synthesis of acetylenes, allenes and cumulenes || miscellaneous reactions of acetylenic and allenic...

[13.1.2004–9:55pm] [353–368] [Page No. 353]

E:/Archive files/4188-Brandsma/Printer-Files/4188-Chapter-19.3d

19Miscellaneous Reactions of Acetylenic

and Allenic Compounds

19.1 ELIMINATION REACTIONS RESULTING IN ADDITIONALUNSATURATION

19.1.1 2-Methyl-1-buten-3-yne from 2-methyl-3-butyn-2-oland acetic anhydride

Scale: 1.0 molar; Apparatus: 1-litre round-bottomed, three-necked flask, equi-

pped with a dropping funnel, a mechanical stirrer and a 40-cm Vigreux column,

connected to a condenser and a receiver, cooled at –20 �C; stirring is carried out

at a moderate rate; all connections should be made gas-tight.

19.1.1.1 Procedure

The flask is charged with 1.3 mol of acetic anhydride and 7 g of p-toluenesul-

phonic acid monohydrate (or, if available, the anhydrous acid). 2-Methyl-3-

butyn-2-ol (1.0 mol, commercially available) is added over 10 min with some

cooling. The flask is then quickly heated until the enyne begins to pass over.

Further heating is carried out in a controlled way, so that the enyne does

not distil too fast. The greater part should pass over below bp 60 �C. With

increasing bath temperature the reaction mixture turns very dark. When, after

45–60 min, the temperature in the head of the column indicates 100 �C,

heating is stopped. The distillate is washed twice in a small separating funnel

with 10–15 ml of a cold KOH solution in order to remove traces of acetic acid.

Redistillation from 5 g of anhydrousMgSO4 gives pure 2-methyl-1-buten-3-yne,

bp � 35 �C/760 Torr, in 60–70% yield. The compound should be stored in a

refrigerator (–20 to –30 �C). Under these conditions polymerisation is slow.

353

[13.1.2004–9:55pm] [353–368] [Page No. 354]


19.1.2 1-Ethynyl-1-cyclohexene from 1-ethynylcyclohexanol andphosphoryl chloride in pyridine

Scale: 0.50 molar; Apparatus: Figure 1.1, 1 litre

19.1.2.1 Procedure

A mixture of 0.50 mol of 1-ethynylcyclohexanol (commercially available) and

90 ml of pyridine (dried over KOH) is heated to 100 �C. A mixture of 33 ml of

phosphoryl chloride and 40 ml of pyridine is then added over 15 min, while

stirring at a moderate rate. The temperature of the reaction mixture is kept

between 105 and 110 �C by occasional cooling or temporarily increasing the

rate of stirring. After the addition heating at 105–110 �C is continued for 15 min.

The mixture is cooled to below 75 �C (partial solidification), and 400 ml of ice

water is poured into the flask. After vigorous stirring, the layers are separated

and five to seven extractions with small portions of a 1:1 mixture of Et2O and

pentane are carried out. The combined organic layers are washed with cold 3 N

hydrochloric acid in order to remove pyridine. After drying over MgSO4,

the greater part of the solvent is distilled off at atmospheric pressure through

a 40-cm Vigreux column. Distillation of the remaining liquid in vacuo (using a

single receiver cooled in an ice bath, Figure 1.10) gives 1-ethynyl-1-cyclohex-

ene, bp 38 �C/12 Torr, in an excellent yield.

1-Ethynyl-1-cyclopentene, bp � 55 �C/120 Torr, and 1-ethynyl-1-cyclohep-

tene, bp 52 �C/12 Torr, are obtained in high yields from the ethynylcarbinols.

The enynes polymerise slowly on storage in a refrigerator.

19.1.3 3,1-Enynes by 1,2-elimination of p-toluenesulphonic acid frompropargylic tosylates using KOH in a water–DMSO mixture

Scale: 0.30 molar; 1-litre round-bottomed, three-necked flask, equipped with a

dropping funnel-gas inlet combination, a mechanical stirrer and a 20-cm

354 19. REACTIONS OF ACETYLENIC/ALLENIC COMPOUNDS

[13.1.2004–9:55pm] [353–368] [Page No. 355]


Vigreux column, connected to a condenser and 250-ml (single) receiver, cooled

in an ice bath; all connections should be made gas-tight.

19.1.3.1 Procedure

Potassium hydroxide (110 g) is dissolved in 120ml of water and 40 ml of DMSO.

After the air in the flask has been completely replaced by nitrogen, the solution is

heated in a bath at 100–110 �C. When the temperature of the solution indicates

90 �C, the flow of nitrogen is adjusted at 100 ml/min and 1-vinyl-3-butynyl-4-

methylbenzenesulphonate (0.30 mol, for the preparation cf. Chapter 20,

exp. 20.5.4) is added over 10–15 min to the efficiently stirred solution. During

the reaction, a mixture of 1,3-hexadien-5-yne and water passes over, after about

half an hour only water. Remaining traces of the dienyne may be forced into the

condenser by vigorously introducing nitrogen during a few seconds. The con-

tents of the receiver are shaken with anhydrous MgSO4 (� 5 g) in order to

remove the water. The receiver is subsequently equipped for a vacuum distilla-

tion (20-cm Vigreux column, condenser, receiving flask, cooled in a bath at

–70 �C, Figure 1.10). The apparatus is evacuated using a water aspirator (10–

20 Torr). A mixture of (E)- and (Z)-1,3-hexadien-5-yne, (E) / (Z) ratio � 1), is

collected in the receiver. The yield is usually greater than 80%.

Other volatile enynes, e.g. 3-penten-1-yne, HC�CCH¼CHMe, and 3-hexen-

1-yne, HC�CCH¼CHEt, can be prepared in >80% yields in a similar

way from the corresponding tosylates. Comparable amounts of the (E)-and

(Z)-isomers are obtained. These compounds can be redistilled at atmospheric

pressure: HC�CCH¼CHMe, bp 44–51 �C/760 Torr; HC�CCH¼CHEt,

bp 73–78 �C/760 Torr.

19.1.4 3,5-Heptadien-1-yne from 5-hepten-1-yn-4-ol in a one-potprocedure using p-toluenesulphonic chloride and potassiumhydroxide as reagents

Scale: 0.25 molar; Apparatus: for the preparation of the tosylate a 1-litre round-

bottomed, three-necked flask, equipped with a powder funnel, a mechanical

stirrer and a thermometer; for the preparation of the enyne the powder funnel

is replaced with a gas inlet and the thermometer by a reflux condenser.

19.1 ELIMINATION REACTIONS 355

[13.1.2004–9:55pm] [353–368] [Page No. 356]


3,5-Heptadien-1-yne is less volatile and presumably more stable than 1,3-

hexadien-5-yne. Distillative separation at atmospheric pressure from Et2O

should therefore be possible without involving the risk of polymerisation or

decomposition. This allows the preparation of 1-(2-propynyl)-2-butenyl-4-

methylbenzenesulphonate and its conversion into 3,5-heptadien-1-yne in the

same pot, using Et2O as solvent. Other enynes having bp >100 �C/760 Torr

can be prepared in a similar way.

19.1.4.1 Procedure

A mixture of 0.25 mol of 5-hepten-1-yn-4-ol, 350 ml of Et2O and 0.35 mol

(excess) of tosyl chloride is placed in the flask. After dissolution of the tosyl

chloride, the solution is cooled to –5 �C (dry ice-acetone bath). Freshly,

machine-powdered KOH (140 g) is added in 5-g portions over 15 min to the

vigorously stirred mixture while maintaining the temperature between �10 and

0 �C. The air is then completely replaced by nitrogen and the cooling bath is

removed. At � 15 �C an exothermic reaction starts, and after an additional

10–15 min the Et2O begins to reflux. The mixture is heated for another 1 h

under reflux. After cooling to rt, the thick slurry is poured into 500 ml of ice

water and the flask rinsed with a small amount of ice water. After vigorous

shaking and separation of the layers, the aqueous layer is extracted three times

with small portions of Et2O. The combined organic solutions are washed with

saturated aqueous ammonium chloride and subsequently dried over MgSO4.

The greater part of the Et2O is then distilled off (under a slow stream of N2) at

atmospheric pressure through an efficient column. The temperature of the

heating bath is kept between 80 and 90 �C in the last stage of this distillation.

After cooling to rt, the remaining liquid is carefully distilled in a partial

vacuum, giving 3,5-heptadien-1-yne, bp � 30–40 �C/40 Torr, ((E)/(Z)-ratio

� 40:60, the double bond between C-5 and C-6 has the (E)-configuration), in

an excellent yield.

1-(1-Buten-3-ynyl)benzene, HC�CCH¼CHPh ((Z)/(E)-ratio � 55:45), bp

� 55 �C/0.4 Torr, is obtained in an excellent yield from 1-phenyl-3-butyn-1-

ol, HC�CCH2CH(OH)Ph, by a similar procedure.

19.1.5 6-Ethynyl-2,3-dihydro-4H-pyran from 3-bromo-2-ethynyltetrahydro-2H-pyran and t-BuOK in tetrahydrofuran


[13.1.2004–9:55pm] [353–368] [Page No. 357]


Scale: 0.10 molar; Apparatus: Figure 1.1, 500 ml

19.1.5.1 Procedure

3-Bromo-2-ethynyltetrahydro-4H-pyran (0.10 mol, Chapter 4, exp. 4.5.20) is

added over 15 min to a solution of 0.20 mol (excess, Note) of t-BuOK in 150 ml

of THF. During this addition the reaction mixture is kept between –10 and

–20 �C. After an additional 15 min the temperature is allowed to rise to

10–20 �C. Finally, the suspension is warmed to 40 �C. A solution of 25 g of

NH4Cl in 200 ml of water is then added with vigorous stirring. After separation

of the layers, three extractions with Et2O are carried out. The combined organ-

ic solutions are washed three times with water and subsequently dried over

MgSO4. After concentration of the solution under reduced pressure, the

remaining liquid is distilled through a 20-cm Vigreux column to give 6-ethy-

nyl-2,3-dihydro-4H-pyran, bp 54 �C/10 Torr, in � 90% yield.

From other alkynyl bromoethers, for example 4-bromo-3-ethoxy-1-butyne,

BrCH2CH(OEt)C�CH, HBr can be eliminated by a similar procedure. In

this example 2-ethoxy-1-buten-3-yne, H2C¼C(OEt)C�CH, the synthetic

equivalent of 3-butyn-2-one, MeC(¼O)C�CH, is obtained. The ethereal

extract obtained as described above is concentrated under normal pressure,

after which the enyne ether is distilled at water-aspirator pressure.

Note

The t-BuOH formed in the elimination gives the 1:1 complex with t-BuOK,

which is much less active.

19.1.6 2-Ethoxy-2-penten-4-yne from 5-bromo-4-ethoxy-1-pentyneand sodamide

Scale: 0.50 molar; Apparatus: Figure 1.1, 1 litre for the reaction with allenyl-

magnesium bromide; 3-litre wide-necked round-bottomed flask with stirrer for

the dehydrohalogenation (Figure 1.5).

19.1 ELIMINATION REACTIONS 357

[13.1.2004–9:55pm] [353–368] [Page No. 358]


19.1.6.1 Procedure [1]

A solution of 0.50 mol of 1,2-dibromo-1-ethoxyethane in 100 ml of Et2O,

prepared by addition of 0.50 mol of bromine to a mixture of 0.70 mol

of ethyl vinyl ether and Et2O at ��40 �C, is added over 30 min to a solution

of allenylmagnesium bromide in 400 ml of Et2O prepared from 0.70 mol of

propargyl bromide (Chapter 2, exp. 2.3.9). During this addition the tempera-

ture of the reaction mixture is kept between –5 and –15 �C. After an additional

30 min the reaction mixture is poured into 250 ml of an aqueous solution of

50 g of ammonium chloride. After extraction and drying over magnesium

sulphate, most of the Et2O is removed under reduced pressure. The remaining

more concentrated solution in � 100 ml of Et2O is added over 15 min to a

suspension of 1.2 mol of sodamide in 1 litre of liquid ammonia. The reaction is

very vigorous. After the addition most of the ammonia is removed by placing

the flask in a bath at 40–50 �C. To the remaining dark brown slurry is cau-

tiously added a solution of 50 g of ammonium chloride in 200 ml of water,

followed by extraction (4 times) with pentane. After drying, most of the solvent

is distilled off at normal pressure through a 30-cm Vigreux column. 2-Ethoxy-

2-penten-4-yne, bp 42 �C/17 Torr, is obtained in � 60% yield. A contamination

of � 10%, presumably 1-ethoxy-2-ethynylcyclopropane, is present. A consider-

able non-volatile residue is left behind.

Similar procedures may be carried out with the homologues

RCHBrCHBrOEt obtained from 1-ethoxy-1-alkenes, RCH¼CHOEt, and

bromine.

19.1.7 4,1-Enynes by elimination of bromine and ethoxy groups from5-bromo-4-ethoxy-1-alkynes with zinc in dimethylsulphoxide

Scale: 0.50 molar; Apparatus: Figure 1.1, 1 litre with a gas-tight mechanical

stirrer, an evacuable dropping funnel, and a reflux condenser instead of the

thermometer. The top of the condenser is connected with a trap cooled in a

bath at –78 �C (preferably liquid nitrogen, in that case the connection is made

in a way such that the vapour of the volatile product can enter the large

annular space).


[13.1.2004–9:55pm] [353–368] [Page No. 359]



In the flask are placed 250 ml of DMSO, 70 g of technical-grade zinc powder

and 5 g of potassium iodide. The system is evacuated using the water aspirator

and the pressure is adjusted at � 50 Torr. The bromoether 5-bromo-4-ethoxy-

1-pentyne or 5-bromo-4-ethoxy-1-hexyne (0.50 mol, prepared as described in

exp. 19.1.6, and freed from any Et2O) is added over 30 min to the gently

refluxing DMSO. After the addition heating is continued for another 2 h

(if after half an hour no condense is present in the trap, the pressure should

be increased so that the temperature of the reaction mixture can rise). The

contents of the trap are washed twice with 10 ml portions of ice water in

order to remove some DMSO and ethanol formed from EtOZnBr and some

water present in the DMSO. A very small amount of magnesium sulphate is

added, after which the enyne is distilled. 1-Penten-4-yne, bp 42 �C and 4-hexen-

1-yne, bp 76 �C, (E)/(Z) � 1, are obtained in � 70% yields. About 10% of

1,2,4-pentatriene, H2C¼C¼CHCH¼CH2 or 1,2,4-hexatriene, H2C¼C¼

CHCH¼CHMe, formed from the allenic bromoethers 5-bromo-4-ethoxy-1,2-

pentadiene (R¼H) or 5-bromo-4-ethoxy-1,2-hexadiene (R¼Me), BrCH(R)

CH(OEt)CH¼CH2, is present. The allenic compounds can be removed by

reaction with maleic anhydride.

19.2 REMOVAL OF PROTECTING GROUPS

19.2.1 1,3-Diynes by potassium hydroxide-catalysed eliminationof acetone from the corresponding diyne carbinols

Scale: 0.10 molar; Apparatus: Figure 1.10, 250 ml (Note)

19.2.1.1 Procedure

2,7,7-Trimethyl-3,5-octadiyn-2-ol (0.10 mol, obtained by Cu(I)-catalysed

coupling between an excess of 2-methyl-3-butyn-2-ol, HC�C(Me)2OH, and

1-bromo-3,3-dimethyl-1-butyne, t-BuC�CBr, followed by thorough removal

of the excess of the acetylenic alcohol at � 1 Torr, see Chapter 14) is mixed

with 50 ml of paraffin oil and 2 g of finely powdered KOH (vigorous

shaking for a few seconds), after which the flask is equipped for a distilla-

tion in vacuo (10–20 Torr). The system is evacuated and the flask heated

19.2 REMOVAL OF PROTECTING GROUPS 359

[13.1.2004–9:55pm] [353–368] [Page No. 360]


during 2 h in a bath at 160–170 �C. The contents of the receiver (cooled in a

dry-ice acetone bath) are washed 5 times with 5-ml portions of 3 N hydro-

chloric acid in order to remove the acetone, and are subsequently trans-

ferred to a 100-ml round-bottomed flask containing � 0.5 g of magnesium

sulphate. The rather volatile 5,5-dimethyl-1,3-hexadiyne is isolated in almost

quantitative yield in a strongly cooled receiver (Figure 1.10) by distillation

at 10–20 Torr and condensation.

The extremely unstable 1-(1,3-butadiynyl)benzene, PhC�CC�CH, can be

obtained in a good yield by a similar procedure from the Cadiot–

Chodkiewicz coupling product 2-methyl-6-phenyl-3,5-hexadien-2-ol, PhC�

CC�CCMe2OH. An essential condition is that during heating with KOH-par-

affin oil the pressure is lower than 0.01 Torr. A very short (<5 cm) column and

a few centimetre long condenser should be used. The parts of the distillation

set-up should have a diameter of at least 1.5 cm (B-29 ground-glass joints)

and the connection with the vacuum pump as short as possible.

Aryl- and hetaryl-acetylenes, Het(aryl)C�CH, for example 2-ethynylthio-

phene, 2-ethynylfuran and 1-ethynyl-4-fluorobenzene are accessible in high

yields from (Het)arylC�CCMe2OH using this procedure [3]. These compounds

can be prepared by Pd/Cu-catalysed coupling between (Het)aryl-Br and

HC�CCMe2OH (Chapter 16).

Note

The elimination of acetone is accompanied by rather strong foaming. For this

reason a relatively big flask (with a short neck) should be used.

19.2.2 General procedure for the base-catalysed desilylation ofsilylated acetylenes

19.2.2.1 Procedure

A solution of 0.10 mol of the silylated acetylene and 2 g of potassium hydrox-

ide in 50–100 ml of methanol is heated for 15 min at 50 �C, then most of the

methanol is removed under reduced pressure if the free acetylene has

bp >50 �C/10 Torr. In the other cases the solution is diluted with 300 ml of

water, followed by extraction with (preferably) pentane.

For base-sensitive acetylenes (polyynes with a conjugated system of triple

bonds, which easily add methanol in the presence of bases, or compounds


[13.1.2004–9:55pm] [353–368] [Page No. 361]


with the systems HC�CCH2C�C and HC�CCH2C¼C, which may undergo

base-catalysed isomerisation to conjugated systems) the AgNO3–methanol

method [4] may be applied.

Other mild reagents for desilylation are potassium fluoride–H2O and

18-crown-6 [9].

A general method for the preparation of (Het)arylC�CSiMe3, consisting of

Pd/Cu-catalysed coupling between ethynyl(trimethyl)silane, HC�CSiMe3, and

(Het)aryl halides, is described in Chapter 16.

19.2.3 Acid-catalysed conversion of O-protected alcoholsinto the free alcohols

19.2.3.1 Procedure

The protected alcohol (0.10 mol) is mixed with 30–50 ml of methanol and a few

drops of concentrated hydrochloric acid are added. The mixture is heated for

10 min at 50–60 �C. In the case of non-volatile (bp >60 �C/15 Torr) alcohols

the greater part of the methanol is removed on the rotary evaporator after

neutralisation of the HCl with a very small amount of aqueous ammonia. The

alcohol is then isolated (mostly in quantitative yield) applying the usual opera-

tions in the work-up. In the case of rather volatile alcohols the reaction mixture

is diluted with a sufficient amount of water prior to carrying out the work-up.

A similar procedure of deprotection may be followed with tetrahydropyr-

anyl-protected alcohols.

19.2.4 Acid-catalysed conversion of acetylenic acetalsinto the aldehydes

Scale: 0.10 molar; Apparatus: 250-ml round-bottomed flask and thermometer

19.2 REMOVAL OF PROTECTING GROUPS 361

[13.1.2004–9:55pm] [353–368] [Page No. 362]


19.2.4.1 Procedure

To a mixture of 4 g of 96% H2SO4 and 20 ml of water are successively added

40ml ofDMSO and 0.10mol of 1,1-diethoxy-2-heptyne (Chapter 4, exp. 4.5.23).

The mixture is heated for 15–20 min at 75 �C with occasional swirling. After

5 min (at 75 �C) the solution has become homogeneous. The solution is cooled

to rt and subsequently poured into 150 ml of ice water, saturated with NH4Cl.

The product is isolated by extraction with Et2O, washing the solution with

saturated aqueous NH4Cl, drying over MgSO4 and distillation through a

short Vigreux column. 2-Heptynal, bp 55 �C/10 Torr, is obtained in an

excellent yield.

In the case of lower homologues, less DMSO should be used.

19.3 PARTIAL REDUCTIONS OF CONJUGATED SYSTEMSOF TRIPLE BONDS

19.3.1 Partial reduction of 3,5-octadiyne with activated zinc powder

Scale: 0.20 molar; Apparatus: 250-ml round-bottomed, three-necked flask,

equipped with a gas inlet (for introduction of N2), a mechanical stirrer and a

reflux condenser.

There are several reports on the successful partial reduction of acetylenic

compounds with activated zinc powder. The formation of (Z)-double bonds

has been explained by assuming that after adsorption of the acetylene on the

metal two electrons are successively transferred from the metal to the triple

bond [5]. Protolysis of the resulting three-membered metallocycle, in which

the double bond has the (Z)-configuration, finally gives the (Z)-olefinic com-

pound. The reaction is usually carried out in ethanol. Non-conjugated triple

bonds are not reduced unless they are in the terminal position or the molecule

contains a OH group, an amino, ether or ester function. It has appeared

that systems in which these functions are close to the triple bond (e.g. in the

‘propargylic’ position) are reduced more easily than the isomers in which the

heteroatom-containing group is in a more remote position. In a series of homo-

logues, the time required for complete conversion increases with increasing

length of the carbon chain. These experimental facts support the adsorption

mechanism.


[13.1.2004–9:55pm] [353–368] [Page No. 363]


Triple bonds in conjugated unsaturated systems are reduced relatively easily.

Depending upon the degree of activation of the zinc-powder, one or both of the

triple bonds in a conjugated diyne can be reduced to a double bond. We have

found that the reduction can be completely stopped at the stage of the enyne by

using zinc that is activated by boiling with a small amount of 1,2-dibromo-

ethane in ethanol. Further activation of the metal by addition of copper(I) bro-

mide (added as a solution of CuBr �LiBr in THF) leads to reduction of

the second triple bond with formation of a conjugated diene. In the case of

hydrocarbon-diynes, these partial reductions give almost pure (Z)-enynes

and (Z,Z)-dienes.


Ethanol (100%, 60 ml) and zinc powder (50 g, Merck, analytical grade) are

placed in the flask. 1,2-Dibromoethane (5 g) is added and the mixture is

heated until refluxing begins. The heating bath is then removed. When the

exothermic reaction (evolution of ethene and refluxing of the solvent) has

subsided, an additional amount of 5 g of 1,2-dibromoethane is added. After

this has reacted, the mixture is heated for an additional 10 min under reflux.

The suspension is then cooled to � 50 �C, while introducing N2. 3,5-

Octadiyne (0.20 mol, Chapter 4, exp. 4.5.3) is then added over 10 min in

two or three portions. The reaction is markedly exothermic and refluxing of

the ethanol may ensue. After the reaction has subsided, the mixture is heated

for an additional 2 h (technical zinc powder reacts less easily) under reflux

while a weak flow of N2 is passed through the apparatus. The mixture is then

cooled to rt, after which the suspended zinc is allowed to settle down. The

supernatant layer is decanted and poured into a solution of 35 g of NH4Cl in

200 ml of water and 30 ml of concentrated aqueous ammonia. The zinc slurry

in the flask is rinsed four times with 30-ml portions of hot (� 50 �C) ethanol

and the alcoholic solutions added to the hydrolysed mixture. Subsequently

four extractions with pentane are carried out. The combined extracts are

washed with 2 N hydrochloric acid and subsequently dried over MgSO4.

The greater part of the solvent is then distilled off at atmospheric pressure

through a 40-cm Vigreux column. After cooling to rt, the remaining liquid is

distilled in vacuo and the distillate collected in a single receiver, cooled at 0 �C

(Figure 1.10). (Z)-3-Octen-5-yne, bp � 40 �C/15 Torr, is obtained in greater

than 70% yield.

(Z)-4,6-Decen-6-yne, n-PrCH¼CHC�Cn-Pr, bp 75 �C/15 Torr, can be pre-

pared in a similar way from 4,6-decadiyne. The period of reflux (�5 h in the

case of using the same molar amounts of reagents as above) can be shortened

to �3 h if only 35 ml of ethanol is used.

19.3 PARTIAL REDUCTIONS OF CONJUGATED SYSTEMS 363

[13.1.2004–9:55pm] [353–368] [Page No. 364]


19.3.2 Regiospecific and stereospecific partial reduction of1-trimethylsilyl-1,3-heptadiyne with activated zinc powder

Scale: 0.05 molar; Apparatus: 100-ml three-necked, round-bottomed flask,

equipped with a gas inlet (for the introduction of N2), a magnetic stirring

bar and a reflux condenser.

Russian chemists have found that trimethylsilyl groups protect adjacent

triple bonds against hydrogenation with poisoned Pd-catalysts [7]. A similar

effect has been shown in reductions of trimethylsilylated 1,3-diynes with (acti-

vated) zinc powder [6]. One disadvantage of the zinc method is that the zinc

salts present in the reaction mixture can cause cleavage of the �C–Si bond.

This was shown in a separate experiment in which a trimethylsilylated

1,3-diyne was heated with a solution of zinc bromide or chloride in ethanol

[10]. It seems therefore important to keep reaction times of the reductions

with zinc as short as possible and to activate the zinc powder with a limited

amount of 1,2-dibromoethane.

19.3.2.1 Procedure

To a mixture of 30 g of zinc powder (Merck, analytical grade) and 30 ml of

absolute ethanol is added 3.5 ml of 1,2-dibromoethane. The mixture is heated

until an exothermic reaction (evolution of ethene and temporary reflux) starts.

The activation is completed by heating the mixture for an additional 10 min

under reflux. After cooling to about 50 �C, 1,3-heptadiynyl(trimethyl)silane

(0.05 mol, Chapter 7 for silylation methods) is added in one portion. The

introduction of N2 is started and the mixture is heated for 30 min under

reflux. After cooling to rt, the work-up is carried out in a way similar to

that in exp. 19.3.1 (no aqueous ammonia is used). (Z)-3-Hepten-1-ynyl(tri-

methyl)silane, n-PrCH¼CHC�CSiMe3, bp 75 �C/20 Torr, is obtained in a

high yield.

(Z,Z)-1-(1-Ethylthio)-7,7-dimethyl-1,3-octadien-5-yne, EtSCH¼CHCH¼

CHC�C-t-Bu (not distilled), is obtained from (Z)-1-(1-ethylthio)-7,7-

dimethyl-1-octen-3,5-diyne, EtSCH¼CHC�CC�C-t-Bu, by a similar proce-

dure (1 h reflux). The starting compound is prepared by Cadiot–Chodkiewicz

coupling of (Z)-1-ethylthio-1-buten-3-yne, EtSCH¼CHC�CH, with 1-bromo-

3,3-dimethyl-1-butyne, BrC�C–t-Bu (Chapter 14).


[13.1.2004–9:55pm] [353–368] [Page No. 365]


19.3.3 Regiospecific and stereospecific partial reduction of2,5-octadiyn-1-ol with activated zinc powder

Scale: 0.10 molar; Apparatus: 100-ml three-necked, round-bottomed flask,

equipped with a gas inlet, a magnetic stirring bar and a reflux condenser.

For the influence of a propargylic OH group see introduction of exp. 19.3.1.

19.3.3.1 Procedure

Zinc powder (30 g, Merck, analytical grade) is activated in 30 ml of 100%

ethanol as described in exp. 19.3.1. 3,5-Octadiyn-1-ol (0.10 mol, Note) is

added at �50 �C, after which the mixture is heated under reflux for about 2 h.

Nitrogen is introduced during this period. After cooling to rt, the reaction mix-

ture (including the excess of zinc powder) is poured into a solution of 30 g of

NH4Cl in 200 ml of water. The aqueous mixture is extracted seven times with

Et2O. The combined extracts are washed once with a saturated solution of

NH4Cl and subsequently dried overMgSO4. The liquid remaining after removal

of the solvent under reduced pressure is distilled through a shortVigreux column.

(Z)-2-Octen-5-yn-1-ol, bp �60 �C/1 Torr, is obtained in an excellent yield.

Note

3,5-Octadiyn-1-ol can be prepared by removal of the protecting group from

1-(1-ethoxyethoxy)-3,5-octadiyne, EtC�CCH2C�CCH2OCH(Me)OEt. This

compound is obtained by Cu(I)-catalysed reaction of 2-pentynyl-4-methylben-

zenesulphonate, EtC�CCH2OTs, with 1-magnesium bromide-3-(1-ethox-

yethoxy)1-propyne, BrMgC�CCH2OCH(Me)OEt (cf. Chapter 4, exp. 4.5.31).

19.3.4 Regiospecific and stereospecific partial reduction ofO-protected diyne alcohols and acetals with a conjugateddiyne system using activated zinc

19.3 PARTIAL REDUCTIONS OF CONJUGATED SYSTEMS 365

[13.1.2004–9:55pm] [353–368] [Page No. 366]


Scale: 0.10 molar; Apparatus: 100-ml, three-necked, round-bottomed flask,

equipped with a dropping funnel-gas inlet combination, a magnetic stirring

bar and a reflux condenser.

Although treatment of diyne alcohols RC�CC�CCH2OH with activated

zinc powder in ethanol results in specific reduction of the triple bond that is

closest to the OH group, the reduction is not stereospecific. In the various

experiments we found significant amounts of (E)-RC�CCH¼CHCH2OH

(up to 30%). It might be possible that a satisfactory stereoselectivity can be

achieved by using non-activated zinc (with analytical zinc powder activated

with dibromoethane the reaction is very fast). Satisfactory and reproducible

results can be obtained, however, by carrying out the reduction with the

protected diyne alcohol (either as the adduct with H2C¼CHOEt or as an

O–SiR3 derivative, cf. [8]).

19.3.4.1 Procedure

Zinc powder (30 g, Merck, analytical grade) is activated with 1,2-dibro-

moethane (3.5 ml) in 100% ethanol (35 ml) as described in the preceding

experiments. After cooling to �30 �C, the protected diyne alcohol (0.10 mol,

Chapters 14 and 20, exp. 6.7) is added in three equal portions over 5 min,

while introducing N2. The temperature of the suspension rises fast and (as a

rule) after some 10 min the ethanol begins to reflux: Spontaneous refluxing

ceases after 5 to 10 min. The mixture is heated for an additional 30 min

under reflux (for compounds with a longer carbon chain, see introduction of

exp. 19.3.1). After cooling to rt and settling of the zinc powder, the super-

natant solution is decanted and poured into 200 ml of an aqueous solution

of 30 g of NH4Cl to which 10 ml of concentrated aqueous ammonia has

been added. The zinc powder in the flask is rinsed three times with 20-ml

portions of hot (40–50 �C) ethanol, the ethanolic solutions being added to the

NH4Cl-solution. The product is isolated by extraction with Et2O (five times),

washing the organic solution with saturated aqueous NH4Cl, drying the

solution over MgSO4 and concentrating the solution under reduced pressure.

Removal of the protecting group, as described in exp. 19.2.3 gives (Z)-2-

decen-4-yn-1-ol, C5H11C�CCH¼CHCH2OH, bp �80 �C/0.5 Torr, in �80%

yield.

1,1-Diethoxy-2,4-heptadiyne, EtC�CC�CCH(OEt)2,is converted into (Z)-

1,1-diethoxy-2-hepten-4-yne, EtC�CCH¼CHCH(OEt)2, (yield �75%) by a

similar procedure (� 45 min reflux, using the same molar amounts of

reagents).


[13.1.2004–9:55pm] [353–368] [Page No. 367]


REFERENCES

1. M. Bertrand and C. Rouvier, Bull. Soc. Chim. France, 220 (1968).

2. J. Grimaldi and M. Bertrand, Bull. Soc. Chim. France, 947 (1971).

3. A. G. Mal’kina, L. Brandsma, S. F. Vasilevsky and B. A. Trofimov, Synthesis, 589 (1996).

4. H. Schmidt and J. F. Arens, Recl. Trav. Chim., Pays-Bas 86, 1138 (1967).

5. F. Naf, R. Decorzant, W. Thommen, B. Wilhalm and G. Ohloff, Helv. Chim. Acta 58, 1016

(1975).

6. M. H. P. J. Aerssens, R. van der Heiden, M. Heus and L. Brandsma, Synth. Commun. 20,

3421 (1990).

7. B. G. Shakovskoi, M. D. Stadnichuk and A. A. Petrov, J. Gen. Chem., USSR 34, 2646 (1964).

8. W. Oppolzer, C. Fehr and J. Warneke, Helv. Chim. Acta 60, 48 (1977).

9. R. Diercks and K. P. C. Volhardt, J. Amer. Chem. Soc. 108, 3150 (1986).

10. Unpublished results and observations from the author’s laboratory.

REFERENCES 367

synthesis of acetylenes, allenes and cumulenes || miscellaneous reactions of acetylenic and allenic...

Documents