set, forming a six-element {ZnCSiOLiN} ring which displays a
pseudo-boat conformation. As found in 3, both metals
have distorted trigonal planar geometries; Zn binds to a
two monometallic reagents must form mixed-metal reagent
analogues of 1 but with a molecule of trimethyl(phenoxy)-
silane acting as a donor on the lithium reagent. Once formed in
solution it must react rapidly with some non-coordinated
(free) trimethyl(phenoxy)silane to afford the metallated
intermediate 4.
t
terminal Bu ligand, whereas lithium is now coordinated to a
non-metallated molecule of 2 which acts as Lewis donor
through the oxygen atom (Li1–O4, 2.075(3) A) mimicking
the role previously played by THF in 3. Remarkably in
contrast with 3, the lithium–carbon contact in 4 is significantly
weaker (as indicated by the elongated value found for
the Li1–C11 bond length, 2.776(3) A). This is probably a
consequence of the increase in steric hindrance about Li by
coordinating a molecule of 2 which contains a highly sterically
demanding SiMe3 group instead of THF, which prevents it
approaching closer to C11.
We thank the Royal Society (University Research
Fellowship to E. H.) for its generous sponsorship of this
research and also thank Professor R. E. Mulvey for helpful
discussions.
Notes and references
z Crystal data for 3: C26H48LiNO2SiZn, Mr = 507.05, triclinic, space
ꢀ
group P1, a = 11.2813(5), b = 11.3700(6), c = 13.0987(6) A,
a = 79.264(4)1, b = 79.106(4)1, g = 61.575(5)1, V = 1441.91(12) A3,
Z = 2, l = 0.71073 A, m = 0.913 mmꢀ1, T = 123 K; 20 418 reflections,
7908 unique, Rint 0.0326; final refinement to convergence on F2 gave
R = 0.0314 (F, 6081 obs. data only) and Rw = 0.0725 (F2, all data),
GOF = 0.967. Crystal data for 4: C31H54LiNO2Si2Zn, Mr = 601.23,
monoclinic, space group P21/c, a = 12.4930(3), b = 12.8910(3),
c = 20.8841(5) A, b = 92.494(2)1, V = 3360.14(14) A3, Z = 4,
l = 0.71073 A, m = 0.828 mmꢀ1, T = 123 K; 55 721 reflections, 9627
unique, Rint 0.0426; final refinement to convergence on F2 gave
R = 0.0352 (F, 7223 obs. data only) and Rw = 0.0742 (F2, all data),
GOF = 0.997.
The yield of 4 can be improved (to 37%) when two
equivalents of silane 2 are employed. The fact that 4 is
obtained, even when only one molar equivalent of 2 is used,
implies that the rate of metallation of 2 is faster than the
rate of formation of the putative mixed-metal intermediate
[(PhOSiMe3)Li(TMP)(tBu)Zn(tBu)] (I) (Scheme 2) which once
formed must react with the remaining non-coordinated silane
2 to generate 4. In order to detect the formation of I, the
reaction was carried out at ꢀ78 1C; however, this resulted in
the precipitation of a white solid which was isolated and
identified as LiTMP using NMR analysis. Thus, these results
suggest that LiTMP, tBu2Zn and 2 in hexane solution must be
in equilibrium with the mixed-metal compound I (Scheme 2).
The latter as soon as it is formed reacts with some
non-coordinated 2, driving the equilibrium towards I and
therefore towards the formation of the metallated product 4.
At low temperatures, the solubility of LiTMP in hexane
decreases, causing its precipitation, and therefore shifting the
equilibrium towards the monometallic reagents (left) and
inhibiting metallation of 2.
1 For a historical essay on organozinc compounds see: D. Seyferth,
Organometallics, 2001, 20, 2940.
2 P. Knochel and P. Jones, in Organozinc Reagents:
A
Practical Approach, ed. L. H. Harwood and C. J. Moody, Oxford
University Press, Oxford, 1999.
3 (a) R. E. Mulvey, Organometallics, 2006, 25, 1060;
(b) R. E. Mulvey, F. Mongin, M. Uchiyama and Y. Kondo,
Angew. Chem., Int. Ed., 2007, 46, 3802.
4 (a) Y. Kondo, M. Shilai, M. Uchiyama and T. Sakamoto, J. Am.
Chem. Soc., 1999, 121, 3539; (b) W. Clegg, S. H. Dale, E. Hevia,
G. W. Honeyman and R. E. Mulvey, Angew. Chem., Int. Ed., 2006,
45, 2370; (c) M. Uchiyama, Y. Matsumoto, D. Nobuto,
T. Furuyama, K. Yamaguchi and K. Morokuma, J. Am. Chem.
Soc., 2006, 128, 8748.
5 W. Clegg, S. H. Dale, A. M. Drummond, E. Hevia,
G. W. Honeyman and R. E. Mulvey, J. Am. Chem. Soc., 2006,
128, 7434.
In summary this work has revealed a new application of
1 in AMMZn, by isolating and structurally defining the
first intermediates of direct lateral zincation (DlZn) of
the trimethyl(phenoxy)silane 2. These compounds are not
accessible by a conventional two-step metathesis reaction,
using monometallic reagents, illustrating the large potential
that mixed-metal chemistry can offer to synthetic chemists. In
addition the reaction of the unco-complexed LiTMP and
tBu2Zn with 2 has also been explored, which shows that these
6 W. Clegg, S. H. Dale, R. W. Harrington, E. Hevia,
G. W. Honeyman and R. E. Mulvey, Angew. Chem., Int. Ed.,
2006, 45, 2374.
7 T. H. Chan and D. Wang, Chem. Rev., 1995, 95, 1279.
8 (a) P. W. Weber, Silicon Reagents for Organic Synthesis, Springer
Verlag, New York, 1983; (b) D. Ager, Organic Reactions,
ed. L. Paquette, Wiley, New York, 1990, vol. 38.
9 T. F. Bates, S. A. Dandekar, J. L. Longlet, K. A. Wood and
R. D. Thomas, J. Organomet. Chem., 2000, 595, 87.
10 When this reaction was carried out in hexane a white solid was
obtained that could only dissolve in neat d8-THF and which NMR
analysis showed was a complex mixture of products, with no
resonances at negative chemical shifts, which are usually indicative
that a-metallation has occurred.
11 The formation of 3 proved to be quantitative when equimolar
amounts of 1 and 2 were reacted in C6D6 as was monitored by 1H
and Li NMR spectroscopy.
7
12 T. Tatic, H. Ott and D. Stalke, Eur. J. Inorg. Chem., 2008,
3765.
13 D. R. Armstrong, W. Clegg, S. H. Dale, E. Hevia, L. M. Hogg,
G. W. Honeyman and R. E. Mulvey, Angew. Chem., Int. Ed., 2006,
45, 3775.
14 In non-polar solvents the presence of a Lewis donor is usually
required to yield a mixed-metal compound: M. Westerhausen,
B. Rademacher and W. Schwarz, Z. Anorg. Allg. Chem., 1993,
619, 675.
15 T. F. Bates and R. D. Thomas, J. Organomet. Chem., 1989, 359,
285.
Scheme 2
ꢁc
This journal is The Royal Society of Chemistry 2009
3242 | Chem. Commun., 2009, 3240–3242