Synthesis of diastereomerically enriched cyclic homoallylic alcohols using molybdenum π-allyl complexes

Article abstract of DOI:10.1081/SCC-120006039

Condensation of CpMo(NO)(π-allyl)I complexes bearing a tethered aldehyde gave cyclic homoallylic alcohols with a varying degree of diastereoselectivity. Cyclohexyl and tetrahydronaphthyl derivatives were prepared in moderate to good yields. The diastereos

Full text of DOI:10.1081/SCC-120006039

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SYNTHESIS OF DIASTEREOMERICALLY ENRICHED CYCLIC
HOMOALLYLIC ALCOHOLS USING MOLYBDENUM π-ALLYL
COMPLEXES
Marie E. Krafft a & Peter Schmidt a
a Department of Chemistry , The Florida State University , Tallahassee, FL, 32306-4390,
U.S.A.
Published online: 20 Aug 2006.
To cite this article: Marie E. Krafft & Peter Schmidt (2002) SYNTHESIS OF DIASTEREOMERICALLY ENRICHED CYCLIC
HOMOALLYLIC ALCOHOLS USING MOLYBDENUM π-ALLYL COMPLEXES, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 32:17, 2723-2732, DOI: 10.1081/SCC-120006039
To link to this article: http://dx.doi.org/10.1081/SCC-120006039
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SYNTHETIC COMMUNICATIONS
Vol. 32, No. 17, pp. 2723–2732, 2002
SYNTHESIS OF DIASTEREOMERICALLY
ENRICHED CYCLIC HOMOALLYLIC
ALCOHOLS USING MOLYBDENUM
n-ALLYL COMPLEXES
*
Marie E. Krafft and Peter Schmidt
Department of Chemistry, The Florida State University,
Tallahassee, FL 32306-4390, USA
ABSTRACT
Condensation of CpMo(NO)(p-allyl)I complexes bearing a
tethered aldehyde gave cyclic homoallylic alcohols with a
varying degree of diastereoselectivity. Cyclohexyl and tetrahy-
dronaphthyl derivatives were prepared in moderate to good
yields. The diastereoselectivity is highly dependent on the
substituents on the ring.
Key Words: p-Allyl complexes; Homoallylic alcohols;
Molybdenum; Carbocyclization
INTRODUCTION
Homoallylic alcohols are important intermediates required for natural
product synthesis.[1–4] Therefore, stereocontrolled syntheses of these com-
pounds are of consequence. Faller obtained homoallylic alcohols with regio-
*Corresponding author. E-mail: mek@chem.fsu.edu
2723
DOI: 10.1081/SCC-120006039
Copyright & 2002 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
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2724
KRAFFT AND SCHMIDT
and diastereocontrol by condensation of monosubstituted Z3-allylmolybde-
num complexes and aldehydes.[5–7] An enantioselective version of this reac-
tion was carried out by using enantiomerically pure CpMo(NO)(p-allyl)X
(X¼halogen) complexes.[8,9] Liu performed inter- and intramolecular reac-
tions of p-allyl complexes with aldehydes to obtain the homoallylic alcohol
moiety in an array of products.[10–21] Organomolybdenum complexes bear-
ing a p-allyl ligand tethered to a chelating heteroatom were converted to
homoallylic alcohols containing an amine or sulfide with high regio- and
diastereoselectivity upon reaction with an aldehyde.[22,23] In this work, we
investigated the influence of different ring-substituents on the diastereoselec-
tivity of the resulting cyclic homoallylic alcohols. New cyclohexyl and tetra-
hydronaphthyl derivatives were prepared and characterized.
RESULTS AND DISCUSSION
CpMo(NO)(p-allyl)I complexes 1 bearing an aldehyde are unstable
and give cyclization products 3 via intermediate 2, formed by a p to ꢀ
allyl shift (Scheme 1). Due to the coordination of the aldehyde to the
metal, a preferred conformation of a cyclic intermediate 2 leads to diaster-
eomerically enriched homoallylic alcohols 3.
For the preparation of the pre-cyclization complexes, a variety of
allylic acetates 8, 9, 13, 18 and 22 (Scheme 2) were prepared. Mono-
protection[24] of diols 4 and 5 afforded the alcohols 6 and 7, which were
oxidized[25,26] to the corresponding aldehydes (Scheme 2). Subsequent treat-
ment with vinylmagnesium bromide[27] and acetylation gave the substituted
allylic acetates 8 and 9. Alkylation of diethyl malonate with bromide 10
furnished the monosubstituted malonate derivative 11.[28] A second alkyla-
tion with 4-bromo-1-butene resulted in the formation of alkene 12, which
was oxidized to the corresponding aldehyde using ozonolysis. A Grignard
reaction followed by acetylation gave allylic acetate 13. Analog 18 was
synthesized from 4-tert-butylcyclohexanone, 14. A Baeyer-Villiger oxida-
tion[29] led to the formation of lactone 15, which was then hydrolyzed[30]
Scheme 1. Formation of cyclic homoallylic alcohols 3 from CpMo(NO)(p-allyl)I
complexes 1.

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CYCLIC HOMOALLYLIC ALCOHOLS
2725
Scheme 2. Reagents and conditions: (i) NaH (1eq.), THF, reflux, 1 h, then TBSCl, rt,
0.5 h, n ¼ 1 : 68%, n ¼ 2 : 58%; (ii) Swern ox., n ¼ 1 : 84%, n ¼ 2 : 80%; (iii) vinylmag-
nesium bromide (1.5 eq.), THF, ꢀ78ꢁC to rt, 1.5 h, n ¼ 1 : 57%, n ¼ 2: 60%; (iv)
Ac2O, pyridine, DMAP, CH2Cl2, 5 h, n ¼ 1: 90%, n ¼ 2: 97%; (v) sodium diethyl-
malonate (1 eq.), THF, reflux, 24 h, 73%; (vi) NaH (1.5 eq.), DMF, 15 min., rt, then
4-bromo-1-butene (1.3 eq.), reflux, 4 h, 88%; (vii) O3, CH2Cl2, ꢀ78ꢁC, then Me2S
(5 eq.), NEt3 (2 eq.), rt, 24 h, 61%; (viii) vinylmagnesium bromide (1.2 eq.), Et2O,
ꢀ78ꢁC to rt, 25 min., 54%; (ix) Ac2O, pyridine, DMAP, CH2Cl2, 5 h, 90%; (x)
m-CPBA (2 eq.), CH2Cl2, 0ꢁC to rt, 8 h, 98%; (xi) MeOH/conc. H2SO4
(40/1 ¼ v/v), rt, 24 h, 97%; (xii) TBSCl, imidazole, DMF, rt, 24 h, 78%; (xiii)
DIBAL-H (1.0 M in toluene), pentane/Et2O (9/1 ¼ v/v), ꢀ78ꢁC, 1 h, 97%; (xiv) vinyl-
magnesium bromide (1.3 eq.), THF, ꢀ78ꢁC to rt, 1.5 h, 65%; (xv) Ac2O, pyridine,
DMAP, CH2Cl2, 5 h, 94%; (xvi) EtOH/conc. H2SO4 (2.3/1 ¼ v/v), reflux, 7 h, 84%;
(xvii) LiAlH4, Et2O, reflux, 1 h, 92%; (xviii) NaH (1 eq.), THF, rt, 1 h, then TBSCl,
reflux, 1 h, 79%; (xix) Swern ox., 63%; (xx) vinylmagnesium bromide (1.3 eq.), THF,
ꢀ78ꢁC to rt, 1.5 h, 58%; (xxi) Ac2O, pyridine, DMAP, CH2Cl2, 5 h, 61%.
yielding hydroxy ester 16. After a Swern oxidation to the aldehyde, a reac-
tion sequence as outlined above produced allylic acetate 18. To synthesize
allylic acetate 22, dinitrile 19 was converted to the corresponding diester,
then reduced to diol 20.[31] Monoprotection afforded alcohol 21 which was
then oxidized to the corresponding aldehyde. Treatment with vinylmagne-
sium bromide followed by acetylation gave allylic acetate 22.
The requisite aldehyde complexes were prepared in situ from the
related compounds 25a–e bearing an alcohol instead of an aldehyde
(Scheme 3). Thus, the allylic acetates 8, 9, 13, 18 and 22 were heated with
Mo(CO)3(CH3CN)3, generated in situ by thermolysis of Mo(CO)6 in aceto-
nitrile. Oxidative addition produces orange (p-allyl)MoII(CO)2(CH3-

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2726
KRAFFT AND SCHMIDT
Scheme 3. Synthesis of the CpMo(NO)(p-allyl)I complexes 25a–e.
CN)2(OAc) complexes, which were then treated with NaCp to give the air-
sensitive dicarbonyl complexes 23a–e as yellow oils. Addition of NOBF4 in
acetonitrile resulted in replacement of one of the carbonyl ligands by a
nitrosyl group giving cationic complexes 24a–e, which were used directly
in the next step without further purification. Then, an excess of NaI was
added to solutions of 24a–e in acetone, which caused TBS cleavage and
replacement of a CO with iodide. After column chromatography,
the complexes 25a–e were obtained as orange-red oils, that appeared to be
a mixture of isomers in solution.[32] The compounds may be handled in
air, but decompose when stored at room temperature for longer periods
of time.
The hydroxyl groups in complexes 25a–e were oxidized to aldehydes
using a Swern oxidation at low temperature. After quenching with triethyl-
amine, the reaction mixture was warmed slowly to room temperature.

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CYCLIC HOMOALLYLIC ALCOHOLS
2727
Scheme 4. Cyclic homoallylic alcohols obtained from complexes 25a–e after Swern
oxidation and decomplexation.
Before reaching room temperature, the initially orange-red solution turned
dark brown. After additional stirring at room temperature, the
cyclic homoallylic alcohols were isolated and purified by column chromato-
graphy. Scheme 4 shows the cyclization products obtained from the
complexes 25a–e.
The formation of the 2-vinylcyclopentanol from compound 25a pro-
ceeds via the favored 9-membered boat–chair-like transition state 37 shown
in Scheme 5, which leads to the cis-isomer 26.[10] Indeed, only 26 could be
1
isolated and detected by H-NMR spectroscopy. However, due to the low
yield of 7% (average), the formation of small amounts of the trans-isomer
27 cannot be ruled out. To form a six-membered ring, complex 25b was

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2728
KRAFFT AND SCHMIDT
Scheme 5. Proposed transition states that lead to diastereomerically enriched cyclic
homoallylic alcohols.
synthesized. The yield of cyclization products increased to 49% and the
trans-isomer 28 of 2-vinylcyclohexanol was favored over the cis-isomer
29 in a ratio of 6 : 1. The chair–chair-like transition state 38 is evidently
preferred and leads primarily to the trans-isomer 28.[10] To investigate the
influence of tether substituents on the diastereoselectivity and the yield of
the reaction, complexes 25c–e were prepared. It was expected that two ester
groups on the cyclohexyl system would facilitate the cyclization so complex
25c was studied. While the yield increased to 62%, almost all diastereo-
selectivity was lost and isomers 30 and 31 were isolated as a 1:1 mixture
of diastereomers. A tert-butyl group on the tether could theoretically provide
the four isomers 32–35. The major product from the reaction of 25d was
identified as the trans-isomer 32, presumably formed through transition
state 39. At least two of the other isomers were isolated as a mixture and
not further characterized. The combined yield of cyclization products from
complex 25d was 65%.
A Swern oxidation of complex 25e with subsequent condensation leads
exclusively to a single isomer of 36 in 36% yield. The relative configuration
could not be determined unequivocally since the 1H NOE spectra were
inconclusive.
In summary, it was shown that the diastereoselectivity in the resulting
cyclic homoallylic alcohols 3 is highly dependent on the tether substituents
on the ring. The unsubstituted cyclopentyl derivative was obtained in poor
yield, probably caused by the volatility of the compound, whereas the new
cyclohexyl and tetrahydronaphthyl derivatives were isolated in moderate to
good yields.
EXPERIMENTAL
Typical Procedure for the Synthesis of the Complexes
CpMo(CO)2(n-allyl) 23a–e
Freshly sublimed Mo(CO)6 (1.11 g, 4.20 mmol) was refluxed in anhy-
drous CH3CN (35 mL) for 3 h under an argon atmosphere. Allylic acetate 18

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CYCLIC HOMOALLYLIC ALCOHOLS
2729
(1.50 g, 4.21 mmol, neat) was then added to the boiling yellow solution via
syringe. After refluxing for 12 h, the dark orange solution was cooled to 0ꢀC,
followed by the dropwise addition of a 2 M solution of NaCp in THF
(2.52 mL, 5.04 mmol). The resulting mixture was stirred for 1.5 h at rt.
Removal of the solvent in vacuo gave a dark brown semi-solid, which was
then treated with ether and filtered through a glass filter funnel under an
inert atmosphere. The filtrate was concentrated in vacuo and the resulting
brown oil was purified by column chromatography under nitrogen (SiO2,
degassed Et2O : hexanes ¼ 1 : 6), which gave complex 23d as a yellow, air-
sensitive oil in 79% yield.
Typical Procedure for the Synthesis of the Complexes
CpMo(NO)(n-allyl)I 25a–e
Complex 23d (1.59g, 3.09mmol) was dissolved in 20 mL of dry
CH3CN under nitrogen. To the cooled solution (0ꢀC) was added NOBF4
(397 mg, 3.40 mmol) in one portion and the mixture was stirred for 5 min
after the CO evolution had ceased. After removal of the solvent at 0ꢀC, the
residue consisting of 24d was dissolved in degassed, anhydrous acetone
(20 mL). NaI (1.16 g, 7.74 mmol) was added at ꢁ25ꢀC, the mixture was
stirred for 15 min and then slowly warmed up to 0ꢀC, where it was stirred
for a further 30 min under nitrogen. After removal of the solvent in vacuo,
the red-brown residue was treated with CH2Cl2 and filtered through a short
plug of SiO2 with ether. The solvent was removed at rt followed by purifica-
tion of the crude product by column chromatography (SiO2, degassed
Et2O). The resulting complex 25d was obtained as a red oil in 36% yield.
Typical Procedure for the Synthesis of the
Cyclization Products 30–32 and 36
To a cold (ꢁ78ꢀC) solution of oxalyl chloride (128 mg, 88.0 mL,
1.01 mmol) in CH2Cl2 (8 mL) was added slowly DMSO (144 mg, 131 mL,
1.84 mmol). After 5 min, complex 25d (420 mg, 0.84 mmol) in CH2Cl2
(14 mL) was added dropwise. After 1 h at ꢁ78ꢀC under nitrogen, the reac-
tion was quenched with triethylamine (424 mg, 584 mL, 4.19mmol), stirred
for 5 min, then warmed slowly to rt. After 2.5 h, the dark brown mixture was
filtered through a plug of silica gel and washed with ether. The solvent was
removed and the crude material was purified by column chromatography
(SiO2, Et2O : hexanes ¼ 1 : 1) to yield 37% of 32 and 28% as a mixture of at
least two of the isomers 33–35).

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2730
KRAFFT AND SCHMIDT
Characterization of the Cyclization Products 30–32 and 36
The compounds 26–29 are described elsewhere.[33,34] The conforma-
1
tions of the products were determined by H NOE spectroscopy, the peaks
assigned by homonuclear decoupling.
1
30: H NMR (500 MHz, CDCl3): ꢁ 5.65 (ddd, J ¼ 17.4, 10.2, 8.5 Hz,
1H), 5.16 (td, J ¼ 0.9, 17.4 Hz, 1H), 5.13 (td, J ¼ 0.9, 10.2 Hz, 1H), 4.22–4.13
(m, 4H), 3.46 (ddd, J ¼ 10.2, 10.2, 4.0 Hz, 1H), 2.63 (ddd, J ¼ 13.0, 4.0,
2.3 Hz, 1H), 2.32 (dddd, J ¼ 13.6, 3.2, 2.8, 2.3 Hz, 1H), 1.94 (s, 1H), 1.93
(obscured dddd, J ¼ 12.5, 10.2, 8.5, 3.4 Hz, 1H), 1.75 (obscured dddd,
J ¼ 13.8, 3.8, 3.4, 2.8 Hz, 1H), 1.68 (m, 1H), 1.65 (dd, J ¼ 13.0, 10.2 Hz,
1H), 1.36 (dddd, J ¼ 13.8, 13.4, 12.5, 3.2 Hz, 1H), 1.25 (t, J ¼ 7.0 Hz, 3H),
1.22 (t, J ¼ 7.0 Hz, 3H). 13C NMR (75 MHz, CDCl3): ꢁ 171.6, 170.9, 139.7,
117.4, 69.4, 61.5, 61.3, 55.4, 49.8, 37.7, 29.9, 26.9, 13.9, 13.8. Anal. calcd for
C14H22O5: C, 62.20; H, 8.20. Found: C, 62.42; H, 8.36.
1
31: H NMR (500 MHz, CDCl3): ꢁ 5.91 (ddd, J ¼ 17.4, 10.6, 6.6 Hz,
1H), 5.14 (td, J ¼ 1.5, 10.6 Hz, 1H), 5.11 (td, J ¼ 1.5, 17.4 Hz, 1H), 4.22–4.13
(m, 4H), 3.96 (very br s, 1H), 2.47 (dd, J ¼ 14.2, 4.0 Hz, 1H), 2.34 (br ddd,
J ¼ 13.6, 4.7, 3.6 Hz, 1H), 2.22 (br dddd, J ¼ 14.5, 6.6, 4.0, 3.8 Hz, 1H), 2.09
(dd, J ¼ 13.2, 3.0 Hz, 1H), 2.01 (br s, 1H), 1.89(dddd, J ¼ 14.5, 11.3, 11.3,
3.6 Hz, 1H), 1.71 (ddd, J ¼ 13.6, 11.3, 4.0, 1H), 1.56 (dddd, J ¼ 13.6, 4.7, 4.0,
4.0 Hz, 1H), 1.24 (t, J ¼ 7.0 Hz, 3H), 1.22 (t, J ¼ 7.2 Hz, 3H). 13C NMR
(75 MHz, CDCl3): ꢁ 172.3, 172.1, 139.1, 116.3, 67.8, 61.4, 61.3, 52.4, 44.4,
36.2, 29.6, 21.8, 13.9, 13.8. Anal. calcd for C14H22O5: C, 62.20; H, 8.20.
Found: C, 62.21; H, 8.20.
1
32: H NMR (500 MHz, CDCl3): ꢁ 5.68 (ddd, J ¼ 17.6, 10.3, 8.8 Hz,
1H), 5.17 (td, J ¼ 1.5, 17.6 Hz, 1H), 5.13 (td, J ¼ 1.5, 10.3 Hz, 1H), 3.26
(ddd, J ¼ 9.8, 9.8, 4.4 Hz, 1H), 2.09 (dddd, J ¼ 12.2, 4.4, 2.5, 2.5 Hz, 1H),
1.89(s, 1H), 1.82 (obscured dddd, J ¼ 12.2, 3.4, 3.4, 2.5 Hz, 1H), 1.83
(obscured dddd, J ¼ 12.2, 9.8, 8.8, 3.4 Hz, 1H), 1.72 (ddddd, J ¼ 12.2, 2.9,
2.9, 2.5, 2.5 Hz, 1H), 1.18 (dddd, J ¼ 12.2, 12.2, 12.2, 2.9Hz, 1H), 1.12
(dddd, J ¼ 12.2, 12.2, 2.9, 2.5 Hz, 1H), 1.02 (partly obscured ddd, J ¼ 12.2,
12.2, 9.8 Hz, 1H), 1.01 (partly obscured dddd, J ¼ 12.2, 12.2, 2.9, 2.5 Hz,
1H), 0.87 (s, 9H). 13C NMR (75 MHz, CDCl3): ꢁ 140.8, 116.8, 73.3, 51.1,
46.5, 35.0, 32.3, 30.8, 27.5, 26.0. Anal. calcd for C12H22O: C, 79.06; H,
12.16. Found: C, 78.87; H, 12.23.
36: 1H NMR (500 MHz, CDCl3):
ꢁ 7.05–6.98 (m, 4H), 5.96
(ddd, J ¼ 17.2, 10.4, 7.2 Hz, 1H), 5.09(td, J ¼ 1.5, 17.2 Hz, 1H), 5.05 (td, J ¼
1.5, 10.4 Hz, 1H), 4.09(ddd, J ¼ 5.0, 4.7, 2.7 Hz, 1H), 2.97 (dd, J ¼ 17.2,
4.7 Hz, 1H), 2.90 (dd, J ¼ 16.3, 9.1 Hz, 1H), 2.79 (dd, J ¼ 17.2, 5.4 Hz, 1H),
2.76 (dd, J ¼ 17.2, 5.4 Hz, 1H), 2.56–2.52 (m, 1H). 13C NMR (75 MHz,
CDCl3): ꢁ 147.1, 138.6, 134.7, 133.2, 129.5, 128.9, 126.0, 116.5, 68.4, 42.6,

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CYCLIC HOMOALLYLIC ALCOHOLS
2731
36.5, 30.1. Anal. calcd for C12H14O: C, 82.72; H, 8.10. Found: C, 82.63;
H, 8.20.
ACKNOWLEDGMENTS
The National Science Foundation and the donors to the Krafft
Research Fund are gratefully acknowledged for support of this work.
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32. An equilibrium of endo and exo isomers was detected in CDCl3 for
25a–e as well as for 23a–e. It should be noted that the presence of a
mixture of isomers should not have an influence on the diastereoselec-
tivity of the homoallylic alcohols obtained from the cyclization reac-
tion. This is explained by the reaction pathway outlined in Scheme 1
involving a p-ꢀ-p interconversion mechanism.
33. Hoffmann, R.W.; Gilles, N. Liebigs Ann. Chem. 1991, 1195.
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Received in the USA September 14, 2001