Organometallics 1983,2, 184-185
184
stannylcuprates enter into a dynamic equilibrium with ethyl butynoate. The equilibrium can be driven to the product side only with protons as electrophiles. The stannylcuprates are protonated by trifluoroacetic acid (100 % ) and acetic acid (-50%) but not by methanol. Thus, contrary to intuition, the conjugate acid of a stannylcuprate has a pKa of -5.
Piers discovered that stannylcuprates can be added to alkynoate esters.lB2 When the reaction was carried out at low temperature, it was found to produce the E ester (1) stereospecifically and exclusively.
b
Figure 2. Molecular structure of RuzCoz(CO)ll(PhzCz)(2): small circles, CO ligands and C atoms of PhzCz. Bond lengths (pm): Rul-Ru2 = 275.7 (l), Rul-Col = 260.7 (l), RuS-Col = 258.7 (l), Rul-Co2 = 261.4 (l), Ru2-Co2 = 257.2 (l),C0l-C02 = 352.4 (1). Acetylenic ligand distances (pm): Rul-C = 216.6 (3), Col-C = 210.1 (3), C02-C = 210.2 (3) (front C), Ru2-C = 227.8 (3), Col-C = 204.8 (3), Co2-C = 202.4 (3) (rear C), C-C = 143.2 (5).
occupying the "best" position bridging the Ru-Ru edge and the other one bridging a RuCo2face.'* Both reactions are basic steps of substrate activation by clusters. And in both products the reactive sites for the respective reagent are still available. The relation of these observations to bimetallic catalysis is obvious. Subsequent reactions such as the expulsion of H2 from 1 by adding CO or the addition of acetylenes to 1 and of hydrogen to 2 suggest themselves. These reactions require more forcing conditions, thereby leading to complicated product mixtures. Further work in optimizing and extending the model character of the Ru2C02clusters is in progress.
R3Sn(L)CuLi"
t
-
R'C E C - C O z E t
A
1
Piers also noted that the quenching reagent (MeOH) could be added concurrently with the alkynoate to the stannylcuprate without sacrifice in yield or stereoselectivity but did not comment further on this observation. When we tried to apply this reaction to the preparation of compounds of general formula 2, by addition of an electrophile (Y-C1 or Y-Br) to cuprate A at -78 "C, followed by quenching with methanol, we were surprised to observe the results tabulated in Table I. The salient R'
P
'c=c
Z
E
'
\v
E3S/ n
2, y = Br, R,Sn, HgCl
Acknowledgment. This work was supported by the Fonds der Chemischen Industrie and by the Rechenzentrum der Universitat Freiburg.
features were that (a) only protons seemed to quench A efficiently, something that Piers had also observed when Registry No. 1, 83830-89-1; 2, 83830-90-4; R u , C O ~ ( C O ) ~ ~ , he tried to trap A with various organic electrophiles (CH,I, 78456-90-3; Hz,1333-74-0; PhzC2, 501-65-5; CO, 7440-48-4; Ru, ketones, et^),^ and (b) even though formation of A is es7440-18-8; 3-hexyne, 928-49-4. sentially complete in ca. 15 min at -78 to -48 "C, substantial amounts of alkynoate were recovered when powSupplementary Material Available: Listings of the complete erful electrophiles such as bromine or mercuric chloride crystallographic details, all positional and anisotropic thermal were used. parameters, all bond lengths and angles, observed and calculated To account for these observations, we were forced to structure factors, and complete molecular drawings with atom numbering for both structures (47 pages). Ordering information suggest two unprecedented phenomena: (1)addition of is given on any current masthead page. "R3Sn(L)CuLi" to CH,C=C-CO2Et is reversible and (2) the conjugate acid of "R3Sn(L)Cu-Li+"("R,Sn(L)CuH", R = Me, Bu) is a relatively strong acid (or conversely (18)A "better" position for H2, e.g., Ru-Ru bridging or RuPCo "R,Sn(L)Cu-Li+" is a very weak base in THF). This is bridging, seems unlikely due to steric reasons. depicted in Scheme I. Scheme I R3SnLi t CuLBr
Two Unprecedented Observations in Organocuprate Chemistry: Reverslbllity of Addition to an Alkyne and Low pK, of a Stannylcuprate
-
"R3SnLCuLi"
H3C,
S. D. Cox and F. Wudl'
"R3Sn(L)CuLi" t
MeCEC-COPEt
Bell Laboratories Murray Hill, New Jersey 07974
"R3Sn(L)CuLi"
/c=C R3Sn t
Received August 17, 1982 H3C,
Summary: The addiiion of organocuprates to unsaturated carbonyl-substituted systems is assumed always to proceed irreversibly and stereospecifically and to proceed probably via electron transfer. Here we report that
* To whom correspondence should be addressed at the Department of Physics, University of California, Sank Barbara, CA 93106.
0276-7333/83/2302-0l84$01.50/0
( 2)
i = P h S . "PhzP(OZ)", Me23 - 3 r
/c=c
R3Sn
/COZE~
t >C"LI
Ht
MeOH
-
-
h3C
YR
\
/c=c GjSr
COZEt / \ L-CULI
(3)
(4) / \
COzEt
(5) H
L
(1)Piers, E.; Morton, H. E. J. Org. C h e n . 1980, 45, 4263. (2) Piers, E.; Chong, J. M.; Morton, H. E. Tetrahedron Lett. 1981,22, 4905.
(3) Piers, E.; Chong, J. M. J. Org. Chem. 1982, 47, 1602.
0 1983 American Chemical Society
Organometallics 1983, 2, 185-187
185
Table I entry
quenching reagent
reactn time, h
CH,C=C+!O,Et,
(CH, ),Sn( CH,)C= C(H)CO,Et, %
%
60 50
20 26
e
20
< 5c
0 0 0
e
5a
100 100 60-70d
a -78 “C.
-48 “C. About 30% of organomercurials was obtained; the structure of these is currently being determined, 30-40% of (E)-distannylcrotonate was formed. This same compound was obtained by Piers when excess stannylcuprate was allowed t o react with ethyl butynoate., e Reaction of stannylcopper reagent with ethyl butynoate was allowed t o proceed for 2 h a small aliquot was quenched with MeOH and worked up. Table I1 quenching reagent
entry 1
2 3 4 5 6 7
CH,OH CH,OH CH,COOH CF,COOH (TFA) CF,COOH (TFA) CF,COOH (TFA) CF,COOH (TFA) CF,COOH (TFA)
addn. reactn, ratioC time: time,b sm/ pKSd min min prod 16 16 5 0 0 0 0 0
-2 120 -10 -10 -0.1 0.5 120 1000
30 10 30 30 60 30 60 30
30170 5/95 45/55 100/0 85/15 55/45 20/80 25/75
a Addition time relative t o addition of ethyl 2-
butynoate. Reaction time a t -45 t o -50 “C after last Ratio from integration of methyl peaks in addition. NMR of product mixture, SM, MeC=CCO,Et; prod, ( E ) Me,SnC(Me)=C(H)CO,Et. Not determined in THF but for comparison purposes.
The stannylcuprate is in quotations because its structure is unknown. It probably is a cluster compound. Suffice it to say that it has the reactivity pattern of an organocuprate. Whereas RBSnLiadds to enones? it does not add to ethyl butynoate under the conditions of reaction 1 in the absence of CUI;thus, copper’ is essential for this reaction, but the nature of ligands (L)4is not nearly as critical but has some effect on stereochemistry.2 To learn if indeed the reaction described by eq 3 is an equilibrium, we performed experiments shown as entries 3 and 4 in Table I. That is, if one removes an aliquot from the reaction mixture after addition of the butynoate but before addition of Br2 or HgCl, and quenches it with methanol, it shows no butynoate left (by GPC only 1 is present). Yet, after the addition of 1 equiv of either electrophile to the original reaction mixture (followed by methanol quench and workup, there is 60% of butynoate and 20% 1 ! The only explanation consistent with these results is that both reactions 3 and 5 are fast, that ( 5 ) is irreuersible, that ( 3 ) is reversible (Br, and HgCl, destroy the stannylcuprate more rapidly than they react with A, thus “driving” the equilibrium to the left), and that the stannylcuprate is stable to methanol so that with a fast and irreversible reaction (eq 5),2 equilibrium 3 is “driven” to the right by methanol. The question then arose: how low must the acidity of the medium be before a stannylcuprate is quenched as fast as its addition product A? The surprising answer was that it required trifluoroacetic acid (entries 3 and 4, Table 11) to quench the stannylcuprate rapidly. Entries 6 and 7 represent a “snapshot” of the equilibrium mixture of stannylcuprate and A, assuming that TFA reacts as rapidly with A as with the stannylcuprate. Therefore, on the basis of the experiments presented above and the results shown in the tables, stannylcuprates enter into a fast, reversible reaction with butynoates in (4) Bertz,
S. H.; Dabbagh, G.; Villacorta, G. M., in press.
0276-7333/83/2302-0185$01.50/0
THF at -50 to -45 “C and the “pK,” of “Me3SnCu(Me2S)H”in THF is close to that of acetic acid in the same solvent. We have no explanation at this time for the behavior of A toward electrophiles other than protons. Clearly A cannot have a simple structure as drawn by Piers or by US.^ Furthermore, the stereoselectivity of the formation of the kinetic contro11t2product 1, as opposed to thermodynamic contro11p2product (2isomer of l ) , is dependent not only on the possible equilibrium proposed by Piers but also on equilibrium 3. Me
,
/c=c
/COZE+
\
R3Sn
-
L-CULI
Me
L-CULI /
‘c=c /
\
R3Sn
COZEt
B
A
Acknowledgment. We thank Steven H. Bertz and Gary Dabbagh for a sample of Ph2PCwO2and numerous important discussions. Registry No. (E)-(CH,),Sn(CH3)C=C(H)CO2Et, 74854-51-6; Me,SnLi, 17946-71-3;ethyl 2-butynoate, 4341-76-8. ( 5 ) Marino, J. P.; Linderman, R. J. J. Org. Chem. 1981, 46, 3696. These authors also found that a-carbethoxyvinyl cuprates do not exhibit the usual reactivity expected of organocuprates and interpreted their results as the basis of equilibrium i ii. /C02Et R&=C
-
.-
/OEt R,C=C=C
‘C”-Ll+
‘OCu-Lt’
/
/
L
i
L
11
Coupling of Methylidyne and Carbonyl Ligands on the Triosmium Cluster Framework. Crystal Structure of (P-H ~ * ~ ~ 3 ~ ~ ~ ~ 0 ~ P 3 - 1 7 1 ’ ~ ~ ~ ~ John R. Shapley,’ Debra S. Strickland, and George M. Si. George Department of Chemistry, University of Illinois Urbana, Illinois 6 180 1
Melvyn Rowen Churchlll’ and Clifford Bueno Department of Chemistry State University of New York at Buffalo Buffalo, New York 14214 Received July 12, 1982 Summary: The compound H,Os,(CO),(CCO)
is formed more readily from HOs,(CO),,(CH) than from Os3(CO),,(CH,). The crystal structure of H,Os,(CO),(CCO) shows the CCO ligand to be in an upright rather than a tilted orientation. 0 1983 American Chemical Society
