bond of the substrate is activated by ring strain. Thus, e. 9.. 3,3-dimethylcyclopropene 1 is cyclooligomerized, sometimes highly chemoselectively, on palladium(0) catalysts"! Palladacycloalkanes of various ring size have been discussed as intermediates.We have now succeeded in preparing five-and ninemembered palladacycloalkanes in stoichiometric reactions, and have simulated a catalytic cycle by their thermal decomposition. We synthesized the colorless palladacycloalkanes 3 to 5 by reaction of the dark-red complex (q3-al-0 Verlag Chemie GmhH. 6940 Weinheim. 1982 0S70-083~/82/~101-0064 $ OZ,SO/O Angeu,. Chem. I I~I . Ed. Engf. 21 (19821 No. I

The triphenylphosphenium ion in 1, can be looked upon as a triphenylphosphane complex of phosphorus. 1 is formed by the joint action of Ph3P and AlCl3 on PCl3. Surprisingly, the central P atom in 1 can be protonated; the “ligands” can be successively replaced by more strongly basic phosphanes, so that unsymmetrical triphosphenium ions are also accessible. The length of the PP bond in 1 lies between that of a PP single bond and a PP double bond.

Pyridinium bromides 3 prepared by alkylation of 2-methyl-, 2-ethyl-, and 2-benzylpyridines 1 with methyl bromides 2 bearing an electron-withdrawing group (COPh, CN, C02Et, C6H4NO2) are condensed with PC1, in the presence of Et3N to give 2-phosphaindolizines 4. The 1-unsubstituted representatives 9 (prepared from 8) may undergo a substitution reaction at this position with an excess of PC13 1-and 2-phosphaindolizines are 1,3-azaphospholes with a pyridine ring a-annuAzaphospholes constitute a relatively recent group of fivemembered aromatic heterocycles'). Among these systems representatives with and without a nitrogen atom adjacent to the phosphorus atom in the ring may be distinguished. While the former can usually be prepared by a condensation reaction using PC1, or P(NMe2),, the latter (1,3-azaphospholes and 1,2,4-diazaphospholes) are obtained by condensation with phosphines or silylphosphines or by cycloaddition reactions. Thus, the monocyclic 1,3-azaphospholes as well as their benzo and pyrido derivatives B7-12) and D4) have been prepared. The latter may be viewed as 1 -phospha analogs of indolizines E.are not yet known. In this paper we report on their synthesis by means of a PC1, c~ndensationl~). This is the first example of a heterophosphole with carbon atoms on both sides of the two-coordinate phosphorus atom in the ring being formed in this way27). As the positions 1 and 2 of the indolizine ring are different in ~haracter'~) the phosphorus ring member in these positions and hence the systems D and C should differ characteristically, e.g. in their NMR spectra and in their reactivity. SynthesisThe PCI3 condensation of a suitable four-membered chain presents a facile route to heteropho~pholes'~). For the synthesis of 2-phosphaindolizines of type C 1,2-dialkylpyridilated to the 1,2 and 1,5 bond, respectively. The different types of annulation result in characteristic differences in charge distribution, 31P-NMR shift, and chemical behavior. 2-Phosphaindolizines 4 and 9 are stable in dry air and do not undergo alkylation. Compounds 9 hydrolyze to give zwitterionic ' (1-alkyl-2-pyridino)methylphosphinates 13. The phosphorus atom of a 2-phosphaindolizine can act as a ligand atom as shown by the example of a Cr(CO), complex.nium salts 3 offer themselves as the starting material. Their 2-methylene group is sufficiently reactive for a triethylarninemediated condensation; its deprotonation results in a stable enamine16'. In contrast, the 1-methylene group needs activation by an electron-withdrawing substituent R2. COPh, CN, C02Et, and C6H4N02-4 have been found to be effective for this purpose. The respective pyridinium salts 3 are readily obtained17-19) from 2-ethyl-or 2-benzylpyridine (1 a, c) and phenacyl bromide (2a), bromoacetonitrile (2 b), ethyl bromoacetate (2c), or 4-nitrobenzyl bromide (2d). For the condensation, equimolar amounts of 3 and PC1, are treated with a fourfold molar amount of Et3N in toluene at ambient temperature or preferably in acetonitrile at 0 -5°C. The reaction is usually complete within...

3: 0.09 g (0.5 mmol) Ferrocen, 0.06 g (0.25 mmol) I2 und 0.25 g (0.5 mmol) Sbl, werden nacheinander in 150 mL CH,CN unter Erwarmen gelost. Innerhalb van I d bei Raumtemperatur kristallisieren schwarze, kompakte Kristalle in 82% Ausbeute; befriedigende C,H,Fe,l-Analyse. Eingegangen am 21. Oktober 1988 [Z 30211[I] K.

The reaction of phosphonium ylides with phosphorus trihali-to the phosphonio group. In one case the P(II1)-C rotation des has been studied for the synthesis of ylidyl-dihalophos-barrier has been estimated from VT-31P-NMR spectra. By Xphanes (= dihalophosphanyl ylides) Ph3P=CR-PX2 3, X = ray crystallography the structures of 3, R = Me, 2,6-C12C6H,, C1, and 9, X = Br. Compounds 3, R = aryl, are readily prepa-4-NOzC6H5, PC12, of 9, R = Me (two molecules), %Me3, of an red from the phosphonium bromides [Ph3P-CH2R]Br, com-ylidyl-selenophosphonyl dichloride (11 b), and of 12 have pounds 3, R = alkyl, SiMe3 or PC12, and 9 are obtained from been analyzed. They provide representatives for the full silylylides Ph3P=CR-SiMe,, compound 3, R = PPh; results range of rotation from the symmetric conformer with two from the addition of PC1, to the hexaphenylcarbodiphospho-equal P-X bonds to the conformer with one P-X bond perrane. A (P-morpholinoviny1)dichlorophosphane 12 has also pendicular to the PCP plane and with this bond being exbeen prepared. Ylides 3 are oxidized by sulfur and selenium tremely elongated. Thus, they map out the pathway to P-X and are converted to ylidyl-chlorophosphenium (= chloro-bond breaking. On this way the initial charge transfer from phosphaalkenyl-phosphonium) salts [Ph3P-CR=PC1]A1C14 the ylidic carbon to the antibonding P-X orbital ends up in 10. In the ,'P-NMR spectra of 3 and 9 the geminal coupling a TC donation and P-X dissociation. 'Jpp indicates the phosphorus lone pair to be synperiplanar ,,Although direct observation of a molecule along the reaction pathways does not seem feasible, its visualization at least does". Das zu leisten verspricht die Strukturkorrelalions-Hypothese: ,,If a correlation can be found between two independent parameters describing the structure of a given structural fragment in a variety of environments, then the correlation function maps a minimum energy path in the corresponding parameter space" [']. Bei dem hier zu beschreibenden Vorgang handelt es sich um die Heterolyse einer Phosphor(II1)-Halogen-Bindung oder vielmehr um die Einleitung dieser Heterolyse. Zur Wegmarkierung dienen die Strukturen von sieben Dihalogenphosphanen. Die ,,Schnappschusse entlang des Wegs", die sie bieten, sind einerseits hinreichend verschieden und passen andererseits gut genug zusammen, um daraus einen uberzeugenden vorgangsbeschreibenden Film zu machen. Die Reihenfolge, in der die Schnappschiisse dazu angeordnet werden mussen, ist uberdies nicht zufallig, sondern ist in den meisten Fallen einsichtig.Unabhangig von der Art der sonstigen Substituenten sind Phosphor(II1)-halogenide in aller Regel kovalent, und ionische Vertreter sind die seltene Ausnahme. Letztere finden sich dort, WO die bei der Dissoziation am Phosphoratom induzierte Kationladung effektiv auf die ubrige Molekel verteilt werden kann, wie z.B. in bestimmten Azaphos-

Genügend starke anionische Nucleophile X− bauen weißen Phosphor unter mehr oder weniger weitgehender Disproportionierung ab. Mit Kronenether‐Alkali‐, Ammonium‐ oder Phosphoniumcyaniden, X− = CN−, entsteht dabei selektiv das entsprechende Dicyanphosphid, P(CN)2−, und ein Polyphosphid, bevorzugt P15−. [18] Krone‐6‐KP(CN)2 entsteht auch bei der Umsetzung von P(CN)3 mit KF und Kronenether. Im Kristall dieses Salzes sind die am Phosphor (um 95°) gewinkelten Dicyanphosphid‐Anionen mit beiden Stickstoffenden an je ein Kation koordiniert. Der PC‐Abstand ist (mit durch‐schnittlich 168 pm) so kurz wie in Phosphaalkenen. δ13C und JPC von P(CN)2− ordnen sich gut in einen für Cyanphosphorverbindungen allgemein geltenden Zusammenhang mit der Ladungsdichte am Phosphor ein.

Salts of the title anion series have been prepared with the sodium-[l8]crown-6 cation (n = 1 and 2) ([18]crown-6 = 1,4,7,10,13,16-hexaoxacyclo-octadecane) or tetra-alkylammonium cations (n = 4) by addition of Br-to P(CN), or PBr,, or by Br, or BrCN oxidation of P(CN),-.Contrary to this, CN-addition to P(CN),, in a reductive elimination, gives P(CN),-. X-Ray crystal-structure determinations of the above mentioned three salts are reported and their anion structures are discussed. The P(CN),Br,-anion shows the $-trigonal-bipyramidal coordination expected on the basis of Valence Shell Electron Pair Repulsion Theory (V.S.E.P.R.), P(CN),Br-has $-octahedralco-ordination in a dimeric, bromide double-bridge structure, and PBr,-a structure intermediate between the two. Here the trigonal-bipyramidal co-ordination is distorted towards a tetrahedral geometry. This is in contradiction to V.S.E.P.R. and presumably the result of ligand-ligand repulsion. The structures are of interest as transition-state models of phosphorus(ll1) nucleophilic substitution.

Die 'H-NMR-Spektren der Reaktionsldsungen enthalten uberraschenderweise (siehe Molekulstruktur in Abb. 1 !) nur ein Singulett, dessen chemische Verschiebung vom Verhaltnis der Reaktionspartner abhlngt (bei x= 2: 1 : 6 = 1.30 (s); 2: 6= 1.32 (s), 60 MHz, 28 "C, Tolu01)[~~. Die-[**I ~bergangsmetallcarbin-Komplexe, 78. Mitteilung. -77. Mitteilung: E. 0. Fischer, J. Schneider, D. Neugebauer, Angew. Chem. 96 (1984) 814: Angew. Chem. Int. Ed. Engl. 23 (1984) 820. Angew. Chem. 97 (1985) Nr. 3 0 I'CH Verlagsgesellschafi mbH. 0-6940 Weinherm. 1985 0044-8249/85/0303-0215 0 02.50/0