Reactive Intermediate Chemistry

are synthesized by the related reductive coupling of bulky dichlorodiarylgermanes using LiNp or Mg/MgBr2. Although cyclotrigermane (7) is isolated as a stable compound at room temperature, thermolysis of 7 at 80 C readily generates the expected dimesitylgermylene.27,28

2.5. Silylenes from Branched Cyclic Silylsilanes

Branched trisilacycloalkanes such as 8 undergo extrusion of a silylsilylene, for example, (Me3Si)(Me)Si: (9), to produce the ring-contracted disilacycloalkane.29–31 In addition to loss of silylene 9, the five-membered ring gives a linear silylene (10) that is trapped by Et2SiMeH via the competing migration of a Me3Si group and a cleavage of the ring. Irradiation of the disilanyl-substituted 7-silanorbornene (11) with a 450-W high-pressure mercury lamp in n-hexane at 15 C produces the disilanylsilylene (12), which is trapped by 2,3-dimethyl-1,3-butadiene to give adduct 13 in 31% yield (Scheme 14.10).

10

Me3SiMe2Si Me

Scheme 14.10

Photochemical irradiation of (i-Pr3Si)3SiH (14) with light of 254 nm in either 2,2,4-trimethylpentane or pentane leads to the elimination of i-Pr3SiH and the generation of bis(triisopropylsilyl)silylene (i-Pr3Si)2Si: (15). Silylene 15 can also be generated by the thermolysis of the same precursor 14 at 225 C in 2,2,4-trimethyl- pentane (Scheme 14.11).32 Reactions of 15 include the precedented insertion into an Si H bond, and additions to the p bonds of olefins, alkynes, and dienes.

658 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)

rt = room temperatnre

Scheme 14.11

2.6. Silylenes from Metal-Induced a Eliminations

There are also many routes to silylenes involving reductions of halo compounds with alkali metals. In many of these reactions, it is doubtful whether ‘‘free’’ silylenes are formed at all; the silylene may be complexed with metal or held in a solvent ‘‘cage’’ with a salt, or the intermediate may be an organometallic compound rather than a silylene, which is usually called a silylenoid; that is, a silylene-like

intermediate. Silylenoids have been postulated as reaction intermediates in reduction of dihalosilanes5h,13,33 with alkali metals, a reaction that affords polysilanes.

With t-Bu2SiBr2 and t-Bu2SiI2, cyclotrisilanes were obtained, but with t-Bu2SiCl2, a disilane and a cyclotetrasilane were produced. The reactions of dichlorosilane, such as t-Bu2SiCl2, with lithium under ultrasound activation in the presence of Et3SiH (Scheme 14.12) gave Et3Si SiH(t-Bu)2 in 60% yield. This result is rather persuasive evidence in favor of silylene formation, since Si H bond insertion is characteristic for silylenes. It is difficult to see how the disilanes could arise from the reaction of Et3SiH with silylenoids.33b Possibly, the halogen-containing silylenoids are in equilibrium with a very small amount of free silylene (17). The formation of silacyclopentanes (18) was reported in the reaction of dichlorosilanes and alkali metals under irradiation of ultrasound, in which the intermediary R2Si: is intercepted by 2,3-dimethyl-1,3-butadiene. Although products are formed in low yields, the result was interpreted in terms of a silylene intermediate.34 Reactions of diorganodihalosilanes with lithium in the presence of olefins gave silacyclopropanes, and decamethylsilicocene (19) was obtained by reduction of dichloroor

dibromo bis(Z1-pentamethylcyclopentadienyl)silane (Scheme 14.13).33–36 Reactions of t-Bu3Si(i-Pr3Si)SiBr2 (20)37,38 with activated magnesium or the

reagents derived by the reduction of 1,3-dienes with magnesium gave rise to

Scheme 14.13

products that look like silylene adducts. Selective inversions between metal-free and organometallic reaction systems for the generation of silylenes and their equivalents suggest that the silylenoid reactions are initiated by treatment of 20 with Mg* (Scheme 14.14).

A similar reaction of a bulky diaryl-substituted dihalogermane generates the corresponding diarylgermylene. Halogermylenes are also produced in the reaction of trihalogermanes with magnesium (Scheme 14.15).39

660 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)

45%

Scheme 14.15

3. STRUCTURES OF SILYLENES AND GERMYLENES

3.1. Singlet and Triplet States

While carbenes are found with both singlet and triplet ground electronic states, only singlet silylenes and germylenes are well documented.

Figure 14.1

The ground states of silylenes and germylenes are the closed-shell singlet with the triplet state lying higher in energy, as evidenced by a variety of experimental and theoretical methods.40–44 The different multiplicities of the ground state of the parent carbene (:CR2) and other divalent species containing group 14 (IVA) elements (:MR2) can be understood in terms of the energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in such species. The ground state has a pair of electrons in the orbital of s symmetry as shown in structure A (Fig. 14.1).

In the triplet state shown in structure B, these electrons are not paired and one electron is in the orbital of p symmetry and the other remains in the orbital of s symmetry. In the case of :SiH2, the HOMO–LUMO energy gap is much larger than that of :CH2 and the singlet electronic configuration is the ground state.41

Steric effects can also influence the singlet–triplet (S–T) gap, and that suggests different curvature of the potential curves for the singlet and triplet state as a function of the angle around the central metal.44 In silylene chemistry, S–T crossing occurs at H Si H angle of 130 . This result indicates the possibility of formation of a silylene having triplet ground state with the R Si R angle >130 . According to calculations, the singlet ground state of the parent compound H2Si: lies 21 kcal/ mol lower than the first triplet state. In the carbon analogues the situation is reversed. The first singlet state is calculated to lie 9 kcal/mol above its triplet ground state,45 in agreement with experimental data.43,46 In addition to the pairing energy and electrostatic effects, the small extent of s, p mixing in silicon seems to play an important role. The first factor is manifested not only in the large stabilization of the singlet state but also the bond angle of H2Si:, which calculated to be as small as 93.4 ,41 which is a consequence of the relatively large difference between the sizes of the 3s and 3p orbitals compared with the difference between 2s and 2p.47

During the 1980s, several organosilylenes have been isolated and studied in argon or hydrocarbon matrices at very low temperatures such as 77 K, above which temperature they react rapidly with themselves or with solvent. By contrast, the divalent dicoordinate species of germanium, tin, and lead have been well characterized as stable molecules.

The enlargement of the bond angle caused by the triisopropylsilyl groups was encouraging with regard to the prospects of [(i-Pr)3Si]2Si: having a triplet ground state. It was found, however, that the addition of [(i-Pr)3Si]2Si: is totally stereospecific as seen for the reaction with cis-2-butene (Eq. 1).38 One explanation is that the

662 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)