Rhodium allenyl complexes

An intramolecular allenylidene-alkynyl coupling was also observed in the reaction of the mixed alkynyl-allenylidene rhodium(I) complex 73 with carbon monoxide (Scheme 25). In this case, the initially formed thermally unstable allenyl derivative 74 evolved into the metallated cyclobutenone 75 when an excess of CO was present [276]. 

Zwitterionic rhodium(I) complex, Rh (CODX -CfiHsBPtn), is also found to be an efficient catalyst for the silylformylation of 1-alkynes at 40 °C and 40 atm of CO in CH2C12 (equation 125) although no reaction occurs with internal alkynes327. However, silylhydroformylation takes place when the reaction is carried out under hydroformylation conditions, i.e. in the presence of CO and H2 (CO/H2 = 1/1), to give (E)-2-silylmethyl-2-alkenals (319) in 54-92% isolated yields (equation 128). The intermediacy of 7r-allenyl-Rh species is proposed to account for the formation of 31 9327. When 4-acetoxy-l-butyne and 4-(p-tosyloxy)-l-butyne are used as the substrates, saturated silylhydroformylation products are obtained327. 

The hydroboration of enynes yields either of 1,4-addition and 1,2-addition products, the ratio of which dramatically changes with the phosphine ligand as well as the molar ratio of the ligand to the palladium (Scheme 1-8) [46-51]. ( )-l,3-Dienyl-boronate (24) is selectively obtained in the presence of a chelating bisphosphine such as dppf and dppe. On the other hand, a combination of Pdjldba), with Ph2PC6p5 (1-2 equiv. per palladium) yields allenylboronate (23) as the major product. Thus, a double coordination of two C-C unsaturated bonds of enyne to a coordinate unsaturated catalyst affords 1,4-addition product On the other hand, a monocoordination of an acetylenic triple bond to a rhodium(I)/bisphosphine complex leads to 24. Thus, asymmetric hydroboration of l-buten-3-yne giving (R)-allenyl-boronate with 61% ee is carried out by using a chiral monophosphine (S)-(-)-MeO-MOP (MeO-MOP=2-diphenylphosphino-2 -methoxy-l,l -binaphthyl) [52]. 

Trisubstituted furans were obtained via an unprecedented 1,4-shift of the sulfanyl group of allenyl sulfides in high yields employing ruthenium complexes as catalyst, as depicted below. Furan products can also be provided in a one-pot reaction from a-diazocarbonyls and propargyl sulfide using both rhodium- and ruthenium-complexes or only a ruthenium-complex as catalyst <07AGE1905>. 

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