Stereoselective pinacol coupling of (propargyl aldehyde)Co2(CO)6 complexes

Article abstract of DOI:10.1021/om030392t

Propargyl aldehyde complexes, (RC≡CCHO)Co₂(CO)₆ (1a-c; R = Ph, Me, H), undergo pinacol coupling with a variety of reductants, exclusively giving syn (dl)-diacetylenic diol complexes 2a-c. The X-ray structures of 2a and 2c have been determined. The free diol syn-PhC≡C-CH(OH)CH(OH)-C≡CPh (4a) is obtained efficiently from its cobalt complex 2a upon treatment with (NH₄)₂Ce(NO₃)₆.

Full text of DOI:10.1021/om030392t

4260
Organometallics 2003, 22, 4260-4264
Ster eoselective P in a col Cou p lin g of (P r op a r gyl
a ld eh yd e)Co2(CO)6 Com p lexes
K. Lake, M. Dorrell, N. Blackman, M. A. Khan, and K. M. Nicholas*
Department of Chemistry and Biochemistry, University of Oklahoma,
Norman, Oklahoma 73019
Received May 27, 2003
Propargyl aldehyde complexes, (RCtCCHO)Co2(CO)6 (1a -c; R ) Ph, Me, H), undergo
pinacol coupling with a variety of reductants, exclusively giving syn (dl)-diacetylenic diol
complexes 2a -c. The X-ray structures of 2a and 2c have been determined. The free diol
syn-PhCtC-CH(OH)CH(OH)-CtCPh (4a ) is obtained efficiently from its cobalt complex
2a upon treatment with (NH4)2Ce(NO3)6.
In tr od u ction
important issue since pinacols (head/head coupling), 1,6-
dicarbonyl (tail/tail coupling), and γ-hydroxycarbonyls
(head/tail coupling) are possible. Regioisomers are fre-
quently obtained in such reactions, although the Lewis
acidity and complexing power of the low-valent metal
can enable 1,2-coupling in some cases. Reductive cou-
pling of R,â-unsaturated carbonyl compounds is usually
not highly stereoselective. Exceptions are provided by
the ill-defined reductants Cp2TiCl2/sec-BuMgCl5 and
TiI4,6 which are reported to give excellent syn (d,l)-
selectivity in the coupling of acrolein derivatives.
The pinacolization of propargyl aldehydes/ketones has
received rather little attention (eq 2). Conventional
reductants (e.g., Zn/HOAc; Zn-Cu/HOAc) effect these
reactions with only low to moderate yields and variable
meso-stereoselectivity,7 probably a result of competing
regiochemical coupling pathways and the modest steric
demand of the linear acetylenic unit. The aforemen-
tioned reagent, TiI4, has been found to induce syn se-
lective pinacolization of phenylpropynal.6 The diacety-
lenic diol products are attractive intermediates for the
preparation of densely functionalized natural (and un-
natural) products via established regio- and stereocon-
trolled transformations of the alkyne units.
The reductive (pinacol) coupling of carbonyl com-
pounds is a useful method for the creation of C-C bonds
with 1,2-difunctionality (eq 1).1 Although detailed mecha-
nistic studies of pinacol coupling are lacking, the reac-
tions are generally considered to involve the generation
and reaction of the substrate ketyl (radical anion) with
either the neutral substrate or another ketyl species.
Unfortunately, traditional metal reductants for pina-
colization rarely afford useful stereoselectivity and are
intolerant of many functional groups. Recently, more
efficient and stereoselective pinacol couplings of aro-
matic aldehydes (including enantioselective variants)
have been developed using milder stoichiometric reduc-
tants (e.g., Zn, Mn, etc.) in combination with transition
metal catalysts (e.g., Cp2TiCl2, Cp2VCl2, etc.).2 The more
difficult-to-reduce and less sterically biased aliphatic
aldehydes and ketones typically react more sluggishly
and exhibit poorer stereoselectivities.1,3,4
When R,â-unsaturated carbonyl compounds are re-
ductively coupled, regioselectivity also becomes an
(1) Review(s): Robertson, G. M. Pinacol Coupling Reactions. In
Comprehensive Organic Synthesis; Trost, B. M., Ed.; Vol. 3, Pergamon
Press: New York, 1991; pp 563-612. Avalos, M.; Babiano, R.; Cintas,
P.; J imenez, J . L.; Palacios, J . C. Recent Research Developments in
Organic Chemistry; 1997; Vol. 1, pp 159-178. Wirth, T. Angew. Chem.,
Int. Ed. 1996, 35, 61.
(2) Lipski, T. A.; Hilfiker, M. A.; Nelson, S. G. J . Org. Chem. 1997,
62, 4566. Bandini, M.; Cozzi, P. G.; Morganti, S.; Umani-Ronchi, A.
Tetrahedron Lett. 1999, 40, 1997. Nomura, R.; Matsuno, T.; Endo, T.
J . Am. Chem. Soc. 1996, 118, 11666. Gansauer, A. Chem. Commun.
1997, 457. Gansauer, A.; Bauer, D. J . Org. Chem. 1998, 63, 2070.
Gansauer, A.; Bauer, D. Eur. J . Org. Chem. 1998, 2673.
Our group and others have begun to investigate the
potential stabilizing and reactivity-altering effects of
coordinating transition metal fragments to organic
radicals. In this context (propargyl)Co2(CO)6 radicals
have been found to undergo novel and highly stereose-
lective 1,1-dimerizations8,9 and regioselective atom-
(3) Hirao, T.; Hatano, B.; Asahara, M.; Muguruma, Y.; Ogawa, A.
Tetrahedron Lett. 1998, 39, 5247.
(5) Inanaga, J .; Hanada, Y. Tetrahedron Lett. 1987, 28, 5717.
(6) Shimizu, M.; Goto, H.; Haykawa, R. Org. Lett. 2002, 4, 4097.
(7) Durand, M. H. Compt. Rend. 1958, 246, 3469. Piaux, L.; Durand,
M. H. C. R. Acad. Sci. Chim. 1956, 243, 1774. Durand, M. H. Compt.
Rend. 1958, 246, 1562.
(8) Melikyan, G. G.; Combs, R. C.; Lamirand, J .; Khan, M.; Nicholas,
K. M. Tetrahedron Lett. 1994, 35, 363. Melikyan, G. G.; Khan, M. A.;
Nicholas, K. M. Organometallics, 1995, 14, 2170.
(4) Dunlap, M. S.; Nicholas, K. M. J . Organomet. Chem. 2001, 630,
125. Dunlap, M.; Nicholas, K. M. Synth. Commun. 1999, 29, 1097.
Mukaiyama, T.; Yoshimura, N.; Igarashi, K.; Kagayama, A. Tetrahe-
dron 2001, 57, 2499. Groth, U.; J eske, M. Angew. Chem., Int. Ed. 2000,
39, 574. Yamamoto, Y.; Hattori, R.; Miwa, T.; Nakagai, Y.; Kubota,
T.; Yamamoto, C.; Okamoto, Y.; Itoh, K. J . Org. Chem. 2001, 66, 3865.
10.1021/om030392t CCC: $25.00 © 2003 American Chemical Society
Publication on Web 09/16/2003

Stereoselective Pinacol Coupling
Organometallics, Vol. 22, No. 21, 2003 4261
transfer cyclizations,10 suggesting the operation of
significant stabilizing and stereodirecting effects on the
radical by the -Co2(CO)6 unit. Stereoselective reactions
presumed to involve intermediate R-(aryl)Cr(CO)3 radi-
cals have also been reported recently11 and evaluated
computationally;12 these include SmI2-induced, syn-
selective pinacol coupling of (aryl aldehyde)Cr(CO)3
complexes.13
The potential for achieving regio- and stereoselective
reactions of (R-CtC-CHO)Co2(CO)6 complexes has
been demonstrated, including the addition enol deriva-
tives and chiral allyl boranes as nucleophiles.14 In view
of the synthetic potential of 3,4-difunctionalized-1,5-
diynes and the growing interest in the chemistry of
radicals derived from transition metal complexes, we
report herein an investigation of the pinacol coupling
of (propargyl aldehyde)Co2(CO)6 complexes.
Ta ble 1. P in a coliza tion of (P r op a r gyl
a ld eh yd e)Co2(CO)6 Com p lexes
aldehyde reductant (equiv) solvent pinacol (2) % yielda,b
1a
Cp2TiCl2 (2),
sec-BuMgCl (2)
TiCl3
THF
syn-2a
54
1a
1a
1a
CH2Cl2
HOAc
THF
syn-2a
syn-2a
syn-2a
51 (69)
8c
Zn
Cp2TiCl2 (0.1),
Mn (0.55),
Me3SiCl (1.1)
Cp2TiCl2 (0.1),
SmI2 (2.0)
43d
1a
THF
syn-2a
45 (72)
1a
1b
SmI2
THF
THF
syn-2a
syn-2b
32 (71)e
50 (67)
Cp2TiCl2 (1),
sec-BuMgCl (1)
Cp2TiCl2 (0.1),
Mn (0.55),
Me3SiCl (1.1)
SmI2 (2.5)
1b
THF
syn-2b
49 (74)
45
1c
THF
syn-2c
a
b
Isolated yield. Number in parentheses is the yield based on
recovered aldehyde 1. c 30% of alcohol 3a also formed. 18% of
d
3a formed. e 41% of 3a formed.
Resu lts a n d Discu ssion
The representative acetylenic aldehyde complexes
1a -c were prepared in good yield either by hydrolysis
of the corresponding acetal derivatives (for 1a ,b) or by
direct complexation of the aldehyde (1c) with Co2(CO)8.
The aldehyde complexes were then subjected to reaction
with several pinacolization reagents, including Zn/
HOAc, Cp2TiCl2/sec-BuMgCl,5 TiCl315 Cp2TiCl2/Mn/Me3-
SiCl,16 SmI2,17 and Cp2TiCl2/SmI2 (eq 3). In general, the
reactions proceeded at or below room temperature.
Following workup the products were separated by
chromatography and were identified spectroscopically
and (in two cases) by X-ray diffraction. A summary of
the results is provided in Table 1.
Key features of these reactions include (1) pinacols
(2a -c) are the exclusive product complexes in most
systems, with the exception of the reactions of 1a with
Zn/HOAc, SmI2, and Mn/Cp2TiCl2/Me3SiCl, which pro-
duced significant amounts of the alcohol 3a resulting
from simple reduction; (2) a single pinacol diastereomer
was isolated in all instances, established as the syn
isomer (vide infra); (3) the yields of 2 are moderate and
largely independent of the substrate complex or the
reductant. The mass balance in the reactions was only
fair (60-70%), the result of significant demetalation. In
contrast to the selectivity observed in the pinacolization-
complexed aldehydes 1a -c, free PhCtCCHO afforded
a complex mixture of products upon treatment with
Cp2TiCl2/sec-BuMgCl.
1
Although it was apparent from the H NMR spectra
of the products 2a -c that single isomers were formed,
their relative stereochemistry was uncertain. To address
this issue, crystals of 2a and 2c were produced and
subjected to X-ray structure determination. As can be
seen from Figures 1 and 2, the relative stereochemistry
of these compounds is syn (d,l). In the solid state 2a
adopts a conformation in which the two bulky -(C2Ph)-
Co2(CO)6 units are anti and the two hydroxyl groups
are gauche. Interestingly, although 2c is also the syn
diastereomer, both the OH groups and the organocobalt
units are gauche to each other. The reason for the
different conformations of 2a and 2c is not certain but
could reflect the differing steric demand of the Co2C2
clusters when capped by -Ph versus -H. As is typical
of (alkyne)Co2(CO)6 complexes,18 the coordinated alkyne
units in 2a and 2c are severely bent, with angles of
143-145° defined by the coordinated alkyne carbon
atoms and the R-carbons. This feature, and the at-
tendant lengthening of the C-C bond of the coordinated
alkyne (1.33 vs 1.21 Å), reflects substantial rehybrid-
ization of and strong back-bonding to the alkyne from
the cobalt carbonyl moiety. The methanol molecule of
crystallization with 2c is involved in hydrogen bonding
with the diol’s two hydroxyl groups.
(9) Melikyan, G. G.; Deravakian, A.; Myer, S.; Yadegar, S.; Hard-
castle, K. I.; Ciurash, J .; Toure, P. J . Organomet. Chem. 1999, 578,
68. Gruselle, M.; Malezieux, B.; Andres, R.; Amouri, H.; Vaissermann,
J .; Melikyan, G. G. Eur. J . Inorg. Chem. 2000, 2, 359. Melikyan, G.
G.; Deravakian, A. J . Organomet. Chem. 1997, 544, 143.
(10) Salazar, K. L.; Khan, M. A.; Nicholas, K. M. J . Am. Chem. Soc.
1997, 119, 9053. Salazar, K. L.; Nicholas, K. M. Tetrahedron 2000,
56, 2211.
(11) Merlic, C. A.; Walsh, J . C. J . Org. Chem. 2001, 66, 2265. Merlic,
C. A.; Walsh, J . C. Tetrahedron Lett. 1998, 39, 2083. Kuendig, E. P.;
Ratni, H.; Crousse, B.; Bernardinelli, G. J . Org. Chem. 2001, 66, 1852.
Hoffmann, O.; Schmalz, H.-G. Synlett 1998, 12, 1426. Schmalz, H.-G.;
Siegel, S.; Schwarz, A. Tetrahedron Lett. 1996, 37, 2947. Schmalz, H.-
G.; Siegel, S.; Bats, J . W. Angew. Chem., Int. Ed. Engl. 1995, 34, 2383.
(12) Merlic, C. A.; Walsh, J . C.; Tantillo, D. J .; Houk, K. N. J . Am.
Chem. Soc. 1999, 121, 3596. Merlic, C. A.; Hietbrink, B. N.; Houk, K.
N. J . Org. Chem. 2001, 66, 6738. Merlic, C. A.; Miller, M. M.; Hietbrink,
B. N.; Houk, K. N. J . Am. Chem. Soc. 2001, 123, 4904.
(13) Taniguchi, N.; Uemura, M. Tetrahedron 1998, 54, 12775.
Taniguchi, N.; Uemura, M. Tetrahedron Lett. 1998, 39, 5385. Tanigu-
chi, N.; Kaneta, N.; Uemura, M. J . Org. Chem. 1996, 61, 6088.
(14) J u, J .; Reddy, B. R.; Khan, M.; Nicholas, K. M. J . Org. Chem.
1989, 54, 5426. Ganesh, P.; Nicholas, K. M. J . Org. Chem. 1993, 58,
5587. Roush, W. R.; Park, J . C. Tetrahedron Lett. 1991, 32, 6285.
(15) Clerici, A.; Porta, O. Tetrahedron Lett. 1982, 23, 3517. Clerici,
A.; Porta, O. Tetrahedron 1982, 38, 1293. Clerici, A.; Porta, O.; Zago,
P. Tetrahedron 1986, 42, 561.
(16) Dunlap, M. S.; Nicholas, K. M. Synth. Commun. 1999, 29, 1097.
(17) Annunziata, R.; Benaglia, M.; Cinquini, M.; Raimondi, L. Eur.
J . Org. Chem. 1999, 3369, 9. Pedersen, H. L.; Christensen, T. B.;
Enemaerke, R. J .; Daasbjerg, K.; Skrydstrup, T. Eur. J . Org. Chem.
1999, 565, 5. Nomura, R.; Matsuno, T.; Endo, T. J . Am. Chem. Soc.
1996, 118, 11666. Namy, J . L.; Souppe, J .; Kagan, H. B. Tetrahedron
Lett. 1983, 24, 765.
Importantly, the free pinacols can be recovered from
their cobalt complexes efficiently and without loss of
(18) Dickson, R. S.; Fraser, P. J . Advances in Organometallic
Chemistry 12; Academic Press: New York, 1974; p 323.

4262 Organometallics, Vol. 22, No. 21, 2003
Lake et al.
hyde-cobalt complexes stands in contrast to the limited
or meso preference for the pinacolization of free acety-
lenic aldehydes.7 This probably is a result of the greatly
increased steric demand of the (alkynyl)Co2(CO)6 unit
vis a` vis the free alkynyl group. Although the exact
transition state for these reactions is uncertain, a cyclic
arrangement involving the metal (or organometallic)
reductant and the coupling aldehydes (ketyls) is likely
(Figure 3). The trans orientation of the bulky (alkynyl)-
Co2(CO)6 () R′) groups would minimize their steric
interaction in the transition state and lead to the
formation of the observed syn-diol. The insensitivity of
the stereoselectivity to the nature of the reductant is
noteworthy in light of the pinacolizations of aromatic
aldehydes, which show significant variation of the
stereoselectivity with the particular reducing system.19
Even the low pinacolization stereoselectivity often found
F igu r e 1. ORTEP diagram of 2a . Selected bond lengths
(Å) and angles (deg): Co(1)-C(13) 1.967(4), Co(1)-C(14)
1.958(4), Co(1)-Co(2) 2.4647(9), Co(2)-C(13) 1.954(4), Co-
(2)-C(14) 1.956(4), O(1)-C(1) 1.129(6), C(13)-C(14) 1.336-
(5), C(14)-C(15) 1.503(5), C(2)-Co(1)-C(1) 101.0(2), O(1)-
C(1)-Co(1) 177.9(5), C(13)-C(14)-C(15) 142.8(4).
16
with SmI2 is overridden in its reaction with complex
1c. The extraordinary syn selectivity for all three com-
plexes 1a -c is synthetically appealing but not com-
pletely anticipated since the bent coordinated alkyne
unit could place the sterically variable acetylenic sub-
stituent R in proximity of the reacting carbonyl group.20
The partial decomposition suffered by the cobalt
complexes under the reaction conditions indicates the
intervention of competing reactions during the forma-
tion of and/or subsequent coupling of the complexed
aldehyde ketyl. (Alkyne)Co2(CO)6 complexes are known
to undergo electrochemical reduction,21 which is chemi-
cally reversible with electron-deficient alkynes but
irreversible (with decomposition to Co(CO)4-) with more
electron-rich alkynes. These observations raise the
question of the unpaired electron (and charge) density
in the (presumed) intermediate ketyl complexes. EPR
studies of the (alkyne)Co2(CO)6 radical anions and
supporting extended Hu¨ckel MO calculations indicate
substantially delocalized unpaired electron density,
including at the cobalt atoms and the associated car-
bonyl ligands.21 Additionally, we have calculated the
LUMO of aldehyde complex 1c and the HOMO of its
radical anion 1c• using the semiempirical PM3 method.22
Both calculations suggest that addition of an electron
to 1 will result in a delocalized species with only a small
fraction of radical character at the aldehydic carbon,
F igu r e 2. ORTEP diagram of 2c. Selected bond lengths
(Å) and angles (deg): Co(1)-C(7) 1.942(3), Co(1)-C(8)
1.950(3), Co(1)-Co(2) 2.4648(9), Co(2)-C(7) 1.950(4), Co-
(2)-C(8) 1.953(3), O(1)-C(1) 1.118(5), C(7)-C(8) 1.325(5),
C(8)-C(9) 1.490(4), C(3)-Co(1)-C(2) 97.54(18), O(1)-
C(1)-Co(1) 175.9(4), C(7)-C(8)-C(9) 145.6(3).
(19) Handa, Y.; Inanaga, J . Tetrahedron Lett. 1987, 46, 5717.
Barden, M. C.; Schwartz, J . J . Am. Chem. Soc. 1996, 118, 5484. Clerici,
A.; Clerici, L.; Porta, O. Tetrahedron Lett. 1996, 37, 3035. Szymoniac,
J .; Besancon, J .; Moise, C. Tetrahedron 1994, 50, 2841. Kammermeier,
B.; Beck, G.; J acobi, D.; J endralla, H. Angew. Chem., Int. Ed. Engl.
1994, 33, 685. Kempf, D. J .; Sowin, T. J .; Doherty, E. M.; Hannick, S.
M.; Codavoci, L.; Henry, R. F.; Green, B. E.; Spanton, S. G.; Norbeck,
D. W. J . Org. Chem. 1992, 57, 5692. Konradi, A. W.; Kemp, S. J .;
Pedersen, S. F. J . Am. Chem. Soc. 1994, 116, 1316. Barden, M. C.;
Schwartz, J . J . Org. Chem. 1997, 62, 7520.
F igu r e 3. Possible transition states for pinacolization.
(20) Examples of R-dependent stereoselectivity have been reported
in nucleophilic additions to (R-CtC-CHR′)Co2(CO)6 complexes; see:
Schreiber, S. L.; Sammakia, T.; Crowe, W. E. J . Am. Chem. Soc. 1986,
108, 3128, and ref 14.
(21) Peake, B. M.; Rieger, P. H.; Robinson, B. H.; Simpson, J . J .
Am. Chem. Soc. 1980, 102, 156. Arewgoda, M.; Rieger, P. H.; Robinson,
B. H.; Simpson, J .; Visco, S. J . J . Am. Chem. Soc. 1982, 104, 5633.
(22) Cundari, T. J . Chem. Inf. Comput. Sci. 1999, 39, 376.
stereochemical integrity upon treatment with mild
oxidants. For example, complex 2a reacted with (NH4)2-
Ce(NO3)6/acetone at -78 °C to give syn-1,6-diphenyl-
1,5-hexadiyne-3,4-diol (4a ) in 89% yield (eq 4).
The high (complete) syn (d,l) stereoselectivity ob-
served in the pinacol coupling of the acetylenic alde-

Stereoselective Pinacol Coupling
Organometallics, Vol. 22, No. 21, 2003 4263
Ta ble 2. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
for 2a a n d 2c
possibly accounting for the incomplete C-C coupling
chemoselectivity found in the reactions. It is possible
that electronic variations of R or of auxiliary ligands
could enhance the coupling chemoselectivity by concen-
trating radical character at the carbonyl group.
In summary, (propargyl aldehyde)Co2(CO)6 complexes
have been found to undergo completely regio- and syn-
stereoselective pinacol coupling reactions with several
reducing agents, providing new illustrations of the
extraordinary stereoselectivities associated with pro-
pargyl-cobalt radical species. The derived 1,5-diyne-syn-
3,4-diols hold considerable promise as intermediates in
the synthesis of densely functionalized products bearing
multiple adjacent stereocenters.
2a
2c
empirical formula
fw
temperature
wavelength
cryst syst
C
30H14Co4O14
C
19H10Co4O15
834.13
713.99
173(2) K
0.71073 Å
triclinic
P1h
188(2) K
0.71073 Å
monoclinic
C2/c
space group
unit cell dimens
a ) 12.203(3) Å, a ) 9.262(3) Å
b ) 12.011(2) Å, b ) 10.983(3) Å
c ) 21.905(5) Å
R ) 90°
c ) 13.797(3) Å
R ) 89.613(18)°
â ) 72.026(18)°
γ ) 69.918(19)°
1246.0(6) Å3
2
â ) 95.795(16)°
γ ) 90°
volume
3194.0(12) Å3
4
Z
density (calcd)
abs coeff
F(000)
1.735 Mg/m3
2.110 mm-1
1656
1.903 Mg/m3
2.689 mm-1
704
Exp er im en ta l Section
All reactions of the cobalt complexes were conducted under
a nitrogen atmosphere. 1H NMR spectra were obtained at
300 MHz with CDCl3 or C6D6 as solvent. (Phenylpropynal
diethylacetal)Co2(CO)6 and (2-butynal)Co2(CO)6 were prepared
from the reaction of the commercial diethyl acetals and Co2-
(CO)8 as reported.23 Compounds 1a and 1b were prepared by
hydrolysis of the corresponding diethyl acetal complexes;24 1c
was prepared by the reaction of propynal with dicobalt
octacarbonyl.23 PM3 MO calculations22 were carried out using
the MacSpartan Plus program (Wavefunction).
cryst size
0.24 × 0.32 ×
0.42 × 0.36 ×
0.34 mm3
0.32 mm3
θ range for data collection 1.87 to 26.00°
index ranges
2.39 to 25.00°
-10 < h <0,
-12 < k <12,
-16 < l <15
4638
4350 [R(int) )
0.0613]
99.5%
empirical
0 < h < 15,
0 < k < 14,
-27 < l < 26
3298
3149 [R(int) )
0.0467]
no. of reflns collected
no. of ind reflns
completeness to θ ) 25.00° 99.9%
abs corr
none
P in a coliza tion of (P h en ylp r op yn a l)d icoba lt Hexa ca r -
bon yl (1a ). Cp 2TiCl2/sec-Bu MgCl. Cp2TiCl2 (0.25 g, 1.0
mmol) was placed in a sidearm flask with a stirring bar under
nitrogen atmosphere, and THF (10 mL) was added. The flask
was cooled to -78 °C, and then sec-BuMgCl in ether (0.48 mL,
1.0 mmol) was transferred to the flask. The solution was
stirred for 30 min, allowed to warm slowly to room tempera-
ture, and then recooled to -78 °C. A solution of the 1a (0.20
g, 0.50 mmol) dissolved in THF (15 mL) was then added. The
mixture was stirred for 30 min and allowed to warm to room
temperature, and then NaOH (5M, 0.5 mL) was added; this
mixture was stirred for 45 min. The solution was filtered
through a short column of potassium carbonate topped with a
layer of Celite, eluting with THF (250 mL). Column chroma-
tography on silica gel (95:5 petroleum ether/ether) afforded
the diol 2a (0.11 g).
TiCl3. Aldehyde 1a (1.35 g, 3.25 mmol) was dissolved in
CH2Cl2 (10 mL) in a sidearm flask containing a stirring bar
and fitted with a septum under nitrogen. A solution of TiCl3
in HCl/water (10 wt %, 10 mmol) was added by syringe, and
the mixture was stirred at room temperature for 5 h. The
reaction was quenched by the addition of water (10 mL) and
stirred for 18 h. The mixture was transferred to a separatory
funnel, and the phases were separated. The aqueous layer was
washed twice with ethyl acetate, and the combined organic
phases were dried over MgSO4. Silica gel flash chromatography
of the concentrated organic residue (98:2 petroleum ether/
ether) provided unreacted aldehyde 1a (0.25 g) and diol 2a
(0.56 g).
refinement method
full-matrix least- full-matrix least-
squares on F2
squares on F2
no. of data/restraints/
params
3149/0/221
4350/3/353
goodness-of-fit on F2
final R indices
[I>2σ(I)]
1.096
1.036
R1 ) 0.0465,
wR2 ) 0.1154
R1 ) 0.0661,
wR2 ) 0.1342
0.673 and
R1 ) 0.0415,
wR2 ) 0.1005
R1 ) 0.0549,
wR2 ) 0.1085
0.803 and
R indices (all data)
largest diff peak and hole
-1.015 e‚Å-3
-0.744 e‚Å-3
Sm I2. 1a (0.078 g, 0.19 mmol) was placed in a sidearm flask
under N2 with a magnetic stir bar. A 0.10 M solution of SmI2
in THF (5.6 mL, 0.56 mmol) was added to the solid 1a , and
the mixture was stirred for 3 h at room temperature. The
reaction was terminated by addition of 5 M NaOH (5 mL).
Diethyl ether (50 mL) was added to the reaction mixture, and
the organic phase was washed with saturated NaCl solution
(3 × 50 mL) and then dried over MgSO4. Volatiles were
removed by rotary evaporation, and the products were isolated
by preparative TLC on silica gel, affording aldehyde 1a
(0.0023 g), diol 2a (0.025 g), and alcohol 3a (0.0032 g).
Sm I2/Cp 2TiCl2. Cp2TiCl2 (0.12 g, 0.50 mmol) was placed in
a sidearm flask with a stirring bar under nitrogen. The flask
was cooled to -78 °C, and then a 0.1 M solution of SmI2 in
THF (4.9 mL, 0.5 mmol) was transferred to the flask. The
mixture was stirred for 30 min at -78 °C, slowly warmed to
room temperature, and then recooled to -78 °C. The cooled
mixture was then transferred via cannula to a chilled flask
(-78 °C) containing 1a (0.10 g, 0.25 mmol), and the mixture
was stirred for 30 min. After warming to room temperature,
0.1 M HCl (5 mL) was added and stirring was continued for
15 min. Diethyl ether (50 mL) was added, and the organic
phase was washed three times with saturated NaCl solution
and then dried over MgSO4. Rotary evaporation of the solution
volatiles and silica gel preparative TLC gave 1a (0.027 g) and
diol complex 2a (0.046 g).
Zn /HOAc. Aldehyde 1a (0.090 g, 0.22 mmol) was placed in
a sidearm flask containing HOAc (10 mL) and a stirring bar.
The mixture was stirred while Zn (0.10 g, 1.5 mmol) was added
to the solution; stirring was continued for 20 h. The residual
zinc was removed by filtration, and the solution was washed
with water, followed by 10% NaHCO3. The organic phase was
rotary evaporated and the residue purified by preparative TLC
(95:5 petroleum ether/ether): recovered 1a (0.010 g), diol 2a
(0.007 g), and alcohol 3a (0.030 g) were isolated.
Mn /Cp 2TiCl2/TMSCl. Into a sidearm flask 4 Å molecular
sieves (2.0 g), Mn (0.0060 g, 0.10 mmol), Cp2TiCl2 (0.0050 g,
0.018 mmol), and a stirring bar were added under nitrogen
followed by 10 mL of THF. This mixture was allowed to stir
for 20 min, followed by the addition of TMSCl (0.026 mL, 0.20
mmol). A solution of aldehyde 1a (0.076 g, 0.18 mmol) in 10
(23) Stuart, J . G.; Nicholas, K. M. Synthesis 1989, 6, 454. Tester,
R.; Varghese, V.; Montana, A. M.; Khan, M.; Nicholas, K. M. J . Org.
Chem. 1990, 55, 186.
(24) Ganesh, P.; Nicholas, K. M. J . Org. Chem. 1997, 43, 1737.

4264 Organometallics, Vol. 22, No. 21, 2003
Lake et al.
mL of THF was added, and the mixture was stirred for 4 h.
The mixture was filtered through a short column of silica gel,
rinsing with a 3:1 petroleum ether/ether solution. The eluant
was concentrated by rotary evaporation and the residue
subjected to preparative TLC (97:3 petroleum ether/ether). The
diol 2a (0.033 g) and alcohol 3a (0.014 g) were isolated.
2a . 1H NMR (CDCl3, 300 MHz): δ 7.40 (m, 10H), 5.35 (d,
J ) 6.6 Hz, 2H), 2.85 (d, J ) 6.6 Hz, 2H). IR (CHCl3): 2091,
2056, 2027 cm-1 (M - CO).
Decom p lexa tion of Bis(d icoba lt h exa ca r bon yl)-1,6-
d ip h en yl-1,5-d i-yn e-3,4-d iol. The diol complex 2a (0.063 g,
0.075 mmol) dissolved in 1 mL of acetone was cooled to
-78 °C and stirred while (NH4)2Ce(NO3)6 (0.215 g, 0.48 mmol)
was added with stirring. After approximately 3 h the mixture
was warmed to room temperature. Saturated NaCl solution
(2 mL) was added, and the mixture was extracted three times
with 2 mL portions of ethyl acetate. The combined organic
phase was dried over MgSO4, and the solvent was removed
by rotary evaporation. The diol 7 (0.017 g, 89% yield) was
obtained as a white solid after preparative silica gel TLC (1:1
petroleum ether/diethyl ether). The 1H NMR spectrum of
1,6-diphenyl-5,6-hexadiyn-3,4-diol [(CDCl3) 7.2 (m, 4H), 6.9
(m, 6H), 4.6 (bs, 2H), 2.2 (bs, 2H)] was identical to that
reported.6 MS (EI, 70eV) 262 (M, 1), 261 (M-1, 3), 131 (1/2M,
100), 103 (50), 77(47).
P in a coliza tion of (2-Bu tyn a l)d icoba lt Hexa ca r bon yl
(1b). Cp 2TiCl2/sec-Bu MgCl. Aldehyde 1b (0.12 g, 0.34 mmol)
was treated with Cp2TiCl2/sec-BuMgCl according to the pro-
cedure described for 1a . Column chromatography (97:3 petro-
leum ether/ether) was used to separate the two cobalt com-
plexes: aldehyde 1b (0.020 g) and diol 2b (0.050 g).
Mn /Cp 2TiCl2/Me3SiCl. Aldehyde 1b (0.17 g, 0.48 mmol)
was treated with Mn (0.015 g, 0.25 mmol), Cp2TiCl2 (0.014 g,
0.051 mmol), Me3SiCl (1.0 mmol), and 4 Å molecular sieves
(2.0 g) as described above. Preparative TLC (97:3 petroleum
ether/ether) separated the two components, aldehyde 1b
(0.043 g) and diol 2b (0.061 g).
4:1 petroleum ether/ether. In addition to recovered 1c, diol 2c
was obtained (Rf 0.5) as a dark red solid in 40-50% yield.
2c. 1H NMR (d6-benzene): 5.25 (s, 2H), 3.9 (s, 2 H), 2.3
(s, 2H). MS (FAB): 654 (M - CO), 598 (M - 3CO), 570 (-4CO),
542 (M - 5CO), 514 (M - 6CO), 486 (M - 7CO), 458
(M - 8CO).
X-r a y Str u ctu r e Deter m in a tion of 2a a n d 2c. X-ray
quality crystals of 2a were obtained by cooling a saturated
solution in toluene for 2 days at -10 °C. Crystals of 2c were
obtained by cooling a saturated methanol solution at -10 °C
over a few days.
The data for 2a and 2c were obtained on a Siemens/Brucker
P4 diffractometer using Mo KR (λ ) 0.71073 Å) radiation. The
data were corrected for Lorentz and polarization effects; an
absorption correction based on ψ-scans was applied. The
structures were solved by the direct method using the
SHELXTL system25 and refined by full-matrix least-squares
on F2 using all reflections. All the non-hydrogen atoms were
refined anisotropically. All the hydrogen atoms were included
with idealized parameters except for 2c, for which the hydro-
gen atoms at O7 and O8 were located and refined isotropically.
For 2a the final R1 ) 0.047 is based on 2509 “observed
reflections” [I > 2σ(I)], and wR2 ) 0.134 is based on all
reflections (3149 unique data). Details of the crystal data are
given in Table 2. Thermal ellipsoids are drawn at the 50% level
for Figure 1.
The asymmetric unit for 2c contains one molecule of C18H6-
Co4O1 and one CH3OH solvent molecule. The hydroxyl groups
from 2c and the solvent molecule form H-bonds as listed in
Table 2. For 2c the final R1 ) 0.047 is based on 3575 “observed
reflections” [I > 2σ(I)], and wR2 ) 0.109 is based on all
reflections (4350 unique data). Details of the crystal data are
given in Table 1. Thermal ellipsoids are drawn at the 40% level
in Figure 2.
Ack n ow led gm en t. We are grateful for partial sup-
port provided by the National Science Foundation for
REU fellowships (M.D. and N.B.) and by the Petroleum
Research Fund of the American Chemical Society. We
also thank Ms. Oanh Pham for technical assistance.
2b. 1H NMR (CDCl3): δ 4.94 (d, J ) 7.2 Hz, 2H), 2.72 (s,
6H), 2.55 (d, J ) 7.2 Hz, 3H). IR (CH2Cl2): 2090, 2055, 2042
cm-1. MS (EI, 12 eV): 626 (M - 4CO), 570 (M - 5CO), 542
(M - 6CO), 514 (M - 7CO).
P in a coliza t ion of (P r op yn a l)d icob a lt H exa ca r b on yl
(1c). Sm I2. Aldehyde 1c (0.117 g, 0.343 mmol) in a sidearm
flask under nitrogen was cooled to 0 °C and then treated with
8.6 mL of 0.1 M SmI2 in THF with stirring. The reaction was
allowed to warm to room temperature and stirred overnight.
The mixture was then treated with 5 mL of 0.1 M HCl and
5 mL of saturated NaCl and extracted three times with
10 mL portions of ethyl acetate, and the combined organic
phase was dried over MgSO4. The solution was concentrated
and the residue subjected to preparative TLC, developing with
Su p p or tin g In for m a tion Ava ila ble: Details of crystal
data and structure refinement for 2a and 2c including atom
coordinates, isotropic displacement factors, bond lengths and
angles, and hydrogen coordinates. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM030392T
(25) Bruker. SHELXTL, Version 5.1; Bruker AXS: Madison, WI,
1997.