3
4
H. C. Brown, K. W. Kim, M. Srebnik and B. Singaram, Tetrahedron,
To understand this lower phenyl migratory aptitude when R =
Ph, we analysed the electronic structure of the transition states for
the phenyl migration using NBO second order perturbation theory
1
987, 43, 4071.
H. C. Brown, S. K. Gupta and B. Richard, J. Am. Chem. Soc., 1971, 93,
802.
5 (a) H. C. Brown, H. Nambu and M. M. Rogie, J. Am. Chem. Soc.,
969, 91, 6852–6854; (b) H. C. Brown, H. Nambu and M. M. Rogie,
2
16
analysis, at the B3LYP/6-31G* level of theory.{ Using the
bonding pattern of the reactant as a reference, the transition state
is stabilised by two key orbital interactions: donation of an
electron pair from the sB–C(Ph) orbital and the pC(Ph) orbital,
respectively, into the s*C1–S orbital. The stabilization arising from
the second interaction is analogous to the neighbouring group
1
J. Am. Chem. Soc., 1969, 91, 6854–6855; (c) H. C. Brown, H. Nambu
and M. M. Rogie, J. Am. Chem. Soc., 1969, 91, 6855–6856.
J. Hooz and D. M. Gunn, Tetrahedron Lett., 1969, 10, 3455–3458.
6
7 (a) K. Nakamura and Y. Osamura, Tetrahedron Lett., 1990, 31,
251–254; (b) K. Nakamura and Y. Osamura, J. Am. Chem. Soc., 1993,
115, 9112–9120; (c) S. Winstein, C. R. Lindegren, H. Marshall and
L. L. Ingraham, J. Am. Chem. Soc., 1953, 75, 147–155; (d) E. Wistuba
and C. Ruchardt, Tetrahedron Lett., 1981, 22, 4069–4072.
8 (a) H. C. Brown and C. J. Kim, J. Am. Chem. Soc., 1968, 90,
2082–2096; (b) S. Winstein, B. K. Morse, E. Grunwald, K. C. Schreiber
and J. Corse, J. Am. Chem. Soc., 1952, 74, 1113–1120; (c) T. H. Phan
and H. Dahn, Helv. Chim. Acta, 1976, 59, 335–348.
1
7
effect found in phenonium ions, and varies with the electrophilic
character of the C1 carbon. It is larger for the R = H transition
2
1
state (E(2) = 18.0 kcal mol ) than for the R = Ph transition state
2
1
(
7.1 kcal mol ), where benzylic positive charge delocalization at
C1 makes this position less electrophilic. For Wagner–Meerwein
+
rearrangement of Me
2 2 2
PhCCH (OH ) , in which exclusive phenyl
9
The reaction of B-phenyl-9-BBN with haloketones and haloacetonitriles
has been reported. Exclusive phenyl migration was observed in these
cases, exploiting the fact the 9-BBN is a non-migrating group: see
ref. 5a,b.
8b
21
migration is known to occur, E(2) is even larger (22.9 kcal mol
,
details in Supplementary Material{). We can thus now understand
why phenyl is a better migrating group than methyl when R = H
but a poorer one when R = Ph. In the case of R = Ph, the
developing positive charge on the migrating terminus is delocalized
and so the benefit gained from the aromatic ring using its p system
to aid migration is strongly attenuated. Furthermore, migration of
the larger phenyl group (compared to methyl) towards the
hindered migration terminus will result in increased steric
interactions which may now be responsible for the increased
barrier to phenyl migration.
10 For a computational study on the rearrangement of chloromethyl
borate species, see: (a) A. Bottoni, M. Lombardo, A. Neri and
C. Trombini, J. Org. Chem., 2003, 68, 3397–3405. For a computational
study on the rearrangement of dimethylsulfoxoniummethyl borate
species, see: (b) J. M. Stoddard and K. J. Shea, Chem. Commun., 2004,
830–831.
1
1 Methyl group should be an adequate model for primary alkyl groups
since its migration aptitude has been found to be very similar to the one
of ethyl in related systems (see ref. 8).
1
2 For a discussion of reactivity in reactions of onium ylides with
organoboranes, see: V. K. Aggarwal, J. N. Harvey and R. Robiette,
Angew. Chem., Int. Ed., 2005, 44, 5468–5471.
In summary, we have shown that the following factors impact
on which group migrates in mixed aryl–dialkylborates:
13 (a) M. Meot-Ner and C. A. Deakyne, J. Am. Chem. Soc., 1985, 107,
469–474; M. Meot-Ner and C. A. Deakyne, J. Am. Chem. Soc., 1985,
(1) Conformation of the ate complex. There is a preference for
107, 474–479; (b) D. A. Dougherty, Science, 1996, 271, 163–168. For a
recent review on CH p hydrogen bonds in organic reactions, see:
the phenyl group to be syn to the sulfonium group because of
…
20
stabilising electrostatic interactions. This favours alkyl group
migration.
M. Nishio, Tetrahedron, 2005, 61, 6923–6950.
4 Crystal structures in the following references have Ar HCS distances
…
+
1
˚
˚
(
2) The presence of a phenyl group on boron inherently impedes
the migration of the other (alkyl) groups.
3) The phenyl group is usually a better migrating group than
of 2.86 A and 2.89 A respectively. (a) G. M. Iskander, I. E. Khawad,
G. Yousif, K. Fisher, C. K. Fair and E. O. Schlemper, Helv. Chim. Acta,
1
985, 68, 2216; (b) A. F. Cameron, F. D. Duncanson and D. G. Morris,
Acta Crystallogr., Sect. B, 1976, B32, 2002.
5 M. M. Midland, A. R. Zolopa and R. L. Halterman, J. Am. Chem.
Soc., 1979, 101, 248–249.
(
methyl because it can stabilise the transition state by donation of
1
an electron pair from its p system into the s*C1–S orbital
1
6 E. D. Glendening, J. K. Badenhoop, A. E. Reed, J. E. Carpentier,
J. A. Bohmann, C. M. Morales and F. Weinhold, NBO 5.0, Madison,
WI, 2001.
(
neighbouring effect). This effect however is highly attenuated when
the migrating terminus (C1) is less electrophilic (e.g. when R = Ph).
4) Steric effects also play a role. Larger groups will suffer
(
17 E. del R ´ı o, M. I. Men e´ ndez, R. L o´ pez and T. L. Sordo, J. Phys. Chem.
increased barrier to migration when the migrating terminus is
hindered.
A, 2000, 104, 5568–5571.
8 Jaguar 4.0, Schr o¨ dinger, Inc., Portland, OR, 1991–2000.
1
1
9 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin,
J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone,
B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson,
H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene,
X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. Ochterski, P. Y. Ayala, K. Morokuma,
G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski,
S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui,
A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu,
A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox,
T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen,
M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN 03
The authors thank Dr J. P. H. Charmant for useful discussions,
Merck and Pfizer for unrestricted grants, EPSRC for funding, and
the Universit e´ catholique de Louvain for an F.S.R. grant to R.R.
Notes and references
{
The MP2/6-311+G**//B3LYP/6-31G* method has been selected to be
the more adequate for the studied system after investigation of the model
reaction of BMe with CH SMe at a variety of different levels of theory
see Supporting Information for details{). DFT calculations were carried
3
2
2
(
18
out using the Jaguar 4.0 pseudospectral program package and MP2
single-point calculations using the Gaussian 03 program package.
19
1
2
(a) A. Pelter, K. Smith and H. C. Brown, in Borane Reagents, Academic
Press, London, 1988, p. 192; (b) T. Hayashi, in Comprehensive
Asymmetric Catalysis, ed. E. N. Jacobsen, A. Pfaltz and
H. Yamamoto, Springer, Berlin, 1999, vol. 1, p. 351; (c)
D. S. Matteson, Tetrahedron, 1998, 54, 10555–10607.
(
Revision B.04), Gaussian, Inc., Pittsburgh, PA, 2003.
0 Reactions of hindered sulfur ylides e.g. ylide from Scheme 1 (Ar = 3-F-
MeOC ) with Et PhB also results in preferential migration of the
ethyl group indicating that this is a general phenomenon.
2
4
6
H
3
2
V. K. Aggarwal, G. Y. Fang and A. Schmidt, J. Am. Chem. Soc., 2005,
127, 1642–1643.
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 741–743 | 743