Vinyl ligands

Iron porphyrins containing vinyl ligands have also been prepared by hydromet-allation of alkynes with Fe(TPP)CI and NaBH4 in toluene/methanol. Reactions with hex-2-yne and hex-3-yne are shown in Scheme 4. with the former giving two isomers. Insertion of an alkyne into an Fe(III) hydride intermediate, Fe(TPP)H, formed from Fe(TPP)Cl with NaBH4, has been proposed for these reactions. " In superficially similar chemistry, Fe(TPP)CI (present in 10 mol%) catalyzes the reduction of alkenes and alkynes with 200 mol% NaBH4 in anaerobic benzene/ethanol. For example, styrene is reduced to 2,3-diphenylbutane and ethylbenzene. Addition of a radical trap decreases the yield of the coupled product, 2,3-diphenylbutane. Both Fe(lll) and Fe(II) alkyls, Fe(TPP)CH(Me)Ph and [Fe(TPP)CH(Me)Ph] , were propo.sed as intermediates, but were not observed directly. ... 

A pathway that can lead to loss of specificity involves hydronation of the vinyl ligand at the carbon atom remote from the metal. This gives rise to an alkylidene species and consequent formation of a carbon-carbon single bond. Of course, dehydronation can occur to regenerate the vinyl species, but free rotation about this carbon-carbon single bond in the alkylidene would be expected to lead to a mixture of cis- and irans-vinyl species. Surprisingly, this need not be the case. 

Vinyl(aryl)iodonium salts with cyclic vinyl ligands... 

The vinyliodonium salts 52,55 and 56 exist as head-to-tail dimers in the solid state. The dimeric unit of 52 exhibits nearly equidistant intra- and intermodular P—0--T contacts and has been described as a pseudo-octahedral 12-1-4 system95. The dimeric structures of 55 and 56 are held together by P=0--T contacts collinear with the vinyl ligands, while the fluoroborate and perchlorate ions are nearly collinear with the phenyl ligands140. 

When (Z)-(j5-fluoro-j5-trifluoromethylvinyl)phenyliodonium triflate is mixed with sodium phenoxide or sodium 2-phenylethoxide in dichloromethane, substitution is directed to the jft-carbon atom of the vinyl ligand, while the iodonium function remains intact (equation 200)136. With the 4-ter/-butylbenzenethiolate ion, however, substitution occurs at both vinyl carbon atoms (equation 201)136. 

Palladium(II)-copper(I) cocatalyzed couplings of several (/ ,/ -dialkylvinyl)phenyliodo-nium triflates with 4-isopropoxy-3-tri- -butylstannyl-3-cyclobutene-l,2-dione (71) in DMF at room temperature have recently been described (equation 259)157. As observed in earlier studies of Cu(I)- or Pd(II)-promoted reactions of vinyliodonium salts, the vinyl ligands are introduced with retention of configuration. Since iodobenzene is a byproduct of ligand coupling, the production of 3-phenyl-4-isopropoxy-3-cyclobutene-1,2-dione might also be expected. However, the (/ ,/ -dialkylvinyl)iodonium ions are much more reactive... 

Similar Pd(II)/Cu(I) cocatalyzed couplings of (/ ,/ -dialkylvinyl)iodonium triflates with alkynylstannanes afford conjugated enynes, again with retention of configuration in the vinyl ligands (equation 260)157. 

Other neutral cyclopentadienyl alkyne complexes have been prepared by chloride substitution reactions with sulfur nucleophiles pentafluoro-phenylthiolate products have been reported [Eq. (35)] (95). These reactions are sensitive to the identity of the sulfur nucleophile (96), and attack at an alkyne carbon to yield 772-vinyl ligands is common (97). 

Salient structural features of CpL2Mo(rj2-CRCR2) complexes include (1) the orientation of the two Cg substituents roughly orthogonal to the MCaCj3 plane (this is opposite to the location of Cp substituents of rj -vinyl ligands which lie in the MC C plane), (2) short M—Ca distances (1.94-1.96 A) appropriate for a molybdenum-carbon double bond (174),... 

Unambiguous identification of r 2-vinyl ligands is often possible based on 13C-NMR spectra other less definitive spectroscopic probes may not reveal the presence of a metallocyclopropene. The carbenoid character of Ca is reflected in low-field chemical shifts of 230-290 ppm, whereas the four-coordinate Cp signal appears at much higher field, typically near 20-30 ppm (169). For comparison consider the chemical shifts of 151 and 128 ppm for the Ca and Cp carbons of the rj vinyl ligand in CpL3Mo-(V-CHCHBu ) [L = P(OMe)3] (170). 

A much larger barrier, about 20 kcal/mol, is calculated for complete rotation of the tj2-vinyl ligand in the model complex. In other words, a 90°... 

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