Abstract:The reaction of Li/Cl phosphinidenoid tungsten(0) complex 2 [R = CH(SiMe 3 ) 2 ] with 1,3,4,5-tetramethylimidazol-2-ylidene (3) yielded the zwitterionic phosphaalkene complex 4. A comparative study on chloro-(5) or dichloro(organo)phosphane complexes 1 [R = CH(SiMe 3 ) 2 ] and NHC 3 revealed the formation of 4 at ambient temperature. First evidence for the forma-

Contentsp.2 Crystal data for 1a,b p.3 Crystal structure determination and molecular structure of 1a (Figure S1) p.4 Crystal structure determination and molecular structure of 1b (Figure S2)

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...

Reaction of the Li/Cl phosphinidenoid pentacarbonyltungsten complex 2 (R = CH(SiMe 3 ) 2 ) with unsaturated aldehydes 3, 4, 9, 10, and 13 yielded the new oxaphosphirane complexes 5, 7, 11, 12, and 14 and thus revealed a clear preference for CO vs CC bond addition (i) and for 1,2-vs 1,4 addition (ii). Complexes were characterized by NMR, IR spectroscopy, and mass spectrometry and, in the case of 14a, by single-crystal X-ray analysis.

The reaction of Li/Cl phosphinidenoid pentacarbonylmetal(0) complexes with bis(2-pyridyl)ketone led to overcrowded oxaphosphirane derivatives. They can undergo deoxygenation by Ti(iii) reagents or ring expansion via P–C cleavage followed by P–N cyclization.