observed under the conditions used. We believe that the ratios of
the (Z)-vinyl formate 2a to the saturated aldehyde 3a might
depend on the leaving ability of the ammonio groups, which
will increase in the order HN+Pri2 < HN+Et2 < HN+Me2
<
HN+(Me)Ar. These hypotheses are in a good agreement with
the observed ratios shown in Table 1.
Base-induced a-eliminations generating free alkylidene car-
benes are a feasible process for b-alkylvinyliodonium salts.10,11
The a-elimination pathway to yield the alkyne 4a, which was
established by analysis of the deuterium content (88%) of
[1-2H]dec-1-yne obtained by the reaction of b-deuterated 1a
with N,N-diisopropylformamide at 50 °C, would compete with
the vinylic SN2 reaction leading to the formation of 2 and 3. The
ratios of the vinylic SN2 reaction versus the a-elimination
shown in Table 1 clearly indicate that sterically demanding N,N-
diisopropylformamide and N,N-dicyclohexylformamide will
lead to steric retardation of the SN2 pathway, which, in turn,
results in formation of a large amounts of the alkyne 4.
In general, bimolecular nucleophilic substitutions are fa-
voured when a good nucleophile is present; however, for-
mamides with rather low nucleophilicity undergo the vinylic
SN2 reaction under mild conditions. The origin of this unusual
reaction is attributable to the hyper-leaving group ability of the
phenyliodonio group.
Scheme 2
Notes and references
† Selected data for 2a: dH(400 MHz, CDCl3) 0.88 (t, J 6.3, 3H), 1.2–1.44
(m, 12H), 2.16 (q, J 6.9, 2H), 4.99 (dt, J 7.5 and 6.9, 1H), 7.07 (d, J 7.5, 1H),
8.08 (s, 1H); dC(100 MHz, CDCl3) 14.1, 22.7, 24.5, 29.1, 29.2, 29.3, 29.4,
31.9, 116.1, 132.7, 158.0; nmax(CHCl3)/cm21 2920, 2850, 1725, 1665,
1460, 1380, 1170, 910; m/z (EI) 184 (M+, 1%), 138 (4), 110 (8), 96 (19), 82
(38), 68 (32), 57 (100); HRMS calc. for C11H20O2 (M+), 184.1463, found
184.1470.
reaction is highly stereoselective and no formation of the (E)-
vinyl formate 2f was detected by GC and 400 MHz H NMR.
The saturated aldehyde 3a and the terminal alkyne 4a were
obtained as minor products in 12 and 5% yields.
1
N,N-Diethylformamide and N-formyl cyclic amines, i.e. N-
formyl-pyrrolidine, -piperidine and -morpholine, similarly gave
the (Z)-vinyl formate 2a as a major product but with an
increased amount of the aldehyde 3a (entries 2–5). It is noted
that the use of aromatic formamides such as N-formyl-N-
methylanilines resulted in reactions almost free of the byproduct
3a (entries 8–10). In contrast, the reaction course was
dramatically changed when sterically demanding N,N-diisopro-
pylformamide was used and a large amount of dec-1-yne 4a
(52%) was produced via elimination. With N,N-dicylohex-
ylformamide, this elimination is the sole detectable pathway.
Acetamides also act as good nucleophiles in this reaction; for
instance, treatment of 1a with N,N-dimethylacetamide at 50 °C
for 6 h afforded stereoselectively (Z)-dec-1-enyl acetate in 62%
yield, along with formation of 3a (26%) and 4a (7%).
The concerted vinylic SN2 pathways are highly sensitive to
the nature of b-substituents of the vinyliodonium salts and are
inhibited or retarded when (E)-b-tert-butylvinyl- 1d and (E)-b-
phenylvinyliodonium salt 1e were used in the reaction with
halides.6,8 This tendency was also kept in the reaction of
iodonium salts 1d and 1e with DMF at 50 °C, which afforded no
SN2 products 2d and 2e (entries 17 and 18). Reaction of the (Z)-
isomer with DMF resulted in extensive anti b-elimination to
give 4a in 80% yield, as reported previously.6
Scheme 2 illustrates a possible mechanism for formation of
the (Z)-vinyl formate 2a and the aldehyde 3a from (E)-
vinyliodonium salt 1a by the reaction with DMF. Formamide on
treatment with alkyl tosylates undergoes exclusive O-alkylation
with no N-alkylation;9 therefore, it seems reasonable to assume
that the oxygen atom of DMF selectively attacks the (E)-
vinyliodonium salt 1a to produce the inverted (Z)-O-vinylimi-
donium salt 6 (Vilsmeier–Haack salt) via the vinylic SN2
transition state 5, which was stabilized by delocalization of the
nitrogen lone-pair electrons. The hyper-leaving group ability of
the phenyliodonio group would be responsible for the vinylic
SN2 reaction.7 Subsequent attack of water will produce N-
protonated tetrahedral species 7, which collapses to 2a with the
ammonio group being released, or O-protonated tetrahedral
species 8, which collapses to 3a with DMF being released.
The formate 2a and the aldehyde 3a seem to be kinetic
products, since neither isomerization of the double bond of 2a
yielding 2f nor hydrolysis of 2a to the saturated aldehyde 3a was
1 F. Nakatsubo, Y. Kishi and T. Goto, Tetrahedron Lett., 1970, 381; L.
Syper, Tetrahedron, 1987, 43, 2853.
2 P. Magnus and G. Roy, J. Chem. Soc., Chem. Commun., 1978, 297.
3 For reviews of organoiodanes, see: G. F. Koser, in The Chemistry of
Functional Groups, Supplement D, Wiley, New York, 1983, ch. 18; M.
Ochiai and Y. Nagao, J. Synth. Org. Chem. Jpn., 1986, 44, 660; R.
Moriarty and O. Prakash, Acc. Chem. Res., 1986, 19, 244; M. Ochiai,
Rev. Heteroatom Chem., 1989, 2, 92; Y. Kita, H. Tohma and T. Yakura,
Trends Org. Chem., 1992, 3, 113; A. Varvoglis, The Chemistry of
Polycoordinated Iodine, VHC, New York, 1992; G. F. Koser, in The
Chemistry of Functional Groups, Supplement D2, Wiley, New York,
1995, ch. 21; T. Kitamura, J. Synth. Org. Chem. Jpn., 1995, 53, 893; P. J.
Stang and V. V. Zhdankin, Chem. Rev., 1996, 96, 1123.
4 For reviews of nucleophilic vinylic substitutions, see: Z. Rappoport,
Acc. Chem. Res., 1981, 14, 7; Z. Rappoport, Recl. Trav. Chim. Pays-
Bas, 1985, 104, 309.
5 For theoretical studies, see: D. R. Kelsey and R. G. Bergman, J. Am.
Chem. Soc., 1971, 93, 1953; M. N. Glukhovtsev, A. Pross and L.
Radom, J. Am. Chem. Soc., 1994, 116, 5961; V. Lucchini, G. Modena
and L. Pasquato, J. Am. Chem. Soc., 1995, 117, 2297.
6 M. Ochiai, K. Oshima and Y. Masaki, J. Am. Chem. Soc., 1991, 113,
7059; T. Okuyama, T. Takino, K. Sato and M. Ochiai, J. Am. Chem.
Soc., 1998, 120, 2275; T. Okuyama T. Takino, K. Sato, K. Oshima, S.
Imamura, H. Yamataka, T. Asano and M. Ochiai, Bull. Chem. Soc. Jpn.,
1998, 71, 243.
7 For leaving group ability of the aryliodonio groups, see: M. Ochiai, in
Chemistry of Hypervalent Compounds, ed. K. Akiba, Wiley-VCH, New
York, 1999, ch. 12; T. Okuyama, T. Takino, T. Sueda and M. Ochiai,
J. Am. Chem. Soc., 1995, 117, 3360.
8 T. Okuyama and M. Ochiai, J. Am. Chem. Soc., 1997, 119, 4785; T.
Okuyama, H. Oka and M. Ochiai, Bull. Chem. Soc. Jpn., 1998, 71,
1915.
9 B. C. Challis and J. A. Challis, The Chemistry of Amides, ed. J. Zabicky,
Interscience, New York, 1970, ch. 13.
10 Formation of dec-1-yne 4a from the vinyliodonium salt 1a by the
reaction with Et3N has been firmly established to involve a-
elimination–rearrangement pathways (ref. 11).
11 M. Ochiai, Y. Takaoka and Y. Nagao, J. Am. Chem. Soc., 1988, 110,
6565; M. Ochiai, M. Kunishima, S. Tani and Y. Nagao, J. Am. Chem.
Soc., 1991, 113, 3135; M. Ochiai, K. Uemura and Y. Masaki, J. Am.
Chem. Soc., 1993, 115, 2528; M. Ochiai, T. Sueda, K. Uemura and Y.
Masaki, J. Org. Chem., 1995, 60 2624.
Communication 9/04220B
1364
Chem. Commun., 1999, 1363–1364