Osmium with nucleophiles

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.,... 

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 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. 

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]... 

Intermediate Cases In the Os complex 11.12, Roper has a carbene with character intermediate between the Fischer and Schrock extremes because it reacts both with electrophiles [e.g., SO2 (Eq. 11.36) or H ] and with nucleophiles [e.g., CO (Eq. 11.37) or CNR]. This is reasonable based on our bonding picture. The osmium has ir-donor (Cl) as well as ir-acceptor (NO) ligands, the metal is in an intermediate oxidation state [Os(II) if we count the carbene as L, Os(IV) if X2], and the carbene carbon has non-ir-donor substituents (H). 

As with chlorine-containing oxidants, JV-bromo species have been used to oxidize sulphoxides to sulphones (with no bromine incorporation) through the initial formation of a bromosulphonium ion, by nucleophilic attack of the sulphoxide sulphur atom on the electrophilic halogen atom. Such reactions involve JV-bromosuccinimide ° bromamine-T, iV-bromoacetamide ° and iV-bromobenzenesulphonamide. All reported studies were of a kinetic nature and yields were not quoted. In acid solution all oxidations occurred at or around room temperature with the nucleophilic attack on the electrophilic bromine atom being the rate-limiting step. In alkaline solution a catalyst such as osmium tetroxide is required for the reaction to proceed . ... 

Thus the reactivity of transition metal-carbene complexes, that is, whether they behave as electrophiles or nucleophiles, is well explained on the basis of the frontier orbital theory. Studies of carbene complexes of ruthenium and osmium, by providing examples with the metal in either of two oxidation states [Ru(II), Os(II) Ru(0), Os(O)], help clarify this picture, and further illustrations of this will be found in the following sections. 

The first attempt to effect the asymmetric cw-dihydroxylation of olefins with osmium tetroxide was reported in 1980 by Hentges and Sharpless.54 Taking into consideration that the rate of osmium(VI) ester formation can be accelerated by nucleophilic ligands such as pyridine, Hentges and Sharpless used 1-2-(2-menthyl)-pyridine as a chiral ligand. However, the diols obtained in this way were of low enantiomeric excess (3-18% ee only). The low ee was attributed to the instability of the osmium tetroxide chiral pyridine complexes. As a result, the naturally occurring cinchona alkaloids quinine and quinidine were derived to dihydroquinine and dihydroquinidine acetate and were selected as chiral... 

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. 

In contrast to ruthenium and osmium, the reactivity of iron allenylidenes remains almost unexplored. Only the behavior of the cationic diphenylallenylidene-Fe(II) derivative frans-[FeBr(=C=C=CPh2)(depe)2]" has been studied in detail. Thus, it has been found that this complex reacts exclusively at Cy with both neutral (amines, phosphines) and anionic (H , MeO , CN ) nucleophiles [105-107]. This behavior contrasts with that of the neutral Fe(0) derivative [Fe =C=C=C(f-Bu)2 (CO)5] which undergoes PPhs-attack at Co- to afford the zwitterionic phosphonio-allenyl species [Fe C(PPh3)=C=C(f-Bu)2 (CO)5] [104]. 

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