Synthesis of Boryl Metal Complexes with Additional Agostic Stabilization by Hydroboration of Fischer Carbyne Complexes.

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~
Synthesis of Boryl Metal Complexes
with Additional Agostic Stabilization
by Hydroboration of Fischer Carbyne
Complexes**
Table 1. Spectroscopic data for the compounds 5 and 6.
~
Hubert WadepohI,* Ulrich Arnold, and Hans Pritzkow
Dedicated to Professor Walter Siebert
on the occasion of his 60th birthday
After a long hiatus,". 21 interest in boryl transition metal complexes has recently revived.[3- 7 ] This is principally due to their
importance as intermediates in hydroborations catalyzed by
metal complexes.['] It has been suggested that boryl groups are
capable of forming element-metal multiple bonds. A n backbonding contribution to the transition metal-boron bond had
been proposed in earlier worksf91,but was not considered to be
of particular imp~rtance!~-~'Virtually all boryl metal complexes that have been structurally characterized have heterosubstituents (OR, NR,) with x-donor properties attached to the
boron atom. Therefore possible M B back bonding is masked
by the dominating n interaction between the boron and the
heteroatom. Weak M-B-n bonding contributions were discussed
for the only metal complexes known to us containing
diorganoboryl ligands, lf.lal
and ZL5=],but these did not result in
a shortened bond.
-
fac-~(PMe3),(H),Ir(borabicyclo[3.3.1]nonyl)]
1
[ C P ( C O ) , F ~ ( B P ~ ~ )2I
Our investigations into the hydroboration of metal-carbon
multiple bonds [''I have now resulted in the synthesis of the first
low-valent metal complexes containing dialkylboryl ligands.
The Fischer carbyne complexes 3 and 4 [tpb = tris(3,S.dimethylpyrazol-1-yl)hydroborate] react readily at room tem[(tpb')(CO)tMECRI
'HNMR (200MH2, in C,D,, 6[J(H,H)/Hz]): 5 (in C,D,): 1.54[8] (t, 3H,
CHZCH,). 1.99 ( s , 6 H , tpb-CH,), 2.05 ( s , 3H, tpb-CH,), 2.07 ( s , 3H, tpb-CH,),
2.11 (s, 6H, tpb-CH,), 2.33[8] (q, 2H, CH,CH,), 2.41 (s, 2H, CH,-p-tolyl), 2.71
(s, 3H, Me-4), 5.41 (s, 1H, tpb-CH), 5.48 (s, 2H, tpb-CH), 6.87[8]/7.11[I](AABB
system,4H, C6H,).6a: 1.03[7](q, 2H, CH,CH,,, 1.28[8](t,3H,CH,CH3, 1.44[7]
(t, 3H, CH,CH,), 1.92[8] (4, 2H, CH,CH,), 2.27 (s, 6 H , tpb-CH,), 2.34 (s, 3H,
tpb-CH,), 2.40 ( s , 6 H , tpb-CH,), 2.50 ( s , 3H, tpb-CH,), 5.87 (s, 1H, tpb-CH),
5 93(s,2H, tpb'-CH).6b: 1.27[8](t, 3H,CH,CH3), 1.97(s,6H, tpb'-CH,),2.00[8]
(q, 2H, CH,CH,), 2.32 (s, 3H, Me-4), 2.36 (s, 3H, tpb-CH,), 2.42 (s, 6H, tpbCH3),2.56(s,3H, tpb-CH,).2.65(~,2H,CH,-p-toIyl), 5.89(s,2H, tpb-CH),5.90
(s, 1 H, tpb-CH), 7.05[8]/7.11[S] (AABB' system, 4 H , C,H,).
I3CC('H}NMR (in CD,CI,, 6): 5: -11.0 (br, BCH,-p-tolyl), 9.2 (CH,CH,), 12.6,
13.1, 14.4, 14.9 (tpb-CH,), 20.1 (br, BCH,CH,), 20.7 (Me-4), 106.8, 107.2 (tpbCH), 128.9, 130.6 (CH[C,H,]), 135.5, 136.3, 145.4, 145.5, 152.0, 153.6 (C[C6H,,
tpb-CCHJ), 221.8 (CO). 6 a : -30.2 (br, BCH,CH3), 9.4 (CH,CH,), 12.7, 13.2,
15.2, 15.6 (tpb-CH,). 15.9 (CH,CH,), 19.0 (br, BCH,CH,), 107.4 (tpb-CH),
145.2, 145.6, 152 6, 154.3 (tpb-CCH,), 216.7 (CO). 6b: - 14.8 (br, BCH,-p-tolyl),
9.4(CH,CH3), 12.5, 13.0, 14.5, 15.2(tpb-CH3),19.3(br,BCH2CH,),2O.6(Me-4),
106.7. 106.9 (tpb-CH), 128.5, 130.2 (CH[C,H,]), 135.2,136.3,145.0,145.3,152.0,
153.5 (C[C6H,, tpb-CCH,]), 216.4 (CO).
llB{lH} NMR (in CD,CI,, 6): 5 : -9.3 (tpb-BH), 76 (br, MOB). 6 a : -9.3 (tpbBH), 77 (hr, WB). 6b: -9.3 (tpb-BH), 78 (br, WB)
IR(a(C0) in cm-', in toluene): 5 : 1908, 1830; 6a: 1893, 1814; 6b: 1896, 1817
rings with an intensity ratio 2:l). In addition to the signal for
the t p b ligands, the "B NMR spectra reveal a low-field signal
(6 z 77) that lies in the region for resonances attributed to triorganoboranes.["I
X-ray crystal structure analysis['3f 14] shows that 6 b contains
an ethyl(p-tolylmethy1)boryl ligand which, in addition to the
M- B bond, has an agostic interaction between the tungsten
atom and the methylene group attached to the benzene ring
(Figure 1). IR and NMR spectra indicate that the same basic
?
3,M = M o , R=p-Tolyl
40, M = W , R = M e
4b, M = W , R =p-TolyI
perature (R = Me) or 60 "C (R = p-tolyl) with the hydroborating agent ''Et,BH".["l IR and NMR spectra of the products 5
5. M = Mo. R = p-Tolyl
60,M = W , R = M e
6b, M = W. R = p-Tolyl
Figure 1. Molecular structure of 6b in the crystal. Selected bond lengths[A] and
angles["]: W1-N1 2.240(9), Wl-N3 2.251(9), W1-N5 2.185(8), Wl - C l 1.90(1),
WI-Cl2 1.96(1), C1-01 1.21(1), C12-02 l.16(1), BI-C2 1.64(2), B1-C3
1.58(2),C2-C5 I.Sl(1); C2-BI-C3 121(1), C2-Bl-Wl 81.9(7),C3-BI-Wl 157.0(9),
W1 -C1-01 176.1(8), W1 -C12-02 178.1(9).
and 6, which were isolated in yields of 65-85 % (Table l), indicate the presence of (tpb)(CO),M fragments (two vco bands,
two sets of 'H and I3C NMR signals for the three pyrazolyl
[*] Priv. -Doz. Dr. H. Wadepohl, DipLChem. U. Arnold, Dr. H. Pritzkow
Anorganisch-chemisches Institut der Universitat
Im Neuenheimer Feld 270, D-69120 Heidelberg (Germany)
Fax: Int. code +(6221)544197
e-mail : [email protected] uni-heidelberg.de
p*] This work was supported by the Sonderforschungshereich 247 der Universitat
Heidelberg and the Fonds der chemischen Industrie. H. W. thanks the
Deutsche Forschungsgemeinschaft for a Heisenberg Stipendium.
974
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structure also exists for 5 and 6a. The W-B bond length of
2.07(1) A in 6 b is the shortest of all known W-B bonds;['61
comparable boryl complexes with hetero-substituted boryl
ligands have considerably longer W- B bond lengths ([Cp,W(H)(BCat)]
2.190(7)
[Cp,W(BCat),]
2.19(2)
and
2.23(1) A[sd] (BCat = catecholboryl); [Cp(CO),W{B(NMe,)B(NMe,)Cl}] 2.370(8)
The relatively short W1 -C2 distance (2.45(1) A) and the acute WI-Bl-C2 angle [81.9(7)"] are
characteristic of a agostic interaction
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Although the ‘H NMR signals of the agostic methylene
groups are not anomalously shifted (singlets at 6 = 2.41 (5),
2.65 (6b), and a quartet at 6 =1.03 (6a)), the same groups do
show unusual high-field shifts in the I3C NMR spectra
(6 = - 1 1 (5),-30.2 (6a), -14.8 (6b)). The spectra are not
noticeably temperature dependent between room temperature
and 210 K. This implies a low barrier to interchange of the
agostic and terminal hydrogen atoms of the methylene
groups.[’*]The two ethyl groups in 6 a do not change places.
The boryl complexes 5 and 6 are closely related to the cationic
carbene complexes 7f201.The complexes can be interconverted
Keywords: agostic interactions
hydroboration
[(tpb)(CO),W=C(Ph)(CHzR)]+7a , R = H, 7b, R = Me
according to [Eq. (l)]by isolobal and isoelectronic exchange of
the boryl ligand R,B with the cationic carbene ligand R,C’.
The structures of 6b and 7 a are very similar. The short W-C
distance in 7 a (1.94(2) A, 94 YOof Cr,,, , the sum of the covalent
radii‘2iJ)is consistent with a strong metal-carbon 71 bond.[221
The W-B bond length in 6 b is only 95% of Xrcov,.In contrast,
the M- B distances in 1 and 2 correspond to almost exactly the
sum of the covalent radii (98 and 99%, respectively). Furthermore, all boryl metal complexes [L,MBR,] known up to now
have B NMR signals that are shifted to low-field with respect
to the signal of the corresponding free borane B(alkyl)R,,
whereas in 5 and 6 high-field shifts (A6 z 10) are observed. From
these results we deduce that there is significant W-B multiplebond character in 5 and 6. The low CO stretching frequencies
show, however, that the metal atom is affected by the high
electron density[231from the t p b ligand, which is principally
delocalized over the carbonyl ligands.
Without wishing to imply a mechanism, the formation of 5
and 6 can be viewed as an addition of “EtBH,” to the M-C
triple bond, by which the carbyne carbon atom is reduced to a
methylene group and the borylene :BEt is inserted into the
M-C bond. Triethylborane was detected in the reaction solution, which supports the idea that active ethylborane is probably
formed by dismutation of “EtBH,” [Eq. (2)].[241
(Et,BH),
4
(2)
BEt, +“EtBH,”
The Fischer carbyne complexes [L(CO),MCR] (M = Mo, W)
react with “Et,BH” to yield very different products depending
on L and R. For instance, cyclopentadienyl derivatives
(L = C,H,, C,Me,) are hydroborated at the carbyne carbon
atom when R is an aryl group.“’]
R5
The latter group plays an important
role in stabilizing the products 8 and
9 through partial coordination to
oc
+co..M“’
the metal. In contrast, [(C,Me,)Et,B J
(CO),WCMe] is reduced to the boronfree ethylene(hydrid0) complex
8, M=Mo, R = M e
probably via the coordinatively unsat9. M = W . R = H . M e
urated intermediate ll.[25.261
The
&
I(C,Me,)(CO),W(C,H,)(H)I
10
[(C,Me,)(Co),W(C,H,)l
11
substrates [(tpb)(CO),MC(p-tolyl)] with the unsubstituted hydrotris(pyrazoly1borate) ligand (tpb) decompose in the presence
of “Et,BH” at 0°C. Therefore, it can be seen that in the hydroboration reaction carbyne-metal complexes behave very differently to their isolobal organic analogues, the alkynes.
Angew. Chem. Inr. Ed. Engl. 1997, 36, N o . 9
Q
Experimental Section
Typical synthesis procedure: Tetraethyldiborane (310 mg, 4.43 mmol hydride)[281
was added to 4 b (530 mg, 0.83 mmol)[27] in toluene (20 mL). The mixture was
heated to 60°C for 15 min (color change to yellow). After cooling, the solution was
concentrated under vacuum and allowed to crystallize at - 20 “C. The microcrystailine precipitate was dissolved in a small volume of toluene and chromatographed
on a silica column with toluene at - 45 “C. The solvent was removed from the eluate
under reduced pressure, the residue washed several times with pentane, and then
dried under vacuum. Yield 380 mg (67%) of pale yellow powder. Correct C,H,N
analysis.
Received: October 22, 1996 [Z9679IE]
German version: Angew. Chem. 1997, 109,1009-1011
- boron
carbyne complexes
-
111 G. Schmid, Angew. Chem. 1970,82,920; Angew. Chem. In(. Ed. Engl. 1970,9,
819, and references therein.
[2] a) C. S. Cundy, H. Noth, J. Organomet. Chem. 1971, 30, 135; b) H. Kono,
K. Ito, Y Nagai, Chem. Lett. 1975, 1095; c) M. Fishwick. H. Noth, W. Petz,
M. G. H. Wallbridge, Inorg. Chem. 1976, 15, 490
131 J. R. Knorr, J. S . Merola, Organometallics 1990, 9, 3008.
[4] a) R. T. Baker, D. W. Ovenall, J. C. Calabrese, S . A. Westcott, N. J. Taylor,
1. D. Williams, T. B. Marder, J. Am. Chem. SOC.1990, ff2, 9399; b) S . A.
Westcott, N . J. Taylor, T. B. Marder, R. T. Baker, N J. Jones, J. C. Calabrese,
J. Chem. SOC.Chem. Commun. 1991,304; c) S. A. Westcott, T. B. Marder, R. T.
Baker, J. C. Calabrese, Can. J. Chem. 1993, 71,930; d) K. Burgess, W. A. van
der Donk, S. A. Westcott, T. B. Marder, R. T. Baker, J. C. Calabrese, J Am.
1992,114,9350: e) R. T. Baker, J. C. Calabrese, S A. Westcott, P.
Chem. SOC.
Nguyen, T. B. Marder, ibid. 1993, 115, 4367; f ) P. Nguyen, H. P. Biom, S. A.
Westcott, N. J. Taylor, T. B. Marder, zbrd. 1993, 115, 9329.
[5] a) J. F. Hartwig, S. Huber, J. Am. Chem. SOC.
1993, 115,4908. b) J. F. Hartwig,
S . Bhandari. P. R. Rablen, ibid. 1994, 116. 1839; c) I. F. Hartwig, S. R. De
Gala, ibid. 1994, 116, 3661; d) P. R. Rablen, J. F. Hartwig, S. P. Nolan, hid.
1994, 116,4121; e) X. He, J. F. Hartwig, Organomerailics 1996, 15, 400; f) J. F.
Hartwig, X. He, Angew. Chem. 1996, 108, 352; Anger,. Chem. Int. Ed. Engl.
1996, 35, 315.
161 D. R. Lantero, D. H. Motry, D. L. Ward, M R. Smith 111, J Am. Chem. Soc.
1994, f16, 10811.
[7] C N. Iverson, M. R. Smith 111, J. Am. Chem. SOC.1995, 117.4403.
[S] D. Mannig, H. Noth, Angew. Chem. 1985,97,854; Angew Chum. h i . Ed. Engl.
1985,24,878. A more up-to-date overview: K. Burgess, M. J. Ohlmeyer, Chem.
Rev. 1991, 91, 1179. Mechanism: A. E. Dorigo, P. von R. Schleyer. Angeic
Chem. 1995, 107.108; Angew. Chem. Int. Ed Engl. 1995, 34. I1 5 .
191 G. Schmid, H. Noth, Chem. Ber. 1967, 100, 2899.
[lo] a) H. Wadepohl, G. P. Elliot, H. Pritzkow, F. G .A . Stone, A. Wolf, J.
Organomet. Chem. 1994,482, 241; b) U. Arnold, Diploma thesis, Universitat
Heidelberg, 1995; c) H. Wadepohl, U. Arnold, H. Pritzkow, A. Wolf. in Advances in Boron Chemisrry (Ed.: W. Siebert). The Royal Society of Chemistry,
London, in press.
(111 H. I. Schlesinger, L. Horvitz, A. B. Burg, J. Am. Chem. Soe. 1936,58,407. As
is the case for all noncyclic organodiborane-6 compounds B,H.R+,,
tetraethyldiborane-6 is an equilibrium mixture of diboranes differing in their
number of alkyl substituents, since the exchange of alkyl groups between the
boron atoms is catalyzed by the BH functional groups: R. Koster. G. Bruno,
P. Binger, Justus Liebigs Ann. Chem. 1961,644, 1.
[I 21 “Nuclear Magnetic Resonance Spectroscopy of Boron Compounds”: H.
Noth, B. Wrackmeyer, N M R Basic Princ. Prog. 1978, 14, Chap. 4.
[13] The extremely air-sensitive single crystals that are obtained by slow cooling of
the reaction mixture contain one (disordered) toluene molecule and half a
molecule of tetraethylborane-6 per assymmetric unit [d(B~-C)= 1.58(2),
1.62(2) A; d(B-B) =1.82(4) A, C-B-C angle 120(1)l.
[14] Crystallographic data for 6b.O.S(Et2BH),.to1uene: triclinic, Pi, a =
10.153(7), b =11.039(8), c =18.546(14) A, z = 98.39(5),
= 96.66(5),
y = 102.29(5)”, V = 1986(3)A3. 2 = 2. Data collection: Stoe Siemens four-circle diffractometer (203 K, Mo,, radiation, graphite monochromator). o-scan,
3<28148”, - 11 s h < 11. - 1 2 1 k < 12,0<1520, p = 2.94 mm I . 6006measured, 6002 independent reflections. Structure solution: direct methods[l5a];
Refinement: least-squares against F 2 (full matrix) All non-H atoms anisotropic, H atoms of the BH and the BHB groups as well as the methylene unit of the
benzyl group were localized by difference Fourier synthesis, all other H atoms
in calculated positionsll5 b]. One disordered molecule of toluene per asymmetric unit was refined isotropically with a rigid, idealized benzene ring in two
positions (occupation factors 0.6/0.4). A relatively close intramolecular contact exists between B1 and C1 (2.06(2) A). R = 0.057 (4461 reflections with
E 2 4 4 K ) ) , wR2 = 0.131 (relative to F 2 , all reflections). w = [[email protected]) +
(0.0526P)* + 1.70P]-’, P = [max(F:) +2F2)/3, GOOF= 1.025 Crystallographic data (excluding structure factors) for the structure reported in this
paper have been deposited with the Cambridge Crysrdllographlc Data Centre
VCH Verlagsgesellschafr mbH, D-69451 Weitlheim, 1997
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as supplementary publication no. CCDC-100156. Copies of the data can be
obtained free of charge on application to The Director, CCDC. 12 Union
Road, Cambridge CB2 lEZ, UK (fax: Int. code +(1223) 336-033; e-mail:
[email protected]).
a) SHELXS-86: G. M. Sheldrick, Acta CrjstuNogr. Sect. A 1990, 46, 467;
b) SHELXL-93: G . M. Sheldrick, Universitat Gottigen, 1993.
The Cambridge Crystallographic Database contains W- B bond lengths between 2.170 and 2.557A
H. Braunschweig, B. Ganter, M. Koster, T. Wagner, Chem. Brr. 1996, 129.
1099.
The proton-decoupled 13C NMR spectrum of 6 b contains a triplet with
J(C,H) =115 Hzdueto themethylenegroupofthe benzylligand Therefore6b
does not differ substantially from normal alkyiboranes in that respect. although a reduction in the C-H coupling constant would be expected for an
agostic methylene group. Due to the fast exchange between the two methylene
protons however, the value obtained is an average of the I3C-’H coupling
constants of the terminal and agostic protons. The relatively large value can be
explained by the usual increase in ‘J(C,H(terminal)) when ‘J(C,H(agostic))
becomes smaller[l9]; this has been experimentally verified for the carbene
complexes 7[20]. The obvious comparison with the signal of the methylene
group of the nonagostic ethyl group in 6 is not possible due to overlap with the
methyl signals.
M. Brookhart, M. L. H. Green, J: Organornet. Chrm. 1983.250, 395.
S. G. Feng, P. S. White, J. 2. Templeton, J: Am Chem. SOC.1990. 112, 8192;
ihid. 1992, 114, 2951.
J. Emsley, The Elements, 2nd ed., Clarendon, Oxford, 1991.
N. M. Kostic, R. F. Fenske. Organomerallics 1982, 1 . 974.
M. D. Curtis, K:B. Shiu. W M. Butler, J. C. Huffman, J Am. Chern.SOC.1986,
108, 3335.
We would like to thank a referee for pointing out that the boryl complexes 5
and 6 could also be produced by the addition of “Et,BH”, which is more
reactive than “EtBH,” (extrusion of BEt, from an intermediate). Our observation that (PhBH,), reacts with 4 b to yield the corresponding agostic complex
[(tpb)(CO),WB(Ph)CH,-p-tolyl] under milder conditions (room temperature)
and faster ( < 5 min) would tend to indicate that this is not the case here.
R. J. Kazlauskas, M. S. Wrighton, J Am. Chem. SOC.1982,104.6005.
The photochemical cleavage of CO from matrix-isolated [(CsMes)(CO),W(C,H,)] leads via several intermediates to 10[25]. We observed magnetization transfer between the hydridic and olefinic protons in I0 [lob] by NMR
spectroscopy. This can be explained by a dynamic equilibrium between 10
and 11.
[271 J. C. Jefferey, F. G. A. Stone, G. K. Williams, Polyhedron 1991, 10, 215
[28] R. Koster, P. Binger, Inorg. Synth. 1974, IS, 142.
Mechanistic Insights into the Very Efficient
[ReO,OSiR,]-Catalyzed Isomerization of
Ally1 Alcohols
Stephane Bellemin-Laponnaz, Herve Gisie,
Jean Pierre Le Ny, and John A. Osborn*
Dedicated to the late Geoffrey Wilkinson,
mentor, inspirer, and friend
The isomerization of allyl alcohols by the 1,3 transposition of
an hydroxy group (Scheme 1a) is catalyzed by certain high oxidation state transition metal 0x0 complexes, and has been carried out industrially by using [VO(OR),] or yWO(OR),] catalysts
at high temperatures (ca. 130-200°C) for the production of
terpenic alcohols.[‘] Recently Mo and V catalysts have been
reported[*. 31 that are active at 25 “C. However, we find that in
the case of the MoO,X, complexes (X = C1, OtBu), slow reduction of the Mo”’ center by the alcohol takes place,131thereby
I”]
Prof. J. A. Osborn, S. Beilemin-Laponnaz, Dr. H. Gisie, Dr. J. P. Le Ny
Laboratone de Chimie des Metaux de Transition et de Catalyse
Universite Louis Pasteur, Institut Le Be1
U R A 424 CNRS, 4 rue Blaise Pascal, F-67070 Strasbourg Cedex (France)
Fax: Int. code +388416171
e-mail: osborn(~chimie.u-strasbg.fr
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causing a loss of catalytic activity with time. Based on these
observations we have developed dioxomolybdenum(v1) catalysts that under appropriate conditions oxidize allyl and benzyl
alcohols selectively to aldehyde^.'^]
The proposed mechanism of the isomerization process, however, is still largely dependent on the original insight of
Charbardes et al.>’l and involves a cyclic transition state
(Scheme 1 b), akin to that of a Claisen type rearrangement, incorporating a metal 0x0 unit. Further, we have proposed[31that
OH
OH
0
6’
H
b)
Scheme 1. Catalytic isomerization of allyl alcohols (a) and the proposed cyclic
transition state (b).
the efficiency of the [MoO,(OR),] catalysts results, in part, from
the second 0x0 group serving as a spectator liga~~d,’~]
helping to
stabilize negative charge developed on the metal in the transition state, thereby lowering the activation barrier for the rearrangement. We reasoned that a further increase in the number
of 0x0 ligands around the metal may lead to an even greater
stabilization of the charge and thus a more active catalyst system. We present here some catalytic studies on the trioxorhenium complexes [ReO,(OSiR,)] 1 (R = Mer61)and 2 (R = Ph[’’),
which we found are by far the most efficient catalysts yet known
for this isomerization reaction, and thereby have allowed us to
obtain further details on the mechanism. Additionally these
complexes are considerably more stable towards reduction,
giving long-lived catalysts.
For example, using 2.2 x I O - , M of the rhenium catalyst 1 in
acetonitrile at 25 “C, 50 equivalents of hex-1-en-3-01are isomerized at an initial rate (vi) of 8 turnovers per mi, (graphically
determined by extrapolation to initial time); the equilibrium[’]
with trans-hex-2-en-1-01 is reached in less than 10 minutes. This
rate is over 10’ times higher than that observed for the previously described dioxomolybdenum(v1) catalyzed systems under
similar conditions which require about 24 h to reach this equilibrium. Even at 0°C vi with 1 is 2.5 turnovers per min. The
analogous triphenylsiloxy derivative 2 is found to be even more
active (ui> 10 turnovers per min at 0 “C;equilibrium is reached
in about 5 min). In contrast to the molybdenum catalysts, no
degradation of these rhenium systems is observed over 50 h.
Further, using 1 and 2-methylbut-3-en-2-01 as substrate, equilibrium with 3-methylbut-2-en-1-01is obtained in 2 min at 22 “C
but, in contrast to the molybdenum catalysts, no concomitant
diallyl ether formation is observed!g1 Only after leaving the
products in contact with the catalyst over a further two hours
are such ethers detectable.
The greater catalytic activity of 2 in comparison to that of 1
results from inhibition caused by the formation of water when
1 is used. ‘H NMR, studies show that, as expected, the allyl
alcohol substrate displaces the R,SiOH from 1 and 2 but unlike
triphenylsilanol, trimethylsilanol condenses rapidly to form
hexamethyldisiloxane and water. Indeed, addition of small
quantities of water was found to inhibit the catalytic process,
3 1?.S0+ S0jO
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Angen,. Chem. In!. Ed. Engl. 1991, 36, No. 9