Reactive Intermediate Chemistry

Scheme 14.58

5.2. Generation and Reactions of an Overcrowded Diarylsilylene

Until recently, the intrinsic nature of silicon–silicon double bond has not been fully disclosed, and there had been no report concerning the thermal dissociation of disilenes into the corresponding silylenes, in contrast to the facile thermal dissociation for the germanium and tin analogues.142 The high thermodynamic stability of the C C and Si Si double bonds to relative to those for the Ge Ge and Sn Sn dou-

ble bonds is in good agreement with the computed dissociation energies for the process H2E EH2 !2H2E: ( 140 kcal/mol for E ¼ C,143 52–58 kcal/mol for E ¼ Si,144 30–45 kcal/mol for E ¼ Ge,145 and 22–28 kcal/mol for E ¼ Sn.145a,146

Meanwhile, Tokitoh et al.147 reported the first example of thermal dissociation of extremely hindered disilenes [Tbt(Mes)Si Si(Mes)Tbt; Tbt ¼ 2,4,6-tris[bis(tri- methylsilyl)methyl]phenyl, Mes ¼ 2,4,6-trimethylphenyl (mesityl); (Z)-137: cis isomer, (E)-137: trans isomer] into the corresponding silylene [Tbt(Mes)Si:, 138] under very mild conditions ( 70 C) as shown in Scheme 14.59.

Mes; R = Me

R

Scheme 14.59

Kinetic studies on the thermal dissociation of 137 revealed that the unusual behavior of 137 in contrast to the stable disilenes obviously resulted from the presence of extremely bulky Tbt groups that weaken the Si Si double bond.147 The

multiple bond compounds such as carbon disulfide,149 nitriles,150 phosphaalkynes,150 and isonitriles (Scheme 14.61).151 It is especially interesting that the reaction of 138 with isonitriles bearing a bulky aromatic substituent afforded the corresponding silylene–isonitrile complexes 139, the first stable silylene–Lewis base complexes. The 13C and 29Si NMR spectroscopic data and theoretical calculations on the triphenyl-substituted model compound lead to the conclusion that the silylene–isonitrile adducts 139 do not have a cumulene structure (silaketenimine) but a zwitterionic structure (i.e., a silylene–isonitrile complex).

Scheme 14.61

Since Spitzer and West152 reported a similar pyrolysis of tetraalkyldisilene [(Me3Si)2CH]2Si Si[CH(SiMe3)2]2 leading to the generation of corresponding dialkylsilylene [(Me3Si)2CH]2Si: at somewhat higher temperature ( 100 C), thermal dissociation of disilenes into silylenes may be a more general phenomenon than has previously been thought.

5.3. Reactions of a Silylene–Isonitrile Complex as a Masked Silylene

As mentioned above, there have been no examples of stable diarylsilylene although dialkylor diamino-substituted silylenes are now available as stable crystalline compounds. However, Tokitoh et al.151 found that the overcrowded diarylsilylene– isonitrile complexes 139 act as a free silylene 138 in solution even at room temperature. Since there is a severe limitation in a study concerning the reactivities of silylenes under the conventional generating conditions, the reactivity of silylene– isonitrile complexes 139 as a masked silylene is of great interest.

The reactions of silylenes with 1,3-dienes giving the corresponding 3-silolenes

are typical of the cycloaddition reactions of silylenes.2,153 The mechanism of these reactions has been investigated in detail,114,154 and it has been proposed that the

reactions of silylenes with 1,3-dienes proceed via initial ½1 þ 2& addition followed by the isomerization of the resulting 2-vinylsiliranes to the corresponding 3-silo- lenes. However, the observation and isolation of the intermediary 2-vinylsiliranes has been limited to only a few examples155 because vinylsiliranes readily isomerize

690 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)

to 3-silolenes under the conditions of generation of the silylenes. Tokitoh and coworkers156 attempted the synthesis and isolation of a 2-vinylsilirane intermediate by using silylene–isonitrile complex 139a as a silylene source (Scheme 14.62).

Scheme 14.62

Thus, complex 139a reacted with isoprene at room temperature to give the corresponding 2-vinylsilirane (140a, 84%). 2-Vinylsilirane (140a) was also isolated in 89% yield by the reaction of disilene 137 with isoprene at 70 C, which indicates that these reactions proceeded via silylene 138 as an intermediate. The exclusive formation of 140a is probably the result of steric repulsion between bulky substituents on the silicon atom and the methyl group of isoprene. Although 2-vinylsili- rane (140a) was stable in the solid state even on exposure to air, it underwent hydrolysis on silica gel to give the corresponding silanol 141a. Thermolysis of 140a in C6D6 at 160 C for 12 days resulted in the formation of the formal ½1 þ 4& adduct 142a in 88% yield. This reaction is the first isolation of the initial intermediate in the reaction of thermally generated silylene with a 1,3-diene, that is, the 2-vinylsilirane derivative, although the intermediacy of this type of ½1 þ 2& adduct in the reactions of photochemically generated silylenes has been already evidenced by NMR spectroscopy.155a

On the other hand, when complex 139a was allowed to react with 2,3-dimethyl- 1,3-butadiene in C6D6 in a sealed tube at room temperature, the formation of both ½1 þ 2& adducts 140b and ½1 þ 4& adduct 142b was observed by 29Si NMR spectroscopy (Scheme 14.63; 14b: dSi ¼ 76:3; 72:9, 16b: dSi ¼ 5:3). Since heating the reaction mixture at 50 C for 7 h resulted in no isomerization of 140b into 142b, silylene 138 seems to undergo ½1 þ 2& and ½1 þ 4& addition reactions with 2,3-dimethyl-1,3-butadiene competitively. This is the first clear evidence for the occurrence of direct ½1 þ 4& addition of a silylene to a 1,3-diene. This phenomenon is most likely due to the suppression of ½1 þ 2& addition by steric repulsion between the methyl groups of the butadiene and the bulky substituents on the silicon atom of 138. This interpretation is also supported by the exclusive production of 140a in the reaction with isoprene. Thus, the results obtained here suggest that silylenes bearing very bulky substituents may undergo direct ½1 þ 4& addition to dienes. Vinylsilirane

692 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)

succeeded in the synthesis and isolation of very stable dialkylgermylene (135) (Scheme 14.64). In this case, germylene 135 was found to show an identical UV–vis spectrum in THF or hexane, indicating that this cyclic germylene 135 does not form a complex with THF because of severe steric hindrance around the central germanium atom. This result is in sharp contrast to other germylenes that are known to form complexes with bases such as ethers and amines and to cause significant blue shift of the n–p transition of the germylenes.

6.2. Synthesis of Stable Diarylgermylenes

As for stable diarylgermylenes, three examples (146,159 147,160 and 148,161 Scheme 14.65) have been structurally analyzed by X-ray crystallographic analysis. The solid-state structure of germylene 146 indicated that its remarkable stability is mainly the result of intramolecular coordination of the fluorine atoms of the -CF3

Scheme 14.65

substituents to its electron-poor germanium center, while the latter two germylenes (147 and 148) were found to be only kinetically stabilized by their extremely bulky aromatic substituents. However, the stability of germylene 148 was only marginal. In solution, 147 underwent intramolecular insertion of the germanium atom into one of the methyl groups in the substituents in the presence of a Lewis acid or at high temperature to give a germaindane derivative 149 (Scheme 14.65).162

Another stable diarylgermylene (150)163 bearing extremely bulky aromatic substituents (Tbt group) together with 2,4,6-triisopropylphenyl group (Tip group) has been reported. That there is no color change on concentration of the blue solution of 150 in hexane (lmax ¼ 581 nm) strongly suggests that germylene 150 is stable as a monomeric species both in solution and in the solid state, although the solid-state structure of 150 has not been established yet. Germylene 150 was readily synthesized by either the reductive debromination of the corresponding dibromogermane,

Tbt(Tip)GeBr2,164a–c or the thermal retrocycloadditon of the 2,3-diphenylgermirene bearing Tbt and Tip groups on the germanium atom (Scheme 14.66).164d The latter

reaction was found to be a useful synthetic method for germylene 150 under neutral conditions. In spite of the presence of such bulky substituents, germylene 150

showed versatile reactivity toward a variety of reagents such as alcohols, butadienes, acetylenes, hydrosilanes, and elemental chalcogens (sulfur and selenium) (Scheme 14.66). This effective combination of bulky aromatic ligands was success-

fully applied to the synthesis and isolation of a series of stable germanium–chalco- gen double-bond species Tbt(Tip)Ge X (X ¼ S,164a,165 Se,165,166 Te,166,167).

X = S, Se

Tip = 2,4,6-triisopropylphenyl

Scheme 14.66

On the other hand, when a slight modification from Tip to Mes was made in the ligand attached to the germanium center in Tbt-substituted diarylgermylene, the stability of the germylene drastically changed.168 Thus, the less hindered diarylgermylene Tbt(Mes)Ge: (151) was found to be in equilibrium with its dimer, that is, digermene 152 (Scheme 14.67). Germylene 151 showed interesting thermochromism in hexane [blue at 295 K vs. yellow at 190 K], which is in sharp contrast

to that of the temperature independence of the UV–vis spectra observed for 150.164a,165 The absorptions observed at 190 K (lmax ¼ 439 nm, e 2:0 104) and 295 K (lmax ¼ 575 nm, e 1:6 103) are assignable to the p–p* transition of digermene 152 and the n–p transition of germylene 151, respectively. The isosbestic points observed at 335, 390, and 509 nm indicate the quantitative interconversion between 152 and 151.

The thermodynamic parameters ( H ¼ 14:7 0:2 kcal/mol and S ¼ 42:4 0:8 cal/mol/K) for the dissociation of digermene 152 to germylene 151 were obtained from the temperature dependence of the absorptions. The bond dissociation energy (BDE) (14.7 kcal/mol) of 152 into 151 is much smaller than the calculated value (30–45 kcal/mol145) for the parent system (H2Ge GeH2), indicating that the germanium–germanium double bond in 152 is considerably weakened

694 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)

Scheme 14.67

because of the severe steric repulsion between the bulky substituents. Indeed, X- ray crystallographic analysis of the orange crystal of (E)-152 revealed that it has

which is in good agreement with the ready dissociation into germylene 151 in solution.

Meanwhile, when the orange solid of (E)-disilene (137) bearing the same substituents as 152 was dissolved in pentane, the generation of the corresponding silylene 138 was not observed as judged by UV–vis spectra.147 The fact that the disilene 137 exists essentially as a dimer of 138 and dissociates into 138 in solution only to a minor extent at ambient temperature is also noteworthy, although a similar

phenomenon is known for the bis(trimethylsilyl)methyl-substituted disilene– digermene pairs: [(Me3Si)2CH]2M M[CH(SiMe3)2]2 (M Si,170 Ge157). On the

other hand, (E)-disilene (137) dissociates into the corresponding silylene 138 upon heating with activation parameters ( H ¼ 25:0 0:6 kcal/mol and S ¼ 1:3 1:7 cal/mol/K) the H of the equilibrium between 137 and 138 was not determined because of the low concentration of 138 (Scheme 14.67). The most reasonable explanation for the difference between disilene 137 and digermene 152 is that the bond dissociation energy of 137 into 138 is larger than that of 152 into 151. In the case of tin analogues, the equilibrium between distannene (153) and the corresponding stannylene (154) was reported and the thermodynamic parameters ( H ¼ 11:2 0:3 kcal/mol and S ¼ 28:0 1:3 cal/mol/K) for the equilibrium were obtained from the temperature dependence of the NMR chemical shifts (Scheme 14.67).142d The BDE of distannene (153) is smaller than those of digermene (152) and disilene (137), and the BDE of the double-bond species of heavier group 14 (IVA) elements were found to decrease on going from Si to Sn as the theoretical calculations predicted.

6.3. Reactions of Kinetically Stabilized Germylenes

Although very little is known for the reactions of isolated germylenes 146, 147, and 148, diarylgermylene (150) was found to undergo a variety of reactions with olefins, dienes, acetylenes, alcohols, isothiocyanates, elemental chalcogens, and hydrosilanes as in the cases of less hindered, transient germylenes previously reported.

Furthermore, germylene (150) showed a unique reactivity toward some hetero- atom-containing compounds such as carbon disulfide,163 nitrile oxide,171 and gemdihalogenated olefins.172 Treatment of 150 with carbon disulfide resulted in a formation of a novel heteroatom-substituted germene 155 (Scheme 14.68).163 The germaketenedithioacetal structure of the core part of 155 was crystallographically established and the oxidation of 155 with molecular oxygen was found to lead to the diarylgermanone 156 (Scheme 14.68).173

Scheme 14.68

Germylene (150) was allowed to react with a bulky nitrile oxide such as mesitonitrile oxide to give the corresponding ½1 þ 3& cycloaddition product 157, which is the first stable example of an oxazagermete derivative.171 Alternative formation of germanone (156) was achieved by thermolysis of this strained germaheterocycle 157 (Scheme 14.68).171 The first stable alkylidenetelluragermirane 158 was synthesized by the reaction of 151 with 9-(dichloromethylene)fluorene, followed by addition of triphenylphosphine telluride (Scheme 14.69).172 The formation of 158 is most likely interpreted in terms of the initial insertion of germylene 151 into the C Cl bond of 9-(dichloromethylene)fluorene followed by dechlorination with more germylene 151 leading to 1-germaallene and subsequent telluration. The selenium and sulfur analogues of 158, that is, selenaand thiagermiranes 159 and 160,

696 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)

Scheme 14.69

were synthesized by a similar synthetic approach (Scheme 14.69).172 Complexation of germylene (150) with some transition metal carbonyl complexes was also examined to give the first base-free examples of a germylene-group 6 (VIB) metal mononuclear complex, 161 and 162 (Scheme 14.70).164b

150

161; M = Cr

162; M = W

Scheme 14.70

Thus, Tbt-substituted germylenes 150 and 151 have enough space for functionalization, thus enabling them to be versatile synthetic building blocks for a variety of new organogermanium compounds.

7. SYNTHESIS, STRUCTURES, AND REACTIONS OF STABILIZED STANNYLENES

The structures of some stable stannylenes, such as several amino-,174 alkoxy-,175 and arylthio-substituted176 intermediates, have been revealed by X-ray crystallography. They are monomeric crystals and the tin atom has the coordination number 2. The divalent tin in such compounds is stabilized by the effects of electronegativity of the ligand atoms and by the donation of the lone-pair electrons to the vacant 5p orbital of the tin. Although the first monomeric dialkyland diarylstannylenes in