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The [RhCl(PH3)2]-catalyzed hydroboration reaction C2H4 + HBR2 → H3CCH2BR2 (R = OH, 2R = OCHCHO) was investigated by means of density functional theory type calculations using the Amsterdam density functional (ADF) program. In the first step, the borane adduct [RhCl(η2-HBR2)(PH3)2] (1) forms from [RhCl(PH3)2] and the borane HBR2. Subsequently, C2H4 adds to 1 to give either [RhClH(BR2)(C2H4)(PH3)2] (2) (associative pathway I) or [RhCl(η2-HBR2)(C2H4)(PH3)] (23) (dissociative pathway II). Further branching arises because on both pathways either boron migration (I.B, II.B) or hydride migration (I.H, II.H) may occur as initial product-forming steps. It is found that the associative mechanisms, I.B and I.H, have rather similar energy profiles and the formation of product complexes [RhCl(H3CCH2BR2)(PH3)2] (9, 15) by reductive elimination requires overcoming the highest activation barriers (∼9 kcal mol-1). Overall, the I.H pathway may be slightly favored over I.B for an associative mechanism. In contrast, for the dissociative mechanism migration and elimination reactions are kinetically strongly differentiated. On the II.B pathway, C−B bond formation is hindered by a high activation barrier (19.5 kcal mol-1), while reductive C−H coupling furnishing the product complex [RhCl(H3CCH2BR2)(PH3)] (31) has a low barrier (6.5 kcal mol-1). On the II.H pathway the reverse is found:  C−H formation has a low barrier (8.4 kcal mol-1), while reductive C−B formation has a high barrier (15.8 kcal mol-1). In summary, the II.B pathway may be slightly more favorable for a dissociative reaction. Since side products (i.e., vinyl boranes, alkanes) are formed on the I.B and II.B pathways, we suggest that rhodium-catalyzed hydroborations are driven into a dissociative II.H reaction channel which is easier to control kinetically by using bulky electron-withdrawing phosphines as ligands in the catalyst.

Keywords: Phosphorus / Iminophosphorane / Staudinger reaction / Heterocycles / Ab initio calculations The Staudinger-model reaction H 3 P (1) + HN 3 (2) Ǟ H 3 P=NH H 3 P=NH and N 2 . This decomposition can be effectively hampered by intramolecular donor-acceptor interactions as (5) + N 2 (6) has been investigated at the CCSD(T)/6-31G**// MP2(Full)/6-31G* level. Primary products formed in this shown by calculations on model compounds as well as by experiments. Thus the reaction of a methylene-σ 3 ,λ 3 -reaction are the phosphazides H 3 P=N-N=NH (3) which exist as trans and cis isomers. In contrast to some previous phosphanyl-σ 5 ,λ 5 -phosphorane with PhN 3 led to a new fourmembered heterocycle containing a thermally remarkable assumptions, cis-3 is 8.2 kcal mol -1 more stable than trans-3 but decomposes rather easily into the expected products stable cis-phosphazide moiety. Since 1919 the reaction between azides and phosphanesThe structures of some phosphazides have been determined experimentally.[3] [4] Most of these compounds con-(Staudinger reaction) has been widely used by phosphorus chemists for the preparation of λ 5 -iminophosphoranes tain a trans-configured phosphazide moiety [3] while the cisconformation [4] has rarely been observed. This led to the (phosphazenes).[1] In the course of the reaction the primary imination products, phosphazides, are formed, which are assumption that the trans conformers are more stable than their corresponding cis isomers. However, at least for the rarely stable under ambient conditions.[2] In a few cases, where R is strongly electron donating, RЈ strongly electron parent model system H 3 PϭNϪNϭNH our calculations show a different picture: cis-3 is more stable than trans-3 by withdrawing, or where both R and RЈ are sterically bulky, remarkable stable phosphazides R 3 PϭNϪNϭNRЈ have 8.2 kcal mol Ϫ1 . The isomerisation of trans-3 to cis-3 proceeds via the transition state TS1 and requires an activation been isolated. [3] [4] Here we wish to report on the first detailed ab initio study of the Staudinger reaction and a new energy of 6.4 kcal mol Ϫ1 . Indeed, the cis isomer, though thermodynamically more stable, decomposes rather easily possibility to stabilize phosphazides. The Staudinger-model reaction H 3 P (1) ϩ HN 3 (2) Ǟ into H 3 PϭNH (5) and N 2 (6). On the reaction coordinate, the transition state TS2 has to be overcome, which is 8.2 H 3 PϭNH (5) ϩ N 2 (6) has been investigated at the CCSD(T)/6-31G**//MP2(Full)/6-31G* level of theory.[5] In kcal mol Ϫ1 higher in energy than cis-3. This reaction can be described as an intramolecular nucleophilic attack of the the first step of this reaction the trans-configured and cisconfigured phosphazides trans-3 and cis-3 are formed, negatively charged nitrogen centre N3 (Ϫ0.55 e) on the positively charged phosphorus centre (ϩ0.95 e). As a result, respectively. Subsequently, decomposition of these intermediates leads to the phosphazene 5 and dinitrogen (6) the PϪN3 distance is shortened from 2.402 Å in cis-3 to 1.880 Å in TS2. The...

A 35-fold excess of methyl triflate (2) is required to quantitively prepare 3, the first phosphanyl phosphenium ion, from diphosphene 1. Experimental data and calculations indicate that the P=P bond becomes stronger upon alkylation.

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