Allenyl-vinylidene complex

The unsaturated hydride Mn2(li-H)2(CO)6(lt-dppm) reacts with RCMHH (R = H, Ph, Bu , COOMe) to form either hydrido-alkenyls Mn2(M.-H)(ji2. n. l -CR=CH2XCO)6(ii-dppm) (R = H, Ph), or alkenylidene species 2(tt-H)(il2,tl3,Ti2-CR=CHBuO(CO)6(M.- pm) and (OC)3Mn(ji-dppm) (i-C=CHCOOMe Mn(CO)3. The photochemical reactions of Mn2(CO)8(p.-dppm) with alkynes results in vinylidene complexes. 35 The photochemical reaction between Mn2(CO)io and MeCsCNEt2 affords the ynamine complex 86. This isomeiizes into an aminoallene complex on warming. 36 The corresponding riienium complex Re2(CXD)8(p.-MeC2NMe2) is transformed into three complexes on thermolysis these are the (dimethylamino)allenyl Re2(CO)7(p-T 3-... 

The need for a base additive in this reaction implies the intermediacy of acetylide complexes (Scheme 9.10). As in the Rh(III)-catalyzed reaction, vinylidene acetylide S4 undergoes a-insertion to give the vinyl-iridium intermediate 55. A [l,3]-propargyl/ allenyl metallatropic shift can give rise to the cumulene intermediate 56. The individual steps of Miyaura s proposed mechanism have been established in stoichiometric experiments. In the case of ( )-selective head-to-head dimerization, vinylidene intermediates are not invoked. The authors argue that electron-rich phosphine ligands affect stereoselectivity by favoring alkyne C—H oxidative addition, a step often involved in vinylidene formation. 

Mononuclear osmium half-sandwiches, with rf-cyclopentadienyls and 7]5-indenyls alkenyls and allenyls with t/-M-C bonds, 6, 558 alkenyl vinylidenes, 6, 593 alkyl, aryl, acyl complexes, 6, 552 with alkylidyne complexes, 6, 599 alkynyl and enynyl complexes, 6, 567 allenylidene and cumulenylidene complexes,... 

Extending the carbon chain of vinylidenes by one or three carbons provides linear cumulenated ligands, LwM=(C=C)w=CR2 (n = 1,2). These compounds have been studied in considerable detail, especially allenyl-idenes (n = 1) and arise in most cases from the rearrangement and dehydration (often spontaneous) of hydroxyalkyl alkynes or diynes (Figure 5.51). Various intermediates (Figure 4.18) may be envisaged (and occasionally isolated) which are related by elimination/addition of H+ or hydroxide, e.g. y-hydroxyalkynyl complexes have been isolated and subsequently dehydroxylated with Lewis or Bronsted acids. 

The formation of allenylidene derivatives from ethynyl-hexanol and alkenyl-vinylidene mononuclear complexes (9), the formation of mononuclear ruthenium allenyl complexes from terminal alkynes (10), the intermediacy of ruthenium-allenylidene complexes in forming propargylic alcohols (II), and in the cyclization of propargyl alcohols (12), and the use of mononuclear ruthenium compounds in allylic alkylation catalysis (13) have also been reported. 

Finally, Hill et al reported the synthesis of vinylidene and allenylidene complexes of the types [TpRu( = C = CHR)(L)2] (L = PPh3 or L2 = 1,1 -bis(diphenylphosphino)-ferrocene R = Ph, C6H4Me), [TpRu( = C = C = CR2)(L)2] and TpRuCl( = C = C = CPh2)(PPh3). The eationie complexes underwent an unprecedented eoupling with dithiocarbamates, whieh led to metallacyclic vinyl and allenyl complexes (Seheme 34). Similar allenylidene eomplexes but featuring redox-active substituents and ligands based on ferrocene were reported by Hartmann et al ... 

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