A new class of phosphinine/rhodium catalysts for the hydroformylation of terminal and internal alkenes is presented in this study. A series of phosphabenzenes 1-14 has been prepared by condensation of phosphane or tris(trimethylsilyl)phosphane with the corresponding pyrylium salt. Trans-[(phosphabenzene)2RhCl(CO)] complexes 21-25 have been prepared and studied spectroscopically and by X-ray crystal-structure analysis. The hydroformylation of oct-1-ene has been used to identify optimal catalyst preformation and reaction conditions. Hydroformylation studies with 15 monophosphabenzenes have been performed. The catalytic performance is dominated by steric influences, with the phosphabenzene 8/rhodium system being the most active catalyst. Turnover frequencies of up to 45370 h(-1) for the hydroformylation of oct-1-ene have been determined. In further studies, hydroformylation activity toward more highly substituted alkenes was investigated and compared with the standard industrial triphenylphosphane/rhodium catalyst. The reactivity differences between the phosphabenzene and the triphenylphosphane catalyst increase on going to the more highly substituted alkenes. Even tetrasubstituted alkenes reacted with the phosphabenzene catalyst, whereas the triphenylphosphane system failed to give any product. In situ pressure NMR experiments have been performed to identify the resting state of the catalyst. A monophosphabenzene complex [(phosphinine 8)Ir(CO)3H] could be detected as the predominant catalyst resting state.

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Phosphaalkynes readily participate in ene reactions both with alkylidenecyclopropanes and with allenes to furnish phosphaalkenes (the products of a simple ene reaction) and phosphanes (the products of a tandem ene reaction). Thus, irrespective of the stoichiometry, the thermal reactions of the phosphaalkynes la-c with isopropylidenecyclopropane (8) or cyclopropylidenecyclopentane (1 1) proceed by a tandem ene process with retention of the three-membered ring unit to furnish the corresponding bis(cyclopropy1)phosphanes 10a-c or 12. In contrast, with cyclopropylidenecyclohexane (13) as the H-donor, the specific formation of either the monoadducts 14a,c or of the bisadducts 15a,c can be controlled. A crystal structure analysis of 15a confirmed the structures of the tandem ene products while the constitutions and configurations of the novel P-cyclopropylphosphaalkenes 14a,c were unequivocally elucidated by an X-ray crystal structure analysis of the corresponding, W(CO),-complexed system 16. Alkylated allenes such as 1,l-dimethylallene (17), vinylidenecyclohexane (20), tetramethylallene (22), or 1,2-cyclononadiene (24) undergo thermal addition by way of a double ene process to afford the phosphaalkynes 1. In each case, regiospecific attack of the phosphorus atom at the central C-atom of the cumulene system results in the formation of the structurally unique phosphanes 21,23a,b, and 25a,b. On the other hand, thermal reactions of la,b in the presence of 1,l-diethylallene (26) come to a standstill after the first ene addition and furnish the cross-conjugated phosphatrienes 27a,b as the first representatives of the previously unknown phospha[3]dendralene system. The thermally initiated tandem ene reactions of the phosphaalkynes 1 with propadiene (28) or tert-butylallene (30) each proceed through transfer of a vinylic hydrogen atom from the ene to the enophile to afford the bis(propargy1)phosphanes 29a,b and 31, respectively, as addition products.

The ene reaction belongs to one of the simplest and most versatile reaction categories in organic chemistry. Recently ene reactions involving compounds of low-coordinate phosphorus, phospha-ene reactions, have found wide application in the synthesis of different classes of organophosphorus compounds. For phospha-ene reactions four main types are currently known which involve the employment of phosphaalkenes, iminophosphines and phosphaalkynes. Also included are retro-phospha-ene reactions, which reveal access to short-lived and highly reactive phosphaalkenes. The experimentally observed regiochemistry is in agreement with MO calculations, carried out on the simplest parent reactions of type I-IV. Neilson has recently shown that 5 avoids the also feasible [2+2] cycloaddition in reactions with 1,1-dimethylallene, Table 1. Products and reaction conditions for type I phospha-ene reactions of 5 with all-carbon enes (entries 1-5, 21 6-7 19 and 8 20)

Phospha-ene reactions of the type II between methylidenephosphanes as enophiles and Caminophosphaalkenes possessing allylic hydrogen atoms proceed by P-P bond formation to furnish the corresponding functionalized diphosphanes. Thus, the reaction of the methylidenephosphane 1 with the C-amino-substituted ethylidenephosphane 3a runs smoothly at room temperature to afford the unsymmetrical 1,2-diphosphane 4 as a 60:40 mixture of two diastereomers in 64% yield. Rotation about the P-N bond in 4 is hindered at room temperature, but free enthalpies of activation of ∆G q (PN) ) 16.3 kcal‚mol -1 (major diastereomer) and 15.2 kcal‚mol -1 (minor diastereomer) were determined by variable temperature 1 H NMR spectroscopy. In contrast, reactions of the ethylidenephosphanes 3a-c with the chlorophosphane 2 as enophile follow an unusual course involving a regiospecific ene reaction of the type II and a subsequent intramolecular ring closure reaction to furnish the 3-amino-1,2-dihydro-1,2-diphosphetes 7a-c in good yields (53-79%). The reactivity of this new class of heterocyclic compounds has been studied exemplarily for product 7a. The trans form of 7a obtained by synthesis can be converted to the corresponding cis form photochemically. Complexation of both phosphorus centers in 7a is possible by reaction with 2 equiv of diiron nonacarbonyl and gives rise to complex 8, the constitution of which has been confirmed by X-ray crystallography. Unexpectedly, hydrolysis of 7a does not proceed by attack at the endocyclic enamine unit but rather by way of P-P bond cleavage to afford the functionalized phosphaalkene 9. On the other hand, oxidation of 7a by bis(trimethylsilyl) peroxide takes place with retention of the P 2 C 2 skeleton: a selective increase in coordination at P-1 results in the formation of the 1-oxo-1,2-dihydrodiphosphete 10, the structure of which was confirmed by X-ray crystallography. The reactions of 7a with the electron-poor acetylenes 11a-d also follow an unusual course. Nucleophilic attack of P-1 at the CtC triple bond and proton shift lead stereospecifically to the 1-methylene-1,2-dihydro-1,2-diphosphetes 12a-d as thermolabile addition products. The constitution and configuration of 12c were unambiguously confirmed by X-ray crystallography.