Multiple Metal-Carbon Bonds for Catalytic Metathesis Reactions - PowerPoint PPT Presentation

Multiple Metal-Carbon Bonds for Catalytic Metathesis Reactions Nobel Lecture December 8, 2005 1

Metal-carbon double and triple bonds in which the transition metal is in a "low oxidation state" were discovered by E. O. Fischer. CO CO CO CO OCH 3 δ + OC Cr C Br W C δ - OC OC CO CO 1964 1973 "carbyne" "carbene" 2

Beta hydride elimination in an ethyl complex H H C H H H C H H H M M + C C L 3 L 3 L 1 L 1 H H L 2 L 2 3

Known Group 4 Peralkyl Complexes (M = Ti, Zr, Hf) in 1973. H H H Me Me β β Me β Me Si H C α Me Me α α CH 2 CH 2 CH 2 H M M C 6 H 5 CH 2 M Me 3 SiCH 2 Me 3 CCH 2 CH 2 C 6 H 5 CH 2 SiMe 3 C 6 H 5 CH 2 CH 2 CMe 3 Me 3 SiCH 2 Me 3 CCH 2 All alkyls lack one or more hydrogen atoms on the atom β with respect to the metal. 4

The first relatively stable permethyl complex Me pentane Me Me WCl 6 + 6 AlMe 3 W Me Me Me 4 5 6 7 8 Ti V Cr Mn Fe A. J. Shortland and G. Wilkinson Zr Nb Mo Tc Ru J. Chem. Soc., Dalton Trans. 1973 , 872. Hf Ta W Re Os “* Note added in proof . Hexamethylrhenium (K. Mertis and G. Wilkinson) and pentamethyl[t]antalum (R. Schrock, DuPont, Wilmington, private communication) have recently been synthesized.” Geoffrey Wilkinson, Nobel Lecture, December 11, 1973 5

Synthesis of tantalum pentaalkyls H H H C pentane 2 LiMe TaMe 3 Cl 2 TaCl 5 + 1.5 ZnMe 2 Ta CH 3 H 3 C ether - 1.5 ZnMe 2 H 3 C CH 3 (Juvinall) Decomposes above 0 °C bimolecularly SiMe 3 C CH 2 SiMe 3 Me 3 SiCH 2 + 5 Me 3 SiCH 2 MgCl Ta Ta TaCl 5 1/2 Me 3 SiCH 2 CH 2 SiMe 3 C SiMe 3 "It is assumed that a penta-alkyl complex cannot exist for steric reasons." (Mowat, W.; Wilkinson, G. J. Chem. Soc, Dalton Trans. 1973 , 10 , 1120.) 6

Neopentyls yield a stable product of α hydrogen abstraction. t-BuCH 2 2 LiCH 2 CMe 3 H Ta(CH 2 CMe 3 ) 3 Cl 2 Ta C t-BuCH 2 CMe 3 - CMe 4 t-BuCH 2 J. Am. Chem. Soc. 1974 , 96 , 6796 Distills in a good vacuum at 75°C. δ - δ + CH 2 -t-Bu CH 2 -t-Bu δ + δ - - CMe 4 H δ + δ + (t-BuCH 2 ) 3 Ta (t-BuCH 2 ) 3 Ta (t-BuCH 2 ) 3 Ta C H H α hydrogen t-Bu C H δ - C H α hydrogen abstraction activation t-Bu t-Bu (deprotonation) 7

Alkylidenes can be deprotonated to yield tantalum-carbon triple bonds. δ + Li + t-BuCH 2 t-BuCH 2 LiButyl δ - H δ + - Ta C Ta C CMe 3 t-BuCH 2 t-BuCH 2 - Butyl H CMe 3 t-BuCH 2 t-BuCH 2 Guggenberger, L. J.; Schrock, R. R. J. Am. Chem. Soc. 1975 , 97 , 2935. 8

Alkylidenes decompose bimolecularly. Me CH 2 L Me H Me Me δ + + Ta Cp 2 Ta TaCp 2 2 Ta CH 2 Ta δ - C L CH 2 CH 2 Me H 18 electrons L = CO, C 2 H 4 , PR 3 - H + base Schrock, R. R. J. Am. Chem. Soc. 1975 , 97 , 6577. [Cp 2 TaMe 3 ] + Bimolecular decomposition of alkylidenes, especially methylenes, is difficult to prevent, especially in electron deficient species. 9

Olefin metathesis and the Chauvin mechanism (1971) 2 RCH=CHR' RCH=CHR + R'CH=CHR' + RCH=CHR' - R'CH=CHR' R H - RCH=CHR + RCH=CHR' R M=CHR M M=CHR' H H R' M = Mo, W, or Re 10

Alkyne metathesis and the metalacyclobutadiene mechanism R'C ≡ CR' + RC ≡ CR 2 RC ≡ CR' R R RC CR' RC CR - M CR' M CR M R M R R' R' (suggested by T. Katz; 1975) M = Mo, W 11

Reaction of tantalum alkylidenes with olefins. 2 RCH=CH 2 Ta Ta + 4 olefins Cl Cl CH 2 CH-t-Bu Cl Cl CHR CH-t-Bu β H RCH=CHCH 2 -t-Bu + CH 2 CpCl 2 Ta RCH 2 CH=CH-t-Bu CHR CH-t-Bu β H CH 2 =CHRCH 2 -t-Bu + CHR CpCl 2 Ta CH 3 CHR=CH-t-Bu CH 2 12

Modification of Nb and Ta yields metathesis catalysts 4 products of rearrangement M(CH-t-Bu)L 2 Cl 3 + H 2 C=CHR of metallacyclobutanes M = Nb or Ta L = PMe 3 M(CH-t-Bu)(O-t-Bu) 2 Cl(PMe 3 ) + olefins also metathesis products (~35 turnovers for cis-2-pentene) Alkoxides "prevent reduction" and "promote metathesis." J. Molec. Catal. 1980 , 8, 73; J. Am. Chem. Soc. 1981 , 103 , 1440. 13

An oxo neopentylidene complex of tungsten t-Bu t-Bu H H O C C Cl L Cl Cl t-BuO W O-t-Bu + Ta t-BuO Ta O-t-Bu + Cl W O L t-BuO O-t-Bu Cl t-BuO O-t-Bu L L Cl L = a phosphine, e.g. PEt 3 R t-Bu H H C C RCH=CHR L L Cl W O Cl W O L L - t-BuCH=CHR Cl Cl (AlCl 3 cat) Even R = H 14

A sterically demanding diisopropylphenyl imido group might be a desirable "ancillary" ligand. i-Pr X X = N W RO CH-t-Bu RO i-Pr The OR group should be a sterically demanding tertiary alkoxide. 15

A sterically demanding diisopropylphenyl imido group might be a desirable "ancillary" ligand. i-Pr i-Pr N W (CF 3 ) 2 MeCO CH-t-Bu (CF 3 ) 2 MeCO Hexafluoro-t-butoxide was chosen as a highly electron withdrawing alkoxide. 16

Synthesis of a tungsten neopentylidyne complex t-Bu Cl C Cl MeO W + 6 ClMgCH 2 -t-Bu W MeO Cl CH 2 CH 2 CH 2 t-Bu OMe t-Bu t-Bu Volatile yellow crystals. Thermally stable, distilling at 75°C in a good vacuum. (1978) 17

Tungsten-carbon triple bonds and alkyne metathesis t-Bu t-Bu t-Bu C C C 3 HCl in dme 3 LiO-t-Bu Cl Cl W W W CH 2 O Me O Cl CH 2 O - 3 CMe 4 CH 2 O t-Bu t-Bu O t-Bu t-Bu Me t-Bu t-Bu purple crystals The tri-t-butoxide compound is a powerful catalyst for the alkyne metathesis reaction. R'C ≡ CR' + RC ≡ CR 2 RC ≡ CR' 18

Metal-metal bonds and "metathesis" reactions. t-BuO t-BuO O-t-Bu t-BuO t-BuO R-C C-R W C R W W + 2 O-t-Bu t-BuO t-BuO O-t-Bu (1982) R R R R-C C-R X 3 W WX 3 X 3 W WX 3 X 3 W WX 3 R 19

Synthesis of a tungsten imido alkylidene complex Ar Ar Ar H N Et 3 N catalyst N N Cl Me t-Bu Cl 2 LiOR Me t-Bu O W C-t-Bu O W C W C Cl Cl H RO H O RO O Me Me ("14 electron" species) OR = O-t-Bu, OCMe 2 (CF 3 ), OCMe(CF 3 ) 2 , and various bulky phenoxides W(NAr)(CH-t-Bu)(OR) 2 species are "well-defined" i-Pr catalysts for the metathesis of olefins and the NAr = N activity can be varied systematically by varying OR. i-Pr 20

Structure of syn -W(NAr)(CH-t-Bu)(O-t-Bu) 2 1.87Å 145° 000 00 00 1.76Å 169° 000 000 00 00 000 000 00 21

Tungstenacyclobutanes can be isolated, but can be too stable toward loss of olefin. 000 000 000 000 00 000 000 Molybdacyclobutane intermediates lose an olefin more readily. 22

Two isomers ( anti and syn ) are available in any system through rotation about the M=C bond. R' R' N N k a/s R' R' RO RO M M RO RO C H C t-Bu k s/a t-Bu H anti syn (usually favored) 23

Olefin metathesis variations RCM ROM/CM - CH 2 =CH 2 + RCH=CH 2 H H H H H H H H H R H H H H ROMP (Ring-Opening Metathesis Polymerization) Control! x 24

Polymerization of bistrifluoromethylnorbornadiene via enantiomorphic site control. R R Ph N Me Si Me CMe 3 Mo O O H Me Si Me Ph CF 3 CF 3 CF 3 CF 3 CF 3 CF 3 CF 3 CF 3 CF 3 CF 3 H H H H H H H H CF 3 H H CF 3 x all cis and isotactic through enantiomeric site control when R = CH 3 When R = CH(CH 3 ) 2 the polymer structure has a relatively random (71% cis) structure. 25

Alkynes are polymerized to yield polyenes. EtO 2 C CO 2 Et CO 2 Et EtO 2 C H or CO 2 Et CO 2 Et H H H H H head-to-tail tail-to-tail Soluble, highly conjugated (purple), and relatively air-stable; both rings observed in polymer made with Mo(NAr)(OR F6 ) 2 catalyst. E E E E E E E E * * E E E E E E >95% 5-membered rings produced with Mo(NAr)(O-t-Bu) 2 catalyst. 26

Ring-closing metathesis with Mo catalyst (4-5 mol%) (Catalyst = Mo(NAr)(CHCMe 2 Ph)[OCMe(CF 3 ) 2 ] 2 ) O O O O Ph Ph Ph - C 2 H 4 N - 2 butene Ph N 15 min, 92% 2 hr at 50°, 81% Me Me Me Me Ph O O Ph O O - propylene - C 2 H 4 Ph Me Ph Me Me 180 min, 93% 15 min, 92% Me Me Me Me Fu, G. C.; Grubbs, R. H. J. Am.Chem. Soc. 1992, 114 , 5426; 7324. 27

Synthesis of Fluvirucin-B 1 OAc Me H 2 N OH Me OAc AcO Me Me O N(H)COCF 3 N(H)COCF 3 HO Me O O OAc Mo cat O O O O O O C 6 H 6 NH Et Et Et Et Et Me Et - CH 2 =CH 2 HN HN 92% yield Fluvirucin-B 1 A. F. Houri, Z. M. Xu, D. A. Cogan, A. H. Hoveyda, J. Am. Chem. Soc. 1995 , 117 , 2943. 28

Mo or W catalyzed alkyne metathesis reactions are useful in organic chemistry. O O O Hydrog. W cat + H 2 - MeC CMe W cat = W(CCMe 3 )(OCMe 3 ) 3 H H Civetone ( Fürstner) Olefins do not appear to react with M-C triple bonds. 29

Other examples of alkyne metathesis in organic synthesis O S H 2 N N N OH O N O H O HO O O OH Motuporamine C PGE 2 -1,15-lactone Epothilone C O O O O O O MeO MeO HO W cat 1. TsOH O O O 2. 9-I-9- BBN - 2-butyne MeO HO MeO 80% S-(+)-citreofuran 30

An enantiomerically pure Mo catalyst Alexander, J. B.; La, D. S.; Cefalo, D. R.; Hoveyda, A.; Schrock, R. R. J. Am. Chem. Soc. 1998 , 120 , 4041. 31

Asymmetric catalyst design; a modular approach Imido Groups Diolates TRIP t-Bu Mes O O O Me i-Pr Me i-Pr Mo Mo Mo O O O N N Mo Mo TRIP t-Bu Mes Mes CHPh 2 t-Bu O O Cl Cl O Mo Mo Mo O O O N N Mo Mo CHPh 2 Mes t-Bu 24 catalysts! 32