Allylboration of alkenes with allyldihaloboranes

Full text of DOI:10.1021/ja961616k

9986
J. Am. Chem. Soc. 1996, 118, 9986-9987
Table 1. Allylborations with Allyldihaloboranes
Allylboration of Alkenes with Allyldihaloboranes
Daniel A. Singleton,* Stephen C. Waller, Zongren Zhang,
Doug E. Frantz, and Shun-Wang Leung
Department of Chemistry, Texas A&M UniVersity
College Station, Texas 77843
ReceiVed May 14, 1996
Chemists have exerted considerable effort to develop carbo-
metalations of alkenes that would complement the well-
developed areas of hydrometalations (e.g., hydroboration) or
organometal additions to carbonyls. There has been great
success with electron-deficient alkenes (e.g., cuprate chemistry)
and isolated alkynes,1 but much less general progress has been
made with the carbometalation of unactivated and electron-rich
alkenes. Examples include polymerizations, intramolecular
cyclizations,2 additions to allylic alcohols or amines,3 and
uncatalyzed4 and catalyzed5 additions to terminal and highly
strained alkenes.6 Carbometalations with allylic reagents are
often more facilessimple allyl Grignards and zincs can undergo
metallo-ene additions to monosubstituted and strained alk-
enes.4,7 Due to problems with selectivity and efficiency, these
reactions have found best utility in intramolecular examples.8
Allylic boranes can also effect carbometalations of alkynes and
specialized alkenes.9 However, the low reactivity of the
allylboranes employed (usually triallylborane) has been a
critically limiting feature. Triallylborane reacts rapidly with
alkoxyacetylenes and methylcyclopropene, but much more
slowly with alkynes and allenes in reactions troubled by the
instability of the initial products. Vinyl ethers are also
moderately reactive with triallylborane, forming 1,4-dienes at
110-140 °C in variable yields.10
We report here the carbometalation of alkenes with allyldiha-
loboranes. These electrophilic boranes are highly reactive
allylborating reagents and react with electron-rich alkenes regio-
and stereospecifically in high yields.
The previously unreported allyldichloroborane (1) can be
generated in hexanes solution by the reaction of BCl3 with
allyltributyltin (0 °C, <15 min), as indicated by the clean
appearance of 1H NMR peaks at δ 5.85 (d of d of t, 1 H), 5.02
(d, 1 H), 4.99 (d, 1 H), and 2.40 (br d, 2 H). Although 1 could
not be isolated, it decomposes only slowly in solution over the
course of 2-3 days at 25 °C. Methallyldichloroborane (2) was
generated similarly. Crotyldichloroborane (3) was generated
as a undetermined mixture of cis and trans isomers (based on
(1) Normant, J. F.; Alexakis, A. Synthesis 1981, 841. Negishi, E. Acc.
Chem. Res. 1987, 20, 65-72.
(2) Shaughnessy, K. H.; Waymouth, R. M. J. Am. Chem. Soc. 1995,
117, 5873-4. Negishi, E.; Jensen, M. D.; Kondakov, D. Y.; Wang, S. J.
Am. Chem. Soc. 1994, 116, 8404. Zhang, Y.; Negishi, E. J. Am. Chem.
Soc. 1989, 111, 3454.
(3) Richey, H. G., Jr.; Erickson, W. F.; Heyn, A. S. Tetrahedron Lett.
1971, 48, 2183-2186. Eisch, J. J.; Merkley, J. H.; Galle, J. E. J. Am. Chem.
Soc. 1979, 101, 1148-1155. See also: Hoveyda, A. H.; Xu, Z.; Morken,
J. P.; Houri, A. F. J. Am. Chem. Soc. 1991, 113, 8950.
a Unless otherwise noted, the reaction was carried out at 25 °C for
the indicated time. The substrate was added to a solution of the indicated
allylborane at either -45 or 0 °C before warming to 25 °C. b See ref
12 for stereochemical assignments. c Worked up by addition of excess
NaOH or pyridine or triethylamine and then stirring with aqueous
NaHCO3. d The crude product was benzoylated with benzoyl chloride/
pyridine. e Worked up by treatment with H2O2/excess NaOH. f Worked
up by treatment with anhydrous NaOAc followed by refluxing in HOAc
for 1 h.
(4) Lehmkuhl, H. Bull. Chim. Soc. Fr. II 1981, 87-95.
(5) Dzhemilev, U. M.; Vostrikova, O. S. J. Organomet. Chem. 1985,
285, 43-51. Hoveyda, A. H.; Xu, Z. J. Am. Chem. Soc. 1991, 113, 5079.
Takahashi, T.; Seki, T.; Nitto, Y.; Saburi, M.; Rousset, C. J.; Negishi, E. J.
Am. Chem. Soc. 1991, 113, 6266-6268.
(6) Nakamura, E.; Nakamura, M.; Miyachi, Y.; Koga, N.; Morokuma,
K. J. Am. Chem. Soc. 1993, 115, 99.
(7) Lehmkuhl, H. In Organometallics in Organic Synthesis; De Meijere,
A., Dieck, T., Eds.; Springer-Verlag: New York, 1987; pp 185-202.
Nakamura, M.; Arai, M.; Nakamura, E. J. Am. Chem. Soc. 1995, 117, 1179-
1180. Yamamoto, Y.; Asao, N. Chem. ReV. 1993, 93, 2207-2293.
(8) Oppolzer, W. Angew. Chem., Int. Ed. Engl. 1989, 28, 38-52.
(9) For reviews, see: Mikhailov, B. M. Russ. Chem. ReV. 1976, 45, 557.
Bubnov, Y. N. Pure Appl. Chem. 1987, 59, 895.
a complex 1H NMR pattern centered at δ 5.5), with no
observable 3-boryl-1-butene isomer, from either purified trans-
crotyltributylstannane or an ∼1:1:1 mixture of trans,cis, and
3-buten-2-yl isomers. The NMR observation of allyldibro-
moborane (4) from the reaction of BBr3 with allylic stannanes
(10) Mikhailov, B. M.; Bubnov, Y. N. Zh. Obshch. Khim. 1971, 41, 2039.
S0002-7863(96)01616-2 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 41, 1996 9987
had been previously reported,11 although 4 was most conve-
niently generated from allyltrimethylsilane (-20 °C, 15 min).
The allylborations of vinyl and silyl enol ethers readily afford
1,4-dienes. These can be understood as arising from an initial
allylboration followed by elimination of the â-alkoxy- or
â-(silyloxy)borane. The allylboration of dihydropyran (17) with
1-3 afforded purely the cis products, consistent with a syn
allylboration (as observed above) followed by an anti elimina-
tion. The allylboration of phenylacetylene also produced a 1,4-
diene after protodeboronation. The instability of adducts
obtained from the allylboration of alkynes with triallylborane9
is not a problem here, and the reaction with 1 is much more
efficient.
While the dienylsilane 15 reacts readily and regiospecifically
with 1 and 2, simple dienes failed to react faster than the
decomposition of the allyldichloroboranes. However, the more
electrophilic 4 appears to be much more reactive. In a promising
initial result, the reaction of 4 with 1,3-cyclohexadiene afforded
24 regiospecifically after oxidation.
The most striking observation was the facility of the allylbo-
rations of allylic silanes. This was discovered when the reaction
of BCl3 with allyltrimethylsilane, an independent synthesis of
1, afforded 6 after oxidative workup. Although 1 could be
observed as an intermediate by NMR, it appeared to allylborate
the allyltrimethylsilane to form 5 at a rate comparable to that
of its formation.
Ab initio calculations provide several insights into these
reactions. RHF calculations with a 6-31G* basis set predict
the transition structure 2514 for the reaction of 1 with ethylene.
The chairlike cyclic six-membered transition structure is
consistent with the allylically rearranged products observed with
3 (cf, 8 and 19) and with the stereospecificity of the reactions.
The addition is highly asynchronous, and 25 is polarized. The
advanced bonding of C1 with B results in a relatively high
positive charge on C2 compared to ethylene (0.21 e difference
in Mulliken charges). Thus, donor substituents on C2 accelerate
the reaction and control the regiochemistry.
The allylborations of allylic silanes are uniformly regiospe-
cific, affording after oxidation alcohols derived from addition
of the boryl group distally to the silyl group (Table 1).12,13 The
additions to (trimethylsilyl)cyclohexene and (trimethylsilyl)-
cylopentene (9 and 11) in each case produce a single isomeric
product, derived from a syn allylboration, anti to the silyl group.
The reaction of pure cis-crotyltrimethylsilane (13) with 2
afforded a single isomeric product, while a 47:53 cis/trans
mixture of crotylsilanes afforded a 47:53 mixture of the product
from the pure cis and a diastereomeric product. Together, these
results indicate the stereospecificity of the reaction.
(11) Harston, P.; Wardell, J. L.; Marton, D.; Tagliavini, G.; Smith, P. J.
Inorg. Chim. Acta 1989, 162, 245-50.
(12) The stereochemistry of 12 (R ) Me) was assigned from an X-ray
crystal structure of the corresponding 3,5-dinitrobenzoate, and 12 (R ) H)
was assigned from a similarity of its 1H NMR spectrum to that of 12 (R )
Me). The stereochemistry of 10 was assigned from an H2-H3 coupling of
12.4 Hz and the lack of any large coupling to H1 (all <5 Hz). For 18 (R
) H), 18 (R ) Me), and 19, the complex AB pattern in the 1H NMR of the
internal double bond could be simulated using a vinylic vicinal coupling
constant of 10 Hz, indicative of the Z stereochemistry. The structure of 18
(R ) H) was further confirmed by hydrolysis to the known alcohol
(Andrews, G. D. Macromolecules 1984, 17, 1624-1626). The stereochem-
istry of 24 was assigned from a similarity of its NMR spectrum to that of
the known cis-2-methyl-3-cyclohexen-1-ol (Singleton, D. A.; Martinez, J.
P.; Watson, J. V.; Ndip, G. M Tetrahedron 1992, 48, 5831). The
stereochemistry of 14 could only be speculatively assigned on the basis of
the stereospecificity of the reaction (see text) and the observation of syn
addition in the other reactions.
(13) In a typical procedure, 1.47 g (4.3 mmol) of allyltributyltin was
added slowly to 3.5 mL (3.5 mmol) of a 1.0 M solution of BCl3 in hexanes
at -78 °C, and the mixture was warmed to 25 °C for 30 min. The mixture
was then cooled to -45 °C, and 0.325 g (2.0 mmol) of 9 was added. The
reaction was then allowed to warm slowly to 25 °C and stirred for 18 h. In
the workup there were added successively 5 mL of 3 M aqueous NaOH, 5
mL of 30% aqueous H2O2, and 5 mL of THF to the cooled mixture, which
was then stirred at 25 °C overnight. After a normal ethereal extraction,
chromatography of the residue on a 0.75 in. × 14 in. silica gel column
with 6% EtOAc in hexanes as eluent afforded 0.295 g (70%) of 10.
The combination of the high reactivity and selectivity of
allyldihaloboranes with the versatility of the borane adducts
should significantly extend the range of products available from
carbometalations of alkenes.
Acknowledgment. We thank the NIH and The Robert A. Welch
Foundation for support of our programs.
JA961616K
(14) E ) -1138.21609. Structure 25 is fully optimized and exhibited
one imaginary frequency. The predicted activation energy is 31.0 kcal/mol.
For comparison calculations on simple ene reactions, see: Loncharich, R.
J.; Houk, K. N. J. Am. Chem. Soc. 1987, 109, 6947.