A stannylene/aryl iodide reagent for allylic CH activation and double bond addition chemistry

Article abstract of DOI:10.1021/om8000108

Allylic CH bond activation and addition across the C=C double bond are both observed in the reactions of alkenes with a stannylene/phenyl iodide reagent. The relative amounts of each of these products are sensitive to the steric bulk of the alkene and of the aryl halide.

Full text of DOI:10.1021/om8000108

Organometallics 2008, 27, 1041–1043
1041
A Stannylene/Aryl Iodide Reagent for Allylic CH Activation and
Double Bond Addition Chemistry
Ajdin Kavara, Kandarpa D. Cousineau, Ahleah D. Rohr, Jeff W. Kampf, and
Mark M. Banaszak Holl*
Department of Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109-1055
ReceiVed January 8, 2008
The amount of oxidative addition product ArISn[C2(SiMe3)4-
C2H4] and double bond addition products formed is dramatically
reduced.
Summary: Allylic CH bond actiVation and addition across the
CdC double bond are both obserVed in the reactions of alkenes
with a stannylene/phenyl iodide reagent. The relatiVe amounts
of each of these products are sensitiVe to the steric bulk of the
alkene and of the aryl halide.
Activation of Allylic CH Bonds. Activation of the allylic
CH bond was achieved when a 1:1 mixture of 1 and PhI was
added to tetramethylethylene, cyclohexene, cyclopentene, 1-pen-
tene, 1-hexene, or 3,4-dihydro-2H-pyran (Scheme 1, Table 1).
Formation of the CH activation products ranged from 5 to 80%
when employing ∼0.05 M solutions and slow addition of
reagents using a syringe pump. The other major species
produced in the reaction were those of the oxidative addition
of PhI to 1 to form PhISn[C2(SiMe3)4C2H4] (2) and of the
unexpected addition of 1/PhI across the double bond (vide infra)
to form 6, 8, and 10. Separation of the CH activation, double
bond addition, and oxidative addition products could be achieved
by fractional crystallization or column chromatography. Column
chromatography on silica gel was typically the most convenient
and gave the highest yields. The structures of all compounds
CH activation of hydrocarbons and ethers to form Sn-C and
Ge-C bonds can be accomplished under mild conditions using
EY2/PhX reagents (E ) Ge, Sn; Y ) N(SiMe3)2, CH(SiMe3)2;
X ) I, Br).1,2 The direct reaction of germylenes and stannylenes
with functional groups, particularly double bonds,3,4 represents
a major limitation of the chemistry developed to date. When
the products of the CH activation contain a primary Sn-C bond,
they can be directly employed for Stille cross-coupling
chemistry2,5,6 using the conditions reported by Fouquet.7
However, these cross-coupling conditions only work with
primary alkyl groups and attempts at other types of coupling
reactions have often failed in our hands because the -N(SiMe3)2
substituent on tin transfers instead of the desired -R group.
1
were assigned via a combination of H, 13C, and 119Sn NMR
spectroscopy, elemental analysis, infrared spectroscopy, and, in
the case of 3, 6, 10, and 11, single-crystal X-ray diffraction
(Figure 1). In addition, the bromine derivative of the cyclohex-
ene product 4, [C2H4(SiMe3)4C2]SnBr(C6H9) (4′), was synthe-
sized independently by the reaction of 1 with 3-bromocyclo-
hexene and fully characterized.
The stannylene synthesized in 1991 by Kira et al.,
SnC(SiMe3)2CH2CH2C(SiMe3)2 (1),8,9 provides a possible solu-
tion to both of these limitations. 1 has been shown not to react
with alkenes, including ethylene, although it does readily
undergo 4 + 2 cycloaddition with 1,3-butadiene.10 Furthermore,
the ligand contains a chelated ring with Sn-C bonds which
should be much less susceptible to transfer under coupling
conditions. When stannylene 1 has been employed, allylic CH
activation chemistry has been successfully achieved. Surpris-
ingly, the 1/PhI reagent is also observed to add across CdC
double bonds. The regiochemistry of the reaction is anti-
Markovnikov, consistent with an initial attack by a phenyl
radical and subsequent trapping by the iodostannyl radical.
Replacing PhI with the more bulky mesityl iodide (MesI) reagent
has a dramatic impact on the distribution of products formed.
In previous studies employing germylenes, varying the steric
bulk of the aryl iodide did not have a large impact on the ratio
of CH activation/oxidative addition product formed.11 However,
for the chemistry employing stannylene 1, increasing the steric
bulk of the aryl halide had a major impact on the product
distributions. Employing mesityl iodide (MesI) in reactions with
tetramethylethylene, cyclohexene, and cyclopentene resulted in
a dramatic reduction in the formation of oxidative addition
product. Furthermore, the use of MesI also reduced the amount
of double bond addition product observed with cyclopentene
and 1-hexene. The use of PhBr dramatically reduced the rate
of both the CH activation and double bond addition reactions.
Tin compounds derived from stannylenes containing primary
and aromatic Sn-C bonds have previously been employed for
Stille-type cross-coupling reactions using chemistry developed
by Fouquet et al.2,7,12 In order to assess the utility of secondary
allylic Sn-C bonds derived from the CH activation chemistry
for Stille-type cross-coupling, compound 4 was mixed with 1.5
equiv of PhI, 3 equiv of Me4NF, and 10 mol % of Pd(PPh3)4 in
10 mL of dioxane with a 1,3,5-trimethoxybenzene integration
standard. After heating at 77 °C for 16 h, 3-phenylcyclohexene
was obtained in 70% yield.
(1) Miller, K. A.; Bartolin, J. M.; O’Neill, R. M.; Sweeder, R. D.; Owens,
T. M.; Kampf, J. W.; Banaszak Holl, M. M.; Wells, N. J. J. Am. Chem.
Soc. 2003, 125, 8986–8987.
(2) Bartolin, J. M.; Kavara, A.; Kampf, J.; Banaszak Holl, M. M.
Organometallics 2006, 25, 4738–4740.
(3) Ando, W.; Ohgaki, H.; Kabe, Y. Angew. Chem., Int. Ed. 1994, 33,
659–661.
(4) Sita, L. R.; Bickerstaff, R. D. J. Am. Chem. Soc. 1988, 110, 5208–
5209.
(5) Cardenas, D. J. Angew. Chem., Int. Ed. 2003, 42, 384–387.
(6) Luh, T. Y.; Leung, M. K.; Wong, K. T. Chem. ReV. 2000, 100, 3187–
3204.
(7) Herve, A.; Rodriguez, A. L.; Fouquet, E. J. Org. Chem. 2005, 70,
1953–1956.
(8) Kira, M.; Ishida, S.; Iwamoto, T.; Yauchibara, R.; Sakurai, H. J.
Organomet. Chem. 2001, 636, 144–147.
(9) Kira, M.; Yauchibara, R.; Hirano, R.; Kabuto, C.; Sakurai, H. J. Am.
Chem. Soc. 1991, 113, 7785–7787.
(11) Bartolin, J. M.; Banaszak Holl, M. M.,Unpublished results.
(12) Fouquet, E.; Pereyre, M.; Rodriguez, A. L. J. Org. Chem. 1997,
62, 5242–5243.
(10) Kira, M.; Ishida, S.; Iwamoto, T. Chem. Rec. 2004, 4, 243–253.
10.1021/om8000108 CCC: $40.75
2008 American Chemical Society
Publication on Web 02/27/2008

1042 Organometallics, Vol. 27, No. 6, 2008
Communications
Scheme 1
Table 1. Product Distributions As Measured by 1H NMR Spectroscopya
a All reactions were performed using syringe pump techniques. Other unidentified products were also present for cases where the total sums to less
than 100%.
Addition to Double Bonds. In addition to the anticipated
but not attached to the tin atom, as could be ascertained by the
allylic CH activation and oxidative addition products, a third
species was also present in the reactions with tetramethyleth-
ylene, cyclohexene, cyclopentene, 1-pentene, and 1-hexene. 1H
NMR spectroscopy indicated that the phenyl group was present
absence of the characteristic ortho protons at δ 7.92 with
J Sn-H ) 16.5 Hz. In addition, vinylic protons in the region
δ 6.0–5.8 indicative of double bonds were absent. The implica-
3
119
tion of phenyl and tin addition across the double bond and the

Communications
Organometallics, Vol. 27, No. 6, 2008 1043
Figure 1. Ball and stick plots of [C2H4(SiMe3)4C2]SnI(C6H11) (3), [C2H4(SiMe3)4C2]SnBr(C6H9) (4′), [C2H4(SiMe3)4C2]SnI(C11H13) (6),
[C2H4(SiMe3)4C2]SnI(C12H17) (10), [C2H4(SiMe3)4C2]SnI(C5OH7) (11),and [C2H4(SiMe3)4C2]SnI(C14H15) (12). Full experimental details and
ORTEP diagrams are provided in the Supporting Information.
anti-Markovnikov regiochemistry of this addition were con-
firmed by a single-crystal X-ray diffraction study of products 6
and 10. In the case of 4-methylstyrene, no CH activation was
observed at the methyl group and only double bond addition to
give 12 was observed. The reaction is sensitive to the steric
constraints of the substrates, as indicated by the formation of
both CH activation and double bond addition products for
cyclopentene, in 67% and 32% conversion, respectively, as
indicated by 1H NMR spectroscopy, whereas little to no double
bond addition was observed in the reaction with cyclohexene
(6%) or 3,4-dihydro-2H-pyran (0%). The CdC-C angle in
cyclopentene is 112°, as opposed to 123° in cyclohexene and
122 and 124° in 3,4-dihydro-2H-pyran, thus substantially
reducing the steric restriction around the cyclopentene double
bond. 1-Pentene, 1-hexene, and 4-methylstyrene, which contain
only one R substituent on the double bond, are substantially
less sterically constrained and give double-bond addition in
yields of 67, 60, and 90%, respectively (Table 1). The double
bond addition chemistry follows the behavior expected for a
radical process in terms of the anti-Markovnikov regiochemistry
of addition to the terminal olefins and the antiaddition to the
cyclopentene.13
Conclusions. The direct formation of Sn-C bonds from
allylic CH bonds has been demonstrated by employing the
mixed reagent Sn[C2(SiMe3)4C2H4]/PhI. The products of these
reactions are viable for C-C bond forming reactions under
Stille-type cross-coupling conditions. The unexpected addition
of phenyl and the tin complex across double bonds was also
discovered. We are currently exploring the utility of this
chemistry, particularly with respect to the development of novel
routes for the synthesis of heterocycles.
Acknowledgment. This work was supported by National
Science Foundation Grant No. CHE-0453849. We thank
Prof. J. P. Wolfe for helpful discussions.
Supporting Information Available: Text giving complete
experimental preparation details and ORTEP plots for 3, 4′, 6, and
10-12 and CIF files giving crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM8000108
(13) March, J., AdVanced Organic Chemistry, 4th ed.; Wiley: New York,
1992; pp 743–745..