Alkynes, coordination, dimerization, and

Alkyne Coordination, Dimerization, and Scission on a Tungsten—Triiridium Cluster... 

Alkynes, coordination, dimerization, and scission on tungsten-triiridium cluster core, 127-134 Alloys... 

In contrast to cycloaddition reactions of phosphaalkenes, cycloaddition reactions between phosphaalkynes and other unsaturated systems are comparatively rare. Indeed, there are only a limited number of reports for monophos-phacyclobutadiene) complexes, which are obtained from the corresponding phosphaalkyne. Relatively recently, the reaction of phosphaalkynes with highly electron deficient alkynes was reported <19990M4838>. Treatment of a CF3C=CGF3-coordinated dimeric rhodium complex with phosphaalkynes in hexane at — 20°C followed by warming to room temperature afforded the red air- and moisture-stable phosphete complexes 60 in ca. 50% isolated yields. When phosphaalkynes are allowed to react with a kinetically stabilized cyclobutadiene, 2-Dewar-phosphinines, for example 93 (Equation 30), are obtained <1998S1305>. 

Three effects are observed in the reactions of Os6(CO)18 with diphenylacetylene and ethylene. There is modification of the metal framework, rupture of a C=C triple bond, and dimerization of ethylene (228-230). When the activated clusters Os6(CO)17(MeCN) (230) and Os6(CO)20(MeCN) (231) react with mono- and disubstituted alkynes, different penta- and hexanuclear framework geometries are obtained, and the alkyne-derived ligands adopt a range of coordination modes (Fig. 8). 

The [MH(H2)(pp3)]+ system, which was studied extensively by Bianchini and coworkers,94 105 highlights the importance of differences in M-H2 bond strengths (Os > Fe > Ru) on the selective hydrogenation of alkynes to alkenes. The Fe congener is one of the most stable H2 complexes and reactions with alkynes under 1 atm ofH2 produce only alkenes (with one exception) via the mechanism in Eq. (9.54), where the H2 remains attached while the alkyne coordinates. In contrast, the complex [RuH(H2)(pp3)]+ catalyzes dimerization of 1-alkynes to nearly 100% Z-l,4-disubstituted enynes.107... 

The coordinated alkene or alkyne ligand can be attacked by other alkene or alkyne molecule to accomplish some metal-catalyzed synthetically useful transformations. Typical examples include dimerization and polymerization of alkenes catalyzed by highly electrophilic cations [PdL2(MeCN)2] + (L = MeCN, PR3) (e.g. Scheme 8.35) [57], and Cope rearrangement of 1,5-hexadiene derivatives catalyzed by PdCl2 (Scheme 8.36) [58], It was proposed that the key step in these reactions was the C-C bond formation via attack of the external alkene at the alkene carbon which was made highly electron-dehcientby coordination to Pd(II) ion. 

Sanders (14) has exploited the strong and selective coordination of phosphine donor groups to Ru(II) to construct hetero-dimetallic porphyrin dimers (17, Fig. 5). An alkyne-phosphine moiety introduced on the periphery of a free base or metalloporphyrin (M = Zn or Ni) spontaneously coordinates to a Ru(II)(CO) porphyrin when the two porphyrins are mixed in a 1 1 ratio. Coordination is characterized by a downfield shift of the 31P resonance (A<531P = 19 ppm). There is no evidence of self-coordination of the zinc porphyrin at 10 6 m in toluene, there is no shift in the Soret band in the UV-Vis absorption spectrum. The Ni-Ru dimer was observed by MALDI-TOF mass spectrometry. Heating the Ru(II)CO porphyrin with 2 equivalents of the phosphine porphyrins led to quantitative formation of trimeric assemblies. 

The higher catalytic activity of the cluster compound [Pd4(dppm)4(H2)](BPh4)2 [21] (20 in Scheme 4.12) in DMF with respect to less coordinating solvents (e.g., THF, acetone, acetonitrile), combined with a kinetic analysis, led to the mechanism depicted in Scheme 4.12. Initially, 20 dissociates into the less sterically demanding d9-d9 solvento-dimer 21, which is the active catalyst An alkyne molecule then inserts into the Pd-Pd bond to yield 22 and, after migratory insertion into the Pd-H bond, the d9-d9 intermediate 23 forms. Now, H2 can oxidatively add to 23 giving rise to 24 which, upon reductive elimination, results in the formation of the alkene and regenerates 21. 

Cp Ru [14] and TpRu [20] complexes have also been studied in depth. As represented in Scheme 2c, the catalytic alkyne dimerization proceeds via coordinatively unsaturated ruthenium alkynyl species. Either a direct alkyne insertion and/or previous vinylidene formation are feasible pathways that determine the selectivity. The head-to-tail dimer cannot be formed by the vinylidene mechanism, whereas the E or Z stereochemistry is controlled by the nature of the alkynyl-vinylidene coupling. 

Apart from the construction of phenanthrenes, carbene complexes have also been used for the synthesis of more extended polycyclic arenes. An unusual dimerization of chromium coordinated ortbo-ethynyl aryl carbenes results in the formation of chrysenes (Scheme 37) [81]. This unusual reaction course is presumably due to the rigid C2 bridge that links the carbene and alkyne moieties, and thus prevents a subsequent intramolecular alkyne insertion into the metal-carbene bond. Instead, a double intermolecular alkyne insertion favored by the weak chromium-alkyne bond is believed to occur forming a central ten-membered ring that may then rearrange to the fused arene system. For example, under typical benzannulation conditions, carbene complex 97 affords an equimolar mixture of chrysene 98a and its monochromium complex 98b. The peri-interactions between the former alkyne substituent (in the 5- and 11-positions) and the aryl hydrogen induce helicity in the chrysene skeleton. 

---