3127
Organometallics 1996, 14, 3127-3128
Hydroboration of Alkynes with Pinacolborane Catalyzed by HZrCpzCl Schubert Pereira and Morris Srebnik" Department of Chemistry, University of Toledo, Toledo, Ohio 43606 Received April 18, 1995@ Summary: Pinacolborane (1.05 equiv) hydroborates both terminal and internal alkynes, i n the presence of 5 mol % zirconocene chloride hydride (HZrCpzCl) at 25 "C, in excellent yields and with excellent regio- and stereoselectivity. Without HZrCpzCl and under otherwise identical conditions, hydroboration proceeds to a n extent of only 2-20%. Catecholborane (CBH),l like other dioxaborolane or dioxaborinane2 hydrides, is a sluggish hydroborating reagent, requiring elevated temperatures t o assure an adequate rate of hydroboration of alkynes and alkenes. In 1985 Mannig and Noth reported that Rh(PPh&Cl catalyzes hydroboration of alkenes with CBH at ambient temperature and in the process different chemo- and regioselectivity was ~ b s e r v e d . ~Mechanistically, an oxidative-additiodreductive-elimination mechanism was p ~ s t u l a t e d . In ~ 1987 the rhodium-catalyzed hydrobo~ 1988 ration of acetylene by borazine was r e p ~ r t e d .In the enantioselective version of this reaction was reported,6 and in the same year diastereoselective studies with this reagent system were initiated.7 In 1992 organolanthanides were reported to catalyze hydroboration of alkenes with CBH.* A different mechanism was presented. Metal-catalyzed hydroboration of alkynes or alkenes with other dioxaborolane or dioxaborinane hydrides apparently has not been in~estigated.~ However, in many instances the preferred boronic ester is pinacol since these esters are stable to aqueous workup and chromatography.1° Pinacol boronates can be prepared by hydroboration with HBBrz-SMez, followed by hydrolysis and esterification of the boronic ester with pinacol, or by transesterification.ll However, HBBrz is quite Lewis acidic and may not be compatible with acidsensitive groups. Recently, Knochel reported the preparation and reactivity of pinacolborane (PBH; 1).lzHy@Abstractpublished in Advance ACS Abstracts, June 1, 1995. ( 1 ) (a)Brown, H. C.; Gupta, S. K. J. Am. Chem. SOC.1976,97,5249. (b) Ibid. 1972, 94, 4370. (2) (a)Woods, W. G.; Strong, P. L. J . Am. Chem. Soc. 1966,88,4667. (b) Fish, R. H. J. Am. Chem. SOC.1968,90, 4435. (3) Miinnig, D.; Noth, H. Angew. Chem., Int. Ed. Engl. 1986, 25, 878. (4)The oxidative-addition product with Wilkinson's catalyst was isolated: Kono, H.; Ito, K. Chem. Lett. 1975, 1095. (5) Lynch, A. T.; Sneddon, L. G. J. Am. Chem. SOC.1987,109,5867. (6) Burgess, K.; Ohlmeyer, M. J. J. Org. Chem. 1988, 53, 5178. (7)Evans, D. A.; Fu, G. C.; Hoveyda, A. H. J.Am. Chem. SOC.1988, 110,6917. (8) Harrison, K. N.; Marks, T. J . J. A m . Chem. Soc. 1992,114,9220. (9)For a review, see: Burgess, K; Ohlmeyer, M. J. Chem. Rev. 1991, 91, 1179. (10)For a review of the chemistry of organoboranes, see: (a) Matteson, D. S. In The Chemistry ofthe Metal-Carbon Bond; Hartley, F. R., Ed.; Wiley: Chichester, U.K., 1987; Vol 4, p 307. (b) Pelter, A.; Smith, K.; Brown, H. C. Borane Reagents. In Best Synthetic Methods; Katritzky, A. R., Meth-Cohn, O., Rees, C. W., Eds.; Academic Press: London, 1988. (11) Brown, H. C.; Bhat, N. G.; Somayaji, V. Organometallics 1983, 2, 1311. (12)Tucker, C. E.; Davidson, J.; Knochel, P. J . Org. Chem. 1992, 57, 3482.
Table 1. Hydroboration of Alkynes with PBH in the Presence of HZrCp&la entry no.
1 2 3 4 5 6 7 8 9 10
11 12 13
2a 2b 2c 2d 2e 2f 2g 2h 2i 2j 2k 21
Ri CHdCHd5-
R2
PhPh(CHd3-
H H H H H H H
i -Pr-
CH3-
Me&-
CH3Et-
Cl(CHd3Me3Sicyclopentyl
Me&-
Et-
(Et0)zCHMeOCH22m Me3SiCzCCH2-
H
H H
product ratiob 3:4:6:6 yield' (%)
98:2:0:0 96.8:2.4:0:0.8 90.3:2.2:3.2:4.3 97.5:0.7:0:1.8
1oo:o:o:o
97.2:0.8:0.7:0.9 98.3:1.7:0:0 96.9:2.2:0:0.9 1oo:o:o:o
1oo:o:o:o
81.9:10.8:7.3:0 95.0:2.5:2.5:0 96.3:2.5:0.8:0.4
93.1 94.3 86.7 93.7 95.2 74.7 87.2 93.8 91.5 92.6 82.2 86.8 81.5
For general experimental conditions, see ref 14. Determined by GC on a 30 m methylsilicone column and by integration of the corresponding methyl peaks in the 400 MHz 1H NMR spectra. Isolated yields.
droboration of alkynes under Knochel's conditions requires 2 equiv of PBH. In the course of our studies on the reactions of borazirconocenes, we observed very facile boron migrations (apparently involving PBH) that suggested zirconium-mediated catalysis.13 In this communication we present our results on HZrCpzClcatalyzed hydroboration of alkynes with PBH.14 When 1-octyne (1 equiv) was added to a solution of PBH (2 M, 1.05 equiv, CHzClz) containing 5 mol % HZrCpzCl, an orange color developed over time (2 h) that slowly disappeared. After the mixture was stirred for 24 h a t 25 "C, aqueous workup afforded 3a (Table 1, entry 1)in 93% yield as essentially pure material.15(The same reaction without HZrCpzCl afforded 3a in 2% yield when PBH prepared in situ was used or in 20% yield when PBH distilled and isolated was utilized in the hydroboration.) The sequence is outlined in Scheme 1. In addition to the desired alkenylboronates 3,other products were detected. The products of anti addition, 4, were produced in small quantities. Indeed, syn (13) Pereira, S.; Srebnik, M. Unpublished results. (14)For other catalytic reactions involving zirconium, see: (a) Negishi, E.; Takahashi, T. ACC.Chem. Res. 1994,27, 124. (b) Dzhemilev, U.M.; Vostrikova, 0. S.; Ibragimov, A. G. Russ. Chem. Rev.(Engl. Transl.) 1986, 55, 66. (15) General experimental procedure for the hydroboration of alkynes by pinacolborane in the presence of HZrCpsCl: A 2 mL round-bottom flask equipped with a side arm and stirring bar is charged with alkyne (0.5 mmol) and CHZClz (0.25 mL), under a n inert atmosphere, and cooled to 0 "C. Pinacolborane (0.525 mmol, 0.067 g) is then added dropwise via a syringe. After the mixture is stirred for 1 min, the reaction mixture is transferred by a double-ended needle to another 2 mL flask, immersed in an ice bath, containing CpzZrHCl (0.025 mmol, 6.5 mg). After this mixture is warmed to ambient temperature, stirring is continued for 16 h, at which point the reaction is quenched with HzO (0.5 mL). The mixture is extracted with ether (2 x 5 mL), washed with water (3 x 5 mL), dried with anhydrous MgSOd (0.5 g), filtered, and analyzed by GC. Isolation of products is achieved by removing volatile matter under reduced pressure followed by chromatography on a silica gel column if necessary.
0276-733319512314-3127$09.0010 0 1995 American Chemical Society
Communications
3128 Organometallics, Vol. 14,No. 7, 1995
Scheme 1. Hydroboration of Alkynes with PBH Catalyzed by HZrCpzCl
Scheme 2. Proposed Cycle for HZrCpzCI-Catalyzed Hydroboration of Alkynes with Pinacolborane
Cp2ZrHCI (5 mol YO) -2
1
CH2CI,
H
selectivity in the present case is among the best observed for hydroboration16and is comparable to the results obtained with the uncatalyzed reaction.12 Regioisomers 5 were occasionally observed, but in negligible quantities. The exception is 5k (7.3%). This may be due to coordination of the hydroborating reagent with the oxygen atoms of the acetal.17 Nevertheless, the results indicate that the reaction is compatible with acid-sensitive functional groups. Hydrogenation products 6 were sometimes detected, but generally in very small amounts.18 Certainly, the lack of side products demonstrates the benefits associated with PBH over catechol, which often is accompanied by side products.lg The reaction works well for a variety of alkynes, both terminal and internal. Of importance is that only a slight excess of PB-H is required. The results are summarized in Table 1. Zirconocene dichloride was ineffective as a hydroboration catalyst and, indeed, appeared to retard the reaction.20 Cole has demonstrated that vinyl groups from 1-alkenylzirconocenechlorides readily transfer to (16)For a review, see: Zaidlewicz, M. In Comprehensive Organometallic Chemistrv: Wilkinson. G.. Stone. F. G. A,. Abel. E. W.. Eds.: Pergamon Press: "Oxford, U.K., 1982;Vol. 7,p 161. (17)(a) Brown, H. C.; Unni, M. K. J . Am. Chem. SOC.1968,90,2902. For additional examples of directive effects in hydroboration, see: (b) 1983,105, 2489. (c) Still, W. C.; Barrish, J. C. J. Am. Chem. SOC. Harada, T.; Matsuda, Y.; Wada, I.; Uchimura, J.; Oku, A. J. Chem. SOC. Chem. Commun. 1990,21. (18)Compounds 6 presumably arise from hydrolysis of the corresponding gem-borazirconocenealkanes: (a) Zheng, B.; Srebnik, M. J. Organomet. Chem. 1994,474,49.(b) Deloux, L.;Skrzypczak-Jankun, E.; Cheesman, B. V.; Sabat, M.; Srebnik, M. J. Am. Chem. SOC. 1994, 116, 10302. (19)(a)For a discussion of the side reactions that occur in transitionmetal-catalyzed hydroborations, see ref 9, p 1181.(b) Hydroboration of 2a with 1.1 equiv of catecholborane in the presence of 5 mol % HZrCpzC1, using our general experimental procedure, gave products 3-6 in the ratio of 94.0:3.4:2.6:0, in 92% yield. Thus, syn selectivity is not as good as with PBH. Catecholboronates are water-sensitive and partially decompose on column chromatography. For the above reasons, PBH seems to be the reagent of choice in this reaction. (20)Teuben has recently demonstrated that other zirconocene compounds, Le., CpzZrMez, are not effective catalysts in the hydroboration of alkenes with CBH: Bijpost, E. A,; Duchateau, R.; Teuben, J. H. J . Mol. Catal. 1995,95,121.
ZrCp2Cl
B-chlorocatecholborane.21 IlB NMR of an equimolar mixture of zirconocene dichloride and PBH in CH~Clz showed no change in the chemical shift or multiplicity of PBH over the course of 24 h.22 PBH, therefore, does not reduce zirconocene chloride under our conditions. Mechanistically, these observations are consistent with initial hydrozirconation of the alkyne to give a transient alkenylzirconocene chloride. Vinyl and hydride exchange then occur to give the product and regenerate HZrCpzCl in a manner not unlike the mechanism of lanthanide-catalyzed hydroboration of alkenes with catecholborane proposed by Markse7 A proposed cycle is outlined in Scheme .ZZ3 In conclusion, pinacolborane may be used in stoichiometric quantities to hydroborate alkynes at 25 "C by the novel use of HZrCpzCl as a catalyst. syn selectivity and regioselectivity are excellent. This intriguing catalysis is being further developed in our laboratories on related and other systems. Acknowledgment. S.P. thanks the University of Toledo for a teaching fellowship. We thank Dr. Dean Giolando for useful discussions. Supporting InformationAvailable: Text giving general experimental details and figures giving lH NMR and 13CNMR spectra for pinacolboronates 3a-m (34 pages). Ordering information is given on any current masthead page.
OM9502749 (21)Cole, T. E.; Quintanilla, R.; Rodewald, S. Organometallics 1991, 10, 3777.
(22)Catecholborane: llB NMR (CD3C1, 128.3 MHz) b 27.9 (d, J = 175 Hz)(cf. ref 12). (23)Indeed, hydrozirconation of the alkyne with 5 mol 3 ' HZrCpzC1 followed by hydroboration with PBH yielded results identical with those obtained by using the protocol described in the general experimental procedure, in support of the contention that hydrozirconation is the first step.
