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Migration of alkyl groups from zirconium to various haloboranes
Tetrahe&on Letters, Vol. 33, No. 37, pp. 5295-5298, 1992 Printed in Great Britain
0040-4039/92 $5.00 + .00 PergamonPress Ltd
Migration of Alkyl Groups from Zirconium to Various Haloboranes Thomas E. Cole*, Stephan Rodewald and Cynthia L. Watson Department of Chemistry San Diego State University San Diego, California 92182-0328
Key Words: hydrozirconation;isomerization;transmetallation;or&anoboranes;organozirconiuras Abstract: The hydrozirconations of acyclic alkeues are known to yield the dicyclopentadienyl-1alkyI-zirconium chlorides regioselectively and in high yields. These alkyl groups may readily be transmetallated to the haloboranes forming regioisomeric pure alkylboranes in good yields. In principle, this migration reaction combines the facile hydrozirconation and unique tsomerization reactions of organozirconiums with the established chemistry of boron, expanding the utility of both metals for use in organic synthesis. Hydrozirconation is one of the more promising organometallic reactions for use in organic synthesis. Schwartz and co-workers demonstrated this utility in a series of papers during the 1970's. 1-5 Although these organozirconiums showed much promise, their inability to undergo carbon-carbon bond formation necessitated the exploration of migrations of organic groups from the zirconium to metals whose carbon-carbon bond forming reactions were well established. 5,6 This transmetallation was shown to readily occur for more electronegative metals such as aluminum, 5.g copper,9,10 palladium, 9 and mercury. II Surprisingly, migration to boron was not explored by Schwartz and co-workers. There is only one previously reported example of a migration of an alkyl group from zirconium, using the related dimethyl complex Cp2Zr(CH3)2, to BH3.THF.I2 The hydrozirconation reaction was recognized to have a number of Similarities to the hydroboration reaction. The zirconium hydride readily undergoes a cis addition across carbon-carbon double and triple bonds placing the zirconium at the least sterically hindered position. The reaction of electrophiles with organozirconiums is similar to that of the organoboranes, although there are some differences. Organoboranes isomerize at temperatures generally greater than i50 °C, however organozirconiums isomerize at room temperature or lower. 13,14 This facile isomerization is more selective than that of the organoboranes. The isomerization occurs rapidly at room temperature, placing the zirconium at the terminal end of an alkyl chain, provided the migration is not blocked by a tertiary or quaternary carbon. 1,5,15 Unfortunately, this promising reaction has limited applications, partly due to the restricted number of reactions that organozirconiums can undergo to form new products. More recently, related organozirconiums have been actively investigated by a number of research groups and a large number of potentially useful reactions mediated by zirconium have been developed. 16-19 We report here the migration of l-hexyl as a representative alkyl group for the migration of alkyl groups from zirconium to haloboranes. The dicyclopentadienyl-1-hexylzirconium chloride 20 is readily prepared by the hydrozirconation of 1hexene using Schwartz's reagent. 21 The regioselectivity of this reaction was determined as previously reported by reacting with N-bromosuccinimide and subsequent analysis of the bromohexane by gas chromatrography. The regioisomeric purity was found to be greater than 99.8% 1-bromohexane. 4 We have previously reported the transmetallation of 1-alkenyl groups from zirconium to a variety of haloboranes with complete retention of both regio- and stereochemistry. 22 The migration of the 1-alkenyl group 5295
5296
proceeds to completion within 2 h at room temperature. Schwartz reported that alkyl group migration from zirconium to aluminum is significantly slower than for alkenyl groups, thus we expected the alkyl groups to migrate slowly from zirconium to boron.5 In our initial reactions we chose B-chlorocatecholborane because it had given high yields in the transrnetallafion of the 1-hexenyl group from zirconium to boron.22 One equivalent of B-chlorocatecholborane was added to the 1-hexylzirconocene chloride at room temperature. The 8hexylcatecholborane forms as the only boron containing material observed in the lIB NMR specu'um, while the zirconocene dichloride slowly precipitates. B-bromocatecholborane also reacted in a similar fashion, but with lower conversion, forming 70% of the boronic ester. Other di'alk0xyhaloboranes, chlorodimethoxyborane, Bchlorodiisopropoxyborane and B-chloro-l,3,2-dioxoborinane showed little reaction, forming less than 10% product. In a representative alkyl tranm~etallation reaction, 3.0 mmol of a 1.0 M solution of the haloborane in methylene chloride was added to an oven-dried round-bottom flask fitted with a magnetic stirring bar and gasinlet adaptor under a nitrogen atmosphere. After cooling to 0 °C a methylene chloride solution of dicyclopentadienyl- 1-bexylzirconium chloride was added via syringe. The reaction was stirred for the indicated time, followed by removal of the insoluble zirconocene dihalide to afford the organoborane product. The lrihaloboranes all readily reacted with the l-hexylzirconocene chloride. Both boron trichloride and boron tribromide cleanly gave the 1-hexyldihaloborane within 2 h after the addition of 1 equiv of the zirconium complex at 0 °C. However, both boranes yielded a mixture of hexylhaloboranes when reacted with 2 equiv of the organozirconium at 0 °C. Similar selectivities were obtained at -25 °C. The best results were found by mixing the reagents at -78 °C and slowly warming to 0 °C over a period of 2 h to give 90% of the dibexylchloroborane and approximately 10% other boron containing materials. The addition of 3 or more equiv cleanly formed the tri-n-hexylborane. Boron trifluoride etherate was not expected to accept the 1-hexyl group since this haloborane was found to be completely unreactive with the more reactive alkenyizirconium complexes.22 To our surprise we found that the transmetallafion did occur. The reaction with one equiv of the 1-hexylzirconocene chloride yielded the dihexylfluoroborane, with the remainder as starting material as observed in the lIB NMR spectrum. There was no indicadon of any other boron species formed in this reaction. Two equivalents solely formed the dialkyiflnoroborane, which yielded 73% 1-hexanol after oxidation. A slight excess of 3 equiv gave the Irihexylborane as the only observable species in the IIB NMR spectra. The results of the transmetallation of 1-hexyl groups to trihaloboranes are summarized in Table 1. Table L Transmetallation to Trihaloborane~ BX 3 + y CP2Z~C1)-l-hexyl Borane
y Equiv
CH2C12 ~ 2h
Temp, °C
1-hexyiyBX3.y + y CP2Zr(CI)X $ Product
Yield, %
BFyOEt2 1 0 (n-hexyl)2BF + BF3OEt2 71 b BFyOEt2 2 0 (n-bexyl)2BF 73 b BFyOEt2 3.5 0 (n-hexyl)3B 100a BCI3 1 0 (n-hexyl)BCl2 77b BCI3 2 0 Mixture 77 b BCI3 2 -78 ~ 0 (n-hexyl)2BCl 90a BCI3 3.8 0 (n-hexyl)3B 100a BBr3 1 0 (n-hexyi) BCl2 78* BBr3 2 0 Mixture 74 a BBr3 3.5 0 (n-hexyl)3B 100a a Yieldsdeaenninedby liB NMR integration. b GC ananlysisof 1-hexanolfromIh¢oxidationof the organoborane,undecanousedas internal standard.
5297
Alkyldihaloboranes and dialkylhaloboranes were also examined for transmetallation with 1-hexylzirconocene chloride. The reaction of n-octyldibromborane with I equiv of the 1-hexylzirconocene chloride yielded the dialkylbromoborane in an approximate 80% conversion within 2 h at 0 °C. This result is in contrast to the reaction of 2 equiv of 1-hexylzirconocene chloride with BBr3 at the same conditions, which formed a mixture of products as shown in Table 1. The hexylzirconium complex underwent essentially quantitative reaction with the phenyldibromoborane, forming a mixture of phenyldihexylborane (14%), phenylhexylbromoborane (67%) and starting material (19%). B-bromo-, B-choro-9-BBN and dicyclohexylbromoborane are also good acceptors, forming the trialkylboranes in yields of 63 to 74%. B-chlorodiisopinocampheylborane did not show indications of any reaction, presumably due to the larger steric hinderance of the large isopinocampheyl groups. These transmetallations are best carded out at 0 °C, reaction at room temperature give reduced yields. The alkylzirconocenes unlike the organoboranes readily react with H20 or the HX formed from the hydrolysis of a haloborane, to give the corresponding alkane. These hydrolysis products are the single other major product formed in this reaction as analyzed by GC and GC-MS. The reactions of these organohaloboranes are summarized in Table 2. These transmetallations appear to proceed by electrophilic exchange between the Lewis acidic haloboranes and alkylzirconocene chlorides.5 Therefore, the presence of a Lewis base such as dimethylsulfide, may be expected to complex with either the borane or zirconium, inhibiting the exchange. This was demonstrated by the reaction of bromo-9-BBN with the 1-hexylzirconocene chloride in the presence of 1 equiv of dimethylsulfide. In the absence of the sulfide the reaction was complete within 2 h, whereas in the presence of dimethylsulfide the reaction proceeds to only approximately 50% after 24 h. Table 2. Transmetallation to Organohaloboranes Borane
Alkene
n-OctylBBr2 PhBBr2
1-hexene 1-hexene
j
1-hexene
B--Cl
Product n-OctylB(n-hexyl)Br mixture ~
B
~
Time, h Temp, °C Yield, % 2 2
0 0
82a,79b >95 b
48
25
40 a
~B'"-~"
1-hexene ~ B ~
2
0
63b
[~2
1-hexene
24
0
74 b
a Yieldsdeterminedby I1B NMR integration. b GC ananlysisof l-hexanolfrom the oxidationof the organoborane,undecaneusedas internalslandard. One of the most attractive features of the alkylzirconiums is their facile isomerization. We report here two examples of this unique isomerization. First, an equal molar mixture of 1-, 2- and 3-hexene was hydrozirconated with Schwartz's reagent in THF, followed by transmetallation to boron trichloride. The hexyldichloroborane was then oxidized with hydrogen peroxide under alkaline conditions to afford an 84% yield of 1-hexanol, as confirmed by gas chromatography and GC-MS. We were unable to detect any isomeric hexanol within the limits of detectability, <0.5%. Therefore, the regiochemical purity of the product was determined to be greater than 99.5%. The hydrozirconation of 2-methyl-2-pentene and migration to trichloroborane further illustrates this potentially useful reaction. Oxidation of the organoborane yielded 4methyl- 1-pentanol as the single product. These reactions are summarized in Table 3.
5298
Table 3. Hydrozirconation/Isomerization and Transmetallation to Boron Trichloride Borane
BCI3
BCI3
Alkene
~
~
Product
Time, h
Temp, °C Yield, %
C I 2 B ~
2
0
84 b
Ci2B~J,,,~
24
0
68a
a Yieldsdeterminedby liB NMR integration. b GC ananlysisof I-hexanolfromthe oxidationof the organoborane,undecaneusedas internal standard.
Acknowledgement. We thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
References and Notes 1. Schwartz, J.; Hart, D. W. J. Am. Chem. Soc. 1974, 96, 8115. 2. Schwartz, J.; Labinger, J. A. Angew. Chem. Int. Ed. Engl. 1976, 15, 333. 3. Hart, D. W.; Blackburn, T. F.; Schwartz, J. J. Am. Chem. Soc. 1975, 97, 680. 4. Bertelo, C. A.; Schwartz, J. J. Am. Chem. Soc. 1976, 98, 262. 5. Carr, D. B.; Schwartz, J. J. Am. Chem. Soc. 1979, 101, 3521. 6. Carr, D. B.; Yoshifuji, M.; Shoer, L. I.; Gell, K. I.; Schwartz, J. Annals New York Acad. Sci. 1977, 295, 127. 7. Cart, D. B.; Schwartz, J. J. Am. Chem. Soc. 1977, 99, 638. 8. Negishi, E.; Yoshida, T. Tetrahedron Lett. 1980, 21, 1501. 9. Yoshifuji, M.; Loots, M. J.; Schwartz, J. Tetrahedron Lett. 1977, 37, 1303. 10. Lipshutz, B. H.; Ellsworth, E. J. Am. Chem. Soc. 1990, 112, 7440. 11. Budnik, R. A.; Kochi, J. K. J. Organomet. Chem. 1976, 116, C3. 12. Marsella J. A.; Caulton, K.G.J. Am. Chem. Soc. 1982, 104, 2361. 13. Brown, H. C.; Zwiefel, G. J. Am. Chem. Soc. 1966, 88, 1433. 14. Brown, H. C.; Zwiefel, G. J. Am. Chem. Soc. 1967, 89, 561. 15. Labinger, J. A.; Hart, D. W.; Sielbert, W. F.; Schwartz, J. J. Am. Chem. Soc. 1975, 97, 3851. 16. Negishi, E.; Takahashi, T. Synthesis 1988, 1. 17. Douglas, R. S.; Negishi, E. Organometallics 1991, 10, 825. 18. Buchwaid, S. L.; Nielsen, R. B. Chem. Rev. 1988, 88, 1047. 19. Cuny, G. D.; Gutierrez, A.; Buchwald, S. L. Organometallics 1991, 10, 537. 20. The 1-hexylzirconocene chloride was prepared by the addition of 1.05 equiv of 1-hexene to a mixture of Schwartz's reagent in methylene chloride under a nitrogen atmosphere at room temperature. After stirring 5 rain, the insoluble zirconium hydride completely dissolved to yield a yellow homogenous solution, 21. Buchwald, S. L.; LaMaire, S. J.; Nielsen, R. B.; Watson, B. T.; King, S. M. Tetrahedron Lett. 1987, 28, 3895. 22. Cole, T. E.; Quintanilla, R.; Rodewald, S. Organometallics 1991, 10, 3777.
(Received in USA 20 December 1991; accepted 16 June 1992)
0040-4039/92 $5.00 + .00 PergamonPress Ltd
Migration of Alkyl Groups from Zirconium to Various Haloboranes Thomas E. Cole*, Stephan Rodewald and Cynthia L. Watson Department of Chemistry San Diego State University San Diego, California 92182-0328
Key Words: hydrozirconation;isomerization;transmetallation;or&anoboranes;organozirconiuras Abstract: The hydrozirconations of acyclic alkeues are known to yield the dicyclopentadienyl-1alkyI-zirconium chlorides regioselectively and in high yields. These alkyl groups may readily be transmetallated to the haloboranes forming regioisomeric pure alkylboranes in good yields. In principle, this migration reaction combines the facile hydrozirconation and unique tsomerization reactions of organozirconiums with the established chemistry of boron, expanding the utility of both metals for use in organic synthesis. Hydrozirconation is one of the more promising organometallic reactions for use in organic synthesis. Schwartz and co-workers demonstrated this utility in a series of papers during the 1970's. 1-5 Although these organozirconiums showed much promise, their inability to undergo carbon-carbon bond formation necessitated the exploration of migrations of organic groups from the zirconium to metals whose carbon-carbon bond forming reactions were well established. 5,6 This transmetallation was shown to readily occur for more electronegative metals such as aluminum, 5.g copper,9,10 palladium, 9 and mercury. II Surprisingly, migration to boron was not explored by Schwartz and co-workers. There is only one previously reported example of a migration of an alkyl group from zirconium, using the related dimethyl complex Cp2Zr(CH3)2, to BH3.THF.I2 The hydrozirconation reaction was recognized to have a number of Similarities to the hydroboration reaction. The zirconium hydride readily undergoes a cis addition across carbon-carbon double and triple bonds placing the zirconium at the least sterically hindered position. The reaction of electrophiles with organozirconiums is similar to that of the organoboranes, although there are some differences. Organoboranes isomerize at temperatures generally greater than i50 °C, however organozirconiums isomerize at room temperature or lower. 13,14 This facile isomerization is more selective than that of the organoboranes. The isomerization occurs rapidly at room temperature, placing the zirconium at the terminal end of an alkyl chain, provided the migration is not blocked by a tertiary or quaternary carbon. 1,5,15 Unfortunately, this promising reaction has limited applications, partly due to the restricted number of reactions that organozirconiums can undergo to form new products. More recently, related organozirconiums have been actively investigated by a number of research groups and a large number of potentially useful reactions mediated by zirconium have been developed. 16-19 We report here the migration of l-hexyl as a representative alkyl group for the migration of alkyl groups from zirconium to haloboranes. The dicyclopentadienyl-1-hexylzirconium chloride 20 is readily prepared by the hydrozirconation of 1hexene using Schwartz's reagent. 21 The regioselectivity of this reaction was determined as previously reported by reacting with N-bromosuccinimide and subsequent analysis of the bromohexane by gas chromatrography. The regioisomeric purity was found to be greater than 99.8% 1-bromohexane. 4 We have previously reported the transmetallation of 1-alkenyl groups from zirconium to a variety of haloboranes with complete retention of both regio- and stereochemistry. 22 The migration of the 1-alkenyl group 5295
5296
proceeds to completion within 2 h at room temperature. Schwartz reported that alkyl group migration from zirconium to aluminum is significantly slower than for alkenyl groups, thus we expected the alkyl groups to migrate slowly from zirconium to boron.5 In our initial reactions we chose B-chlorocatecholborane because it had given high yields in the transrnetallafion of the 1-hexenyl group from zirconium to boron.22 One equivalent of B-chlorocatecholborane was added to the 1-hexylzirconocene chloride at room temperature. The 8hexylcatecholborane forms as the only boron containing material observed in the lIB NMR specu'um, while the zirconocene dichloride slowly precipitates. B-bromocatecholborane also reacted in a similar fashion, but with lower conversion, forming 70% of the boronic ester. Other di'alk0xyhaloboranes, chlorodimethoxyborane, Bchlorodiisopropoxyborane and B-chloro-l,3,2-dioxoborinane showed little reaction, forming less than 10% product. In a representative alkyl tranm~etallation reaction, 3.0 mmol of a 1.0 M solution of the haloborane in methylene chloride was added to an oven-dried round-bottom flask fitted with a magnetic stirring bar and gasinlet adaptor under a nitrogen atmosphere. After cooling to 0 °C a methylene chloride solution of dicyclopentadienyl- 1-bexylzirconium chloride was added via syringe. The reaction was stirred for the indicated time, followed by removal of the insoluble zirconocene dihalide to afford the organoborane product. The lrihaloboranes all readily reacted with the l-hexylzirconocene chloride. Both boron trichloride and boron tribromide cleanly gave the 1-hexyldihaloborane within 2 h after the addition of 1 equiv of the zirconium complex at 0 °C. However, both boranes yielded a mixture of hexylhaloboranes when reacted with 2 equiv of the organozirconium at 0 °C. Similar selectivities were obtained at -25 °C. The best results were found by mixing the reagents at -78 °C and slowly warming to 0 °C over a period of 2 h to give 90% of the dibexylchloroborane and approximately 10% other boron containing materials. The addition of 3 or more equiv cleanly formed the tri-n-hexylborane. Boron trifluoride etherate was not expected to accept the 1-hexyl group since this haloborane was found to be completely unreactive with the more reactive alkenyizirconium complexes.22 To our surprise we found that the transmetallafion did occur. The reaction with one equiv of the 1-hexylzirconocene chloride yielded the dihexylfluoroborane, with the remainder as starting material as observed in the lIB NMR spectrum. There was no indicadon of any other boron species formed in this reaction. Two equivalents solely formed the dialkyiflnoroborane, which yielded 73% 1-hexanol after oxidation. A slight excess of 3 equiv gave the Irihexylborane as the only observable species in the IIB NMR spectra. The results of the transmetallation of 1-hexyl groups to trihaloboranes are summarized in Table 1. Table L Transmetallation to Trihaloborane~ BX 3 + y CP2Z~C1)-l-hexyl Borane
y Equiv
CH2C12 ~ 2h
Temp, °C
1-hexyiyBX3.y + y CP2Zr(CI)X $ Product
Yield, %
BFyOEt2 1 0 (n-hexyl)2BF + BF3OEt2 71 b BFyOEt2 2 0 (n-bexyl)2BF 73 b BFyOEt2 3.5 0 (n-hexyl)3B 100a BCI3 1 0 (n-hexyl)BCl2 77b BCI3 2 0 Mixture 77 b BCI3 2 -78 ~ 0 (n-hexyl)2BCl 90a BCI3 3.8 0 (n-hexyl)3B 100a BBr3 1 0 (n-hexyi) BCl2 78* BBr3 2 0 Mixture 74 a BBr3 3.5 0 (n-hexyl)3B 100a a Yieldsdeaenninedby liB NMR integration. b GC ananlysisof 1-hexanolfromIh¢oxidationof the organoborane,undecanousedas internal standard.
5297
Alkyldihaloboranes and dialkylhaloboranes were also examined for transmetallation with 1-hexylzirconocene chloride. The reaction of n-octyldibromborane with I equiv of the 1-hexylzirconocene chloride yielded the dialkylbromoborane in an approximate 80% conversion within 2 h at 0 °C. This result is in contrast to the reaction of 2 equiv of 1-hexylzirconocene chloride with BBr3 at the same conditions, which formed a mixture of products as shown in Table 1. The hexylzirconium complex underwent essentially quantitative reaction with the phenyldibromoborane, forming a mixture of phenyldihexylborane (14%), phenylhexylbromoborane (67%) and starting material (19%). B-bromo-, B-choro-9-BBN and dicyclohexylbromoborane are also good acceptors, forming the trialkylboranes in yields of 63 to 74%. B-chlorodiisopinocampheylborane did not show indications of any reaction, presumably due to the larger steric hinderance of the large isopinocampheyl groups. These transmetallations are best carded out at 0 °C, reaction at room temperature give reduced yields. The alkylzirconocenes unlike the organoboranes readily react with H20 or the HX formed from the hydrolysis of a haloborane, to give the corresponding alkane. These hydrolysis products are the single other major product formed in this reaction as analyzed by GC and GC-MS. The reactions of these organohaloboranes are summarized in Table 2. These transmetallations appear to proceed by electrophilic exchange between the Lewis acidic haloboranes and alkylzirconocene chlorides.5 Therefore, the presence of a Lewis base such as dimethylsulfide, may be expected to complex with either the borane or zirconium, inhibiting the exchange. This was demonstrated by the reaction of bromo-9-BBN with the 1-hexylzirconocene chloride in the presence of 1 equiv of dimethylsulfide. In the absence of the sulfide the reaction was complete within 2 h, whereas in the presence of dimethylsulfide the reaction proceeds to only approximately 50% after 24 h. Table 2. Transmetallation to Organohaloboranes Borane
Alkene
n-OctylBBr2 PhBBr2
1-hexene 1-hexene
j
1-hexene
B--Cl
Product n-OctylB(n-hexyl)Br mixture ~
B
~
Time, h Temp, °C Yield, % 2 2
0 0
82a,79b >95 b
48
25
40 a
~B'"-~"
1-hexene ~ B ~
2
0
63b
[~2
1-hexene
24
0
74 b
a Yieldsdeterminedby I1B NMR integration. b GC ananlysisof l-hexanolfrom the oxidationof the organoborane,undecaneusedas internalslandard. One of the most attractive features of the alkylzirconiums is their facile isomerization. We report here two examples of this unique isomerization. First, an equal molar mixture of 1-, 2- and 3-hexene was hydrozirconated with Schwartz's reagent in THF, followed by transmetallation to boron trichloride. The hexyldichloroborane was then oxidized with hydrogen peroxide under alkaline conditions to afford an 84% yield of 1-hexanol, as confirmed by gas chromatography and GC-MS. We were unable to detect any isomeric hexanol within the limits of detectability, <0.5%. Therefore, the regiochemical purity of the product was determined to be greater than 99.5%. The hydrozirconation of 2-methyl-2-pentene and migration to trichloroborane further illustrates this potentially useful reaction. Oxidation of the organoborane yielded 4methyl- 1-pentanol as the single product. These reactions are summarized in Table 3.
5298
Table 3. Hydrozirconation/Isomerization and Transmetallation to Boron Trichloride Borane
BCI3
BCI3
Alkene
~
~
Product
Time, h
Temp, °C Yield, %
C I 2 B ~
2
0
84 b
Ci2B~J,,,~
24
0
68a
a Yieldsdeterminedby liB NMR integration. b GC ananlysisof I-hexanolfromthe oxidationof the organoborane,undecaneusedas internal standard.
Acknowledgement. We thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
References and Notes 1. Schwartz, J.; Hart, D. W. J. Am. Chem. Soc. 1974, 96, 8115. 2. Schwartz, J.; Labinger, J. A. Angew. Chem. Int. Ed. Engl. 1976, 15, 333. 3. Hart, D. W.; Blackburn, T. F.; Schwartz, J. J. Am. Chem. Soc. 1975, 97, 680. 4. Bertelo, C. A.; Schwartz, J. J. Am. Chem. Soc. 1976, 98, 262. 5. Carr, D. B.; Schwartz, J. J. Am. Chem. Soc. 1979, 101, 3521. 6. Carr, D. B.; Yoshifuji, M.; Shoer, L. I.; Gell, K. I.; Schwartz, J. Annals New York Acad. Sci. 1977, 295, 127. 7. Cart, D. B.; Schwartz, J. J. Am. Chem. Soc. 1977, 99, 638. 8. Negishi, E.; Yoshida, T. Tetrahedron Lett. 1980, 21, 1501. 9. Yoshifuji, M.; Loots, M. J.; Schwartz, J. Tetrahedron Lett. 1977, 37, 1303. 10. Lipshutz, B. H.; Ellsworth, E. J. Am. Chem. Soc. 1990, 112, 7440. 11. Budnik, R. A.; Kochi, J. K. J. Organomet. Chem. 1976, 116, C3. 12. Marsella J. A.; Caulton, K.G.J. Am. Chem. Soc. 1982, 104, 2361. 13. Brown, H. C.; Zwiefel, G. J. Am. Chem. Soc. 1966, 88, 1433. 14. Brown, H. C.; Zwiefel, G. J. Am. Chem. Soc. 1967, 89, 561. 15. Labinger, J. A.; Hart, D. W.; Sielbert, W. F.; Schwartz, J. J. Am. Chem. Soc. 1975, 97, 3851. 16. Negishi, E.; Takahashi, T. Synthesis 1988, 1. 17. Douglas, R. S.; Negishi, E. Organometallics 1991, 10, 825. 18. Buchwaid, S. L.; Nielsen, R. B. Chem. Rev. 1988, 88, 1047. 19. Cuny, G. D.; Gutierrez, A.; Buchwald, S. L. Organometallics 1991, 10, 537. 20. The 1-hexylzirconocene chloride was prepared by the addition of 1.05 equiv of 1-hexene to a mixture of Schwartz's reagent in methylene chloride under a nitrogen atmosphere at room temperature. After stirring 5 rain, the insoluble zirconium hydride completely dissolved to yield a yellow homogenous solution, 21. Buchwald, S. L.; LaMaire, S. J.; Nielsen, R. B.; Watson, B. T.; King, S. M. Tetrahedron Lett. 1987, 28, 3895. 22. Cole, T. E.; Quintanilla, R.; Rodewald, S. Organometallics 1991, 10, 3777.
(Received in USA 20 December 1991; accepted 16 June 1992)