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Synthesis, structure, and high catalytic activity of cationic alkoxytitanium(IV) complexes in the Diels-Alder reaction
ELSEVIER
InorganicaChimicaActa256 (1997) 161-L63
Preliminary Communication
Synthesis, structure, and high catalytic activity of cationic alkoxytitanium(IV) complexes in the Diels-Alder reaction Yuldhiro Motoyama ~, Masako Tanaka b, Koichi Mikami
a,*
~'Departme~lt vf Chemicut Engineering. Facuhy of Engineerlng, Tokyo Institute of Techrtotog~. Megaro-ku. Tokyo 152, Japan h Research Laboratory ~Resoarccs Ulitizatioo. Tokyo I~titute ufTechmdogy. Midori-ku. Yokohama 227. Japan
Recei'~ed 12 September 1996;revised4 December 1996;a~c'epled8 January 1997
Abstract The cationic alkoxytitanium complexes, [(i-PrO)2Ti((Yl'f),(i-PrOH)2l (la), [ti-PtO)2Ti(bpy)(i-PrOH)2](OTf) 2 (3a) arm [(iPrO)~.Ti(bpy)(OTf) (i-PFOH)]lOTf) (4a), are synthesized as highly effective catalysts for the Diels-Alder reaction of l-acetexy-l,3butadiene with methacrolein, X-ray crystal analysis of the tert-butoxy derivative reveals the dinuclear, six-coordinated, and di storted o~:.ahedral structure [ (t-BuO)2Ti2(#-O) (p-OTf)2(OTf),.( H20)21 (2b) with a #-O and two bt-OTf ligands.
Kpywordx: Crystalstructures; Dials Alderreaction:Titaniumcomplexes;Alkoxycomplexes
The development of Lewis acid catalysts is the key to catalysis of the Diels-Alder reaction in order to attain higher stercoselectivity incleding endo- and cnantiaseleclivities,and
higher regioselectivity th,m those in thermal reactions [ I ]. The most commonly used I.ewis acid catalysts possess halide ligands. For the construction of ehiral Lewis acid catalysts, ehiral alcohols are usually introduced as chiral ligands. However, this usually results in a decrease in the Lewis acidity. Cationic alkoxymetal complexes 1 that have the anionic ligand of the conjugate base of a strong Breasted acid such as trifluuromethanesulfonic acid may provide a means of maintaining high Lewis acidity. We report herein the synthesis of cationic alkoxytitanium complexes that have trifluoromethanesulfonate (triflate) as the anionic ligand and demonstrate their effectiveness as Lewis acid catalysts for the Diels-Alder reaction of l-acetoxy-l,3-butadiene with methacrolein. First, the isopropoxytitanium triflate complex ( l a ) was prepared by reaction of titanium(IV) isopropoxide ((iPrO),Ti) and trifluuromethanesulfoalc acid (HOTf) in pentune at 0°C (Eq. (1)). * Correspondingauthor. ' The only reports on the cationic alkoxymetalcomplexare the t~em ~ports given in Ref. [2]. The X-i'aycrystal analysisof a cationica!koxyaluminumcomplex has been reported 12a]. CationicalkoxytitaniLmanti -zti~oniumcomplexes have been reported as effectiveLewisacid catalysts for the Diels-Alder,Mukaiyamaaldol,and allyltincarbonyladditionreactions [2h]. 0020-t693/97/$17~00~ 1997ElsewerScienceS.A. All rightsreserved PlISOO20-1693t97)05471-6
(RO)4Ti
HOTf --, (RO)2Ti(OTf)2(ROH)2 r~.n~ ~ pea~/ether. O°C l a : R =i-Pr (l) Ib:
R =
t-Bu
The isopropoxytitanium complex ( l a ) was obtained in an 86% yield as a white solid that was highly moisture sensitive. In order to obtain a single crystal, the tert-butoxydtanium derivative ( l b ) was then prepared in a manner similar to Eq. (1) except in pentane and ether. X-ray analysis o f the colorless single crystal obtained from dichloromethane/ether/ pentane, surprisingly reveals the p,-O and ~-OTf complex 2b (Fig. 1) 2. Fig. I shows the dinuclear, six-coordinated, and distorted octahedrM geometry with a ~-O and two p.-OTf ligands, wherein t~-O and H20 are coordinated in a tmns configuration to each other. Since the bond distance of Ti l Os is 0.166 A longer than the Tit--e6 distance, Oa is apparently bonded to titanium, although this bond is weak. Note that the structure of complex l b changes during crystallization to the p-oxo complex 2b [3] due to the lability o f the alkoxy ligand. Then, the isopropoxytitanium complex ( l a ) is transformed to the 2,2'-bipyridine (bpy)-stabilized complexes -'Crystal data for 2b: C~eH~20~Ftz'fiz, monoclinic. C2/c. a m II.86M3), b=19.490(i), c=20.173(6) /~..~=I02.35{2) °, V~ 4555t2) .$,~,Z=4. Of 3183 teP~ectiom collected tMo Kc¢, 20(lrmx) =
45.0°.296 K), 1952wereol~eP~ed.R=0.051 aed R~=0.051.
E Motoyama el aL/Inorganica Chimica Acta 256 (1997) 161-163
162
Table I IH chemical shift differences tA 8) of bpy on comptexation with 1 or TiCI4 •
Lewis acid
AHa
AHb
AHc
AHd
1 TIC[4
0.37 0A I
0.76 0.81
0.35 0.47
0.35 0.88
LFree bpy: 6 7.29 (Hal, 7.39 (lib). 7.80 (He), 8.67 (Hd). Positive A~$ means a downfield shill (A ~ = 8 ~ p ~ -- ~l,~ ~y ). Table 2 Diels-Alder reactions of methaerolein with I -ace~c.xy-1.3-botadiene a
Fig 1. ORTEP drawing of 2b. Selected bond distances (~): Ti le l = 1.798(2),Tic-Oz=2.059(d),Til~)3-1,995(4),TiL~O~=2.059(4). Ti143a = 2.225(4), Til~09 = 1.7fi6(4). 09.-{23= IA58(7l, Hc-Oio = 1.6fi. Selected bond angles (°): O,-Tild)2= 162.9(2), Oi-Tii-O3=95.9(I), O~-Ti~4:l~ = 89.3( 1), O~-Tixd:)a= 84A(2), Oa-Tixd)9 = 108.2(2).
~, "-o~ o~.~,,~. ..
/
I~., ~o~
/ I~O~ I
1
.~..~zl
L ,z "c J L%8o
/ / o-,,~
J
Fig. 2. Structure of 3a and 4a. ( 3 a and 4 a ) which can be separated by crystallization in dichloromethane (Eq. ( 2 ) ) . hpy (i-PrO) 2Tit O T f ) 2 ( i - P r O H ) 2 --* CH2CI2 la [ ( i - P r O ) z T i ( b p y ) (i-PrOH)2] (OTf)~ 3a (4:1 ) + [ ( i - P r O ) 2 T i ( b p y ) ( O T D (i -PrOH) l ( O T O
4a
(2)
T h e ~H N M R spectra of 3 a and 4 a showed their highly symmelrieal, six-coordinated structures (Fig. 2) 3, with one or two i-PrOH to stabilize the cationic titanium centers. The 3 The two i-PrO and bpy ligands are therefore located in the equatofial plane, tH NMR (300 MHz, CDCl3): 3a: 6 1.21 (d, J = 6.1 Hz, 12H). 1.56 (d, J=fi.l Hz. 12H),4.03 (hept, J=6.l Hz, 2H), 5.32 (hept, J=6A Hz. 2H), 7.66 (m, 2H), 8,15 (m, 4H), 9.02 (m, 2H);da: 81.22 (d, J = 6.l Hz, 61t), 1.55 (d, J~6.2 Hz, 12H), 4.01 (hept, J=6.1 Hz, 2H). 5.32 (hept, J - 6 . 2 Hz, 2H), 7.66 (m, 2H), 8.15 (rn.4H), 9.02 (m, 9H).
e¢~,
exo
EntrJ
Catalyst (rnol%)
Conditions
Yield (%)
endo b
I 2 3 4 5
la (3) 3a(l) 4a(l) (i-PrO)~TiCl2 ( t ) CpzTi(OTf)~ ( l )
r.t., 5 rain r.t., 6 h r.t., l ft r.t., 12 h r.t., l h
50 99 98 0 85
88 98 99
(%)
99
All reactions were carried out using 1.0 mmol of diene and 2~0 mmol of methacrolein. Determined by ~H NMR and GLC analysis. ~ignal of the methiue proton o f the isopropoxy (i-PrO) ligand (5.32 p p m ) appears in a field ~ 1.3 p p m lower than those o f isopropanols ( i - P r O H ) at 4.01 ( 3 a ) and 4.03 ( 4 a ) ppm, respectively. Thus, the Lewis acidity o f l a can he compared as a bpy-complexed form ( 3 a ) to that o f TiCI4 (Table 1) [ 4 ] . The magnitude o f the downfield shift o f the Ha, H b and He peaks of bpy on complexation with l a is quite similar to that of TiCI4 4. Therefore, the Lewis acidity o f l a appears to be as high as that of TiCId. T h e high catalytic activities o f these complexes ( l a , 3a, 4 a ) were confirmed in the D i e l s - A l d e r reaction o f 1-acetoxy1,3-butadiene with methacrolein ( T a b l e 2 ) . These alkoxytitanium triflate complexes showed dramatically increased catalytic activity compared to the corresponding halide complex, (i-PrO)2TiCI 2 (entries 1-3 versus 4 ) . U s i n g l a as a catalyst, the chemical yield o f the Diels--Alder adduet was only moderate due to decomposition o f the diene, presumably resulting from the high Lewis acidity o f l a (entry 1). The bpy-stabilized complexes ( 3 a and 4 a ) appear to be efficient 4 The Hd pimton resonance of bpy on complexation with l a is in a higher field tbaa Ihet of TiCI4 because the Hd proton is located near the stexicolly
hindered i-PrO group.
E Motoym~m el aL/Inorganica Chimica Acts 256 (1997) 161-163
catalysts that are capable of providing almost perfect e n d o selectivity and quantitative yields, even within I h, using 4a (entries 2 and 3). Particularly, the catalytic activity of 4 a is higher than that o f Cp2Ti(OTf)2, one of the most efficient Lewis acid catalysts available for the D i e l s - A l d e r reaction (entries 3 versus 5) [ 5 ].
Acknowledgements This work was supported by a Grant-in-Aid fcr Scientific Research on Priority Aica (~f Asym~ci~ic Synthesis from the Ministry of Education, Science and Culture, Japan. We arc gra)cful to Professor Hiroharu Suzuki of Tokyo Institute of Technology for his helpfu! discussion.
References [ [ I (a) L.A. Paqueae, in JD. Molrison (ed.),AsymmelricSynthexis, V01. 3B, Academic press. New York. 1984; (b) W. Oppolzer.Ange~ Chem. Ira. Ed. Engl..23 (1984) 876; (c) M J. Taschner, inT. Hudlicky led ),
163
Organic Synthesis: Theory ~dAppliealians. Vol, I. JAI Press, Loadon, 1989; (d) A, Taff~cehi,Dleaes in the Diel.¢-Alder Reaction. Wiley, New York, 19901 (e) W. 0!~polzer. in B.M. TtosI and I. Flendng (eds.), Comprehensive Organic Synthesis, Vol. 5. Petg'm~on, Oxford. 1991; if) H.B. Kagall alld O. Rioat. Chem. Bey,, 92 (1992) 1007; (g) U. Pinata, G. Lets sad C. Otto, Chem. Rev., 93 (1993) 741; {g) T~Oh ~nd M. R~illy,Org. Prep. prnc. Inc., 26 (1994-) 129. [2] (a) D.A. Atwood. J.A. Jegier ~ D. Rutheffold, J. Am. Chem. S¢)c., 117 (1995) 6"r'/9; (b) P. Giotgio and C. Flotle,ni, J. Chem. StJc.. Per~ir. Trine.. I (1995) 2557, t3l (a) W.P. Griffilh, Cmsrd. Chem. Rev., 3 ! 1970,~ 459; (h) B.O~Wcsl, Polyhedran, 8 (1989) 219; (c) R.H. !!aim, Che~. Rev., 87 0987) 1401; (d) R.H. Holm and J.P. Donahue, Polyhedro~° 12 (1993) 571;
(e) V.W. Day, T.A. Eberspacher, Y. C'nell.J. Ha(:,andW.G. Klcrapeter, Int~rg. Chlm, Acts. 229 (1995) 391; (f) J. Blanchard, $.H..Doeaff, I. Maqucl and C, Sar,(:lmz,New J. Chem., 19 { 1995) 929.
[4] (a) J. Furukawa, E. Kobayasbi, S. Nagala a~1¢1T.Mo,dlani../. Po/y~ ScL. Polym. Chem. F~,. 12 (19"/4) 1799: (b) W. Ktc'aa, S. Pasynkiewlez. K. Flol'j~cz-ik and E, Lusz~k, Makromo(. Chem., 177 (1976) 2627; (c) R.F, Cbilds, D.L Mulholland and A. Nixoa, Can- .£ Chem., 60 (1982) gOI. [5] T.K. Hollis, N.P. Robinson and B. B~nich, Organot~e~llics, I1 ( ! 992 ) 2745: S. Collins. B E. Knenc, R. Ramachandr~ and NJ.Taylor, Organnmetallics, IC ( 1991 ) 2{192.
InorganicaChimicaActa256 (1997) 161-L63
Preliminary Communication
Synthesis, structure, and high catalytic activity of cationic alkoxytitanium(IV) complexes in the Diels-Alder reaction Yuldhiro Motoyama ~, Masako Tanaka b, Koichi Mikami
a,*
~'Departme~lt vf Chemicut Engineering. Facuhy of Engineerlng, Tokyo Institute of Techrtotog~. Megaro-ku. Tokyo 152, Japan h Research Laboratory ~Resoarccs Ulitizatioo. Tokyo I~titute ufTechmdogy. Midori-ku. Yokohama 227. Japan
Recei'~ed 12 September 1996;revised4 December 1996;a~c'epled8 January 1997
Abstract The cationic alkoxytitanium complexes, [(i-PrO)2Ti((Yl'f),(i-PrOH)2l (la), [ti-PtO)2Ti(bpy)(i-PrOH)2](OTf) 2 (3a) arm [(iPrO)~.Ti(bpy)(OTf) (i-PFOH)]lOTf) (4a), are synthesized as highly effective catalysts for the Diels-Alder reaction of l-acetexy-l,3butadiene with methacrolein, X-ray crystal analysis of the tert-butoxy derivative reveals the dinuclear, six-coordinated, and di storted o~:.ahedral structure [ (t-BuO)2Ti2(#-O) (p-OTf)2(OTf),.( H20)21 (2b) with a #-O and two bt-OTf ligands.
Kpywordx: Crystalstructures; Dials Alderreaction:Titaniumcomplexes;Alkoxycomplexes
The development of Lewis acid catalysts is the key to catalysis of the Diels-Alder reaction in order to attain higher stercoselectivity incleding endo- and cnantiaseleclivities,and
higher regioselectivity th,m those in thermal reactions [ I ]. The most commonly used I.ewis acid catalysts possess halide ligands. For the construction of ehiral Lewis acid catalysts, ehiral alcohols are usually introduced as chiral ligands. However, this usually results in a decrease in the Lewis acidity. Cationic alkoxymetal complexes 1 that have the anionic ligand of the conjugate base of a strong Breasted acid such as trifluuromethanesulfonic acid may provide a means of maintaining high Lewis acidity. We report herein the synthesis of cationic alkoxytitanium complexes that have trifluoromethanesulfonate (triflate) as the anionic ligand and demonstrate their effectiveness as Lewis acid catalysts for the Diels-Alder reaction of l-acetoxy-l,3-butadiene with methacrolein. First, the isopropoxytitanium triflate complex ( l a ) was prepared by reaction of titanium(IV) isopropoxide ((iPrO),Ti) and trifluuromethanesulfoalc acid (HOTf) in pentune at 0°C (Eq. (1)). * Correspondingauthor. ' The only reports on the cationic alkoxymetalcomplexare the t~em ~ports given in Ref. [2]. The X-i'aycrystal analysisof a cationica!koxyaluminumcomplex has been reported 12a]. CationicalkoxytitaniLmanti -zti~oniumcomplexes have been reported as effectiveLewisacid catalysts for the Diels-Alder,Mukaiyamaaldol,and allyltincarbonyladditionreactions [2h]. 0020-t693/97/$17~00~ 1997ElsewerScienceS.A. All rightsreserved PlISOO20-1693t97)05471-6
(RO)4Ti
HOTf --, (RO)2Ti(OTf)2(ROH)2 r~.n~ ~ pea~/ether. O°C l a : R =i-Pr (l) Ib:
R =
t-Bu
The isopropoxytitanium complex ( l a ) was obtained in an 86% yield as a white solid that was highly moisture sensitive. In order to obtain a single crystal, the tert-butoxydtanium derivative ( l b ) was then prepared in a manner similar to Eq. (1) except in pentane and ether. X-ray analysis o f the colorless single crystal obtained from dichloromethane/ether/ pentane, surprisingly reveals the p,-O and ~-OTf complex 2b (Fig. 1) 2. Fig. I shows the dinuclear, six-coordinated, and distorted octahedrM geometry with a ~-O and two p.-OTf ligands, wherein t~-O and H20 are coordinated in a tmns configuration to each other. Since the bond distance of Ti l Os is 0.166 A longer than the Tit--e6 distance, Oa is apparently bonded to titanium, although this bond is weak. Note that the structure of complex l b changes during crystallization to the p-oxo complex 2b [3] due to the lability o f the alkoxy ligand. Then, the isopropoxytitanium complex ( l a ) is transformed to the 2,2'-bipyridine (bpy)-stabilized complexes -'Crystal data for 2b: C~eH~20~Ftz'fiz, monoclinic. C2/c. a m II.86M3), b=19.490(i), c=20.173(6) /~..~=I02.35{2) °, V~ 4555t2) .$,~,Z=4. Of 3183 teP~ectiom collected tMo Kc¢, 20(lrmx) =
45.0°.296 K), 1952wereol~eP~ed.R=0.051 aed R~=0.051.
E Motoyama el aL/Inorganica Chimica Acta 256 (1997) 161-163
162
Table I IH chemical shift differences tA 8) of bpy on comptexation with 1 or TiCI4 •
Lewis acid
AHa
AHb
AHc
AHd
1 TIC[4
0.37 0A I
0.76 0.81
0.35 0.47
0.35 0.88
LFree bpy: 6 7.29 (Hal, 7.39 (lib). 7.80 (He), 8.67 (Hd). Positive A~$ means a downfield shill (A ~ = 8 ~ p ~ -- ~l,~ ~y ). Table 2 Diels-Alder reactions of methaerolein with I -ace~c.xy-1.3-botadiene a
Fig 1. ORTEP drawing of 2b. Selected bond distances (~): Ti le l = 1.798(2),Tic-Oz=2.059(d),Til~)3-1,995(4),TiL~O~=2.059(4). Ti143a = 2.225(4), Til~09 = 1.7fi6(4). 09.-{23= IA58(7l, Hc-Oio = 1.6fi. Selected bond angles (°): O,-Tild)2= 162.9(2), Oi-Tii-O3=95.9(I), O~-Ti~4:l~ = 89.3( 1), O~-Tixd:)a= 84A(2), Oa-Tixd)9 = 108.2(2).
~, "-o~ o~.~,,~. ..
/
I~., ~o~
/ I~O~ I
1
.~..~zl
L ,z "c J L%8o
/ / o-,,~
J
Fig. 2. Structure of 3a and 4a. ( 3 a and 4 a ) which can be separated by crystallization in dichloromethane (Eq. ( 2 ) ) . hpy (i-PrO) 2Tit O T f ) 2 ( i - P r O H ) 2 --* CH2CI2 la [ ( i - P r O ) z T i ( b p y ) (i-PrOH)2] (OTf)~ 3a (4:1 ) + [ ( i - P r O ) 2 T i ( b p y ) ( O T D (i -PrOH) l ( O T O
4a
(2)
T h e ~H N M R spectra of 3 a and 4 a showed their highly symmelrieal, six-coordinated structures (Fig. 2) 3, with one or two i-PrOH to stabilize the cationic titanium centers. The 3 The two i-PrO and bpy ligands are therefore located in the equatofial plane, tH NMR (300 MHz, CDCl3): 3a: 6 1.21 (d, J = 6.1 Hz, 12H). 1.56 (d, J=fi.l Hz. 12H),4.03 (hept, J=6.l Hz, 2H), 5.32 (hept, J=6A Hz. 2H), 7.66 (m, 2H), 8,15 (m, 4H), 9.02 (m, 2H);da: 81.22 (d, J = 6.l Hz, 61t), 1.55 (d, J~6.2 Hz, 12H), 4.01 (hept, J=6.1 Hz, 2H). 5.32 (hept, J - 6 . 2 Hz, 2H), 7.66 (m, 2H), 8.15 (rn.4H), 9.02 (m, 9H).
e¢~,
exo
EntrJ
Catalyst (rnol%)
Conditions
Yield (%)
endo b
I 2 3 4 5
la (3) 3a(l) 4a(l) (i-PrO)~TiCl2 ( t ) CpzTi(OTf)~ ( l )
r.t., 5 rain r.t., 6 h r.t., l ft r.t., 12 h r.t., l h
50 99 98 0 85
88 98 99
(%)
99
All reactions were carried out using 1.0 mmol of diene and 2~0 mmol of methacrolein. Determined by ~H NMR and GLC analysis. ~ignal of the methiue proton o f the isopropoxy (i-PrO) ligand (5.32 p p m ) appears in a field ~ 1.3 p p m lower than those o f isopropanols ( i - P r O H ) at 4.01 ( 3 a ) and 4.03 ( 4 a ) ppm, respectively. Thus, the Lewis acidity o f l a can he compared as a bpy-complexed form ( 3 a ) to that o f TiCI4 (Table 1) [ 4 ] . The magnitude o f the downfield shift o f the Ha, H b and He peaks of bpy on complexation with l a is quite similar to that of TiCI4 4. Therefore, the Lewis acidity o f l a appears to be as high as that of TiCId. T h e high catalytic activities o f these complexes ( l a , 3a, 4 a ) were confirmed in the D i e l s - A l d e r reaction o f 1-acetoxy1,3-butadiene with methacrolein ( T a b l e 2 ) . These alkoxytitanium triflate complexes showed dramatically increased catalytic activity compared to the corresponding halide complex, (i-PrO)2TiCI 2 (entries 1-3 versus 4 ) . U s i n g l a as a catalyst, the chemical yield o f the Diels--Alder adduet was only moderate due to decomposition o f the diene, presumably resulting from the high Lewis acidity o f l a (entry 1). The bpy-stabilized complexes ( 3 a and 4 a ) appear to be efficient 4 The Hd pimton resonance of bpy on complexation with l a is in a higher field tbaa Ihet of TiCI4 because the Hd proton is located near the stexicolly
hindered i-PrO group.
E Motoym~m el aL/Inorganica Chimica Acts 256 (1997) 161-163
catalysts that are capable of providing almost perfect e n d o selectivity and quantitative yields, even within I h, using 4a (entries 2 and 3). Particularly, the catalytic activity of 4 a is higher than that o f Cp2Ti(OTf)2, one of the most efficient Lewis acid catalysts available for the D i e l s - A l d e r reaction (entries 3 versus 5) [ 5 ].
Acknowledgements This work was supported by a Grant-in-Aid fcr Scientific Research on Priority Aica (~f Asym~ci~ic Synthesis from the Ministry of Education, Science and Culture, Japan. We arc gra)cful to Professor Hiroharu Suzuki of Tokyo Institute of Technology for his helpfu! discussion.
References [ [ I (a) L.A. Paqueae, in JD. Molrison (ed.),AsymmelricSynthexis, V01. 3B, Academic press. New York. 1984; (b) W. Oppolzer.Ange~ Chem. Ira. Ed. Engl..23 (1984) 876; (c) M J. Taschner, inT. Hudlicky led ),
163
Organic Synthesis: Theory ~dAppliealians. Vol, I. JAI Press, Loadon, 1989; (d) A, Taff~cehi,Dleaes in the Diel.¢-Alder Reaction. Wiley, New York, 19901 (e) W. 0!~polzer. in B.M. TtosI and I. Flendng (eds.), Comprehensive Organic Synthesis, Vol. 5. Petg'm~on, Oxford. 1991; if) H.B. Kagall alld O. Rioat. Chem. Bey,, 92 (1992) 1007; (g) U. Pinata, G. Lets sad C. Otto, Chem. Rev., 93 (1993) 741; {g) T~Oh ~nd M. R~illy,Org. Prep. prnc. Inc., 26 (1994-) 129. [2] (a) D.A. Atwood. J.A. Jegier ~ D. Rutheffold, J. Am. Chem. S¢)c., 117 (1995) 6"r'/9; (b) P. Giotgio and C. Flotle,ni, J. Chem. StJc.. Per~ir. Trine.. I (1995) 2557, t3l (a) W.P. Griffilh, Cmsrd. Chem. Rev., 3 ! 1970,~ 459; (h) B.O~Wcsl, Polyhedran, 8 (1989) 219; (c) R.H. !!aim, Che~. Rev., 87 0987) 1401; (d) R.H. Holm and J.P. Donahue, Polyhedro~° 12 (1993) 571;
(e) V.W. Day, T.A. Eberspacher, Y. C'nell.J. Ha(:,andW.G. Klcrapeter, Int~rg. Chlm, Acts. 229 (1995) 391; (f) J. Blanchard, $.H..Doeaff, I. Maqucl and C, Sar,(:lmz,New J. Chem., 19 { 1995) 929.
[4] (a) J. Furukawa, E. Kobayasbi, S. Nagala a~1¢1T.Mo,dlani../. Po/y~ ScL. Polym. Chem. F~,. 12 (19"/4) 1799: (b) W. Ktc'aa, S. Pasynkiewlez. K. Flol'j~cz-ik and E, Lusz~k, Makromo(. Chem., 177 (1976) 2627; (c) R.F, Cbilds, D.L Mulholland and A. Nixoa, Can- .£ Chem., 60 (1982) gOI. [5] T.K. Hollis, N.P. Robinson and B. B~nich, Organot~e~llics, I1 ( ! 992 ) 2745: S. Collins. B E. Knenc, R. Ramachandr~ and NJ.Taylor, Organnmetallics, IC ( 1991 ) 2{192.