In order to synthesize chiral palladacycles for stereoselective C-C coupling reactions we studied the cyclopalladation of P-chiral phosphanes 2 and 3. New palladium complexes of the type LzPdXz (6, 8) and LXPd-p-X2-PdXL (5, 9, X = C1; L = 2, 3) were isolated. A detailed study of the reactivity of all intermediates towards cyclopalladation proved the mechanism of cyclometalation reactions of o-tolylphosphanes for the first time. Different deuteration experiments clearly demonstrated the higher reactivity of dimeric palladium complexes towards metalation compared to monomeric spec -C coupling reactions catalyzed by transition metals are of fundamental interest in modern synthetic chemistry. In this respect, the arylation and vinylation of olefins catalyzed by palladium complexes (Heck reaction) are widely usedL21.Scheme 1. Palladium-catalyzed arylation of olefins with aryl halides (Heck reaction); X = I, Br, N2BF4, C(=O)Cl, CF3S03; B = base: NR3, K2C03, NaOAc; R, R ' = substituentsRecently, because of its excellent control of regio-and stereoselectivity intra-L3] and interm~lecular[~] asymmetric variations of Heck reactions were developedL51. Here standard catalyst systems use chiral bisphosphanes as auxiliary reagents to stabilize the catalytically active palladium (0) species. So far enantioselective Heck reactions are limited to aryl triflates and aryl iodides as arylating agents. In the case of aryl iodides a stoichiometric amount of silver salts is necessary to obtain good yields and high enantiomeric excessesL61. All stereoselective Heck reactions suffer from insufficient catalyst turnover frequencies (TOF < 10 h-l) and catalyst turnover numbers (TON < 100). problems we initiated a program for the development of new chiral palladium catalysts for stereoselective C -C coupling reactions.Recently, we discovered palladacycles 1 as superior catalysts in Heck and related reactions of aryl bromides with catalyst turnover numbers up to 500000r71. These structural well-defined single-site catalysts offer new opportunities to design optically pure palladacycles for asymmetric C-C coupling reactions, especially Heck reactions. Looking at the structure of 1 there are two possibilities with either C or P chirality. On the one hand the metalated methylene group can be replaced by a suitable alkyl or aryl group, on the other hand a phosphane containing a stereogenic P atom with an o-tolyl substituent suitable for cyclopalladation can be used.Because of the easy access we started to study systems of the latter type. Thus, we synthesized phosphanes 2 and 3 as
The halogen atoms in the 1-chloro-1H-phosphirenes, 5a-c, are easily substituted on reaction with organolithium and Grignard reagents, 14a-p, with formation of the corresponding 0-, N-, C-, Si-, and Ge-substituted 1H-phosphirenes, 15a-t. Cl/H exchange reactions also occur on reaction with lithium metal hydrides (5a 4 17). Furthermore, substitution reactions are also realized with the alkali metal borates,
18a-g (-19a-g);the same is true for reactions with tnmethylsilyl cyanide and azide (-+22a-d). Some of the substitution products have been chardcterized in the form of their metal complexes, 16, 23, 24, and 25.
Photolysis of the silyl-substituted 1H-phosphirene 3 a proceeds selectively with cleavage of a silicon ± silicon bond and ring expansion to furnish the 1,2-dihydro-1,2-phosphasilete 4 a. The corresponding heterocyclic germanium product, 4 b, is prepared analogously from 3 b. The preparative significance of 4 a is reflected not only in its numerous addition reactions to multiple-bond systems, such as alkynes 8 and 12 and ketene 10, but also in its substitution reactions with the chloro compounds 15, 18, and 21. The latter reactions proceed through the formation of chlorotrimethylsilane and the novel 1,2-dihydro-1,2-phosphasiletes (16, 19, and 22) which, depending on the substitution pattern at the phosphorus atom, undergo various isomerization reactions (16 317, 19 320). The hydrolysis of 4 a to 23 by mere traces of water in carefully purified tetrahydrofuran demonstrates the extreme sensitivity of this compound towards moisture.
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