5017
Organometallics 1995, 14,5017-5018
-is( trimethylsily1)germane as a Radical-Based Reducing Agent C. Chatgilialoglu* and M. Ballestri ICoCEA, Consiglio Nazionale delle Ricerche, Via P. Gobetti 101,40129 Bologna, Italy Received August 22, 1995@ Summary: Tris(trimethylsily1)germane is an effective reducing agent for a variety of organic substrates. Among the group 14 hydrides, it has proven to be one of the most efficientH atom donors toward primary alkyl radicals. The majority of radical reactions of interest to synthetic chemists are chain processes under reducing conditions.lI2 A feature associated with these reactions is hydrogen transfer from the reducing agent to a radical (eq 1). The resulting M radical undergoes further
MH
+ R'-
M
+ RH
(1)
propagation step(s) to generate fresh R' radicals, thus completing the cycle of the chain reactions. Until a few years ago, the organometallic hydrides employed were limited to the R3SnH ~ e r i e s .The ~ continuing demand for new tools for synthetic chemists prompted us and others to introduce a variety of group 14 hydrides and to invent new ~ t r a t e g i e s .For ~ ~ ~example, today (MeaSi)sSiH is, in most cases, an attractive alternative to the popular BusSnH. Herein we report our preliminary results on (Me3Si)sGeH as a reducing agent.6 The reduction of a variety of organic derivatives was carried out using (Me3Si)sGeH. A toluene solution (2.02.5 mL) containing the compound to be reduced (0.2 M), the germane ( 2 equiv), and a,a'-azoisobutyronitrile (AIBN; 10 mol %) was heated at 82 "C for 20-60 min. GC analysis using decane or undecane as internal standard showed the formation of the corresponding hydrocarbon in excellent yield (Table lh7 Reduction of chlorides, bromides, and iodides, deoxygenation of secondary alcohols via thiono esters (Barton-McCombie reaction), deamination of primary amines via isocya@Abstractpublished in Advance ACS Abstracts, October 1, 1995. (1)For reviews, see: (a) Curran, D. P. Synthesis 1988,417, 489. (b) Jasperse, C. P.; Curran, D. P.; Fevig, T. L. Chem. Rev. 1991,91, 1237. (c) Curran, D. P. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford, U.K., 1991; Vol. 4, pp 715-831. (d) Smadja, W. Synlett 1994,1. (2) For books, see: (a) Giese, B. Radicals in Organic Synthesis: Formation of Carbon-Carbon Bonds; Pergamon Press: Oxford, U.K., 1986. (b) Motherwell, W. B.; Crich, D. Free Radical Chain Reactions in Organic Synthesis; Academic Press: London, 1992. (3) Neumann, W. P. Synthesis 1987,665. (4) For reviews, see: (a) Chatgilialoglu, C. Acc. Chem. Res. 1992, 25,188. (b) Chatgilialoglu, C.; Ferreri, C. Res. Chem. Intermed. 1993, 19,755. ( c ) Barton, D. H. R. Tetrahedron 1992,48, 2529. (5) For some more recent work, see: (a) Curran, D. P.; Xu, J.; Lazzarini, E. J. Am. Chem. SOC.1995,117,6603. (b) Gimisis, T.; Ballestri, M.; Ferreri, C.; Chatgilialoglu, C.; Boukherroub, R.; Manuel, G. Tetrahedron Lett. 1995, 36, 3897. ( c ) Clive, D. L. J.; Yang, W. J. Org. Chem. 1995,60, 2607. (d) Rai, R.; Collum, D. B. Tetrahedron Lett. 1994,35,6221. (e) Oba, M.; Nishiyama, K. J.Chem. SOC.,Chem. Commun. 1994,1703. (DBarton, D. H. R.; Jang, D. 0.;Jaszberenyi, J. Cs. Tetrahedron 1993,49,7193. (6) Tris(trimethylsily1)germane is commercially available from Aldrich. To our knowledge, the following two articles dealing with this compound exist only in the literature: (a) Brook, A. G.; Abdesaken, F.; Sollradl, H. J. Organomet. Chem. 1986,299,9. (b) Mallela, S. P.; Geanangel, R. A. Inorg. Chem. 1994,33, 1115. (7) The products of interest were identified by comparison of their retention times with those of authentic materials.
Table 1. Reaction of a Variety of Organic Compounds with (MesSi)SGeH" yield of
RX
Oh
99
0 100
O*OTC' 0
DBr
99
cH300Br 9a
98 B l 99 3
99
97
96
97 a Conditions: 0.2 M of RX,0.4 M of (MeaSi)aGeH, a n d AIBN (10 mol %) in toluene were heated at 82 "C for 20-60 min. Based on product formation in t h e crude reaction mixture; determined by GC analysis using decane or undecane as intemal standard.
nides, removal of the PhSe group, and replacement of a tertiary nitro group by hydrogen were extremely effective.!j Evidence for a free radical chain mechanism (eqs 2 and 3) was provided by the observations (i)that in most cases the reactions started only in the presence of radical initiators such as AIBN and dibenzoyl peroxide and (ii) that the reactions were retarded by 2,6-di-tertbutyl-4-methylphenol and nitroxides such as TEMPO. (Me,Si),GeH
-
+ R' (Me,Si),Ge' + RH + RX - (Me,Si),GeX + R'
(2)
(Me,Si),Ge' (3) One of the intriguing aspects of reaction 1 is the ability of MH to donate hydrogen. In his review, (8) For mechanistic details of these reactions with other reducing agents, see refs 1-4.
0276-733319512314-5017$09.00/00 1995 American Chemical Society
Communications
5018 Organometallics, Vol.14,No. 11, 1995 Newcomb discussed this topic regarding a number of carbon-centered radicals with the most common hydrogen atom transfer agent^.^ Furthermore, one of us reviewed this topic within the family of silicon hydrides.1° In order t o quantify the observed reactivity of (Me3Si)sGeH and to provide a rate constant for H atom abstraction from this germane by primary alkyl radicals (eq 2), we utilized the 5-hexenyl cyclization as a free radical clock.g Under conditions in which the hydride concentration changed significantly during the experiments, the 5-hexenyl radical, which is formed as an intermediate in the reduction of the corresponding bromide, either abstracted H from the germane or cyclized prior to the hydrogen transfer. By measurement of the concentrations of 1-hexene and methylcyclopentane by GC analysis,ll the mean value Of KGeH/Kc = 12.55 M-l was obtained from four independent experiments at 25 "C.12 Taking K, = 2.5 x lo5 s-l,13* we calculate K G ~ H= 3.1 x lo6 M-l s-l at 25 "C. Figure 1schematically illustrates the hydrogen donation abilities of the most representative group 14 hydrides.13 (Me3Si)sGeHis slightly more reactive than BusSnH. It is worth mentioning that the replacement of n-butyl groups in Bu3GeH by MeaSi groups produces a 33-fold increase of the rate constant. On the basis of (9) Newcomb, M. Tetrahedron 1993,49, 1151. (10)Chatgilialoglu, C. Chem. Rev. 1996,95, 1229. (11)The relative quantities of 1-hexene and methylcyclopentane varied in the expected manner as the concentration of germane changed. Minor quantities of cyclohexane (1.3%)were also formed. (12)The k&kC ratio was determined by using the equation [AHBI = l/[B]{[C] + kJko,a}{l - e-k&H[BYkc} - 1, where [AI, [Bl, and [Cl are the concentrations of 1-hexene, methylcyclopentane and (MeaSihGeH (initial), respectively.
B u ~ ~ H
Id F&H
104
Me3SiSi(H)Me2 (Me3Si)&i)Me
i !05 (MeaSihiiH
G%Si)@H
LO'
Ru3SnH
Figure 1. Rate constants for hydrogen abstraction ( k ~ , M-l s-l) from a variety of group 14 reducing agents by primary alkyl radicals at 25 "C (taken from ref 13). the experimental and theoretical studies of analogous silanes where this increase is ca. 600,1° we suggest that the extra stability of the (Me3Si)sGe' radical is mainly due to a through-space (hyperconjugation) interaction between the bonding and/or antibonding Si-C B-bond. In conclusion, we have shown that of the known group 14 hydrides (Me3Si)sGeHis one of the most efficient H atom donors. It also rivals the effectivenessof the other group 14 hydrides in reduction processes. Further work on the kinetics and synthetic scope of these reactions is in progress.
Acknowledgment. We thank the Progetto Strategic~ "Tecnologie Chimiche Innovative" (CNR, Rome) for some financial support. OM950656E (13) (a) For Bu3SnH: Chatgilialoglu, C.; Ingold, K. U.;Scaiano, J. C. J . Am. Chem. SOC.1981,103,7739. (b) For BusGeH: Lusztyk, J.; Maillard, B.; Lindsay, D. A.; Ingold, K. U. J . Am. Chem. SOC.1983, 105,3578. ( c ) For EtsSiH: Chatgilialoglu, C.; Ferreri, C.; Lucarini, M. J . Org. Chem. 1993,58, 249. (d) For MesSiSi(H)Mez: Ballestri, M.; Chatgilialoglu, C.; Guerra, M.; Guerrini, A.; Lucarini, M.; Seconi, G. J . Chem. SOC., Perkin Trans. 2 1993,421. (e) For (Me3Si)zSi(H)Me: Chatgilialoglu, C.; Guerrini,A.; Lucarini,M. J . Org. Chem. 1992, 57,3405. (0For (Me3Si)sSiH: Chatgilialoglu, C.; Dickhaut, J.; Giese, B. J . Org. Chem. 1991,56,6399.
