Reduction of alkyl halides by triethylsilane based on a cationic iridium bis(phosphinite) pincer catalyst: Scope, selectivity and mechanism

Article abstract of DOI:10.1002/adsc.200800528

A highly efficient procedure for the reduction of a broad range of alkyl halides by triethylsilane based on a cationic iridium bis(phosphinite) pincer catalyst has been discovered and developed. This reduction chemistry is chemoselective and has unique selectivities compared with conventional radical-based processes and the aluminum trichloride/ triethylsilane (AlCl ₃/Et₃SiH) and triphenylmethyl tetrakis[pentafluorophenyl] borate/triethylsilane {[Ph₃C] [B(C₆F₅) ₄]/Et₃SiH} systems. Reductions use three equivalents of triethylsilane relative to the halide and can be carried out with very low catalyst loadings and in a solvent-free manner, which may provide an environmentally attractive and safe alternative to many currently practiced methods for reduction of alkyl halides. Mechanistic studies reveal a unique catalytic cycle. The cationic iridium hydride 2,6-bis[di-(tert-butyl) phosphinyloxy)phenyl-(hydrido)iridium, (POCOP)IrH⁺ {POCOP= 2,6-[OP(t-Bu)₂]₂C₆H₃} binds and activates the silane. This complex serves as a potent silylating reagent to generate silyl halonium ions, Et₃SiXR⁺, which are reduced by the neutral iridium dihydride to yield alkane product and regenerate the cationic (POCOP)IrH⁺, thus closing the catalytic cycle. All key intermediates have been identified by in situ NMR monitoring and kinetic studies have been completed. An application of this reduction system to the catalytic hydrodehalogenation of a metal chloride complex is also described.

Full text of DOI:10.1002/adsc.200800528

FULL PAPERS
DOI: 10.1002/adsc.200800528
Reduction of Alkyl Halides by Triethylsilane Based on a Cationic
Iridium Bis(phosphinite) Pincer Catalyst: Scope, Selectivity and
Mechanism
Jian Yanga and Maurice Brookharta,*
a
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, USA
Fax : (+1)-919-962-2476; phone: (+1)-919-962-0362; e-mail: mbrookhart@unc.edu
Received: August 23, 2008; Published online: December 19, 2008
Abstract: A highly efficient procedure for the reduc- 2,6-bis
tion of a broad range of alkyl halides by triethylsi- (hydrido)iridium,
lane based on a cationic iridium bis(phosphinite) 2,6-[OP(t-Bu)2]2C6H3} binds and activates the silane.
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
pincer catalyst has been discovered and developed. This complex serves as a potent silylating reagent to
This reduction chemistry is chemoselective and has generate silyl halonium ions, Et3SiXR+, which are re-
unique selectivities compared with conventional radi- duced by the neutral iridium dihydride to yield
cal-based processes and the aluminum trichloride/ alkane product and regenerate the cationic
triethylsilane (AlCl3/Et3SiH) and triphenylmethyl
tetrakis[pentafluorophenyl]borate/triethylsilane
ACHTUNGTRENNUNG
{[Ph3C] [BACHTUNGTRENNUNG(C6F5)4]/Et3SiH} systems. Reductions use NMR monitoring and kinetic studies have been com-
three equivalents of triethylsilane relative to the pleted. An application of this reduction system to
halide and can be carried out with very low catalyst the catalytic hydrodehalogenation of a metal chlo-
loadings and in a solvent-free manner, which may ride complex is also described.
provide an environmentally attractive and safe alter-
native to many currently practiced methods for re-
duction of alkyl halides. Mechanistic studies reveal a Keywords: alkyl halides; chemoselectivity; mechanis-
unique catalytic cycle. The cationic iridium hydride tic studies; reduction; silanes; solvent-free process
Introduction
ficient hydrogen donor and was developed by Chatgi-
lialoglu et al. as a new free radical reducing reagent.[2]
Reduction of alkyl halides to alkanes is a frequently However, this TTMSS reagent is not economically at-
practiced synthetic transformation. The most common tractive and not easily made. It has also been reported
method employed is the use of Bu3SnH in a radical that Et2SiH2 and Et3SiH can reduce certain classes of
chain process.[1] While this is an efficient process, al- alkyl halides to the corresponding alkanes in the pres-
ternative reduction methods are desired owing to the ence of a suitable initiator and alkanethiols as polarity
toxicity of tin reagents and difficulties in work-up and reversal catalysts.[2,4]
separation of tin halide by-products from the desired
Strong Lewis acids like AlCl3 have been explored
organic products.[2] Other organometallic compounds to mediate the reduction of alkyl halides by Et3SiH.[5a]
such as Bu3GeH and RHgH complexes have been ex- Extensive skeletal rearrangements or Friedel–Crafts
plored as alternatives to Bu3SnH as halide reducing alkylation can occur accompanying this reduction
agents, but they are not viable options for many appli- chemistry. High loadings of Pd(II) salts have been re-
cations due to high cost or high toxicity.[2,3]
ported to induce Et3SiH reduction of certain classes
Although thermodynamically feasible, the use of of alkyl halides.[5b] P,N-chelated Pt(II) complexes in
readily available and inexpensive trialkylsilanes in combination with HSiMe2Ph have also been shown to
place of tin hydrides for reduction is not effective due reduce alkyl chlorides and the following order of re-
À
to the high bond energy of the Si H bond which will activity was observed: CCl4 >CHCl3 >CH2Cl2 >
not support a radical chain process.[2] It was demon- CH3Cl.[5c]
strated that silicon-hydrogen bonds can be weakened
by substitution of silyl groups on the Si H group.
Tris(trimethylsilyl)silane (TTMSS), therefore, is an ef- to be capable of catalytic reduction of C
The combination of [Ph3C] [BACHTUNGTERNU(NG C6F5)4] and Et3SiH
[2]
À
was recently demonstrated by Ozerov and co-workers
3
À
N
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Jian Yang and Maurice Brookhart
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bonds via a mechanism involving generation of carbo- Results and Discussion
cations through fluoride abstraction by Et3Si+ fol-
lowed by hydride transfer to the carbocation by Initial Observations and Synthesis of an Active Well-
Et3SiH (Scheme 1).[6a,b] Mꢁller has described similar Defined Iridium Catalyst
chemistry using hydride- and fluoride-bridged disilyl
cations.[6c]
Previously we have shown that iridium bis(phosphin-
ite) pincer complexes are precursors to highly active
catalysts for the transfer dehydrogenation of alkanes[9]
[10]
À
and are capable of N H bond activation. Abstrac-
tion of chloride from (POCOP)Ir(H)(Cl) {3,
POCOP=2, 6-[OPACTHNUTRGENN(UG t-Bu)2]2C6H3} with NaBACHTUNGTRENNUNG(ArF)4
[ArF =3,5-(CF3)2C6H3], generates a solvated cationic
iridium monohydride in situ, which was found to cata-
lyze the reduction of dichloromethane to methane by
Et3SiH at room temperature with 100 turnovers
(TOs) (Scheme 2).
Scheme 1. Proposed mechanism for reduction of alkyl fluo-
rides with the [Ph3C] [BACHTNUTGRNEUNG
(C6F5)4]/Et3SiH system.[6a]
As a part of our continuing interest in incorpora-
À
tion of Si H bond activation chemistry into catalytic
Scheme 2. Initial discovery of iridium-catalyzed reduction of
CD2Cl2 to CD2H2 by Et3SiH.
cycles we have discovered and recently communicat-
ed[7] that the cationic iridium bis(phosphinite) pincer
complex 1 (Figure 1) is a highly efficient catalyst for
Interestingly, since CD2HCl should be the initial re-
duction product of CD2Cl2, the observation that only
traces of CD2HCl were identified during the reduc-
tion of CD2Cl2 indicates that the reduction of
CD2HCl is much more rapid than that of CD2Cl2.
This observation suggests that this reduction chemis-
try probably does not go through the conventional
radical-based process. The combination of a novel
mechanism and practical utility strongly encouraged
us to explore the scope, selectivities and mechanism
of this reduction chemistry.
The inactivity of IrACTHNUTRGNEU(GN POCOP)HCl (3) under identi-
cal conditions for reduction of CD2Cl2 excludes the
possibility of 3 as the catalytically active species for
Figure 1. Cationic iridium pincer catalysts 1 and cationic iri-
dium h1-silane complex 2.
this reduction. Similarly, NaB
the active species. Thus, the catalytically active species
the reduction of a broad range of alkyl halides by tri- must be the in situ generated cationic monohydride
ethylsilane reagents. Our preliminary studies argue complex from the reaction between Ir(POCOP)HCl
against a radical pathway and the catalytic cycle ap- and NaB(ArF)4.
pears to operate by a unique process involving an un- Catalysis with the in situ generated complex some-
precedented highly electrophilic h1-silane complex, times produces inconsistent results; therefore, the cat-
2[8] (Figure 1). In the present paper, we report a full ionic
iridium complex,
dichlorometÀhane
account of this chemistry including catalyst discovery (POCOP)Ir(H) (ArF)4 , 4, was synthe-
(CH2Cl2)+
and development, substrate scope and selectivity and sized and isolated by treatment of 3 with a slight
mechanistic studies. In addition, this reduction excess of NaB(ArF)4 in dichloromethane [Eq. (1)].
chemistry has also been extended to the catalytic hy- Complex 4 was characterized by NMR spectroscopy
drodehalogenation of a metal halide complex. and elemental analysis (see Experimental Section).
ACHTUGNTRENUN(NG ArF)4 was excluded as
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
BACHTUNGTRENNUNG
AHCTUNGTRENNUNG
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Reduction of Alkyl Halides by Triethylsilane
FULL PAPERS
A broad spectrum of alkyl halides was conveniently
reduced by triethylsilane in the presence of 0.5 mol%
(or less) of 1 at 23–608C in chlorobenzene. Fluoro-
benzene, dichlorobenzene or even neat alkyl halides
(see below) have also proved to be suitable solvents.
Results of typical reactions are illustrated in Table 1.
Conversions were determined by NMR spectroscopy.
Entries 1 and 2 show that highly efficient reduction of
benzyl halides can be achieved at 238C with 0.5% cat-
alyst loading. With 0.01% 1 3200 turnovers (TOs) for
Complex 4 was found to be an efficient catalyst for the reduction of benzyl chloride were obtained after
the reduction of a variety of alkyl chlorides and bro- 29 h at 238C. At 0.075% catalyst loading rapid and
mides. However, at higher reaction temperatures complete reduction of benzyl bromide to toluene is
(608C or 1108C) and with certain substrates, a small accomplished in 2.5 h.
amount of Et3SiF was identified by 13C and 19F NMR,
Results for the reduction of primary alkyl halides
presumably due to the attack of “Et3Si+” on the are shown in entries 3–7. Rapid reductions of 1-bro-
À
BACHTUNGTRENNUNG
catalyst, À we sought to replace B
ACHTUGNTRENUN(NG ArF)4 by the that the bromide (1.5 h) is reduced faster than the
B
G
chloride (7 h). Surprisingly, reduction of iodide is very
slow; 48 h are required for complete conversion of 1-
iodopentane to pentane at 608C with 0.5% loading of
1. Entry 6 shows that 1-fluoropentane can also be re-
duced at 2% catalyst loading at 608C although with
Reduction of Alkyl Halides with Et3SiH Catalyzed by
Iridium Acetone Complex (1)
À
reduced efficiency (46 TOs based on loss of C F
bonds) and selectivity (unidentified products in addi-
À
Synthesis and isolation of 4 possessing a B
G
counteranion is not straightforward. The chloride ab- tion to pentane are observed). Similarly, efficient re-
straction route analogous to that described in Eq. (1) ductions of secondary halides can be accomplished at
is not very efficient under several conditions presuma- either 23 or 608C (entries 8–11) and again bromides
bly due to the poor solubility of KB
(C6F5)4 in CH2Cl2 are more reactive than chlorides. Bromocycloheptane
À
and the strong Ir Cl bond. An alternate synthetic is efficiently and selectively reduced to cycloheptane
route (involving hydride transfer) was then employed. without observation of any skeletal rearrangement
Acetone complex 1 is readily prepared by treatment product, methylcyclohexane (entry 11). The tertiary
of dihydride 5[12] with [Ph3C] [B
ACHTUNGTRENNUNG
than 4 due to the strong coordinating ability of ace-
Catalytic reductions can also be carried out in a sol-
tone and can be stored in a dry box at room tempera- vent-free manner as illustrated in Table 2. Thus com-
ture for at least 1 year. Exposure of 1 to Et3SiH re- plete reduction of neat benzyl chloride can be ach-
sults in rapid hydrosilation of acetone and forms non- ieved with 0.5 mol% catalyst loading in less than
coordinating (CH3)2CHOSiEt3 and the highly reactive 20 min at room temperature (entry 1). Similarly, pri-
solvated complex which initiates reduction reactions mary and secondary chlorides and bromides are re-
[Eq. (2)]. Thus 1 is an ideal initiator for catalytic re- duced efficiently at 238C without a solvent (entries 2–
ductions.
5).
Competition Experiments to Determine Relative
Reactivities: Comparison with Conventional Radical-
Based Reduction Chemistry
In radical-based reduction processes the general order
of reactivities is RI>RBr>RCl and 28 RX>18 RX
(28 RX=secondary alkyl halides; 18 RX=primary
alkyl halides).[1] Quite different selectivities are seen
for the 1/Et3SiH system. Relative reduction rates of
18 RX vs. 28 RX were determined by carrying out
head-to-head competition experiments in one flask.
These results were previously described in detail[7]
and provided the following relative reactivities:
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Jian Yang and Maurice Brookhart
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Table 1. Iridium-catalyzed reduction of alkyl halides by Et3SiH.[a]
Entry
1
Catalyst [mol%]
RX
Temperature [8C]
Time [h]
Conversion[c][%]
Product
0.5
23
23
0.3
29
>99
32
0.01[b]
0.5
0.075
23
23
0.3
2.5
>99
>99
2
3
0.5
0.5
0.5
2.0
60
60
60
60
7
>99
>99
>99
92
4
1.5
48
50
5
6[d]
7
1.0
0.5
0.5
23
60
60
51
>99
>99
>99
8[e]
9[e]
2.3
0.7
10[e]
11[e]
12[e]
0.5
0.5
60
23
16
>99
>99
0.3
0.5
0.5
23
23
0.3
0.3
>99
>99
13
[a]
Reaction conditions: 3 equiv. of Et3SiH.
Reduction was carried out in C6D4Cl2.
Determined by loss of alkyl halides by NMR.
Products in addition to pentane are observed.
[b]
[c]
[d]
[e]
In addition to alkane, traces of olefin and H2 were observed which convert to alkanes at longer reaction times.
Table 2. Solvent-free reduction of alkyl halides with Et3SiH catalyzed by 1.[a]
Entry
1
RX
Time [h]
0.3
Conversion[b] [%]
99
Product
2
3
20
86
98
3.3
4[c]
9
99
98
5[c]
3.3
[a]
Reaction conditions: 3 equiv. of Et3SiH.
Determined by loss of alkyl halides by NMR.
In addition to alkane, traces of olefin and H2 were observed and converted to alkanes at longer reaction times.
[b]
[c]
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Reduction of Alkyl Halides by Triethylsilane
FULL PAPERS
18 RCl:28 RCl=2.6:1.0, 18 RBr:28 RBr=2.0:1.0, 18
RI:28 RI=1.6:1.0.
Entries 3–5 in Table 1 show surprising relative reac-
tivities of primary chloride vs. primary bromide vs.
primary iodide with RBr>RCl>RI when reductions
are carried out in separate flasks. Entries 8 and 9
show a similar order for secondary bromide vs. secon-
dary chloride. Quite different results were obtained
when head-to-head competition experiments were
carried out in the same flask. Using primary halides,
relative reactivities were found to be RI:RCl=
1200:1, RBr:RCl=260:1, RI:RBr=80:1.[7] Only
traces of CD2HCl were observed during the reduction
of CD2Cl2 by 1/Et3SiH which shows that CD2HCl is
reduced much faster than CD2Cl2, a result which is
also inconsistent with a radical mechanism. Qualita-
tive comparisons of selectivities of radical-based re-
duction of alkyl halides and reductions employing 1/
Et3SiH are shown in Scheme 3.
Scheme 3. Selectivities for alkyl halide reductions observed
in the 1/Et3SiH system compared with conventional radical-
based reductions.
Scheme 4. Comparison of reductions using 1/Et3SiH versus
AlCl3/Et3SiH.[5a]
Comparison of 1/Et3SiH with AlCl3/Et3SiH
yielded solely 1-deuteriohexane while (bromomethyl)-
cyclohexane gave (deuteriomethyl)cyclohexane as the
Use of the strong Lewis acid AlCl3 in combination major reduction product and 1-deuterio-1-methylcy-
with Et3SiH was explored by Doyle et al.[5a] to medi- clohexane as the minor product (ca. 6:1 ratio as deter-
ate the reduction of alkyl halides. No major difference mined by 13C{1H} NMR spectroscopy). These results
in reactivities was found between comparable alkyl show that extensive carbocation isomerizations in-
bromides and chlorides.[5a] Reduction by triethylsilane volving both hydride and alkide shifts are observed in
was often accompanied by Friedel–Crafts alkylation the AlCl3 system in contrast to the (POCOP)IrH+
reactions or extensive skeletal rearrangements system where only minor rearrangement occurs in the
(Scheme 4).[5a] Rearrangement to methylcyclohexane most favorable circumstances, i.e., 18 to 38 conver-
was observed in the reduction of bromocyclohep- sions via a 1,2 hydride shift. (Additional comments
ACHTUNGTRENNUNG
tane.[5a] Formation of 2-deuteriohexane or 1-deuterio- concerning these isomerizations appear below in dis-
1-methylcyclohexane from reductions of 1-bromohex- cussing the overall mechanism.)
ane and (bromomethyl)cyclohexane,[5a] respectively,
using AlCl3/Et3SiD suggested a mechanism involving
carbocation-like intermediates in which the carbocat- Comparison of 1/Et3SiH with [Ph3C] [BACTHNUTRGEN(UNG C6F5)4]/
ion rearrangement precedes the reduction event.
In contrast to the AlCl3-catalyzed process, we ob-
Et3SiH
served quite different chemistry for the 1/Et3SiH The combination of [Ph3C] [BACHTNUTRGNEUNG(C6F5)4]/Et3SiH was re-
system (Scheme 4). Cycloheptane was the only ob- cently reported to catalytically reduce alkyl fluorides
servable reduction product (>98%) of bromocyclo- via a mechanism involving generation of carbocations
heptane. When treated with Et3SiD, 1-bromohexane through fluoride abstraction by Et3Si+ followed by hy-
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Jian Yang and Maurice Brookhart
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dride transfer to the carbocation by Et3SiH.[6a] To gain reduced more rapidly but only up to 80% conversion
more insight into the (POCOP)IrH+ system studied presumably due to the decomposition of the catalyst.
here, we compared the catalytic reactivities of 1 with If formation of a carbocation is rate-determining,
[Ph3C] [BACHTUNGTRENNUNG(C6F5)4] for alkyl chloride reductions. Thus a then this is the expected order. In contrast, in the re-
mixture of a 1:1 molar ratio of 1-chloropentane and duction system based on 1 (Figure 2, right), head-to-
2-chloropentane was treated with 6 equiv. of Et3SiH head competition establishes that the primary chlo-
and 1% catalyst (a: [Ph3C] [B
and the reaction was monitored by H NMR spectros- which is inconsistent with rate-determining formation
ACHTNUGTENR(NUNG C6F5)4]; b: complex 1) ride is more reactive than the secondary chloride,
1
copy [Eq. (3)].
of a carbocation intermediate.
Mechanistic Studies
1
In situ 31P and H NMR Monitoring of the Working
Catalyst System: Identification of Catalytic Resting
State(s) and Key Intermediates
Potential catalytic intermediates were generated inde-
As shown in Figure 2 (left), in the [Ph3C] pendently using methods described in Scheme 5 and
[B
ACHTUNGTRENNUNG
Figure 2. Plot of substrate concentration vs. time for reduction of alkyl halides catalyzed by catalyst a: [Ph3C] [B
(left), and by catalyst b: iridium complex 1 (right).
ACHTUNGTRENNUNG(C6F5)4]
Scheme 5. Methods for generating potential intermediates.
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Reduction of Alkyl Halides by Triethylsilane
FULL PAPERS
potential catalyst resting states and key intermediates the concentrations of silane complex (2) and alkyl
for this process, the reduction of alkyl halides was halide complex (8 or 9) could be measured the equi-
performed under catalytic conditions and monitored librium constants were determined to be 7.1 and 240
1
by H, 13C{1H} and 31P{1H} NMR spectroscopy.
for reductions of 1-chloropentane and 1-bromohex-
The catalyst resting state(s) were found to depend ane, respectively, consistent with the proposition that
on the relative binding affinities of the silane and the equilibrium is maintained between the silane complex
alkyl halide.[13] Following the in situ reduction of CH3I 2 and the halide complex (8 or 9) throughout the cat-
at 238C shows that the dominant iridium species is alytic reduction.
the CH3I complex, 7. However, following the reduc-
tion of either 1-chloropentane or 1-bromohexane
shows that the Ir species exist as an equilibrium mix- The Catalytic Cycle
ture of the halide complex (8 or 9) and the silane
complex (2) with the ratio depending on the ratio of A plausible catalytic cycle accounting for all observa-
silane:halide and the nature of the halide [Eq. (4) and tions is shown in Scheme 6. The cationic iridium hy-
Eq. (5)]. At several stages of conversion when both dride (POCOP)IrH+ binds and activates the silane.
The h1-silane complex, 2, serves as a potent silylating
reagent to generate silyl halonium ions, Et3SiXR+,
which are reduced by the neutral iridium dihydride, 5,
to yield alkane product and regenerate the cationic
(POCOP)IrH+, thus closing the catalytic cycle. This
mechanism has parallels to that proposed by Piers
et al.[14] for hydrosilylation of ketones using (C6F5)3B/
Et3SiH wherein the silane is activated by (C6F5)3B
and transfers Et3Si+ to ketone to produce R2C=
+
OSiR3 and H
(C6F5)3BÀ. The catalytic cycle is closed
+
by hydride reduction of R2C=OSiR3 .
The cationic silane complex is in rapid equilibrium
with the halide complexes and the position of equilib-
rium depends on the relative binding affinities and
concentrations of the silane and the alkyl halide. The
observation that equilibrium is maintained between
the silane complex (2) and the chloride or bromide
complex (8 or 9) throughout the catalytic reduction
indicates that this equilibrium is established rapidly
relative to the reduction. The equilibrium between
alkyl iodide complexes and the s-silane complex was
Scheme 6. Proposed catalytic cycle for iridium-catalyzed reduction of RX by Et3SiH.
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Jian Yang and Maurice Brookhart
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not observable by NMR spectroscopy due to the very
The presumed intermediates in the AlCl3/Et3SiH
low equilibrium concentration of the iridium s-silane and 1/Et3SiH systems are Cl3AlXR and Et3SiXR+.
complex due to the much tighter binding of alkyl The Lewis acidity of Et3Si+ is greater than that of
iodide versus silane to IrACHTNUGRTNEUNG
(III). However, a low temper- AlCl3 so it would seem that Et3SiXR+ would be more
ature NMR experiment using 13C-labeled iodome- prone to carbocation rearrangements than Cl3AlXR.
thane shows that the reaction between silane complex In the case of AlCl3/Et3SiH system, Et3SiH is the hy-
(2) and 13CH3I initially yields the iridium 13CH3I com- dride donor while in the 1/Et3SiH system
plex (7) with no appreciable formation of reduction
ACHTUNGTRENN(UNG POCOP)Ir(H)2 (5) is the hydride donor. We have es-
product 13CH4 [Eq. (6)]. Thus the equilibrium be- tablished that 5 is 104 times more reactive for reduc-
tween the iodomethane complex (7) and silane com- tion of Et2OSiEt3 than Et3SiH.[17] Thus, more rapid
plex (2) is established prior to the reduction event.
+
reduction of Et3SiXR+ by 5 relative to Et3SiH reduc-
tion of Cl3AlXR likely explains the observation of a
lower fraction of reduction products resulting from
carbocation rearrangements in the 1/Et3SiH system.
Generation and Reactivity of the Triethylsilyl-
+
AHCTUNGTRENNUNG
(methyl)iodonium Ion,[18–20] Et3SiICH3
To gain deeper insight into the catalytic mechanism
we sought to generate the proposed intermediate
+
Et3SiICH3 , 11, and explore its reactivity toward
ACHTUNGTRENUN(NG POCOP)Ir(H)2, 5. Observation of the kinetic prod-
Kinetic studies[7] of the reduction of CH3I show
that the turnover frequency is zero-order in [CH3I] ucts of reduction of 11 could reveal whether step I or
and first-order in [Et3SiH], consistent with the pro- step II was turnover-limiting. If step II were turnover-
posed catalytic cycle where 7, [Ir]HACTHNUTRGNEUNG
(ICH3)+, is the cat- limiting, Et3SiH and CH3I would be initially observed
alyst resting state. The proposed mechanism explains while if step I were turnover-limiting CH4 and Et3SiI
the differing relative reactivities of halides in separate would be initially observed. Unfortunately, all at-
flasks versus the same flask. Alkyl iodides bind tightly tempts to cleanly generate 11 at low temperatures
to Ir and result in very low concentrations of complex always resulted in a mixture of two species. Thus,
2 thus retarding the overall rate. However, the silane when in situ generated arene-stabilized triethylsilyl
complex reacts preferentially with alkyl iodides when cation [Et3SiACHTNUTRGENNUG HCATUNGTRENNGUN
(arene)]+ [B(C6F5)]À (arene=C6D6 or
offered a choice among alkyl iodides, bromides and C6D5Cl)[21,19] is treated with a solution of 13CH3I in
1
chlorides in the “same flask” experiments. Similarly, C6D5Cl at À408C both the H and 13C NMR spectra
alkyl bromides are favored over alkyl chlorides in exhibit two sets of 13CH3 resonances. One species ex-
head-to-head competition. This reactivity order is hibits a sharp 13C resonance at d=10.3 and a sharp
consistent with observations of Reed who has shown doublet at d=2.18 (1JC,H =158 Hz) in the 1H NMR
that the binding affinities of hexahalo-carborane spectrum. This species can be assigned to dimethylio-
+
anions to i-Pr3Si+ are in the order I>Br>Cl.[15]
donium ion 13CH3I13CH3 , 12, previously prepared by
Kinetic studies cannot distinguish step I (silylation) Olah.[22] This species presumably forms from attack of
or step II (hydride transfer) as the turnover-limiting 13CH3I on the initially formed Et3SiI13CH3 , 11, as
+
step, and this may well depend on the nature of the shown in [Eq. (7)].[22] A second set of methyl resonan-
substrate (RCl vs. RBr vs. RI and 18 RX vs. 28 RX vs. ces is observed which consists of a broadened 13C res-
38 RX). It is tempting to suggest that the relatively onance at d=À8 together with a methyl doublet in
small difference in competitive rates of reduction of the H NMR spectrum at d=1.95 (1JC,H =157 Hz). We
1
18 and 28 halides indicates that step I, Et3Si+ transfer, assign these signals to rapidly averaging signals of
is turnover-limiting. The basicity of the halide in 18 vs.
28 systems should differ little and the steric differen-
ces between 18 and 28 substrates should not signifi-
cantly influence rates of Et3Si+ transfer. However, it
is known that when Y is an exceptionally good leaving
group, relative rates of nucleophilic attack on 18 vs. 28
RY systems are compressed due to the high carbocat-
ionic character in the transition state.[16] Certainly
XSiEt3 species will be excellent leaving groups thus
step II cannot be completely discounted as the turn-
over-limiting step.
182
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Adv. Synth. Catal. 2009, 351, 175 – 187

Reduction of Alkyl Halides by Triethylsilane
FULL PAPERS
+
Et3SiI13CH3 , 11, and free 13CH3I in the degenerate
exchange equilibrium shown in Eq. (8). In support of
this assignment we note that increasing the concentra-
tion of free 13CH3I results in moving the averaged 13C
shift upfield toward that of 13CH3I (d=À23.4). As the
1
13C NMR shift moves upfield, the JC,H, as measured
1
by the H doublet, decreases consistent with the lower
1
value of JC,H in free 13CH3I (151 Hz). It is interesting
+
to note that CH3ICH3 does not engage in CH3I ex-
change which implies that CH3I attacks 11 at Si [Eq.
+
(8)] much more rapidly than it attacks CH3ICH3 .
Upon treating the À358C C6D5Cl solution contain-
ing 11, 12 and free 13CH3I with iridium dihydride 5
1
rapid formation of 13CH4 (13C NMR: d=À3.7, JC,H
=
126 Hz), iridium iodomethane complex, 7 (13C NMR:
d=À4.0, JC,H =155 Hz) and free 13CH3I (13C NMR:
1
1
d=À23.4, JC,H =151 Hz) occurs accompanying disap-
pearance of 11 and 12. Reduction of 12 will result in
production of CH4 and CH3I, and any of the
ACHTUNGTRENNUNG
(POCOP)IrH+ generated will rapidly bind CH3I to
produce 7. We cannot quantitate the species suffi- Conclusions
ciently accurately to determine if 12 is the sole source
of methane. In addition, there is sufficient Et3SiH In summary, we have discovered and developed a
present from generation of Et3Si
(arene)+ that the highly efficient procedure for the reduction of a
AHCTUNGTRENNUNG
change in concentration of Et3SiH cannot be accu- broad range of alkyl halides using three equivalents
rately determined. Thus, these experiments do not of triethylsilane and a cationic iridium bis(phosphin-
provide an answer to which step, I or II, is turnover- ite) pincer catalyst. This reduction chemistry is che-
limiting. They do however establish that both 11 and moselective and has unique selectivities compared
12 rapidly react with dihydride 5 at low temperatures. with conventional radical-based processes, and the
In addition, the observation of the facile formation of AlCl3/Et3SiH and [Ph3C] [BACHTUNGTRNEUNG(C6F5)4]/Et3SiH systems.
12 suggests a further potential addition to the catalyt- In addition, catalyst loadings as low as 0.01% have
ic scheme: the initially produced Et3SiXR+ species proved successful and the process can be carried out
(see Scheme 6) may react with RX to produce RXR+ in a solvent-free manner, which in many cases may
which could then be reduced by 5 to give RH and provide an environmentally attractive and safe alter-
RX.
native to currently practiced reductions of alkyl hal-
ides.
In-depth mechanistic studies have been carried out
which have revealed a unique catalytic cycle. The
electrophilic iridium hydride complex binds and acti-
Catalytic Hydrodechlorination of 3 with Et3SiH
To further broaden the scope of this reduction vates the silane. This complex transfers “Et3Si+” to
chemistry, we have extended it to the catalytic hydro- the halide forming a highly active bridged halonium
dehalogenation of a metal halide, 3. While a control ion which is rapidly reduced by the iridium dihydride
experiment shows that there is no reaction of iridium remaining following silyl transfer and the cationic iri-
hydrochloride 3 with excess (320 equiv.) Et3SiH for dium hydride complex is thus regenerated. In certain
21 h [Eq. (9)], it could be readily converted to iridium case the intermediate alkyl silyl halonium ions can ex-
dihydride 5 in the presence of catalytic amounts of 1 hibit b-elimination and carbocationic rearrangements,
at room temperature in 40 min [Eq. (10)].
but generally these are not competitive with reduction
Adv. Synth. Catal. 2009, 351, 175 – 187
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
183

Jian Yang and Maurice Brookhart
FULL PAPERS
177.9; anal. calcd. for C55H54BF24O2P2IrCl2 (1538.96): C
42.92, H 3.54; found: C 42.74, H 3.45.
by the iridium dihydride, 5. This reduction chemistry
has also been extended to catalytic hydrodehalogena-
tion of a metal chloride complex.
General Procedure for the Reduction of Alkyl
Halides Using an in situ Generated Catalyst
Experimental Section
Chlorobenzene-d5 (0.5 mL) was added to a J-Young NMR
tube charged with 3 (6.3 mg, 0.01 mmol, 1 mol%) and
NaBACTHNUTGRNEUGN(ArF)4 (8.9 mg, 0.01 mmol, 1 mol%). The NMR tube
General Considerations
was agitated at room temperature for 1 hour. Triethylsilane
(240 mL, 1.50 mmol, 1.5 equiv.) and substrate (1.00 mmol,
1.0 equiv.) were then added by syringe under argon. The re-
actions were allowed to stand at room temperature or
heated in an oil bath and the progress was followed by
NMR spectroscopy. Conversions were determined by loss of
All manipulations were carried out using standard Schlenk,
high-vacuum and glove-box techniques. Argon and nitrogen
were purified by passage through columns of BASF R3–11
catalyst (Chemalog)[23] and 4 ꢂ molecular sieves. THF was
distilled under a nitrogen atmosphere from sodium benzo-
phenone ketyl prior to use. Pentane, dichloromethane and
toluene were passed through columns of activated alumina
and degassed by either freeze-pump-thaw methods or by
purging with argon. Acetone was dried with 3 ꢂ molecular
sieves and degassed by freeze-pump-thaw methods. Et3SiH
was dried with LiAlH4 and vacuum transferred into a sealed
flask. Et3SiD was dried with LiAlD4 and vacuum transferred
into a sealed flask. All the substrates, 1,3,5-tris(trifluorome-
thyl)benzene, and all the haloarene solvents were dried with
either CaH2 or activated 4 ꢂ molecular sieves and vacuum
transferred to a sealed flask. NMR spectra were recorded
on Bruker spectrometers (DRX-400, AVANCE-400, AMX-
300 and DRX-500). 1H and 13C NMR spectra were refer-
enced to residual protio solvent peaks. 31P NMR chemical
shifts were referenced to an external H3PO4 standard.
19F NMR shifts were referenced to external C6F6 [d
1
alkyl halides by H NMR spectroscopy. Reduction products
1
were identified using H and 13C{1H} NMR in comparison to
literature data or authentic samples.
General Procedure for the Reduction of Alkyl
Halides Catalyzed by 4
Triethylsilane (240 mL, 1.50 mmol, 1.5 equiv.) was added to a
solution of 4 (14.5 mg, 0.0094 mmol, 1 mol%) in C6D5Cl
(0.5 mL) in a J. Young NMR tube and the contents were
well shaken. The substrate (1.00 mmol, 1.0 equiv.) was then
added. The reactions were allowed to stand at room temper-
ature or heated in an oil bath and the progress was followed
by NMR spectroscopy. Conversions were determined by loss
1
of alkyl halides by H NMR spectroscopy. Reduction prod-
1
ucts were identified using H and 13C{1H} NMR in compari-
19F
were referenced to external (CH3)4Si. K[B
(ArF)4 [ArF =3,5-(CF3)2C6H3] were purchased from Boulder
ACHTUNGTRENNUNG
son to literature data or authentic samples.
AHCTNUGERTG(NNUN C6F5)4] and NaB-
ACHTUNGTRENNUNG
General Procedure for the Reduction of Alkyl
Halides Catalyzed by 1
Scientific and dried under vacuum at 1208C for 24 h. All
other reagents were purchased from Sigma–Aldrich or
Strem. (POCOP)Ir(H)2 (5)[12], (POCOP)Ir(H)(Cl) (3)[9b] and
Triethylsilane (480 mL, 3.00 mmol, 3 equiv.) was added to a
solution of 1 (6.7 mg, 0.005 mmol, 0.5 mol%) in C6D5Cl
(0.3 mL) in a J. Young NMR tube and the contents were
well shaken. The substrate (1.00 mmol, 1 equiv.) was then
added. The reactions were allowed to stand at room temper-
ature or heated in an oil bath and the progress was followed
by NMR spectroscopy. Conversions were determined by loss
[Ph3C] [BACHTUNGTRENNUNG
(C6F5)4][6a] were prepared according to published
procedures.
Synthesis of [(POCOP)Ir(H)ACHTUNTRGENNUG AHCTUNGTRENNGUN
(acetone)]+ [B(C6F5)4]À, 1, in
situ generation of 6–10, competition experiments to deter-
mine relative reactivities of RX (X=Br, Cl, I) and 18 RX
vs. 28 RX, and kinetic studies of reduction of iodomethane
have been previously described in a preliminary communica-
tion.[7]
1
of alkyl halides by H NMR spectroscopy (conversions for
alkyl fluoride reductions were determined by 19F NMR spec-
1
troscopy). Reduction products were identified using H and
13C{1H} NMR in comparison to literature data or authentic
samples.
Synthesis of [(POCOP)Ir(H)ACHTUNTRGENNUG AHCTUNGTRENNGUN
(CH2Cl2)]+[B(ArF)4]À
[ArF =3, 5-(CF3)2C6H3] (4)
A flame-dried Schlenk flask was charged with (POCO-
P)Ir(H)(Cl) (3) (200 mg, 0.319 mmol), and NaB(ArF)4
(311 mg, 0.351 mmol) under argon. Dichloromethane
(20 mL) was added and the mixture was stirred at room
temperature for 15 h. The solution was cooled in an ice
General Procedure for the Reduction of Alkyl
Halides Catalyzed by 1 without Solvent
AHCTUNGTRENNUNG
Triethylsilane (480 mL, 3.00 mmol, 3 equiv.) was added to a
J. Young NMR tube with 1 (6.7 mg, 0.005 mmol, 0.5 mol%)
and a sealed capillary tube with C6D6 as internal standard.
To this suspension was then added the substrate (1.00 mmol,
1 equiv.), and the tube was quickly inverted to ensure com-
plete mixing. The reactions were allowed to stand at room
temperature and the progress was monitored by NMR spec-
troscopy. Conversions were determined by loss of alkyl hal-
ides by 1H NMR spectroscopy. Reduction products were
bath, and NaCl and excess NaBACTHNUTRGNEUNG(ArF)4 were removed via
cannula filtration. The filtrate was concentrated before pen-
tane (40 mL) was added to precipitate an orange solid. The
solid was filtered, washed with pentane (5 mL, three times),
and dried under vacuum for 2 h to give 4 as an orange
powder; yield: 300 mg (61%). 1H NMR (300 MHz, 238C,
CD2Cl2): d=7.72 [s, 8H, BACTHUNGTRENN(GU ArF)4], 7.56 [s, 4H, BAHCTUNGTRENN(UNG ArF)4],
1
7.02 (t, 1H), 6.69 (d, 2H), 5.33 (s), 1.36 (m, 36H), À42.34 (t,
identified using H and 13C{1H} NMR in comparison to liter-
1H, IrH); 31P{1H} NMR (162 MHz, 238C, CD2Cl2): d=
ature data or authentic samples.
184
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Adv. Synth. Catal. 2009, 351, 175 – 187

Reduction of Alkyl Halides by Triethylsilane
FULL PAPERS
Reduction of 1-Bromohexane with Et3SiD Catalyzed
by 3
Reaction between Cationic Silane Complex (2) and
13CH3I at Low Temperature
Et3SiD (160 mL, 1.00 mmol, 2 equiv.) was added to a solution
of 1 (6.7 mg, 0.005 mmol, 1 mol%) in C6D4Cl2 (0.3 mL) in a
J. Young NMR tube. The contents were well shaken and 1-
bromohexane (70.2 mL, 0.50 mmol, 1 equiv.) was added. The
reaction was allowed to stand at room temperature and the
Triethylsilane (320 mL, 2 mmol, 200 equiv.) was added to a
solution of 1 (13.3 mg, 0.01 mmol, 1 equiv.) in C6D5Cl
(0.5 mL) in a screw-cap NMR tube in a dry-box and the
contents were well shaken. A solution of 13CH3I in C6D5Cl
(0.2M, 0.1 mL, 2 equiv.) was added by syringe at À788C and
the NMR tube was placed in the pre-cooled NMR probe at
À508C. The progress of the reaction was followed by 1 H,
31P and 13C NMR spectroscopy. Characteristic shifts (in
1
progress was followed by H and 13C{1H} NMR spectroscopy.
CH
determined
3ACHTUNGTRENNUNG(CH2)4CH2D was the only observable alkane product as
by
13C{1H} NMR.
CH3ACHTUNGETRNNU(G CH2)4CH2D:
13C{1H} NMR (C6D4Cl2, 100.6 MHz): d=31.71 (s), 31.68 (s),
22.75 (s), 22.66 (s), 14.0 (s), 13.7 (t, JD,C =19.1 Hz).
C6D5Cl/Et3SiH at À508C) used to determine 7, 13CH3I and
1
13CH4: 7 [1H NMR (coordinated 13CH3I): d=2.17 (d, JC,H
=
155 Hz), 13C NMR (coordinated 13CH3I): d=À4.0, 31P NMR:
d=178.6.]; 13CH3I [1H NMR: d=1.70 (d, 1JC,H =151 Hz),
Reduction of (Bromomethyl)cyclohexane with Et3SiD
Catalyzed by 1
1
13C NMR: d=À23.4]; 13CH4 [1H NMR: d=0.21 (d, JC,H
=
126 Hz), 13C NMR: d=À3.7]
Et3SiD (240 mL, 1.50 mmol, 3 equiv.) was added to a solution
of 1 (20 mg, 0.015 mmol, 3 mol%) in C6D4Cl2 (0.5 mL) in a
J. Young NMR tube and the contents were well shaken.
(Bromomethyl)cyclohexane (70 mL, 0.50 mmol, 1 equiv.) was
then added. The reaction was allowed to stand at room tem-
Attempted in situ Generation of Triethylsilyl
Iodonium Ion, [Et3SiICH3] [BACTHUNTGRNENUG(C6F5)4] (11)
Triethylsilane (3.5 mL, 0.022 mmol, 1.1 equiv.) was added to
solution of [Ph3C] [B(C6F5)4] (18.4 mg, 0.02 mmol,
1
perature the progress was followed by H and 13C{1H} NMR
a
ACHTUNGTRENNUNG
spectroscopy. The NMR spectra of the reaction products
were analyzed by comparison with 13C{1H} NMR data of un-
deuterated methylcyclohexane. (Deuteriomethyl)cyclohex-
ane was identified as the major reduction product and 1-
deuterio-1-methylcyclohexane was the minor one (ca. 6:1
ratio as determined by 13C{1H} NMR spectroscopy). (Deuter-
iomethyl)cyclohexane: 13C{1H} NMR (C6D4Cl2, 100.6 MHz):
d=35.4 (s), 32.7 (s), 26.5 (s), 26.4 (s), 22.5 (t, JD,C =19.1 Hz).
The minor product 1-deuterio-1-methylcyclohexane was
identified and quantified by a triplet for 13C1-D (d=32.2,
1.0 equiv.) in C6D5Cl (0.5 mL) in a screw-cap NMR tube.
The tube was quickly inverted to ensure complete mixing
and put into a À788C bath. Iodomethane-13C (0.1 mL of
0.2M stock solution in C6D5Cl, 1.0 equiv.) was added by sy-
ringe and the NMR tube was placed in the pre-cooled NMR
1
probe at À408C. Both the H and the 13C NMR spectra ex-
hibited formation of Ph3CH and two sets of CH3 resonances.
The sharp resonance at d=10.3 (1JC,H =158 Hz) in 13C NMR
+
was assigned to 13CH3I13CH3 , 12. A broadened 13C reso-
nance at d=À8 (1JC,H =157 Hz) was attributed to rapidly
J
D,C =19.0 Hz) and a 13CH3 signal at d 22.6 overlapping the
+
averaging signals of Et3SiI13CH3 , 11, and free 13CH3I in the
triplet of -CH2D (d=22.5, JD,C =19.1 Hz).
degenerate exchange equilibrium. Similar results have also
been obtained by treating in situ formed [Et3Si
[B
(C6D6)]+
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Competition Experiments to Determine Relative
Reactivities of Primary and Secondary Chlorides with
1/Et3SiH System
Reactions of 11 and 12 with 5
A stock solution of 1 (16.7 mm) was prepared in C6D4Cl2 in
a glove-box. Triethylsilane (480 mL, 3.00 mmol, 6 equiv.) was
then added to an aliquot (300 mL, 1 mol% Ir) of this stock
solution in a J. Young NMR tube and the contents were
well shaken. 1-Chloropentane (0.50 mmol, 60.5 mL, 1 equiv.),
2-chloropentane (0.50 mmol, 61.3 mL, 1 equiv.) and 1,3,5-
A stock solution of 5 (0.067m) was prepared in C6D5Cl in a
glove-box. An aliquot of the stock solution of 5 (300 mL,
0.02 mmol, 1.0 equiv.) was added by syringe to the mixture
of 11 and 12 in a À788C bath. The NMR tube was then
placed in the pre-cooled NMR probe at À358C. The prog-
ress was monitored by NMR spectroscopy. Observations are
summarized in the text.
tris(trifluoromethyl)benzene
(0.16 mmol,
30.0 mL,
0.32 equiv.; used as an internal standard) were then added.
The reaction was allowed to stand at room temperature, and
the progress was followed by NMR spectroscopy.
Hydrodechlorination of (POCOP)Ir(H)(Cl) (3) with
Et3SiH Catalyzed by 1
Catalysis with 25% 1: (POCOP)Ir(H)(Cl) (3) (12.52 mg,
0.02 mmol, 1 equiv.) and 1 (6.7 mg, 0.005 mmol, 25 mol%)
were dissolved in 0.5 mL of C6D5Cl in a J. Young NMR
tube. Triethylsilane (16 mL, 0.1 mmol, 5 equiv.) was added
and the contents were well shaken. In less than 40 min 3
was quantitatively converted to 5.
Catalysis with 10% 1: A stock solution of 1 (10 mm) was
prepared in C6D4Cl2 in a glove-box. An aliquot (150 mL,
0.0015 mmol, 10 mol% Ir) of this stock solution was added
to a solution of (POCOP)Ir(H)(Cl) (3) (9.8 mg, 0.016 mmol,
1 equiv.) and Et3SiH (15 mL, 0.094 mmol, 6 equiv.) in 650 mL
of C6D4Cl2 in a J. Young NMR tube. The reaction was al-
Competition Experiments to Determine Relative
Reactivities of Primary and Secondary Chlorides in
[Ph3C] [BACHTUNGTRENNUNG(C6F5)4]/Et3SiH System
Triethylsilane (480 mL, 3 mmol, 6 equiv.) was added to a so-
lution of [Ph3C] [B(C6F5)4] (4.6 mg, 0.005 mmol, 1 mol%) in
ACHTUNGTRENNUNG
C6D4Cl2 (0.3 mL) in a J. Young NMR tube and the contents
were well shaken. 1-Chloropentane (60.5 mL, 0.50 mmol,
1 equiv.) and 2-chloropentane (61.3 mL, 0.50 mmol, 1 equiv.)
were then added. The reaction was allowed to stand at room
temperature, and the progress was followed by NMR spec-
troscopy.
Adv. Synth. Catal. 2009, 351, 175 – 187
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
185

Jian Yang and Maurice Brookhart
FULL PAPERS
lowed to stand at room temperature and monitored by the
1H and 31P{1H} NMR. In less than 40 min 3 was quantitative-
ly converted to 5.
[7] J. Yang, M. Brookhart, J. Am. Chem. Soc. 2007, 129,
12656.
[8] J. Yang, P. S. White, C. K. Schauer, M. Brookhart,
Angew. Chem. 2008, 120, 4209; Angew. Chem. Int. Ed.
Angew. Chem. Int. Ed. Engl. 2008, 47, 4141.
[9] a) I. Goettker-Schnetmann, M. Brookhart, J. Am.
Chem. Soc. 2004, 126, 9330; b) I. Goettker-Schnetmann,
P. White, M. Brookhart, J. Am. Chem. Soc. 2004, 126,
1804.
ACHTUNGTRENNUNG
(POCOP)IrH2 (5): 1H NMR (C6D5Cl, 400 MHz, 238C):
3 3
d=7.08 (t, JH,H =7.6 Hz, 1H, 4-H), 6.87 (d, JH,H =7.6 Hz,
2
2H, 3- and 5-H), 1.30 (m, 36H, 4ꢃt-Bu), À17.0 (t, JP, H
=
5.8 Hz, 1H, IrH); 31P{1H} NMR (C6D5Cl, 162 MHz, 238C):
d=204.8.
[10] a) A. C. Sykes, P. White, M. Brookhart, Organometal-
lics 2006, 25, 1664; b) W. H. Bernskoetter, M. Broo-
khart, Organometallics 2008, 27, 2036.
[11] C. A. Reed, Acc. Chem. Res. 1998, 31, 325.
[12] I. Goettker-Schnetmann, P. White, M. Brookhart, Or-
ganometallics 2004, 23, 1766.
Acknowledgements
We gratefully acknowledge funding by the STC Program of
the National Science Foundation (Center for Environmental-
ly Responsible Solvents & Processes) under Agreement No.
CHE-9876674.
[13] Stable iridium alkyl halide and aryl halide complexes
have been previously reported: a) M. J. Burk, B. Seg-
muller, R. H. Crabtree, Organometallics 1987, 6, 2241;
b) R. H. Crabtree, J. W. Faller, M. F. Mellea, J. M.
Quirk, Organometallics 1982, 1, 1361; c) B. A. Arndt-
sen, R. G. Bergman, Science 1995, 270, 1970; d) P. J. Al-
bietz, B. P. Cleary, W. Paw, R. Eisenberg, J. Am. Chem.
Soc. 2001, 123, 12091.
[14] a) D. J. Parks, J. M. Blackwell, W. E. Piers, J. Org.
Chem. 2000, 65, 3090; for closely related studies, see:
b) V. Gevorgyan, M. Rubin, S. Benson, J.-X. Liu, Y.
Yamamoto, J. Org. Chem. 2000, 65, 6179; c) E. A. Ison,
E. R. Trivedi, R. A. Corbin, M. M. Abu-Omar, J. Am.
Chem. Soc. 2005, 127, 15374; d) G. Du, P. E. Fanwick,
M. M. Abu-Omar, J. Am. Chem. Soc. 2007, 129, 5180;
e) V. K. Dioumaev, R. M. Bullock, Nature 2003, 424,
530.
[15] Z. Xie, J. Manning, R. W. Reed, R. Mathur, P. D. W.
Boyd, A. Benesi, C. A. Reed, J. Am. Chem. Soc. 1996,
118, 2922.
[16] F. A. Carey, R. J. Sundberg, Advanced Organic Chemis-
try Part A: Structure and Mechanisms, 4th edn., Kluwer
Academic/Plenum Publishers, New York, 2000, Chapter
5.
References
[1] a) W. P. Neumann, Synthesis 1987, 665; b) M. Pereyre,
J.-P. Quintard, A. Rahm, Tin in Organic Synthesis, But-
terworth, London, 1987; c) T. V. RajanBabu, in: Ency-
clopedia of Reagents for Organic Synthesis, (Ed.: L. Pa-
quette), Wiley, New York, 1995, Vol.7, p 5016; d) F.
Alonso, I. P. Beletskaya, M. Yus, Chem. Rev. 2002, 102,
4009.
[2] a) C. Chatgilialoglu, Acc. Chem. Res. 1992, 25, 188;
b) C. Chatgilialoglu, Chem. Rev. 1995, 95, 1229; c) C.
Chatgilialoglu, Chem. Eur. J. 2008, 14, 2310; d) C. Chat-
gilialoglu, V. I. Timokhin, Adv. Organomet. Chem.
2008, 56, 117.
[3] a) K. B. Clark, D. Griller, Organometallics 1991, 10,
746, and references cited therein; b) B. Giese, Angew.
Chem. 1985, 97, 555; Angew. Chem. Int. Ed. Engl. 1985,
24, 553 and references cited therein.
[4] a) S. J. Cole, J. N. Kirwan, B. P. Roberta, C. R. Willis, J.
Chem. Soc., Perkin Trans. 1 1991, 103; b) J. N. Kirwan,
B. P. Roberts, C. R. Willis, Tetrahedron Lett. 1990, 31,
5093; c) R. P. Allen, B. P. Roberta, C. R. Willis, J.
Chem. Soc., Chem. Commun. 1989, 1387.
[17] J. Yang, PhD thesis, University of North Carolina at
Chapel Hill, USA, 2008.
+
[18] DFT studies of Et3SiBrCH3 : G. A. Olah, G. Rasul,
G. K. S. Prakash, J. Organomet. Chem. 1996, 521, 271.
[19] An X-ray structure of the aryl chloride coordinated
[5] a) AlCl3/Et3SiH system: M. P. Doyle, C. C. McOsker,
C. T. West, J. Org. Chem. 1976, 41, 1393; b) Pd(II)/
Et3SiH system: R. Boukherroub, C. Chatgilialoglu, G.
Manuel, Organometallics 1996, 15, 1508; c) catalysis
with P,N-chelated Pt(II) complexes and HSiMe2Ph: F.
Stçhr, D. Sturmayr, U. Schubert, Chem. Commun.
2002, 2222; reduction of bromides and iodides by si-
species, [(i-Pr)3SiACHTNUTRGEN(UNG C6H4Cl2)] [CHB11Cl11], has been re-
ported: S. P. Hoffmann, T. Kato, F. S. Tham, C. A.
Reed, Chem. Commun. 2006, 767.
[20] R3Si(X6-CB11H6) (R=Et, i-Pr; X=Cl, Br, I): a) Z. W.
Xie, R. Bau, C. A. Reed, Organometallics 1995, 14,
3933; b) ref.[15]
[21] a) J. B. Lambert, Y. Zhao, H. Wu, J. Org. Chem. 1999,
64, 2729; b) J. B. Lambert, S. Zhang, S. M. Ciro, Orga-
nometallics 1994, 13, 2430; c) J. B. Lambert, S. Zhang,
C. L. Stern, J. C. Huffman, Science 1993, 260, 1917;
d) J. B. Lambert, L. Kania, S. Zhang, Chem. Rev. 1995,
95, 1191.
lanes in the presence of InACTHNUGTRNEUNG(III): d) K. Miura, M.
Tomita, Y. Yamada, A. Hosomi, J. Org. Chem. 2007,
72, 787; e) N. Hayashi, I. Shibata, A. Baba, Org. Lett.
2004, 6, 4981.
[6] a) V. J. Scott, R. C¸ elenligil-C¸ etin, O. V. Ozerov, J. Am.
Chem. Soc. 2005, 127, 2852; b) C. Douvris, O. V.
Ozeroc, Science 2008, 321, 1188; c) R. Panisch, M.
Bolte, T. Mꢁller, J. Am. Chem. Soc. 2006, 128, 9676;
d) M. Klahn, C. Fischer, A. Spannenberg, U. Rosenthal,
I. Krossing, Tetrahedron Lett. 2007, 48, 8900; e) K. Fu-
chibe, Y. Ohshima, K. Mitomi, T. Akiyama, Org. Lett.
2007, 9, 1497.
[22] a) G. A. Olah, J. R. DeMember, J. Am. Chem. Soc.
1970, 92, 718; b) G. A. Olah, J. R. DeMember, J. Am.
Chem. Soc. 1969, 91, 2113.
+
[23] A mixture of CH3ICH3 and CH3IH+ was obtained in
the efforts of generating CH3IH+, which was proposed
to occur by nucleophilic attack of CH3I on CH3IH+:
186
asc.wiley-vch.de
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 175 – 187

Reduction of Alkyl Halides by Triethylsilane
FULL PAPERS
G. A. Olah, Y. Yamada, R. J. Spear, J. Am. Chem. Soc.
1975, 97, 680.
Giardello, R. H. Grubbs, R. K. Rosen, F. J. Timmers,
Organometallics 1996, 15, 1518.
[24] a) P. J. Alaimo, D. W. Peters, J. Arnold, R. G. Bergman,
J. Chem. Educ. 2001, 78, 64; b) A. B. Pangborn, M. A.
Adv. Synth. Catal. 2009, 351, 175 – 187
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
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