, which consist exclusively of endocyclic double bonds , the radialenes 3, polyolefins which are characterized by semicyclic double bonds, the fulvenes 4, hybrids containing endo-and semicyclic double bonds, and finally, the dendralenes 5 [2] which are acyclic cross-conjugated polyenes... 

A considerable number of processes have been described for the preparation of the two simplest dendralenes however, most of them are rather means of formation than efficient preparative methods. As Scheme 2 shows for [3]dendralene 7, thermal methods of preparation predominate (1,2-eliminations and periyclic processes) however, these methods do not always start from readily accessible precursors [5], The most effective procedure - which can hence be employed in subsequent reactivity studies (see below) - consists in the cheletropic decomposition of 11 carried out by Cadogan and Gosney et al. [6] although the preparation of the sulfolene derivative also requires several steps. 

The situation was even more critical for [41] dendralene 15 [2, 7], especially since the thermal decompositions shown in the lower half of Scheme 3 have all been carried out in connection with mechanistic studies. 

Before the first general synthetic concept for the preparation of the dendralenes is presented, it will be shown that these 7i-systems are interesting not only preparatively but also from a structural point of view. The application of dendralenes in Diels-Alder additions holds particular promise in synthetic chemistry. This is demonstrated in general form in Scheme 4 for the two simplest dendralenes. The [2+4]cycloaddition of 7 with a dienophile 20, not only leads to the expected l l-adduct 21 but also generates a new conjugated diene system which... 

As expected, the possibilities for [4]dendralene 15, which until now has only been used occasionally in diene-transmissive additions [7c], to participate in [2+4]cycloadditions are much greater (Scheme 4, b). Two possibilities arise for the first addition step, depending on whether 20 adds to a terminal diene unit (formation of 23) or to the central diene unit (formation of 25). For 23, two further alternatives are possible, which could either lead via 24 to... 

The masked dendralenes 36 are crystalline compounds, stable at room temperature, from which, as hoped, the hydrocarbons 37 could be released on demand in good yields by high-temperature pyrolysis. No solvent is required in these cheletropic reactions which facilitates the work-up. The dendralenes 37 obtained, up to [8]dendralene, have been completely characterized by the usual spectroscopic and analytical methods and can, although they tend to polymerize, be handled under the usual laboratory conditions (see below). The sulfolene decomposition route has recently been applied to the synthesis of many other cross-conjugated compounds, among them the hydrocarbons 39-42 (Scheme 7) [12]. 

Having sufficient amounts of these novel dienes in hand opens the field for further study. Cycloaddition of acetylenedicarboxylic acid dimethyl ester (ADDE) to [3]dendralene 7 in toluene first afforded the l l-adduct 43, which by a second addition of ADDE provided the expected 2 l-adduct 44. After this had been aromatized by treatment with DDQ to the naphthalene derivative 45, a double benzannulation could be carried out as described in Scheme 8 [13]. 

A similar sequence was carried out with [4]dendralene 14 and via the expected cycloaddition products 49 and 50 the tetraester 51 was isolated after aromatization, again a compound which can be employed for further extension of its aromatic core (Scheme 9) [13]. 

In principle an [n]dendralene can undergo [n - 1] cycloaddition reactions. To test whether this potential is actually used by these multi dienes Sherburn and co-workers have treated various dendralenes with 4-phenyl-l,2,4-trizoline-3,5-dione (PTAD) [12]. Since this is one of the most reactive dienophiles known, chances are high that indeed the full addition potential of the dendralene is used. Indeed [4]-, [5]-, and [6]dendralene added PTAD 3, 4, and 5 times, as expected. With [8]dendralene the cycloaddition process stopped after fivefold PTAD-addition, very likely because the lower mass cycloadducts were too poorly soluble to engage in further cycloaddition steps. 

Dendralene-type TTF vinylogues 837-842 containing tellurium or selenium atoms were obtained by condensation of the 2-diformylmethylene-l,3-diselenole or -1,3-ditellurole with phosphorus olefination reagents 834-836 under basic conditions (Scheme 126) <2000CEJ1955, 2001JOC7757, 20020L2581>. 

The three dendralenes 1, 2, and 39 show their longest-wavelength absorption maxima at A ax 407, 404, and 409 nm, respectively, in dichloromethane <2001JOC7757>. 

The electrochemistry of Te-containing dendralenes 50-52 is collected in Table 3 <20020L2581>. The first oxidation for 51 occurs at a lower potential than for the corresponding diselenole analog 2. 

Vilsmeier-Haack diformylation of 2-methylene-l,3-diselenole 140 gave 2-diformylmethylene-l,3-diselenole 141 (Equation 20) that served as a precursor for the dendralene-type vinylogs of tetrathiafulvalene 1, 2, 39, 142, and 143 by condensation with suitable dithiolium phosphonium salts or phosphonates in the presence of base <2001JOC7757>. 

The dialdehyde 150 was condensed with malononitrile to give the donor-acceptor compound 151 <20020L2581>. Condensation with different phosphoranes and phosphonates gave the dendralene-type vinylogs of tetrathiafulvalene 50-52 and the donor-acceptor compound 152 <20020L2581>. 

Compound 22 is the simplest of a family of cross-conjugated aUcenes, called dendralenes. For a review of these compounds, see Hopf, H. Angew. Chem. Int. Ed. 1984, 23, 948. 

---