Osmylation

The soccer ball shaped framework of Cgg was confirmed for the first time by the X-ray crystal structure of 13 [52]. The bridging of the OSO4 unit occurs in the characteristic 1,2-mode to a [6,6] double bond of the fullerene core. The 17 sets of carbons in the C2v-symmetric 13 were assigned using the 2D NMR INADEQUATE technique on the basis of their connectivities [53]. In these experiments C-enriched material was used. The coupling constants fall into three categories, 48, 54—57 and 65-71 Hz. These values can be attributed to the three types of C-C bonds present in 13, namely C(sp )-C(sp ), the longer [5,6], and the shorter [6,6] bonds, respectively [53]. 

Reactions with Strong Oxidizing Reagents and Acids 

A problem in all these oxidations is the extreme instability of C50 cations under the applied conditions. Cgo and the higher cations are susceptible to oxidative degra- 

Photoinduced oxidation of Cjq has been achieved by electron transfer from excited to a strong electron acceptor such as p-chloranil [72, 73], p-benzoquinone [73], tetracyano-p-quinodimethane (TCNQ) or tetracyanoethylene (TONE). This electron transfer proceeds efficiently only by addition of promoters such as Sc(OTf)3 or triflic acid, both of which strongly enhance the electron-transfer process [72, 73]. Another possibility to produce the cation is the electron transfer from to the singlet excited state of a strong electron acceptor such as N-methylacridinium hexafluorophosphate (NMA ) [74, 75], triphenylpyriliumtetrafluoroborate (TPP ) 

The EPR spectrum shows a signal at g= 2.002 with a line width AHpp of 3-8 G. In the N1R spectrum the typical absorption at 980 nm is accompanied by a weaker 

---

The most important members of this class are the osmium nitrido, and the osmyl complexes. The reddish-purple K2[OsNCl5] mentioned above is the result of reducing the osmiamate. The anion has a distorted octahedral structure with a formal triple bond Os=N (161pm) and a pronounced /ram-influence (pp. 1163-4), i.e. the Os-Cl distance trans to Os-N is much longer than the Os-Cl distances cis to Os-N (261 and 236 pm respectively). The anion [OsNCls] also shows a rram-effect in that the Cl opposite the N is more labile than the others, leading, for instance, to the formation of [Os NCl4] , which has a square-pyramidal structure with the N occupying the apical position. 

I osmyl complexes (a) 3-cenire rr bond formed hy overlap of ligand p, and nilar tv>nd is produced hy p, and d,. ovcrlapi, and Ihi MO diagram Isee lexij,... 

The axial O-An bonds are clearly very strong. They cannot be protonated and are nearly always shorter than the equatorial bonds. In the case of U02 ", for instance, it is likely that the U 0 bond order is even greater than 2, since the U-O distance is only about 180 pm in spite of the difference in the ionic radii of the metal ions (U = 73 pm. Os " = 54.5 pm), this is close to that of the 0s=0 double bond found in the isostructural, osmyl group (175 pm, see p. 1085). It is usually assumed that combinations... 

The interest in asymmetric synthesis that began at the end of the 1970s did not ignore the dihydroxylation reaction. The stoichiometric osmylation had always been more reliable than the catalytic version, and it was clear that this should be the appropriate starting point. Criegee had shown that amines, pyridine in particular, accelerated the rate of the stoichiometric dihydroxylation, so it was understandable that the first attempt at nonenzymatic asymmetric dihydroxylation was to utilize a chiral, enantiomerically pure pyridine and determine if this induced asymmetry in the diol. This principle was verified by Sharpless (Scheme 7).20 The pyridine 25, derived from menthol, induced ee s of 3-18% in the dihydroxylation of /rcms-stilbene (23). Nonetheless, the ee s were too low and clearly had to be improved. 

These compounds give characteristic osmyl IR bands (840 cm 1 in 0s02Cl2(PPh3)2) [151]. 

Racemic pinitol from benzene di-dihydrodiol benzoate by snccessive epoxidation and osmylation (Fignre 8.7a) (Ley et al. 1987). 

Enantiomencally pure (+)- and (-)-diphenylethylenediamines have recently been used for highly stereoselective Dlels-Alder, aldol,8 allylation,9 osmylation,10 and epoxidafion11 reactions. Other synthetic applications involve enantioselective Michael addition12 and asymmetric hydrogenation.13... 

Annual Volume 71 contains 30 checked and edited experimental procedures that illustrate important new synthetic methods or describe the preparation of particularly useful chemicals. This compilation begins with procedures exemplifying three important methods for preparing enantiomerically pure substances by asymmetric catalysis. The preparation of (R)-(-)-METHYL 3-HYDROXYBUTANOATE details the convenient preparation of a BINAP-ruthenium catalyst that is broadly useful for the asymmetric reduction of p-ketoesters. Catalysis of the carbonyl ene reaction by a chiral Lewis acid, in this case a binapthol-derived titanium catalyst, is illustrated in the preparation of METHYL (2R)-2-HYDROXY-4-PHENYL-4-PENTENOATE. The enantiomerically pure diamines, (1 R,2R)-(+)- AND (1S,2S)-(-)-1,2-DIPHENYL-1,2-ETHYLENEDIAMINE, are useful for a variety of asymmetric transformations hydrogenations, Michael additions, osmylations, epoxidations, allylations, aldol condensations and Diels-Alder reactions. Promotion of the Diels-Alder reaction with a diaminoalane derived from the (S,S)-diamine is demonstrated in the synthesis of (1S,endo)-3-(BICYCLO[2.2.1]HEPT-5-EN-2-YLCARBONYL)-2-OXAZOLIDINONE. 

Organometals and metal hydrides as electron donors in addition reactions 245 Oxidative cleavage of carbon-carbon and carbon-hydrogen bonds 253 Electron-transfer activation in cycloaddition reactions 264 Osmylation of arene donors 270... 

Despite the extensive utility of osmylation reactions, the mechanism remains controversial - generally focusing on either a concerted [3 + 2] cycloaddition to directly yield the cyclic osmate ester or a two-step mechanism where [2 + 2] cycloaddition forms a 4-membered osmaoxetane which then undergoes a ligand-assisted ring expansion to yield the cyclic osmate ester216 (Scheme 21). 

Unfortunately, the fast rates of 0s04 addition to most alkenes preclude the observation of D/A complexes, and they are not readily characterized. However, a variety of aromatic electron donors form similar (colored) D/A complexes with Os04 that are more persistent and the observation of ArH/ 0s04 complexes forms the basis for examining the electron-transfer paradigm in osmylation reactions. 

Thermal osmylation. Upon standing in the dark, the purple CT color of anthracene complex [ANT, 0s04] slowly diminishes to afford the 1 2 (insoluble) osmium adduct as the sole product, i.e.,... 

Photochemical osmylation. The irradiation of the charge-transfer bands (Fig. 13) of the EDA complex of 0s04 with various benzenes, naphthalenes, anthracenes, and phenanthrene yields the same osmylated adducts as obtained in the thermal reactions. For example, irradiation of the purple solution of anthracene and 0s04 in dichloromethane at k > 480 nm yields the same 2 1 adduct (B) together with its syn isomer as the sole products, i.e.,... 

Note that the charge-transfer irradiation of the same anthracene/Os04 complex in hexane solution yields only a small amount of the adduct (B) together with considerable amounts of anthraquinone. The latter probably arises from osmylation at the (9,10) positions followed by decomposition of the unstable osmium adduct.218... 

Furthermore, kinetic analysis of the decay rate of anthracene cation radical, together with quantum yield measurements, establishes that the ion-radical pair in equation (76) is the critical reactive intermediate in osmylation reaction. Subsequent rapid ion-pair collapse then leads to the osmium adduct with a rate constant k 109 s 1 in competition with back electron-transfer, i.e.,... 

Such a charge-transfer osmylation of aromatic donors constitutes the critical... 

The identical stoichiometries and the color changes that are observed in thermal and photochemical aromatic osmylations point to the ion-radical pair Ar+, OsO T as the seminal intermediate in both activation processes. It is similarly possible that the osmylation of olefinic donors may proceed via the same types of reactive intermediates as delineated for the aromatic osmylation. 

The olive-green osmium(VI) octaethylporphyrin complex 0s02(0EP) (IR i/(Os—O) 825 cm-1) is representative of a number of osmyl porphyrins [185] they can readily be transformed into a number of osmium porphyrins in lower oxidation states (Figure 1.73). 

---