Iridium olefines

The reactions of iridium olefin complexes are not restricted to reactions with phosphines. Amines have also employed in bridge-splitting and substitution reactions with [Ir(COD)Cl]2, especially chelating diamines. The reactions proceed to yield [Ir(COD)N-N]2 compounds. A fertile chemical area involves the irw(pyrazolyl)borate (see Tris(pyrazolyl)borates) family of compounds with the monoethylene and bisethylene complexes serving as reactive entries in this field. ... 

Ir Me3tpa)(ethene)] + have indicated that MeCN coordination to the metal can be sufficient to overcome the unfavorable bending of the olefin, thus yielding an iridium-olefin species with an electronic structure intermediate between a slipped olefin Ir —CH2=CH2 and an alkyl radical Ir —CH2—CH2 description (see Section IV.D). In addition to the energy gain from MeCN coordination, delocalization of the unpaired spin to iridium renders formation of such species less unfavorable in this case. 

Therefore 4d and 5d electron metals interact with ligands in a more effective manner and thus form more covalent compounds. Because of valence orbital energy and orbital sizes, compounds of these elements in their lower oxidation states, particularly organometallic ones, are more stable than analogous complexes of M electron metals. The increased stability of olefin and acetylene compounds with increasing atomic number in a given group may serve as an example. Olefin complexes of cobalt are few and very unstable, while rhodium and iridium olefin compounds are quite common and usually air-stable. 

The first transition metal-catalyzed hydroamination of an olefin was reported in 1971 by Coulson who used rhodium(I), rhodium(III) or iridium(III) catalysts (Eq. 4.8) [105,106]. 

Scheme 8.18 Hydrogenations of olefins with iridium complexes containing dithio-ether ligands with a pyrrolidine backbone.

In recent years, the asymmetric hydrogenation of prochiral olefins have been developed in the presence of various chiral sulfur-containing ligands combined with rhodium, iridium or more rarely ruthenium catalysts. The best results have been obtained by using S/P ligands, with enantioselectivities of up to 99% ee in... 

According to the proposed mechanism, the hydrogenation of olefin by iridium catalyst should conform to the following rate expression,... 

The functional form of this rate expression is consistent with the behavior of the iridium system observed throughout the kinetic investigations. The coordination of nitrile to iridium is anticipated to produce more than a simple inhibitory effect. Being the dominant equilibrium in the mechanism, nitrile coordination may produce the observed first order dependence of the reaction rate with respect to hydrogen. Given Kcn[RCN] is the predominant term in the denominator, the rate expression may be reduced to the form of (8) which is first order with respect to both olefin and [H2]. 

Chiral phosphinodihydrooxazole iridium ligands are used to hydrogenate trisubstituted olefins in moderate yields and high enantioselectivity albeit of... 

Successive hydrogen transfers within 60, followed by coordination of olefin and then H2 (an unsaturate route), constitute the catalytic cycle, while isomerization is effected through HFe(CO)3(7r-allyl) formed from 59. Loss of H2 from 60 was also considered to be photoinduced, and several hydrides, including neutral and cationic dihydrides of iridium(III) (385, 450, 451), ruthenium(II) (452) and a bis(7j-cyclopentadienyltungsten) dihydride (453), have been shown to undergo such reductive elimination of hydrogen. Photoassisted oxidative addition of H2 has also been dem-... 

Incorporation of rhodium triphenylphosphine moieties into carboranes has led to HRh(C2B9Hn)(PPh3)2 complexes, which are formally hydri-dorhodium(III) dicarbollides and which catalyze olefin hydrogenation under mild conditions (527). Iridium and ruthenium analogs are also known, including complexes with carboranylphosphine ligands, e.g., HRuCl(PPh3)(l-P(CH3)2-l,2-C2B, Hn]2 (,527-530). 

Another area of high research intensity is the catalytic dehydrogenation of alkanes to yield industrially important olefin derivatives by a formally endothermic (ca. 35 kcal mol-1) loss of H2. Recent results have concentrated on pincer iridium complexes, which catalytically dehydrogenate cycloalkanes, in the presence of a hydrogen accepting (sacrificial) olefin, with turnover numbers (TONs) of >1000 (Equation (23)) (see, e.g., Ref 33,... 

The reaction is thought to proceed via an iridium hydride, with the olefin group acting as a directing group. Metallacycle intermediates have also been implicated in this reaction (Scheme 22).96... 

The most fundamental reaction is the alkylation of benzene with ethene.38,38a-38c Arylation of inactivated alkenes with inactivated arenes proceeds with the aid of a binuclear Ir(m) catalyst, [Ir(/x-acac-0,0,C3)(acac-0,0)(acac-C3)]2, to afford anti-Markovnikov hydroarylation products (Equation (33)). The iridium-catalyzed reaction of benzene with ethene at 180 °G for 3 h gives ethylbenzene (TN = 455, TOF = 0.0421 s 1). The reaction of benzene with propene leads to the formation of /z-propylbenzene and isopropylbenzene in 61% and 39% selectivities (TN = 13, TOF = 0.0110s-1). The catalytic reaction of the dinuclear Ir complex is shown to proceed via the formation of a mononuclear bis-acac-0,0 phenyl-Ir(m) species.388 The interesting aspect is the lack of /3-hydride elimination from the aryliridium intermediates giving the olefinic products. The reaction of substituted arenes with olefins provides a mixture of regioisomers. For example, the reaction of toluene with ethene affords m- and />-isomers in 63% and 37% selectivity, respectively. 

Numerous studies aimed at the understanding of the mechanism of these processes rapidly appeared. In this context, Murai examined the behavior of acyclic linear dienyne systems in order to trap any carbenoid intermediate by a pendant olefin (Scheme 82).302 A remarkable tetracyclic assembly took place and gave the unprecedented tetracyclo[6.4.0.0]-undecane derivatives as single diastereomer, such as 321 in Scheme 82. This transformation proved to be relatively general as shown by the variation of the starting materials. The reaction can be catalyzed by different organometallic complexes of the group 8-10 elements (ruthenium, rhodium, iridium, and platinum). Formally, this reaction involves two cyclopropanations as if both carbon atoms of the alkyne moiety have acted as carbenes, which results in the formation of four carbon-carbon bonds. 

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