Polarity-reversal catalysis by thiols of radical-chain hydrosilylation of alkenes

Tetrahedron Letters, Vol. 36, No. 16, pp. 2875-2878, 1995 Elsevier Science Ltd

Pergamon

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Polarity-Reversal Catalysis by Thiols of Radical-Chain Hydrosilylation of Alkenes H a i - S h a n D a n g and Brian P. R o b e r t s *

Christopher lngold Laboratories, Department of Chemistry, University College London, 20 Gordon Street, LONDON WCIH OAJ, U.K.

Abstract: The radical-chain hydrosilylation of alkenes by triethylsilane under very mild conditions is promoted by thiols, which act as polarity-reversal catalysts for the abstraction of electron-rich hydrogen.t?om the silane by nucleophilic ~-silylalkyl radicals'. The key role of organosilicon compounds in organic synthesis is now welt established.~ Hydrositylation of alkcnes [eqn. (1)] is an important method for the formation of Si-C bonds 2 and such addition rcactions can proceed by a radicalchain mechanism 3-6 (see Scheme) or under the influence of various transition metal catalysts, in particular rhodium, palladium and platinum complexes. 2'4

X3SiH

I I I I

+

\ C=C / / \

I I I I

~,

X3Si--C--C--H

X3Si--C--C--H

(1)

/ (ii)

\

(i)

X3SiH

I

\

,Scheme However, radical-chain hydrosilylation of alkencs using trialkylsilanes is not very useful in synthesis because the hydrogen-atom abstraction step (ii) is relatively slow at moderate temperatures and competing telomerisation of the alkene can also be a problemY' For example, the attempted reaction between triethylsitane and Ocl-l-cne ;.It 40 ~'C, initiated by the UV photolysis of di-tert-butyl peroxide (DTBP), gave no triethyloctylsilanc, 5 and only a 15% yield of Et3SiOct was obtained after triethylsilane, oct-l-ene and DTBP in thc molar proportions 5:5:1 had been heated at 110 °C lor 96 h. ~ At both temperatures, the addition of Et3Si" to the alkcnc [step (i)] is f,'tst and irreversible. 7 However, a 90% yield of Pr3SiOct could be obtained from radical addition of Pr3SiH to oct-l-ene by slowly dropping a mixture of the alkcnc (1 mol) and DTBP (0.2 mol) onto the silanc (6 tool) hcatcd at 140 "C. 6

2875

2876

Hydrogen-atom transfer to a non-conjugatively stabilised alkyl radical from silicon in a trialkylsilane is usually exothermic and we have argued that the slowness of this reaction can be attributed to unfavourable polar effects which operate in the transition state for abstraction of electron-rich hydrogen by a nucleophilic alkyl radical. 8'9 Furthermore, we have shown that this reaction is promoted by thiols which act aspolarity-reversal catalysts, 8-1° such that the slow direct abstraction is replaced the cycle of relatively rapid reactions (2) and (3), both of which benefit from favourable polar

R" + YSH YS" + Et3SiH_

-~ RH + YS"

(2)

*" YSH + Et3Si"

(3)

effects because the thiyl radical is electrophilic and the sulflaydryl hydrogen atom is electron-deficient.

Hence, it would

be expected that thiols will catalyse the radical-chain hydrosilylation of alkenes by trialkylsilanes, provided that possible competing radical-chain addition of the thiol to the alkcne [reactions (4) and (5)1 can be suppressed. The known ready

YS"

+

I I YS--(~--C" / \

\

C=C

I +

/

\

YSH

"

I / YS--C--C"

(4)

•

I I YS--C--C-- H

(5)

I

I

\

I

reversibility of the addition of thiyl radicals to alkenes 1~ should help to favour the desired reaction. Because the kinctics and thermodynamics of reactions (2)-(5), especially of reactions (3) and (4), will change in response to the electronic and steric properties of the group Y, the efficiency of thiol-catalysed alkene hydrosilylation could depend significantly on the nature of the thiol. 9 A solution in hexane (6.0 cm 3) containing oct- 1-ene ( 10.0 retool), triethylsilane (20.0 mmol), tridecane (2.0 mmol) and di-tert-butyl hyponitrite 12 (TBHN; 0.5 mmol) as initiator was heated at 60 °C lor 2 h under an atmosphere of nitrogen. GLC analysis of the reaction mixture, using the tridecane as internal rcfcrence, showed that only a trace amount of Et3SiOct had been formed (< 1% yield based on octene), in accord with the previous observations. 5'6 However, whcn the experiment was repeated and tert-dodecanethio113 (0.5 mmol) was added in one portion when the temperature of thc reaction mixture had reached 60 °C, the yield of Et3SiOct increased to ca.40%. Consideration of reactions (2)-(5) suggests that the yield of EtaSiOct might be improved by adding the thiol slowly during the reaction, rather than in a single portion, because competitive (and overall irreversible) addition of the thiol to the alkene should then be suppressed. Using this procedure, under otherwise identical conditions, when the thiol in hexane (1.0 cm 3) was added over a period of 1.8 h (conveniently with the aid of a syringe pump), the yield of tetraalkylsilane was increased to ca.60%. Oct-l-ene and a number of other representative alkenes were hydrosilylated with Et3SiH in hexane or using the EtaSiH as solvent, in the presence of tert-dodecanethiol as polarity reversal catalyst, and the alkyltriethylsilane adducts were isolated by distillation or by flash chromatography on silica gel) 4'~5 The yields of purified products are given in the Table.

2877

TABLE: Isolated yields of hydrosilylation products from the reactions of triethylsilane with alkenes in the presence of tert-dodecanethiol a at 60 °C

Alkene

Conditionsb

Product

B.p. ( °C/Torr)

.

~

B

~

S

i

E

t

3

34 52 65

63-65 / 0.02

3O 54

98 - 100 / 0.02

55 81

87-89/25

63

110-112/20

31 55

6%6910.1)2

28

110-113/0.02

85

52 / 0.02

61

C ~

Me~S i ~

A C

C

AN

O

C

(EtO2C)2C ~ ' ~

C

C

S

i

E

t ' slEt~

Me3S i @

~

3

''SiEt3

~ S i E t 3

C y

SiEt3

(EtO2C)2C ~ , ~ , . , , I

~

Si Et3

siEt3

(%)

76-78 / 0.05

A ~

Yield

a 5 Mol% based on alkene, b A = Thiol added in one portion at the start of the reaction. B = Thiol in hexane (1.0 cm3) added during 1.8 h to the stirred reaction mixture. C = Et3SiH as solvent; thiol added in one portion. Yields were improved by working in Et3SiH as solvent, presumably because the efficiency with which YS" is converted into EtaSi" [eqn. (3)] is increased, and this represents a practical alternative for this relatively cheap, volatile and easily recycled silane. However, certain aspects of thiol-catalysed hydrosilylation are not well understood at present and there are indications that the success of the reaction depends critically on the extent to which addition of YS" to the alkene is reversible [eqn. (4)]. For example, although diethyl allylmalonate gave an excellent yield of hydrosilylation product (see Table), diethyl diaUylmalonate gave very little product. With the lauer alkene we had hoped to obtain a cyclic silane, resulting from 5-exo-ring closure of the initially-formed adduct of EtaSi', and it is possible that corresponding rapid cyclisation of the thiyl radical adduct 16is responsible for the irreversible removal of YS" from the system. However, since the balance between the reversible reactions (3) and (4) will depend on the nature of Y in YS'/YSH, there should be considerable scope for improving the efficiency and generality of the thiol-catalysed hydrosilylation.

Radical-chain

addition of trialkylsilanes to other unsaturated functional groups, under conditions of polarity-reversal catalysis by thiols and related compounds, remains to be investigated.

2878

REFERENCES AND NOTES

1.

(a) Colvin, E.W. Silicon in Organic Synthesis, Butterworths, London, 1981. (b) Colvin, E.W. Best Synthetic Methods: Silicon

Reagents in Organic Synthesis, Academic Press, London, 1988. (c) Weber, W.P. Silicon Reagents for Organic Synthesis, Springer-Verlag, Berlin, 1983. 2.

(a) Ojima, I. in The Chemistry of Organic Silicon Compounds, eds. Patai, S.; Rappoport, Z., Wiley, Chichester, 1989, part 2, ch. 25. (b) Hiyama, T.; Kusumoto, T. in Comprehensive Organic Synthesis, eds. Trost, B.M.; Fleming, I., Pergamon, Oxford, 1991, vol. 8, ch. 3.12.

3.

Wilt, J.W. in Reactive Intermediates, ed. Abramovitch, R.A., Plenum Press, New York, 1983, vol. 3, oh. 3.

4.

Eaborn, C.; Boa, R.W. in Organometallic Compounds of Group IV Elements, ed. MacDiarmid, A.G., Marcel Dekker, New York, 1968, vol. 1, ch. 2.

5.

Eaborn, C.; Harrison, M.R.; Walton, D.R.M.J. Organometal. Chem., 1971, 31, 43.

6.

E1-Durini,N.M.K.; Jackson, R.A.J. Organometal. Chem., 1982, 232, 117.

7.

Chatgilialoglu, C.; Ingold, K.U.; Scaiano, J.C.J. Am. Chem. Soc., 1983, 105, 3292.

8.

(a) Allen, R.P.; Roberts, B.P.; Willis, C.R.J. Chem. Soc., Chem. Commun., 1989, 1387. (b) Kirwan, J.N.; Roberts, B.P.; Willis, C.R. Tetrahedron Lett., 1990, 31, 5093.

9.

Cole, S.J.; Kirwan, J.N.; Roberts, B.P.; Willis, C.R.J. Chem. Soc., Perkin 7)'ans. 2, 1991, 103.

10. (a) Paul, V.; Roberts, B.P.J. Chem. Soc., Chem. Commun.. 1987, 1322. (b) Paul, V.; Roberts, B.P.J. Chem. Soc., Perkin Trans. 2, 1988, 1183. (c) Paul, V.; Roberts, B.P.; Willis, C.R..t. Chem. Sot., Perkin l'rans. 2, 1989, 1953. 11. Kellogg, R.M. in Methods in Free Radical Chemistry, ed. Huyser, E.S., Marcel Dekker, New York, 1970, vol. 2, ch. 1. 12. Kiefer, H.; Traylor, T.G. 7~trahedron Left., 1966, 6163. Mendenhall, G.D. Tetrahedron Lett., 1983, 24, 451. 13. This is the mixture of isomers tert-C12H2~SH as obtained from the Aldrich Chemical Company. 14. Typical procedure: All operations were carried out under an atmosphere of dry nitrogen; all liquid reagents and solvents were dried and deoxygenated.

Diethyl allylmalonate (2.00 g, 10.0 mmol) and TBHN (90 rag, 0.5 mmol) were dissolved in

triethylsilane (6 cm3) in a flask containing a magnetic stirrer bar, The reaction flask was fitted with a short reflux condenser, equipped at the top with a septum inlet and a nitrogen by-pass bubbler. The flask was immersed in an oil bath which had been preheated to 60 °C and, after allowing a few minutes for the solution to achieve thermal equilibrium, terl-dodecanethiol (118 ~1, 0.5 retool) was added in a single portion via the septum and the mixture was heated tor a further 2 h. Removal of the excess triethylsilane under reduced pressure and isolation of the product by distillation atforded diethyl (3-triethylsilylpropyl)malonate (2.70 g, 85%), b.p. 110-113 °C/0.02 Ton'. Found: C, 60.4; H, 10.3. C16H3204Si requires C, 60.7; H, 10.2%. NMR spectra (CDCI3 solvent, J in Hz), ~5n 0.48 (8H, m), 0.90 (9H, t, .t 8.0), 1.26 (6H, t, J 7.2), 1.32 (2H, m), 1.90 (2H, apparent q, J 7.5), 3.35 (1H, t, J 7.5), 4.19 (4H, q, J 7.2); ~c 3.2, 7.4, 11.0, 14.1,21.7, 32.7, 51.7, 61.2, 169.6. All hydrosilylation products showed 1H/13C NMR spec~a and elemental analysis data in accord with the proposed structures; boiling points were consistent with literature values, where these are available. 15. As with the thiol-catalysed reduction of alkyl halides and xanthates by trialkylsilanes, 8'9 the choice of initiator is important.

Bis(4-tert-butylcyclohexyl)

peroxydicarbonate 9 (at 7(1 "C in hexane) or dilauroyl peroxide (at 80 °C in cyclohexane) were

generally less efficient initiators than TBHN and were ineffective with some alkenes. 16. Hach6, B.; Gareau, Y. Tetrahedron Lett., 1994, 35, 1837. Brumwell, J.E.; Simpkins, N.S.; Terrett, N.K. Tetrahedron, 1994, 50, 13533.

(Received in UK 15 February 1995; accepted 24 February 1995)