We demonstrate here that even the most electron-poor olefins undergo Os0,-catalyzed oxidations. The reduced reactivity of chloroolefins relative to their fluorine counterparts is explained by a size effect; in all other respects the chloroolefins behave analog~usly.~~] The procedure is also applicable to strained fluoroolefins, for example norbornene derivatives.Our results cannot be rationalized with the frequently held conception of the mechanism of the osmylation as the electrophilic attack of osmium tetroxide on the olefin. The pyridine base effect discussed by Corey et al.
1 19) Exper.imenfa1 procedure: A 90: 10 mixture of 3a and 4a (10 mmolj was added to a solution of methanol (5 mL) in methanesulfonic acid (20 nlL). The reaction mixture was stirred at 20°C for 18 h and then poured onto crushed ice and extracted with dichloromethane. The organic phase was washed with water and a 2% NaHCO, solution and dried over sodium sulfate. Evaporation of the solvent in vacuo gave 2.8 g (75%) of 2-(S)- We have now found that the photolytic dehydrogenation of the cis-dihydrides (pp3)FeH2 1[6b1 and (pp3)RuH2 2@'l (both of which are stabilized by our "pp3" ligand P(CH2CH2CH2PMe2)316a1) in C6H6 leads, depending on the central metal atom, to either intra-or intermolecular C-H activation. In the iron system, internal addition to H-CH2P results in formation of the three-membered-ring derivative 5 . In the ruthenium system, on the other hand, H-C6HS cleavage results in formation of the phenyl hydride 6.['.]The structures of 5 and 6 were established by elemental analysis and s p e c t r o s~o p y . '~~-~ The dt splitting of the 'Hcoupled FeC resonance of 5 (J(PC) = 13, J(CH2)= 148 Hz) rules out a CH2 cleavage product formed by metalation of the trimethylene chains, as has been discussed, for example, for ( d m p p ) ( w e H (dmpp= Me2P(CH2),PMe2, For both reactions, we postulate intermediate 16e dS fragments (pp3)M, 3 and 4, respectively, because these incorporate particularly advantageous structural features for the oxidative C-H addition owing to their bent, cis-unsaturated configuration."' The rneta/-dependent transition be-C7H,8P2).14C1
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