Speeches

37 views

A Suzuki Coupling Based Route to 2,2'Bis(2-indenyl)biphenyl Derivatives

A Suzuki Coupling Based Route to 2,2'Bis(2-indenyl)biphenyl Derivatives
of 8

Please download to get full document.

View again

All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Share
Tags
Transcript
  A Suzuki CouplingBased Routeto2,2 ′ -Bis(2-indenyl)biphenylDerivatives Edwin G. IJ peij,* ,† Felix H. Beijer, † Henricus J . Arts, † Claire Newton, ‡ J ohannes G. de Vries, § and Gert-J an M. Gruter †, | DSM Research, Department of Polyolefins, Chemistry and Catalysis, P.O. Box 18,6160 MD Geleen, The Netherlands, Millennium Pharmaceuticals (formerly Cambridge Discovery Chemistry), Granta Park, Great Abington, Cambridge, CB1 6ET, United Kingdom, and DSM Research,DSM Fine Chemicals, Advanced Synthesis and Catalysis, P.O. Box 18, 6160 MD Geleen, The Netherlands Edwin.ijpeij@dsm.com  Received August 17, 2001  Because of the promising performance in olefin polymerization of 2,2 ′ -bis(2-indenyldiyl)biphenylzirconium dichloride, we developed a new and broadly applicable route to 2,2 ′ -bis(2-indenyl)biphenylderivatives. Reaction of the known 2,2 ′ -diiodobiphenyl ( 26 ) with the new 2-indenyl boronic acid( 23 ) did not result in the desired 2,2 ′ -bis(2-indenyl)biphenyl ( 10 ); instead an isomer thereof, (spiro-1,1-(2,2 ′ -biphenyl)-2-(2-indenyl)indane) ( 27 ), was obtained. It was found that compound  10 couldbe made via a palladium-catalyzed reaction of 2,2-biphenyldiboronic acid ( 31 ) with 2-bromoindene( 21 ) under standard Suzuki reaction conditions. However, the yield of this reaction was low at low palladium catalyst loadings, due to a competitive hydrolysis reaction of 2,2-biphenyldiboronic acid( 31 ). HTE techniques were used to find an economically viable protocol. Thus, use of thecommercially available 1.0 molar solution of ( n  -Bu) 4 NOH in methanol with cosolvent toluene ledto precipitation of the pure product in a fast and clean reaction, using only 0.7 mol %(0.35 mol %per C - C) of the expensive palladium catalyst. Introduction For the isotactic polymerization of propylene, an iso-specific catalyst is required. Many examples have beenreported in the literature, e.g., the  ansa   bis(1-indenyl)-dimethylsilane ( 1 ) derivatives of Hoechst and BASF  1 andthe  ansa   bis-cyclopentadienyl compounds of Chisso. 2 Themain disadvantage of most of these systems is that twoisomers ( rac   ( 2 ) and  meso   ( 3 )) are formed in the synthesisof these organometallic complexes, often in a ratio of near1:1 depending on the solvent used in the synthesis (eq 1).Only the  rac  -isomer ( 2 ) is isospecific and catalyzesformation of isotactic polypropylene. The  meso  -isomer ( 3 )is nonspecific and produces atactic polypropylene (a-PP).To avoid the presence of undesired, sticky a-PP, aseparation of the  rac  / meso   mixture is often necessary.These separations are usually not straightforward andoften rely on difficult crystallizations to obtain pure  rac  -isomer. The unwanted  meso  -isomer is often discarded atthe end of a long route.The synthesis of an isospecific metallocene is moreefficient if the  rac  / meso   separation can be avoided bysmart design of the ligand-system. An example is thefamily of  ansa  -fluorenyl-cyclopentadienyl complexes de-scribed by Ewen et al. 3 Another example is the use ofbiaryl bridged ligands and/or complexes (Figure 1). Thespatial arrangement of the biaryl-bridge prohibits forma-tion of the  meso  -form.To avoid the introduction of additional isomers, the Cpor indenyl moiety needs to have  C  2 v   symmetry. This  C  2 v  * To whom correspondence should be addressed. † DSM Research. ‡ Millennium Pharmaceuticals. § DSM Fine Chemicals. | Current address: Avantium Technologies, P.O. Box 2915, 1000 CXAmsterdam.(1) Spaleck, W.; Kueber, F.; Winter, A.; Rohrmann, J .; Bachmann,B.; Antberg, M.; Dolle, V.; Paulus, E. F.  Organometallics  1994 ,  13  , 954.(2) (a) Yoshimura, T.; Mise, T.; Miya, S.; Yamazaki, H.; EP 0316155A2, Chisso Corp. (b) Mise, T.; Miya, S.; Yamazaki, H.  Chem. Lett  .  1989 ,1853.(3) Ewen J . A.  J. Mol. Catal. A: Chem  .  1998 ,  128  , 103 and referencestherein. Figure 1.  Published examples of biaryl-bridged metal-locenes:  4 , 4 5 , 5 6 , 6 7 , 7 8, 7 9( R  )  H,  tert  -Bu). 8 169 J. Org. Chem.  2002, 67,  169 - 176 10.1021/jo016040i CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 12/13/2001  symmetry is present in the 2-indenyl moieties that weredeveloped by us, 9 and later by Waymouth, 10 Halterman, 11 and Montell, 12 as ligands for homogeneous Ziegler - Natta(Z - N) polymerization. To the best of our knowledge,biaryl-bridged 2-indenyl-based compounds were reportedin only one publication by Bosnich et al. ( 7  and  8 ). 7 However, the application of these biaryl-bridged 2-inde-nyl catalysts in olefin polymerization has not beendescribed. Bosnich et al. studied these complexes ascatalysts for enantioselective Diels - Alder reactions. Resultsand Discussion The synthesis of 2,2 ′ -bis(2-indenyl)biphenyl ( 10 ) ac-cording to Bosnich et al. was repeated by us (Scheme 1).Conversion of  10 into the zirconium dichloride complex 7 proceeded in 95%yield.Initial results in olefin polymerization using zirconiumcomplex  7 were very promising. 9 Therefore, we decidedto further develop this type of catalyst system. Unfortu-nately, the route by Bosnich et al. was not amenable toscale-up, as the yield of bis-Grignard  11 is only high atlow concentration (max 0.075 M). 13 We also found thatincrease of the scale to 0.60 mol (8 L) gave a significantlower yield (10%) at this concentration of bis-Grignard 11 , although we could reach similar yields of diol  12 atsmall scale (37 mmol). It thus became necessary todevelop a new route to  10 . RouteExploration. Logical disconnections (Figure 2)can be made in the biphenyl - indene bond (disconnectionA)or between the two phenyl rings of the biphenyl moiety(disconnection B). Since biphenyl is an attractively cheapstarting material and 2-indene derivatives are easilyaccessible, we chose disconnection A. Synthesis of 10 Using Noncatalyzed Reactions. Reaction of 2,2 ′ -dilithiobiphenyl bis TMEDA adduct 14 ( 16 )or its corresponding bis-Grignard  17 , obtained via trans-metalation of  16  with magnesium bromide, with 2-in-danone ( 18 ) in diethyl ether did not result in formationof diol  12 . Instead, the condensation product of 2-in-danone, 1-(2 ′ -indanylidene)-2-indanone ( 19)  (eq 2) wasisolated after workup in 46% yield (see SupportingInformation). This compound has also been reported byTreibs et al. 15 Synthesis of 10 Using Metal-Catalyzed Cross-CouplingReactions. Cross-coupling reactions are ef-fective methods to prepare biaryls. 16 However, the doubleKumada coupling of 2-(bromomagnesio)indene ( 20 ) (pre-pared by reaction of 2-bromoindene ( 21 ) 17 with magne-sium in THF) with 2,2 ′ -bis(trifluoromethylsulfonyl)-biphenyl ( 22 ) 18 was unsuccessful with various catalysts 19 (4) (a) Hu¨ttenloch, M. E.; Diebold, U.; Rief, U.; Brintzinger, H. H.;Gilbert, A. M.; Katz, T. J .  Organometallics   1992 ,  11  , 3600. (b)Brintzinger, H. H.; Schmidt, K.; Prosenc, M.-H.; Rief, U.; Dorer, B.;Hu¨ttenloch, M. E . J. Organomet. Chem  .  1997 ,  541,  219. (c) Brintzinger,H. H.; Stu¨rmer, R.; Ringwald, M.  J. Am. Chem. Soc  .  1999 ,  121  , 1524.(5) Halterman, R. L.; Ramsey, T. M.  Organometallics  1993 ,  12  , 2879.(6) Burk, M. J .; Colletti, S. L.; Halterman, R. L.  Organometallics  1991 ,  10  , 2998.(7) Bosnich, B.; Ellis, W. W.; Hollis, T. K.; Odenkirk, W.; Whelan,J .; Ostrander, R.; Rheingold A. L.  Organometallics   1993 ,  12  , 4391.(8) Alt, H. G.; Zenk, R.,  J. Organomet. Chem  .  1996 ,  512  , 51.(9) (a) Beek van, J . A. M.; Vries de, J . G.; Arts, H. J .; Persad, R.;Doremaele van, G. H. J . NL9201970, DSM N.V. (b) Beek van, J . A.M.; Vries de, J . G.; Arts, H. J .; Persad, R.; Doremaele van, G. H. J .WO9411406., DSM N.V. (c) Beijer, F. H.; IJ peij, E. G.; Arts, H. J .;Gruter, G. J . M.; Kranenburg, M.; Meijers, R. H. A. M. EP1059300,DSM N.V.(10) (a) Coates, G. W.; Waymouth, R. M.  Science  1995 ,  267  , 217. (b)Hauptman, E.; Waymouth, R. M . J. Am. Chem. Soc.  1995 ,  117  , 11586.(c) Coates, G. W.; Mogstad, A.-L.; Hauptman, E.; Bruce, M. D.;Waymouth, R. M.  Polym. Prepr  .  36,  545.(11) (a) Schumann, H.; Karasiak, D. F.; Mu¨hle, S. H.; Halterman,R. L.; Kaminsky, K.; Weingarten, U . J. Organomet. Chem  .  1999 ,  579  ,356. (b) Halterman, R. L.; Zhu, C.  Tetrahedron. Lett.  1999 ,  40,  7445.(12) Resconi, L., WO 0029415, Montell Techonolgy Company B.V.(13) Lappert, M. F.; Martin, T. R.; Raston, C. L.; Skelton, B. W.;White A. H.  J. Chem. Soc., Dalton Trans  .  1982 , 1959.(14) Neugebauer, W.; Kos, A. J .; Schleyer, R.  J. Organomet. Chem  . 1982 ,  228  , 107.(15) Treibs, W.; Schroth, W.  Justus Liebigs Ann. Chem., 639   1961 ,204.(16) (a) Metal-catalyzed Cross-coupling Reactions, Diederich, F.,Stang, P. J ., Eds.; Wiley-VCH Verlag GmbH., 1998. (b)  Palladium Reagents and Catalysts  - Innovations in Organic Synthesis  ; Tsuji, J .,Ed.; J ohn Wiley and Sons: New York, 1995.(17) MacDowell, D. W. H.; Lindley, W. A.  J. Org. Chem  .  1982 ,  47  ,705.(18) (a) Ogasawara, M.; Yoshida, K.; Hayashi, T.  Organometallics  2000 ,  19  , 1567. (b) Higashizima, T.; Sakai, N.; Nozaki, K.; Takaya, H. Tetrahedron Lett.  1994 ,  35  , 2023. (c) Percec, V.; Okita, S.  J. Polym.Sci., Part A: Polym. Chem.  1993 ,  31  , 877. (d)  Ibid.  1992 ,  30  , 1037.(19) We tested Ni(acac) 2  with 3 equiv of LiBr (to prevent decomposi-tion of the catalyst), NiCl 2 (dppp), and [Pd(PPh 3 ) 4 ] with and without 3equiv of LiBr in THF. Scheme1. Synthesisof 2,2 ′ -Bis(2-indenyl)-biphenyl AccordingtoBosnich et al. Figure2.  Two of the possible disconnections. 170  J. Org. Chem., Vol. 67, No. 1, 2002   IJ peij et al.  (eq 3). All starting materials were still present in thereaction mixture, according to the GC-MS spectra.To explore the use of a Suzuki approach, 20 2-indenylboronic acid ( 23 ) was prepared. This was done by thereaction of  20 with an excess (5 equiv) of triisopropoxybo-rane (at  - 30 °C) or trimethoxyborane (at  - 100 °C) andsubsequent warming to room temperature, followed byacidic hydrolysis (eq 4).The yields of the 2-indenyl boronic acid  23  obtainedvia this reaction varied between 50 and 60%. Higherreaction temperatures ( - 30 °C) can be applied whentriisopropoxyborane is used instead of the more commontrimethoxyborane ( - 100 °C)to reach comparable yields. 21 Attempted Suzuki couplings of boronic acid  23  withbistriflate  22 or the bismesylate of 2,2 ′ -biphenol ( 24 ) inrefluxing toluene, however, did not give the desiredproduct under catalysis of [Pd(PPh 3 ) 4 ] with K 2 CO 3  andLiBr. As indicated by GC-MS, the monosubstitutedproduct ( 25 )was formed when  22 was used in 1,4-dioxanewith K 3 PO 4  (eq 5).Reaction of 2,2 ′ -diiodobiphenyl 14 ( 26 ) with 2 equiv ofboronic acid  23 and K 2 CO 3  in DME/water gave a mixtureof 4 products (eq 6) as indicated by GC-MS analysis. Scheme2. Proposed Mechanism(strongly simplified)for theFormation of 27 2,2 ′ -Bis(2-indenyl)biphenyl Derivatives  J. Org. Chem., Vol. 67, No. 1, 2002   171  The major product was isolated and characterized asspiro-1,1-(2,2 ′ -biphenyl)-2-(2-indenyl)indane ( 27 ) by  1 H, 13 C,  1 H - 1 H-COSY, and  13 C - 1 H correlation NMR spec-troscopy (see Supporting Information).For the mechanism for the formation of  27 , we proposea cascade as depicted in Scheme 2. IdenticalDisconnection,ReversedApproach. Sincethe approach above suffered from the occurrence of anintramolecular Heck-type insertion ( 26b  to  26c ), theopposite approach was performed. This translates to aSuzuki-coupling of 2 equiv of  21  with 2,2 ′ -biphenyldiboronic acid ( 31 ). Diboronic acid  31 was prepared bythe reaction of  16 with trimethoxyborane in diethyl etherat  - 30 °C (eq 7  )  .Yields dropped significantly if the temperature ex-ceeded  - 20 °C. The use of triisopropoxyborane insteadof trimethoxyborane in diethyl ether gave only low yields( ∼ 10%). These lower yields may be explained by theformation of the monoboronic acid ( 32 ) and side products,similar to those in the formation of the described binaph-thyldiboronic acid. 22 Reaction of  31 with 2 equiv of  21 in DME/water withK 2 CO 3  as base under catalysis of 7 mol % [Pd(PPh 3 ) 4 ]indeed resulted in the formation of  10 (equation 7), whichwas isolated after crystallization from ethanol - acetonein 79%yield. We wanted to reduce the amount of [Pd-(PPh 3 ) 4 ] significantly, since this catalyst is expensive.However, reduction of the Pd-catalyst concentration to3 mol %resulted in significant lower yields (50%of thedesired product). Mono-substituted product  28 was formedas the single side product. HTE ApproachintheOptimizationoftheSuzukiCoupling.  Many aspects, such as base, solvent, cosol-vent, and catalyst, may have an influence on thisstandard Suzuki reaction. For this reason, a HTE ap-proach was used for the optimization of the amount ofthe catalyst. In the first experiment an array of sixsolvents and eight bases was explored in a custom build96-well heating block located on the bed of a TECANGenesis 150 automated pipetting robot. The results aredepicted in Figure 3.It is clear that for most bases, toluene is the bestsolvent. Additionally, use of K 2 CO 3  as base has hot spotsin DME and dioxane. Remarkably, common Lewis basesfor Suzuki reactions, such as CsF and Bu 4 NF, 16a gave low yields, even in toluene.Base, solvent, and catalyst were varied in the secondgeneration 4 by 3 by 4 array: four bases (K 2 CO 3 , LiOH ‚ H 2 O, KOAc, and BaOH ‚ H 2 O all as aqueous solutions),three solvents (DMF, DME, and toluene), four catalysts([Pd(PPh 3 ) 4 ], Pd(OAc) 2 , [PdCl 2 (dppf)], and 5%Pd/C. [Pd-(PPh 3 ) 4 ] appeared to be the best catalyst, toluene the bestsolvent, and LiOH ‚ H 2 O and Ba(OH) 2 ‚ H 2 O the best bases.Furthermore, an inert atmosphere (nitrogen) appearedto be very important to avoid degradation.An important observation made during a controlexperiment (no catalyst present) was that  31 decomposedto the mono-boronic acid ( 32 ) under the reaction condi-tions, presumably due to hydrolysis (proto-deboronation).Usually, boronic acids are stable toward hydrolysis, (20) (a) Suzuki, A.  J. Organomet. Chem  .  1999 ,  576  , 147. (b) SuzukiA. In  Metal-catalyzed Cross-coupling Reactions  ; Diederich, F., Stang,P. J ., Ed.; Wiley-VCH Verlag GmbH.: New York, 1998; Chapter 2. (c)Gro¨ger, H. J  . Prakt  .  Chem  .  2000 ,  342  , 334.(21) To avoid multiple alkylation, see: (a) Brown, H. C.; Cole, T. E. Organometallics  1983 ,  2  , 1316. (b) Brown, H. C.; Bhat, N. G.; Srebnik,M.  Tetrahedron Lett  .  1988 ,  29  , 2631. (c) Brown, H. C.; Rangaishenvi,M. V.  Tetrahedron Lett.  1990 ,  49  , 7113 and 7115.(22) Schilling, B.; Kaiser, V.; Kaufmann, D. E.  Chem. Ber  .  1997 , 130,  923. Figure3.  First generation (6 by 8) array for the optimization of the synthesis of  10 . The red balls indicate the hot spots (where 10 was formed). Reactions performed on 50 mg (0.256 mmol) of 2-bromoindene and 31 mg (0.128 mmol) of 2,2 ′ -biphenyl diboronicacid in 1 mL of solvent. All bases except Bu 4 NF (1.0 M) added as 2.0 M aqueous stock solutions in 1.5 equiv per coupling undercatalysis of [Pd(PPh 3 ) 4 ] (1 mol %per coupling). The reaction vessels were heated at 70 °C during 6 h and analyzed by LC-MS. 172  J. Org. Chem., Vol. 67, No. 1, 2002   IJ peij et al.  although some boronic acids are reported to hydrolyzeunder Suzuki reaction conditions. 23 - 27 We believe thatthe decomposition of  31 (and reported compounds in refs20 - 24) is related to the high acidity of the boronic acidgroups: (i) the protons of the OH groups of diboronic acid 31 have a very low-field chemical shift at 9.15 ppm inDMSO- d  6 , indicating exceptionally high acidity; (ii) thep K  a  was determined by base titration and appeared tobe 5.4 (!) (in comparison: the p K  a  of phenylboronic acidis 8.8 16 ). Diboronic acid  31 and the reported boronic acidsin refs 20 - 24 have in common that they have a neigh-boring acceptor atom, which can induce hydrogen bondformation that causes increased acidity. Modeling studiesof diboronic acid  31 (and the boronic acids from refs 20 - 24) with Spartan Pro both semiempirical (PM3) and DFT(pBP/DN**) showed qualitatively significant intramo-lecular hydrogen bond formation with the neighboringacceptor atoms.Apparently, reduction of the catalyst amount leads toa slow Suzuki reaction. Consequently, the decompositionof  31 becomes more important, resulting in decreasingyields of the desired product. Obviously, to obtain highyields of  10  at low catalyst loadings, the side reaction(presumably proto-deboronation) that leads to the de-composition of  31 has to be prevented by using anhydrousconditions. Remarkably, anhydrous conditions, like DMF/Et 3 N, also led to decomposition of  31 to  32 . As anotheroption to prevent proto-deboronation, diboronic acid  31 was esterified with 2 equiv of pinacol to give  33,  inanalogy to the method of Gronowitz et al. (eq 8). 23 Remarkably, we found that pinacol ester  33  washydrolyzed very rapidly to 31 in DME/water with K 2 CO 3 ,in contrast to what was observed by Gronowitz.A much better solution found in the screening was insitu esterification of  31 to  34 in methanol (Scheme 3). 27 This is in contrast to the method described by Suzuki etal., 25 where the boronic esters are prepared in an extrareaction step.Use of the commercially available 1.0 molar solutionof (n-Bu) 4 NOH in methanol with toluene as cosolventappeared to be a good choice. Accordingly, catalystconcentrations could be decreased to 0.7 mol %Pd (0.35mol %per C - C), and yields up to 82%were obtained (alsoon 0.35 molar scale). A second advantage is that theproduct precipitates out of the reaction mixture. Thus,filtration of the reaction mixture at room temperatureyielded  10 in high purity after washing the precipitatewith toluene and methanol. Also, the reaction appearedto be very fast, being finished after approximately 10 - 15 min. On the basis of these observations, even lowerPd-concentrations might be possible. ScopeoftheMethod. To vary the indenyl moiety ofthe ligand, the 2-bromoindene derivatives  35 ,  36 , and  37 were prepared via the route as depicted in Scheme 4.Bromoindenes and  35 ,  36 , and  37 28 were successfullyconverted to the corresponding biphenyl-bridged bisin-denes  46 ,  47 , and  48 , using the new Suzuki conditions(Figure 4). Conclusions A new route to 10 has been developed, which is broadlyapplicable and economically feasible. This route involvesa Suzuki coupling of 2 equiv of 2-bromoindene ( 21 ) with2,2 ′ -biphenyl diboronic acid ( 31 ) with [Pd(PPh 3 ) 4 ]. Stan-dard Suzuki reaction conditions (in DME/water withK 2 CO 3  as base) proved unsatisfactory for our needs dueto a competitive proto-deboronation of the 2,2 ′ -biphenyldiboronic acid ( 31 ), necessitating the use of high loadingsof the expensive Pd-catalyst.HTE techniques proved a very useful tool to rapidlyoptimize the scale-up of this Suzuki reaction. By use ofthe commercially available 1.0 molar solution of ( n  -Bu) 4 -NOH in methanol optionally with cosolvent toluene, pureproduct could be easily obtained in a fast and cleanreaction, using only 0.7 mol %(0.35 mol %per C - C) ofthe expensive palladium catalyst. Experimental Section Experiments were performed under a dry and oxygen-freenitrogen atmosphere using Schlenk-line techniques.  1 H NMR(200, 300 or 400 MHz) and  13 C NMR spectra (50, 75 or 100MHz) were measured on a Bruker AC200, Bruker Avance 300,Varian Unity 300 or Bruker DPX 400. GC-MS spectra weremeasured on a Fisons MD-800 GC-MS equipped with a CPSil8low bleed column (dimensions: 30 m  ×  0.25 mm, film thick-ness: 1.0  µ m) or on a HP5890 - HP5971-MSD equipped witha CPSill8 low bleed column (dimensions: 25m  × 0.25 mm; filmthickness: 0.4  µ m). Diethyl ether and ligroin were distilledfrom sodium/potassium alloy; THF and toluene from potassiumand sodium, respectively, all having benzophenone as indica- (23) a) Gronowitz, S.; Bobosˇik, V.; Lawitz, K.  Chem. Scripta   1984, 23  , 120; b) Gronowitz, S.; Bobosˇik, V.; Lawitz, K.  Chem. Scripta   1988 , 28,  281.(24) Muller, D.; Fleury, J .-P.  Tetrahedr. Lett  .  1991 ,  32,  2229.(25) Watanabe, T.; Miyaura, N.; Suzuki, A.  Synlett.  1992 , 207.(26) Fukuyama, Y.; Kiriyama, Y.; Kodama, M.  Tetrahedron Lett  . 1993 ,  34  , 7637.(27) Zhang, H.; Chan, K. S.  Tetrahedron Lett  .  1996 ,  37,  1043 describesimilar reaction conditions for Suzuki reactions; however, they mentiona base effect instead of an in situ protection.(28) (a) Schumann, H.; Karasiak, D. F.; Mu¨hle, S. H.; HaltermanR. L.; Kaminsky, W.; Weingarten U.  J. Organomet. Chem.  1999 ,  579  ,356. (b) Halterman, R. L.; Fahey, D. R.; Bailley, E. F.; Dockter, D. W.;Stenzel, O.; Shipman, J . L.; Khan, M. A.; Dechert, S.; Schumann, H. Organometallics   2000 ,  19  , 5464. Scheme3. Equilibriabetween 31,34and theIrreversibleDecomposition of 31to32and TheirSubsequent Suzuki Reactions 2,2 ′ -Bis(2-indenyl)biphenyl Derivatives  J. Org. Chem., Vol. 67, No. 1, 2002   173
Advertisement
Related Documents
View more
Related Search
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks