P P N Cl Cl Ru C C Ph Ph P N Cl Ru PPh 3 C HCCC Ph OH Ph HC CC R OH Me P P N Cl Cl Ru C C H C R C H H P Cl P P N Cl Cl Ru CO H H Ph Ph H H Me R PPh 3 H 2 O PPh 3 H 2 O 1 H 2 O H 2 O 2 R = Me, 3; Ph, 4 5 THF, reflux THF, reflux THF or CH 2 Cl 2 THF or CH 2 Cl 2 Selective C a –C b bond cleavage by water in allenylidene and alkenylvinylidene ruthenium complexes Claudio Bianchini,* Maurizio Peruzzini,* Fabrizio Zanobini, Carlos Lopez, Isaac de los Rios and Antonio Romerosa Istituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione, ISSECC, CNR, Via J. Nardi, 39 - 50132 Firenze, Italy. E-mail: bianchin@fi.cnr.it; peruz@fi.cnr.it Received (in Basel, Switzerland) 4th January 1999, Accepted 27th January 1999 The Ru(ii) complex mer,trans-[(PNP)RuCl 2 (PPh 3 )] [PNP = MeCH 2 CH 2 N(CH 2 CH 2 PPh 2 ) 2 ] reacts in refluxing THF with propargyl alcohols HC·CCRRAOH (R = RA = Me, Ph; R = Me, RA = Ph) yielding either the allenylidene complex fac,cis-[(PNP)RuCl 2 {CNCNCPh 2 }] or the alkenylvinylidene derivatives fac,cis-[(PNP)RuCl 2 {CNC(H)C(R)NCH 2 }] (R = Me, Ph); treatment of all these complexes in CH 2 Cl 2 or THF with water results in the formation of the ruthenium carbonyl fac,cis-[(PNP)RuCl 2 (CO)] and free alkenes H 2 CNCRRA (R = RA = Me, Ph; R = Me, RA = Ph) via regioselective C a –C b bond cleavage. Understanding and rationalizing the reactivity of transition metal complexes containing allenylidene ligands, MNCNCNCR 2 , is a topic of much current interest in organome- tallic chemistry. 1–3 Sound motivations arise from the increasing applications of allenylidene complexes, especially of ruthenium derivatives, to various catalytic reactions involving C–C bond formation. 4 As a general trend, nucleophiles may attack either the C a or C g carbon atom in allenylidene ligands affording Fischer-type carbenes or alkynyl compounds, respectively. 5 The addition of water has been found to take place selectively at C a to give unsaturated hydroxycarbenes. 6 At least in one case, however, there was the suspicion that water might be able to cleave a C–C bond in an allenylidene ligand but no experimental evidence was reported supporting this hypothesis. 7 The question regard- ing the hydrolytic C–C bond cleavage in allenylidenes and homologous ligands is not of trivial importance given the wide use of their metal complexes as catalyst precursors in a variety of homogeneous processes, especially in reactions where new C–C bonds are formed. 4,8 For this reason, we decided to investigate the reactions of both allenylidene and alkenylvinyli- dene complexes with water. Ruthenium was the metal of choice as it is known to effectively stabilize cumulene complexes and it also constitutes the essential ingredient in many C–C bond forming reactions. 4,8 Following the well known Selegue’s synthetic protocol, 9 the neutral allenylidene complex fac,cis-[(PNP)RuCl 2 - {CNCNCPh 2 }] 2 was obtained in 78% yield by refluxing mer,trans-[(PNP)Ru(Cl) 2 (PPh 3 )] 1 10 and 1,1-diphenylprop- 2-yn-1-ol in THF (Scheme 1).† Complex 2 11 was authenticated by means of standard spectroscopic techniques as well as a single-crystal X-ray analysis (Fig. 1).‡ The crystallographic study confirmed the fac stereochemistry of the PNP ligand and the trans disposition of the diphenylallenylidene ligand and the nitrogen donor atom. 12 While p-alkyne coordination (A) and alkyne to vinylidene tautomerization (B) are common steps to any propargyl alcohol activation, the eventual dehydration process of the hydroxy- vinylidene intermediate may occur with different regioselectiv- ity (Scheme 2). Either allenylidene (C) or alkenylvinylidene derivatives may indeed form depending on the presence of hydrogens on the carbon atom proximal to the OH group. If there are no hydrogens, the allenylidene products are stable, otherwise they may tautomerize to alkenylvinylidenes (E). Alternatively, the alkenylvinylidene ligand may form via direct elimination of water from the hydroxyvinylidene intermediate (D). 13 Within this mechanistic picture, it was not surprising to find that 1 reacts with either 1,1-dimethyl- or 1-methyl,1-phenyl- propyn-1-ol yielding the alkenylvinylidenes fac,cis- [(PNP)RuCl 2 {CNC(H)C(R)NCH 2 }] (R = Me, 3; Ph, 4). 11 ‡ It was surprising instead to discover that both the allenylidene Scheme 1 Fig. 1 ORTEP drawing of complex 2. 18 For sake of clarity only the ipso carbon atoms of the phenyl substituents in the PNP ligand are reported. Selected distances (Å) and angles (°): P(1)–Ru(1) 2.314(2), P(2)–Ru(1) 2.289(2), N(1)–Ru(1) 2.335(5), Ru(1)–Cl(1) 2.449(2), Ru(1)–Cl(2) 2.461(2), Ru(1)–C(8) 1.858(7), C(8)–C(9) 1.221(9), C(9)–C(10) 1.376(10), N(1)–Ru(1)–P(1) 82.69(14), N(1)–Ru(1)–P(2) 83.11(15), N(1)–Ru(1)– Cl(1) 90.15(14), N(1)–Ru(1)–Cl(2) 87.96(15), N(1)–Ru(1)–C(8) 173.4(2), P(1)–Ru(1)–P(2) 100.17(8), P(1)–Ru(1)–Cl(1) 171.95(6), P(1)–Ru(1)– Cl(2) 88.49(8), P(1)–Ru(1)–C(8) 90.66(19), P(2)–Ru(1)–Cl(1) 82.52(7), P(2)–Ru(1)–Cl(2) 166.62(6), P(2)–Ru(1)–C(8) 98.35(19), Cl(1)–Ru(1)– Cl(2) 87.59(8), Cl(1)–Ru(1)–C(8) 96.5(2), Cl(2)–Ru(1)–C(8) 91.67(19), Ru(1)–C(8)–C(9) 175.8(6), C(8)–C(9)–C(10) 171.1(7). Chem. Commun., 1999, 443–444 443