7
residue was purified by flash chromatography on silica using hexane–
EtOAc (10:1) as eluent. This affords N-tosyl-2,5-dihydro-1H-pyrrole as
zations (ROMP) upon treatment with diazo alkanes, whereas
Hafner et al. initiated ROMP reactions with 3 after exposure to
UV light (200 W Hg lamp). These latter procedures, however,
either require hazardous additives or special photochemical
equipment and have not yet been applied to RCM.
We now report an even more convenient method for the
activation of this key compound. Thus, heating a solution of a
diene in the presence of catalytic amounts of 3 or, preferably, a
8
colorless crystals (799 mg, 90%), mp 123-124 °C; d
7
H
(300 MHz, CDCl
.70 (d, J 8, 2H), 7.29 (d, J 8, 2H), 5.63 (s, 2H), 4.10 (s, 4H), 2.41 (s, 3H);
(75 MHz, CDCl ) 143.8, 134.7, 130.1, 127.8, 125.8, 55.2, 21.9; m/z 223
3
)
d
C
3
+
(35, [M ]), 155 (32), 91 (77), 68 (100), 41 (24).
1
For reviews see: R. H. Grubbs and S. Chang, Tetrahedron, 1998, 54,
4413; A. Fürstner, Top. Organomet. Chem., 1998, 1, 37; S. K.
Armstrong, J. Chem. Soc., Perkin Trans. 1, 1998, 371; M. Schuster and
S. Blechert, Angew. Chem., Int. Ed. Engl., 1997, 36, 2036; A. Fürstner,
Top. Catal., 1997, 4, 285; K. J. Ivin and J. C. Mol, Olefin Metathesis and
Metathesis Polymerization, Academic Press, New York, 1997.
mixture of commercially available [(p-cymene)RuCl
2 2
] 4 and
PCy effects a clean and very efficient cyclization of the
3
substrate, provided that the reactions are exposed to neon light
or strong daylight.† Several aspects of this method are
noteworthy:
i) Light emitted by neon tubes commonly used for the
lighting of laboratories is sufficient.
ii) The reactions are performed in standard laboratory
2
S. T. Nguyen, R. H. Grubbs and J. W. Ziller, J. Am. Chem. Soc., 1993,
1
15, 9858; S. T. Nguyen, L. K. Johnson, R. H. Grubbs and J. W. Ziller,
(
J. Am. Chem. Soc., 1992, 114, 3974; P. Schwab, R. H. Grubbs and J. W.
Ziller, J. Am. Chem. Soc., 1996, 118, 100.
(
3
For recent reviews on applications in polymer chemistry see: D. Tindall,
J. H. Pawlow and K. B. Wagener, Top. Organomet. Chem., 1998, 1, 180;
L. L. Kiessling and L. E. Strong, Top. Organomet. Chem. 1998, 1, 196
and references cited therein.
®
glassware (DURAN ) which absorbs 50% of UV light with l =
9
3
00 nm and more than 95% of UV light with l @ 285 nm.
(
iii) Control experiments carried out in the dark clearly prove
10
4 J. Wolf, W. Stüer, C. Grünwald, H. Werner, P. Schwab and M. Schulz,
Angew. Chem., 1998, 110, 1165; T. R. Belderrain and R. H. Grubbs,
Organometallics, 1997, 16, 4001; T. E. Wilhelm, T. R. Belderrain, S. N.
Brown and R. H. Grubbs, Organometallics, 1997, 16, 3867.
the photochemical effect on the rate of reaction.
turned out to be the best solvent.11
2 2
Cl
(
(
iv) Refluxing CH
v) Under these photo-assisted conditions, the reactions
proceed very cleanly and reproducibly and neither activation by
hazardous diazo alkanes nor any special photochemical devices
or equipment is necessary.
5
For structural modifications of 1 see: S. Chang, L. Jones, C. Wang,
L. M. Henling and R. H. Grubbs, Organometallics, 1998, 17, 3460; E.
L. Dias and R. H. Grubbs, Organometallics, 1998, 17, 2758; T.
Weskamp, W. C. Schattenmann, M. Spiegler and W. A. Herrmann,
Angew. Chem., 1998, 110, 2631.
(
vi) Although the rate of reaction is slower than that obverved
with Grubbs carbenes, the isolated yields are good to excellent
and comparable to those obtained with 1 in all RCM
experiments carried out so far. Note that a slow initiation is a
disadvantage in polymerization reactions, whereas it is of minor
importance for RCM.
6 A. Fürstner, M. Picquet, C. Bruneau and P. H. Dixneuf, Chem.
Commun., 1998, 1315. This complex also responds to hn, cf. M. Picquet,
C. Bruneau and P. H. Dixneuf, Chem. Commun., 1998, 2249.
7
A. Demonceau, A. W. Stumpf, E. Saive and A. F. Noels, Macromole-
cules, 1997, 30, 3127; A. W. Stumpf, E. Saive, A. Demonceau and A. F.
Noels, J. Chem. Soc., Chem. Commun., 1995, 1127; A. Demonceau,
A. F. Noels, E. Saive and A. J. Hubert, J. Mol. Catal., 1992, 76, 123.
A. Hafner, A. Mühlebach and P. A. van der Schaaf, Angew. Chem.,
1997, 109, 2213.
(
vii) As can be seen from the examples compiled in Table 1,
this new procedure tolerates a wide range of functional groups
in the substrates, including esters, amides, ketones, ethers, silyl
ethers, acetals, glycosides, sulfones, sulfonamides, and even
unprotected secondary hydroxy groups.
8
9 Constant irradiation of the reaction mixture with a UV emitting quartz
lamp dipped into the solution at room temperature is inappropriate,
leaving 98% of the substrate unchanged after 16 h reaction time.
(
viii) This new procedure applies to all ring sizes including
medium-sized and macrocyclic ones. Furthermore it allows the
preparation of even trisubstituted olefins in the five- and six-
membered ring series as shown in entries 3–5 of Table 1.12
Finally—and most importantly—we would like to stress that
this new protocol is distinguished by a hitherto unrivaled ‘low
tech’ nature. It requires only commercially available reagents,
avoids the (multi-step) preparation of peculiar complexes as
well as the use of hazardous additives and needs no special
equipment whatsoever; importantly, however, its efficiency and
scope favorably compare with all current alternatives described
in the literature. Therefore we believe that this extremely
simple, yet highly productive methodology may help to increase
the popularity of RCM even further and should find many
applications in advanced organic synthesis.
1
1
0 Reaction of N,N-bis(allyl)toluene-p-sulfonamide in refluxing CH
2 2
Cl
with 4 (2.5 mol%) and PCy (5.5 mol%) in the dark shows that 92% of
3
the substrate (GC) is unchanged after 16 h. Note that after this period of
time, the photoassisted reaction is quantitative.†
1 RCM can also be carried out in toluene at 80 °C; in this medium,
however, the reactions tend to be less selective as shown by the
following prototype experiment:
Studies to elucidate the mechanism of the photochemical
activation step on a molecular level are underway and will be
reported in due course.
1
2 For an in-depth study on the effect of substitution on RCM see: T. A.
Kirkland and R. H. Grubbs, J. Org. Chem., 1997, 62, 7310.
13 A. Fürstner and T. Müller, Synlett, 1997, 1010.
1
1
4 A. Fürstner, T. Gastner and H. Weintritt, J. Org. Chem., in the press.
5 A. Fürstner and K. Langemann, J. Org. Chem., 1996, 61, 3942; A.
Fürstner and K. Langemann, Synthesis, 1997, 792; A. Fürstner and K.
Langemann, J. Am. Chem. Soc., 1997, 119, 9130.
Notes and references
†
Representative procedure: A solution of N,N-bis(allyl)toluene-p-sulfon-
amide (1.00 g, 3.99 mmol), [(p-cymene)RuCl 4 (61 mg, 0.1 mmol) and
PCy (62 mg, 0.22 mmol) in CH Cl (40 ml) was refluxed under Ar for 16
h in a well-illuminated hood. The solvent was removed in vacuo and the
2
]
2
16 A. Fürstner and T. Müller, J. Org. Chem., 1998, 63, 424.
3
2
2
Communication 8/08810A
96
Chem. Commun., 1999, 95–96