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Tandem ROMP-hydrogenation via a single Ru precursor permits catalytic reduction of ROMP polymers under exceptionally mild, homogeneous conditions (1 atm of H 2 , 60°C). Repeated catalyst cycling between metathesis and hydrogenation chemistry is effected in a one-pot procedure.Interest in rapid-throughput chemical synthesis has spurred development of technologies in which one catalyst supports several functions. 1 Particularly attractive in this context is the ability to simply, controllably, and reversibly "turn on" different modes of catalysis. Here we describe facile cycling of a single catalyst precursor between metathesis and hydrogenation activity (eq 1). Use of methanol in the hydrogenation step permits complete polymer hydrogenation under extraordinarily mild conditions, at 1 atm of H 2 . This approach is equally relevant to reduction of unsaturated molecular species derived from ring-closing metathesis (RCM) or cross-metathesis.Industrial demand continues to increase for lightweight, moldable optical-grade materials. Ring-opening metathesis polymerization (ROMP) of cycloolefins, followed by hydrogenation, enables synthesis of highmolecular-weight, narrow-polydispersity polyolefins with desirable optical characteristics. 2 Hydrogenation expands the range of applications open to ROMP materials by eliminating the inherent susceptibility of olefinic linkages to oxidative and thermal degradation. 3,4 Hydrogenation of all unsaturated polymers typically requires forcing conditions, as local steric effects are exacerbated by diffusion constraints associated with the random coil. 3 Steric problems are particularly acute in ROMP polymers, in which the olefinic groups are part of the polymer backbone; 5 in ROMP via 1, they are also of predominantly trans geometry. 6 Nickel and rhodium -Olefins in polybutadiene are reduced faster than 1,4-olefins and cis-1,4 units faster than trans-1,4 units. 3a (6) Schwab, P.; Grubbs, R. H.; Ziller, J. W.

RuCl 2 (dcypb)(CO)] 2 2 (dcypb ) 1,4-bis(dicyclohexylphosphino)butane) was prepared in high yield via phosphine exchange between dcypb and RuCl 2 (CO)(PPh 3 ) 2 (DMF) (1). Reaction of 2 with 8 equiv of KBH s Bu 3 affords [fac-RuH 3 (CO)(dcypb)] -(3), stabilized by interactions with a K + counterion and an intact KBH s Bu 3 molecule in the third coordination sphere. Substantial ion pairing accounts for the stability and high hydrocarbon solubility of 3. Complex 3 effects reduction of benzophenone under unprecedentedly mild conditions, at 1 atm of H 2 in refluxing 2-propanol. It is also active for ortho functionalization of benzophenone under 20 atm of ethylene. Stoichiometric experiments reveal facile formation of orthometalated RuH(CO)[OC(C 6 H 4 )(Ph)](dcypb) (5), an intermediate proposed in both types of catalysis. The catalytic activity of isolated 5 supports this hypothesis in the case of hydrogenation but not of Murai catalysis. The X-ray crystal structures of 3 and 5 are reported.

Reaction of RuHCl(PPh3)3 4 with 3‐chloro‐3‐methyl‐1‐butyne effects transformation into RuCl2(PPh3)2(CHCHCMe2) 1c. Starting 4 is available commercially, or via quantitative reaction of RuCl2(PPh3)3 with one equivalent of alkali phenoxides or isopropoxides in refluxing benzene‐2‐propanol. Phosphane exchange between 1c and PCy3 or 1,3‐(CH2PCy2)2C6H4 is rapid at RT, affording RuCl2(PCy3)2(CHCHCMe2) 1b or the novel alkylidene complex RuCl2[1,3‐(CH2PCy2)2C6H4](CHCHCMe2) 7. Much slower exchange occurred on use of RuCl2(PCy3)2(CHPh) (1a) as precursor. Complex 1c is stable indefinitely (months) in the solid state at RT under N2, but dimerizes slowly in solution to give RuCl(PPh3)2(μ‐Cl)3Ru(PPh3)2(CHCHCMe2) 6a. 2,7‐Dimethyl‐octa‐2,4,6‐triene, the formal product of carbene coupling, is observed by 1H NMR. Dimerization does not compete with phosphane exchange. A side‐product arising from use of excess 3‐chloro‐3‐methyl‐1‐butyne in the synthesis of 1c was identified as Ru(IV) carbyne complex RuCl3(PPh3)2(≡CCHCMe2) 5, the structure of which was confirmed by X‐ray crystallography.