Reactions of Ruthenium Cp Phosphine Complex with 4,4-Disubstituted-1,6-Enynes and Diyne
Date Issued
2010
Date
2010
Author(s)
Chung, Chia-Pei
Abstract
We studied chemical reactions of Cp(PPh3)2RuCl with nine 1,6-enyne compounds (1-4, 8, 12, 19, 21, and 22) in which the triple bond is associated with propargylic alcohol and the olefinic group has various substituted methyl groups. For the enyne compounds 1-3 with no substituted methyl group, the reaction takes place at the propargylic alcohol first giving the allenylidene complex 6 which could undergo a skeletal rearrangement to yield the disubstituted vinylidene complex 7. By changing the propargylic alcohol to propargylic ether, the reaction gives the carbene complex 10 as the major product and the butadiene complex 9 by a cyclization reaction as the minor product. For the enyne 12 with two methyl groups at the terminal carbon of the olefinic part, formation of either of the carbene complexes 15 and 16 with a substituted cyclopentenyl ring at Cα or the vinylidene complex 17 is controlled by the use of solvent. For the formation of 15 and 16, a C-C bond-forming cyclization reaction is proposed to occur at Cβ in an intermediate where the triple bond is π-coordinated. However, for the vinylidene intermediate, the reaction may proceed by the formation of the allenylidene, which undergoes a retro-ene reaction to bring about cleavage of the dimethyl substituted allyl group giving 17. For two enynes 21 and 22 where each olefinic portion is internally substituted with one methyl group, two vinylidene complexes 23 and 24 each with a five-membered ring bonded at Cβ are isolated. The reaction proceeds via formation of an allenylidene intermediate followed by a cyclization at Cγ. Stabilization of the cationic charge by the presence of methyl subsituents clearly controls the reaction pathway to give different products.
On the other hand, reactions of the four 1,6-diynes 30 and 31−33, each with one terminal propargylic alcohol and one internal triple bond containing Me3Si groups, with [Ru]Cl ([Ru] = Cp(PPh3)2Ru) led to two types of products. In the first type, only the propargylic group is involved in the reaction leading to vinylidene, allenylidene, or acetylide complexes. A C−C bond formation of two triple bonds in 1,6-diynes gave allylcarbene products of the second type. The reaction of diyne 30 with [Ru]Cl gave both types of complexes, namely the vinylidene complex 36 and the allylcarbene complex 35. The formation of 35 proceeds by a cyclization reaction involving two triple bonds on the metal accompanied by a migration of a phosphine ligand to Cα. Addition of HCl to 35 transforms the five-electron-donor allylcarbene ligand to the four-electron-donor diene ligand along with formation of a Ru−Cl bond, giving complex 38. The reaction of 31 with [Ru]Cl yielded only the first type, giving a mixture of two cationic complexes, the allenylidene complex 39 and the phosphonium acetylide complex 40, the latter resulting from further addition of a phosphine molecule to Cγ of 39. The same reaction in the presence of excess phosphine gave 40 only. However, with an additional methyl group, the 1,6-diyne 32 reacted with [Ru]Cl to give the allylcarbene complex 41 also with a phosphonium group on the ligand. In both reactions of 31 and 32, strong affinity between alkyne and phosphine was observed, resulting in formations of P−C bonds with different regioselectivity. From the reaction of [Ru]Cl with diyne 33 containing a tert-butyl group at the propargylic carbon, both the allenylidene complex 43 and the allylcarbene complex 44 were obtained.
We also studied chemical reactions of Cp(PPh3)2RuCl and Cp(dppe)RuCl with three 1,6-enyne compounds (1, 3 and 12) in which the triple bond is associated with propargylic alcohol and the olefinic group has substituted methyl groups. Herein, we focus our attentions on the influence of substituents, PPh3 and dppe, mediated on the ruthenium center with the same 1,6-propargyl enynes. The vinylidene complexes 5g and 5g'', tethering a dimethyl allyl moiety at C4 were prepared from the reaction of the 4,4-diphenylsubstituted propargylic alcohol 12 with Cp(PPh3)2RuCl and Cp(dppe)Cl in moderate to high yield, respectively. Deprotonation of 5g and 5g'' yields the acetylide complexes 46 and 46'', respectively. When 46 and 46’ were exposed to atmospheric condition, molecular oxygen readily reacts possibly via a [2+2+2] cyclization with the 1,6-enyne ligand of complexes 46 and 46’ to yield the acyl complexes 47 and 47’. Especially, the reaction of oxygen with the 1,6-enyne ligand in 46’ is achieved under mild condition in less than 10 min even in dark. When changed the dimethyl allyl moiety to allyl moiety or replaced O2 by other reactants containing activated C=C double bonds, this transformation would not occur. Treatment of the acetylide complex 46e with HBF4 in ether/H2O at 0 oC generated vinylidene complex 5e. However, treatment of the acetylide complexes 46’, 46e’ and 46h’ with the same procedure generated no vinylidene complex, but surprisingly, the corresponding ruthenium hydroxycarbene complexes 48’, 48e’ and 48h’ were obtained. It is suggested that the steric effect of the coordinated dppe ligand is significant in exposing the acetylide Cα atom to be attacked by a water molecule in acidic environment. These chemical reactions and their mechanisms are corroborated by structure determinations of ruthenium complexes using deuterium labeling experiments, 2D-NMR and single crystal X-ray diffraction analysis.
Subjects
Enyne
ruthenium
vinylidene
allenylidene
cyclization
allylcarbene
phosphonium acetylide
acetylide
acyl
oxidative addition
Type
thesis
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