Organometallics 1995, 14, 2977-2986
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Regioselectivity of the Insertion of Dienes into Pd-R Bonds. Diastereoselection in the Isomerization of an (q1-q2-Enyl)palladium Complex to an (q3-Ally1)palladium Complex Ana C. Albbniz, Pablo Espinet,*9+and Yong-Shou Lin Departamento de Quimica Inorganica, Facultad de Ciencias, Universidad de Valladolid, 47005-Valladolid, Spain Received May 4, 1994@ The regioselectivity of the olefin insertion reaction into Pd-pentafluorophenyl bonds has been examined by studying the reactions of [Pd(CsFb)Br(NCMe)21and dienes with two nonequivalent double bonds, namely, isoprene, vinylcyclohexene, R-(+)-limonene, a-terpinene, and y-terpinene. Analysis of the structure of the final (q3-allyl)palladium derivatives formed shows that in each case attack occurs selectively on the least substituted carbon of the least substituted double bond. The selectivity is very high except in the case of isoprene which leads to two products as a result of insertion of the least and the most substituted double bond in a 3:l ratio, respectively. Total regioselective attack on the least substituted carbon of each double bond is still maintained for this diene. Vinylcyclohexene and E-(+)limonene give at low temperature q1-q2-enyl derivatives as a 1:l mixture of two diastereoisomers in each case. These q1-q2-enylderivatives are intermediates in the allyl formation and isomerize to the corresponding q3-allylderivatives via Pd migration. Isomerization occurs simultaneously for the two diastereomeric q1-v2-enylcomplexes derived from vinylcyclohexene, but the q1-q2-enyl derivatives from &(+)-limonene form the corresponding y3-allyl complexes with different activation energies. This selective isomerization is attributed to the difference in stability of the two diastereomeric y1-y2-enylcomplexes formed from I?-(+)limonene.
Introduction The Heck reaction (palladium-catalyzed arylation of olefins) and related processes are examples of cis addition of Pd-R to olefins, and they are widely used in organic syntheskl Cis addition of Pd-R (also referred t o as endo attack of the R group) is believed to proceed via the “in situ” formation of ”PdFtX”intermediates from the palladium salt and the arylating agent; coordination of the olefin cis t o the R group will then allow insertion of the former into the Pd-R bond via a four center intermediate. The mechanism has been studied theoretically by Hoffmann et a1.2 When dienes are used as the substrates in this reaction (q3-allyl)palladium c ~ m p l e x e s ~or- ~(q1-q2-enyl)palladiumcomp l e ~ e sare ~,~ obtained. Further reaction of these complexes with nucleophiles gives access to a wide variety of organic derivatives.1° E-mail:
[email protected]. Abstract published in Advance ACS Abstracts, April 15, 1995. (1)(a)Trost, B. M.; Verhoever, T. R. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G., Abel, E. W., Eds.; Pergamon Press: London, 1982; Vol. 8, p 854 and references therein. (b) Heck, R. F. Palladium Reagents in Organic Syntheses; Academic Press: New York, 1985. (c) Hegedus, L. S. In Organometallics in Synthesis; Schlosser, M., Ed.; Wiley: New York, 1994; Chapter 5. (2) Thorn, D. L.; Hoffman, R. J . A m . Chem. SOC.1978,100, 2029. (3)Mabbot, D. J.; Maitlis, P. M. J . Chem. SOC.,Dalton Trans. 1978, 2156. (4) Heck, R. F. J . A m . Chem. SOC.1968,90,5542. (5)Stakem, F. G.; Heck, R. F. J . Org. Chem. 1980,45, 3584. (6) Larock, R. C.; Takagi, K. Tetrahedron Lett. 1983,24(33), 3457. (7) Larock, R. C.; Takagi, K. J . Org. Chem. 1984,49,2701. (8)Segnitz, A,; Bailey, P. M.; Maitlis, P. M. J . Chem. SOC.,Chem. Commun. 1973,698. (9) (a) Albeniz, A. C.; Espinet, P.; Jeannin, Y.; Philoche-Levisalles, M.; Mann, B. E. J . A m . Chem. SOC.1990,112,6594. (b) Albeniz, A. C.; Espinet, P. Organometallics 1991,10,2987. +
A stable species corresponding to the stoichiometry of the intermediates proposed in the Heck reaction, namely [Pd(C6F5)Brln,was prepared in modest yield by Klabunde using the metal atom technique.ll More recently we described a purely chemical method which made possible the preparation of “Pd(C6F5)Br” solutions12 and [ P ~ ( C ~ F E J B ~ ( N C M in ~high ) ~ I yield. ~~ We could show that in fact any of these “Pd(CsF5)Br” synthons react with dienes to give [Pd@-Br)2(l-3-q3CsF5)-ally1)21 c o m p l e ~ e s . ~The ~ , ~diolefins ~ we used were linear or cyclic with two equivalent double bonds, and the Pd-CsFs addition proved to be fully stereoselective cis and fully regioselective: The C6F5 group always added to an external carbon of the diene system, and the allyl moiety was always formed at the position of the unattacked double bond. Using this model reaction, in this paper we look at the regio- and stereoselectivity of the insertion and subsequent Pd migration on dienes that offer two nonequivalent double bonds, hence four different carbon atoms susceptible to undergo C6F5 endo attack. When nonconjugated dienes undergo insertion of one double bond into a Pd-R bond, an (q1-q2-enyl)palladium complex can be formed. These derivatives are intermediates in the formation of the thermodynamically (10)Backvall, J.-E. Adu. Met. Org. Chem. 1989,1, 135. (11)(a) Klabunde, K. J.; Low, J. Y. F. J . Organomet. Chem. 1973, 51,C33. (b) Klabunde, K. J.; Low, J . Y. F. J . A m . Chem. SOC.1974, 96,7674. (c) Klabunde, K. J.;Anderson, B. B.; Neuenschwander, K. Inorg. Chem. 1980,19,3719. (12)Uson, R.; Fomies, J.; Nalda, J. A.; Lozano, M. J.; Espinet, P.; Albeniz, A. C. Inorg. Chim. Acta 1989,156,251. (13)Albeniz, A. C.; Espinet, P.; Foces-Foces, C.; Cano, F. H. Organometallics 1990,9,1079.
0276-733319512314-2977$09.00/0 0 1995 American Chemical Society
2978 Organometallics, Vol. 14,No.6,1995
Albeniz et al.
Table 1. Summary of Reactivitpb diene
allyl moiety
compd lnyn,lb
y
1
ratio 19
l*ml
5
1-n
7
1MnIi
1
2., 2b
7.8
B 2
(-y-
2c
3
4%-
1
34 3b
12.9
PfCH2 k
p% P -d
1
4., 4b
b
Pf
I
Sa,5b
l?
