910 J. Am. Chem. Soc., Vol. 118, No. 4, 1996
Communications to the Editor
equiv of PBH at 25 °C. This is a significant advance in
polyhalogenated methane addition to alkenes, as this reaction
can be effected at 25 °C. The products obtained from this
reaction are useful synthetic intermediates that can be converted
to R,â-unsaturated acids.10b Yields of CCl4 addition to a series
of alkenes are summarized in Table 1. The yields are good to
excellent. In all cases (where applicable), the CCl3 moiety
attaches itself to the terminal carbon of the alkene. The reaction
of trans-4-octene is especially interesting. Unlike the hydrobo-
ration of trans-4-octene, the internal chloro trichlormethylated
compound was obtained as a 2:1 mixture of diastereomers (eq
2).
prepared genuine pinacol-4-octylboronate (16) by hydroboration
of trans-4-octene with HBBr2‚SMe2, followed by treatment with
pinacol. Reaction of 16 with Rh(PPh3)3Cl or Rh(PPh3)3Cl/PBH
did not provide 1. Furthermore, rhodium-catalyzed hydrobo-
ration of cis-2-methyl-3-hexene furnished 8. It seems, therefore,
that with internal alkenes, rhodium-catalyzed hydroboration with
PBH behaves similarly to hydrozirconation of internal alkenes.17
In the case of cis-2-methyl-3-hexene, isomerization had occurred
at the nonbranched carbon of the chain. This contrasts with
the report in the literature for the enantioselective hydroboration
of cis-3-hexene with catecholborane, which gave the expected
internal boronate.18 We attribute the difference between PBH
and CBH in the hydroboration of internal alkenes to the much
larger steric requirements of pinacolborane, apparently resulting
in a fast â-hydride elimination/recomplexation sequence that
places rhodium on the least hindered carbon of the chain,
followed by a relatively slow boron insertion. Careful examina-
tion of the reaction mixture failed to reveal any PBH degradation
products, indicating that it is much more stable than CBH in
metal-catalyzed hydroborations. Degradation of catecholborane
is a major problem.15e Metal-catalyzed hydroboration of alkenes
with PBH requires 1.05 equiv of borane versus 1.5 equiv with
catecholborane to produce the same yield over an identical
period of time. The products of PBH hydroboration are air-,
moisture-, and chromatography-stable boronates, facilitating the
separation of isomers. It is well known that catecholboronates
are notoriously sensitive to moisture and cannot be chromato-
graphed. For these reasons, PBH is superior to CBH.
We next turned our attention to CCl4 addition to alkenes in
the presence of rhodium.19 As was the case in the zirconium-
boron system, Rh(PPh3)3Cl by itself was ineffective, and a
complex reaction mixture was obtained, dominated by oligo-
mers. However, Rh(PPh3)3Cl (1 mol %) in the presence of
catalytic amounts of PBH (10-15 mol %) gave an almost
quantitative yield of CCl4 addition product at 25 °C. Only trace
amounts of hydroborated products were observed. Results are
summarized in Table 1. Again, the final boron-containing
compound was B-chloropinacolboronate, which was completely
ineffective in catalyzing CCl4 addition to alkenes in the presence
of Rh(PPh3)3Cl. As was the case with the zirconium system,
various free radical inhibitors did not inhibit or slow the rate of
the reaction. It is premature to speculate on a mechanism,
especially regarding the role of PBH in the course of the
reaction.
Thus, hydrozirconation is apparently not the initial step in
CCl4 addition.11 Both galvinoxyl free radical and butylated
hydroxytoluene did not inhibit the reaction.12 In addition,
zirconium hydride and boron hydride are required.13 Cp2ZrCl2/
PBH, HZrCp2Cl/R3B, HZrCp2Cl/(RO)3B, PBH, and R3B or
(RO)3B were ineffective in catalyzing the CCl4 addition to
alkenes. The boron-containing species identified at the end of
the CCl4 reaction is B-chloropinacolboronate.14 The final
zirconium product is ZrCp2Cl2. However, use of HZrCp2Cl or
ZrCp2Cl2 and B-chloropinacolboronate in the CCl4 addition to
1-octene did not result in any product. The mechanism,
therefore, is complex and may involve a non free radical
pathway; if a free radical mechanism does occur, it is a step in
the catalytic process that does not involve the alkene.9,10,12,13
Boron-Rhodium. Metal-catalyzed hydroboration is gener-
ally done with catecholborane (CBH).15 However, 1.5 equiv
of CBH is required due to degradation during the course of the
reaction.15e It has been reported that hydroboration with PBH
is not catalyzed by rhodium.5 Having successfully used PBH
in conjunction with zirconium, it appeared to us that PBH should
serve as a suitable substrate in rhodium-catalyzed hydroboration
of alkenes.15 Indeed, this has now proven to be the case. As
little as 0.2 mol % Rh(PPh3)3Cl is effective. The reaction is
very fast (10 min at 25 °C) and essentially quantitative in
methylene chloride. Aqueous workup of the water- and
chromatography-stable pinacolboronates gave almost pure prod-
uct (Table 1). As is the case with CBH, metal-catalyzed
hydroboration of styrene with PBH gave a mixture of boronates
(C1, 35%; C2, 50%; and a â-vinyl boronate, 15%).16 Two
internal alkenes were investigated. Surprisingly, 1 was also
obtained when rhodium was used as the catalyst in the
hydroboration of trans-4-octene (Table 1) in CH2Cl2. We
In summary, we have developed two systems consisting of
metal/PBH. In hydroboration, PBH is used stoichiometrically.
In CCl4 addition, PBH is used catalytically, and the reaction
occurs at ambient temperature. Hydroboration of alkenes
furnishes terminal boronates. Chloro trichloromethylation, on
the other hand, provides the internal addition products. We have
also clarified the literature surrounding PBH and predict that
PBH will enter organic synthesis alongside such venerable
hydroborating reagents as CBH and 9-BBN.
(11) Addition of 5 mol % HZrCp2Cl to the alkene, followed by the
addition of PBH after 2 h, leads to results identical to those obtained under
the conditions in Table 1. Other transition metal catalysts that we tried in
the hydroboration of 1-octene are Rh(CO) (PPh3)2Cl (80% 1) and Ru(PPh3)3-
Cl (0% 1).
(12) Hydroquinone was shown not to inhibit the rate of addition of CX4
to alkenes: Suzuki, T.; Tsuji, J. J. Org. Chem. 1970, 35, 2982.
(13) However, in a study using RuH3(SiMe2Ph)(PPh3)3 and RuH2(PPh3)4,
no enhanced catalytic activity for haloalkene addition to alkenes was
observed: Davis, R.; Furze, J. D.; Cole-Hamilton, D. J.; Pogorzelec, P. J.
Organomet. Chem. 1992, 440, 191.
(14) We prepared B-chloropinacolboronate from pinacol and BCl3
according to the procedure of: Gennari, C.; Colombo, L.; Poli, G.
Tetrahedron Lett. 1984, 25, 2279. 11B NMR (CDCl3): δ +28.2.
(15) (a) Ma¨nnig, D.; No¨th, H. Angew. Chem., Int. Ed. Engl. 1985, 25,
878. (b) For a review, see: Burgess, K.; Ohlmeyer, M. J. Chem. ReV. 1991,
91, 1179. (c) Bijpost, E. R.; Duchateau, R.; Teuben, J. H. J. Mol. Catal. A:
Chem. 1995, 121. (d) With lanthanides, see: Harrison, K. N.; Marks, T. J.
J. Am. Chem. Soc. 1992, 114, 9220. (e) For mechanistic studies and
degradation of catecholborane, see: Wescott, S. A.; Blom, H. P.; Marder,
T. B.; Baker, R. T.; Calabrese, J. C. Inorg. Chem. 1993, 32, 2175. (f) New
rhodium catalysts for hydroboration: Westcott, S. A.; Blom, H. P.; Marder,
T. B.; Baker, R. T. J. Am. Chem. Soc. 1992, 114, 8863.
Acknowledgment. We thank the University of Toledo for support
of this work. Special thanks to Dr. Craig Blankenship of Boulder
Scientific for providing generous quantities of Cp2ZrCl2.
Supporting Information Available: General experimental proce-
dures and representative spectra for compounds in Table 1 (39 pages).
This material is contained in many libraries on microfiche, immediately
follows this article in the microfilm version of the journal, can be
ordered from the ACS, and can be downloaded from the Internet; see
any current masthead page for ordering information and Internet access
instructions.
JA952653Z
(17) Hart, D. W.; Schwartz, J. J. Am. Chem. Soc. 1974, 96, 8115.
(18) Sato, M.; Miyaura, N.; Suzuki, A. Tetrahedron Lett. 1990, 31, 231.
(19) RhBr(CO)(PMe3)2 was reported to catalyze CBrCl3 addition to
1-octene: Cable, C. J.; Adama, H.; Bailey, N. A.; Crosby, J.; White, C. J.
Chem. Soc., Chem. Commun. 1991, 165. In our hands, RhCl(CO)(PMe3)2/
PBH gave <10% addition product with CCl4 and 1-octene, while Ru(PPh3)3-
Cl was completely ineffective.
(16) (a) For a study of selectivity of hydroboration of vinylarenes with
rhodium catalysts, see: Burgess, K.; Donk, van der, W.; Westcott, S. A.;
Marder, T. B.; Baker, R. T.; Calabrese, J. C. J. Am. Chem. Soc. 1992, 114,
9350. (b) Westcott, S. A.; Marder, T. B.; Baker, R. T. Organometallics
1993, 12, 975.