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Unusual reactivity of isocyanates with dioxo- and diimidomolybdenum(VI) complexes
Polyhedron Vol. IO, No. 415, pp. 4&465,
1991
Printed in Great Britain
0
0277-5x37/91 s3.00+.00 1991 Pergamon Press plc
UNUSUAL REACTIVITY OF ISOCYANATES WITH DIOXOAND DIIMIDOMOLYBDENUM(V1) COMPLEXES RICHARD LAI,* SYLVAIN MABILLE,
ALAIN CROUX and SYLVIE LE BOT
ENSSPICAM, URA 1410, Universitt Aix-Marseille III, Marseille, France (Received 4 October 1990; accepted 24 October 1990)
Abstract-Phenyl isocyanate reacts with dioxo- and diimidomolybdenum(V1) complexes such as Mo(O),(mes), (1) (mes = mesityl = 2,4,6-C6H1Me3), Mo(mes)2CH2PBuj(0)2 (2), Mo(mes){CPBu,(mes)}(O), (3) and Mo(NBu’),(mes), (4), selectively affording phenyl mesityl amide. This reaction proceeds via insertion into the Mo-mesityl bond, leading to an unstable oxazamolydenacycle which has been identified by 13CNMR. This intermediate is probably hydrolysed’with generation of the amide during chromatographic work-up.
There are many reports in the literature of reactions involving heteroallenes such as isocyanates and transition metal complexes. The objectives of these studies were predominantly to better understand metal-promoted reactivity of CO,-like molecules and also to develop reactions of synthetic utility. ‘J As part of our study of the synthesis and reactivity of multiple-bonded complexes,3 it seemed appropriate to compare the reactivity of dioxodimesitylmolybdenum(VI), Mo(0)z(mes)2 (1) (where mes = mesityl = 2,4,6-C6H2Me3), with the isoelectronic di(phenylimido)dimesityhnolybdenum(VI) complex. Therefore, we attempted to prepare this compound by reacting 1 with phenyl isocynate, a synthetic route which seems to have potential application to a wide variety of metals.4 However, the reaction took an entirely different pathway and phenyl mesityl amide was found to be the sole identified product : Mo(O),(mes),+2PhNCO Mo(NPh),(mes), <
+ 2 CO2
PhNHCO(mes).
We report here on the unusual reactivity of isocyanates with dioxo- and diimidomolybdenum(V1) complexes.
*Author to whom correspondence
should be addressed. 463
RESULTS
AND DISCUSSION
When a yellow solution of M(O),(mes), (1) in pyridine or tetrahydrofuran was treated with one equivalent of PhNCO, an instantaneous colour change occurred. After stirring for several hours, a brown solution was obtained. Analysis of the oily brown residue, obtained after vacuum evaporation of the solvent, did not reveal the presence of a diimido species, but column chromatography of the reaction mixture afforded an organic compound identified as phenyl mesityl amide by IR, NMR and mass spectrometry analysis. One equivalent of amide was obtained per mesityl group of the starting dioxo complex. Bu’NCO reacted similarly affording Bu’NHCO(mes). However, only 0.6 equivalent of amide per mesityl group was isolated. The same behaviour was observed with the phosphorus ylide complex Mo(mes), CH,PBU~(O)~ (2). However, in this case, elution of the amide from the column was only possible after on-column hydrolysis by dilute aqueous acid. The dinitrene complex, Mo(NBut),(mes)z (4), obtained by an indirect method,5 also reacted with PhNCO to give phenyl mesityl amide in good yield (92%). But in this case, the reaction was less selective since gas chromatographic analysis showed the presence of small amounts of other products. Two of them have been identified as tertiary butyl mesityl amide (1%) and a phenyl isocyanate trimer (5%). It is worth noting here that elimination of tertiary butyl mesityl amide has been observed upon treatment of Cr(NBu’),(mes)z with CO.5
464
R. LA1 et al.
When the dioxophosphinomethylidene complex, Mo(mes){CPBu,(mes))(0), (3), was reacted with PhNCO under the same conditions, no reaction was apparent after several hours at room temperature. Phenyl mesityl amide was detected only when the reaction was performed under more forcing conditions (70°C for 4 h) and no interaction with the Mo=C double bond was observed, in contrast with
graphic work-up. We could neither isolate nor identify such an intermediate from 2 or from 4. However, when 1 was reacted in a sealed NMR tube with one equivalent of PhNCO in dry CSD5N at 20°C we were able to obtain good 13C NMR evidence for a species tentatively described as an oxazamolybenacycle, represented here as two mesomerit forms :
Scheme 1. the results reported by Cramer et al. for Cp, U=CHPMe*Ph. 6 The generation of amides (or thioamides) from isocyanates (or isothiocyanates) and Grignard or organolithium reagents has been reported.7.8 This type of reactivity with transition metal 0x0 and imido complexes is unprecedented, although the formation of cyclohexyl mesityl amide by reacting &Hi *NC0 and a vanadium complex has been mentioned recently.’ Generally, the interaction of transition metal complexes with isocyanates leads to stable compounds resulting from insertion of RNCO into M-C, lo M-N, ’ I M=C6 or M=C” bonds. As far as 0x0 complexes are concerned differences in reactivity have been reported depending on the starting compound. Thus, 0x0 complexes either give imido species in a “classical way”, or adducts. ’ 3 An oxo-imido complex has been reported from the reaction between Mo(0)$12 ($(MeCN) 2 and Bu’NCO.‘~ In contrast, C5H&Mo0 reacts with PhNCO to form a cyclometallacarbamate, Mo(q5-C5H5)2{0C(0)N(Ph)), resulting from a formal [2 f 23 cycloaddition of the C=N bond to the 0x0 linkage. I5 In the present case, it is obvious that the reaction leading to the imido complex is very unfavourable, if not completely inoperative. Indeed, quantitative gas chromatography analysis of the atmosphere above the reaction mixture revealed the presence of CO, [less than 5% with a lo-fold excess of PhNCO vs l] and of CO (1 “A). Carbon monoxide formation could arise from the decomposition of PhNCO adducts, although this type of decomposition is very rare. I6 Even with an excess of phenyl isocyanate, a single isocyanate was inserted into the Mo-mesityl bond, as observed with other systems. ” We believe that the amide is formed by hydrolysis of an unstable intermediate during the chromato-
Two low-field signals at 187.3 and 181.3 ppm are, respectively, assigned to the carbonyl carbon atom and to the ipso-carbon of the q’-mesityl ligand. At higher field, four signals are observed at 27.95,21.8, 20.80 and 18.70 ppm. The peaks at 27.95 and 20.80 ppm are associated with the signal at 181.3 ppm. They are assigned, by analogy with the 13C NMR spectrum of 1, to the ortho- andpara-methyl groups of the rl ‘-mesityl ligand. The two other signals at 21.8 and 18.7 ppm are assigned to the para- and ortho-methyl groups, respectively, of the mesityl group in the q*-amide ligand. The ortho-methyl group is shielded as in phenyl mesityl amide due to the proximity of the carbonyl group. The assignment of the signal at 187.3 ppm to the carbonyl group is, furthermore, consistent with the spectra of other complexes with related structures.2b~12Formation of such a species resulting from the insertion of the isocyanate into the MO-C a-bond is certainly favoured by the polarization of the MO-C structure. ” This insertion occurs at the N=C bond rather than at the CL0 bond, reflecting the difference in bond strength between these two bonds.17 Depending on the structure of the starting complex, this reaction would be more or less easy as shown for the dioxophosphinomethylidene complex, 3. EXPERIMENTAL General
Preparations of complexes 1,” 2 Iv 3” and 45 have been described earlier. PhNCd and Bu’NCO were purchased from Janssen and distilled and stored under nitrogen. All operations were conducted under oxygen-free N2 by standard Schlenk techniques and solvents were distilled before use from sodium/benzophenone, except for pyridine which was distilled from KOH under N,. Anhy-
Isocyanates with dioxo- and diimidomolybdenum(V1) drous CH2C12 was kept on molecular sieves (4 A)
and degassed prior to use. NMR spectra were recorded on a Bruker spectrometer AM-200 and GC/MS coupling experiments were performed on a Ribernag R.10.10.c. apparatus. IR spectra were obtained using a FT-IR Nicolet MX-S spectrometer. Gas chromatography analyses were conducted on an Intersmat 1GC 120FB apparatus and an Intersmat ICR 1B integrator, on 5m x l/8” columns of 5% OV 17 on Chromosorb W. Gas chromatography of gases were carried out on an Intersmat 1GC 120MB using a thermal conductivity detector on 2m x l/8” columns of Prorapak N (CO,) or of molecular sieves, 3 A (CO). Benzamide was used as an internal standard for quantitative GLC analyses.
3.
4.
5.
6. 7.
8. Description
of a typical experiment
with isocyanates
9.
The complex (3.2 x lop3 mol) was dissolved in 10. tetrahydrofuran or pyridine (20 cm3) and after cooling at -30°C 0.36 cm3 (3.2 x lop3 mol) of PhNCO was added. The solution was allowed to warm slowly to room temperature and after 5 h stirring the reaction mixture was evaporated under 11. reduced pressure to a small volume and passed 12. down a silica gel column (0.063-0.2 mm, Merck). Elution of the amide was obtained with CH2C12. 13. For complex 3, after deposition of the reaction mixture on silica gel, 10 cm3 of H2S04 (5% v/v in H20) was passed down the column before CH2C12 14. elution. Acknowledgement-We wish to express our thanks to Professor Sir Geoffrey Wilkinson for helpful discussions.
15.
16.
REFERENCES (a) G. Fachinetti, C. Biran, C. Floriani, A. ChiesiVilla and C. Guastini, Znorg. Chem. 1978,17,2995 ;
17.
(b) S. Gambarotta, M. Pasquali, C. Floriani, A. Chiesi-Villa and C. Guastini, Znorg. Chem. 1981,20, 1173 ; (c) S. Gambarotta, S. Strologo, C. Floriani, A. Chiesi-Villa and C. Guastini, Znorg. Chem. 1985, 24, 654. (a) H. Hoberg, K. Radine and A. Milchereit, J. Organomet. Chem. 1985, 280, C60; (b) H. Hoberg,
18. 19.
20.
complexes
465
K. Summerman and A. Milchereit, J. Organomet. Chem. 1985,28?3,237. R. La&Advances in Metal Carbene Chemistry (Edited by U. Schubert), Nato Asi Series, Series C, Vol. 269, p. 171. Kluwer Academic Publishers, Dordrecht (1989) ; R. Lal, S. Le Bot and R. Faure, J. Organomet. Chem. 1987,336, C5. M. L. H. Green and K. J. Moynihan, Polyhedron 1986, 5, 921; W. A. Nugent and B. L. Haymore, Coord. Chem. Rev. 1980,31, 123. A. C. Sullivan, G. Wilkinson, M. Motavelli and M. B. Hursthouse, J. Chem. Sot., Dalton Trans. 1988, 53. R. E. Cramer, J. H. Jeong and J. W. Gilje, Organometallics 1987,6, 2010. Y. Zhang, J. Jiang and Y. Chen, Tetrahedron Lett. 1987,3815; Y. Zhang, J. Jiang and Z. Zhang, Tetrahedron Lett. 1988, 651. N. A. Lebel, R. M. Cherluck and E. A. Curtis, Synthesis 1973,678. M. Vivanco, J. Ruiz, C. Floriani, A. Chiesi-Villa and C. Guastini, Organometallics 1990,9, 2185. I. Tkatchenko, Comprehensive Organometallic Chemistry (Edited by G. Wilkinson, F. G. A. Stone and E. W. Abel), p. 743. Pergamon Press, Oxford (1982) ; J. D. Wilkins, J. Organomet. Chem. 1974, 65, 383. Y. Takahashi, N. Onoyama, Y. Ishikawa, S. Motojima and K. Sugiyama, Chem. Lett. 1978, 525. K. Weiss, U. Schubert and R. R. Schrock, Organometallics 1986. 5, 397. F. Bottomley and L. Sutin, Adv. Organomet. Chem. 1988,28,1988 ; S. F. Pedersen and R. R. Schrock, J. Am. Chem. Sot. 1982,104,7483. D. Ehrenfeld, J. Kress, B. D. Moore, J. A. Osborn and G. Schoettel, J. Chem. Sot., Chem. Commun. 1987,129. P. Jernakoff, G. L. Geoffroy, A. L. Rheingold and S. J. Geib, J. Chem. Sot., Chem. Commun. 1987, 1610. H. Hoberg, B. W. Oster, C. Kruger and Y. H. Tsay, J. Organomet. Chem. 1983,252, 365 ; J. G. Brennan and R. A. Andersen, J. Am. Chem. Sot. 1985, 107, 514. P. Jernakoff and N. J. Cooper, J. Am. Chem. Sot. 1987,109,2173. B. Heyn and R. Hoffmann, Z. Chem. 1976,16, 195. R. Lai, S. Le Bot, A. Baldy, M. Pierrot and H. Arzoumanian, J. Chem. Sot., Chem. Commun. 1986, 1208. H. Arzoumanian, A. Baldy, R. Lai, J. Metzger, M. L. Nkeng Peh and M. Pierrot, J. Chem. Sot., Chem. Commun. 1985, 1151.
1991
Printed in Great Britain
0
0277-5x37/91 s3.00+.00 1991 Pergamon Press plc
UNUSUAL REACTIVITY OF ISOCYANATES WITH DIOXOAND DIIMIDOMOLYBDENUM(V1) COMPLEXES RICHARD LAI,* SYLVAIN MABILLE,
ALAIN CROUX and SYLVIE LE BOT
ENSSPICAM, URA 1410, Universitt Aix-Marseille III, Marseille, France (Received 4 October 1990; accepted 24 October 1990)
Abstract-Phenyl isocyanate reacts with dioxo- and diimidomolybdenum(V1) complexes such as Mo(O),(mes), (1) (mes = mesityl = 2,4,6-C6H1Me3), Mo(mes)2CH2PBuj(0)2 (2), Mo(mes){CPBu,(mes)}(O), (3) and Mo(NBu’),(mes), (4), selectively affording phenyl mesityl amide. This reaction proceeds via insertion into the Mo-mesityl bond, leading to an unstable oxazamolydenacycle which has been identified by 13CNMR. This intermediate is probably hydrolysed’with generation of the amide during chromatographic work-up.
There are many reports in the literature of reactions involving heteroallenes such as isocyanates and transition metal complexes. The objectives of these studies were predominantly to better understand metal-promoted reactivity of CO,-like molecules and also to develop reactions of synthetic utility. ‘J As part of our study of the synthesis and reactivity of multiple-bonded complexes,3 it seemed appropriate to compare the reactivity of dioxodimesitylmolybdenum(VI), Mo(0)z(mes)2 (1) (where mes = mesityl = 2,4,6-C6H2Me3), with the isoelectronic di(phenylimido)dimesityhnolybdenum(VI) complex. Therefore, we attempted to prepare this compound by reacting 1 with phenyl isocynate, a synthetic route which seems to have potential application to a wide variety of metals.4 However, the reaction took an entirely different pathway and phenyl mesityl amide was found to be the sole identified product : Mo(O),(mes),+2PhNCO Mo(NPh),(mes), <
+ 2 CO2
PhNHCO(mes).
We report here on the unusual reactivity of isocyanates with dioxo- and diimidomolybdenum(V1) complexes.
*Author to whom correspondence
should be addressed. 463
RESULTS
AND DISCUSSION
When a yellow solution of M(O),(mes), (1) in pyridine or tetrahydrofuran was treated with one equivalent of PhNCO, an instantaneous colour change occurred. After stirring for several hours, a brown solution was obtained. Analysis of the oily brown residue, obtained after vacuum evaporation of the solvent, did not reveal the presence of a diimido species, but column chromatography of the reaction mixture afforded an organic compound identified as phenyl mesityl amide by IR, NMR and mass spectrometry analysis. One equivalent of amide was obtained per mesityl group of the starting dioxo complex. Bu’NCO reacted similarly affording Bu’NHCO(mes). However, only 0.6 equivalent of amide per mesityl group was isolated. The same behaviour was observed with the phosphorus ylide complex Mo(mes), CH,PBU~(O)~ (2). However, in this case, elution of the amide from the column was only possible after on-column hydrolysis by dilute aqueous acid. The dinitrene complex, Mo(NBut),(mes)z (4), obtained by an indirect method,5 also reacted with PhNCO to give phenyl mesityl amide in good yield (92%). But in this case, the reaction was less selective since gas chromatographic analysis showed the presence of small amounts of other products. Two of them have been identified as tertiary butyl mesityl amide (1%) and a phenyl isocyanate trimer (5%). It is worth noting here that elimination of tertiary butyl mesityl amide has been observed upon treatment of Cr(NBu’),(mes)z with CO.5
464
R. LA1 et al.
When the dioxophosphinomethylidene complex, Mo(mes){CPBu,(mes))(0), (3), was reacted with PhNCO under the same conditions, no reaction was apparent after several hours at room temperature. Phenyl mesityl amide was detected only when the reaction was performed under more forcing conditions (70°C for 4 h) and no interaction with the Mo=C double bond was observed, in contrast with
graphic work-up. We could neither isolate nor identify such an intermediate from 2 or from 4. However, when 1 was reacted in a sealed NMR tube with one equivalent of PhNCO in dry CSD5N at 20°C we were able to obtain good 13C NMR evidence for a species tentatively described as an oxazamolybenacycle, represented here as two mesomerit forms :
Scheme 1. the results reported by Cramer et al. for Cp, U=CHPMe*Ph. 6 The generation of amides (or thioamides) from isocyanates (or isothiocyanates) and Grignard or organolithium reagents has been reported.7.8 This type of reactivity with transition metal 0x0 and imido complexes is unprecedented, although the formation of cyclohexyl mesityl amide by reacting &Hi *NC0 and a vanadium complex has been mentioned recently.’ Generally, the interaction of transition metal complexes with isocyanates leads to stable compounds resulting from insertion of RNCO into M-C, lo M-N, ’ I M=C6 or M=C” bonds. As far as 0x0 complexes are concerned differences in reactivity have been reported depending on the starting compound. Thus, 0x0 complexes either give imido species in a “classical way”, or adducts. ’ 3 An oxo-imido complex has been reported from the reaction between Mo(0)$12 ($(MeCN) 2 and Bu’NCO.‘~ In contrast, C5H&Mo0 reacts with PhNCO to form a cyclometallacarbamate, Mo(q5-C5H5)2{0C(0)N(Ph)), resulting from a formal [2 f 23 cycloaddition of the C=N bond to the 0x0 linkage. I5 In the present case, it is obvious that the reaction leading to the imido complex is very unfavourable, if not completely inoperative. Indeed, quantitative gas chromatography analysis of the atmosphere above the reaction mixture revealed the presence of CO, [less than 5% with a lo-fold excess of PhNCO vs l] and of CO (1 “A). Carbon monoxide formation could arise from the decomposition of PhNCO adducts, although this type of decomposition is very rare. I6 Even with an excess of phenyl isocyanate, a single isocyanate was inserted into the Mo-mesityl bond, as observed with other systems. ” We believe that the amide is formed by hydrolysis of an unstable intermediate during the chromato-
Two low-field signals at 187.3 and 181.3 ppm are, respectively, assigned to the carbonyl carbon atom and to the ipso-carbon of the q’-mesityl ligand. At higher field, four signals are observed at 27.95,21.8, 20.80 and 18.70 ppm. The peaks at 27.95 and 20.80 ppm are associated with the signal at 181.3 ppm. They are assigned, by analogy with the 13C NMR spectrum of 1, to the ortho- andpara-methyl groups of the rl ‘-mesityl ligand. The two other signals at 21.8 and 18.7 ppm are assigned to the para- and ortho-methyl groups, respectively, of the mesityl group in the q*-amide ligand. The ortho-methyl group is shielded as in phenyl mesityl amide due to the proximity of the carbonyl group. The assignment of the signal at 187.3 ppm to the carbonyl group is, furthermore, consistent with the spectra of other complexes with related structures.2b~12Formation of such a species resulting from the insertion of the isocyanate into the MO-C a-bond is certainly favoured by the polarization of the MO-C structure. ” This insertion occurs at the N=C bond rather than at the CL0 bond, reflecting the difference in bond strength between these two bonds.17 Depending on the structure of the starting complex, this reaction would be more or less easy as shown for the dioxophosphinomethylidene complex, 3. EXPERIMENTAL General
Preparations of complexes 1,” 2 Iv 3” and 45 have been described earlier. PhNCd and Bu’NCO were purchased from Janssen and distilled and stored under nitrogen. All operations were conducted under oxygen-free N2 by standard Schlenk techniques and solvents were distilled before use from sodium/benzophenone, except for pyridine which was distilled from KOH under N,. Anhy-
Isocyanates with dioxo- and diimidomolybdenum(V1) drous CH2C12 was kept on molecular sieves (4 A)
and degassed prior to use. NMR spectra were recorded on a Bruker spectrometer AM-200 and GC/MS coupling experiments were performed on a Ribernag R.10.10.c. apparatus. IR spectra were obtained using a FT-IR Nicolet MX-S spectrometer. Gas chromatography analyses were conducted on an Intersmat 1GC 120FB apparatus and an Intersmat ICR 1B integrator, on 5m x l/8” columns of 5% OV 17 on Chromosorb W. Gas chromatography of gases were carried out on an Intersmat 1GC 120MB using a thermal conductivity detector on 2m x l/8” columns of Prorapak N (CO,) or of molecular sieves, 3 A (CO). Benzamide was used as an internal standard for quantitative GLC analyses.
3.
4.
5.
6. 7.
8. Description
of a typical experiment
with isocyanates
9.
The complex (3.2 x lop3 mol) was dissolved in 10. tetrahydrofuran or pyridine (20 cm3) and after cooling at -30°C 0.36 cm3 (3.2 x lop3 mol) of PhNCO was added. The solution was allowed to warm slowly to room temperature and after 5 h stirring the reaction mixture was evaporated under 11. reduced pressure to a small volume and passed 12. down a silica gel column (0.063-0.2 mm, Merck). Elution of the amide was obtained with CH2C12. 13. For complex 3, after deposition of the reaction mixture on silica gel, 10 cm3 of H2S04 (5% v/v in H20) was passed down the column before CH2C12 14. elution. Acknowledgement-We wish to express our thanks to Professor Sir Geoffrey Wilkinson for helpful discussions.
15.
16.
REFERENCES (a) G. Fachinetti, C. Biran, C. Floriani, A. ChiesiVilla and C. Guastini, Znorg. Chem. 1978,17,2995 ;
17.
(b) S. Gambarotta, M. Pasquali, C. Floriani, A. Chiesi-Villa and C. Guastini, Znorg. Chem. 1981,20, 1173 ; (c) S. Gambarotta, S. Strologo, C. Floriani, A. Chiesi-Villa and C. Guastini, Znorg. Chem. 1985, 24, 654. (a) H. Hoberg, K. Radine and A. Milchereit, J. Organomet. Chem. 1985, 280, C60; (b) H. Hoberg,
18. 19.
20.
complexes
465
K. Summerman and A. Milchereit, J. Organomet. Chem. 1985,28?3,237. R. La&Advances in Metal Carbene Chemistry (Edited by U. Schubert), Nato Asi Series, Series C, Vol. 269, p. 171. Kluwer Academic Publishers, Dordrecht (1989) ; R. Lal, S. Le Bot and R. Faure, J. Organomet. Chem. 1987,336, C5. M. L. H. Green and K. J. Moynihan, Polyhedron 1986, 5, 921; W. A. Nugent and B. L. Haymore, Coord. Chem. Rev. 1980,31, 123. A. C. Sullivan, G. Wilkinson, M. Motavelli and M. B. Hursthouse, J. Chem. Sot., Dalton Trans. 1988, 53. R. E. Cramer, J. H. Jeong and J. W. Gilje, Organometallics 1987,6, 2010. Y. Zhang, J. Jiang and Y. Chen, Tetrahedron Lett. 1987,3815; Y. Zhang, J. Jiang and Z. Zhang, Tetrahedron Lett. 1988, 651. N. A. Lebel, R. M. Cherluck and E. A. Curtis, Synthesis 1973,678. M. Vivanco, J. Ruiz, C. Floriani, A. Chiesi-Villa and C. Guastini, Organometallics 1990,9, 2185. I. Tkatchenko, Comprehensive Organometallic Chemistry (Edited by G. Wilkinson, F. G. A. Stone and E. W. Abel), p. 743. Pergamon Press, Oxford (1982) ; J. D. Wilkins, J. Organomet. Chem. 1974, 65, 383. Y. Takahashi, N. Onoyama, Y. Ishikawa, S. Motojima and K. Sugiyama, Chem. Lett. 1978, 525. K. Weiss, U. Schubert and R. R. Schrock, Organometallics 1986. 5, 397. F. Bottomley and L. Sutin, Adv. Organomet. Chem. 1988,28,1988 ; S. F. Pedersen and R. R. Schrock, J. Am. Chem. Sot. 1982,104,7483. D. Ehrenfeld, J. Kress, B. D. Moore, J. A. Osborn and G. Schoettel, J. Chem. Sot., Chem. Commun. 1987,129. P. Jernakoff, G. L. Geoffroy, A. L. Rheingold and S. J. Geib, J. Chem. Sot., Chem. Commun. 1987, 1610. H. Hoberg, B. W. Oster, C. Kruger and Y. H. Tsay, J. Organomet. Chem. 1983,252, 365 ; J. G. Brennan and R. A. Andersen, J. Am. Chem. Sot. 1985, 107, 514. P. Jernakoff and N. J. Cooper, J. Am. Chem. Sot. 1987,109,2173. B. Heyn and R. Hoffmann, Z. Chem. 1976,16, 195. R. Lai, S. Le Bot, A. Baldy, M. Pierrot and H. Arzoumanian, J. Chem. Sot., Chem. Commun. 1986, 1208. H. Arzoumanian, A. Baldy, R. Lai, J. Metzger, M. L. Nkeng Peh and M. Pierrot, J. Chem. Sot., Chem. Commun. 1985, 1151.