Bridged acetylene

The acetylene-bridged tris(dichlorostannane) (Equation (135)) achieves five-coordination by intramolecular asso-... 

By comparison of the already discussed redox systems 59qx—63qx with 6 ox— 70qx the different effects of ethylene and acetylene bridges can be evaluated. 

Weber et al. have developed pulsed electron double resonance (PELDOR) at S-band, and used it to probe the dipolar and exchange interactions in the bis-nitroxides 13 - 16, where in the ester-bridged compounds the exchange interactions are diminished relative to the acetylene-bridged compounds, whilst maintaining a similar separation.10 They also demonstrate distance measurements by this technique. 

Fig. 12.8 Synthetic semi-automated solid-phase strategy for the modification of DNA bases by chromophores via an acetylene bridge and Sonogashira-type cross-couplings.

Alkynes can be bound to metals using one or both sets of p-bonds, as 2, 3 or Ae donors. Acetylene bridges of several types are known. In acetylene there are two p-bonds at 90° to each other and both can be bound to a metal. The Co atoms and the acetylene carbon atoms forms a distorted tetrahedron and the C6H5 or other groups [Co2(CO)8(C2R2)] on the acetylene are bent away as shown... 

Homodimerization of aryl iodides (9, 19, 21) with an acetylene bridge was then conducted at room temperature (PdCl2(PPh3)2, Cul, Et3N, 24 h.) to provide symmetrical dimer 15 (58%) accompanied by the acetylenic monomer 13 (27%). Attempts to provide the corresponding ortho and meta monoacetylene-bridged dimers 23 and 26 were troublesome since they were invariably contaminated with doubly bridged-dimers (25, 28, Scheme 5) as inseparable mixtures (2 1 and... 

The acetylene-bridged dicarbyne form N shows both low-field M=C signals (ca. 8 350 ppm) and higher field acetylenic resonances for the C=C nuclei near 8 80-100 ppm.166,227,245... 

At low temperature, (38) reacts with stoichiometric O2 to form a transient diamagnetic adduct (38) 2-(38) (Figure 12) from which oxygen can be transferred to a-diketones. A similar dimer has been observed in which two molecules of (38) are linked via an acetylene bridge, that is, (38)A3sC-(38). 

Addition of pyridine and acetylene results in generation and trapping of the complex shown in equation (87), which has a reduced bond order. In this complex, the acetylene bridges the two metals. This type of chemistry was initially developed by Schrock and coworkers for alkyne metathesis (see Alkyne Metathesis) reactions but has been subsequently developed by a number of other researchers. 

AUcynylchalcogenato ruthenium complexes react with zirconocene to give rise to heterobimetallic early-late dissymmetrically bridged complexes of family (76) (equation 39). In those specific complexes, the two metal centers are linked by /u.-chalcogenido and /x-a, jr-alkynyl moieties. The acetylenic bridge is unsymmetrical because the terminal carbon of alkynyl is ct-bonded to ruthenium, while zirconium interacts with both alkynyl carbons in a side-on fashion. 

Finally, when compared to intervalence electron transfer, Dexter-type energy transfer gives widely different decay laws (expressed in terms of In Ftt vs. R), i.e., from -0.085 to -0.33 A. No simple explanation appears at present, due to the higher complexity of the process. One can only remark that, since the matrix element Ftt appears as the product of matrix elements for electron transfer and hole transfer (Eq. (7)), one can expect the rate of decay with distance to be the sum of the rates of decay for these two processes [46a, 83]. This would explain why the decay is generally faster than for electron transfer, with the curious exception of Ziessel s acetylene-bridged complexes (see Section 1.5.4). 

The fluoro compounds could be obtained by halogen exchange reactions from the corresponding chloro compounds with zinc fluoride in diethylether The acetylene-bridged compound 17 was prepared by reaction of di(/er/-butyl)chlorosilane with sodium acetylide in THF (Eq 2). Remarkably, during such reactions gaseous acetylene was formed, so that the acetylene bridged compounds arose in a one-step synthesis. 

Formal replacement of the acetylene bridge in the compounds mentioned above by an oxygen atom provides the disiloxanyl-bridged tetraorganodistannoxanes shown in entries 35-37. Like their acetylene-bridged analogs, they adopt B-type stmctures, but exclusively crystallize as frani -isomers. The... 

Figure 2.10.11 Molecular structure of the acetylene-bridged tetraorganodistannoxane [R(CI)SnCH2SiMe2 C2SiMe2CH2Sn(0H)R]0)2 (72, R=Me3CCH2)...

As with the identity of the organic substituents R and the spacers Z, the electronegative substituents X and Y also have a profound influence on the structure of spacer-bridged tetraorganodistannoxanes. Like the parent compounds, enhanced kinetic stability in solution is observed for those cases in which the substituents are different (X Y). This is especially pronounced for the phenyl-substituted derivatives 39-43 (entries 1-5) and also valid for the acetylene-bridged compounds (entries 32-34). 

Beginning with the use of acetylene anions, Armstrong, eta/.,18 prepared linear acetylene-bridged C-disaccharides. An example of the synthetic... 

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