Reactions of (tert-butyldimethylsilyl)alkynes with IPy2BF4: Selective synthesis of novel head-to-head dimers [8]

Full text of DOI:10.1021/ja970108n

J. Am. Chem. Soc. 1997, 119, 6933-6934
6933
yield
Table 1. Dimers 2 and 4a
Reactions of (tert-Butyldimethylsilyl)alkynes with
IPy2BF4: Selective Synthesis of Novel Head-to-Head
Dimers
starting
yield starting
material product T (h) (%)b material product T (h) (%)b
1a
1b
1c
1d
2a
2b
2a
2b
2c
2d
4a
4b
20
5
95
99
99
97c
93
98
2c
2d
1a
1b
1c
1d
4c
4d
4a
4b
4c
4d
5
16
24
14
14
50
95
90
95
98
97
85c
Jose´ Barluenga,*,† Isidro Llorente,†
Lorenzo J. Alvarez-Garc´ıa,† Jose´ M. Gonza´lez,†
Pedro J. Campos,‡ M. Rosario D´ıaz,§ and
Santiago Garc´ıa-Granda§
5
16
10
5
Instituto UniVersitario de Qu´ımica Organometa´lica
“Enrique Moles”, Unidad Asociada al C.S.I.C.
Departamento de Qu´ımica F´ısica y Anal´ıtica
UniVersidad de OViedo, E-33071, OViedo, Spain
Departamento de Qu´ımica, UniVersidad de La Rioja
Logron˜o, E-26001, La Rioja, Spain
a See eqs 1 and 3. b Isolated yield. c Excess of IPy2BF4/HBF4 was
used (1:2 molar ratio to the starting material).
IPy2BF4/HBF4; however, at low temperature, TBDMS-alkynes
1 (TBDMS ) t-BuMe2Si) react with IPy2BF4 furnishing 2,4-
diaryl-1-iodo-1-(tert-butyldimethylsilyl)-1,3-enynes 2, resulting
in a new and selective method for the homocoupling of
alkynylsilanes11 (eq 1).
ReceiVed January 15, 1997
The straightforward elaboration of enynes by transition-metal-
mediated linear dimerization of terminal alkynes is, in many
cases, of limited synthetic value, mainly due to the formation
of dimers as mixtures of regio- and stereoisomers with products
arising from competitive further oligomerization reactions.1
Thus, although remarkable examples of successful preparation
of enynes using this approach had been reported,2 alternative
sp2-sp coupling reactions are commonly the choice to assemble
this organic frame.3 The considerable current interest in
acetylene chemistry,4 recent efforts focusing on selective
dimerization of alkynes,5 and our finding of IPy2BF4-catalyzed
head-to-tail dimerization of iodoalkynes6 prompted us to explore
the reactivity of (trialkylsilyl)alkynes toward this reagent that,
eventually, might led to highly functionalized enynes through
a new “C-C” bond-forming reaction.7 Herein, we report an
unprecedented coupling of (trialkylsilyl)acetylenes 1 upon
Head-to-tail dimers 2 were clean and efficiently obtained by
mixing a 1:1:1 molar ratio of 1 to IPy2BF4 and HBF4 (1d
required 1:2 molar ratio of IPy2BF4/HBF4) in CH2Cl2 and
12
stirring the mixture for several hours (Table 1) at -80 to -30
°C (0 °C for 2a and 2d), followed by aqueous workup. Related
aliphatic alkynylsilanes failed to couple under the same condi-
tions. The proposed structure for 2 relies on spectroscopic (1D,
2D, and NOE NMR experiments) and analytical data.13
The above described homocoupling of alkynylsilanes yields
enynes 2 functionalized in a way that makes them valuable for
further synthetic transformations. Thus, for instance, it enables
a short elaboration of enediyne cores from alkynes,14 as depicted
for the synthesis of 315 (eq 2).
8
reaction with IPy2BF4/HBF4 (Py ) pyridine) furnishing the
regio- and diastereoisomerically pure enynes 2. Moreover, at
higher temperature, the enynes 2 further react with IPy2BF4
affording enynes 4 in another selective and efficient process.
Alkynylsilanes easily give substitution products upon reaction
with simple electrophiles,9 nevertheless, the reactivity of the
“Si-C(sp)” σ-bond can be strongly modulated by the remainder
substituents onto silicon.10 TMS-protected terminal acetylenes
(TMS ) Me3Si) led only to iodoalkynes upon reaction with
† Instituto Universitario de Qu´ımica Organometa´lica “Enrique Moles”.
Universidad de Oviedo.
‡ Universidad de La Rioja.
§ Departamento de Qu´ımica F´ısica y Anal´ıtica (X-ray service), Univer-
sidad de Oviedo.
(1) Winter, M. J. In The Chemistry of the Metal-Carbon Bond; Hartley,
F. R., Patai, S., Eds.; Wiley: Chichester, 1985; Vol. 3, p 261.
(2) (a) Trost, B. M.; Chan, C.; Ruther, G. J. Am. Chem. Soc. 1987, 109,
3486. (b) Trost, B. M.; Sorum, M. T.; Chan, C.; Harms, A. E.; Ru¨hter, G.
J. Am. Chem. Soc. 1997, 119, 698.
(3) Sonogashira, K. In ComprehensiVe Organic Synthesis; Trost, B, M.,
Fleming, I., Eds., Pergamon: Oxford, 1991; Vol. 3, pp 521-549.
(4) Modern Acetylene Chemistry; Stang, P., Diederich, F., Eds.; VCH:
Weinheim, 1995.
(5) For recent examples, see: (a) Bianchini, C.; Frediani, P.; Masi, D.;
Peruzzini, M.; Zanobini, F. Organometallics 1994, 13, 4616 and references
cited therein. (b) Matsuzaka, H.; Takagi, Y.; Ishii, Y.; Nishio, M.; Hidai,
M. Organometallics 1995, 14, 2153. (c) Straub, T.; Haskel, A.; Eisen, M.
S. J. Am. Chem. Soc. 1995, 117, 6364. (d) Yi, C. S.; Liu, N. Organometalics
1996, 15, 3968.
(6) Barluenga, J.; Gonza´lez, J. M.; Llorente, I.; Campos, P. J Angew.
Chem., Int. Ed. Engl. 1993, 32, 893.
(7) This reaction would place an iodine atom attached to the functionality
developed along the dimerization, adding synthetic interest to the final
product.
(8) The acid protonates pyridine liberating the active iodine, see:
Barluenga, J; Campos, P. J.; Gonza´lez, J. M.; Sua´rez, J. L.; Asensio, G. J.
Org. Chem. 1991, 56, 2234.
The rich chemistry of the I-C(sp2) bond adds synthetic
potential to this coupling reaction and clearly establishes a
significant difference with respect to methods relying upon
transition-metal-catalyzed reaction of terminal alkynes, where
a new C-H rather than a C-I bond is formed in the final
product.
Moreover, dimers 2 were also unexpected starting materials
for a new and selective process giving (E)-1,4-diaryl-1,2-diiodo-
1,3-butadiynes 4 (eq 3). Pure samples of compounds 2 react
(11) (a) Ohshita, J.; Furumori, K.; Matsuguchi, A.; Ishikawa, M. J. Org.
Chem. 1990, 55, 3277. (b) Heres, H. J.; Teuben, J. H. Organometallics
1991, 10, 1980. (c) Schaverien, C. J. Ibid 1994, 13, 69.
(12) Solutions of IPy2BF4 (5 × 10-2 M) were used.
1
(13) A full list of 13C and H-MR data, together with representative IR
stretching vibrations, MS data, and elemental analyses are provided in the
Supporting Information.
(9) Bassindale, A. R.; Taylor, P. G. In The Chemistry of Organic Silicon
Compounds, Part 2; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, 1989;
p 929.
(10) See: (a) Schmidt-Radde, R. H.; Vollhardt, K. P. C. J. Am. Chem.
Soc. 1992, 114, 9713. (b) Boldi, A. M.; Diederich, F. Angew. Chem., Int.
Ed. Engl. 1994, 33, 468.
(14) For a recent review on enediynes, see: Grissom, J. W.; Gunawar-
dena, G. U.; Klingberg, D.; Huang, D. Tetrahedron 1996, 52, 6453.
(15) X-ray analysis of 3b proves its structure and, furthermore, confirms
the proposed regio- and stereochemistry for the reaction of alkynylsilanes
yielding the homocoupled compounds 2. For crystal data of 3b and a figure,
see the Supporting Information.
S0002-7863(97)00108-X CCC: $14.00 © 1997 American Chemical Society

6934 J. Am. Chem. Soc., Vol. 119, No. 29, 1997
Scheme 1. Proposed Mechanism
Communications to the Editor
Initial steps might comprise chemoselective attack of elec-
trophilic iodine to the triple bond,19 followed by five-membered
ring closure involving 1,4-neighboring iodine participation20,21
giving an iodolium ion22 which has been detected by low-
temperature NMR experiments.23 1H and 13C NMR spectra at
-30 °C of the crude material obtained by reaction of 2a with
1.0 equiv of IPy2CF3SO3 and 2.0 equiv of CF3SO3H in CDCl3
showed a silylated intermediate that underwent rearrangement
to the head-to-head dimer 4a upon warming up to room
temperature, whose NMR data are consistent with the cyclic
iodolium ion A shown in Scheme 1 (Ar ) Ph, X ) CF3SO3).
The assignment of the signals was based on 2D NMR hetero-
nuclear and NOE difference experiments. Deshielding in the
13C NMR spectrum of the Si,I-bearing carbon atom (from 118.0
ppm in 2a to 144.5 ppm), the pair of acetylenic carbons (from
96.3 and 97.6 to 108.1 and 134.5 ppm for the C-I and C-I+,
respectively), and the vinylic carbon bearing the aryl group (Ar
) Ph, from 144.9 to 168.0 ppm) was observed and supports
the existence of the cyclic iodolium ion.24 A reasonable path
for the evolution of this intermediate could involve counterion
attack onto silicon25 with concomitant loss of iodine that would
migrate in a 1,4-sense affording an alkylidenecarbene,26 from
which a 1,2-aryl rearrangement furnishing an alkyne is also well-
documented in hypervalent iodonium chemistry.27
(1:1 molar ratio) with IPy2BF4/HBF4 affording enynes 4, in
excellent yield (Table 1, at -30 °C to room temperature, IPy2-
BF4 2.5 × 10-2 M in CH2Cl2). Interestingly, compounds 4 can
In short, head-to-tail dimers 2 and head-to-head dimers 4 can
be prepared from acetylenes 1 upon reaction with IPy2BF4. This
manifold has not been reported in related coupling of alkynes
mediated by transition metals. Other worthy features of this
transformation include the yields and the noted regio- and
stereoselectivity. The reactivity of the C-I bond formed in the
reaction should enlarge the synthetic utility of this homocoupling
process. Further work on the use of IPy2BF4 as a reagent
simulating the outcome of transition-metal-based reactions of
alkynes is in progress.
Acknowledgment. This research was supported by DGICYT
(Grants PB92-1005 and PB93-0330). Fellowships to I.L. (DGICYT)
and L.J.A.-G. (FICYT) are gratefully acknowledged. The assistance
of Dr. E. Rubio with the spectroscopic elucidation is also acknowledged.
also be prepared from the readily available alkynes 1,16 using a
1:1:1 molar ratio of 1 to IPy2BF4 and HBF4 (IPy2BF4/HBF4 )
1:2, for 1d), where the reaction time (see Table 1) and
temperature (from -80 to 0 °C for 4b,c, and to room
temperature for 4a,d) represent the differences in the experi-
mental conditions with respect to those furnishing dimers 2.
Spectroscopic and analytical data support the proposed structure
for dimers 4, which was confirmed by an X-ray analysis of 4c.17
To gain further insights into the conversion of enynes 2 to 4,
the cross-coupled dimer 5 was prepared by linear dimerization
of the parent-substituted alkynes using IPy2BF4.18 Upon reaction
with IPy2BF4/HBF4 (1:1 molar ratio to 5) from 0 °C to room
temperature for 16 h, only the enyne 6 was obtained, in 76%
isolated yield, as depicted in Scheme 1. The reaction pathway
includes several steps, namely, removal of the TBDMS group,
migration of the aryl group labeled with chlorine from an
internal to a terminal position, and migration of the already
present iodine atom together with incorporation of an additional
one. This transformation, resulting in the formation of three
new σ-bonds and altering two π-bonds in the frame, is
remarkably selective. A tentative mechanistic interpretation
accounting for the conversion of dimers 2 into 4 is outlined in
Scheme 1.
Supporting Information Available: Characterization data for 2-7
and A, X-ray figures, and relevant features for 3b and 4c, NMR spectra
for A, and X-ray crystallographic data for compounds 3b and 4c
including tables of atomic coordinates, bond lengths and bond angles
(40 pages). See any current masthead page for ordering and Internet
access instructions.
JA970108N
(19) Barluenga, J.; Rodr´ıguez, M. A.; Campos, P. J. J. Org. Chem. 1990,
55, 3104. For the relative reactivity of CdC and CtC toward electrophiles,
see: Melloni, G.; Modena, G.; Tonellato, U. Acc. Chem. Res. 1981, 14,
227.
(20) For previous 1,4-halogen shifts, see: (a) Peterson, P. E.; Bopp, R.
J.; Ajo, M. M. J. Am. Chem. Soc. 1970, 92, 2834. (b) Reich, I. L.; Reich,
H. J. J. Am. Chem. Soc. 1974, 96, 2654.
(21) The cis-relationship of iodine and the acetylene is crucial. Reaction
of 2b as a mixture of E:Z (2.6:1) gives 4b and the product of I-Si exchange
(2:1 ratio), proving its implication in the process.
(22) Iodolium ions: (a) Beringer, F. M.; Ganis, P.; Avitabile, G.; Jaffe,
H. J. Org. Chem. 1972, 37, 879. (b) Yang, R.-Y.; Dai, L.-X.; Chen, C.-G.
J. Chem. Soc., Chem. Commun. 1992, 1487.
(23) We thank the referees for suggestions, particularly those about the
stability of this iodolium intermediate that encouraged us to its detection
and characterization.
(24) We have not found 13C NMR data for cyclic iodolium ions; however,
chemical shifts for A are concordant with these in alkynyl- and alkenyliodo-
lium, see: Stang, P. J.; Arif, A. M.; Crittell, C. M. Angew. Chem., Int. Ed.
Engl. 1990, 29, 287. Ochiai, M.; Sumi, K.; Takaoka, Y.; Kunishima, M.;
Nagao, Y.; Shiro, M.; Fujita, E. Tetrahedron 1988, 44, 4095.
(25) The unique combination of silicon and positive iodine in one carbon
and a mild source of F- toward silicon provides a driving force to the
formation of 4. Related 2,4-diphenyl-1,1-diiodo-1,3-butenyne does not
undergo this reaction, giving (E)-1,1,3,4-tetraiodo-2,4-diphenyl-1,3-buta-
diene 7.
(16) Both 1,1-diiodo-2,4-disubstituted 1,3-butenynes (ref 6) and (E)-1,2-
diiodo-1,4-disubstituted 1,3-butenynes can be selectively prepared upon
reaction of the alkyne with IPy2BF4, simply by prior substitution at the
terminal position by iodine or TBDMS, respectively.
(17) For crystal data of 4c, including interesting -C-I‚‚‚π and -C-H‚‚‚π
bonding interactions and a figure, see the Supporting Information.
(18) Prepared in an unoptimized 35% isolated yield, adapting the
synthesis above described for 2, starting from 1d and PhCtCI (2:1 molar
ratio, respectively), at -60 °C for 40 h, and 2:1 molar ratio of HBF4 to
IPy2BF4.
(26) Stang, P. J. Chem. ReV. 1978, 78, 383. Stang, P. J. Acc. Chem. Res.
1982, 15, 348.
(27) Unsaturated carbenes are accepted as intermediates in 1,2-rear-
rangement on alkenyl and alkynyl(phenyl)iodonium compounds. For a
review, see: Stang, P. J. Angew. Chem., Int. Ed. Engl. 1992, 31, 274.