Caught in the trap: Two different routes to the thermally unstable phosphinidenoid complex I are described, and chemical evidence for this novel intermediate is provided through selective reactions. For example, methyl iodide, dimethylcyanamide, or butyraldehyde furnished complexes II, III, and IV (see scheme) under very mild conditions.

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a R denotes ubiquitous organic substituents; E denotes O or NR; [M] denotes a W(CO) 5 group. MWB(W) level. Crystallographic data of 6a and 7a have been deposited at the Cambridge Crystallographic Data Centre under the numbers CCDC-671013 (6a) and CCDC-701394 (7a). This data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

b S Supporting Information S piroalkanes such as spiropentanes (I) have been intensively studied by experimentalists as well as theoreticians. 1 Although spiroheterocycles are present in a large number of natural products 2 and show physical properties of particular interest, 3 the area of phosphorus-containing spiroheterocycles is deeply underdeveloped and, moreover, the research has been focused on phosphorusÀcarbon ring systems 4 such as the phospha-spiropentanes IIÀIV (Scheme 1). Whereas most derivatives of II 5,6 and III 7 were bound to a transition-metal center, 8 the only known derivative of IV 9 (all tert-butyl substituted) was obtained nonligated. To the best of our knowledge, nothing is known about spiroheterocycles that have another heteratom (as in V) and, therefore, the question of the effect of a (very) polar ring bond on structure and reactivity has not been addressed before.Theoretical investigations on monocyclic oxaphosphirane derivatives revealed a considerable ring strain of 23.2 kcal/mol for the parent system (RI-CCSD(T)/TZVPP), while for trimethyloxaphosphirane pentacarbonylchromium(0) a slightly smaller mean value of 22.0 kcal/mol was obtained (RI-SCS-MP2/ TZVPP); 10 this transition-metal effect was not studied further. Recent experimental studies on acid-induced ring opening 11 of monocyclic oxaphosphirane complexes have demonstrated that they have become valuable new building blocks which deserve further study. On the basis of the convenient methodology for oxaphosphirane complexes that was developed recently, 12 we felt attracted by the idea to establish a method that enables access to spiroheterocycles of type V as new ligand systems, which have various ring sizes and various heteroatoms E.Here, we report the facile synthesis of the first spirooxaphosphirane complexes (E = O) using the Li/Cl phosphinidenoid complex route and cyclic ketones. Furthermore, a first attempt to access a phospha-spiropentene complex derivative using this particular route is described. ' RESULTS AND DISCUSSIONChlorine/lithium exchange in complex 1, 13 using tert-butyllithium in the presence of 12-crown-4 at low temperature, led to the transient Li/Cl phosphinidenoid complex 2, 12a which was reacted in situ with cyclohexanone, cyclopentanone, and cyclobutanone, thus yielding the spirooxaphosphirane complexes 3a,bÀ5a,b (Scheme 2).In all reactions the two isomers a and b were formed (3a,b, 23:77; 4a,b, 55:45; 5a,b, 69:31; determined via integration of the 31 P{ 1 H} NMR spectra); only in the case of complexes 3a,b was the major isomer the b isomer, while in all other cases (4a,b and 5a,b) was the minor isomer. Column chromatography yielded only isomer 3b in pure form (see below), while the other complexes could not be separated and thus were purified and characterized as mixtures. Despite this, the structures of complexes 4a and 5a were unambiguously confirmed by single-crystal X-ray analysis; for selected data see Figures 1 and 2. Scheme 1. Spiropentane (I), Phospha-Spiropentane Derivatives IIÀIV, and Phospha-S...

Synthesis of the first oxaphosphirane chromium(0) and molybdenum(0) complexes of the type [{(R(1)PCH(R(2))-O}M(CO)(5)] (R(1) = C(5)Me(5)) (8a-e, 15a-e) and (R(1) = CH(SiMe(3))(2)) (9a-e, 16a-e) via reaction of dichloro(organo)- (1, 2, 10, 11) and chloro(organo)phosphane complexes (3,4,12) with lithium bases and aldehydes 7a-e is reported. Furthermore, bicyclic 1,3-oxaphospholane complexes 17 and 18 have been obtained via O-protonation of oxaphosphirane complexes 8a and 15a with HCl. All complexes were characterized by NMR, IR spectroscopic, mass spectrometric investigations and, in addition, single-crystal X-ray structures of complexes 8a-e, 9a,c, 15a,b,e, 16a-c, 17, 18 are presented and discussed.