Cyclization of 5-alkynones with chromium alkylidene equivalents generated: In situ from gem -dichromiomethanes

Article abstract of DOI:10.1039/d0cc03986a

A rare example of cyclization with 5-alkynones, which possess non-polar alkynyl and less electrophilic polar keto carbonyl groups, was demonstrated. The key to promoting carbene/alkyne metathesis followed by alkylidenation with pendant CO double bonds was the Schrock-type nucleophilic reactivity of the generated chromium carbene equivalents from readily available diiodomethanes and CrCl2 by simple heating.

Full text of DOI:10.1039/d0cc03986a

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Cyclization of 5-Alkynylketones with Chromium Alkylidene
Equivalents Generated in situ from gem-Dichromiomethanes
Masahito Murai,*a,b Ryuji Taniguchi,a Takashi Kurogi,a Shunsuke Moritani,a and Kazuhiko Takai*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A rare example of cyclization with alkynylketones, which possess chemistry.7 Surprisingly, however, few reports exist on the
non-polar alkynyl and less electrophilic polar keto carbonyl groups, generation and use of Schrock-type chromium carbene, i.e.,
was demonstrated. The key to promoting carbene/alkyne chromium-alkylidene species.8
metathesis followed by alkylidenation with pendant C=O double
bonds was the Schrock-type nucleophilic reactivity of the generated
chromium carbene equivalents from readily available
diiodomethanes and CrCl2 by simple heating.
Metal carbenes are well-established reagents in modern organic
synthesis.1 Strategies, including -hydrogen abstraction of
dialkylmetal species (Figure 1, Path A), complexation of metal species
with carbene equivalents, such as diazoalkanes (Path B), and
elimination of ethylene via ring-cleavage of metallacyclobutanes
(Path C), have been reported for the reliable generation protocols of
metal carbene species.2 Another approach involves conversion of
gem-metalloalkanes (M1−CR2−M2), possessing two metal atoms on
the alkylene carbon (Path D).3,4 Although Grubbs et al. realized the
possibility of the route by the generation of titanium carbene species
from the Tebbe reagent (Ti−CH2−Al) upon treatment with 4-
(dimethylamino)pyridine in 1982,4 few studies expanding on this
Figure 1. Generation of metal carbene species
The present study demonstrates the reactivity of gem-
dichromiomethane species, which are as useful dianion species,5,9 as
Schrock-type chromium carbene equivalents via coupling and
cyclization with 5-alkynylketones. The in situ preparation of gem-
dichromiomethane species from reaction of CrCl2 (8 equiv) and
(diiodomethyl)trimethylsilane,10 followed by treatment with 5-
alkynylketone in THF at 70 °C, afforded alkynylcycloalkene 1a as a
single stereoisomer in 70% yield (Eq 1). Note that this cyclization of
method have been reported. In the current study,
a gem-
dichromiomethane species (Cr−CR2−Cr), which can be readily
prepared from CrCl2 and dihalomethanes, was chosen as a readily
available and stable metal carbene generator via Path D, and was
used as a precursor for the facile in situ generation of Schrock-type
carbene equivalents.5,6 The group 6 metal carbene species stabilized
by heteroatom or aryl group substitution at the -carbon,
represented by chromium Fischer-type carbenes, [R1R2C=M(CO)5]
(e.g., R1, R2 = alkyl, alkoxy, amino, halogen), possessing an
electrophilic carbene carbon, are historically important
organometallic reagents in organic syntheses and organometallic
alkynylketones, which possess non-polar C≡C and polar keto C=O
bonds, is very rare,11 while the corresponding cyclization of
alkynylaldehydes, which have more electrophilic formyl groups, is
relatively common.12 Previously, cyclization with Fischer-type
electrophilic ruthenium carbene complexes generated in situ
provided dihydropyrans (Eq 2, Si = SiMe3, W = CO2Me, Ru =
Cp*RuCl).11b In contrast, formation of 1a in the current study could
be explained by the sequential metathesis reaction13 of carbene
species to C≡C and C=O bonds (see Scheme 2 (vide infra)), implying
that the generated carbene equivalents possessed Schrock-type
a. Division of Applied Chemistry, Graduate School of Natural Science and
Technology, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700-
8530, Japan
b. Department of Chemistry, Graduate School of Science, Nagoya University, Furo,
Chikusa, Nagoya 464-8602, Japan
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
nucleophilic
reactivity.
Although
reactivity
of
gem-
dichromiomethane species toward alkynes has been demonstrated
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in our recent work on the transformation of 1,n-enynes,14 the position or alkyne terminal prevented the triple bond from
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present results provide direct information about the reactivity of the participating in the present reaction.19,20DIOnI:c1o0n.1t0r3a9s/tD, 0sCeCq0u3e9n8t6iaAl
generated carbene equivalents. The yield of 1a increased to 85% by cyclization with a diyne containing a keto carbonyl group provided
changing the solvent to DME (1,2-dimethoxyethane) (see Table S1 in the conjugated triene 2 in 64% yield (Scheme 1). The double bond
ESI for optimization details). The amount of CrCl2 could be reduced geometry of 1 and 2 was E regardless of the structure of the
to 4 equiv using manganese powder to reduce the generated CrX3 (X precursor.
= Cl or I) back to CrX2 (see Scheme 2),15,16 resulting in isolation of 1a
When 5-alkynylaldehyde was used as a precursor, direct
in 80% yield (Table 1). The yield of 1a decreased to 32% when the alkylidenation furnished 3 without affecting the terminal triple bond
reaction was conducted at 50 °C, and an inseparable mixture was (Eq 3). The results obtained so far indicated that the affinity of the
obtained with TMEDA, which was an effective additive for previously gem-di(chromio)silylmethane species was in the order of formyl
reported cyclopropanation of alkenes.17
group (CHO) > ethynyl group (C ≡ CH) > keto carbonyl group
(RC(=O)R’).
Cyclization initiated by coupling of a chromosilylcarbene
equivalent with C≡C bonds proceeded smoothly with phenyl
ketones to produce 1b and 1c in 74% and 71% yield, respectively
(Table 1). Note that aryl ketones could not be used as substrates in
the previous intermolecular alkylidenations of carbonyl
compounds.18 Despite the more sterically hindered environment of
ketocarbonyl groups, cyclization proceeded efficiently to afford
fused bi- and tricycles 1d and 1e in high yields. The composition of
the linkers between the alkynyl and carbonyl groups did not affect
cyclization efficiency, and azacycle 1f and spirocycle 1g, respectively,
were obtained, respectively. However, substituents at the propargyl
The reaction mechanism for the current coupling and cyclization
reaction is proposed in Scheme 2. gem-(Dichromio)silylmethane
species 4 generated in-situ by sequential single-electron transfers
from 4 equiv of CrCl2 may generate chromium alkylidene species A.
Chromacyclobutene intermediate
B
generated via formal
[2+2]cycloaddition of alkynylketones with A is then converted to
alkenylchromacarbene species C,14 which subsequently undergoes
intramolecular alkylidenation of the pendant keto carbonyl group to
furnish alkenylcycloalkene 1.21 Note that the generated C is a
Schrock-type carbene species, and formation of the corresponding
dihydropyrans as shown in Eq 2 was not observed.
Table 1. Coupling and cyclization of 5-alkynylketones with gem-
(dichromio)silylmethane
Scheme 2. Plausible reaction mechanism for the formation of 1 (Cr
= CrCl(dme)(ClCrCl2(dme)), X = Cl or I)
Although attempts to obtain direct information about the
generation of chromium alkylidene species A was unsuccessful, the
isolation and structural characterization of a key reactive species,
gem-(dichromio)silylmethane 4, stabilized by coordination of DME,
was achieved. Note that only a trace amount of 4 was detected in
DME at 70 °C due to its competitive decomposition to (E)-1,2-
bis(trimethylsilyl)ethene in the absence of alkynylketones (Eq 4).22
However, an authentic sample of 4 was obtained by ligand exchange
of the THF-coordinated gem-(dichromio)silylmethane 4’ prepared in
THF at 25 °C (Eq 5).23
A single crystal suitable for X-ray
crystallographic analysis was obtained by recrystallization from a
DME solution layered with hexane. The solid-state structure of 4 in
Scheme 1. Sequential cyclization initiated by coupling with diyne
2 | J. Name., 2012, 00, 1-3
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Conflicts of interest
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DOI: 10.1039/D0CC03986A
There are no conflicts to declare.
Notes and references
1
(a) K. H. Dötz, H. C. Jahr, In Carbene Chemistry; G. Bertrand,
Ed.; Fontis Media S. A.: New York, 2002; (b) F. Zaragoza
Dörwald, Metal Carbenes in Organic Synthesis; Wiley VCH:
Weinheim, Germany, 1999. (c) P. de Frꢀmont, N. Marion and
S. P. Nolan, Coord. Chem. Rev., 2009, 253, 862; (d) Handbook
of Metathesis Applications in Organic Synthesis, 2nd ed.; R. H.
Grubbs, D. J. O’Leary, Eds.; Wiley-VCH: Weinheim, Germany,
2015.
Figure 2. POV-ray drawing of 4 with thermal ellipsoids at the 50%
probability level. Hydrogen atoms with the exception of H1 and co-
crystallized solvent molecules have been omitted for clarity.
2
3
R. Beckhaus, Angew. Chem. Int. Ed. Engl., 1997, 36, 686.
Representative reviews on gem-dimetalloalkanes, see: (a) I.
Marek and J.-F. Normant, Chem. Rev., 1996, 96, 3241; (b) S.
Matsubara, K. Oshima and K. Utimoto, J. Organomet. Chem.,
2001, 617-618, 39; (c) I. Marek and J. F. Normant, In
Organozinc Reagents. -A Practical Approach; P. Knochel, P.
Jones, Eds.; Oxford University Press: Oxford, 1999; pp 119-
137; (d) J. F. Normant, Acc. Chem. Res., 2001, 34, 640; (e) K.
Endo, Bull. Chem. Soc. Jpn. 2017, 90, 649.
Figure 2 contained a dinuclear chromium coordinated with a DME
bridged by a methine carbon. Comparison of a previously isolated
gem-(dichromio)silylmethane complex having a TMEDA ligand5c
indicated that bond the lengths for C−Cr(dme) (2.051(4) Å, 2.062(4)
Å) were slightly shorter than that for C−Cr(tmeda), (2.082(3) Å).
Despite the shorter C−Cr distances and less hindered environments
around the chromium center of 4 compared to those for the TMEDA-
coordinated species, the weaker -donation ability of DME may
result in facile ligand dissociation to provide a vacant site for the
coordination of alkynylketones. This is one of the reasons why DME-
4
5
(a) D. A. Straus and R. H. Grubbs, Organometallics, 1982,
1658; (b) J. D. Meinhart, E. V. Anslyn and R. H. Grubbs,
Organometallics, 1989, , 583. For a Tebbe reagent, see: (c) F.
1,
8
N. Tebbe, G. W. Parshall and G. S. Reddy, J. Am. Chem. Soc.,
1978, 100, 3611. For the generation of related zirconium
alkylidene species, see: (d) F. W. Hartner Jr., J. Schwartz and S.
M. Clift, J. Am. Chem. Soc., 1983, 105, 640. (e) S. M. Clift and
J. Schwartz, J. Am. Chem. Soc., 1984, 106, 8300.
coordinated gem-(dichromio)silylmethane
4
showed better
performance for the transformation of alkynylketones than TMEDA-
coordinated species (Table S1 in ESI, entries 4 vs 8). Solution
susceptibility measurements obtained using Evans’ method24
revealed that complex 4 had S = 3 (eff = 6.64 B), where each
chromium(III) center had a d3 high-spin configuration with no metal-
metal bond, similar to the TMEDA-coordinated species (eff = 6.75
B).5c The reactivity of 4 as a key precursor to the chromium
alkylidene species was demonstrated in the addition and cyclization
leading to 1a (Eq 6). The reaction proceeded efficiently at 70 °C,
which indicated that heating was required for both the generation of
For gem-dichromiomethane species, see: (a) K. Takai, K. Nitta
and K. Utimoto, J. Am. Chem. Soc., 1986, 108, 7408; (b) T.
Okazoe, K. Takai and K. Utimoto, J. Am. Chem. Soc., 1987, 109
,
951. (c) M. Murai, R. Taniguchi, N. Hosokawa, Y. Nishida, H.
Mimachi, T. Oshiki and K. Takai, J. Am. Chem. Soc., 2017, 139
13184.
,
6
7
For a seminal work on Schrock-type alkylidene complexes,
see: (a) R. R. Schrock, J. Am. Chem. Soc., 1974, 96, 6796. For
reviews, see: (b) R. R. Schrock, Chem. Rev., 2002, 102, 145. (c)
R. R. Schrock and C. Copꢀret, Organometallics, 2017, 36, 1884.
For a seminal work, see: (a) E. O. Fischer and A. Maasbꢁl,
Angew. Chem. Int. Ed. Engl., 1964, 3, 580. For reviews, see: (b)
4
and the addition to alkynylketones. As expected, (E)-1,2-
D. F. Harvey and D. M. Sigano, Chem. Rev., 1996, 96, 271; (c)
M. A. Sierra, Chem. Rev., 2000, 100, 3591; (d) R. Aumann, Eur.
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56, 1257; (f) A. de Meijere, H. Schirmer and M. Duetsch,
Angew. Chem. Int. Ed., 2000, 39, 3964; (g) J. Barluenga, J.
Santamaría, and M. Tomꢂs, Chem. Rev., 2004, 104, 2259; (h)
H. K. Dꢁtz and J. Stendel, Jr. Chem. Rev., 2009, 109, 3227; (i)
D. I. Bezuidenhout, S. Lotz, D. C. Liles and B. van der
Westhuizen, Coord. Chem. Rev., 2012, 256, 479.
(a) S. Hao, J.-I. Song, P. Berno and S. Gambarotta,
Organometallics, 1994, 13, 1326; (b) M. P. Coles, V. C.; Gibson,
W. Clegg, M. R. J. Elsegood and P. A. Porrelli, Chem. Commun.,
1996, 1963; (c) J. A. N. Ajjou, S. L. Scott and V. Paquet, J. Am.
Chem. Soc., 1998, 120, 415; (d) P. Wu, G. P. A. Yap and K. H.
Theopold, J. Am. Chem. Soc., 2018, 140, 7088; (e) P. Wu, G. P.
A. Yap and K. H. Theopold, Organometallics, 2019, 38, 4593.
Although Cr(VI) salts are highly toxic, Cr(II)Cl2 possesses lower
toxicity than MnCl2, FeCl2, NiCl2, and PdCl2. See: (a) A. K. Steib,
O. M. Kuzmina, S. Fernandez, D. Flubacher and P. Knochel, J.
Am. Chem. Soc., 2013, 135, 15346; (b) K. S. Egorova and V. P.
Ananikov, Organometallics, 2017, 36, 4071. For reviews on
chromium-promoted reactions, see: (c) A. Fürstner, Chem.
Rev., 1999, 99, 991; (d) L. A. Wessjohann and G. Scheid,
Synthesis 1999, 1; (e) K. Takai, Org. React., 2004, 64, 253; (f)
bis(trimethylsilyl)ethene was formed quantitatively in the absence of
alkylketones.
8
9
In conclusion, the present study demonstrated the rare generation
of alkylidene equivalents from gem-dimetalloalkanes (Path D in
Figure 1). gem-Dichromiomethanes acted as a chromiumalkylidene
equivalent to promote carbene/alkyne metathesis13 followed by
capture by polar C=O bonds to yield functionalized carbo- and
heterocycles. Formation of alkeynylcycloalkenes via the attack of an
alkenylcarbene intermediate to a carbonyl group indicated that the
carbene equivalents generated possessed Schrock-type nucleophilic
reactivity. Application of this strategy for preparation of early
transition metal-based alkylidene species is ongoing.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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K. M. Smith, Coord. Chem. Rev., 2006, 250, 1023; (g) H. 21 Sequential alkyne insertion occurred in the reaction with
Yamamoto and G. Xia, Chem. Lett., 2007, 36, 1082; (h) K. Takai,
In Comprehensive Organic Synthesis, 2nd ed.; G. A. Molander,
simple terminal alkynes without forming the cVoierwreAsrtpicolenOdnilninge
cyclopropenes. On the other hand, reaDcOtioI:n10o.1f0s3i9m/Dp0leCkCe0t3o9n86eAs
gave the corresponding olefins albeit in low yields. Given this
high reactivity toward ethynyl groups, cycloisomerisation of
enynes generated via the direct alkylidenation of keto
carbonyl group in the initial stage is unlikely as a reaction
P. Knochel, Eds. Elsevier, Ox-ford, 2014, 1, 159-203.
10 (a) J. Villieras, C. Bacquet and J.-F. Normant, Bull. Soc. Chim.
Fr., 1975, 1797; (b) J. A. Bull and A. B. Charette, J. Org. Chem.,
2008, 73, 8097; (c) D. S. W. Lim and E. A. Anderson, Org. Lett.,
2011, 13, 4806.
mechanism.
The
possibility
of
intramolecular
followed by
11 For cyclization of alkynylketones, see: (a) T. Jin and Y.
[2+2]cycloaddition
of alkynylketones
Yamamoto, Org. Lett., 2007,
9
, 5259. Cyclization with Fischer-
alkylidenation might be another potential mechanism, but
this route is also unlikely because formation of the key
intermediate, 1-cyclopentene-1-carbaldehydes, was never
observed.
type electrophilic ruthenium carbene complexes generated in
situ gave dihydropyrans. See: (b) F. Cambeiro, S. López, J. A.
Varela and C. Saá, Angew. Chem. Int. Ed., 2014, 53, 5959.
12 For reviews on the catalytic cyclization of alkynals, see: (a) M. 22 The byproduct, Me3SiCH=CHSiMe3, might be formed via
C. Willis, Chem. Rev., 2010, 110, 725; (b) K. Tanaka and Y.
Tajima, Eur. J. Org. Chem., 2012, 3715.
13 For pioneering works on carbene/alkyne metathesis, see: (a)
T. R. Hoye, C. J. Dinsmore, D. S. Johnson and P. F. Korkowski,
J. Org. Chem., 1990, 55, 4518; (b) A. Padwa, D. J. Austin and S.
radical or chromium-mediated coupling reaction of alkylidene
species Because formation of the corresponding
phosphonium ylide, Me3SiCH=PMe3, was not observed in the
presence of PMe3, dimerization of the metal-free carbene
species can be ruled out as a possible reaction mechanism.
A
.
L. Xu, Tetrahedron Lett., 1991, 32, 4103. For reviews, see: (c) 23 gem-(Dichromio)silylmethane was generated at 25 °C in THF,
ꢃ. Torres and A. Pla-Quintana, Tetrahedron Lett., 2016, 57
,
whereas heating to 70 °C was required in DME. The THF-
coordinated gem-(dichromio)silylmethane 4’ could not be
structurally characterized due to the amorphous character of
the solid obtained and the difficulty of separating it from the
byproduct CrCl2I(thf)3.
3881; (d) C. Pei, C. Zhang, Y. Qian, and X. Xu, Org. Biomol.
Chem., 2018, 16, 8677.
14 M. Murai, R. Taniguchi and K. Takai, Org. Lett., 2020, 22, 3985.
15 For the use of manganese to reduce Cr(III)X3 species, see: (a)
A. Fürstner and N. Shi, J. Am. Chem. Soc., 1996, 118, 2533; (b) 24 (a) D. F. Evans, J. Chem. Soc., 1959, 2003; (b) E. M. Schubert,
A. Fürstner and N. Shi, J. Am. Chem. Soc., 1996, 118, 12349;
(c) K. Takai, Y. Kunisada, Y. Tachibana, N. Yamaji and E.
Nakatani, Bull. Chem. Soc. Jpn., 2004, 77, 1581. Addition of
MnCl2, which accumulated during the catalytic process, did
not promote the reaction.
J. Chem. Educ., 1992, 69, 62.
16 Attempted addition of chlorotrimethylsilane to reduce
Cr(=O)Cl2 species back to CrCl2 by deoxygenation decreased
the yield of 1a
.
17 For representative examples, see: (a) K. Takai, S. Toshikawa,
A. Inoue and R. Kokumai, J. Am. Chem. Soc., 2003, 125, 12990;
(b) M. Murai, C. Mizuta, R. Taniguchi and K. Takai, Org. Lett.,
2017, 19, 6104; (c) M. Murai, R. Taniguchi, C. Mizuta and K.
Takai, Org. Lett., 2019, 21, 2668.
18 For chromium-mediated silyl-, boryl-, stannyl-, and
iodoalkylidenation, see: (a) K. Takai, Y. Kataoka, T. Okazoe and
K. Utimoto, Tetrahedron Lett., 1987, 28, 1443; (b) D. M.
Hodgson, Tetrahedron Lett., 1992, 33, 5603; (c) D. M. Hodgson,
A. M. Foley and P. J. Lovell, Tetrahedron. Lett., 1998, 39, 6419;
(d) K. Takai, T. Ichiguchi and S. Hikasa, Synlett, 1999, 1268; (e)
J. R. Falck, R. Bejot, D. K. Barma, A. Bandyopadhyay, S. Joseph
and C. Mioskowski, J. Org. Chem., 2006, 71, 8178; (f) R. Baati,
C. Mioskowski, D. Kashinath, S. Kodepelly, B. Lu and J. R. Falck,
Tetrahedron Lett., 2009, 50, 402.
19 Use of 1-phenyl-5-hexyn-1-one and 3-phenacyloxy-1-propyne
as substrates gave their oligomers via alkyne metathesis
without forming the corresponding alkenylcycloalkenes,
which indicates that substituents on the linker affect the
efficiency of cyclization. Although reaction with 6-
alkynylketones,
including
diethyl
and
(3-oxo-3-
3-(2-
phenylpropyl)(prop-2-ynyl)malonate
ethynylphenyl)-1-phenylpropan-1-one, was also examined,
the expected 6-membered ring alkenylcycloalkenes were not
obtained.
20 Reactions with other gem-dichromiomethanes having boryl,
stannyl, iodo, and alkyl groups were examined, yields of the
expected alkenylcycloalkenes were unfortunately less than
30%. The presence of the relatively bulky silyl group seems to
be important to stabilize the gem-dichromiomethanes and
chromium alkylidene species. Because the present reaction
requires heating to 70 °C, it is likely that the gem-
dichromiomethane species without appropriate substituents
decomposed during the reaction with alkynylketones.
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