was also maintained under bromination and mesylation12
conditions, although the resulting products 6d and 6e,
respectively, were obtained only in moderate to good yields
(entries 4 and 5).
1-Fluoro-1,3-butadiene has been extensively studied for
conformational stability,13 spectral characteristics,14 reactivity
in Diels-Alder reaction,15 and [2 + 2] cycloaddition.16 Its
substituted derivatives are less known, perhaps due to their
laborious synthesis. Our recent Pd-catalyzed preparation of
2-aryl- and 2-alkynyl-substituted 1,1-difluoro-1,3-buta-
dienes12 persuaded us to conduct a similar study with
monofluorovinyl iodide 7a. To our satisfaction, coupling 7a
with aryl boronic acids under Suzuki conditions afforded
substituted-1,3-butadienes 8a-d (entry 8), mostly in good
or very good yields, except when a strong electron-
withdrawing substituent was employed (8d). We have not
succeeded yet at elucidating the stereochemistry of the
fluorine-containing double bond in 8seither by scalar
coupling analysis or NOE experiments. Investigation is
ongoing. The spectral data of 8a-d showed that in each case
the chemical shifts of C-1 and H-1, as well as 1JHF and 1JCF
coupling constants, were in good agreement with the reported
values for E/Z 1-fluoro-1,3-butadiene.16 The chemical shift
of C-4 was assigned on the basis of a DEPT experiment,
but the 13C NMR signals of C-2 and C-3 are interchangeable
with the quaternary carbon of the aromatic ring bonded to
the diene moiety.
Interestingly, one sees allylic isomerization under certain
conditions. For example, treatment of 3a with methyl iodide
and Mitsunobu conditions produced the conjugated diene
isomer 7a (entry 6). Our hypothesis that an allene-containing
allylic halide (4-iodo-1,2-diene) (6, R ) CH2I) was generated
first via an SN2 mechanism but isomerized later to the
thermodynamically more stable 2-iodo-1,3-diene 7a was
confirmed by monitoring the progress of the reaction using
19F NMR. At room temperature, after a reaction time of only
5 min, the signal corresponding to the starting material (δ
-173) had decreased dramatically, and a new signal at δ
-176 ppm, believed to correspond to the 4-iodo-1,2-diene,
was observed, accompanied by two smaller signals of similar
intensity at δ -95 and -97, thought to correspond to E/Z
diastereomers of 7a.
As the reaction time lengthened, the intensity of the 19F
signals at δ -95 and -97 increased, while the δ -176 signal
decreased until its complete disappearance. The final signal
ratio of δ -95 to -97 was approximately 6:1. Decreasing
the temperature slowed the rate of isomerization, yet the
reaction did not take place at -78 °C. Alternatively, under
Finkelstein conditions (entry 7), a bromide attack on mesylate
6e produced a mixture of diene 7b and allene 6d in 3:1 ratio.
The fact that 6d was still present at the end of the reaction
discarded the possibility of a SN2′ allylic transposition,
suggesting instead an apparent SN2 course, followed by
partial allylic isomerization, as in 7a.
In sum, we have found a practical preparation of fluoro-
allenyl alcohol 3a and its derivatives such as aldehydes,
amines, halides, and aryl-substituted conjugated dienes. Their
chain extension reactions and metal-mediated cyclizations
are under study.
Acknowledgment. The Petroleum Research Fund (PRF
no. 36602-AC1) has partially financed this work. Drs. James
A. Golen (UMass Dartmouth) and A. Chandrasekaran
(UMass Amherst) supplied the crystallographic data, and Mr.
Qilong Shen provided helpful experimental advice.
(10) ORTEP view of 6c:
Supporting Information Available: Experimental pro-
cedures and spectroscopic and analytical data for 3a-8 and
CIF files for 6c. This material is available free of charge via
OL026196K
(13) Ribeiro-Claro, P. J. A. Chem. Phys. Lett. 1992, 188, 303-309.
(14) Cederbalk, P. Acta Chem. Scand., Ser. A 1980, A(34), 409-413.
Cederbalk, P. Acta Chem. Scand., Ser. A 1984, A(38), 15-21. Fueno, T.;
Yamaguchi, K. J. Am. Chem. Soc. 1972, 94, 1119-1125.
(15) Apeloig, Y.; Matzner, E. J. Am. Chem. Soc. 1995, 117, 5375-5376.
Imade, M.; Hirao, H.; Omoto, K.; Fujimoto, H. J. Org. Chem. 1999, 64,
6697-6701.
(11) For an analysis of bond lengths and angles in fluorinated allenes,
see: Dixon, D.; Smart, B. J. Phys. Chem. 1989, 93, 7772-7780.
(12) A similar but higher yielding SN2 mesylation was observed with
3b. See: Shen, Q.; Hammond, G. B. Org. Lett. 2001, 3, 2213-2215.
(16) Dolbier, W. R., Jr.; Gray, T. A.; Keaffaber, J. J.; Celewicz, L.;
Koroniak, H. J. Am. Chem. Soc. 1990, 112, 363-367.
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