J . Org. Chem. 1997, 62, 4827-4828
4827
P (CH3NCH2CH2)3N: A Non ion ic Su p er ba se for Efficien t
Deh yd r oh a logen a tion
Subramaniam Arumugam and J ohn G. Verkade*
Department of Chemistry, Iowa State University, Ames, Iowa 50011
Received December 10, 1996X
The commercially available nonionic superbase P(CH3NCH2CH2)3N (1) is far superior to DBU for
the conversion of primary and secondary alkyl halides to alkenes. A reason for the efficacy of
acetonitrile as a solvent for the halides requiring extended reaction times is presented.
In tr od u ction
identical conditions are listed in Table 1. Also listed in
Table 1 are conversions of 3-19 to the corresponding
olefins as measured by 1H NMR spectroscopy using DBU.
Except for substrate 19, which afforded no detectable
product with either 1 or DBU, the isolated yields (85-
98%) of the products formed with the remaining 16
substrates in the presence of 1 exceeded the conversions
using DBU (0-87%) by moderate to very substantial
margins. In Table 1 it is seen that alkyl halides pos-
sessing an electron-withdrawing group â to the halogen
(3-9) gave high isolated yields of olefins with 1 in 5 min
without detectable side reactions. High stereoselectivity
was achieved upon HBr elimination from 7 since only
trans-bromostilbene was formed in 92% yield. The
presence of the activating substituents at the â carbon
atom in 3-9 apparently facilitates rapid E2 elimination
in the presence of strong nonionic bases. Evidence for
the formation of 2 (X ) Br) in these rapid reactions with
1 came from an NMR experiment involving p-nitro-
(bromoethyl)benzene (3) in CD3CN. Thus, the proton
originally bound to the substrate became bound to the
phosphorus in 2, giving a doublet centered at 5.32 ppm
and a one-bond P-H coupling constant of 494.4 Hz, which
is characteristic of this cation.8 That this reaction does
not require a polar solvent was shown by employing C6D6
as a solvent instead of CD3CN. The conversion to
p-nitrostyrene was again 98%, and the formation of 2 was
In our ongoing search1 for new synthetic applications
of the commercially available proazaphosphatrane (1)
first synthesized in our laboratories, we have discovered
that 1 is a superior dehydrohalogenation agent for
primary and secondary alkyl halides. The non-nucleo-
philic yet strongly basic 1 is easily regenerated from its
solid product 2 by treatment with KO-t-Bu.2 The intro-
duction of double bonds into organic systems via the
elimination of hydrogen halides is a well-known trans-
formation3 that has been applied to the synthesis of a
variety of substrates including prostaglandins,4 vitamin
A,5 and polyenes.6 Because typical organic bases, such
as Et3N, N,N-dimethylaniline, pyridine, and quinoline
are often unsatisfactory for such reactions, DBN and
DBU have become prominent owing to their non-nucleo-
philic nature and greater basicity.6,7 The efficacy of the
latter bases was observed over 30 years ago in the
dehydrohalogenation of an intermediate in the synthesis
of vitamin A.5 However, dehydrohalogenations with
DBU or DBN often require heating and excess reagent
(2 or more equiv), and yields are often not high. Here,
we report the efficient dehydrohalogenation of alkyl
halides using only 1.1 equiv of 1 at room temperature in
CH3CN.
1
verified by its characteristic H NMR spectrum.8
Rather than olefin formation as the final product from
10, cyclization to a cyclopropane ring in nearly quantita-
tive yield was observed. Rapid cyclization was also the
case for 11 and 12 wherein the presence of acidic protons
facilitate epoxide and lactone formation, respectively.
Substrates such as 13-15 required more time to give
high yields of the corresponding alkenes. The existence
of an electron-releasing substituent at the â carbon in
17 prevents elimination in the presence of DBU, whereas
1, which is ca. 17 pKa units more basic than DBU,9 reacts
to give an 85% yield of product, albeit after 72 h.
It is of interest to speculate on a possible dehydroha-
logenation pathway for substrates requiring extended
reaction times in the presence of 1 in acetonitrile.
Earlier, we reported that 1 slowly abstracts a proton from
CD3CN to give the deuterio analogue of 2 and -CD2CN
in an equilibrium reaction.1d Because the deuterio
analogue of 2 was the only phosphorus-containing com-
pound isolated in the reaction of 1 with substrates 13-
Resu lts a n d Discu ssion
The isolated yields of olefins obtained in reactions of
1.1 equiv of 1 with a variety of alkyl halides 3-19 under
X Abstract published in Advance ACS Abstracts, J une 15, 1997.
(1) (a) D’Sa, B. A.; Verkade, J . G. J . Am. Chem. Soc., in press. (b)
D’Sa, B. A.; Verkade, J . G. J . Org. Chem. 1996, 61, 2963. (c) Tang, J .
S.; Verkade, J . G. Angew. Chem., Int. Ed. Engl. 1993, 32, 896. (d) Tang,
J . S.; Verkade, J . G. J . Org. Chem. 1994, 59, 7793. (e) Tang, J . S.;
Verkade, J . J . Org. Chem. 1996, 61, 8750.
(2) Tang, J . S.; Verkade, J . G. Tetrahedron Lett. 1993, 36, 2903.
(3) Baciocchi, E. In The Chemistry of Functional Groups, Supplement
D; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1983; Part 2, p
1173.
(4) Martel, J .; Toromanoff, E.; Mathieu, J .; Nomine, G. Tetrahedron
Lett. 1972, 1491.
(5) (a) Oediger, H.; Kabbe, H. J .; Mo¨ller, F.; Eiter, K. Chem. Ber.
1966, 99, 2012. (b) Oediger, H.; Eiter, K. German Patent 1157606,
1963; Chem. Abstr. 1964, 60, 5569.
(6) Oediger, H.; Mo¨ller, F.; Eiter, K. Synthesis 1972, 591.
(7) Hermecz, I. Adv. Heterocycl. Chem. 1987, 42, 100.
(8) Schmidt, H.; Lensink, C. S. K.; Verkade, J . G. Z. Anorg. Allg.
Chem. 1989, 578, 75.
(9) Tang, J .-S.; Dopke, J .; Verkade, J . G. J . Am. Chem. Soc. 1993,
115, 5015.
S0022-3263(96)02310-9 CCC: $14.00 © 1997 American Chemical Society