1.4- donor-substituted

To prepare cyclic tetraynes consisting of 1,4-donor-substituted butadiyne units one can use the Glaser type coupling of terminal dithiadiynes 28(3) or 28(4) (Scheme 7.8). This method produces the 18- and 20-membered rings 29(3) and 29(4), respectively, but no trimerization products were found [40]. 

To prepare macrocycles with three 1,4-donor-substituted butadiyne units the synthesis commences with the stable, commercially available bis-trimethylsilyl pro-... 

Scheme 7.9. One-pot synthesis of macrocycles with 1,4-donor substituted butadiyne units, (a) MeLi/LiBr, THF, -78 °C to rt,...

The coupling of bromo- or iodobenzene to styrene yields regioselectively a mixture of E- and Z-stilbenes 12 and 13. An electron-withdrawing substituent at the olefinic double bond often improves the regioselectivity, while an electron-donor-substituted alkene gives rise to the formation of regioisomers. 

In the 1,4-dihelerocine scries 1 with different heteroatoms only the 1,4-oxazocines 2 are known. As in the case of the 1,4-diazocines, depending upon the substituent on the nitrogen, these systems exhibit either nonplanarity with a localized -electron system for acceptor substituents or planar delocalized structures for donor-substituted or the unsubstituted systems.10... 

Fig. 11-2. Effect of the complexation by crown ethers on the ground and first excited states of electron donor-substituted benzenediazonium ions (after Walkow and Israel, 1990).

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

In the presence of a catalytic amount of triethylamine, a readily enolizable carbonyl compound like acetylacetone (25) can undergo a Michael-type addition onto the triple bond of 23 with C-C bond formation, and subsequent 1,2-addition of the hydroxy group with elimination of an alcohol (MeOH or EtOH) to eventually yield a pyranylidene complex 28 (mode E) [29]. The most versatile access to / -donor-substituted ethenylcarbene complexes 27 is by Michael-type additions of nucleophiles, including alcohols [30-32], primary... 

Aumann et al. showed that 1,2,4-tridonor-substituted naphthalenes, such as 126, are accessible from 3-donor-substituted propenylidenecarbene complexes 124 containing a (Z)-positioned 3-phenyl substituent and isocyanide (Scheme 26). These transformations constitute formal [5+1] cycloadditions [39, 89, 90]. Since isocyanides are strongly coordinating ligands on chromium, at least... 

In accordance with this, the reaction of the electron-donor-substituted butadienes 170 (R=Ph, OMe) with the arylcarbene complexes 163 yields divinylcyclopropane intermediates 168 with high chemoselectivity for the electron-rich double bond in 170, which readily undergo a [3,3]-sigmatropic rearrangement to give the as-6,7-disubstituted 1,4-cycloheptadiene derivatives... 

Catalytic cyclopropanation of alkenes has been reported by the use of diazoalkanes and electron-rich olefins in the presence of catalytic amounts of pentacarbonyl(rj2-ris-cyclooctene)chromium [23a,b] (Scheme 6) and by treatment of conjugated ene-yne ketone derivatives with different alkyl- and donor-substituted alkenes in the presence of a catalytic amount of pentacarbon-ylchromium tetrahydrofuran complex [23c]. These [2S+1C] cycloaddition reactions catalysed by a Cr(0) complex proceed at room temperature and involve the formation of a non-heteroatom-stabilised carbene complex as intermediate. 

In the same way as arylcarbene complexes, alkenylcarbene complexes typically react with alkynes to provide [3C+2S+1C0] Dotz cycloadducts (see Chap. ccChromium-Templated Benzannulation Reactions , p. 123 in this book). However, some isolated examples involving the formation of five-membered rings through [3C+2S] cycloaddition processes have been reported [71]. In this context, de Meijere et al. found that /J-donor-substituted alkenylcarbene complexes react with alkynes to give cyclopentene derivatives [71a]. This topic is also discussed in detail in Chap.ccThe Multifaceted Chemistry of Variously Substituted a,/J-Unsaturated Fischer Metalcarbenes , p. 21 of this book. 

Table 6 Electronic spectral data for selected Ni11 complexes of donor-substituted tacn ligands.

Salaiin, J. R. Y. Synthesis and Synthetic Applications of 1-Donor Substituted Cyclopropanes with Ethynvl, Vinyl and Carbonyl Groups, 144, 1-71 (1987). 

In all the latter cases the easier dimerization reaction is connected with the particular stability of the intermediate diradical species. This is also the reason for the recently found facile dimerization of the 1-donor substituted allylidene-cyclopropane 136a (Scheme 66) [127]. Allylidenecyclopropane 136a cyclodimer-izes to the expected cyclobutane 467 in very mild thermal conditions, due to the stabilization of the intermediate 466. At higher temperature (120 °C) both 136a and 467 give a more complex mixture of products, with the cyclooctadiene dimer 468 being the prevailing one (Scheme 66) [127],... 

Cycloadducts have been successively obtained by reaction of MCP with maleic anhydride (116) and a number of related electron-deficient alkenes (137,486,487) under photolytic conditions in the presence of a sensitizer (Table 38, entries 5-8) [132b]. Analogous cycloadditions in mild conditions with high yields have also been performed with electron-donor substituted alkenes, such as vinylene carbonates 483 and 484 and the imidazolinone 485 (entries 2-4) [132], In the case of the unsymmetrical anhydride 137 (entry 6), an almost equimolar mixture of both the possible regioisomers has been obtained [132b]. In all these cases the reaction has also been proposed to occur via diradical intermediates formed from the reaction of 1 with the alkene in its excited triplet state [132]. 

Malicki M, Guan Z, Ha SD, Heimel G, Barlow S, Rumi M, Kahn A, Marder SR (2009) Preparation and characterization of 4 -donor substituted stilbene-4-thiolate monolayers and their influence on the work function of gold. Langmuir 25(14) 7967-7975... 

The data demonstrate that the electron-transfer rate in donor-substituted fuller-enes can be controlled by the electron-releasing property of the substituent as well as by the electronic structure and/or length of the spacer used. 

In this chapter we shall deal with donor-substituted allenes of type 3, 4, 5 and 6 (Scheme 8.2), among which oxygen-substituted allene derivatives 3 are of major importance. They have been utilized for numerous syntheses of interestingly func-... 

Due to the synthetic versatility of donor-substituted allenes, it is of great importance to modify these compounds by substitution reactions at C-l. The CH-acidity at the a-carbon of alkoxyallenes allows their smooth lithiation [12, 42] which conveniently... 

Cycloadditions and cyclization reactions are among the most important synthetic applications of donor-substituted allenes, since they result in the formation of a variety of carbocyclic and heterocyclic compounds. Early investigations of Diels-Alder reactions with alkoxyallenes demonstrated that harsh reaction conditions, e.g. high pressure, high temperature or Lewis acid promotion, are often required to afford the corresponding heterocycles in only poor to moderate yield [12b, 92-94]. Although a,/3-unsaturated carbonyl compounds have not been used extensively as heterodienes, considerable success has been achieved with activated enone 146 (Eq. 8.27) or with the electron-deficient tosylimine 148 (Eq. 8.28). Both dienes reacted under... 

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