Patent Application: US-1168001-A

Abstract:
the cationic iridium carbene complexes ] x have been synthesized by reaction of 2 ] pf 6 with l or nl ligands . complexes of this type are active hydrogenation catalysts capable of hydrogenating simple olefins at room temperature and atmospheric pressure of hydrogen or by transfer hydrogenation .

Description:
synthesis and characterization of [ ir ( cod )( py )( simes )] pf 6 ( 2 ). complex 2 was prepared by a simple ligand exchange reaction of [ ir ( cod )( py ) 2 ] pf 6 with simes in toluene ( eq . 1 ). due to the steric bulk of simes , this ligand could be used in slight excess to accelerate the kinetics of ligand substitution without fear of obtaining a dicarbene iridium complex . the free carbene was treated with [ ir ( cod )( py ) 2 ] pf 6 directly . after stirring at room temperature for 2 d , 2 was isolated as a yellow - orange solid in 80 % yield . a similar methodology has been used by grubbs to prepare a simes bearing ruthenium olefin metathesis catalyst . 5e the 1 h nmr spectrum of 2 showed two multiplets at δ3 . 15 and 3 . 80 for the vinyl protons of the cod ligand . it has been previously demonstrated for [ ir ( cod )( cl )( l )] pf 6 ( l = monophosphine ) that the downfield signal can be assigned to the vinyl resonance trans to l ( δ ( h a )). by analogy , the signal at δ3 . 80 can be assigned to vinyl protons trans to the simes and that at δ3 . 15 to the two vinyl protons trans to the pyridine ligand . two mulitplets at δ3 . 78 and 3 . 96 were observed for the methylene protons of the simes ligand . in order to unequivocally establish the structure of 2 , a single - crystal x - ray diffraction study was performed on crystals grown from slow diffusion of diethyl ether into a saturated dichloromethane solution of 2 . single crystal x - ray analysis of 1 and 2 . as shown in the ortep ( fig4 ), 2 adopts a square - planar coordination geometry around the iridium center . the structure of 2 is very similar to that of 1 , which had not previously been reported and is presented here for comparative purposes ( fig5 ). 10 selected bond lengths and angles for 1 and 2 are given in table 1 . consistent with the greater trans influences of simes and pcy 3 compared to pyridine , the ir — c bond distances trans to the simes in 2 and pcy 3 in 1 are significantly longer than those trans to pyridine . this is also in accord with the vinyl protons assignments in the 1 h nmr spectrum ( vide supra ). previously , we showed that imes was a stronger σ - donor than pcy 3 , 5a so it is reasonable to assume that the saturated analogue would also have a strong σ - donating capability . consistently , the ir — n bond distance in 2 ( 2 . 1073 å ) was longer than that found in 1 ( 2 . 089 å ) and the c ═ c bond distance trans to the simes in 2 was shorter than that trans to pcy 3 in 1 by 0 . 013 å . a comparison of fig4 and 5 clearly illustrates the simes ligand in 2 occupying significantly more space around the iridium center than the pcy 3 ligand in 1 . catalytic hydrogenation . the catalytic performance of 2 in the hydrogenation of simple alkenes has been investigated . the catalytic activity of 1 was also studied for comparative purposes . selected results are listed in table 3 . the catalytic reactions were performed with 1 mol % of catalyst in 5 ml of ch 2 cl 2 . at ambient temperature and pressure of hydrogen , 2 was an efficient catalyst in the hydrogenation of cyclohexene , although displaying a lower activity than 1 ( entry 1 , 2 ). it is generally significantly more difficult to hydrogenate highly substituted alkenes . as shown in entry 3 and 4 , crabtree &# 39 ; s catalyst gave a final 77 % conversion of 1 - methyl - 1 - cyclohexene while a modest yield of 44 % was obtained when using 2 . the catalytic reaction did not proceed to completion even after prolonged stirring when either 1 or 2 were employed . it had been shown that 1 undergoes an irreversible deactivation process over time involving the formation of the catalytic inactive hydrogen - bridged cluster ( vide supra ). interestingly , 1 and 2 displayed different catalytic behaviors in the hydrogenation of 1 - methyl - 1 , 4 - cyclohexene , which contains both a trisubstituted and a disubstituted non conjugated double bonds . complex 1 gave a mixture of methylcyclohexane and 1 - methyl - 1 - cyclohexene within 1 h ( entry 5 ). the ratio of the fully hydrogenated to the partially hydrogenated product increased slowly over time until a final ratio of 84 : 16 was obtained . the isomeric , partially hydrogenated product , 1 - methyl - 4 - cyclohexene was not observed . presumably , 1 catalyzes a simple two - step process in which the first hydrogenation occurs at the less hindered double bond to form 1 - methyl - 1 - cyclohexene , which is then converted to the fully hydrogenated methylcyclohexane in the subsequent step . surprisingly , 2 gave a mixture of products , methylcyclohexane , 1 - methyl - 4 - cyclohexene , and 1 - methyl - 1 - cyclohexene in a ratio of 3 : 12 : 25 after 1 h ( entry 6 ). the amount of 1 - methyl - 4 - cyclohexene increased to a maximum after 4 h and then gradually decreased with time . the yield of the fully hydrogenated product increased slowly to a final 57 % yield . these results showed that , in contrast to 1 , the relative kinetics of hydrogenation of the least substituted and more substituted olefins are similar . efforts directed at understanding the relative kinetics of these hydrogenation reactions are underway . in an effort to determine the factors leading to an increase of the yield of the fully hydrogenated product for hindered substrates , we conducted catalytic hydrogenations of 1 - methyl - 1 - cyclohexene under 60 psi of h 2 at 50 ° c . since crabtree &# 39 ; s catalyst is not very thermally stable , it may not be surprising that its activity was drastically decreased when the catalytic reaction was conducted under 60 psi at 50 ° c . catalyst degradation is presumably favored at elevated temperatures and conversion into a catalytically inactive species results in poor or no olefin hydrogenation . a poor 34 % yield of methylcyclohexane was obtained after 7 h ( entry 7 ) when using 1 . in contrast , the activity of 2 is significantly improved under these conditions . in fact , a complete conversion to the fully hydrogenated product was obtained within 7 h ( entry 8 ). in summary , a new cationic iridium carbene complex 2 has been prepared and found to be effective in the hydrogenation of simple olefins . although it is less efficient than crabtree &# 39 ; s catalyst at room temperature and atmospheric pressure of hydrogen , ( which may be related to the larger steric bulk of simes in 2 compared to that of pcy 3 in 1 ), it displayed a higher activity under a mild pressure of hydrogen at 50 ° c . this is in contrast to the catalytic property of 1 , which is significantly less active under the same conditions . the difference in activity can be attributed to the presence of the bulky n - heterocyclic carbene ligand in 2 which results in an improved thermal stability . investigations into the use of the new catalyst to a variety of substrates and synthesis of a family of nucleophilic carbene bearing iridium complexes are ongoing . general considerations . all reactions were carried out under an atmosphere of dry argon with standard schlenk tube techniques or in a mbraun glovebox containing less than 1 ppm of oxygen and water . anhydrous hexane was purchased from aldrich and used as received . toluene , thf , and ch 2 cl 2 were dried by passage through activated alumina columns . 11 the nmr solvents were dried from activated molecular sieves ( 4 å ). complex 1 was purchased from strem and used as received . [ ir ( cod )( py ) 2 ] pf 6 3a and simes . hcl 12 were prepared according to the literature procedures . nmr spectra were recorded using a varian 400 mhz spectrometer . elemental analyses were performed by desert analysis , tucson , ariz . gas chromatographic analyses were performed on a hewlett - packard hp 5890 ii equipped with an fid and a hp - 5 column . synthesis of [ ir ( cod )( py )( simes )] pf 6 ( 2 ). a mixture of 423 mg simes . hcl ( 1 . 242 mmol ) and 139 mg potassium tert - butoxide ( 1 . 242 mmol ) in 20 ml of thf was stirred at room temperature for 1 h . the solvent was then removed completely under vacuum . the residue was extracted with 20 ml of toluene . the solution was filtered and charged with 500 mg of [ ir ( cod )( py ) 2 ] pf 6 ( 0 . 828 mmol ). the suspension was then stirred at room temperature for 2 days . the orange precipitate was filtered on a collection frit , washed with hexane , and dried under vacuum . yield : 550 mg ( 80 %). anal . calcd for c 34 h 43 f 6 irn 3 p : c , 49 . 15 ; h , 5 . 22 ; n , 5 . 60 . found : c , 49 . 35 ; h , 5 . 31 ; n , 5 . 80 . 1 h nmr ( 399 . 95 mhz , cd 2 cl 2 ): δ1 . 60 ( m , 4 h , ch 2 of cod ), 1 . 90 ( m , 4 h , ch 2 of cod ), 2 . 33 ( s , 3 h , ch 3 ), 2 . 36 ( s , 6 h ch 3 ), 2 . 39 ( s , 6 h , ch 3 ), 2 . 50 ( s , 3 h , ch 3 ), 1 . 56 ( m , 4 h , ch 2 of cod ), 1 . 87 ( m , 2 h , ch 2 of cod , 1 . 99 ( m , 2 h , ch 2 of cod ), 3 . 15 ( m , 2 h , ch of cod trans to py ), 3 . 78 ( m , 2 h , nch 2 ch 2 n ), 3 . 80 ( m , 2 h , ch of cod trans to simes ), 3 . 96 ( m , nch 2 ch 2 n ), 6 . 93 - 7 . 26 ( m , 6h , aromatic h ), 7 . 70 - 7 . 77 ( m , 3 h , aromatic h ). crystals suitable for x - ray measurements were obtained by slow diffusion of ether into a dichloromethane solution of 2 . the hydrogenation experiments . a solution of 0 . 01 mmol of catalyst , 1 . 0 mmol of olefin in 5 ml of dichloromethane was loaded into a 50 ml scintillation vial equipped with a screw cap and septum inside a glove box . the vial was then purged with hydrogen from a schlenk line for 1 min . the reaction was allowed to stir under atmospheric pressure of hydrogen and the product ratio was monitored by gc . in some of the cases , the catalytic solution was loaded into a 100 ml fisher - porter pressure bottle , which was purged with hydrogen three times and then pressurized with 60 psi of hydrogen . the reaction was allowed to stir in an oil bath at 50 ° c . for 7 h . the reaction flask was cooled to room temperature . the pressure was then carefully discharged and product ratios were determined by gas chromatography . reported experimental yields are the average of two runs . acknowledgment . the national science foundation and the petroleum research fund administrated by acs are gratefully acknowledged for support of this work . johnson matthey is also gratefully acknowledged for their generous gift of [ ir ( cod ) cl ] 2 . supporting information available : tables of crystal data and structure refinement details , atomic coordinates , bond distances and angles , anisotropic thermal parameters , and hydrogen atom coordinates for 1 and 2 . this material is available free of charge via the internet at http :// pubs . acs . org . 1 . herrmann , w . a . ; cornils , b . angew . chem ., int . ed . engl . 1997 , 36 , 1049 - 1067 . 2 . 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( c ) lee , h . m . ; nolan , s . p . org lett . 2000 , 2 , 2053 - 2055 . 7 . huang , j . ; grasa , g . ; nolan , s . p . org lett . 1999 , 1 , 1307 - 1309 . 8 . lee , h . m . ; smith , d . c ., jr . ; he , z . ; stevens , e . d . ; yi , c . s . ; nolan , s . p . organometallics , manuscript accepted for publication ( copy attached to u . s . provisional patent application serial no . 60 / 251 , 075 , filed dec . 4 , 2000 ). 9 . chen , a . c . ; ren , l . ; decken , a . ; crudden , c . m . organometallics 2000 , 19 , 3459 - 3461 . 10 . orange crystals suitable for x - ray measurements were obtained by slow diffusion of ether into a dichloromethane solution of 1 . 11 . pangborn , a . b . ; giardello , m . a . ; grubbs r . h . ; rosen , r . k . ; timmers , f . j . organometallics 1996 , 15 , 1518 - 1520 . 12 . arduengo , a . j ., iii ; krafczyk , r . ; schmutzler , r . ; craig , h . a . ; goerlich , j . r . ; marshall , w . j . ; unverzagt , m . ; tetrahedron 1999 , 55 , 14523 - 14534 . all measurements disclosed herein are at standard temperature and pressure , at sea level on earth , unless indicated otherwise . all materials used or intended to be used in a human being are biocompatible , unless indicated otherwise . the foregoing embodiments are presented by way of example only ; the scope of the present invention is to be limited only by the following claims .