Phosphasilene

B. Calculations of31P and 29Si Chemical Shifts of Parent Phosphasilenes... 

The first evidence for a phosphasilene was reported in 1979.19 Thermolysis of 1,2-phosphasiletane 1 at 100°C gave the transient silylidenephos-phane Me2Si=PPh 2, which undergoes head-to-tail [2 + 21-cyclodimerization, as well as an insertion reaction into the Si—P bond in 1, leading to 3 and 4 (Scheme 1). 

Other indications for the generation of phosphasilenes involved compounds 520 and 6.21 Both derivatives were generated by thermally induced elimination of LiF/THF from corresponding lithium(fluorosilyl)phospha-nylides. Whereas 5 was directly observed by means of 3,P-NMR spectroscopy, surprisingly, the sterically more hindered phosphasilene 6 was not detected due to its lower thermostability. Therefore, evidence for the existence of 6 was achieved solely by trapping experiments. Due to the more crowded substitution in 5 compared with 2, the dimerization process to give 7 is relatively slow (Eq. 1) but is complete within a few hours. 

Scheme 2. Trapping reactions of the transient phosphasilene /Bu2Si=P(Si/Bu3) (6), leading to 8, 9,10, and 11.

The first moderately stable phosphasilene derivatives 12 were synthesized by Bickelhaupt et al. in 1984 (Eq. 2).l0a They possess limited stability (up to 60°C), and were characterized by NMR spectroscopy (see Section IV.C) and chemical reactions (see Section IV.E). 

An alternative approach was achieved by the one-pot reaction of dichlo-rosilane with RPH2 in the presence of two equivalents of nBuLi. However, neither synthetic method yields crystalline phosphasilenes. The first synthesis of the crystalline phosphasilene 13 was achieved by a two-step reaction of fBuSiCl3 with 4 equivalents of RPHLi (R = 2,4,6-/Bu3C6H2) and Ph2PCl by Niecke et al. in 1993 (Scheme 3).10c This reaction proceeds... 

Scheme 3. Synthesis of the phosphasilene 13, proceeding via the 1,3-diphospha-2-silaal-lylide derivative 13. ...

In order to overcome the difficulties of thermal instability, the phosphasilene derivatives 15, which bear a silyl or germyl group attached to phosphorus, were synthesized. Indeed, they proved to have stronger Si=P bonds (stable up to 100°C), thus allowing studies of their structures and reactivity.l0b 14 Phosphasilenes 15a-15i were synthesized from the corresponding Iithium(fluorosilyl)phosphanides 16a-16i by the thermally induced elimination of LiF (see Scheme 4).10b It has been shown that excellent steric protection of the highly reactive Si=P bond in 15 is provided by the 2,4,6-triisopropylphenyl (Is = isityl) substituent attached to the low-coordinate silicon center. The appropriate precursors 16a-16i were synthesized in a multiple-step procedure, starting from 17 (Scheme 4).10b U... 

Scheme 4. Synthesis of the stable P-silylated phosphasilenes 15a-i, proceeding via 16-18.

The analogous decomposition process was observed for related germylide-nephosphanes.32 Although the thermolysis of 12a was carried out in deuterated solvents, no deuterium incorporation was observed. Further work is needed in order to explore the reaction mechanism. In contrast to 12a, the thermolysis behavior of phosphasilenes 15 is quite different. 

Besides disilenes, the metastable phosphasilenes 15 (and the analogous arsasilenes) are the only known compounds with (p-p) 7r-bonds between main-group elements that react with P4 under relatively mild conditions (40oC).10b 33 Thus 15a reacts with P4 in a 2 1 molar ratio to build up the SiP3 butterfly-like compound 22 (Eq. 6). 

The oxidation of the phosphasilenes 12a and 15a with elemental sulfur and tellurium led the corresponding chalcogen heterocycles 23 (Eqs. 7,... 

The ability of the Si=P bond to serve as a dienophile for a [2+4]-cycloaddition (hetero-Diels-Alder reaction) has been verified by the reaction of 15a with cyclopentadiene (Eq. 10).14 The components react at 60°C in benzene to give a quantitative yield of 25, which was isolated as a colorless solid. Interestingly, a Br0nsted acid (C—H)/base (Si=P) reaction, as in the case of 15a and phenylacetylene, was not observed. In the mass spectrum of25, the molecular peak but also the free phosphasilene 15a+ have been detected. Similar behavior was observed for related ad-... 

Scheme 10. Formation of the phosphaalkenes 39 and 40 by reaction of the phosphasilene (15a) with mesitylisocyanide and 1,6-diisocyanohexane, respectively.

The synthesis of compounds 44 containing Si=As bonds was achieved by a route analogous to that used for the phosphasilenes 15 (Eq. 13).12,14 26 The thermolysis of colorless solutions of 45a-d in hexane at 60°C yielded the desired products 44a-d as orange-red oils or orange solids. The starting... 

Hydrolysis of 3a-d under mild conditions leads to the phosphasilenes 5a-d (R = H) [5]. Compound 5c possesses an outstandingly large p si coupling constant (224 Hz). Treatment of 3a,b with chlorodiphenylphosphine yields the phosphasilenes 4a,b (R = PPh2) [5]. 

Compound 4a was isolated as a yellow solid and recrystallized from n-pentane at +4°C to give the first X-ray-structure of a phosphasilene [5] (Figure 1). The P=Si bond (2.094 A) is a little shorter than the P-Si bonds in 3a-c (2.11-2.13 A) but longer than the theoretically calculated values (2.04-2.06 A) [6]. This may be caused by a significant pyramidalization of the Si-atom (E, = 356.7°) and great steric hindrance. The Pl-Sil-P2-P3 atoms almost form a planar trapezoid with a rather short Pl-P3-distance (3.68 A). 

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