The CoCl2/Ph(Et)2N:BH3/CO system: Reactions of the borane and cobalt carbonyl species

Article abstract of DOI:10.1016/S0022-328X(97)00596-2

The CoCl₂/Ph(Et)₂N:BH₃/CO system is useful for the hydroboration and carbonylation of alkenes to obtain the corresponding dialkyl ketones. The cobalt carbonyl species formed in situ in this way is also useful for hydroacylation-cyclisation of norbornene and for the Pauson-Khand reaction.

Full text of DOI:10.1016/S0022-328X(97)00596-2

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Journal of Organometallic Chemistry 553 1998 91–97
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The CoCl2rPh Et 2 N:BH3rCO system: Reactions of the borane and
cobalt carbonyl species
)
M. Lakshmi Narayana Rao, Mariappan Periasamy
School of Chemistry, UniÕersity of Hyderabad, Hyderabad-500 046, India
Received 1 April 1997; received in revised form 6 September 1997
Abstract
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The CoCl2rPh Et 2N:BH3rCO system is useful for the hydroboration and carbonylation of alkenes to obtain the corresponding
dialkyl ketones. The cobalt carbonyl species formed in situ in this way is also useful for hydroacylation–cyclisation of norbornene and for
the Pauson–Khand reaction. q 1998 Elsevier Science S.A.
Keywords: Hydroboration; Alkene; Carbonylation; Dialkylketone; Cobalt carbonyl reagents; Carbonylative cyclisations
1. Introduction
In recent years, there has been immense interest in the utilisation of transition metal organometallic reagents for
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organic synthesis 1,2 . Transition metal salts in combination with hydride reagents have been used for many useful
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new synthetic methods 2 . In this laboratory, several such reagents generated in situ were used for hydrogenation,
hydroboration, isomerisation, dimerisation, reductions and for carbonylation in the presence of carbon monoxide
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3–11 . In continuation of our studies on the development of useful organometallic reagents for organic synthesis, we
report that the boron and cobalt carbonyl reagents, generated in the reaction of BH3 complexes with anhydrous CoCl2
under carbon monoxide atmosphere, are useful for hydroboration–carbonylation, hydroacylation of norbornene and
Pauson–Khand cyclisation.
2. Results and discussion
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2.1. ReactiÕity of borane species generated using PhN Et 2 N:BH3 rCoCl2 rCO system
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We have observed that the reagent generated from the Ph Et N:BH3 complex 1 equiv. in benzene 11 and
2
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anhydrous CoCl2 1 equiv. in THF in the presence of carbon monoxide, is useful for the hydroboration and
carbonylation of alkene 2 equiv. to obtain the corresponding dialkyl ketones after oxidation with H2O2rNaOH 12 .
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The reaction sequence is represented in Eq. 1 .
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1
This new method of carbonylation of alkene through borane species works well with various other alkenes and the
)
Corresponding author.
0022-328Xr98r$19.00 q 1998 Elsevier Science S.A. All rights reserved.

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M.L.N. Rao, M. PeriasamyrJournal of Organometallic Chemistry 553 1998 91–97
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corresponding dialkyl ketones were isolated in 50–70% yield Table 1 . The alcohols 25–35% are also formed as
side products in all cases. However, the dialkyl ketones can be readily separated by column chromatography on silica
gel. Evidently, the present reagent system tolerates the presence of an ester group as indicated by the conversion of
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methyl-10-undecenoate to the corresponding dialkyl ketone entry 5, Table 1 .
The trialkylborane species have been reported to give the corresponding dialkyl ketones with CO in the presence of
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H2O at 100–1208C 13 . We have carried out an experiment using 30 mmol of 1-decene to examine whether R3B is
formed under the reaction conditions. In this run, only 20 mmol of the 1-decene reacted and 10 mmol was recovered.
This indicates that the species generated in the reaction of the BH3 complex with CoCl2 reacts with only two
equivalents of olefins. Presumably, the BH3–amine complex on reaction with CoCl2 would give the ClBH2 and
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cobalt hydride species. Scheme 1 14 . The latter could yield cobalt carbonyl species in the presence of carbon
Table 1
Reaction of Alkenes with Ph Et 2 N:BH3rCoCl2rCO Systema
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a
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All the reactions were carried out using 10 mmol of Ph Et 2 N:BH3 complex and 10 mmol of CoCl2 with 20 mmol of alkene at room temperature
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and the oxidation of organoborane was carried out with H2O2rNaOH. In the case of methyl-10-undecenoate entry No. 5, Table 1 the oxidation
was carried out using H2O2rsodium acetate. b The dialkyl ketone and the alcohol were separated by column chromatography using ethyl acetate
in hexane as eluents respectively. All products are identified by the spectral data IR, 1H, C NMR and in comparison with the reported data.
13
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c Yields are of isolated products and based on the amount of alkene used.

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M.L.N. Rao, M. PeriasamyrJournal of Organometallic Chemistry 553 1998 91–97
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Scheme 1.
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monoxide. The ClBH2 species would be expected to give R2 BCl on reaction with an alkene 14 . However, the
formation of HBCl2 to some extent through further reaction of BH2Cl cannot be ruled out. In this case, the alcohol
would result, if the intermediate boron compound does not undergo carbonylation.
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In a control experiment carried out using Ph Et N:BH3 complex with 1-decene without using CoCl2 , the ketone
2
was not formed after oxidation with H2O2rNaOH. Only, the hydroboration–oxidation product, 1-decanol was
obtained in 84% yield.
Recently, it has been reported from this laboratory that the BH3–THF can be readily generated in situ by mixing
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NaBH4 and I2 in THF 15 . We have carried out an experiment using the BH3–THF generated in this way for further
reaction with CoCl2. In this case, the corresponding dialkyl ketone entry 1, Table 1 was isolated in 40% yield.
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Hence, the BH3–THF can be used in the place of Ph Et N:BH3 complex for this transformation. However, the yields
2
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are better in the latter case Table 1 . It has been reported that the dicobalt octacarbonyl reacts with BH3–THF at
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x
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4
y158C to give H2 BCo CO THF 16 . However, the H2 BCo CO , prepared in this way, has been reported to
4
decompose at 258C with the evolution of H2 in THF to give polymeric products, resulting in a deep reddish brown
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solution. In the present case, a deep green coloured reaction mixture results when the Ph Et N:BH3 in benzene is
2
added to the blue coloured anhydrous CoCl2 in THF under carbon monoxide. The IR spectrum recorded, after stirring
the reaction mixture for 2 h, exhibited a strong absorption band at 1890 cmy1 indicating the presence of cobalt
carbonyl species. This absorption band was also observed after stirring the mixture for 6 h with alkene in the presence
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of CO. This is similar to the reaction of R2 BI with NaCo CO where the dialkyl ketones were obtained after
4
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oxidation 17 .
The present transformation may be visualised by a sequence of reactions involving initial hydroboration of alkene
with H2 BCl complex followed by cobalt assisted carbonylation, subsequent migration of alkyl groups from boron to
carbon of the CO moiety Scheme 1 .
This mechanistic sequence is similar to that suggested for the reaction of R2 BI with NaCo CO and according to
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4
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the mechanism proposed for the reaction of CO with R3B 13,17 . The interesting point is the involvement of
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–Co CO species. Seyferth and Spohn 18,19 noted previously that the Co2 CO cleaved by THF to give –Co CO
4
4
8
and Co2q. The strong absorption in the IR spectrum of the reaction mixture at 1890 cmy1 does indicate the formation
of cobalt carbonyl species in this transformation. So, we have decided to investigate the reactivities of the cobalt
carbonyl species generated in this way by masking the reactivities of the borane species.
2.2. Reactions of the cobalt carbonyl reagent formed
We have carried out a series of experiments with olefins after adding methanol to the reagent system to convert the
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hydroborating ‘BH’ species to ‘BOCH3’ Eq. 2 . Whereas simple olefins such as 1-decene and cyclopentene did not
react under the reaction conditions, norbornene did undergo carbonylation. After workup, the dinorbornylketone 8 and
the enol-lactone 9 were obtained in 15% and 70% yields, respectively. We have also established that the species
generated using BH3 amine complex and methanol under similar conditions does not hydroborate olefins. Clearly, the

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M.L.N. Rao, M. PeriasamyrJournal of Organometallic Chemistry 553 1998 91–97
observed reactivity of norbornene should result from the hydridocobalt carbonyl species formed in situ in the reaction
mixture. It is well-known such species, which are formally acidic in nature, are stable in protic medium.
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2
Previously, it has been observed in this laboratory that the reagent generated by the reduction of CoCl2 with
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NaBH4 and CH3OH in THF under CO, on reaction with norbornene gave the dinorbornylketone 8 and a novel
enol-lactone 9 Eq. 2 20 . The 13C NMR spectrum of enol-lactone 9 indicated the presence of an isomeric mixture
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two closely spaced signals for several carbon atoms . This enol-lactone 9 earlier has been reported in the reaction of
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norbornene with a palladium 0 complex in the presence of CO 21 . Structural assignment of the enol-lactone 9 is
1
13
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based on the spectral data IR, H, C NMR and mass and agrees with the reported data 21 .
Most probably, these products could have formed through the reaction of hydridocobalt species, generated in situ,
with norbornene followed by migratory insertion of another norbornene through intermediate 10. The dinorbornyl
ketone 8 could result from the protonolysisrreduction of the intermediate 10 Scheme 2 . The formation of
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enol-lactone 9 could result from the cyclisation of the intermediate.
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In the above experiment Eq. 2 methanol was added to convert the hydroborating B–H species to B–OCH3 to
mask the reactivity of the borane species. However, this led to the formation of the H–Co species. It was thought that
the cobalt carbonyl species of the type Co2 CO would result if Et3N is used to trap the hydroborating species as
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8
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unreactive Et3N:BH3. The formation of Co2 CO can be readily examined by performing the reaction in the
8
Scheme 2.

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M.L.N. Rao, M. PeriasamyrJournal of Organometallic Chemistry 553 1998 91–97
95
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presence of an alkyne to obtain the alkyne Co2 CO complex. In turn, it is also possible to examine the presence of
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this complex by performing the Pauson–Khand reaction with norbornene 22 . To examine this possibility, an
experiment was performed using triethylamine and phenyl acetylene Eq. 3 . The crude product obtained was oxidised
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with H2O2rNaOH to remove any organoborane that might have formed and to destroy the BH3:N Et . Interestingly,
3
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the epoxy–ketone 11 was obtained in 30% yield Eq. 4 .
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4
Presumably, the epoxide is formed through Michael type addition reaction of the Pauson–Khand cyclopentenone
with the HOOy present in the medium 23,24 . The epoxide ring formed may be expected to be exo to the existing
bicyclooctyl skeleton. The epoxy-ketone was also prepared through the reaction of HOOy with an authentic sample of
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the corresponding cyclopentenone under similar reaction conditions 22 . Obtention of the epoxy-ketone derived from
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the Pauson–Khand cyclopentenone indicates the generation of Co2 CO or equivalent species in situ in the above
8
experiment using triethylamine.
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In summary, a simple, convenient method has been developed using the BH3:N Et PhrCoCl2rCO reagent
2
combination for the hydroboration and carbonylation of alkenes to obtain the corresponding dialkyl ketones after
oxidation. It was further observed that the hydridocobalt species can be generated by the addition of methanol to this
reagent combination as illustrated in the formation of dinorbornyl ketone and novel enol-lactone 9 with norbornene.
Also, the dicobalt octacarbonyl or its equivalent species has been generated in the presence of triethylamine, as
demonstrated by the formation of epoxy-ketone 11 derived from the Pauson–Khand cyclopentenone using pheny-
lacetylene and norbornene. Obtention of the epoxy-ketone from four fragments, i.e., phenylacetylene, norbornene,
carbon monoxide and H2O2 in a single pot operation is interesting from the synthetic point of view.
3. Experimental
3.1. General
All glassware were pre-dried at 1408C in an air oven for 4 h, assembled in hot and cooled under a stream of dry
nitrogen. All the operationsrtransformations of organoborane reagent transferred using standard syringe, septum
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techniques as recommended for handling air sensitive organoboranes 21 . All dry solvents and liquid reagents
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distilled using appropriate drying agents just before use. Sodiumborohydride 97% supplied by LOBA-Chemie, India
and Fluka, Switzerland were utilised and kept under N2 in a desiccator. Anhydrous CoCl2 was prepared from its
hydrated salt supplied by Fluka, Switzerland, LOBA Chemie and E. Merck, India. The hydrated metal salt was kept in
the air oven for 5–6 h further, dried at 1508C for 4 h under vacuum and was kept under nitrogen in a desiccator. The
alkenes utilised were commercial samples supplied by Fluka, Switzerland. 1-alkynes were prepared following a
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literature procedure 22 . IR spectra were recorded on Perkin–Elmer IR spectrometer Model 1310 and JASCO FT
1
5300 with polystyrene as reference. H and 13C NMR spectra were recorded on a JEOL-FX-100 and Bruker AC-200
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spectrometers with chloroform-d as a solvent and TMS as reference ds0 ppm . Column chromatography was
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carried out using Acme’s silica gel 100–200 mesh using hexanerethyl acetate as eluent. Carbon monoxide was
generated by dropwise addition of formic acid 98% to conc. H2 SO4 96% at 80–908C using an apparatus
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recommended for utilisation in the carbonylation of organoboranes 21 .
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3.2. General procedure for the reaction of Ph Et 2 N:BH3 rCoCl2 rCO with 1-decene
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The Ph Et N:BH3 10 mmol complex in benzene 30 ml was prepared according to literature procedure 11 .
2
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This was added to a suspension of anhydrous CoCl2 10 mmol, 1.29 g in THF 50 ml at 258C while bubbling carbon

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M.L.N. Rao, M. PeriasamyrJournal of Organometallic Chemistry 553 1998 91–97
monoxide, through a double ended needle under N2 atmosphere. The reaction mixture was stirred for 2 h under
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carbon monoxide atmosphere. Alkene 1-decene, 20 mmol, 2.8 g was added and carbon monoxide bubbling was
continued for 5–6 h. The contents were further stirred for 36 h at 258C. The resulting mixture was poured into water
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20 ml . The organic phase was separated and the aqueous phase was saturated with NaCl extracted with ether 2=30
.
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ml . The combined organic extracts were washed with brine 30 ml , dried over anhydrous MgSO4 and concentrated.
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The residue was dissolved in THF 30 ml and oxidised with H2O2rNaOH. After oxidation, the organic phase was
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separated and the aqueous phase was saturated with NaCl solution and extracted with ether 2=30 ml . The
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combined organic extracts were washed with 3 N HCl 2=30 ml , water 20 ml , brine 30 ml , dried over anhydrous
MgSO4 and concentrated. The crude product was subjected to chromatography on a silica gel column. Di-n-decylke-
tone 69%, 2.13 g and n-decanol 25%, 0.79 g were isolated using ethyl acetate 3% and 5% in hexane as eluents
respectively. The compound was identified by IR, 1H, 13C NMR and by comparison with the literature data. The same
procedure was followed for other substrates and the spectral data of the products were in 1:1 correspondence with the
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reported data 17 .
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3.3. Reaction of the species generated from Ph Et 2 N:BH3 rCoCl2 rCOrmethanol with norbornene
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The Ph Et N:BH3 10 mmol kendy complex in benzene 30 ml was prepared according to literature procedure
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11 . This was added to a suspension of anhydrous CoCl2 10 mmol, 1.29 g in THF 50 ml at 258C while bubbling
carbon monoxide through a double-ended needle under N2 atmosphere. The reaction mixture was stirred for 2 h under
carbon monoxide atmosphere. Dry methanol 30 mmol was added and the reaction mixture was stirred for 1 h under
carbon monoxide atmosphere. Norbornene 10 mmol, 0.94 g was added and the contents were further stirred for 7 h
at 70–808C. The resulting mixture was poured into water 30 ml . The organic phase was separated and the aqueous
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phase was saturated with NaCl solution and extracted with ether 2=30 ml . The combined organic extracts were
dried over anhydrous MgSO4 and concentrated. The crude product was subjected to column chromatography on silica
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gel. The dinorbornyl ketone 8 was isolated in 15% 0.17 g yield using ethyl acetate 5% in hexane as eluent. Ethyl
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acetate 10% in hexane eluted 70% 0.85 g yield of enol-lactone 9. The spectral data of the products 8 and product 9
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show 1:1 correspondence with the data previously reported 17,21 . Dinorbornyl ketone 8: M.P. 538C Ref. 25 M.P.
1
53–548C ; IR neat 1700 cm ; H NMR d 0.9–1.9 m , 2.1–2.6 m ; 13C NMR d 28.4, 29.2, 29.5, 32.1, 32.9,
y1
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34.2, 35.4, 39.5, 40.1, 52.3, 52.7, 212.3, 212.6 CO . Enol-lactone 9: IR neat 1720, 1780 cmy1;1H NMR d 0.9–1.6
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13
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m , 1.9–2.7 m ; C NMR d 26.9, 27.0, 27.4, 27.5, 27.7, 27.9, 28.8, 28.9, 33.7, 36.0, 39.0, 39.7, 39.8, 39.4, 40.3,
42.7, 44.6, 44.8, 48.4, 48.5, 119.9, 120.0, 141.8, 142.2, 176.8; Mass mrz 244 M .
q
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3.4. Reaction of the Ph Et 2 N:BH3 rCoCl2 rCOrNEt3 reagent system with PhC[CH and norbornene followed by
H2 O2 rNaOH oxidation
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The Ph Et N:BH3 20 mmol complex in benzene 30 ml was prepared according to literature procedure 11 .
2
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This was added to a suspension of anhydrous CoCl2 20 mmol, 2.6 g in THF 50 ml at 258C while bubbling carbon
monoxide through a double-ended needle under N2 atmosphere. The reaction mixture was stirred for 3 h under carbon
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monoxide atmosphere. To this, dry triethylamine 20 mmol, 2.02 g was added and the reaction mixture was stirred
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for 2 h under carbon monoxide atmosphere. Then PhC[CH 5 mmol, 0.5 g was added and the contents were further
stirred for 4 h at room temperature. Norbornene 10 mmol, 0.94 g was added to the reaction mixture and the contents
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were stirred at 60–708C for 7 h. The contents were brought to 258C and the metal carbonyl was oxidised with CAN in
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methanol. Water 20 ml is added and the organic phase was separated. The aqueous phase was saturated with sodium
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chloride and extracted with ether 2=30 ml . The combined organic extract was dried over anhydrous MgSO4 and
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concentrated. The residue was dissolved in THF 30 ml and treated with H2O2rNaOH to oxidise organoborane
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species if any present in the crude product mixture. The organic phase was separated and the aqueous phase was
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saturated with sodium chloride and extracted with ether 2=30 ml . The combined organic extracts washed with 3 N
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HCl 2=30 ml , brine, dried over anhydrous MgSO4 and concentrated. The crude product thus obtained was
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subjected to chromatography on a silica gel column and the epoxy-ketone 11 was isolated in 30% 0.36 g yield using
1
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ethyl acetate 2% in hexane as eluent. The product was identified by IR, H, C DEPT experiments NMR and
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mass spectral data.yC1 ompound 11: IR neat 2874, 2382, 1738, 1628, 1500, 1448, 1278, 1240, 1153,1097, 1043, 883,
1
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756, 696, 609 cm ; H NMR d 1.12–1.68 m, 6H , 2.38–2.45 m, 3H , 2.65 s, 1H , 3.62 s, 1H , 7.26–7.28 m,
13
.
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5H ; C NMR d 28.0, 29.0 and 34.5 -yCH2 , 38.0, 42.6, 45.2 and 53.4 -yCH , 65.8 quaternary , 69.0 -yCH ,
q
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.
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126.8, 128.3 and 128.5 -yCH , 131.0, quaternary , 210.0 CO ; Mass mrz 240 M , 20% , 173 30% , 105
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100% , 77 40% . 41 30% .

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M.L.N. Rao, M. PeriasamyrJournal of Organometallic Chemistry 553 1998 91–97
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Acknowledgements
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We are grateful to the UGC New Delhi and DST for financial support. We also thank the UGC for support under
Special Assistance Programme.
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