On the origin of 1,5-induction in tin(IV) halide-promoted reactions of 4-alkoxyalk-2-enylstannanes with aldehydes

Article abstract of DOI:10.1039/a800781k

A combination of experiment and theory has identified both the allyltin trichlorides 3 (R² = Bn, H) as key intermediates in tin(IV) chloride-promoted reactions of allylstannanes 1 (R² = Bn, H) and the transition structures that are involved in their reactions with aldehydes.

Full text of DOI:10.1039/a800781k

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On the origin of 1,5-induction in tin(iv) halide-promoted reactions of
4-alkoxyalk-2-enylstannanes with aldehydes
Lindsay A. Hobson, Mark A. Vincent, Eric J. Thomas*† and Ian H. Hillier*
The Department of Chemistry, The University of Manchester, Manchester, UK M13 9PL
A combination of experiment and theory has identified both
the allyltin trichlorides 3 (R2 = Bn, H) as key intermediates
in tin(IV) chloride-promoted reactions of allylstannanes 1
(R2 = Bn, H) and the transition structures that are involved
in their reactions with aldehydes.
starting with the 4-hydroxypent-2-enylstannane 1 (R2 = H)
gave the 3-triphenylstannylpentan-2-ol 9 which was benzylated
to give the syn-benzyl ether 7.
The hydroxyalkylstannane 9 could not be converted into a
crystalline material suitable for X-ray diffraction and so its
structure was confirmed by comparison with an authentic
sample (see Scheme 3.) The racemic (E)- and (Z)-epoxides 10
and 14 were obtained from the corresponding alkenes using
NBS and KOH.7 Ring-opening using triphenylstannyllithium
then gave the regioisomeric hydroxystannanes 11/12 and 15/9
which were separated and characterised spectroscopically. The
configurations of these products were assigned on the basis that
analogous epoxide ring-openings are known to proceed with
inversion of configuration.8 The syn-hydroxyalkylstannane 9
obtained from the cis-epoxide 14 was spectroscopically and
chromatographically identical to that obtained from the trapping
reaction of the hydroxystannane 1 (R2 = H). Moreover,
O-benzylation gave a benzyl ether which was identical to that
prepared by reduction of the alkenylstannane 6. The anti-
hydroxyalkylstannane 12 obtained from the trans-epoxide 10
was distinctly different from that obtained from the trapping
reaction of the hydroxystannane 1 (R2 = H), and benzylation
gave the anti-benzyloxystannane 13 which was clearly dis-
tinguishable from its syn-diastereoisomer 7 by 1H and 13C NMR
spectroscopy.
Useful levels of 1,5-, 1,6- and 1,7-asymmetric induction are
obtained in tin(iv) halide-promoted reactions of 4-, 5- and
6-alkoxyalk-2-enylstannanes and aldehydes.1 For example,
tin(iv) chloride-promoted reactions betweeen aldehydes and the
(4-benzyloxypent-2-enyl)stannane 1 (R2 = Bn) (Scheme 1)
give the 1,5-syn(Z)-alkenols 2 with excellent stereoselectivity
(syn:anti ! 97:3).2 Similar results are obtained using the
4-hydroxypent-2-enylstannane 1 (R2 = H).3
This stereoselectivity is consistent with generation of the
allyltin trichlorides 3, which react with the aldehydes via the
chair-like transition structures 4.2 However, alternative expla-
nations are possible, notwithstanding the results from trapping
the allyltin trichlorides generated from the 5-alkoxypent-
2-enylstannanes 5.4,5 For example, participation of the
C(3)-epimers of the allyltin trichlorides 3 and boat-like
transition structures for the reactions with aldehydes, would
account for the overall 1,5-syn (Z)-stereoselectivity. We now
report confirmation of the involvement of the allyltin trichlor-
ides 3 in these reactions, together with high level modelling of
the transition structure 4.
SnPh3
SnPh3
OH
SnCl4
Me
ii
i
Me
Me
Me
R13Sn
1
Me
OR2
R3
R1 = Ph, R2 = Bn
OR2
R3CHO
OBn
OBn
6
7
2
1
iii
SnPh3
SnCl4
SnPh3
ii
i
Cl
O
1
Me
Cl
Me
Me
H
R1 = Ph, R2 = H
Cl
Me
4
3
R3CHO
OH
OH
Sn
R3
8
9
OR2
Cl3Sn OR2
Scheme 2 Reagents and conditions: i, SnCl4, 278 °C, 5 min, then PhLi (6,
54%; 8, 35%); ii, diimide (7, 68%; 9, 69%); iii, NaH, BnBr (68%)
H
Me
4
3
SnPh3
H
O
OH
Scheme 1
Me
Me
Me
Me
i
Me
Me
+
Tin(iv) chloride was added to a solution of the 4-benzyloxy-
pent-2-enylstannane 1 (R2 = Bn) at 278 °C followed by an
excess of PhLi. From this mixture the syn-4-benzyloxypent-
1-en-3-yl(triphenyl)stannane 6, containing less than 5% of its
anti diastereoisomer, was isolated (see Scheme 2.) To avoid
1,3-migration of the triphenyltin moiety,6 the alkenylstannane 6
was reduced using diimide to give (2S,3S)-2-benzyloxypent-
3-yl(triphenyl)stannane 7. The same sequence of reactions
OR
H
SnPh3
12 R = H
10
11
ii
13 R = Bn
OH
SnPh3
Me
H
O
Me
Me
i
Me
ii
Me
H
+
SnPh3
OR
9 R = H
Me
14
15
7 R = Bn
R13Sn
OR2
Scheme 3 Reagents and conditions: i, Ph3SnLi (11, 30%; 12, 20%; 15, 20%;
9, 20%); ii, NaH, BnBr (13, 61%; 7, 72%)
Me
5
Chem. Commun., 1998
899

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It would appear that transmetallation of the 4-benzyloxy- and
4-hydroxy-pent-2-enylstannanes 1 (R2 = Bn, H) with tin(iv)
chloride gives the (3S,4S)-4-benzyloxy- and (3S,4S)-4-
hydroxy-pent-1-en-3-yl)tin trichlorides 3 (R2 = Bn, H) as
originally postulated. By analogy with the transmetallation of
the 5-alkoxypent-2-enylstannanes 5,5 it is likely that this
transmetallation is due to kinetic control. However only low
yields of products were obtained on attempted transmetallation
of the pent-1-en-3-ylstannane 6 using either tin(iv) chloride or
bromide, and so the interconversion of regioisomeric allyltin
trihalides could not be investigated in this case.
The formation of the 1,5-syn (Z)-products from the reactions
between the tin trichlorides 3 and aldehydes is consistent with
participation of chair-like transition structures akin to 4.
However, why does the C(3)–C(4) bond prefer to be axial rather
than equatorial, i.e. why are (Z)-alkenes obtained as the
dominant products from these reactions rather than their
(E)-isomers? To probe the factors involved in this ster-
eoselectivity, high level electronic structure calculations have
been carried out on the allyltin trichloride 16 and the transition
structures for its reactions with formaldehyde which would lead
to (E)- and (Z)-alkenols. A split valence basis was employed,9
with electron correlation included at the density functional
theory level (B3LYP), using GAUSSIAN94.10 Stationary
structures were identified as minima or saddle points by the
calculation of harmonic frequencies.
trans-isomer [Fig. 1(b)], which has a barrier of 12.0 kcal mol21
.
The favoured structure has a six-coordinated tin atom, with Sn–
C and Sn–O bond lengths in the range 2.2–2.4 Å. The transition
structure leading to the unfavoured trans-isomer has one of its
Sn–O bond lengths considerably longer than normal (3.3 Å), so
that the tin is effectively five-coordinated.
Two factors favour the transition structure shown in Fig. 1(a).
Firstly, there is greater steric repulsion between the C(3)–C(4)–
OMe entity and the chlorine atoms in the transition structure for
formation of the unfavoured trans-isomer. Secondly, the
different orientations of the C(3)–C(4) bond in the two
transition structures leads to the long Sn–O bond in the
disfavoured transition structure.
This work confirms the participation of the allyltin trich-
lorides 3 in tin(iv) chloride-promoted reactions of the allylstan-
nanes 1. The theoretical studies provide an insight into the
factors which favour the formation of cis-alkenes in reactions of
these allyltin trichlorides with aldehydes and complement
recently reported computational studies into reactions between
allylsilanes and aldehydes.11,12
We thank the University of Manchester and SmithKline
Beecham for support (to L. A. H.) under the CASE Scheme, Dr
M. Fedouloff of SmithKline Beecham for helpful discussions,
and the EPSRC for support.
Notes and References
We find two conformations of the allyltin trichloride 16,
corresponding to rotamers about the C(2)–C(3) bond, to be
energy minima with Sn–O bond lengths of 2.457 and 2.531 A.
Transition states for reactions of 16 with formaldehyde leading
† E-mail: e.j.thomas@man.ac.uk
1 E. J. Thomas, Chem. Commun., 1997, 411.
2 A. H. McNeill and E. J. Thomas, Synthesis, 1994, 322.
3 G. W. Bradley, D. J. Hallett and E. J. Thomas, Tetrahedron: Asymmetry,
1995, 6, 2579.
4 J. S. Carey and E. J. Thomas, Synlett, 1992, 585.
5 R. L. Beddoes, L. A. Hobson and E. J. Thomas, Chem. Commun., 1997,
1929.
3
4
Cl3Sn OMe
16
6 V. J. Jephcote and E. J. Thomas, J. Chem. Soc., Perkin Trans. 1, 1991,
429.
7 A. T. Bottini, R. L. VanEtten and A. J. Davidson, J. Am. Chem. Soc.,
1965, 87, 755.
8 L. D. Hall, P. R. Steiner and D. C. Miller, Can. J. Chem., 1979, 57,
38.
to both cis- and trans-double-bonded products have been
located and identified as saddle points on the potential energy
surfaces. These are summarised in Fig. 1.
9 W. J. Hehre, R. Ditchfield and J. A. Pople, J. Chem. Phys., 1972, 56,
2257; R. Ditchfield, W. J. Hehre and J. A. Pople, J. Chem. Phys., 1971,
54, 724; M. S. Gordon, Chem. Phys. Lett., 1980, 76, 163; P. C.
Hariharan and J. A. Pople, Theor. Chim. Acta, 1973, 28, 213;
A. Stromberg, O. Gropen and U. Wahlgren, J. Comput. Chem., 1983, 4,
181; an additional d-function with exponent 0.27 was used on tin.
10 M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson,
M. A. Robb, J. R. Cheeseman, T. A. Keith, G. A. Petersson,
J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G.
Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov,
A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen,
M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin,
D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. J. P. Stewart, M. Head-
Gordon, C. Gonzalez and J. A. Pople, GAUSSIAN 94, Revision A.1,
Gaussian, Inc., Pittsburgh, PA, 1995.
There are considerable differences in the transition state
structures and energy barriers leading to the two different
stereoisomers. The transition structure leading to the cis-
double-bonded product [Fig. 1(a)] has a considerably lower
barrier (1.9 kcal mol21) and different structure, particularly
about the tin atom, than the transition structure leading to the
(a)
(b)
Cl
Cl
Cl
Cl
Cl
Sn
Cl
Sn
2.222
2.182
1.284
3.297
2.380
O
1.267
O Me
C
O
C
O
C4
2.046
2.197
C3
C2
C3
C2
1.429
1.434
Me
C1
C1
1.388
2.374
1.376
C4
11 A. Bottoni, A. L. Costa, D. Di Tommaso, I. Rossi and E. Tagliavini,
J. Am. Chem. Soc., 1997, 119, 121131.
2.370
12 K. Omoto, Y. Sawada and H. Fujimoto, J. Am. Chem. Soc., 1996, 118,
1750.
∆E = 1.9 kcal mol–1
∆E = 12.0 kcal mol–1
Fig. 1 Transition structures leading to (a) cis and (b) trans products. Bond
lengths are in Å.
Received in Liverpool, UK, 28th January 1998; 8/00781K
900
Chem. Commun., 1998