Osmium electrophilic addition

The effect of metal basicity on the mode of reactivity of the metal-carbon bond in carbene complexes toward electrophilic and nucleophilic reagents was emphasized in Section II above. Reactivity studies of alkylidene ligands in d8 and d6 Ru, Os, and Ir complexes reinforce the notion that electrophilic additions to electron-rich compounds and nucleophilic additions to electron-deficient compounds are the expected patterns. Notable exceptions include addition of CO and CNR to the osmium methylene complex 47. These latter reactions can be interpreted in terms of non-innocent participation of the nitrosyl ligand. 

Thus methyl and chalcoformaldehyde complexes of osmium are accessible by both electrophilic addition to a neutral d8 methylene complex and nucleophilic addition to a cationic d6 methylene complex. 

The /3-electrophilic additions of pentaamineosmium(ll) complexes bearing various 4,5-tf -coordinated pyrroles to carbonyl compounds have been reported by Harman and co-workers (Scheme 78). 1 1-Methylpyrrole complex, when reacted with benzaldehyde or its dimethylacetal in the presence of /-butyldimethylsilyl triflate (TBSOTf), afforded the corresponding aldol adduct 177 as a 1 1 ratio of diastereoisomers. Pyrrole, 1-methylpyrrole, or 2,5-dimethylpyr-role osmium complexes reacted with an excess of acetone in the presence of TBSOTf to give the O-silylated 377-pyrrolium aldol adducts 178, which may serve as intermediates for various other reactions. 

The synthesis and chemistry of metal complexes of thiophenes have been reported including the electrophilic additions to osmium-thiophene complexes <9902988> and nucleophilic additions to ruthenium-thiophene complexes <99JOMC242>. The selectivity for the insertion of ruthenium into 3-substituted thiophenes was studied <99CC1793>. For example, treatment of 3-acetylthiophene (84) with Ru(cod)(cot) led to a regioselective 1,2-insertion of ruthenium giving thiaruthenacycle 85. 

The preceding sections describe regioselective electrophilic addition of pyrrole complexes at the 3-position with various electrophiles to give ( -substituted lZ/-pyrrole or 3//-pyrrolium isomers. The latter compounds, in contrast to their noncomplexed counterparts, are only moderately acidic (pKa 6) and therefore resist rearomatization and multiple alkylations. For example, when uncomplexed 2,5-dimethylpyrrole is treated with 1 equiv of methyl acrylate and TBSOTf, a statistical 1 2 1 ratio of starting material, monoalkylated 105 (vide infra), and 3,4-bis-alkylated product is found.12 Treatment of 1-methylpyrrole under the same conditions results in at least four alkylated products along with starting material. In contrast, coordination by osmium results in smooth... 

Vinylidene osmium(O) (187) (Section II,B,3,g) reacts with sulfur, selenium, or copper chloride to give complexes 233 via electrophilic addition to the osmium-carbon bond (116) (Scheme 18). Complex 187 also reacts with benzoylazide to form the five-membered metallacycle 234 (Z) which isomerizes 234 (E) on heating in benzene (141). 

Such dearomatization of the arene ligand activates it toward an electrophilic addition. Thus, osmium(ll) was used as a dearomatization agent for the direct 10/3-alkylation of /3-estradiol 70 (equation 29). When the tautomeric mixture 71 72 was placed in acidic methanol and reprecipitated, a 3 1 equilibrium ratio of the phenolic 71 and dienone 72 tautomers was observed . This intermolecular Michael addition to the C(10) position of the aromatic steroid was unprecedented. 

The mechanism of this activation of the C-H bond is unknown although the reaction may proceed by an oxidative addition. Generally, the pentaammine-osmium(II) system is known to activate phenols, anilines, and anisoles toward electrophilic addition and substitution reactions by binding the aromatic ligand in an T -fashion. Protonation, for example, results in the formation of a heterotriene system [30b] ... 

Most osmium complexes of phenols [26,44], anilines [24,45], and anisoles [23, 46,47] undergo electrophilic addition with a high regiochemical preference for para addition. While electrophilic additions to phenol complexes are typically carried out in the presence of an amine base catalyst, the other two classes generally require a mild Lewis or Bronsted acid to promote the reaction. The primary advantage of the less activated arenes is that the 4H-arenium species resulting from electrophilic addition are more reactive toward nucleophilic addition reactions (see below). 

Although C4 addition occurs with phenol complexes even for cases where C4 is substituted, in many cases, ortho addition is thermodynamically favored. In this scenario, the regiochemistry can be effectively controlled by adjusting reaction variables such as temperature, time, and catalyst [44]. Under basic conditions, the active form of the phenol complex is the phenoxide species, which can undergo reversible Michael reactions at C4 and C2, provided that the resulting enolate is not protonated. For instance, the addition of MVK to the osmium complexes of para-cresol (31) or estradiol (27, Fig. 8) occurs at C4 to give the 4H-phenol product (28,32) at -40 °C with an amine base. However, if the reaction is carried out at 20 °C or is run in the presence of a Zn " co-catalyst, the initially formed enolate may undergo retroaddition, and ultimately, the reaction yields the orthoalkylated product (30,33 see Fig. 8). Electrophilic addition at the ortho... 

The reactions outlined in the previous sections all utilized an arene with a heteroatom substituent that could stabilize the arenium intermediates resulting from electrophilic addition. Benzene, alkylated benzenes, and naphthalene are more difficult to activate because they lack this mode of stabilization. Osmium... 

Electrophilic Addition.—The hydroxylation of alkenes, including several steroids, with osmium tetroxide has been reviewed. Addition of HOBr to the 19-functionalized-5a-cholest-6-enes (13) gave mixtures of the corresponding bromohydrins (14) and the 6,19-epoxides (15) whereas the analogous B-homo-compounds (16) gave only the 6,19-epoxides (17). Similar effects were noted for the related HBr- and HC104-catalysed opening of the 6a,7a-epoxides and it was observed that the predominant attack by the 19-0 atom [5(0)" attack] in the B-homo-series lay in the possibility of its linear approach with the C-6—Br or C-6—O bond. The reactions of chromyl chloride and chromyl fluoride with steroidal alkenes and dienes have been reported " and it was observed that the... 

Alkenes are very susceptible to electrophilic addition reactions, which with osmium tetroxide gives the product of 1,2-dihydroxylation. 

The pattern of chemical reactions observed for these compounds clearly sets them apart from "Fischer-type" carbyne complexes of GroupVI e.g., W(hCR)X(CO)4. Whereas the "Fischer-type" complexes typically react with nucleophiles at the carbyne carbon all of the reactions observed for the five coordinate mthenium and osmium complexes, including the cationic examples, are electrophilic additions to the MsC bond. The following sections deal individually with, protonation, addition of halides of the coinage metals, addition of chlorine and chalcogens, and finally an attempted nucleophilic addition where the nucleophile is directed to a remote site on the aryl ring of the carbyne substituent. 

Initially, it was thought more likely that the electron poor metal atom would be involved in the electrophilic attack at the alkene and also the metal-carbon bond would bring the alkene closer to the chiral metal-ligand environment. This mechanism is analogous to alkene metathesis in which a metallacyclobutane is formed. Later work, though, has shown that for osmium the actual mechanism is the 3+2 addition. Molecular modelling lends support to the 3+2 mechanism, but also kinetic isotope effects support this (KIEs for 13C in substrate at high conversion). Oxetane formation should lead to a different KIE for the two alkene carbon atoms involved. Both experimentally and theoretically an equal KIE was found for both carbon atoms and thus it was concluded that an effectively symmetric addition, such as the 3+2 addition, is the actual mechanism [22] for osmium. 

Zinc serves to further reduce osmium and free the diol product. Similar oxidative additions to alkenes occur with bromine, chlorine, IN3, peracids, and many other electrophiles. 

As already indicated, the chemistry of terminal borylene complexes is as yet almost unexplored. In addition to the photochemically induced borylene transfer, which was already discussed in Chapter 3.2, studies of the reactivity of terminal borylene complexes are restricted to two recent reports by Roper.147,148 The base-stabilized borylene complex [Os (=BNHC9H6N)Cl2(CO)(PPh3)2] (26) undergoes a reaction with ethanol to yield the ethoxy(amino)boryl complex [Os B(OEt)NHCgH6N Cl(CO) (PPh3)2] (35) according to Eq. (13) with a 1,2-shift of the quinoline nitrogen atom from the boron to the osmium center. The alcoholysis of 26 indicates that even the boron atom in base-stabilized borylene complexes displays some electrophilic character—a fact already predicted by a theoretical study.117... 

The cyclic intermediate, called an osmate ester, is not isolated instead, the osmium-oxygen bonds are cleaved by using a reagent such as sodium sulfite, Na2S03, resulting in the formation of a 1,2-diol. (The mechanistic details of the cleavage step need not concern us.) Because both the electrophilic and nucleophilic oxygens are attached to the same metal atom, both are delivered from the same side of the plane of the double bond—the reaction is a syn addition. 

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