Osmium reaction with nucleophiles

In carbyne osmium compounds such as [Os3(/i-COMe)(/z-H)(CO)io] nucleophilic attack on the carbyne carbon atom also takes place. By carrying out sequential reactions with nucleophiles and electrophiles, it is possible to break the C—O bond (Table 3.14, footnote reference A). The reaction furnishes an isolable carbene complex, [Os3(M-CHOMe)(/i-H)(CO)io]... 

The intermediacy of cationic d6 osmium methylene complex 6 is implicated in the reactions of OsI(CH2I)(CO)2(PPh3)2 with nucleophiles (23), e.g.,... 

In summary, the reaction of osmium tetroxide with alkenes is a reliable and selective transformation. Chiral diamines and cinchona alkakoid are most frequently used as chiral auxiliaries. Complexes derived from osmium tetroxide with diamines do not undergo catalytic turnover, whereas dihydroquinidine and dihydroquinine derivatives have been found to be very effective catalysts for the oxidation of a variety of alkenes. OsC>4 can be used catalytically in the presence of a secondary oxygen donor (e.g., H202, TBHP, A -methylmorpholine-/V-oxide, sodium periodate, 02, sodium hypochlorite, potassium ferricyanide). Furthermore, a remarkable rate enhancement occurs with the addition of a nucleophilic ligand such as pyridine or a tertiary amine. Table 4-11 lists the preferred chiral ligands for the dihydroxylation of a variety of olefins.61 Table 4-12 lists the recommended ligands for each class of olefins. 

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. 

For example, the thermal reactions of Os6(CO),8 with CO (56, 425) and P(OMe)3 (56, 98, 392) lead to the open raft species Osg(CO)2i and [Os6(CO)2i ) P(OMe)3 J (n = 1-6), respectively, probably by sequential bond hssion in the bicapped tetrahedral cluster. As indicated in Scheme 21, under slightly different reaction conditions, elimination of an osmium vertex occurs, resulting in the formation of clusters containing a trigonal-bipyrami-dal metal framework. These react further with nucleophiles to give open Osj clusters and finally substituted triosmium species. Cluster unfolding has also beeen observed in the reactions of Os5(CO),jH2 with a variety of nucleophiles (Scheme 22). 

Scheme 14 Generation of osmium alkoxy- and aminoboryl complexes via the reaction of dichloroboryl system 8.124 with nucleophiles...

The synthetic methodology outlined in this chapter is fundamentally different than that described elsewhere in this book. It is based on a transition metal acting as an electron-donor (i.e., a 71 base) for the aromatic ligand. Thus, reactions with electrophiles are promoted, and the resulting arenium systems are stabilized such that a nucleophile may then be added. In this regard, the reactions outlined in this chapter complement the more established q -arene methodology. Whereas the chromium q -arene complexes have been known for over 40 years, viable q -arene complexes have been known for the past 15, and only recently has an alternative to osmium emerged. Ultimately the impact that 71 bases... 

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 reluctance of the carbyne carbon to react with nucleophiles is revealed by the reaction with LiEt3BH (see Scheme 6). Here the most electrophilic site is not the carbyne carbon but the ipara position of the aryl ring in the carbyne substituent Both ruthenium and osmium five coordinate, cationic, carbyne complexes undergo this reaction. The structure of a representative example, the osmium compound derived from the p-tolyl carbyne complex, has been determined by X-ray crystallography [16]. The unusual vinylidene complex reacts with HCl to produce a substituted benzyl derivative. The reaction may proceed through the intermediate a-vinyl complex depicted in Scheme 6 although there is also the possibility that the vinylidene compound is in equilibrium with the carbene tautomer as shown below. 

The formal similarity of these ruthenium and osmium compounds to the octahedral Group VI carbyne compounds would lead to an expectation of reactivity towards nucleophiles. In fact the neutral octahedral complexes prove to be rather unreactive compounds but the cationic complexes do react with nucleophiles. Mention has already been made of the hydrolysis of [Ru(=CPh)ClI(CO)(PPh3)2]I to RuPhCl(CO)2g Ph3)2. A very clear-cut example is provided by the reaction of [0s(sCR)Cl2(CNR )(PPh3)2]C104 (R=p-dimethylaminophenyl, R =p-tolyl) with NaSH. 

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