New transformations from a 3-silyloxy 2-aza-1,3-diene: Consecutive Zr- mediated retro-Brook rearrangement and reactions with electrophiles

Article abstract of DOI:10.1016/S0040-4020(00)00310-0

A one-pot procedure for the transformation of the title compound to α- functionalized (silylated) and α,β-unsaturated secondary amides was described. The following steps were involved: a Zr-mediated retro-Brook rearrangement, selective deprotonation with n-BuLi on the organometallic intermediate, and trapping with electrophiles including alkyl and acyl halides and aldehydes. The electrophilic addition step occurred at a stabilized α-silyl carbanion center without affecting the near transition metal residue. In the case of aldehydes, the Peterson alkenation reaction took place on the transition metal complex in a highly stereoselective way. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4020(00)00310-0

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 4467±4472
New Transformations from a 3-Silyloxy 2-Aza-1,3-diene:
Consecutive Zr-Mediated Retro-Brook Rearrangement and
Reactions with Electrophiles
*
Vincent Gandon, Philippe Bertus and Jan Szymoniak
  Â
Reactions Selectives et Applications, CNRS and Universite de Reims, 51687 Reims Cedex 2, France
Received 21 February 2000; accepted 20 April 2000
AbstractÐA one-pot procedure for the transformation of the title compound to a-functionalized (silylated) and a,b-unsaturated secondary
amides was described. The following steps were involved: a Zr-mediated retro-Brook rearrangement, selective deprotonation with n-BuLi on
the organometallic intermediate, and trapping with electrophiles including alkyl and acyl halides and aldehydes. The electrophilic addition
step occurred at a stabilized a-silyl carbanion center without affecting the near transition metal residue. In the case of aldehydes, the Peterson
alkenation reaction took place on the transition metal complex in a highly stereoselective way. q 2000 Elsevier Science Ltd. All rights
reserved.
2-Aza-1,3-dienes have attracted much attention due to their
particular reactivity pattern and synthetic interest.1 Both
electron-rich and neutral azadienes have found a synthetic
use as 4p components in hetero Diels±Alder reactions,2 and
participated in some heterocyclization and nucleophilic
addition reactions.3 The recent development of new practi-
cal methods for the preparation of differently functionalized
2-azadienes could facilitate further investigations.1b,4 In this
respect, the imine and enamine reactivities of these
compounds are almost unknown. The organometallic reac-
tions involving the 2-azadiene system are restricted to the
regiospeci®c generation of lithioenamines.5 It appears of
current interest to explore the early transition-metal
chemistry of 2-azadienes, in order to activate them towards
a selective attack of suitable electrophiles and to induce
insertion reactions through organometallic intermediates.
In this matter, group IV metallocenes should be taken into
account. In fact, simpler titanocene- and zirconocene±imine
complexes have been demonstrated to undergo coupling
reactions with various polar and non-polar unsaturated
molecules.6
Results and Discussion
We recently reported the complexation of electron-rich
2-azadienes having trimethylsilyloxy group at C3 to
zirconocene (`Cp2Zr').7,8 As we have noticed, these reac-
tions involved an unprecedented transition metal mediated
retro-Brook rearrangement. For example, by using
1-phenyl-3-trimethylsilyloxy-2-aza-1,3-butadiene (1), the
reaction with a zirconocene equivalent (Cp2Zr(butene) or
Cp2ZrCl2 and Mg) in THF afforded the a-silylated amide
2 after hydrolysis (Scheme 1).
To gain some information on this surprising reaction, we
have performed deuterolysis and NMR monitoring experi-
ments. Thus, quenching of the reaction with D2O afforded
Scheme 1. (i) Cp2ZrCl2, 2 equiv. n-BuLi, THF, 2788C to rt or Cp2ZrCl2, Mg, THF, rt; (ii) NaHCO3 aq or D2O then NaHCO3 aq.
Keywords: zirconium; retro-Brook; azadiene; electrophilic addition.
* Corresponding author. Tel.: 133-03-2691-3244; fax: 133-03-2691-3166; e-mail: jan.szymoniak@univ-reims.fr
0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)00310-0

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V. Gandon et al. / Tetrahedron 56 (2000) 4467±4472
the Zr±C bond occurred, because of the concomitant easy
formation of the C1±Si bond.10
The above reactivity pattern of the azadiene 1 prompted us
to envisage further synthetic developments. In keeping with
this aim, we considered the presumed organozirconium
structure B. We reasoned that it might be readily deproto-
nated at C4, i.e. in the a position towards both silicon and
zirconaoxazoline moiety. In fact, the ability of silicon to
stabilize adjacent carbanions is well known.11 On the
other hand, 2-substituted 2-oxazolines can be metallated
and alkylated to afford synthons for carboxylic acids.12 By
assuming the structure B, a generation of an ef®ciently
stabilized a-carbanion should be anticipated. Trapping of
the carbanion by several electrophiles would provide
a-functionalized (silylated) secondary amides after
hydrolysis.
Scheme 2. (i) Cp2ZrCl2, 2 equiv. n-BuLi, THF, 2788C to rt; (ii) n-BuLi,
2788C, (iii) D2O, rt.
the monodeuterated a-silylated amide [2H]-2. Monitoring
the reaction by H NMR spectroscopy in THF-d8 indicated
1
a progressive formation of the rearranged organometallic
complex. On the other hand, no oxygen to carbon migration
took place using the nitrogen-free analog of azadiene 1, i.e.
1-phenyl-3-trimethylsilyloxy-1,3-butadiene as a substrate.
Based on these results, the rearrangement process as
shown in Scheme 1 has been proposed. In the initial stage
of the reaction, the zirconaaziridine complex A is formed,
similarly to the reaction involving simpler imines or
1-azadienes.6,9 Thereafter, the retro-[1,3]-Brook rearrange-
ment is indirectly induced by the strongly oxophilic
zirconium to afford the zirconaoxazoline complex B. In
this reaction, the cleavage of the Zr±N bond rather than
In order to test the viability of this hypothesis, the azadiene
1 was reacted with Cp2ZrBu2, as previously described.7 The
reaction mixture was then cooled to 2788C and treated with
1 equiv. of n-BuLi. After stirring for 1 h the electrophile
was added and the reaction carried out at room temperature.
Table 1. Tandem reactions of azadiene 1
a Yields of isolated products after column chromatography; Np: 2-naphthyl.

V. Gandon et al. / Tetrahedron 56 (2000) 4467±4472
4469
Scheme 3.
Initially, a deuterolysis was performed to examine the
metallation step. Quenching of the reaction with D2O gave
the bis(deuterated) a-silylated amide [2H,2H]-2 (>95% D)
(Scheme 2). In comparison with the monodeuteration
observed in the absence of n-BuLi (Scheme 1), this result
proved the complex B to be lithiated at C4 atom. Having
demonstrated the formation of the anion, some alkylation
reactions were performed, the representative examples are
shown in Table 1.
i.e. the dienylamide 11 (entry 10);19 (iv) in all cases, the
C±C double bond was formed in a highly stereoselective
way, leading predominantly (entry 8) or exclusively (entries
6, 7, 9 and 10) to the (E)-con®gurated products. The above
high stereoselectivity is noteworthy, since alkenes formed in
Peterson reactions are often mixtures of (Z),(E)-isomers.
The stereochemistry of the reaction can be explained within
the context of a rapid spontaneous elimination of b-oxido-
silane, often occurring if a stabilized a-silylcarbanion is
involved and lithium used as a counter cation. In our case,
the syn elimination from a markedly less crowded b-silyl
alkoxide may predominantly proceed to give the (E)-a,b-
unsaturated amide (Scheme 3). Finally, as a-silyl
carbanions react with acyl chlorides to afford a-silylketones
rather than the elimination products,20 we also tried to
perform some acylation reactions. Using acetyl chloride,
an almost complete desilylation occurred on hydrolysis to
afford the oxoamide 12 (Table 1). Similarly, the amido ester
of malonic acid (13) was obtained when methyl chlorofor-
mate was employed as the electrophile.
Using alkyl, benzyl and allyl bromides or iodides as
substrates, the reactions proceeded regioselectively at the
carbon atom.13 After basic (NaHCO3 aq) treatment, the
a-silylated amides 3±5 were isolated in 60% yield on
average starting from the azadiene 1 (Table 1, entries 2±
4).14 The above reactions appeared to be synthetically
equivalent to the alkylation of enolates of secondary amides
(a-silylated). In fact, the zironaoxazoline moiety should be
considered as a masked secondary amide functionality,
since the latter has been restored on hydrolysis.15 It is note-
worthy that, while the metal enolates of N,N-dialkylamides
are easily prepared and C-alkylated, those of N-alkylamides
are less common. On the other hand, C-silylated secondary
amides are of potential synthetic utility, as they can undergo
¯uoride promoted aldolization.16 These compounds are
rather dif®cult to obtain by direct silylation of secondary
amides which generally takes place at nitrogen.11b Finally,
an attempt to silylate the anion with Me3SiCl succeeded to
afford the bis(trimethylsilyl)amide 6, albeit in a lower yield
(Table 1, entry 5).
In summary, an example of a one-¯ask procedure for
activating a 3-silyloxy 2-azadiene towards the regio-
selective (at C4) attack of various electrophiles is reported.
The reaction begun with an unprecedented zirconium
induced retro-Brook rearrangement. Although the resulting
organometallic complex could not be isolated, labeling
experiments and NMR monitoring was indicative of a
2-(trimethylsilylmethyl)-zirconaoxazoline structure, which
was further evidenced by the reactivity pattern. Thus, the
intermediate compound could be successively deprotonated
and trapped by several electrophiles. This second addition
step should be formally regarded to be equivalent to the
coupling reactions of a secondary amide enolate, since
a-substituted (silylated) and a,b-unsaturated secondary
amides were isolated after hydrolysis. Within this context,
the zirconaoxazoline moiety appeared as an organometallic
masked secondary amide function. A remarkable feature of
these alkylation, acylation and even ole®nation reactions is
that they have occurred selectively on the complex, without
affecting the transition metal residue. Further studies on the
development of these new tandem reactions and their
application in synthesis are in progress.
The Peterson carbonyl alkenation reaction has proven to be
of general synthetic utility.17 Particularly, the use of
stabilized a-silylcarbanions has led to a number of versatile
transformations. The question arose about the feasibility of
the Peterson reaction by using a silyl anion attached to a
transition metal residue. We thought that it might be
envisaged in our case, as an ef®ciently stabilized a-silyl-
carbanion would be involved.18 Thus, the zirconocene
complex was prepared and lithiated as previously, and
reacted with benzaldehyde at 2788C to rt for 3 h. After
basic (NaHCO3) treatment, the a,b-unsaturated amide 7,
having a unique E con®guration of the double bond, was
obtained (Table 1, entry 6). Some other aldehydes were
tested. The Peterson reaction has been demonstrated to
occur from the complex and aromatic as well as aliphatic
aldehydes to provide the unsaturated amides 7±11.
Examination of Table 1 reveals several interesting features:
(i) the reaction took place starting from the relatively
crowded 2-naphthaldehyde and pivalaldehyde (entries 7
and 9); (ii) it gave a comparatively good yield using an
enolizable aldehyde (entry 8); (iii) the use of an a,b-
unsaturated aldehyde cleanly led to a 1,2-addition product,
Experimental
All reactions were carried out under Ar using vacuum line
techniques. THF was distilled under Ar from sodium-benzo-
phenone ketyl. Zirconocene dichloride was purchased from
Strem Chemicals. Other reagents were purchased from
1
Aldrich, and distilled under Ar prior to use. H and 13C
NMR spectra were recorded in CDCl3 using a Bruker AC

4470
V. Gandon et al. / Tetrahedron 56 (2000) 4467±4472
250 spectrometer. High and low resolution mass spectra (EI,
70 eV) were performed on a JEOL D300 apparatus. Column
¯ash chromatography was performed on silica gel 40±
63 mm or alumina 63±200 mm (Merck). Melting points
were determined with a BuÈchi capillary apparatus and
were not corrected.
Bis-deuterated N-benzyl(trimethylsilyl)acetamide ([2H],
1
[2H]-2). Selected data: H NMR d 4.31 (m, 1H), 1.75 (m,
1H); 13C NMR d 43.1 (CHD), 28.4 (CHD).
N-Benzyl-2-(trimethylsilyl)propionamide (3). 144 mg
1
(61%); oil; IR (®lm) n 1633, 1541, 1248 cm21; H NMR
d 7.35±7.25 (m, 5H), 5.48 (m 1H), 4.43 (d Jˆ5.5 Hz, 2H),
1.80 (q, Jˆ7.0 Hz, 1H), 1.22 (d, Jˆ7.0 Hz, 3H), 0.07 (s,
9H); 13C NMR d 175.3 (CvO), 138.8 (C), 128.4 (2CH),
127.8 (2CH), 127.2 (CH), 43.5 (CH2), 31.9 (CH), 9.8 (CH3),
23.0 (3CH3); MS m/z (%) 236 (M11, 33), 235 (46), 220 (18),
179 (26), 164 (27), 163 (20), 147 (27), 129 (16), 120 (100),
119 (32), 117 (36), 107 (90), 106 (47), 105 (28); High Reso-
lution MS: C13H21NOSi requires 235.1392, found 235.1356.
Reaction of azadiene 1 with zirconocene
To a solution of Cp2ZrCl2 (292 mg, 1 mmol) in THF (4 mL)
was added n-BuLi (0.8 mL, 2 mmol, 2.5 M hexanes solu-
tion) at 2788C. After the solution was stirred for 1 h at
2788C, azadiene 11b (219 mg, 1 mmol) in THF (3 mL)
was added slowly via syringe and the reaction mixture
was allowed to warm to room temperature for 2 h. At this
stage, deuterolysis or electrophilic additions might follow
(see below). Alternatively, the reaction mixture was
quenched by adding H2O or D2O (2 mL) followed by an
additional 1 h stirring. Thereafter, the mixture was treated
with ice-cold saturated aqueous NaHCO3 (4 mL) and
extracted with ether (3£20 mL). The extract was washed
with water and dried over Na2SO4. Column chromatography
on alumina with n-hexane/AcOEtˆ7:3 followed by MeOH
as the eluent afforded silylated amide 2 (H2O quenching) or
[2H]-2 (D2O quenching).
N-Benzyl-3-phenyl-2-(trimethylsilyl)propionamide (4).
187 mg (60%); solid; mp 1548C (from CCl4); IR (KBr) n
1
1630, 1566, 1240 cm21; H NMR d 7.35±7.15 (m, 8H),
7.06±6.94 (m, 2H), 5.35 (m, 1H), 4.46 (dd, Jˆ14.7,
6.4 Hz, 1H), 4.21 (dd, Jˆ14.7, 5.2 Hz, 1H), 3.18 (dd,
Jˆ14.0, 12.0 Hz, 1H), 2.73 (dd, Jˆ14.0, 3.0 Hz, 1H), 1.93
(dd, Jˆ12.0, 3.0 Hz, 1H), 0.14 (s, 9H); 13C NMR d 173.6
(CvO), 142.3 (C), 138.6 (C), 128.4 (2CH), 128.3 (4CH),
127.7 (2CH), 127.1 (CH), 125.9 (CH), 43.5 (CH2), 42.6
(CH), 33.2 (CH2), 22.6 (3CH3); MS m/z (%) 311 (M1X
,
64), 296 (17), 239 (19), 238 (100), 220 (21), 164 (17),
106 (26); High Resolution MS C19H25NOSi requires
311.1705 found 311.1687.
MetallationÐdeuterolysis protocol
The solution of the complex (see above) was cooled to
2788C and treated with n-BuLi (0.4 mL, 1 mmol, 2.5 M
hexanes solution). After stirring for 1 h at 2788C, D2O
(2 mL) was added, and the reaction mixture warmed to rt
(1 h). Workup as above afforded bis (deuterated) a-silylated
amide [2H,2H]-2.
N-Benzyl-2-trimethylsilyl-pent-4-enamide (5). 154 mg
1
(59%); oil; IR (®lm) n 1705, 1655, 1591 cm21; H NMR
d 7.37±7.23 (m, 5H), 5.86±5.78 (m, 1H), 5.45 (m, 1H), 5.04
(d, Jˆ17.2 Hz, 1H), 4.95 (d Jˆ9.9 Hz, 1H), 4.48 (dd,
Jˆ14.6, 5.7 Hz, 1H), 4.37 (dd, Jˆ15.0, 5.7 Hz, 1H), 2.59
(m, 1H), 2.16 (m, 1H), 1.75 (dd Jˆ11.5, 3.3 Hz, 1H), 0.09
(s, 9H); 13C NMR d 173.9 (CvO), 138.7 (C), 138.1 (CH),
128.6 (2CH), 128.0 (2CH), 127.4 (CH), 115.1 (CH2), 43.7
(CH2), 39.5 (CH), 31.2 (CH2), 22.6 (3CH3); MS m/z (%)
261 (M1X, 5), 246 (5), 190 (17), 189 (100), 188 (20), 172
(6), 164 (10), 160 (12); High Resolution MS C15H23NOSi
requires 261.1549 found 261.1557.
General procedure for the electrophilic addition
reactions
The solution of the complex (see above) was cooled to
2788C and treated with n-BuLi (0.4 mL, 1 mmol, 2.5 M
hexanes solution). After the reaction mixture was stirred
for 1 h at 2788C, the solution of electrophile (1 mmol) in
THF (2 mL) was added via syringe. The mixture was stirred
at this temperature for 3 h. The basic (NaHCO3 aq) workup
as above, followed by ¯ash chromatography, afforded
compounds 4±13 (Table 1).
N-Benzyl-bis(trimethylsilyl)acetamide (6). 88 mg (30%);
1
oil; IR (®lm) n 1616, 1541, 1248 cm21; H NMR d 7.35±
7.20 (m, 5H), 5.51 (m, 1H), 4.37 (d Jˆ5.7 Hz, 2H), 1.29 (s,
1H), 0.12 (s, 18H); 13C NMR d 173.4 (CvO), 139.1 (C),
128.5 (CH), 127.9 (CH), 127.2 (CH), 126.2 (CH), 125.3
(CH), 43.8 (CH2), 33.2 (CH), 0.1 (6 CH3); MS m/z (%)
293 (M1X, 100), 278 (78), 187 (51), 164 (51), 147 (74),
135 (42), 107 (97), 106 (55), 105 (50); High Resolution
MS C15H27NOSi2 requires 293.1631 found 293.1631.
Selected experimental data
N-Benzyl(trimethylsilyl)acetamide (2). 151 mg (70%);
1
oil; IR (®lm) n 1630, 1543 cm21; H NMR d 7.35±7.20
N-Benzyl-3-phenylacrylamide (7).21 119 mg (50%);
eluent: n-hexane/AcOEtˆ85:15, Rf 0.20.
(m, 5H), 5.45 (m, 1H), 4.42 (d, Jˆ5.3 Hz, 2H), 1.80 (s,
2H), 0.12 (s, 9H); 13C NMR d 171.9 (CvO), 138.7 (C),
128.6 (2 CH), 127.9 (2 CH), 127.4 (CH), 43.8 (CH2), 29.2
(CH2), 21.3 (3 CH3); MS m/z (%) 222 (M11, 76), 221 (72),
206 (53), 205 (27), 164 (22), 147 (18), 115 (22), 107 (100),
106 (62), 105 (57); High Resolution MS: C12H19NOSi
requires 221.1236 found 221.1236.
N-Benzyl-3-(2-naphthyl)acrylamide (8). 167 mg (58%);
solid; mp 1568C; eluent: n-hexane/AcOEtˆ7:3, Rfˆ0.60;
1
IR (KBr) n 1655, 1618, 1543, 1253 cm21; H NMR d
7.93±7.75 (m, 6H), 7.69±7.59 (m, 1H), 7.56±7.45 (m,
2H), 7.41±7.23 (m, 4H), 6.54 (d, Jˆ15.6 Hz, 1H), 4.61 (d,
Jˆ5.7 Hz, 2H); 13C NMR d 165.8 (CvO), 141.5 (CH),
138.2 (C), 133.9 (C), 132.2 (C), 129.4 (CH), 128.7 (CH),
128.6 (CH), 128.4 (CH), 127.9 (2 CH), 127.7 (CH), 127.6
Mono-deuterated N-benzyl(trimethylsilyl)acetamide ([2H]-
2). Selected data: 1H NMR d 4.40 (m, 1H); 13C NMR d 43.5
(CHD).

V. Gandon et al. / Tetrahedron 56 (2000) 4467±4472
4471
Trost, B. M., Fleming, I., Eds. Pergamon, Oxford, 1991, p 451.
(b) Ghosez, L.; Bayard, P.; Nshimyumukiza, P.; Gouverneur, V.;
Sainte, F.; Beaudegnies, R.; Rivera, M.; Frisque-Hesbain, A.-M.;
Wynants, C. Tetrahedron 1995, 51, 11021. (c) Barluenga, J.;
(CH), 126.9 (CH), 126.6 (CH), 123.5 (CH), 120.5 (CH),
43.9 (CH2); MS m/z (%) 287 (M1X, 55), 182 (100), 181
(88), 154 (42), 153 (59), 152 (70), 127 (19), 106 (61);
High Resolution MS C20H17NO requires 287.1310 found
287.1313.
Â
Joglar, J.; Gonzalez, F. J.; Fustero, S. Synlett 1990, 129.
(d) Gouverneur, V.; Ghosez, L. Tetrahedron 1996, 52, 7585.
(e) Ghosez, L.; Jnoff, E.; Bayard, P.; Sainte, F.; Beaudegnies, R.
Tetrahedron 1999, 55, 3387. (f) Jnoff, E.; Ghosez, L. J. Am. Chem.
Soc. 1999, 121, 2617.
(E)-N-Benzyl-pent-2-enamide (9a). 83 mg (44%); oil;
eluent: n-hexane/AcOEtˆ8:2, Rfˆ0.30; IR (CHCl3) n
1
1667, 1516 cm21; H NMR d 7.40±7.20 (m, 5H), 6.92 (dt,
2. Boger, D. L.; Weinreb, S. M. Hetero Diels±Alder Methodology
in Organic Synthesis, Academic: London, 1987.
Jˆ15.3, 6.1 Hz, 1H), 5.96 (m, 1H), 5.79 (dt, Jˆ15.3, 1.5 Hz,
1H), 2.20 (m, 2H), 1.05 (t, Jˆ7.3 Hz, 3H); 13C NMR d
166.0 (CvO), 146.5 (CH), 138.3 (C), 128.6 (2CH), 127.8
(2CH), 127.4 (CH), 122.4 (CH), 43.5 (CH2), 25.0 (CH2),
12.4 (CH3); MS m/z (%) 190 (M11, 38), 189 (50), 161
(42), 160 (100), 117 (25), 107 (53), 106 (91); High Resolu-
tion MS C12H15NO requires 189.1154 found 189.1165.
3. (a) Bandini, E.; Martelli, G.; Spunta, G.; Panunzio, M.;
Piersanti, G. Tetrahedron: Asymmetry 1999, 10, 1445. (b) Bongini,
A.; Panunzio, M.; Bandini, E.; Martelli, G.; Spunta, G. J. Org.
Â
Â
Chem. 1997, 62, 8911. (c) Barluenga, J.; Carlon, R. P.; Pelaez,
E.; Joglar, J.; Ortiz, F. L. Tetrahedron 1992, 48, 9745.
4. (a) Panunzio, M.; Zarantonello, P. Org. Process Res. Dev. 1998,
2, 49. (b) Lasarte, J.; Palomo, C.; Picard, J. P.; Dunogues, J.;
Aizpurua, J. M. J. Chem. Soc., Chem. Commun. 1989, 72.
5. (a) Wender, P. A.; Schaus, J. M. J. Org. Chem. 1978, 43, 782.
(b) Martin, S. F.; Phillips, G. W.; Puckette, T. A.; Colapret, J. A.
J. Am. Chem. Soc. 1980, 102, 5866.
(Z)-N-Benzyl-pent-2-enamide (9b). 15 mg (8%), oil;
eluent: n-hexane/AcOEtˆ8:2, Rfˆ0.40; IR (CHCl3) n
1
1666, 1504 cm21; H NMR d 7.40±7.20 (m, 5H), 6.02 (dt
Jˆ11.4, 7.6 Hz, 1H), 5.75 (m, 1H), 5.68 (dt, Jˆ11.4, 1.5 Hz,
1H), 2.70 (m, 2H), 1.06 (t, Jˆ7.3 Hz, 3H); 13C NMR d
166.3 (CvO), 147.8 (CH), 138.3 (C), 128.7 (2 CH), 127.9
(2 CH), 127.5 (CH), 121.3 (CH), 43.3 (CH2), 22.2 (CH2),
13.8 (CH3); MS m/z (%) 189 (M1X, 27), 160 (100), 117 (8),
106 (37), 104 (9); High Resolution MS C12H15NO requires
189.1154 found 189.1159.
6. (a) Ito, H.; Taguchi, T.; Hanzawa, Y. Tetrahedron Lett. 1992,
33, 4469. (b) Buchwald, S. L.; Watson, B. T.; Wannamaker, M. W.;
Dewan, J. C. J. Am. Chem. Soc. 1989, 111, 4486. (c) Jensen, M.;
Livinghouse, T. J. Am. Chem. Soc. 1989, 111, 4495.
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41, 3053.
8. For reviews of `Cp2Zr' chemistry, see: (a) Negishi, E.;
Takahashi, T. Bull. Chem. Soc. Jpn 1998, 71, 755. (b) Negishi,
E.; Kondakov, D. Y. Chem. Soc. Rev. 1996, 26, 417. (c) Negishi,
E.; Takahashi, T. Acc. Chem. Res. 1994, 27, 124. (d) Negishi, E.
Chem. Scripta 1989, 29, 457.
N-Benzyl-3-tert-butylacrylamide (10). 102 mg (47%) solid;
mp 1018C (from hexane); eluent: n-hexane/AcOEtˆ85:15,
1
Rfˆ0.25; IR (KBr) n 1667, 1630, 1566 cm21; H NMR d
7.38±7.20 (m, 5H), 6.87 (d, Jˆ15.6 Hz, 1H), 6.08 (m, 1H),
5.70 (d, Jˆ15.6 Hz, 1H), 4.47 (d, Jˆ5.7 Hz, 2H), 1.05 (s,
9H); 13C NMR d 166.3 (CvO), 155.1 (CH), 138.3 (C),
128.7 (2CH), 128.1 (2CH), 127.5 (CH), 118.5 (CH), 43.6
(CH2), 33.4 (C), 28.8 (3CH3); MS m/z (%) 218 (M11, 10),
217 (11), 209 (9), 161 (19), 160 (100), 117 (9), 111 (30), 106
(24); High Resolution MS C14H19NO requires 217.1467
found 217.1459.
È
9. Scholz, J.; Kahlert, S.; Gorls, H. Organometallics 1998, 17,
2876.
10. A retro-[1,4]-Brook rearrangement (C4±Si bond forming
process) exclusively occurred starting from the more crowded
1,4-diphenyl-3-trimethylsilyloxy-2-aza-1,3-butadiene. In this case,
similar structural environment at C1 and C4 in the substrate led
to the generally easier Zr±C bond-breaking.
(E,E)-N-Benzyl-4-methyl-5-phenylpenta-2,4-dienamide
(11). 105 mg (38%); solid; mp 1418C (from CCl4); eluent:
n-hexane/AcOEtˆ85:15, Rfˆ0.20; IR (KBr) n 1655, 1604,
1566 cm21; 1H NMR d 7.48 (d, Jˆ15.6 Hz, 1H), 7.40±7.20
(m, 10H), 6.82 (s, 1H), 6.21 (s, 1H), 5.99 (d, Jˆ15.6 Hz,
1H), 4.54 (d, Jˆ5.7 Hz, 2H), 2.02 (s, 3H); 13C NMR d 166.1
(CvO), 146.4 (CH), 138.3 (C), 137.9 (CH), 136.9 (C),
133.8 (C), 129.4 (2CH), 128.7 (2CH), 128.2 (2CH), 127.9
(2CH), 127.5 (CH), 127.4 (CH), 119.7 (CH), 43.8 (CH2),
13.8 (CH3); MS m/z (%) 277 (M1X, 16), 141 (37), 128 (100),
115 (64), 104 (52); High Resolution MS C19H19NO requires
277.1467 found 277.1470.
11. (a) Panek, J. S. Comprehensive Organic Synthesis, Vol. 1,
Trost, B. M., Fleming, I., Eds.; Pergamon, Oxford, 1991, p. 580.
(b) Colvin, E. W. Silicon in Organic Synthesis, Krieger, R. E.
Publishing Company, Malabar, FL, 1985.
12. Meyers, A. I.; Mihelich, E. D. Angew. Chem. 1976, 88, 321;
Angew. Chem., Int. Ed. Engl. 1976, 15, 270.
13. This feature corroborates the total regioselectivity of the
Meyers C-alkylation of 2-oxazolines, see Ref. 12.
14. The structures of silylated amides were con®rmed by removal
of the a-trimethylsilyl group, which could be readily accomplished
by either brief heating with NaOH±MeOH±H2O or by treatment
with KF in MeOH, see Woodbury, R. P.; Rathke, M. W. J. Org.
Chem. 1978, 881.
N-Benzyl-3-oxobutyramide (12).22 74 mg (39%); eluent:
n-hexane/AcOEtˆ1:1, Rfˆ0.20.
N-Benzyl-carbamoylmethylacetate (13).23 125 mg (61%);
15. We have proved the complex to be quite inert towards
insertion of carbonyl compounds and even isocyanides into the
Zr±C bond.
16. Werner, R. M.; Barwick, M.; Davis, J. T. Tetrahedron Lett.
1995, 36, 7395.
17. Ager, D. J. Synthesis 1984, 384.
eluent: n-hexane/AcOEtˆ1:1, Rfˆ0.30.
18. Lithiated 2-silylmethyl-1,3-oxazines have been demonstrated
to react ef®ciently with aldehydes, see Sachdev, K. Tetrahedron
Lett. 1976, 4041.
References
1. (a) Boger, D. L. Comprehensive Organic Synthesis, Vol. 5,
19. To elaborate the 1,3-diene system via the Peterson reaction,

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V. Gandon et al. / Tetrahedron 56 (2000) 4467±4472
the reactions of a-silylallylanions with saturated aldehydes are
generally employed.
22. Casadei, M. A.; Cesa, S.; Inesi, A.; Franco, M. J. Chem. Res.
1995, 5, 1064.
23. Garcia, M. J.; Rebolledo, F.; Gotor, V. Tetrahedron 1994, 50,
6935.
20. Chan, T. H.; Chang, E.; Vinokur, E. Tetrahedron Lett. 1970,
1137.
21. Kazuyoshi, T.; Kanoko, T.; Hiroaki, T.; Rie, S.; Masumi, T.;
Haruo, O. Chem. Pharm. Bull. 1989, 37, 2334.