Osmium electrophilic addition reactions

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

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. 

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 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). 

The q -phenol complex undergoes conjugate addition at C4 with a variety of Michael acceptors (Fig. 7), including those with p substituents [44]. In most cases, the addition reaction is accomplished with an amine base as catalyst (see 25). Less reactive electrophiles, such as methyl acrylate or acrylonitrile, require a Lewis acid co-catalyst (e.g., 24). An example of the versatility of this reaction is shown in Fig. 7, where the aromatic steroid p-estradiol (26) is complexed (27) and subsequently alkylated exclusively at CIO (i.e.,para) at -40 °C. Since the osmium preferentially binds the a face of the steroid in 27, conjugate addition occurs from the p face, yielding the stereochemistry found in testosterone [26]. The overall yield of this transformation after decomplexation of the dienone product 29 is 69%. 

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... 

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. 

The metal-carbon triple bond chemistry of ruthenium and osmium described in this article bears a close resemblance to the metal-carbon double bond chemistry of these elements as exemplified by the methylene complexes [26]. In both systems two structural classes are found, five coordinate (trigonal bipyramidal, formally zero oxidation state) and six coordinate (octahedral, formally +2 oxidation state). In both systems the five coordinate compounds exhibit multiple metal-carbon bonds which are rather non-polar and typically undergo addition reactions with electrophilic reagents. On the other hand the six coordinate compounds, both M=C and M=C, begin to show electrophilic character at the carbon centres especially in cationic complexes. Further development of the carbyne chemistry of ruthenium and osmium will depend upon the discovery of new synthetic methods allowing the preparation of a broader range of compounds with widely differing carbyne substituents. 

Unlike the above example, the majority of five-coordinate complexes appear to undergo oxidative-addition reactions in two separate steps. Additions to the bis phosphine complexes of ruthenium(O) (36) and osmium(O) (39) (XIV) are the most thoroughly studied examples of this generalization (see Section IV). The configurations of these complexes have been established by infrared spectroscopy and, in the case of the osmium complex, by X-ray diffraction (72), Addition of an electrophile A" " (for example H+, HgX+, or Br+ from Br2) to the five-coordinate complexes... 

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. 

Electrophilic attack at carbyne complexes may ultimately place the electrophile on either the metal or the (former) carbyne carbon, the two possibilities being related in principle by a-elimination/migratory insertion processes (Figure 5.39). The reactions of the osmium carbyne complex are suggestive of an analogy with alkynes. Each of these reactions (hydro-halogenation, chlorination, chalcogen addition, metal complexation see below) have parallels in the chemistry of alkynes. 

We now turn to the stereochemistry governed by a ring system, and we shall look at both nucleophilic and electrophilic attack, since usually they have similar stereochemical preferences rather than contrasting preferences. In addition to several reactions that are straightforwardly electrophilic attack, we shall see several which can be described as electrophilic in nature, like the reactions of alkenes with osmium tetroxide, with peracids, with some 1,3-dipoles, and with boranes, and of dienes with dienophiles in Diels-Alder reactions. Some of these reactions are pericyclic, the pericyclic nature of which we shall meet in Chapter 6. For now, it is only their diastereoselectivity that will concern us. 

CH2BU or p-MeCjH4]. The reaction is considered to be charge controlled and to proceed via electrophilic attack by Sg on the carbyne carbon (178). An additional complex (267, M = W, E = S, L = CO, R = Me) can be obtained from [W=CMe(Cp)(CO)2] and cyclohexenesulfide (179). With the osmium complex 268 (R = /7-tolyl), however, reaction with sulfur does not proceed beyond the t/ -thioacyl complex (269, E = S or Se). Seleno-and telluroacyl complexes result from analogous reactions (180). 

The reactions collected in Schemes 56 and 57 contain overwhelming evidence proving that the direction of the addition to alkynyl and alkenyl acetate complexes of osmium and ruthenium is determined by the electronic nature of the metallic center (Os or Ru), by the electronic properties of the ancillary ligands of the complexes, and also by the source of the electrophile. 

Only a few examples have been obtained through the classical methodologies followed in group 6 metal chemistry. Most rf -Cs Fischer-type ruthenium and osmium carbenes arise from the nucleophilic additions of alcohol and amino groups at the electrophilic carbenic Ca-atom of both allenylidene and vinylidene complexes. The fate of the reaction depends on the electrophilicity as well as the steric hindrance around the Ca-atom, which can control its accessibility, especially for bulky nucleophiles. These features have been thoroughly discussed in a recent review. ... 

---