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Lewis Acid Promoted [2 + 2] Cycloaddition of Allylsilanes and Unsaturated Esters A Novel Method for Cyclobutane Construction.

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the ring closure to give the oxetane does not seem to
play a major role; however. in the case of 7 the formation of the
(complexed) oxetane 8 (Scheme 2, route A) is the simplest hypothesis."] There are some examples[*] in the chemistry of oxetanes['l for the subsequent acid-catalyzed ring opening. They
show, as expected, that the most stable carbocation is formed
after the cleavage of the four-membered ring. Accordingly, 8
should open to give the intermediate 9 with a cationic benzylic
group. which produces 3a or 3 b by a Grob fragmentation."I
The majority of these reaction steps are probably reversible.
Thus, for example, it is known that oxetane 11, which is made
by photoaddition of 1 to 2b and whose structure is regioisomeric with that of 8b. can be cleaved by acid to give acetaldehyde
as well as 1[*"' (Scheme 3). The olefination is, however. not
1 +Zb-
( cMe2C=CHMe)
MeCHO [ + M e 2 C = C W h ]
Scheme 3. Fragmentation of oxetane 11 [gal
dominated exclusively by the thermodynamically determined
position of equilibrium, since model calculations for the reaction 1 + 2 + 3 + 4 (Scheme 1) indicate strongly negative reaction enthalpies ( A H , ) and Gibbs energies (AC,) (with 2 a :
( A H , ) = 4.5;
( A H , ) = - 5.2;
(AC,) =
-4.6 kcalmol- ') that should lead to an almost complete displacement of the equilibrium to the right-hand side. The formation of 5 under extreme conditions seems to proceed via the ally1
cation 10, which can be generated (in a multistep process) from
7 (Scheme 2, route B) or from 9.
Acetone (4), which along with 3 was expected to be the second
fragmentation product, was not detected, presumably because
under these reaction conditions acetone rapidly reacts to form
the aldol condensation product, benzalacetone, which has been
identified in the reaction mixture (Scheme 1 ) . From this, the
first limitation of the new olefination process can be derived
directly: the carbonyl compound to be olefinated may not contain any a-hydrogen atoms, since otherwise, the aldol condensation dominates. Furthermore, the cation formed by protonation
of the carbonyl compound must not be ZOO stable; for instance.
no reaction was observed between benzophenone and 2a even at
100 "C, presumably because the reaction corresponding to the
conversion 6 -7 would be thermodynamically unhvorable for
the intermediate with a cationic diphenylhydroxymethyl unit.
The same applies to the olefin 2 used in the process whose
nucleophilicity may be neither too low nor too high. We draw
this conclusion from the observation that benzaldehyde did not
react either with I-octene o r with 1,l-diphenylethene. In the case
of 1 -octene, the attack of the olefin on 6 leading to the formation
of 7 is probably too unfavorable; for 1,l-diphenylethene, the
Lewis acid could either react directly with the olefin to form a
stable zwitterion, or 7 (with the cationic diphenylmethyl group!)
is so stable that ring closure to give the oxetane 8 does not
Further studies on the mechanism, as well as the optimization
and limitations of the new reaction sequence, are in progress.
Experimentul Procedure
In a typical experiment 1 (20 mmol) and EPZ-I0 (1 g) [3] (heated for about 15 h in
an oven at 130 C) were stirred in nitromethane (20 mL) under nitrogen at room
temperature. 2 (20 mmol) was added and the mixture stirred for an additional 44 h .
in the case of 2a. ii reflux condenser filled with acetone:dry ice was applied. After
VCH I/i,rlufi.igr.\ell.\c/iu~ m h H , D-694.51 Wc,iiihrini, t994
completion of the reaction. the catalyst was separated by filtration. The yield in the
filtrate was determined by GC;MS analysis with nitrobenzene as an internal standard. The identity of the products was confirmed by 'H NMR spectroscopy of
samples purified by preparative gas chromatograhy: 3a (authetic sample); 3 b [lo].
5 a [I I] (because the yield w a s less than 1 'A, identification by mass spectrometry on14
was possible): 5b [12]. (E)-1,3-diphenyl-l-butene[4a]. benzalacetone (authetic sample)
Received: March 14. 1994 [Z 6757IEl
German version. A n p i , . Cheni. 1994. 106. 1703
[I] a ) G. Wittig, G. Geissler. Lwhrgr Ann. Chew. 1953. 580. 44 57: h) G. Wittig.
U. Schollkopf. Chem. Ber. 1954. 87. 1318-1330: c ) B. E. Maryanoff. A. B.
Reitz. Ch<m.Re)'. 1989. K9, 863-927.
1991. 165-167.
[2] S. H. Pine. G. S. Shen, H. Hoang.
[3] a) The best results were obtained by using the zinc chloride containing clak
catalyst Envirocat EPZ-I0 from Contracl Chemicals, Prescot. England. We
would like to thank Contract Chemicals for supplying the catalyst free of
charge. b) Further applications of EPZ-10: T. W. Bastock. J. H. Clark in Sprciulig Ch~viicu/.s,
lnnovuthis in hidustriul Sjnthrri.5 unrl Applications (Ed.: B .
Pearson). Elsevier. London. 1991. pp. 383-396. c) Nation-H (further applications of Nation-H: G . S. Olah. P. S. Iyer. G. K. S. Prakash. Svnfhesis 1986,
513-531) zinc chloride and p-toluenesnlfonic acid have similar product mixtures; however. the yields of 3 were lower.
[4] a) A. R. Taqlor. G. W. Keen, E. J. Eisenbraun, J Org. Chrm. 1977. 42. 3477
3480: h) M. J. Climent. A. C o m a . H. Garcia. S. Ihorra, J. Pnmo. Rec. Trar
Chini. PuwBu.s 1991. 110. 275 27X.
[5] D. R. Adams. S. P. Bhatnagar, Synthrsis 1977. 661-672.
[6] a ) H. W. Scheeren. R. W. M. Ahen. P. H. J. Ooms, R. J. F. Nivard, J Org.
Cheni. 1977. 42. 312X 3132; b) J. W. Scheeren. Reel. Trav. Chiin. Pu?s-Bus
1986. IOS, 71 -84: c) J. Mattay. K. Buchkremer, Heferocyclc,s 1988. 27. 21532165: d ) H. Sugimura. K. Osumi. E,/ruhedron L e / t . 1989, 30. 1571-1574.
[7] A closely related mechanistic variant would he. for example. the reaction o f 7
with a second benialdehyde molecule to give 1.3-dioxane, which i b often found
in Prins reactions. and its fragmentation to 3 and 4. This would. however. not
affect the important aspects of the reaction.
[S] a) G. Buchi, C. C. Inman. E. S. Lipinsky. J Am. Chem. Sor. 1954, 76, 4327
4331: h ) D. Scbarf. F. Korte. Tr.truhrdron Lerf. 1963, 821 -823: c ) S. A. Carr,
W. P. Weber. J O r g Chmi. 1985. SO. 2782-2785.
191 C . A. Grob. Aiigev~.Chmi. 1969, 81. 543 554; Angew Chem. fnr. E d Engl.
1969.8, 535-546.
[lo] C . L. Bumgardner, J A m . C'hein. Soc. 1961. 83. 4420-4423.
[ l l ] EPA!,VIH A4uc.s Specrrul Dutu Buw: V i l 1 [Natl. Stund. Ref: Dutu Ser. U S .
,Vur/. Bur. S f u d NSRDS-NBS 63. 1978. 4031.
[12] R. Gelin. B. Chantegrel. Bull Sor. C'him. Fr. 1971, 2527-2544.
Lewis Acid Promoted [2 21 Cycloaddition
of Allylsilanes and Unsaturated Esters: A Novel
Method for Cyclobutane Construction **
Hans-Joachim Knolker,* Gerhard Baum, and
Regina Graf
The [3+2] cycloaddition of allylsilanes, which was first described by us four years ago,"] has been established as an effcient method for the stereoselective construction of five-membered ring systems.[2- 4 1 The major achievement was the
suppression of the classical Sakurai reaction[51by using bulky
substituents at the silicon atom of the allylsilane.[21We report
herein a novel mode of reactivity for allylsilanes and a,b-unsaturated carbonyl compounds in the presence of a Lewis acid, the
[*I Prof. Dr. H.-J. Knolker. Dip].-Chem. R. Graf
lnstitut fur Organische Chemie der Universitiit
Richard-Willst~tter-Allee,D-76131 Karlsruhe (FRG)
Telehx. In[. code +(721)698-529
G. Baum
Institut fur Anorganische Chemie der Universitit Karlsruhe (FRG)
[**I Cycloadditions of Allylsilanes. Part 6. This work was supported by the
Deutsche Forschungsgemeinschaft (Gerhard-Hess-Forderpreis) and the
Fonds der Chemischen Industrie. Part 5 : 1131
0'70-0833:94;lS1S-16/2 6 IO.OO+ .2S;O
A n g w . Chem. I n ! . Ed. Engl. 1994, 33, N o . 15!1h
[2+ 21 cycloaddition of allylsilanes and a$-unsaturated esters.
The three types of reactions of allylsilanes with unsaturated
carbonyl compounds can be rationalized as presented in
Scheme 1 .
by the present findings, which describe for the first time" that
allylsilanes having bulky substituents at the silicon atom undergo Lewis acid promoted [2+ 21 cycloadditions with a,B-unsaturated esters leading to silylmethylcyclobutanes.
Initially. we investigated the [2 +2] cycloaddition of the allylsilanes 2 and the methyl acrylates 8. The cycloadditions of allyltriphenylsilane (2a) and allyltriisopropylsilane (2 b) with methyl
acrylate (8 a) afforded irrespective of the temperature a mixture
of the diastereomeric silylmethylcyclobutanes onfi-9 and .syn-9
(syn and anti specify the position of the silylmethyl side chain
relative to the methoxycarbonyl group) and minor amounts of
silylcyclopentane 10 (Table 1 ) . However, the range of products
a: R i = H
b: R i = M e
b: R = iPr
Scheme I Possible reactions for the n./l-unsaturated carbonyl compound 1 and
allylsilaney 2 in the presence ofTiC1,: Sakurai reaction (product 5 ; R = Me; X = H.
alkyl). [3+2] cycloaddition (product 6: R = Ph, iPr; X = alkyl). and
[ ? + 2 ] cycloaddition (product 7; R = Ph, iPr: X = Oalkyl)
Fable 1. Results of the cycloadditions of allylsilanes 2 and methyl acrylates 8
Nucleophilic attack of the allylsilane 2 is initiated by the
Lewis acid and takes place at the 8 position of the conjugated
carbonyl system 1. The positive charge of the resulting 8-silyl
cation 3 is stabilized by internal nucleophilic neighboring group
participation of the silicon atom, and a bridged nonclassical
pentavalent silicon cation ( = siliranium ion) 4 is generated. The
pentavalency of silicon in the siliranium ion is possible because
of its d orbitals.r61This contribution to the 8-effect of silicon has
been termed "non-vertical stabilization" by Traylor."' Intermolecular nucleophilic attack at the silicon atom of the cationic
intermediate either in its open form 3 or in the form of the
siliranium ion 4 provides the Sakurai product 5 after hydrolysis
and workup."] Sterically demanding substituents inhibit the nucleophilic attack at the silicon atom of either cationic species
and thus favor cyclization by intramolecular nucleophilic attack
of the titanium enolate. In the case of r,P-unsaturated ketones
nucleophilic attack of the titanium enolate occurs at the primary
carbon atom of the siliranium ion 4 (pathway a, 5-rxo-fet cyclization)I8l and thus results in a [3+2] cycloaddition of the
allylsilane involving a cationic 1,2-silyl shift to the silylcyclopentane 6.
Herein we describe cyclizations that are inititated by
nucleophilic attack of the titanium enolate at the secondary
carbon atom of the intermediate siliranium cation (pathway b,
4-exo-tet cyclization)r81which afford the silylmethylcyclobutanes 7. Two points are worth noting in this context and should
be mentioned in order to avoid any confusion. First, allyltrimethylsilanes failed to give conjugate additions in the
Lewis acid (TiCI,) promoted reaction with a,fi-unsaturdted carboxylic acid esters.[5a1Secondly. all proposed silylmethylcyclobutanes reported as by-products from the Lewis acid promoted reaction of conjugated enonesr9] were reassigned as
silylcyclopentanes.rlolThese reassignments are not challenged
Product ratio [a]
Yield ["h] firrrj-9,\ w d : 10
Reaction conditions
25 C , 1 d - 4 0 ° C I d
- 4 0 C + O <, 1 9 h
40 C 3 h
25 C
40 C. 4 d
-78 C + -20 C , 1 9 h 40
-78 C - 4 0 C, 1 9 h
I? 1
119 1
5 1
l I 1
5 4 1 13
9 1
X h
[a] The product ratios were determined by N M R spectroscopy
in the reaction of 8a with allyltriisopropylsilane (2 b) was significantly dependent on the temperature during the cycloaddition.
Addition of 2 b to the titanium tetrachloride complex of 8a at
-40 'C and subsequent reaction at 0 'C for 19 h quantitatively
provided the diastereomers anti-9 b and syn-9 b (1 : 1.25) together
with small amounts of 10 b. When the reaction was performed in
refluxing dichloromethane, a mixture of anti-9 b, s-yn-9b. and
10b was obtained in a ratio of 1.7:6.3:1 (97% yield). Therefore,
it can be concluded that higher reaction temperatures favor both
the cyclization of intermediate 4 (X = Oalkyl) to the synconfigurated silylmethylcyclobutane over the anti product, and the
formation of the silylcyclopentane product (cyclization according to pathway a). The same trend was observed in the cycloaddition of allyltriisopropylsilane (2 b) and methyl methacrylate
(8b). In this case the regioselectivity for the cyclization of the
intermediate siliranium ion is reversed in favor of the cyclization
leading to the corresponding cyclopentane derivative (path a).
Reaction at 40°C provided the silylcyclopentane 10d and the
two diastereomers of silylmethylcyclobutane 9d in a ratio of
2: 1 . Also for this example a higher proportion of the sjw-silylmethylcyclobutane isomer was obtained at a higher temperature.
Structural assignments of the silylmethylcyclobutanes 9 and
the silylcyclopentanes 10 are based on the 13C N M R data
(Table 2). The signals for the C atom CI to the silicon atom
(nSi-CH) appear for the silylcyclopentanes 10 at roughly 6 = 23.
formed by hydrogen bonding are composed exclusively of the
two opposite enantiomers (Fig. 2). Comparison of the I3C
NMR data of the acid of anti-9c with those of the other silylmethylcyclobutane derivatives confirms the structural assignments made above (cf. Table 2).
Table 2. Selected '.'C N M R data (100 MHr. CDCI,. d values) of the silylmcthylcyclohutancs 9, 12, and 15 [a].
\I 11
11 9
10.2: 13.3 [c]
34 3
31.4. 35.2 [o]
13.6 [d]
43 1
40.7 [d]
[a] Relcvsnt ' " C N M R data of further products for comparison. IOa: 22.Y (aS1CH); lob: 22.4 (rSi-CH): 1Oc: 23.3 (rSi-CH): IOd: 23.5 (2.7-CH): acid ofnnti-Yc:
14.7 ( z S I - C H ~ 37.6
) . ([ISi-CH): 14: 12.6 (rSi-CH,). 38.8 ([ISi-CH). [b] N o assipnment possible because of overlap *ith the signal of the triisopropylsilyl g o u p .
(minor isomer).
[c] ni? (major isomer). [d] <J17..5?.?7-15
Fig. 2. Arrangement of the dimer formed by ihe acid of anti-9c in the crystal.
Selected distances [A] and angles [ 1: 0 V H 1 1.16(6), 02-HI 1.47(6). 0 1 - 0 2
2.619(3); 0 1 - H I - 0 2 1 7 x 5 ) .
The N M R signals of nSi-CH groups in this type of compound
generally show up in the region of 6 = 20-25.[2,3. '"I The 13C
NM R signals of the xSi-CH, groups in the silylmethylcyclobutanes described here are, as expected, significantly shifted to
higher field, while the signal of the PSi-CH group appears in the
normal region. As anticipated, the signals of the anti-silylmethylcyclobutanes are shifted towards higher field relative to
their s j n isomers, in which the silylmethyl group is on the same
side of the four-membered ring as the methoxycarbonyl group.
For an unambiguous confirmation of the relative configuration
we required an X-ray analysis. In contrast to the triphenylsilylcyclopentanes. which crystallize very easily,'21most of the silylmethylcyclobutanes described here were obtained as oils. However, saponification of the triphenylsilylmethylcyclobutanes 9c
(KOH, H 2 0 , reflux. 1 d, 92%) afforded the corresponding acids
from which the major diastereoisomer exhibiting the ariti orientation was easily crystallized. Single crystals of the acid of anti9c were suitable for an X-ray crystal structure determination,
and the four-membered ring with the anti configuration of the
side chain was unequivocally confirmed (Fig. 1) . [ 1 2 ] The
racemic acid of anti-9c crystallized diastereospecifically; dimers
c 12
Addition of allyltriisopropylsilane (2 b) to the titanium tetrachloride complex of dimethyl maleate (11) at room temperature
and subsequent reaction at 40 "C for 19 h afforded in 73 YOyield
a mixture of the diastereomers anti-12 and syn-12 in a ratio of
25°C --C 40T, 19 h
c 17
4: 1. In this case lower reaction temperatures favored the formation of the syn diastereomer (25 "C, 5 d, 61 YOyield; a n t i - l 2 / s p
12 = 1 : 1.3). Structural assignments were again based on the
I3C chemical shift of the CH, group a to silicon, which is significantly shifted downfield for the syn diastereomer (Table 2).
We recently described a remarkable extension of the [3+2]
cycloaddition methodology of allylsilanes-the direct synthesis
of bicyclo[3.3.0]octanes by the domino [3 21 cycloaddition of
allylsilanes and 3-butyn-2-one." 31 This process generates four
C-C a-bonds in a one-pot reaction. Thus we expected that
direct access to the bicyclo[2.2.0]hexane framework should be
feasible by a domino [2 21 cycloaddition of allyltriisopropylsilane (2b) and propynoic ester 13. In this reaction the two
four-membered rings would be formed sequentially. the first by
a [2+2] cycloaddition of 2b to the Lewis acid complex of 13,
and the second by a [2+2] cycloaddition to the Lewis acid
complex of the intermediate cyclobutenecarboxylic acid ester
formed in the first step.
Addition of three equivalents of allyltriisopropylsilane (2 b) to
the titanium tetrachloride complex of methyl propynoate (13) at
-78 "C and subsequent reaction at -20 .'C for 19 h selectively
provided the cyclobutene 14 in 98 YOyield (Table 3). After reac-
Fig. 1. Molecu1;ir structure of the acid of o m - 9 c in the crystal. Selected bond
lengths [A]: C h C 2 1.571(4). C 2 - U 1.544(4). C3-C4 1.536(5). CI-C4 1.541(4),
C1-CS 1.486(4). C1-C6 1.531(4), C2-C7 1.518(3). C7-Si 1.874(3). CX-Si 1.873(3).
Ci4-Si 1.867(3). C20-Si 1.8733). CS-01 1.304(3). C 5 - 0 2 1.227(3).
Angcw. Cheiii. 1992. 104. 335: Aiigeiv. C%eni. h i . E d Eng/. 1992. 31. 313: g)
R. L. Danheiser. B. R. Dixon. R. W. Gleason, J. Urg. C h < w 1992. 57. 6094;
h) J. S. Panek. N. F. Jain, ihid. 1993.58. i) 2345; R. L. Danheiser. T. Takahashi,
B. Bertok. B. R. Dixon. T r f r o l i ~ ( / r of fei r r . 1993. 34. 384S.J)M.-J. Wu. JLY. Yeh.
syn, syn-15
Table 3. C')cloaddirion of allylsilane 2b and methyl propynoate (13)
Reaction conditions
14. Yield
3 equiv 2b. -7X C
-20 C. 19 h
4equi\2h. - 7 8 t - 2 5 C . 5 d
3equivZh.X C -40 C.19h
( i i i r r . \ i +IS:
d c r i w t n e were
15, Yield
46 [a]
64 [a]
3: 1. Traces o f the Sakurai product and a cyclopentane
\ w , . \ j ~ i i - l S=
tion at room temperature for fivedays. 14 and the bicyclo[2.2.0]hexane 15. the product of the domino reaction, were
formed i n approximately equal amounts. Cycloaddition of 2 b
with 13 in dichloromethane at reflux for 19 h provided 15 and
14 in yields of 6 4 % and 34%, respectively. Bicyclo[2.2.0]hexane
15 was obtained as a mixture of the anti,sw and the,syn
diastereomers in a 3:l ratio. The two diastereomers are easily
distinguished by their I3C N M R spectra. The major isomer
rmti.sjn-15 shows two signals for the aSi-CH2 groups, one anti
to the methoxycarbonyl group at 6 =10.2 and one syn at
6 = 13.3. The minor isomer syrz.syn-15 is symmetrical and therefore exhibits only one signal for the aSi-CH, group in the region
typical for the syn diastereoisomer (6 = 13.6). Thus, this
product was assigned the syr,sjn configuration with both silylmethyl groups on the same side as the methoxycarbonyl group.
Ewohi,dron 1994. 50, 1073.
a) A. Hosomi. H. Sakurai. J. A n ? . C / i e r i , . Suc. 1977, 9Y. 1673. G . Majetich.
A . M . Casares, D. Chapman. M. Behnke, E,rruhcr/ron Lcrr. 1983. 24, 1909; bl
T. A. Blumenkopf. C. H. Heathcock, J. Am. Chcm. So(.. 1983. 10.7. 2354. Reviews: H. Sakurai, Pirrr Appl. C/ieni. 1982. 54. 1. A. Hosonii, . 4 w . ('hem. Rc\.
1988. 21. 200: D. Schinrer. .Si~rrrhcsi.\ 1988. 263.
M. R. Ibrahim. W. L. Jorgenson. J Am. Clicm. Soc. 1989. 111. 819: J. B Lambert. Tctru/irdron 1990. 46. 2617; J. B. Lambert. E. C. Chelius. J. Am. C ' h e i n
Suc. 1990, 112. X120; J. M. White. G. B. Robertson. J. Org. C ' / i w i . 1992. 5 4638. J. B. Lambert. R. W. Emblidgc, S. Malany, J. Ain. Chiwi So< 1993. / I
W. Hanstein, H. J. Berwin. T. G. Traylor, J. Am. C/iwi. Soc. 1970. 92. 829: T. G.
Traylor. W Hanstein. H .I Berwin, N . A. Clinton. R. S. Brown. ihid. 1971. Y3,
J. E. Baldwin. J. Chmi. Snc. C/wn?.Ciniiniuii 1976. 734.
R. Pardo. J.-P. Zhara, M. Santelli, Glruhrr/roii f r r i . 1979. 45S7: A. Hosomi.
H. Kobayashi. H. Sakurai. ihid. 1980. 21. 955: S. Danishefsky. M. Knhn, ;hid
1981.72.485: H. 0. House, P. C. Gax. D. VanDerveer. J. Orx. U w f n .1983.4S.
1661; G. Majetich. J. Defauw. C Ringold. ihid. 1988, 53, SO: K Nickisch, H.
Laurent. E~rriihedronLrrr. 1988, 2Y, 1533: G. Majetich. K . Hull. D. Lowery.
C. Ringold. J. Defauw in Se/wrii~irir,.\iw L P Mi s Acid Proiiiorcd Rc~ucrion.\(Ed :
D. Schinrer). Kluwer. Dordrecht. 1989. p. 169; G. Majetich. JLS. Song. c'.
Ringold. G. A. Nemeth, fi,fru/ii4run L E I / .1990. 31. 2239.
H.-J. Knolker. N. Foitrik. R. Graf, J.-B. Pannek. P. G. Jones. 7 i ~ / r u / r r ~ / r o ? i
1993, 49, 9955.
a ) Photochemic:il [2+ 2 ] cycloaddition of allyltrimethylsilane\ and I .4-naphthoquinonea: M . Ochiai. M. Arimoto. E. Fujita. J,Chcm. Soic Chiwi. ~ ' i f r i n r n i i i .
1981, 460: b) [2+ 21 cycloaddition of allyltrimethylsilane\ and tetracyanoethylene: G. D. Hartmann, T. G. Traylor, 7 i ~ t r a l i d r o fLcrr.
1975. 939 and
ref. [4h]: c) Lewis acid mediated [2 +2] cycloaddition of 3-methylthio-4~rimethylsilyl-1.I-butadiene and alkenes: M . Hojo. K tom it;^. Y. Hii-ohai-a,A .
Hosomi. ~ ~ r r u h ~ ~ dLrrr.
r u r i 1993. 34, 8123.
Crystal structure analysis of the acid of unrr-9c:C,,H,,O,Si. monoclinic, space
group P2,:ii. u = 9.445(3). h = 19.035(7), ( ' =12.235(5) A. /J = 96.90(3)
1.' = 2183.7(14) A'.
Z = 4. { J ~ ~ , ~=, , 1.176 gcm-',
7 = 200(1) K. 11 =
0.124mm-', I = 0.71069 A; 20,,,,, = 52 4009 independen! reflections: relinement method: full-matrix least squares on F'; R , = 0.0507. wRZ = 0.1326:
maximal residual electron density: 0.477 e k ' . Programs: C; M Sheldrick.
SHELXS-86 (Gottingen 1986). SHELXL-93 (Gottingen 19931, E. Keller.
SCHAKAL-92 (Freiburg I Br.. 1992). Further details of the crkstal structurc
investigation may be obtained from the Fachinformationszentrum Karlsruhe
D-76344 Eg~enstein-Leopoldshafen.on quoting the depository number CSD58304.
H:J. Knolker. R. Graf. S~.nlrrt1994. 131.
15: A b o l u t i o n of methyl propynoate (13) (137 pL. 339 ing. 4.03 mmol) in dry
dichloromethane ( 5 mL) was added to a stirred solution of titanium tetrachloride
(490 pL. 840 mg. 4.43 inmol) in dry dichloromethane ( 5 mL) at room temperature.
To this ini\ture was added a solution ofallyltriisopropylsilane (2b) (2.92 mL. 2.40 g.
13.08 miiio) i n dry dichloromethane ( 5 mL). The reaction mixture was refluxed for
19 h and tlieii quenched by addition of an aqueous solution ofamrnonium chloride.
The organic layer was separated, the aqueous layer was extracted three times with
dichlorometh;rne. and the combined organic layers were dried over magnesium
sulfatc. E\;iporatioii of the solvent m d flash chromatography (hexane:ether 1 5 : l )
of the rcsidue o n silica gel provided the bicyclo[2.2.0]hexane 15 (1.24 g. 64"/0) and
the cyclobutene 14 (387 mg, 34%) both as colorless oils.
Rcceived: March 15, 1994 [Z6760lE]
German version' Angew. Chmii. 1994, 106. 1705
[ l ] H.-J. Knijlker, P. G. Jones. J.-B. Pannek. Svniftr 1990. 429: H.-J. Knolker in
Orginiw LSmr/ies;.s i,iu Orgrii~omeru//ic.s
(Eds.: K. H . Diitr, R . W. Hoffmann).
Vieweg. Braunschweig, 1991, p. 119: H.-J. Knolker, P. G. Jones. J.-B. Pannek,
A. Wcinkauf, Sjnlerr 1991. 147: H.-J. Knolker, ihid. 1992, 371.
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The Formation and an Unusual Dimerization
of 2-Mesityl-l,l-bis(trimethylsilyl)silene**
Clemens Krempner, Helmut Reinke, and
Hartmut Oehme"
The reaction of tris(trimethy1silyl)silylmagnesium bromide 1
with aldehydes and ketones makes accessible l-hydroxyalkyltris(trimethylsilyl)silanes, which have proven useful as precursors for the synthesis of silenes by a modified Peterson mechanism. Recently we have reported the formation of 2-rertbutyl-l,I -bis(trimethylsilyl)silene by the base-initiated elimination of trimethylsilanolate from 2,2-dimethyl-l-tris(trimethylsily1)silylpropanol and its head-to-head cyclodimerization to the
corresponding 1,2-disila~yclobutane.~~~
[*I Prof. Dr. H. Oehme. DipLChem. C. Krempner. Dr. H. Reinke
Fachbereich Chemie der Universitiit
D-IXOSI Rostock ( F R G )
Telefax: Int. code (381)498-1763
This work was supported by the Dcutsche Forschungsgemeinschaft and the
Fonds der Chcmischen Industrie.
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Размер файла
476 Кб
acid, promote, allylsilanes, cycloadditions, esters, construction, method, unsaturated, novem, lewis, cyclobutane
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