The behaviors of metal acetylides with dinitrogen tetroxide

Article abstract of DOI:10.1002/hlca.200590020

Lithium phenylacetylide (1a) and N₂O₄ (2) at - 78° yield diphenylbutadiyne (6a) by oxidative coupling, phenylacetylene (7a) by oxidation and then solvent H-abstraction, and benzoyl cyanide (8) by dimerizative-rearrangement of nitroso(phenyl)acetylene (23). Nitro(phenyl)acetylene (3, R = Ph) is not obtained. Benzonitrile (9), a further product, possibly results from hydrolytic decomposition of nitroso(phenyl)ketene (27) generated from phenylacetylenyl nitrite (26). Phenylacetylene (7a) and 2 give, along with (E)- and (Z)-1,2-dinitrostyrenes (34 and 35, resp.), 3-benzoyl-5-phenylisoxazole (10), presumably as formed by cycloaddition of benzoyl nitrile oxide (40) to 7a. Further, 2 reacts with other lithium acetylides (1b-1e), and with sodium, magnesium, zinc, copper, and copper lithium phenylacetylides, 1f-1l, to yield diacetylenes 6a-6c and monoacetylenes 7a-7c. Conversions of metallo acetylide aggregates to diacetylenes are proposed to involve generation and addition reactions of metallo acetylide radical cationic intermediates in cage, further oxidation, and total loss of metal ion. Loss of metal ions from metallo acetylide radical cations and H-abstraction by non-caged acetylenyl radicals will give terminal acetylenes. The principal reactions (75-100%) of heavy metal acetylides phenyl(trimethylstannyl)acetylene (44) and bis(phenylacetylenyl)mercury (47) with 2 are directed nitrosative additions (NO⁺) and loss of metal ions to give nitroso(phenyl)ketene (27), which converts to benzoyl cyanide (8).

Full text of DOI:10.1002/hlca.200590020

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Helvetica Chimica Acta ± Vol. 88 (2005)
The Behaviors of Metal Acetylides with Dinitrogen Tetroxide
by Christopher J. Woltermann and Harold Shechter*
Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA
(phone: ‡ 1614-292-6300; fax: ‡ 1614-292-1685; e-mail: shechter@chemistry.ohio-state.edu)
Lithium phenylacetylide (1a) and N2O4 (2) at À 788 yield diphenylbutadiyne (6a) by oxidative coupling,
phenylacetylene (7a) by oxidation and then solvent H-abstraction, and benzoyl cyanide (8) by dimerizative-
rearrangement of nitroso(phenyl)acetylene (23). Nitro(phenyl)acetylene ( 3, R ˆ Ph) is not obtained.
Benzonitrile (9), a further product, possibly results from hydrolytic decomposition of nitroso(phenyl)ketene
(27) generated from phenylacetylenyl nitrite (26). Phenylacetylene (7a) and 2 give, along with (E)- and (Z)-1,2-
dinitrostyrenes (34 and 35, resp.), 3-benzoyl-5-phenylisoxazole (10), presumably as formed by cycloaddition of
benzoyl nitrile oxide (40) to 7a. Further, 2 reacts with other lithium acetylides (1b ± 1e), and with sodium,
magnesium, zinc, copper, and copper lithium phenylacetylides, 1f ± 1l, to yield diacetylenes 6a ± 6c and
monoacetylenes 7a ± 7c. Conversions of metallo acetylide aggregates to diacetylenes are proposed to involve
generation and addition reactions of metallo acetylide radical cationic intermediates in cage, further oxidation,
and total loss of metal ion. Loss of metal ions from metallo acetylide radical cations and H-abstraction by non-
caged acetylenyl radicals will give terminal acetylenes. The principal reactions (75 ± 100%) of heavy metal
acetylides phenyl(trimethylstannyl)acetylene (44) and bis(phenylacetylenyl)mercury (47) with 2 are directed
‡
nitrosative additions (NO ) and loss of metal ions to give nitroso(phenyl)ketene (27), which converts to
benzoyl cyanide (8).
Introduction. ± The present study primarily involves the behaviors of metal
acetylides (1) with dinitrogen tetroxide (N2O4; 2). Of interest is determination if 1
reacts with 2 to give 1) 1-nitroacetylenes (3) [1][2], 1-acetylenyl nitrites (4), and/or 1-
nitrosoacetylenes (5) [3], 2) conjugated diacetylenes 6 by oxidative coupling1) [4], and/
or 3) monoacetylenes 7 by oxidation of 1 and then abstraction of hydrogen. Literature
methods for syntheses of nitro- and nitrosoacetylenes 3 [1][2] and 5 [3]2), respectively,
are unsatisfactory; acetylenyl nitrites 4 are unknown. Investigation has also been made
of 1) other products formed from 1 and 2, and 2) the reactions of 2 with
phenylacetylene (7, R ˆ Ph):
Results and Discussions. ± Addition of N2O4 (2; 1 equiv.) in Et2O to lithium
phenylacetylide (1a) in Et2O at À 788 (Scheme 1) and direct analysis of the reaction
products by GC/MS/IR reveal that diphenylbutadiyne (6a; 80%) and phenylacetylene
(7a; 15%) are obtained. In experiments conducted as above at À 788 in which the
solvent is hexane, 1a and 2 yield phenylacetylene 7a (95%), and diacetylene 6a is not
found. Reactions of 2 with 1a at À 788 in Fluorinert, a perfluoroalkane solvent, give
1
)
For recent review and references concerning oxidative coupling of metal acetylides (1) to diacetylenes (6),
see [4].
Reference [2] reports that ꢁaddition of nitronium salts to a lithio acetylide does not give a nitroacetyleneꢂ.
The products of these reactions were not given.
2
)
ꢀ 2005 Verlag Helvetica Chimica Acta AG, Zürich

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355
diacetylene 6a (75%) and polar products, but no phenylacetylene (7a), nitro(phenyl)-
acetylene (3, R ˆ Ph) [1a], or nitroso(phenyl)acetylene (5, R ˆ Ph) [3]. In further
efforts to determine if 3 (R ˆ Ph) is obtained from 1a and 2 in Et2O at À 788, Et2NH
was added to the cold reaction mixtures within 10 min after introduction of 2 had been
completed. Michael adducts (E)- and (Z)-(Et2)NÀC(Ph)ˆCHÀNO2 and derivatives
thereof containing NO2 groups are not found3).
Scheme 1
a) Isoxazole 10 and (E)- and (Z)-1,2-dinitrostyrenes (34 and 35) are major products.
b) Iodo(phenyl)acetylene is obtained in 74% yield.
The products and yields obtained from 1a and 2 in a specific solvent depend on the
order and the rates of addition and mixing of the reactants, temperature, and reaction
time. Rapid addition of 1a in Et2O to 2 in Et2O at À 788 and direct GC/MS/IR analysis
of the cold reaction mixture reveal monoacetylene 7a (60%) and diacetylene 6a (13%)
to be the volatile products. Slow addition of 1a in Et2O to excess 2 in Et2O at 08, rapid
aqueous washing of the cold reaction mixture, and product isolation by rapid column
chromatography yield diacetylene 6a (>25%), benzoyl cyanide (8; > 5%), benzoni-
3
)
Et2NH adds to 3,3-dimethyl-1-nitrobut-1-yne at 08 to give 2-(diethylamino)-3,3-dimethyl-1-nitrobut-1-enes
(94%) [5].

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Helvetica Chimica Acta ± Vol. 88 (2005)
trile (9; > 3%, [6b]), 3-benzoyl-5-phenylisoxazole (10; > 15%, [6c]), and non-volatile
polar materials. Products 8 ± 10 are always found in reactions in which 1a is added to 2
in Et2O. Use of interface reaction techniques [7] in which 2 in THF is solidified on top
of a 1a/THF glass at À 1968, and the solid mixture is melted slowly yield 7a and 8 as
major products along with benzoyl(phenyl)acetylene (11) and high-molecular weight
materials. Acetylene 11, identified by comparison with an authentic sample, is
presumably formed by displacement of cyanide ion in reaction of 8 with 1a.
Condensation of 2 on solid 1a [7] at À 1968 results in violent reactions.
The behaviors of other lithium acetylides 1b ± 1e (Scheme 1) with N2O4 (2) have
also been investigated. Addition of 2 (1 equiv.) in Et2O to lithium pent-1-ynide (1b) in
Et2O at À 788 and rapid direct GC/MS/IR analysis of the reaction mixture reveal that
deca-4,6-diyne (6b; 30%) and pent-1-yne (7b; 57%) are produced. Similarly, lithium
trimethylsilylacetylide (1c) and 2 yield 1,4-bis(trimethylsilyl)buta-1,3-diyne (6c; 44%)
and (trimethylsilyl)acetylene (7c; 25%); 1-nitro-2-(trimethylsilyl)acetylene (3; R ˆ
Me3Si) [2] is not found. Since 1-nitro-2-(triisopropylsilyl)acetylene [2] decomposes
violently in air at ca. 2008 to give 1,4-bis(triisopropylsilyl)buta-1,3-diyne (50%) and
´ NO2, the possibility that 3 (R ˆ Me3Si) is formed in reactions of 1c and 2 in Et2O at
À 788 and is then converted to 6c was of concern. The behavior of 3 (R ˆ Me3Si)
prepared independently [2] has, thus, been investigated in detail. The nitro(silyl)ace-
tylene can be chromatographed satisfactorily (TLC and GC) and does not yield 6c
under the varied conditions of the present experiments. Further, lithium (phenyl-
sulfonyl)acetylide (1d) and lithium ethoxyacetylide (1e) react selectively with 2 at
À 788 in Et2O to give (phenylsulfonyl)acetylene (7d; 95%) and ethoxyacetylene (7e;
90%), respectively; diacetylenes are not obtained. In none of the above experiments is
a nitroacetylene (3), nitrosoacetylene (5), acetylenyl nitrite (4), or an acetylenyl
nitrate (RÀCꢀCÀONO2) detected.
The mechanisms of the reactions of 2 with lithium acetylides (1; Metal ˆ Li) in
various solvents are of interest. Solid 2 is planar and of structure indicated [8a].
Unhindered ethers form adducts with 2 [8b]. Et2O and 2 yield N2O4 ´ 2 Et2O ( m.p.
À 74.88), THF and 2 give 2 adducts: N2O4 ´ C4H8O ( m.p. À 20.58) and N2O4 ´ 2 C4H8O
(incongruent m.p.), and dioxane and 2 form N2O4 ´ O( CHCH2)2O (m.p. 45.28) [8b].
2
The magnetic moments and the Raman, VIS, and IR spectra of solutions of 2 in ethers
‡
at À 728 to À 558 reveal that the N2O4 (2) is only slightly dissociated into ´ NO2, NO ,
‡
and NOÀ3 , or NO2 and NOÀ2 [8c ± j]. At its boiling point (21.58) at atmospheric pressure,
2 is 0.12% dissociated into ´ NO2 [8h]. The equilibrium constants for decomposition of
2 into ´ NO2 in C6H12, CCl4, and MeCN at 258 are 1.77 Â 10À4, 1.78 Â 10À4, and 3 Â
‡
10À5 mol lÀ1, respectively [8i]. There is much less dissociation of 2 into NO2 and NOÀ2
‡
than into NO and NOÀ3 in the above solvents [8j]. There is much chemistry, however,
in which 2 in ethers and in various non-polar solvents at higher temperatures, possibly

Helvetica Chimica Acta ± Vol. 88 (2005)
357
‡
via ONONO2 (syn and anti) [9a], is a source of 1) the nitrosonium ion (NO ) which
functions as an electron transfer [9b] or/and a nitrosating agent [9b] and 2) ´ NO2 for
varied homolytic reactions.
As is known for many alkyl, alkenyl, and aryllithium reagents, lithium acetylides are
aggregates [10][11]. Lithium phenylacetylide (1a) is a four-membered ring dimer, 12a,
in THF at À 788 and oligomeric, 13a, as a solid [11a ± c]. Lithium tert-butylacetylide is a
cyclic dimer, 12 (R ˆ t-Bu), in hexane, forms the tetramer [t-BuÀCꢀCÀLi]4(THF)4
[11d,e] and the dodecamer [t-BuÀCꢀCÀLi]12(THF)4 [11d] with THF, and is
oligomeric, 13 (R ˆ t-Bu), when dry [11d]. Presumably lithium acetylides 1b and 1c
are self-associated in Et2O, hydrocarbons, and perfluorinated solvents before initiation
of their reactions with 2 at low temperatures. The lithium [10][11] in aggregates 12 and
13 is also coordinated with Et2O and other donor solvents, but such interactions are not
included in representations such as 12 and 13, or in the simplified mechanistic schemes
and equations in this summary.
Production of diacetylenes 6a ± 6c and monoacetylenes 7a ± 7c from 1a ± 1c and 2 at
À 788 in various solvents raises significant mechanistic questions. Oxidations of
‡
solvated aggregates of 1a ± 1c (Scheme 2) are envisaged to involve additions of NO or
´ NO2 to the CꢀC bonds of the lithium acetylides and loss of ´ NO or NOÀ2 and/or direct
‡
electron transfers to NO and/or ´ NO2 from the acetylene p electron systems, CÀLi
bonds or bonded Li-atoms in 1a ± 1c in cage to give solvated-caged and then (or) non-
caged lithium acetylide radical cations 14a ± 14c, respectively. Hybrid representations
:
‡
‡
‡
of non-caged monomeric 14a ± 14c are RÀCˆCÀLi and [RÀCꢀC ´ Li] . Loss of Li
from non-caged 14a ± 14c will yield discrete 1-acetylenyl radicals 15a ± 15c. Formation
of diacetylenes 6a ± 6c (Scheme 2) as major products, however, by 1) direct couplings of
discrete acetylenyl radicals 15a ± 15c, respectively, 2) additions of 15a ± 15c to radical
cations 14a ± 14c to give 2-lithiobut-1-en-3-ynyl cations 16a ± 16c, respectively, and then
‡
loss of Li , and/or 3) reactions of discrete radicals 15a ± 15c with 1a ± 1c to yield 2-
lithiobut-1-en-3-ynyl radicals 17a ± 17c, respectively, followed by oxidation (single-
‡
electron transfer), and Li loss is highly unlikely because of the low concentrations and
the great solvent H-abstraction reactivities predicted for discrete acetylenyl radicals
15a ± 15c [12]4). Reactions of radicals 15a ± 15c to produce or leading to diacetylenes
6a ± 6c (Scheme 2) in high yields would have to take place rapidly in tight cages and not
involve 2 or the solvents. Production of monoacetylenes 7a ± 7c in the above
experiments presumably results (Scheme 2) by H-abstraction from H-containing
4
)
Because of the great strengths of acetylenic CÀH bonds (ca. 125 kcal/mol), uncomplexed acetylenyl free
radicals such as 15a ± 15c are expected to react rapidly with ethers and with hydrocarbon solvents at À 788
with H-abstraction at rates of near-diffusional control [12].

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Scheme 2
solvents, 18, by the non-caged, highly reactive acetylenyl radicals 15a ± 15c eventually
generated4).
Production of diacetylenes 6a ± 6c in reactions of 1a ± 1c with 2 is explained by
‡
oxidation (overall one electron transfer to NO and/or ´ NO2) of a lithium acetylide
unit in each of the cyclic dimers 12a ± 12c (Scheme 3) to give caged, highly delocalized,
radical-cationic reaction intermediates 19a ± 19c. Reorganizations of 19a ± 19c may
then occur (Scheme 3) to yield 2,3-dilithiobuta-1,3-diene (Sequence A) or/and 1,2-
dilithiocyclobutadiene radical cation intermediates [13] (Sequence B), 20a ± 20c and
‡
21a ± 21c, respectively, which undergo oxidation (overall one-electron transfer to NO
‡
and/or ´ NO2) and loss of 2 Li . Dilithiocyclobutadiene radical cations 21a ± 21c in cage
are expected to be highly delocalized and may be of lower energies than their isomeric
dilithiobuta-1,3-diene radical cations 20a ± 20c. Support for Sequence B of Scheme 3 is
:
‡
that acetylene radical cations (RÀCˆCÀR) add to acetylenes (RÀCꢀCÀR) at low
temperatures to give long-lived cyclobuta-1,3-diene radical cation analogs of 21a ± 21c
efficiently [13a][13b], and that such cyclobutadiene radical cations, 21a ± 21c (H
instead of Li), in gaseous and condensed phases are theorized to be more stable than
their buta-1,3-diene radical cation isomers 20a ± 20c (H instead of Li) [13c]. Oxidations
(one-electron transfer) of lithio radical cations 14a ± 14c in cage resulting in cyclo-
dimerizations (Scheme 3, Sequence C) and/or oxidations (one-electron transfer) of
21a ± 21c may yield stabilized 1,2-dilithiocyclobutadiene dications 22a ± 22c5) [14],
‡
which upon loss of 2 Li give 6a ± 6c. Mechanisms similar to those in Scheme 3 also
explain oxidative transformations of higher aggregates of 1a ± 1c by 2 to diacetylenes
6a ± 6c. Conversions of lithium acetylides 1d and 1e in Et2O at À 788 (Scheme 1) by 2
almost totally to monoacetylenes 7d (95%) and 7e (90%), respectively, imply that
lithium acetylide radical cation intermediates 14 (R ˆ EtO and PhSO2) as generated by
5
)
For discussions of the structures, stabilities, and theory of cyclobutadiene dications, see [14] and refs. cit.
therein.

Helvetica Chimica Acta ± Vol. 88 (2005)
359
Scheme 3
oxidation are not held tightly in aggregates, or/and the subsequent acetylenyl radicals
‡
15 (R ˆ EtO and PhSO2) are not as complexed by Li as those from 1a ± 1c and thus 1d
and 1e are proposed to react with 2 as in Scheme 2.
Reaction of 1a and 2 to give benzoyl cyanide (8) is understandable (Scheme 4).
Metal alkylmonoacetylides (RÀCꢀCÀM; R ˆ t-Bu and Bu and M ˆ Hg and Li) are
known to react with nitrosyl chloride (NOCl) at À 788 in CH2Cl2 to give nitro-
soacetylenes 5 as unstable products (which have been detected spectroscopically) that
convert to acyl cyanides (RÀCOÀCN) on warming [3]. Many nitroso compounds
(RÀNˆO) dimerize rapidly to (E)- and (Z)-azo-N,N'-dioxides RÀNOˆNOÀR [15].
Scheme 4

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Helvetica Chimica Acta ± Vol. 88 (2005)
‡
Formation of 8 from 1a and 2 is rationalized (Scheme 4) by nitrosation of 1a by NO
(by directed addition ± elimination and/or displacement) to give nitroso(phenyl)ace-
tylene (23), which dimerizes to (E)-24 that then converts to 25. Dimerization of 23
(Scheme 4) might also give 25 directly. Collapse of cyclodimer 25 as illustrated in
Scheme 4 will then yield acyl cyanide 8. Unimolecular rearrangement of 23 to 8 is
presumed to be a less-likely process.
That benzonitrile (9) is obtained from lithium phenylacetylide (1a) (and many
other metal phenylacetylides) and 2 under varied conditions led to speculation and
study. Of interest is whether 1a and 2 (Scheme 5) react to give phenylacetylenyl nitrite
(26) that isomerizes to nitroso(phenyl)ketene (27) which, upon reaction with H2O and
then decomposition of the (E)- and (Z)-phenylglyoxylic acid oximes (28) generated,
yields 9. Nitrosoketene (OˆNÀCHˆCˆO) has been prepared [16]. As noted
previously, acetylen-1-yl nitrites (4) have not been reported. In present experiments, all
attempts (Scheme 5) to prepare or detect acetylenyl nitrite 26 or nitrosoketene 27 by
varied preparative-separation methods and/or rapid direct gas-chromatographic
analyses of the products from 1a and 2 at À 788 were unsuccessful. Phenylglyoxylic
acid oximes 28 (Scheme 5), however, do convert rapidly in acidic environments to 9,
CO2, and H2O [17].
Scheme 5
In efforts to prepare and determine the behavior of phenylacetylenyl nitrite (26) as
in Scheme 6, lithium phenylethynolate (29) [18] and NOCl at À 788 in Et2O and rapid
aqueous washing of the cold mixture are found to give PhCN (9) and CO2 as major
products. Acetylenyl nitrite 26 was not detected. Whether 29 reacts with NOCl to give
nitrosoketene 27 directly or/and acetylenyl nitrite 26, which rearranges to ketene 27 as
in Scheme 6, cannot yet be concluded.
Scheme 6

Helvetica Chimica Acta ± Vol. 88 (2005)
361
In a further study of possible generation and the chemistry of phenylacetylenyl
nitrite (26; Scheme 7), NaNO2 is found to react with phenyl(phenylacetylenyl)iodo-
nium toluene-4-sulfonate (30) [19a] upon phase transfer in H2O/CHCl3 with loss of
TsONa, H2O, and CO2 to yield benzonitrile (9; > 45%) and iodobenzene (32; 81%).
Addition ± displacement of 30 (Scheme 7) by NOÀ2 to form iodonium ylide 31, which,
upon loss of 32, followed by rearrangement of carbene 33 to give acetylenyl nitrite 26,
is a sequence similar to that for 30 with other nucleophiles [19b]. Conversion of
phenylacetylenyl nitrite (26) to nitrile 9 as in Scheme 7 is identical to that presumed for
26 as generated from 1a (Scheme 5) and from lithium phenylethynolate (29; Scheme 6).
Scheme 7
Since phenylacetylene (7a) is a major product from 1a and 2 (1 equiv.) in Et2O and
hydrocarbon solvents, and, as to be discussed, is expected to be involved in formation of
isoxazole 10, the behavior of 7a with 2 has been investigated. Reactions of 7a and 2 in
Et2O/gasoline at 08 are reported to yield (E)- and (Z)-1,2-dinitrostyrenes (34 and 35;
Scheme 8); other products were not identified [20a]. Similarly, hex-3-yne and 2 in Et2O
at 0 ± 108 give (Z)- and (E)-3,4-dinitrohex-3-enes (4.5 and 35%, resp.) along with 4,4-
dinitrohexan-3-one (8%), hexane-3,4-dione (8%), and propanoic acid (6%) [20b].
Pent-1-yne and 2 in Et2O at 0 ± 108 yield unidentified products [20b]. In present
experiments, phenylacetylene (7a) and 2 in Et2O/ligroin at À 788 yield 1,2-dinitrostyr-
enes 34 and 35 (42%; Scheme 8), isoxazole 10 (25%; Scheme 8), and other polar
materials are, however, also obtained. Isoxazole 10 is possibly formed by [3 ‡ 2] dipolar
cycloaddition of phenylacetylene (7a) to benzoyl nitrile oxide (40) as generated from
7a and 2 via adducts 36 or/and 37 [21], and then 38 and benzoyl methanenitrolic acid
(39) as in Scheme 8. Nitrolic acids (RÀC(ˆNOH)ÀNO2) undergo elimination of
‡
HNO2 readily to form nitrile oxides (RÀCꢀN ÀOÀ) [21]. Cycloadditions of nitrile
oxides to acetylenes to give isoxazoles are well-established [21].
Investigation was then made of the behaviors of other metal phenylacetylides, 1f ±
1l, with 2. Sodium phenylacetylide (1f) is reported to undergo oxidative coupling to
diacetylene 6a (67%) by reaction with KMnO4 in liquid NH3 at À 358 [22]. Rapid
addition of 2 in Et2O to 1f suspended in Et2O at À 788 is now found to yield
phenylacetylene (7a; 16%), (E)- and (Z)-1,2-dinitrostyrenes (34 and 35, resp. are
major products), and isoxazole 10 (23%) along with diacetylene 6a (low yield) and

362
Helvetica Chimica Acta ± Vol. 88 (2005)
Scheme 8
chromatographically unseparable materials. That there is much less conversion of 1f
than of 1a by 2 in Et2O at À 788 to diacetylene 6a is of interest.
Sodium acetylides NaÀCꢀCÀH [23a] and NaÀCꢀCÀCH3 [23b] are tetrameric,
layered aggregates with limited solubilities in Et2O, hydrocarbon, and other non-polar
‡
‡
solvents. Because of the greater electropositivity and size of Na as compared to Li ,
sodium acetylides (1, M ˆ Na) are expected to be more reactive than Li analogs.
Sodium phenylacetylide aggregates 1f are, thus, predicted to be oxidized more readily
‡
by NO , ´ NO2, and/or 2, and less subject to cage maintenance effects than is 1a.
:
‡
Further, sodium phenylacetylenyl radical cations (PhÀCˆCÀNa) are expected to be
less tightly metal-bonded and to decompose to discrete phenylacetylenyl radicals (15a)
more readily than do lithio radical cation analogs 14a. Reactions of 1f with 2 to give
more monoacetylene 7a and much less diacetylene 6a than do 1a and 2 under
comparable conditions are, thus, rationalized.
The behaviors of 2 with more covalent metal phenylacetylides (1) were then
studied. Solid magnesium phenylacetylide/THF ((PhÀCꢀC)2Mg(THF)4) [24], zinc
phenylacetylide ((PhÀCꢀC)2Zn) [25a,b], and other zinc acetylide derivatives
((RÀCꢀC)2Zn) [25c ± f] are highly-bridged metal ± acetylene aggregates. Alkynyl
Grignard reagents are oxidatively homocoupled to diacetylenes by iodine [26a ± c], CuII
and CoII salts [26c ± f], nitrobenzene [26g], and oxovanadium(V) compounds
(VO(OR)Cl2) [26h], respectively. Phenylacetylenylmagnesium bromide (1g) [27]
and 2 in Et2O at À 788 are now found to yield diacetylene 6a (20%) and phenyl-
acetylene (7a; > 60%) along with cyanide 8, nitrile 9, and isoxazole 10. Phenyl-
acetylenylzinc chloride (1h) and phenylacetylenylzinc bromide (1i) in THF at À 788
each also react with 2 on warming slowly (and rapid direct GC/MS/IR analyses of the
mixtures) to give diacetylene 6a (20%) and monoacetylene 7a (76%). The principal
reactions of 1g ± 1i, respectively, with 2 in Et2O apparently involve one-electron
transfer from metallo-acetylene units in the organometallics, cage collapse to give
discrete phenylacetylenyl radicals 15 (R ˆ Ph), and H-abstraction similar to that in

Helvetica Chimica Acta ± Vol. 88 (2005)
363
Scheme 26). In reactions with 2, magnesium acetylide 1g, and zinc acetylides 1h and 1i
yield more coupling product diacetylene 6a than does sodium acetylide 1f but much less
‡‡
‡‡
‡
6a than does lithium acetylide 1a. The divalent metal ions Mg and Zn presumably
do not hold acetylide oxidation intermediates as tightly in cage as does Li in reactions
of phenylacetylides 1g and 1h with 2 to give coupling product diacetylene 6a.
Phenylacetylenylzinc iodide (1j) is converted, however, by 2 in THF (À 788) to
iodo(phenyl)acetylene (Ph ÀCꢀCÀI; 74%) along with diacetylene 6a (17%). The
iodide ion in the reaction mixture from 1j and 2 is rapidly oxidized to iodine, which
then reacts with 1j or/and acetylenylzinc nitrite 41 and/or nitrate 42 to form
iodo(phenyl)acetylene. Diacetylene 6a is also produced from 1j and 2 in significant
yield presumably by oxidation ± reduction (electron transfer) and subsequent reactions
at À 788 similar to that proposed for 1a and 2 (Scheme 3). In none of the above
experiments with 2 and 1g ± 1j, respectively, was nitro(phenyl)acetylene ( 3, R ˆ Ph)
detected.
Copper(I) phenylacetylide (1k) [28] behaves differently from 1h and 1i with 2 in
THF at À 788 in that diacetylene 6a (> 72%) is the major product. Phenylacetylene
(7a), the H-abstraction product, is not found. Similarly, dilithium tris(phenylacetyl-
enyl) cuprate (1l) reacts rapidly with 2 in Et2O at À 788 to give diacetylene 6a (> 71%)
and no 7a. The behaviors of 1k and 1l with 2 are, thus, similar to conversions of varied
copper acetylides (1, M ˆ Cu) to diacetylenes 6 [4] in ammoniacal solution by oxygen
[29a ± c] and by CuII reagents [29d ± h] in pyridine or amine bases, peroxides [29i ± j],
and potassium ferricyanide [29k,l]. Although oxidative couplings of copper acetylides
are widely used for synthesis of conjugated diacetylenes, and the behaviors of such
acetylides with oxygen and with cupric salts have been investigated kinetically and
otherwise7), the detailed mechanisms of these reactions have not been established [4].
The present results with 2 and 1k (and other metal acetylides) allow discussion [4] with
respect to the mechanisms of the oxidative couplings.
Copper(I) phenylacetylide (1k) is a tight, high-molecular-weight oligomer of
known structure [30]. The overall conversion of 1k by 2 (and possibly O2) to 6a is
presently explainable7) [4] by oxidation (one-electron transfer, 1 equiv.) of a dimer unit
in the aggregate [30] to give the resulting delocalized cuprous bis(phenylacetylenyl)
radical cation (43) in cage. Much as in the conversions of 12a ± 12c to 6a ± 6c in
Scheme 3, intramolecular rearrangement of 43, oxidation (one-electron transfer), and
‡
‡
total loss of Cu and/or oxidation of 43, cyclodimerization, and total loss of Cu will
yield diacetylene 6a. Mechanisms similar to the above may operate in oxidation of
lithio cuprate 1l by 2 to diacetylene 6a. In the reactions of 1k and of 1l with 2 to yield 6a,
1) electron transfers are facilitated by the ease of oxidation of the CuI atoms, and 2)
6
)
)
The mechanisms of other electron-transfer reactions of Grignard reagents are reviewed in [27].
Studies of the mechanisms of coupling of copper acetylides by oxygen and by CuII reagents are reviewed in
[4]; see also refs. cit. therein.
7

364
Helvetica Chimica Acta ± Vol. 88 (2005)
coupling or/and addition reactions of uncaged (discrete) phenylacetylenyl radicals
(14a, R ˆ Ph) are not extensively involved4).
Study has been initiated of the behaviors of highly covalent metal acetylides 1 with
2. (tert-Butyl)(trimethylstannyl)acetylene (44) and 2 in Et2O at À 608 are reported to
yield (tert-butyl)(nitroso)acetylene (45; Scheme 9; 40%) [3c]; other products were not
identified. On storage in Et2O at À 308, 45 is converted (Scheme 9) to trimethylacetyl
cyanide (46; 30%) [3c] presumably by reactions similar to those in Scheme 4.
Phenyl(trimethylstannyl)acetylene (47) and 2 in Et2O at À 608 are now found to give
benzoyl cyanide (8; Scheme 9; 76 ± 100%) apparently upon generation of nitrosoace-
tylene 23 and rearrangement via dimerization as in Scheme 4. Efficient generation and
then reactions of 1-nitrosoacetylenes (5) at low temperatures offer excellent
opportunities for developing their chemistries.
Scheme 9
Of further interest is that bis(phenylacetylenyl)mercury (48) and 2 in Et2O at À 788
(Scheme 10) yield benzoyl cyanide (8; > 99%) efficiently. Nitroso(phenyl)acetylene
(23) is apparently formed from mercuric acetylides 48 and 50 with 2 by directed
‡
nitrosative additions (NO ) and eliminations of mercuric derivatives 50 and 52
(Scheme 10), respectively. Dimerization of 23 and collapse as in 25 in Scheme 4 then
yield cyanide 8. It is now clear that reactions of 2 with a metal acetylide (1) are greatly
affected by the coordinated metal ion.
Conclusions. ± Dinitrogen tetroxide (2) may react with metallo acetylides (1) in
Et2O and in hydrocarbon solvents at À 788 to yield 1) conjugated diacetylenes 6 by
oxidative processes in cage, 2) monoacetylenes 7 by H-abstraction reactions of the
acetylen-1-yl free radicals (15) eventually generated, and/or 3) acyl cyanides
(RCOCN) by rearrangements via dimerization of the 1-nitrosoacetylenes (5) initially
formed. Which of the above reactions is dominant depends greatly on the metal of the
metallo acetylides (1), the solvent, and temperature. Benzonitrile (9), a further product
from metallo phenylacetylides (1a and 1f ± 1i) and 2, is presumed to be obtained from
phenylacetylenyl nitrite (26) and then (or) nitroso(phenyl)ketene (27), followed by
hydration and then decarboxylation and loss of H2O. Phenylacetylene (7a) and 2 yield
(E)- and (Z)-1,2-dinitrostyrenes (34 and 35, resp.), and 3-benzoyl-5-phenylisoxazole
(10); 10 arises from cycloaddition of benzoyl nitrile oxide (40) to 7a. The above results
are of mechanistic interest in that the behaviors of various metallo acetylide radical
:
‡
cations (RÀCˆCÀM) are possibly revealed.

Helvetica Chimica Acta ± Vol. 88 (2005)
365
Scheme 10
Studies are to be made of 1) reactions of metal acetylides 1 with nitronium
and other possible nitrating agents8), 2) syntheses and the properties of
acetylen-1-yl nitrites (4), acetylen-1-yl nitrates (RÀCꢀCÀOÀNO2), nitroketenes
(R(O2N)CˆCˆO), and dinitroacetylene (O2NÀCꢀCÀNO2), 3) efficient low-tem-
perature methods for preparing 1-nitrosoacetylenes (5), and their conversions to 1-
nitroacetylenes (3) and other valuable products, and 4) the chemistries of metallo
radical cations as generated by coordination of various alkyl, vinyl, acetylenyl, aryl,
‡
heteroaryl, and non-carbon free radicals with Li and other positive metal ions.
Experimental Part
General. Reaction solvents were dried and purified scrupulously prior to use. Et2O and THF were distilled
from Na or Na/K alloy with benzophenone as an indicator. Benzene and MeCN were each distilled from CaH2.
CH2Cl2, CHCl3, and CCl4 were each distilled from P2O5. Reaction mixtures were usually analyzed cold, rapidly,
and directly by GC/MS/IR. Relative amounts of the components of a reaction mixture were determined by
integration of peak areas in total ion chromatography as obtained mass spectrally. Quantitative measurements
of yields were made by comparisons of peak areas to that of an internal standard (diphenylacetylene unless
noted otherwise) in calibrated equipment. Melting points below 2008 were determined in capillary tubes; those
above 2008 were obtained using glass plates. All melting points are uncorrected. NMR Spectra (peaks reported
in ppm downfield from TMS at d 0.00) were determined at 200.133 MHz for 1H and 50.323 MHz for 13C. High-
resolution mass spectra (HR-MS) were obtained from The Ohio State University Chemical Instrument Center.
Preparation of Solns. of N2O4 (2) in Et2O. Commercial 2 from a steel cylinder was condensed through Tygon
tubing into a flask equipped for distillation and cooled to 08. Compound 2 was then distilled into tared receivers
cooled to 08. The distillations removed plasticizers leached from the Tygon tubing. The receivers were removed
and weighed. A measured volume of dry Et2O cooled to 08 was added to 2. Dry O2 was bubbled through the cold
solns. until the mixtures became amber. The solns. were kept at À 258 and treated with dry O2 before use. Solns.
of 2 were replaced after 1 week or sooner.
Lithium Phenylacetylide (1a) and 2. a) Direct Addition. BuLi (3.7 ml, 1.31m) in hexanes and then 2 (1.72 ml,
2.84m in Et2O) were added by syringe dropwise into a soln. of phenylacetylene (7a; 0.5 g, 4.9 mmol) in Et2O
8
)
Dinitrogen pentoxide (N2O5) converts 1k in CCl4 at À 158 to diyne 6a (55 ± 78%) along with acetophenone
(5%).

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(10 ml) at À 788. A suspension formed immediately. Rapid analyses of the cold mixture by GC/MS/IR revealed
that diphenylbutadiyne (6a; 80%) and 7a (15%) were present. Upon warming to r.t., the mixture was poured
into cold 5% aq. Na2CO3 and extracted rapidly with Et2O. The Et2O extracts were combined, dried (MgSO4),
filtered, concentrated in vacuo, and chromatographed (silica gel; hexanes/AcOEt 4 :1) to give 6a (0.27 g, 55%)
as slightly yellow crystals. Recrystallization from EtOH/H2O yielded 6a, white needles, m.p. 85 ± 878 ([31]: 86 ±
1
878), whose FT-IR (NaCl, melt) and H-NMR (CDCl3) (d 7.55 ± 7.49 (m, 2 H); 7.31 À 7.35 (m, 3 H)) correspond
to those of an authentic sample [31]. The aq. layer gave an instant positive test (blue color) for nitrite ion upon
acidification with 5% aq. H3PO4 and application to KI/starch paper [32]. There was no evidence for production
of nitro(phenyl)acetylene (3, R ˆ Ph) in many such experiments.
b) Inverse Addition. An Et2O soln. of 1a (prepared from 7a (0.50 g, 4.9 mmol) and BuLi (3.74 ml, 1.31m in
hexanes)) was added by cannula (ca. 10 min) to 2 (1.72 ml, 2.85m in Et2O) at À 788 under Ar. Upon warming the
mixture to r.t., addition of diphenylacetylene as an internal standard, and direct analysis by GC/MS/IR, 7a
(60%) and 6a (13%) were found as reaction products. Compound 3 (R ˆ Ph) was not detected.
Lithium Phenylacetylide (1a) and Excess 2. A mixture of BuLi (0.86 ml, 2.27m in hexanes) and 1a (0.20 g,
1.96 mmol) in Et2O (10 ml) at 08 was added by cannula in 50 min to a freshly prepared soln. of 2 (6.6 ml, 2.99m)
in Et2O at À 788. Direct analysis by GC/MS/IR showed 6a (0.0487 g, 25%), benzoyl cyanide (8; 0.0123 g, 5%)
[6a], benzonitrile (9; 0.0049 g, 2 ± 3%) [6b] and 3-benzoyl-5-phenylisoxazole (10; 0.0373 g, 15%) [6c] to be
present. Diyne 6a and isoxazole 10 (yellow crystals) were isolated by column chromatography (CC, silica gel;
hexanes/AcOEt 4 :1). Anal. samples of 10 were obtained by recrystallization from hexanes or EtOH/H2O. M.p.
81 ± 828 ([6c,d]: 898). 1H-NMR (CDCl3): 8.3 (m, 2 H); 7.8 (m, 2 H); 7.5 (m, 6 H); 7.0 (s, 1 H). 13C-NMR (CDCl3):
‡
185.8; 170.8; 162.4; 135.8; 134.1; 130.7; 130.7; 129.2; 128.6; 126.7; 126.0; 100.2. HR-MS: 249.0792 (M ; calc.
249.0790), 105, 77, 51. Anal. calc. for C16H11NO2: C 77.10, H 4.45; found: C 76.99, H 4.50.
Products 6a, 8, and 9 were identified by IR, MS, and GC retention times as compared to that from authentic
samples.
Lithium Phenylethynolate (29) [17] and NOCl. BuLi (0.32 ml of a 2.27m soln. in hexanes) was added to a
soln. of hexamethyldisilazane (0.128 g, 0.79 mmol) in THF (3 ml) at À 788. After 5 min, 1,1-dibromoacetophe-
none [17] (0.20 g) in THF (2 ml) was added dropwise. The mixture was brought to 08, cooled to À 788, and t-
BuLi (2.0 ml of a 1.1m soln. in hexanes) was added slowly. The mixture was stirred 15 min at À 788, and a freshly
prepared soln. of NOCl in Et2O (0.22 ml, 3.33m) was added. After 1 h, the mixture was warmed to 258 and the
reaction was quenched rapidly with cold 10% aq. HCl. The org. layer was analyzed quickly by GC/MS/IR. The
complex mixture contained 9 [6b] as the main constituent. Phenylacetylenyl nitrite (26) was not detected.
(Phenylacetylenyl)(phenyl)iodonium Toluene-4-sulfonate (30) [18] and NaNO2. A soln. of NaNO2 (0.43 g,
6.3 mmol) in H2O (10 ml) was added in one portion to 30 [18a] (0.30 g, 0.63 mmol) in CHCl3 (10 ml), and the
two-phase mixture was stirred vigorously for 3 d. TLC revealed the presence of 2 major products that GC/MS/
IR showed to be iodobenzene (32, 81%) and 9 (45%). There was no evidence for 26. CC (silica gel; hexanes/
AcOEt 4 :1) of the product mixture led to separation of 32 and 9 (>45%), which were identified by
comparisons with authentic samples.
Data of 9: FT-IR (gas phase): 3083w, 2235m, 1492m, 738s, 889s, 543s. MS: 103, 75, 50.
Sodium Phenylacetylide (1f) and 2. N2O4 (2, 11.4 ml, 1.72m in Et2O, 19.6 mmol) was added at À 788 to a
stirred white suspension of 1f prepared by agitation of Na (0.24 g, 10.3 mmol) with 7a (1.00 g, 9.8 mmol) in Et2O
(40 ml) under Ar for 12 h. Reaction occurred rapidly. After slowly warming to r.t., the bright yellow suspension
was poured into sat. aq. NaHCO3 soln. and extracted with Et2O. The Et2O extracts were combined, dried
(MgSO4), concentrated, and chromatographed (silica gel; gradient elution with hexanes/AcOEt) to give 7a
‡
(0.16 g, 16%), 6a (traces), 10 (0.28 g, 23%, yellow needles, m.p. 80 ± 81.58; [6c ± d]: 898, MS: 249 (M ), 105, 77,
51), and (E)- and (Z)-1,2-dinitrostyrenes (34 and 35, resp., are major products) [20a], assigned by FT-IR and
comparison with authentic samples, resp.
Phenylacetylenylmagnesium Bromide (1g) and 2. EtMgBr was prepared by adding 1,2-dibromoethane (1
drop) to a suspension of EtBr (0.216 g, 1.98 mmol), Mg turnings (0.095 g, 3.90 mmol), and anh. Et2O ( 2 ml). The
Et2O soln. was added via cannula to 7a (0.20 g, 2.00 mmol) in Et2O at r.t. After stirring for 20 min, the mixture
was cooled to À 788 and THF (2 ml) was added to dissolve the solids completely. Purified 2 (0.82 ml of a 2.38m
solution in Et2O) was added dropwise. Analysis of the mixture by GC/MS/IR revealed that the major product
was 7a (>60%); 6a (20%), (Z)- and (E)-1,2-dibromostyrenes, and 8 (trace) were also present. (The 1,2-
dibromostyrenes were formed by reaction of 7a with Br2 as produced by oxidation of BrÀ with 2.) No 3 (R ˆ Ph)
was found in the reaction product.
Phenylacetylenylzinc Chloride (1h), Phenylacetylenylzinc Bromide (1i), and Phenylacetylenylzinc Iodide
(1j), Respectively, with 2. Solns. of BuLi (1.7 ml, 1.18m in hexanes) and then anh. ZnCl2 (0.28 g, 2.0 mmol) in

Helvetica Chimica Acta ± Vol. 88 (2005)
367
THF (3 ml) were added to 1a (0.20 g, 2.00 mmol) in THF (5 ml) at À 788 under Ar. The mixture was rapidly
warmed to r.t. and then cooled to À 788. After dropwise addition of 2 (0.82 ml, 2.4m in Et2O) and warming the
mixture to r.t., direct GC/MS/IR analysis revealed 7a as a major component. The mixture was poured into 5%
Na2CO3, washed with brine, and concentrated in vacuo to a yellow liquid (0.19 g), which was separated by CC
(silica gel; 3% AcOEt in hexanes) to give 6a (0.04 g, 20%) and 7a (0.15 g, 76%).
In an experiment similar to that with 1a, BuLi, and ZnBr2 and then 2 at À 788, the reaction products from 1i
are 6a (20%) and 7a (76%). From 1a, BuLi, ZnI2, and then 2 at À 788, 6a (17%) and iodo(phenyl)acetylene
(74%) are obtained. Iodo(phenyl)acetylene was identified by comparison with an authentic sample.
Copper Phenylacetylide (1k) and 2. An Et2O soln. of 2 (1.5 ml, 1.24m, 1.8 mmol) was added dropwise to a
stirred suspension of 1k (0.30 g, 1.8 mmol) [27] in dry Et2O (10 ml) at À 788 under Ar. Reaction began almost
immediately, and the suspension changed from yellow to dark green. The heterogeneous mixture was slowly
warmed to r.t., filtered through Celite, washed rapidly with 5% aq. Na2CO3 and brine, dried (MgSO4), and
concentrated. Nitroacetylene 3 (R ˆ Ph) and 7a were not reaction products. After the concentrate had been
dried at 1 mm Hg, diacetylene 6a (0.13 g, 72%) remained as yellow crystals, m.p. 75 ± 818. The crude 6a (0.11 g,
61%) was chromatographed (silica gel; hexanes/AcOEt 4 :1) and then recrystallized from EtOH/H2O. White
crystals of 6a (0.09 g, 50%) were obtained: m.p. 85 ± 878 ([31]: 86 ± 878).
In a similar experiment in which 1k was added (50 min) to excess 2 at À 788, 6a was formed in 60% yield.
The crude reaction product did not contain 3 (R ˆ Ph) or 7a.
Dilithium Tris(phenylacetylenyl) Cuprate (1l) and 2. To 7a (0.20 g, 2.0 mmol) in Et2O (10 ml) was added
BuLi (0.86 ml, 2.27m in hexanes) at À 788 and 1k (0.16 g, 1.0 mmol) [28]. The mixture warmed to r.t. overnight.
The bright yellow color of 1k had disappeared, and the soln. was almost completely homogeneous. The mixture
was cooled to À 788, and 2 (0.57 ml, 1.73m in hexanes) was added. The soln. became dark immediately and
contained a yellow suspension. After 10 min, the cold mixture was filtered through Celite and washed
thoroughly with CH2Cl2. The filtrate was dried (K2CO3) and concentrated to 6a (0.14 g, 71%), a yellow solid,
which, on recrystallization from EtOH/H2O, gave colorless needles (0.10 g, m.p. 83.5 ± 858), identical with an
authentic sample of 6a [31].
Lithium Pent-1-ynide (1b) and 2. Pent-1-yne (7b, 0.50 g, 7.4 mmol) in Et2O (10 ml) was cooled under Ar to
À 788. BuLi (5.6 ml, 1.31m in hexanes, 7.4 mmol) and then 2 (2.6 ml, 2.85m in Et2O, 7.4 mmol) were added. After
stirring the mixture at À 788 for 10 min, chloro(trimethyl)silane (1 ml) was added to quench any remaining 1b.
The soln. was warmed to r.t. in 1 h, poured into 5% aq. Na2CO3 soln. (50 ml), and extracted with Et2O. The
extracts were combined, dried (MgSO4), and analyzed by GC/MS/IR for deca-4,6-diyne (6b, 0.15 g, 30%) [33].
The extract was treated with AgNO3 in EtOH/H2O to convert 7b to silver pent-1-ynide (white). The white
precipitate, after filtration, washing with H2O, EtOH, and Et2O, and drying weighed 0.73 g, which corresponded
1
to 0.28 g of 7b (57%). Diyne 6b was isolated by prep. GLC. H-NMR (200 MHz, CDCl3): 2.20 (t, J ˆ 7, 4 H ) ;
1.52 (q, J ˆ 7, 4 H); 0.96 (t, J ˆ 7, 6 H). MS: 134, 117, 105, 91, 77, 63, 51, 32. The spectra of 6b are identical with
those reported in [33].
Lithium (Trimethylsilyl)acetylide (1c) and 2. A soln. of 1c (0.20 g, 2.0 mmol) in Et2O (10 ml) was flushed
with Ar and cooled to À 788. BuLi (0.90 ml, 2.27m in hexanes, 2.0 mmol) was added, followed by 2 (1.6 ml, 1.24m
in Et2O, 2.0 mmol). The soln. was stirred for 1 h at À 788 and then poured into 5% aq. Na2CO3 soln. The org.
layer was distilled to remove the 7c and Et2O. The distillate was treated with AgNO3 in EtOH/H2O to convert
the 7c to disilver acetylide (AgCꢀCAg), which, when filtered, was washed with H2O, EtOH, and Et2O, and
dried, weighed 0.12 g, and corresponded to 0.05 g of the initial 7c (25%). Prep. GLC of the initial distillation
residue led to isolation of bis(trimethylsilyl)butadiyne (6c; 44%) [34]. FT-IR (gas phase): 2967w, 2907w, 2064m,
‡
1261w, 850s, 648w. 1H-NMR (200 MHz, CDCl3): 0.16 (s, 18 H). MS: 194 (M ), 179, 163, 149, 121, 97, 73, 67, 32.
The spectra of 6c are identical with those reported in [34].
Lithium (Phenylsulfonyl)acetylide (1d) and 2. To a soln. of (phenylsulfonyl)acetylene (7d; 0.20 g, 1.2 mmol)
in Et2O (10 ml) at À 788 under Ar was added BuLi (0.5 ml, 2.27m in hexanes, 1.1 mmol). A cold soln. of 2
(5.7 ml, 0.21m in Et2O) was then added dropwise. The cooling bath was removed, and the mixture was warmed
slowly to ca. 208. After addition of aq. HCl (10%), the org. layer was separated, dried, and concentrated to give
7d (95%) as identified by its IR and 1H-NMR spectra.
Bis(phenylacetylenyl)mercury (48) and 2. A soln. of 2 (0.32 ml, 3.13m) in Et2O was added dropwise to 48
(0.20 g, 0.50 mmol) in Et2O (10 ml) and stirred at À 788 under Ar. A white precipitate formed immediately.
After warming to r.t. (ca. 0.5 h), the heterogeneous mixture became yellow. The mixture was filtered rapidly
through silica gel and concentrated under reduced pressure. The yellow residue (0.13 g) was analyzed by GC/
MS/IR and found to be essentially totally 8 (> 99% yield) [6a]. FT-IR (gas phase): 3080w, 2218, 1700 (CˆO),

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Helvetica Chimica Acta ± Vol. 88 (2005)
‡
‡
1598, 1451, 1244, 1180, 980, 699. MS: 131 (M ), 105 ([M À CN] ), 77, 51. The spectra of the product are identical
with those of an authentic sample of 8 [6a].
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Received January 6, 2005