The chemistry of silacyclobutenes: Synthesis, reactions, and theoretical studies

Coordination Chemistry Reviews 335 (2017) 58–75

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Review

The chemistry of silacyclobutenes: Synthesis, reactions, and theoretical studies Mitsuo Ishikawa a,⇑, Akinobu Naka b, Hisayoshi Kobayashi c a

Professor Emeritus Hiroshima University, Higashi-Hiroshima 739-8527, Japan Department of Life Science, Kurashiki University of Science and the Arts, Kurashiki, Okayama 712-8505, Japan c Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8586, Japan b

a r t i c l e

i n f o

Article history: Received 3 August 2016 Received in revised form 22 December 2016 Accepted 22 December 2016 Available online 27 December 2016 Keywords: Nickel Thermolysis Photolysis Isomerization Silacyclobutene Silene

a b s t r a c t The synthesis and reactions of various silacyclobutenes developed by our research group are reported in this review. The nickel-catalyzed reactions of phenylethynylpolysilanes and silacyclopropenes generated photochemically from the phenylethynylpolysilanes, with phenyl(silyl)acetylenes afforded the silacyclobutene derivatives. The photolysis and thermolysis of acylpolysilanes in the presence of alkynes readily produced the silacyclobutenes. These reactions provide important routes for the synthesis of various types of the silacyclobutenes. The silacyclobutenes thus obtained underwent thermal reactions to give many types of the products, whose structures highly depend on those of the starting silacyclobutenes. The results on the theoretical studies concerning the synthesis and reactions of the silacyclobutenes have also been reported. Ó 2016 Elsevier B.V. All rights reserved.

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Nickel-catalyzed reaction of (phenylethynyl)polysilanes and silacyclopropenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Transition-metal-mediated formation of silacyclobutene system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Photochemical formation of silacyclobutenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Thermal formation of silacyclobutenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Reaction of acylpolysilanes with mono- and di-alkynes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Reaction of acylpolysilanes with bis(silyl)but-1-en-3-ynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7. Reaction of acylpolysilanes with diphenylketene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reaction of silacyclobutenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Thermolysis of 3-silyl-1-silacyclobutenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Thermolysis of 2-silyl-3-silylethynyl-1-silacyclobutenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Thermolysis of 2-silyl-3-silylethenyl-1-silacyclobutenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Thermolysis of 3-tert-butyl-1-silacyclobutenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Palladium-catalyzed reaction of phenylethynylidenesilacyclobutene with acetylene dicarboxylate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Reaction of acylpolysilanes with mesitylacetylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7. Reaction of acylpolysilanes with bis(trimethylsilyl)acetylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. E-mail address: [email protected] (M. Ishikawa). http://dx.doi.org/10.1016/j.ccr.2016.12.011 0010-8545/Ó 2016 Elsevier B.V. All rights reserved.

59 59 59 62 62 65 65 67 67 68 69 69 70 71 73 73 74 74 74 74

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M. Ishikawa et al. / Coordination Chemistry Reviews 335 (2017) 58–75

The results of the investigation on the synthesis and reactions of silacyclobutenes reported up to 2006, are summarized in a review article published by Mohseni-Ala and Auner [22]. Many examples for the formation of the silacyclobutenes by way of the photolysis of acylpolysilanes in the presence of alkynes have been reported by Brook and his co-workers [7]. In 1984, we reported that the reactions of (phenylethynyl) polysilanes and silacyclopropenes [23a] with alkynes in the presence of a nickel catalyst gave silacyclobutenes, which can best be explained by assuming the transient formation of nickelcontaining reactive species such as 1-silapropadiene-nickel complexes [23b]. On the other hand, the thermolysis of acylpolysilanes in the presence of alkynes afforded the silacyclobutenes, which undergo thermal reactions to give many types of the product. The purpose of this account is to summarize the results obtained in our laboratory on the synthesis and reactions of the silacyclobutenes and the related papers published to date. We also report theoretical investigation on the formation and reactions of the silacyclobutenes.

1. Introduction Numerous examples of the synthesis and reactions of unsaturated silicon compounds, silenes, have been reported in the past several decades [1–6]. For the synthesis of silenes, acylpolysilanes can be used as useful starting materials. In fact, the photolysis [7,8] and thermolysis [9,10] of acylpolysilanes afford various types of the silenes. The Peterson-type reactions of acylpolysilanes also offers a convenient method for the preparation of the silenes [11–16]. Cycloaddition of the silenes thus formed to alkynes provides a simple method for the synthesis of many types of silacyclobutenes, which are not readily available by any other route. The silacyclobutenes are highly reactive, due to the high ring strain, and therefore, show interesting chemical behavior. Further, the silacyclobutenes are useful materials for the synthesis of both cyclic and ring-opened compounds. In 1964, the first example for the synthesis of silacyclobutene was reported by Gilmann and Atwell [17]. They prepared 2,3-benzo-1,1-diphenyl-1-silacyclobutene by the reaction of o-bromobenzylmagnesium bromide with dichlorodiphenylsilane in the presence of magnesium or by the simultaneous addition of o-bromobenzylbromide and dichlorodiphenylsilane to magnesium. Subsequently, many attempts have been made to find a convenient route leading to the silacyclobutenes [18,19]. For examples, vapor phase pyrolysis of appropriate silicon compounds produces silacyclobutenes. Auner and his coworkers [20] found that addition of tert-butyllithium to trichlorovinylsilane in the presence of dialkyl-substituted acetylene gave 1,1-dichloro-2,3-dialkyl-4-neo pentyl-1-silacyclobutene. Takahashi et al. [21]. found the reactions of di-substituted dialkynylsilanes with a low valent zirconocene complex, followed by treatment of the resulting mixture with hydrogen chloride or alkynes, giving various silacyclobutenes.

SiMe3 PhC

C

Si

2. Synthesis 2.1. Nickel-catalyzed reaction of (phenylethynyl)polysilanes and silacyclopropenes In 1984, we reported the first example of the nickel-catalyzed reaction of (phenylethynyl)polysilanes with alkyne [23]. The reaction involves the nickel-catalyzed isomerization of the starting (phenylethynyl)polysilanes and the addition of the resulting nickel complexes to a triple bond of the alkyne. When a mixture of (phe

Ph hν

SiMe3

SiMe3 C

C Si

SiMe3

Me3Si

1

1 and 2

NiCl2 (PEt3 )2

SiMe3 2

Me 3Si

SiMe3 C

C

Ph

SiMe3

Ph C

C

Me

(Et3 P)2 Ni

Si

SiMe2

Si SiMe3 Ni(PEt3) 2

SiMe3

A

B PhC

CSiMe3

Ph Me 3Si

Ph SiMe3

C

Ph

SiMe 3 C

C Me

C

Si

C

C 3

SiMe3

Me2 Si

Si Ni (PEt)2

SiMe3

SiMe3

C PhC Ph

SiMe3 C

C

Me2 Si

Me Si

C Me 3Si

CSiMe3

SiMe3

C Ph 4

Scheme 1. Nickel-catalyzed reaction of phenylethynylpolysilanes 1 and silacyclopropene 2 with phenyl(trimethylsilyl)acetylene [23,24].

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M. Ishikawa et al. / Coordination Chemistry Reviews 335 (2017) 58–75

(8), respectively. In this reaction, 1-silapropadiene-nickel complex (E) gives the silacyclobutene derivative 8, but nickel species (D) and (F) produce the products 6 and 7 [24], respectively, as shown in Scheme 2. When (phenylethynyl)bis(trimethylsilyl)mesitylsilane (9) was heated with phenyl(trimethylsilyl)acetylene in the presence of a catalytic amount of Ni(PEt3)4, 1-mesityl-3-phenyl-2-[phenyl(trime thylsilyl)methylene]-1,4-bis(trimethylsilyl)-1-silacyclobut-3-ene (10) was obtained in 77% yield, along with 1-mesityl-3,4-diphenyl1,2,5-tris(trimethylsilyl)silole (11) in 11% yield (Scheme 3). The product 10 could readily be separated from 11 by fractional recrystallization from methanol. The formation of major product 10 may be best explained by the nickel-catalyzed reaction of

nylethynyl)tris(trimethylsilyl)silane (1) and phenyl(trimethylsilyl) acetylene in the presence of a catalytic amount of NiCl2(PEt3)2 was heated at 200 °C, two crystalline products, 3-phenyl-4-[phenyl(tri methylsilyl)methylene]-1,1,2-tris(trimethylsilyl)-1-silacyclobut-2ene (3) and 1,4,4-trimethyl-3,6-diphenyl-1,2,5-tris(trimethylsilyl)1,4-disilacyclohexa-2,5-diene (4) were obtained in 19% and 58% yield, respectively, as shown in Scheme 1 [24]. The photolysis of (phenylethynyl)polysilanes readily affords isolable silacyclopropenes [6,25]. Thus, irradiation of 1 with a low-pressure mercury lamp gave 3-phenyl-1,1,2-tris(trimethylsi lyl)-1-silacyclopropene (2) [24]. Silacyclopropene 2 thus obtained is less sensitive to air than the usual silacyclopropenes, but it still have to be handled under an inert atmosphere. When a mixture of silacyclopropene 2 and phenyl(trimethylsi lyl)acetylene in the presence of a NiCl2(PEt3)2-catalyst was heated at 135 °C, the products 3 and 4, the same products as those obtained from the nickel-catalyzed reaction of 1 with phenyl(trime thylsilyl)acetylene, were obtained in the ratio of 1:1.4 in 84% yield [24]. The fact that the nickel-catalyzed reactions of 1 and 2 with phenyl(trimethylsilyl)acetylene afford the same products indicates that both reactions involve the common intermediates, silapropadiene-nickel complex (A) and nickelasilacyclobutene (B) (see Scheme 1). The formation of A from 1 may be understood by isomerization of 1 involving a 1,3-trimethylsilyl shift, while the formation of B from 2 can be explained by the insertion of a nickel species into a C-Si bond in the silacyclopropenyl ring. The formation of the silacyclobutene derivative 3 may be best understood in terms of [2+2] cycloaddition of 1-silapropadienenickel complex A with phenyl(trimethylsilyl)acetylene. It seems likely that the product 4 would be produced by the reaction of nickeladisilacyclopentene intermediate (C), arising from isomerization of B, with the acetylene. The nickel-catalyzed reaction of 1-methyl-3-phenyl-1,2-bis(tri methylsilyl)-1-silacyclopropene (5) with phenyl(trimethylsilyl)ac etylene afforded 1-methyl-3,4-diphenyl-1,2,5-tris(trimethylsilyl)s ilole (6), two regioisomers of 1,1,4,4-tetramethyl-3-phenyl-5-[phe nyl(trimethylsilyl)methylene]-2-trimethylsilyl-1,4-disila-cyclo pent-2-ene (7a and 7b), and 1-methyl-3-phenyl-4-[phenyl-(trime thylsilyl)methylene]-1,2-bis(trimethylsilyl)-1-silacyclobut-2-ene

Ph

path a Ph

path b SiMe3 PhC

C

SiMe3

Si

C SiMe3 path a

Mes

C

Me 3Si Ph

9

Mes

path b Ph

Me 3Si

C Ph

Si Mes

+

Me

PhC

SiMe3 C

(Et3 P)2 Ni

Si

Ph

SiMe3 SiMe 3

11 Scheme 3. Nickel-catalyzed reaction of phenylethynylpolysilane 9 with phenyl (trimethylsilyl)acetylene [26,27].

Me C

C

PhC

Me

SiMe3 6

Me

PhC Me 2 Si

Me 3Si Ph

C

SiMe3

Ni(PEt 3) 2 Si Me2

Ph

CSiMe3

Me3 Si Ph

C Si

C

F

C

C Me3 Si

Si

Me 2 Si

Me 3Si

Si Me2 Ni(PEt 3) 2

Ph

CSiMe3

Ph C

Me

E

Ph

SiMe3

C Si

Mes

Me

PhC

10

C

C Me3Si

Me 3Si

D

C

Ph C

Ni(PEt3 )4

C SiMe3

C

SiMe3

CSiMe3

SiMe3

C

Si

Ni(PEt3 )4

5

Ph

C

SiMe3

C Si

C

SiMe3 C

CSiMe3

Ni(PEt3 )4

Mes

SiMe3 C

PhC

Si

C

C

Si

C

SiMe3

C Ph

SiMe3

C

CSiMe3

Ph C

C Si Me2

SiMe3

7a and 7b

8 Scheme 2. Nickel-catalyzed reaction of silacyclopropene 5 with phenyl(trimethylsilyl)acetylene [24].

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M. Ishikawa et al. / Coordination Chemistry Reviews 335 (2017) 58–75

Ph

hν

9

SiMe3 C Si

Mes

SiMe3

Ph Ni(PEt3 )4

C

C

C

(Et3 P)2 Ni

Si Mes

12

PhC

CSiMe3

13

10

11

+

Me 2 Si

Me Ph

Ph

SiMe 2

13

SiMe3

SiMe3

C

C

C

Si

Me 3Si

Ni(PEt 3) 2

C Si

Me 3Si

Mes Ni(PEt3 )2

Me

G Me2 H Si Ni

Ph C

C

Ni(PEt 3 )2 C

CH 2

Si

Me 3Si

Me2 Si

Ph C

CH 2

Me 3Si

Si

Me

Me H

Ph C

Me2 Si

C

Me 3Si

Me 3Si C

CH 2 +

Si

Me2 Si

C

Ph

CH 2

Si

Me

Me H

H

14a

14b

Scheme 4. Formation and reaction of nickelasilacyclobutene 13 [27].

Me

Ph Ph PhC

CSiMe2 SiMe3

PhC

CSiMe3

C

C

C +

Ni(PEt3) 4

15

Ph

C

Me 2Si

C Si Me2

Me3 Si

C

SiMe 3

16

SiMe2 C

C

Me3 Si

Ph 17a and 17b

Me 2 Si

Me 3Si +

C

C

C Si Me2

Ph

Ph C SiMe3

18 Ph 15

hν

SiMe3 C

C

Me

PhC

CSiMe3

16

Ni(PEt3) 4

Si Me 19

Scheme 5. Nickel-catalyzed reaction of phenylethynyldisilane 15 and silacyclopropene 19 with phenyl(trimethylsilyl)acetylene [24].

1-silapropadiene intermediate with phenyl(trimethylsilyl)acety lene (path a), while minor product 11 may be understood by the reaction of silacyclopropene with the acetylene (path b).

In an effort to get more information about the nickel complexes, which are considered to be involved in the reaction, we carried out the stoichiometric reaction of silacyclopropene with a nickel

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M. Ishikawa et al. / Coordination Chemistry Reviews 335 (2017) 58–75

complex [27]. Thus, the reaction of 1-mesityl-3-phenyl-1,2-bis(tri methylsilyl)-1-silacyclopropene (12), generated photochemically from 9, with 1 equiv of tetrakis(triethylphosphine)nickel(0) afforded 2-mesityl-4-phenyl-1,1-bis(triethylphosphine)-2,3-bis(tri methylsilyl)-1-nickela-2-silacyclobut-3-ene (13) quantitatively. Similar treatment of 12 with tetrakis(triethylphosphine)nickel(0) gave 13 in quantitative yield (Scheme 4). Unfortunately, all attempts to isolate nickelasilacyclobutane 13, as a pure form, were unsuccessful. Complicated decomposition products were always obtained. Therefore, we used the solution of 13 for further reactions to examine its chemical behavior. First of all, the reaction of nickelasilacyclobutene 13 with phe nyl(trimethylsilyl)acetylene was examined. When a xylene solution of 13 with a small excess of phenyl(trimethylsilyl)acetylene was heated to reflux for 21 h, silacyclobutene 10 and silacyclopentadiene (silole) 11 were obtained in 6% and 32% yield, respectively. Next, the reaction was carried out in the absence of the acetylene. Thus, when a xylene solution of 13 was heated to reflux in the absence of phenyl(trimethylsilyl)acetylene for 2 h, two regioisomers, 5,6-benzo-1,3-disilacyclohexene derivatives 14a and 14b were obtained in 47% and 41% yield, respectively. The formation of 14a and 14b may be understood by the isomerization of the nickelasilacyclobutene 13 to silapropadiene-nickel complex (G), followed by further isomerization of G to the nickel-containing cyclic complexes shown in Scheme 4 [27]. The nickel-catalyzed reactions of (phenylethynyl)polysilanes bearing less hindered substituents on the central silicon atom in a polysilanyl group, with phenyl(trimethylsilyl)acetylene produce the different types of the products from those obtained in the reaction of 1. For example, the reaction of (phenylethynyl)pentamethyl disilane (15) with phenyl(trimethylsilyl)acetylene in the presence of a nickel catalyst afforded four products, 1,1-dimethyl-3,4-diphe nyl-2,5-bis(trimethylsilyl)silole (16), two regioisomers of 1,1,4,4-t etramethyl-5-(methylphenylmethylene)-3-phenyl-2-(trimethylsi lyl)-1,4-disilacyclopent-3-ene (17a and 17b), and 1,1,3,3-tetrame thyl-2,5-bis[phenyl(trimethylsilyl)methylene]-1,3-disilacyclobu tane (18) in a ratio of 1:0.6:0.6:0.4 [24,25] (Scheme 5). In this reaction, no silacyclobutene was detected in the reaction mixture. The reaction of 1,1-dimethyl-3-phenyl-2-(trimethylsilyl)silacy clopropene (19) prepared photochemically from 15 with phenyl(tri methylsilyl)acetylene in the presence of a catalytic amount of NiCl2(PEt3)2 gave silole 16, as the sole product [24,28]. In the nickel-catalyzed reactions, the (phenylethynyl)polysilanes and silacyclopropenes bearing the bulky substituents such as a mesityl and trimethylsilyl group on the silicon atom, afford silacyclobutenes, but compounds with less bulky substituents produce no silacyclobutenes.

butene-silacyclobutene fused ring compounds, similar structure to 29, have also been reported [30]. As an extension of the zirconocene-mediated coupling, Xi and co-workers carried out the reaction of 1,4-dialkynylbenzene and 1,3,5-trialkynylbenzene with bis(phenylethynyl)silanes in the presence of Cp2Zr(n-Bu)2, and found that 1,4-bis- and 1,3,5-tris(sila cyclobutene)-containing benzenes with a longer p-conjugated system were produced [31]. Many examples for the reactions of zirco nacyclobutene-silacyclobutene fused complexes are described in a review article reported by Xi and co-workers [31c]. Low temperature synthesis of a silacyclobutenyl ring system utilizing the platinum hydride addition has been reported by Lukehart and MacPhail groups [32]. When bis(tert-butylethynyl)diphenylsilane (23) was treated with cationic platinum hydride complex (30), followed by addition of a moderate excess of water, platinum-substituted silacyclobutene (31) was isolated as an aquo complex [32a]. A mechanistic study of this reaction shows that the reaction proceeds through the formation of two distinct intermediates (32) and (33) (Scheme 8) [32b]. The reaction of 23 with 30 in the presence of an excess of acetic acid gives (alkynyl)(alkenyl)diphe nylsilane (34). 2.3. Photochemical formation of silacyclobutenes It is well-known that the photolysis of acylpolysilanes offers a convenient method for the synthesis of many kinds of silene, including stable silenes. This method was found for the first time by Brook, et al. in 1979 [33a], and a new field of silicon chemistry was developed using the reaction of such silenes [33]. For examples, the silenes readily react with alkynes to give silacyclobutenes in good yield. In fact, the photolysis of pivaloyltris(tri methylsilyl)silane in the presence of 1-phenylpropyne in THF produces a [2+2] cycloadduct, 4-tert-butyl-2-methyl-3-phenyl-4-tri methysiloxy-1,1-bis(trimethylsilyl)-1-silacyclobut-2-ene [33a].

R RC 20, R 21, R 22, R 23, R

C

2

SiR' 2 +

H

Cp2 ZrEt 2

= Ph, R' = Me = R' = Ph = MeOC6 H 4, R' = Me = t-Bu, R' = Ph

C C

SiR' 2

C

C R

H 24, R 25, R 26, R 27, R

= Ph, R' = Me = R' = Ph = MeOC6 H 4, R' = Me = t-Bu, R' = Ph

Scheme 6. Reaction of bis(alkynyl)silane 20–23 with Cp2ZrEt2 [29].

2.2. Transition-metal-mediated formation of silacyclobutene system Takahashi et al. have reported that treatment of bis(alkynyl)silanes with a stoichiometric amount of Cp2ZrEt2, followed by hydrolysis of the resulting mixture gives silacyclobutenes in high yield [29]. Treatment of bis(phenylethynyl)dimethylsilane (20), bi s(phenylethynyl)diphenylsilane (21), bis(4-methoxy-phenylethy nyl)dimethylsilane (22), and bis(tert-butylethynyl)diphenylsilane (23) with zirconocene compound affords the respective silacyclobutenes (24–27), after hydrolysis of the reaction mixtures (Scheme 6). Deuterolysis of the mixture obtained from the reaction of 20 with zirconocene compound gives dideuterated product (28). This result indicates that the intermediate involved in the reactions must have the zirconacyclobutenesilacyclobutene ring structure (Scheme 7). In fact, the reaction of (t-BuC5H4)2ZrBu2 with bis(alkynyl)silane 21 gives zirconacyclobutene-silacyclobutene complex (29). The synthesis and reactions of dicyclopentadienyltitanacyclo

Ph D 2O 20

+

D

C

Cp2 ZrEt 2

C

SiMe2

C

C Ph

D 28 Ph 21

+

C

C

SiPh 2

(t-BuC 5H 4) 2Zr

C

C

(t-BuC5 H4 )2 ZrEt 2 29

Ph

Scheme 7. Deuterolysis of the product formed from the reaction of bis(phenylethynyl)diphenylsilane 20 with Cp2ZrEt2 and the reaction of 21 with (t-BuC5H4)2ZrEt2 [30].

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M. Ishikawa et al. / Coordination Chemistry Reviews 335 (2017) 58–75

t -BuC

C

2

[tr ans-Pt(H)(PEt 3) 2(THF)]+[SbF6 ]-

SiPh 2 +

23

30 +

t -Bu PEt3

C

Ph

C

Si Ph

Pt(H 2 O)

C

[SbF6 ]-

PEt3

C t-Bu

H 31

t-Bu

t-Bu

H C C

23

30

+

H C C

Ph2 Si

H

CH3 COOH

+

PtL2

Ph2 Si

C

C C

t-Bu

C

t -Bu

32

34

t -Bu

H C

32

31

C Ph2 Si

C C 33

+

PtL 2

t-Bu

Scheme 8. Formation of platinum-containing intermediates 32 and 33 [32].

hν

Me3 Si

(Me3 Si) 2 SiCOAd Mes 35

Me3 Si

Ad Si

+

C OSiMe 3

Mes

OSiMe3 Si

Ad

Mes 36b

36a RC

Me3 Si Mes

H

CH

C

C

C

RC

Me3 Si

Ad

Si

C

OSiMe3

R

37a, R = Ph 38a, R = Me3Si

Mes

H

CH

OSiMe3

Si

C

C

C

Ad

R

37b, R = Ph 38b, R = Me 3Si

Scheme 9. Photochemical reaction of acylpolysilane 35 with phenyl- and silylacetylene [33d].

Similar photolysis of (adamantoyl)[mesitylbis(trimethylsilyl)]s ilane (35) gives rise to two stable geometric isomers of silene (36a and 36b), with a ratio of 1.4:1, at room temperature. The reaction of a 1.4:1 mixture of the silenes 36a and 36b with phenylacetylene proceeds with high regio- and stereospecificity to afford a 1.4:1 mixture of silacyclobutenes (37a and 37b) [33d]. The reaction of a mixture of 36a and 36b in the ratio of 1.4:1 also reacts with (trimethylsilyl)acetylene to give a 1.4:1 mixture of two silacyclobutenes (38a and 38b) as shown in Scheme 9. The structures of the major adduct 37a and 38a were confirmed by X-ray crystallography, and the parent major silene 36a was determined to have E-geometry on the basis of the X-ray analysis of the adducts.

Baines, et al. [8,34] carried out the reaction of the silenes photochemically generated from pivaloyl- and adamantoyltris(trime thylsilyl)silane with mono-substituted alkynes, such as (trans-2phenylcyclopropyl)ethyne, (trans,trans-2-methoxy-3-phenylcyclo propyl)ethyne, and (trans,trans-2-methoxy-1-methyl-3-phenylcy clopropyl)ethyne, to clarify the mechanism for the formation of the silacyclobutenes. The reaction of (Me3Si)2Si@C(OSiMe3)t-Bu with (trans-2phenylcyclopropyl)ethyne, which contains a-hydrogen, produces a diastereomeric mixture of an ene-adduct, allenic silane (39a and 39b), as the sole product. Similar reaction of (trans,trans-2-m ethoxy-1-methyl-3-phenylcyclopropyl)ethyne, bearing no ahydrogen, with the silene, however, proceeds to give a mixture of two diastereomers of silacyclobutene (40a and 40b) and six diastereomers of silacycloheptene (41a–41f). On the basis of the structure of silacycloheptenes 41a–41f, in which the phenyl substituent locates at the a-position to the former silenic carbon, it is likely that products 41a–41f, and silacyclobutenes 40a and 40b are formed by the mechanism involving a biradical intermediate (Scheme 10). Theoretical treatment for the addition of alkynes to the silenes suggests the radical mechanism [10g,35,36]. For the formation of the silene, two kinds of stereochemistry may be considered to be involved in the migrating silyl radical for the 1,3-silyl shift from silicon to oxygen in the acylpolysilanes; one of them is stereochemically retention and the other is inversion on the migrating silyl radical center. The silene thus formed should retain the stereochemistry of the silyl radical. The retention pathway is energetically preferred to the inversion pathway. The activation energy for the [2+2] cycloaddition of silene and acetylene is evaluated to be 27.6 kcal/mol.

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M. Ishikawa et al. / Coordination Chemistry Reviews 335 (2017) 58–75

OSiMe 3 HC (Me 3Si)2 Si

C(OSiMe3 )t-Bu +

C

H

CH

t-Bu

C

(Me3 Si)2Si

C

H C

HC

C

CH 2

Ph HC

CH 2

Ph 39a and 39b

HC C (Me 3Si)2 Si

Me

C(OSiMe3 )t-Bu +

C HC

CH

Ph

OSiMe 3

OSiMe 3

t-Bu

C

(Me3 Si)2Si

OMe

C

(Me3 Si)2Si

HC HC

C

Me

C

CH

Me

C HC Ph

OSiMe 3

t-Bu

HC

Ph

C

CH OMe

t-Bu

C

(Me3 Si)2Si HC

HC

Ph

C

CH OMe

C

Me

OMe (Me 3Si)2 Si OSiMe 3

OSiMe 3 (Me3 Si)2Si

C

HC

C

(Me3 Si) 2 Si

t-Bu

t-Bu

C

CH Ph

HC Me

HC

HC

C

C

(Me3 Si)2 Si Me 3SiO

CH

Ph

C(OSiMe3 )t-Bu

OMe

C

C H

OMe

CH 2

t-Bu 41a - 41f

40a and 40b

Scheme 10. Reaction of silene with alkynyl-substituted cyclopropanes [8,34a].

t-Bu 2 Si t-BuC

C

C

Ct-Bu

+

t -Bu2 Si

C

C

C

C

t -Bu

t-Bu 42

Si t-Bu2

t -Bu hν

(t-Bu) 2Si

C

C

C

C

Si(t -Bu) 2

t-Bu 43 Scheme 11. Photochemical reaction of silylene with 1,3-diyne [37a].

Weidenbrüch et al. [37a] reported that the reaction of di(tertbutyl)silylene generated photochemically with 1,4-di(tert-butyl)1,3-diyne gave the isolable bis(silacyclopropene) (42), which

upon continued irradiation, underwent rearrangement to yield bis(silacyclobutene) (43) (Scheme 11). They have also reported that the reaction of di(tert-butyl)silylene with 1,8-di-tert-

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M. Ishikawa et al. / Coordination Chemistry Reviews 335 (2017) 58–75

Ph

SiMe3 C

C

Me 2Si C Me 3Si

Me Si

2.4. Thermal formation of silacyclobutenes

Ph hν

SiMe 3

C

Me3 Si

SiMe3

C Ph

C

Si

C

C

Ph

4

SiMe 3

SiMe3

3

Me 3Si C hν

Me3 Si

C C

Si

H

C

Me3 Si

SiMe3

Ph 44

The fact that the thermolysis of acylpolysilanes proceeds cleanly to give silenes is well-known. The thermal formation of the silenes involves a 1,3-shift of a silyl group in the polysilanyl group to the carbonyl oxygen in analogy with the photolysis of the acylpolysilanes. The driving force of the silyl migration is obviously due to the formation of a silicon-oxygen bond with the large bond energy. The first example of the formation of the silene produced thermally from the acylpolysilane was reported by Brook and coworkers [33a]. The thermolysis of acylpolysilanes readily produces silenes, and the reaction of the silenes with many kinds of alkynes proceeds with high regiospecificity, to give the respective adducts in high yield [10,39]. 2.5. Reaction of acylpolysilanes with mono- and di-alkynes

Scheme 12. Photolysis of 1,4-disilacyclohexadiene 4 [12].

butylocta-1,3,5,7-tetrayne [37b], 1,4-bis(cyclohexen-1-yl)buta-1,3 -diyne [37c], and 1,7-octadiene-3,5-diyne [37c] produces the respective bis(silacyclobutene) derivatives. As described above (see Scheme 1), the nickel-catalyzed reaction of (phenylethynyl)tris(trimethylsilyl)silane 1 or silacyclopropene 2 with phenyl(trimethylsilyl)acetylene affords silacyclobutene 3 and 1,4-disilacyclohexadiene 4 [26]. Compounds 3 and 4 thus formed are photochemically active [26a,38]. Irradiation of 1,4-disilacyclohexadiene 4 with a high-pressure mercury lamp gave silacyclobutene 3 in high yield. Irradiation of the silacyclobutene 3 thus obtained, gave 4,5-benzo-1,3-bis(trime thylsilyl)-2-[1-phenyl-2,2-bis(trimethylsilyl)ethenyl]-1-silacyclo pent-2,4-diene (44) (Scheme 12). The photolysis of 3 in the presence of a large excess of methanol produced a 1:1 mixture of two methoxysilanes, 1-methoxy-3-phe nyl-2-[phenyl(trimethylsilyl)methyl]-1,4,4-tris(trimethylsilyl)-1-s ilacyclobutene (45) and 1-methoxy-3-phenyl-2-[phenyl(trimethyl silyl)methylene]-1,4,4-tis(trimethylsilyl)-1-silacyclobutane (46) (Scheme 13). No other products were detected in the reaction mixture. The production of methanol adducts 45 and 46 can best be explained by the reaction of compound (H) generated photochemically from 3, via a 1,2-trimethylsilyl shift, with methanol. Although direct evidence for the formation of H could not be obtained, the photochemical 1,2-silyl shift to the unsaturated carbon atom in alkenyl- and alkynyldisilanes is well-known [25b,c].

When a mixture of acetyltris(trimethylsilyl)silane (47) and alkyne was heated in a degassed sealed tube, (E)-1-{[1-(trimethylsi loxy)ethenyl]bis(trimethylsilyl)silyl}-2-(trimethylsilyl)ethane (48) was obtained (Scheme 14). Similar reaction of isopropionyltris(tri methylsilyl)silane (49) with (trimethylsilyl)acetylene afforded adduct (50), as the sole product [39]. The formation of the products 48 and 50 can be best understood in terms of the [2+2] cycloaddition of the silenes generated thermally from acylpolysilanes 47 and 49 with (trimethylsilyl)acetylene, followed by radical scission of a carbon-carbon bond in the silacyclobutenyl ring, and then disproportionation of the resulting biradical species as shown in Scheme 14. In these reactions, no [2+2] cycloadducts, silacyclobutenes were detected in the reaction mixture. In contrast to acylpolysilanes 47 and 49, the thermolysis of pivaloyl- and adamantoyltris(trimethylsilyl)silane (51) and (52) with the same acetylene gave the respective [2+2] cycloadducts and ring-opened products. Treatment of 51 with (trimethylsilyl) acetylene produced 2-tert-butyl-2-(trimethylsiloxy)-1,1,3-tris(tri methylsilyl)-1-silacyclobut-3-ene (53) and 1-tert-butyl-3-[(trime thylsiloxy)bis(trimethylsilyl)silyl]-1-(trimethylsilyl)-1,2-propa diene (54) (Scheme 15). In this reaction, a small amount of 1:2 adduct (55), arising from the reaction of silacyclobutene 53 with trimethylsilylacetylene was produced. The products 53 and 54 are unstable toward moisture and oxygen in air, and they gradually decompose on a recycling HPLC column [39]. Similar treatment of acylpolysilane 52 with the same acetylene afforded two products, adamantly-substituted silacyclobutene (56) and propadiene (57). Again, a small amount of 1:2 adduct (58) was

Ph

Me3 Si

Ph

C 3

hν

OMe

C

Si

C

C

Ph

SiMe 3

Me3 Si

MeOH

CH

SiMe3

C

Si

C

C

SiMe 3 SiMe3

Ph

SiMe3

SiMe3

H

45 Ph OMe Me3 Si

C C

Si

C H

C

SiMe 3

+ Ph

SiMe3

SiMe3 46 Scheme 13. Photolysis of silacyclobutene 3 in the presence of MeOH [26a,38].

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M. Ishikawa et al. / Coordination Chemistry Reviews 335 (2017) 58–75

Me 3Si Me 3Si (Me3 Si) 3SiCOR

Si

C

CH

Me3 Si

R

Me3 Si

47, R = Me 49, R = i-Pr

OSiMe3 Me SiC 3

OSiMe3

Si

C

C

C SiMe3

H

Me 3Si Me3 Si

Me3 SiO

OSiMe3

Si

R

C

C

C

CR' 2

C

C

(Me 3Si) 2Si

C SiMe3

H

R

H

H

SiMe3

48, R' = H 50, R' = Me Scheme 14. Thermal reaction of acylpolysilanes 47 and 49 with silylacetylene [39].

Me 3Si 140°C (Me3 Si) 3SiCOR

+

Me3 SiC

Me3 Si

CH

51, R = t -Bu 52, R = Ad

OSiMe3

Si

C

C

C

(Me3 Si) 3SiCOR

SiMe3 H 53, R = t -Bu 56, R = Ad

OSiMe 3 R

+ (Me3 Si) 2Si C

C

SiMe 3

Me3 Si +

Me3 SiC

C

H

OSiMe3 R C

Si

C

C

C SiMe 3

H

H SiMe3 55, R = t -Bu 58, R = Ad

54, R = t -Bu 57, R = Ad

Scheme 15. Thermal reaction of acylpolysilanes 51 and 52 with silylacetylene [39].

Me 3Si (Me3 Si) 3SiCOR

+

t-BuMe2 SiC

120°C

Me3 Si

CH

R

C

C C SiMe2 t-Bu H 59, R = t -Bu 60, R = Ad

51, R = t -Bu 52, R = Ad

Me 3Si

51 and 52 +

OSiMe3

Si

120°C PhMe2 SiC

Me3 Si

CH

OSiMe3

Si

C

C

C

R

SiMe2 Ph H 61, R = t -Bu 63, R = Ad Me3 Si + PhMe2 SiC

C

SiMe 3

OSiMe3 R C

Si C

C

Me 3Si

R

H

H SiMe2 Ph 62, R = t -Bu 64, R = Ad

Scheme 16. Thermal reaction of 51 and 52 with t-butyldimethylsilyl- and phenyldimethylsilylacetylene [39].

produced in low yield. The products 56 and 57 are unstable in analogy with adducts 53 and 54, but somewhat more stable than those. The structures of cycloadducts 53 and 56 were once reported to be 2-tert-butyl- and 2-adamantyl-2-trimethylsiloxy-1,1,4-tris(tri methylsilyl)-1-silacyclobut-3-ene [39]. However, this has turned out to be an erroneous assignment. Careful studies of their spectral

51, R = t -Bu 52, R = Ad

+

t -BuC

140°C CH

Me3 Si

H

OSiMe3

Si

C

C

C

R

t-Bu 65, R = t -Bu 66, R = Ad

Scheme 17. Thermal reaction of 51 and 52 with t-butylacetylene [40].

data and the chemical reactions indicated that these compounds must be 2-tert-butyl- and 2-adamantyl-2-trimethylsiloxy-1,1,3-tr is(trimethylsilyl)-1-silacyclobut-3-ene [10e], as described here. As shown in the reaction of acylpolysilanes with trimethylsilylacetylene, the silenes generated thermally from 51 and 52 readily react with the acetylene to produce the respective silacyclobutenes. Therefore, we used mainly 51 and 52 for the further reactions with alkynes, to obtain silacyclobutenes. The thermal reaction of 51 with tert-butyldimethylsilylacetylene proceeded with high regiospecificity to give a [2+2] cycloadduct, 2tert-butyl-3-tert-butyl-dimethylsilyl-2-trimethylsiloxy-1,1-bis(tri methylsilyl)-1-silacyclobut-3-ene (59) in high yield [39] (Scheme 16). No other volatile products were detected in the reaction mixture. Similarly, the reaction of 52 with tert-butyldimethylsilylacetylene gave [2+2] cycloadduct (60). Likewise, phenyl-substituted silylacetylenes such as dimethylphenylsilyl- and triphenylsilylacetylene react with 51 and 52, to afford silacyclobutenes, together with 1:2 adducts [10e]. The reaction of 51 with dimethylphenylsilylacetylene produced 2-tert-butyl-3-dimethylphenylsilyl-2-trimethylsiloxy-1, 1-bis(trimethylsilyl)-1-silacyclobutene (61) and 1:2 adduct (62) in 52% and 20% yield, respectively. The reaction of 52 with dimethylphenylsilylacetylene again afforded silacyclobutene (63) and 1:2 adduct (64) in 53% and 13% yield. With triphenylsilylacetylene, 51 and 52 also reacted, to give the respective silacyclobutenes and 1:2 adducts. The reaction of 51 and 52 with monosilyl-substituted acetylenes always afforded the silacyclobutenes bearing a silyl group at the 3-position in the silacyclobutenyl ring. No other regioisomers were detected in the resulting reaction mixtures. Silacyclobutenes that have an alkyl substituent on the sp2hybridized carbon atom at the 3-position in the silacyclobutenyl ring can be prepared by the thermal reactions of 51 and 52 with alkyl-substituted acetylene. In fact, the reaction of 51 with tertbutylacetylene gave 2,3-di-tert-butyl-2-trimethylsiloxy-1,1-bis(tri methylsilyl)-1-silacyclobut-3-ene (65) in high yield [40]. For the location of hydrogen on the silacyclobutenyl ring of 65, NOE-FID

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M. Ishikawa et al. / Coordination Chemistry Reviews 335 (2017) 58–75

difference experiments at 300 MHz clearly indicate that the hydrogen on the olefinic carbon is located on the carbon atom at the 4position in the silacyclobutenyl ring, in analogy with that of the silacyclobutenes produced from the reaction with silylacetylenes. The reaction of 52 with the same acetylene again produced silacyclobutene derivative (66) (Scheme 17). In these reactions, no other isomers such as the regioisomers and the ring-opened products were detected by spectrometric analysis. Brook, et al. [33d] have found that the cothermolysis of 51 with di-substituted acetylene, phenylpropyne at 170 °C affords 2-tertbutyl-4-methyl-3-phenyl-2-trimethylsiloxy-1,1-bis(trimethylsi lyl)-1-silacyclobut-3-ene (67), arising from [2+2] cycloaddition of the silene generated thermally from 51 to the alkyne, in contrast to the reaction with bis(silyl)acetylene (Scheme 18). The cothermolysis of 51 with bis(trimethylsilyl)butadiyne at 120 °C, proceeded with high regiospecificity, to produce 2-tert-bu tyl-2-trimethylsiloxy-1,1,4-tris(trimethylslilyl)-3-trimethylsilyle thynyl-1-silacyclobut-3-ene (68) [35,41], as the sole product. Similar thermolysis of 52 in the presence of the butadiyne yielded 2admanyl-2-trimethylsiloxy-1,1,4-tris(trimethylslilyl)-3-trimethyl silylethynyl-1-silacyclobut-3-ene (69) (Scheme 19). The fact that no isomers were detected in the resulting mixture in both reactions indicates that the [2+2] cycloaddition of the silene to butadiyne proceeds with high regiospecificity. Phenyl-substituted bis(silyl)butadiynes react with 51 and 52 under the same conditions, to give the respective silacyclobutenes in high yield. The reaction of 51 with bis(dimethylphenylsilyl)buta diyne gave 2-tert-butyl-4-dimethylphenyl-silyl-3-[2-(dimethylphe nylsilyl)ethynyl]-2-trimethylsiloxy-1,1-bis(trimethylsilyl)-1-silacy clobut-3-ene (70), and the reaction of 52 with bis(dimethylphenyl silyl)butadiyne produced 2-adamantyl-4-dimethyphenylsilyl-3-[2 -(dimethylphenylsilyl)ethynyl]-2-trimethylsiloxy-1,1-bis(trime thylsilyl)-1-silacyclobut-3-ene (71) [41]. The reaction of 51 and 52 with bis(silyl)butadiyne bearing a bulky-substituent on the silicon atom, such as bis(tert-

Me 3Si (Me3 Si) 3SiCOR +

170°C PhC

CMe

Me3 Si

51, R = t -Bu

OSiMe3

Si

C

C

C

t -Bu

2.6. Reaction of acylpolysilanes with bis(silyl)but-1-en-3-ynes In order to learn more about the regioselectivity of the silene in the [2+2] cycloaddition to the unsaturated compounds involving a carbon-carbon double bond and a triple bond simultaneously in a molecule, the reaction of 51 and 52 with silyl-substituted but-1-en-3-ynes was investigated [41]. The reaction of 51 with (E)-1,4-bis(dimethylphenylsilyl)but-1-en-3-yne proceeded with high regiospecificity to give 2-tert-butyl-4-dimethylphenylsilyl-3[(E)-2-(dimethylphenylsilyl)ethenyl]-2-(trimethylsiloxy)-1,1-bis (trimethylsilyl)-1-silacyclobut-3-ene (74) (Scheme 20). No regioisomer of 74 was detected in the reaction mixture. The reaction of 52 with (E)-1,4-bis(dimethylphenylsilyl)but-1-en-3-yne under the same conditions gave 2-adamantyl-substituted 2-(dime thylphenylsilyl)ethenyl-1-silacyclobut-3-ene (75). In the reactions of 51 and 52 with (E)-1,4-(pentamethyldisila nyl)but-1-en-3-yne, 2-tert-butyl- and 2-adamantyl-4-pentamethyl disilanyl-3-[(E)-2-(pentamethyl-disilanyl)ethenylenyl]-2-(trime thylsiloxy)-1,1-bis(trimethylsilyl)-1-silacyclobut-3-ene (76) and (77), whose structures are similar to those of 74 and 75, were produced, as the sole product [41]. The results clearly indicate that the addition of silenes generated from 51 and 52 occurs only to the carbon-carbon triple bond, but not double bond in the 1,4-bis (silyl)but-1-en-3-yne. 2.7. Reaction of acylpolysilanes with diphenylketene The reactions of silenes with carbonyl compounds have been investigated to date [7a,10b,41–43]. In these reactions, the siloxetanes are considered to be formed as the intermediates. In fact, the siloxetanes can be isolated in the reaction of silenes

Ph

Me

butyldimethylsilyl)butadiene at 120 °C afforded no adducts, but the starting acylpolysilanes 51 and 52 were recovered unchanged. At higher temperature, however, the reactions proceeded cleanly to give the silacyclobutenes in high yield. When compound 51 was heated in the presence of bis(tert-butyldimethylsilyl) butadiyne at 160 °C, 2-tert-butyl-4-(tert-butyldimethylsilyl)-3-[2(tert-butyldimethylsilyl)ethynyl]-2-trimethylsiloxy-1,1-bis(trime thylsilyl)-1-silacyclobut-3-ene (72) was obtained. Likewise, the reaction of 52 with the butadiyne at 160 °C gave silacyclobutene (73). In all reactions of 51 and 52 with bis(silyl)butadiynes, the silacyclobutenes with a silylethynyl group at the 3-position in the silacyclobutenyl ring are produced as the sole product.

67 Scheme 18. Thermal reaction of 51 with phenylpropyne [33d].

Me 3Si (Me3 Si) 3SiCOR 1 +

R2 Me2 SiC

C

C

CSiMe2 R 2

120°C

51, R1 = t-Bu 52, R1 = Ad

Me3 Si

Si

C

C

C

R 2 Me2 Si 68, 69, 70, 71,

C

C

CSiMe2 t-Bu

160°C

Me3 Si

t-BuMe2 Si

R1

CSiMe2 R 2

C R1 = R1 = R1 = R1 =

Me 3Si 51 and 52 + t-BuMe2 SiC

OSiMe3

t-Bu, R 2 = Me Ad, R 2 = Me t-Bu, R 2 = Ph Ad, R 2 = Ph

OSiMe3

Si

C

C

C

R

C

CSiMe2 t-Bu

72, R = t -Bu 73, R = Ad Scheme 19. Thermal reaction of 51 and 52 with bis(silyl)butadiynes [35,41].

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M. Ishikawa et al. / Coordination Chemistry Reviews 335 (2017) 58–75

Me 3Si Me3 Si

120°C

(Me3 Si) 3SiCOR 1 +

C

51, R1 = t-Bu 52, R1 = Ad

OSiMe3

H

R 2Me 2Si C

C

H

CSiMe2 R 2

Si

C

C

C

R1 H

R 2 Me2 Si

C

C SiMe 2R 2

H 74, 75, 76, 77,

1

2

t-Bu, R = Ph Ad, R 2 = Ph t-Bu, R 2 = Me Ad, R 2 = Me

R = R1 = R1 = R1 =

Scheme 20. Thermal reaction of 51 and 52 with 1,4-bis(silyl)but-1-en-3-yne [41].

Me 3Si OSiMe3 Ph C 2

Me 3Si (Me3 Si)3 SiCOR

Si Me 3Si

51, R = t -Bu 52, R = Ad 80, R = Mes

O

C

Me 3Si

R

OSiMe3

Si

C

O

C

R

C

Me2 Si

OSiMe3 Me

Me3 SiO Si

C

R

Si Me3 SiO

C

OSiMe3 C R

C C

C Ph

Ph

Ph

R = t-Bu, Ad

Ph J

I

Me R = Mes

R

Me

Si

C

Ph

C

C

Me 3Si Me3 SiO

Ph

Ph

Ph

Me2 Si

Me

C

Si

O

C

C

OSiMe3 Si

SiMe3

C Mes

OSiMe 3

Ph

Me 78, R = t -Bu 79, R = Ad

Ph 81 Scheme 21. Thermal reaction of acylpolysilanes 51, 52 and 80 with diphenylketene [43].

with nonenolaizable ketones [41a]. In addition to the siloxetanes, [2+4] cycloadducts are observed in some cases [10a,41]. The thermal reaction of 51 and 52 with acetone produces two types of the siloxymethylcyclopropane derivatives and silyl enol ethers, arising from a 1,2-trimethysiloxy shift from carbon to silicon in the siloxetane intermediates [10b]. The reaction of 51 and 52 with diphenylketene, however, produced silacyclobutenes [43]. Treatment of 51 with diphenylketene afforded 2-tert-butyl-1,1-dimethyl-3-[methylbis(trimethylsiloxy)s ilyl]-4,4-diphenyl-1-silacyclobut-2-ene (78), whose structure was confirmed by X-ray crystallographic analysis. The reaction of 52 with diphenylketene under the same conditions again gave silacyclobutene (79). The formation of the products 78 and 79 may be best explained by a series of the reactions: formal [2+2] cycloaddition of the silenes to a carbon-oxygen double bond in diphenylketene, ring contraction of the resulting siloxetanes, giving silacyclopropane intermediate (I), a 1,2-methyl shift from the trimethylsilyl group to the ring silicon atom in the intermediate I to produce disilacyclobutane (J), and skeletal rearrangement of J, as shown in Scheme 21. In marked contrast to 51 and 52, the cothermolysis of mesitoyl tris(trimethylsilyl)silane (80) with diphenylketene at 140 °C

yielded cis-3-diphenylmethylene-4-mesityl-2-trimethylsiloxy-2,4 -bis(trimethylsilyl)-2-siloxetane (81), together with 76% of the starting mesitoylpolysilane 80. At 160 °C, the reaction of 80 with diphenylketene afforded siloxetane 81 in 44% yield, in addition to 17% of the starting acylpolysilane 80. Neither trans-2-siloxetane isomer nor other volatile product was detected in the reaction mixture. The energetics were investigated using DFT calculations from 51 to J, and from 80 to 81 via intermediate I (Scheme 21). Complete reactions were divided into three steps, i.e., addition of the silenes produced from 51 and 80 to the ketene, ring contraction of the siloxetanes thus formed, to the silacyclopropanes (I) and isomerization of I to J or 81. The rate-determining step was observed in the last step [43].

3. Reaction of silacyclobutenes Zircona- and titanacyclobutene-silacyclobutene fused-ring compounds have proved to be useful reagents for the synthesis of various organic compounds, and the reactions of these reagents with unsaturated organic compounds such as nitriles, alkynes, and ketones have been reported to date [30,44,31a]. In this article,

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M. Ishikawa et al. / Coordination Chemistry Reviews 335 (2017) 58–75

clobutene 59 was heated at 160 °C, 1-tert-butyl-1-(tert-butyldime thylsilyl)-3-[(trimethylsiloxy)bis(trimethylsilyl)silyl-1,2-propa diene (82) was obtained, as a single product. Similar reaction of 52 with tert-butyldimethylsilylacetylene again gave the ring-opened product, 1-adamantyl-1-adamantyl-1-tert-butyldimethylsilyl-3[(trimethylsiloxy)bis(trimethylsilyl)silyl]-1,2-propadiene (83) (Scheme 22). In these reactions, no trace amount of hexamethyldisiloxane was detected in the reaction mixtures. The cothermolysis of 51 and 52 with phenyldimethylsilylacetylene also afforded the respective 1,2-propadienes in high yield. With triphenylsilylacetylene, 51 and 52 again gave the 1,2-propadiene derivatives. The results indicate that the 3-silyl-1-silacyclobut-3-enes formed from the reaction of 51 and 52 with monosilyl-substituted acetylenes undergo isomerization to give the ring-opened products.

however, we discuss only the reaction of the silacyclobutenes which do not involve any metals. 3.1. Thermolysis of 3-silyl-1-silacyclobutenes The reactions of acylpolysilanes 51 and 52 with silylacetylenes at 120 °C produce silyl-substituted silacyclobutenes, as discussed above. We initially thought that the thermal reaction of silacyclobutenes might proceed to give the 1-silacyclo-1,3-butadiene derivatives with extrusion of hexamethyldisiloxane from the starting silacyclobutenes, and investigated their thermal behavior to confirm this idea. However, all thermolyses of the silacyclobutenes produced from the reaction of 51 and 52 with silylacetylenes failed to afford any 1-silacyclo-1,3-butadienes, but produce the ringopened 1,2-propadiene derivatives [39]. Indeed, when silacy-

3.2. Thermolysis of 2-silyl-3-silylethynyl-1-silacyclobutenes Me 3Si Me3 Si

OSiMe3

Si

C

OSiMe 3

R

160°C

C C SiMe2 t-Bu H 59, R = t -Bu 60, R = Ad

In contrast to the thermal isomerization of 3-silyl-1silacyclobut-3-enes 59–61 and 63, the silacyclobutenes obtained from the reaction of 51 and 52 with bis(silyl)butadiynes readily isomerize to produce the products consisting of the oxygencontaining five-membered ring [40,35]. No trace amounts of the ring-opened products are detected in the reaction mixtures. When silacyclobutene 68 was heated at 160 °C, 5-tert-butyl-2,5-dihydro2,2,3,5-tetrakis(trimethylsilyl)-4-[2-(trimethyl-silyl)ethynyl]-1,2oxasilole (84) was obtained in almost quantitative yield (Scheme 23). The thermolysis of 69, 70, and 71 under the same conditions also gave the respective 2,5-dihydro-1,2-oxasiloles (85–87) in high yield. When a mixture of 51 and bis(tert-butyldimethylsilyl) butadiyne was heated at 220 °C, 2,5-dihydro-1,2-oxasilole (88) was produced. Likewise, the reaction of 52 with the same butadiyne gave 2,5-dihydro-1,2-oxasilole derivative (89), as the sole product. In an effort to trap the intermediate, which might be involved in these reactions, the reaction of silacyclobutenes 68 and 69 with methanol was carried out. Treatment of 68 with an excess of methanol at 160 °C afforded (E)-1-tert-butyl-2-{[(methoxybis(tri methylsilyl)silyl-2-trimethylsilyl]methyl}-1-trimethylsiloxy-4-(tri methylsilyl)but-1-en-3-yne (90), as the sole product [35]. Similar reaction of 69 with methanol under the same conditions gave but-1-en-3-yne derivative (91). No other stereoisomers were detected in the reaction mixture. The formation of 90 and 91 can be best explained in terms of the reaction of the ring-opened intermediate, 1-silabuta-1,3-diene (K, R = t-Bu or Ad) with methanol. It seems reasonable to assume that the 2,5-dihydro-1,2-oxasiloles

R

(Me3 Si) 2Si C

C

C

H

SiMe 2t -Bu

82, R = t -Bu 83, R = Ad

Scheme 22. Thermolysis of silacyclobutenes 59 and 60 [39].

Me 3Si Me3 Si

OSiMe3

Si

C

C

C

R 2 Me2 Si 68, 69, 70, 71,

t-BuMe2 Si

Me3 Si

O

84, 85, 86, 87,

OSiMe3

Si

C

C

C

R = R1 = R1 = R1 =

220°C

CSiMe2 t-Bu

t-Bu, R = Me Ad, R 2 = Me R2 = Ph Ad, R 2 = Ph SiMe3 R

C C

t-BuMe2 Si

72, R = t -Bu 73, R = Ad

CSiMe2 R 2

2

O Si C

C

C C

1

Me 3Si Me3 Si

R1

C

Si C

R

SiMe3

R 2 Me2 Si

t-Bu, R 2 = Me Ad, R 2 = Me R2 = Ph Ad, R 2 = Ph

Me 3Si Me3 Si

160°C

CSiMe2 R 2

C

R1 = R1 = R1 = R1 =

Me 3Si

R1

CSiMe2 t-Bu

C 88, R = t -Bu 89, R = Ad

Scheme 23. Thermal behavior of silacyclobutenes 68–73 produced from the reaction of 51 and 52 with bis(silyl)butadiynes [40,35].

Me3 Si 68 and 69

160°C

Me3Si

R

Si

Me3 Si OMe R

C C

OSiMe 3

Me 3Si

C

CSiMe 3

K, R = t -Bu, R = Ad

Si

Me 3Si

C C H

C

88 and 89

C C

C

90 , R = t-Bu 91, R = Ad

R

C

OSiMe 3

C

O

Si

Me 3Si

Me3Si

SiMe 3

Me3 Si Me3Si

MeOH

C

CSiMe 3

L, R = t-Bu, R = Ad Scheme 24. Thermal behavior of silacyclobutenes 68 and 69 [35].

CSiMe 3

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M. Ishikawa et al. / Coordination Chemistry Reviews 335 (2017) 58–75

nylsilyl)but-1-en-3-yne at 200 °C yielded 2-tert-butyl-2,5-dihydro3-(dimethylphenylsilyl)-4-[(E)-2-(dimethyphenyl-silyl)ethenyl]-2, 2-5-tris(trimethylsilyl)-1,2-oxasilole (92), while treatment of 52 with the same but-1-en-3-yne afforded 5-adamantyl-2,5-dihy dro-1,2-oxasilole (93), as the sole product (Scheme 25). The thermal reaction of 51 with (E)-1,4-bis(pentamethyldisila nyl)but-1-en-3-yne at 200 °C proceeded with high regio- and stereospecificity to give 5-tert-butyl-2,5-dihydro-3-(pentamethyl disilanyl)-4-[(E)-2-(pentamethyldisilanyl)ethenyl]-2,2,5-tris(trime thylsilyl-1,2-oxasilole (94) in high yield. Similarly, the reaction of 52 with (E)-1,4-bis(pentamethyldisilanyl)but-1-en-3-yne gave 2,5-dihydro-1,2-oxasilole (95), as the sole product. In these reac-

may be formed by isomerization of intermediate K to bicyclic intermediate (L), followed by ring enlargement of L to 88 and 89, as shown in Scheme 24. 3.3. Thermolysis of 2-silyl-3-silylethenyl-1-silacyclobutenes In analogy with the thermal reactions with bis(silyl)butadiynes, the thermolysis of 51 and 52 in the presence of bis(silyl)-1-en-3ynes yielded 2,5-dihydro-1,2-oxasiloles, arising from isomerization of the silacyclobutene derivatives. Interestingly, these thermolyses proceeded with high regio- and stereospecificity to give the adducts [35]. Thus, the reaction of 51 with (E)-1,4-bis(dimethylphe

Me 3Si (Me3 Si) 3SiCOR 1 +

C

51, R1 = t-Bu 52, R1 = Ad

OSiMe3

H

R 2Me 2Si

Me3 Si

C CSiMe2 R 2

C

H

Si

C

C

C

2

R Me2 Si

R1 H C

C SiMe 2R 2

H Me 3Si 200°C

O

C

H

C

2

R Me2 Si 92, 93, 94, 95,

R1

C

Si

Me3 Si

OSiMe3

C R1 = R1 = R1 = R1 =

C

H SiMe 2R 2 2 t-Bu, R = Ph Ad, R 2 = Ph t-Bu, R 2 = Me 3Si Ad, R 2 = Me3 Si

Scheme 25. Thermal reaction of 51 and 52 with (E)-1,4-bis(silyl)but-1-en-3-yne [35].

Me3 Si 65

190°C Me3Si

Me3 Si

OSiMe 3

Si C

Me3Si

t-Bu

C

Si C

C

H

t -Bu

H

250°C

t-Bu

C

t-Bu

Me3 Si

OSiMe3

Me3 SiO

Si

C

C

C

t -Bu

C

H 96

t -Bu

97

SiMe3

t-BuOH Me3 Si Me3Si

OSiMe3

Si Ot-Bu

H 2C C

t-Bu

t-Bu Me3 Si

66

250°C

a C

H

Ad

SiMe 3 Si

path b

C H

H

C

C

C

Ad Ad

C t -Bu

Me 3SiO

Me3 SiO

path a C

t-Bu

Me 3Si

SiMe3 C

t-Bu

Me3 Si

Si

b

Me3 SiO Si

SiMe 3

Me 3SiO

SiMe 3

98

C

Ad

C t -Bu

Me3 Si Me3 SiO

Ad

Si

C

C

C

t -Bu

SiMe3

H 100

Scheme 26. Thermal behavior of silacyclobutenes 65 and 66 [10f,h,j,k].

Si

C

C

C 99

SiMe3

H

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M. Ishikawa et al. / Coordination Chemistry Reviews 335 (2017) 58–75

utyl-1-trimethylsiloxy-1,2-bis(trimethylsilyl)-1-silacyclobut-3-en e (96) was produced in almost quantitative yield [10f,h,j,k] (Scheme 26). In this reaction, no stereoisomer of 96, cis-isomer, was detected in the reaction product. In order to get more information about the thermal behavior of 65, we investigated the thermolysis of 65 under various conditions, and found that when 65 was heated at 190 °C, an isomerization product, 1,2-di-tert-butyl3-[bis(trimethylsilyl)(trimethylsiloxy)-silyl]cycloprop-2-ene (97) was obtained quantitatively. As expected, the product 97 thus produced was transferred cleanly into silacyclobutene 96 at 250 °C. On the other hand, the reaction of 65 with tert-butyl alcohol at 210 °C produced (Z)-1-tert-butoxy-1,2-di-tert-butyl-2-{[(trimethyl siloxy)bis(trimethylsilyl)silyl]-methyl}ethene (98), arising from

Thermal behavior of 3-tert-butyl-1-silacyclobut-3-ene 65 and 76 differs significantly from that of 3-silyl-1-silacyclobut-3-enes, in which the ring-opened 1,2-propadienes are produced. When compound 65 was heated at 250 °C, its isomer, trans-2,4-di-tert-b

00

31.7 (31.2)

tBu 3

14.5 (14.2) 65b

99

TS65'b tBu

1

3.750

Me3SiO

2 1 .3 7

1 . 36

4

Si(SiMe3)2

tBu

5

SiMe3 1

Si 5

H

C

tBu

1.488 C 2

C3

4

1

3.57 5 5

-27.9 97 (-29.1)

50 1.7

C

1.487C 2

C3

51 1 .7

4

tBu

H

C 4 1.535 2C Me3Si

tBu

H

C

1 .2

1.412

1.465 1. 560

TS65'b

12.4 (12.0) 65a

65

OSiMe3

5

40.0 (39.7) TS65c

0.0 (0.0)

Si(SiMe3)2

35

65

tBu

1

91

5 1.

tBu

2.0 32

1 .7

TS65c

21.8 (21.7) TS65a

Si(SiMe3)2

2.8 97

5O SiMe3

TS65b

1

4

C

37 1. 8

4

C

4 1.9 90 1 Si(SiMe3)2 63 1 .9

3 70

Me3SiO

1.355 C 2

C3

tBu

36.5 (36.5)

68 1 .8

5

tBu

C

TS65a

H

tBu

Si(SiMe3)2

1.373C 2 3

C

1.

TS65b

2.882

98 1.8

5

5

1

H

tBu

9 97

Me3SiO

OSiMe3

4

C

1.370C C 2 3

1.

80 1 .3

98

Si(SiMe3)2

62 1.5

tBu

1

2.990

1 .7

32 1.8

4

C

1. 397C C3 2

H

tBu

H

tBu

1.390C C3 2

1 .5

H

tBu

9

3.4. Thermolysis of 3-tert-butyl-1-silacyclobutenes

1. 4 6

tions, the intermediates analogous to 1-sila-1,3-butadiene K and bicyclic compound L are probably involved in the formation of the products 92–95. For the formation of the 2,5-dihydro-1,2oxasilole structure, the presence of the unsaturated group, such as an ethenyl or ethynyl group, at the 3-position in the silacyclobutenyl ring seems to be important.

OSiMe3

97

Si(SiMe3)2

OSiMe3

65b

65a

Fig. 1. Optimized geometries and energy diagrams for the ring-opening reaction, the 1,2-siloxyl shift, and the 1,4-siloxyl shift at the B3LYP/6-31+G⁄⁄ level of theory. The values in parentheses are relative energies from single-point calculations at the B3LYP/6-311+G⁄⁄ level. Bond distances and energies are in Å and kcal/mol, respectively [46].

t-Bu t-Bu 140°C Me3Si

t-Bu(Me 3Si)2 SiCOt-Bu + t-BuC

Si

C

C

C

OSiMe Me3Si +

CH

101

t-Bu OSiMe

t -Bu

H

Si

C

C

C

H tr ans-102

cis -102

t-Bu SiMe3 cis -102 + tr ans-102

250°C

Me3 SiO

Si

C

C

C

t -Bu

t-Bu

H 104

t-Bu 190°C

SiMe 3 250°C

Me3 SiO Si C

C H

t-Bu

C 103

t -Bu

Scheme 27. Thermal isomerization of cis- and trans-silacyclobutenes [10f,h].

t-Bu

t -Bu

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M. Ishikawa et al. / Coordination Chemistry Reviews 335 (2017) 58–75

by a 1,4-trimethylsiloxy shift. The other comprises a direct 1,2trimethylsiloxy shift in silacyclobutene 65. The energetics of these reaction pathways are shown in Fig. 1 [45]. The concerted route involves the pathway 65 ? TS65b ? 65b ? TS650 b ? 97, and the direct route involves 65 ? TS65c ? 97. Under the thermal conditions, both routes are likely to occur, however, the route 65 ? TS65a ? 65a does not connect to 97. In contrast to 65, the thermolysis of 66 under the same conditions afforded two regioisomers, trans-4-adamantyl-2-tert-butyland trans-2-adamantyl-4-tert-butyl-1-trimethylsiloxy-1,2-bis(tri methylsilyl)-1-silacyclobut-3-ene (99 and 100) in a ratio of 1.5:1. The structure of 100 was confirmed by X-ray crystallographic analysis [10h]. The cothermolysis of pivaloyl[tert-butylbis(trimethylsilyl)]sila ne (101) with tert-butylacetylene produced a mixture consisting of cis- and trans-1,2,3-tris(tert-butyl)-2-trimethylsilyl-1-trimethyl siloxy-1-trimethylsilyl-1-silacyclobut-3-ene (cis-102 and trans102) in a ratio of 0.7:1 [10f,h]. When a mixture of cis- and trans102 was heated at 190 °C, 2,3-di(tert-butyl)-1-[tert-butyl((trime thylsiloxy)(trimethylsilyl)silyl)cycloprop-2-ene (103) was obtained, as the sole product (Scheme 27). The thermolysis of silylcyclopropene 103 at 250 °C proceeded with high stereospecificity to give trans-1,2,4-tri(tert-butyl)-1-trimethylsiloxy-2-trimethylsi lyl-1-silacyclobut-3-ene (104), as a single isomer. As expected,

the scission of a carbon-carbon bond in the cyclopropenyl ring, along with the isomerized silacyclobutene 96. These results obviously indicate that product 96 is produced via silacyclopropene 97. Theoretical studies concerning the isomerization of silacyclobutene 65 to silacyclopropene 97 have been reported [10j, k,45,46]. For the thermal isomerization of silacyclobutene 65 to silylcyclopropene 97 is considered to occur via two possible reaction pathways. One of these involves a concerted ring-opening of the silacyclobutenyl ring in 65, to give 3,4-di(tert-butyl)-4-trime thylsiloxy-1,1-bis(trimethylsilyl)-1-silabuta-1,3-diene, followed

Ph Ph

C

C

2

SiHMe 2

+

Cp2 ZrEt2

105

C

C

SiMe2

Cp 2Zr

C

C Ph

106 Ph H 2O

HC

C

SiMe2

HC

C Ph

107

Scheme 28. Synthesis of phenylethynylidenesilacyclobutene [47].

MeOOC path a

COOMe C

Ph HC

SiMe2

C HC

path a path b

Ph C

C

SiMe2

Cp2 Zr

C

C

C Ph

108a

MeOOCC

CCOOMe Ph

Ph

106

C

Me2 Si

HC C path b

COOMe C

HC

C C

COOMe

Ph 108b Scheme 29. Formation of phenylidenesilacyclohexadienes [48].

C 160°C (Me3Si) 3SiCOR

+

MesC

CMes H2 C

(Me 3Si) 2Si

CH

C

C

51 , R = t-Bu 52, R = Ad 80, R = Mes

H

R C

C OSiMe3

Mes Mes 109, R = t -Bu 111, R = Ad 113, R = Mes

C +

CMes H2 C

(Me 3Si) 2Si C H

C

H C

C

Mes Mes 110, R = t -Bu 112, R = Ad 114, R = Mes

R C

Mes

C OSiMe3

Scheme 30. Reaction of acylpolysilanes 51, 52 and 80 with mesitylacetylene [49].

73

M. Ishikawa et al. / Coordination Chemistry Reviews 335 (2017) 58–75

when a mixture of cis- and trans-102 was heated at 250 °C, the silacyclobutene 104 was produced quantitatively.

gave no 1:1 adducts, but 1:3 and 1:4 adducts were always produced [49]. When a mixture of 51 and a 4-fold excess of mesitylacetylene was heated in a sealed tube at 160 °C, two products, consisting of 51 and mesitylacetylene in the molar ratio of 1:3 and 1:4, (1E,4Z)-5tert-butyl-2,4-dimesityl-1-[(mesitylethynyl)-bis(trimethylsilyl)sily

3.5. Palladium-catalyzed reaction of phenylethynylidenesilacyclobutene with acetylene dicarboxylate Takahashi et al. and Xi et al. reported that various alkylidenesilacyclobutenes can be synthesized by the reaction of bis(alkenyl)silanes with 1 equiv of Cp2ZrEt2, followed by hydrolysis [47,48] and the alkylidenesilacyclobutenes thus formed react with alkynes in the presence of a catalytic amount of Pd(PPh3)4 to give alkylidenesilacyclohexadienes in high yield. For example, treatment of bis(p henylethynyl)dimethylsilane (105) with Cp2ZrEt2 gives zirconacyclobutene-fused complex (106). After hydrolysis of the complex 106, silacyclobutene derivative (107) is produced [47] (Scheme 28). The reaction of phenylethynylidenesilacyclobutene 107 with acetylene dicarboxylate in the presence of a Pd-catalyst affords two-types of acetylene insertion products, phenylidenesilacyclo hexadienes (108a) and (108b), as shown in Scheme 29 [48].

Me 3Si (Me3Si) 3SiCOR + Me 3SiC

Me3 Si

CH

51, R = t-Bu 52, R = Ad 80, R = Mes

(Me3Si) 3SiCOR

Mes

C

C

C

Si

C

C

C

Me3 Si

CSiMe3

C

Mes

Si(SiMe3 )2 C

R

C Mes

C H

C

Mes

C OSiMe3

Mes C

C

H C

109, 111, and 113

C

MesC

R

C C

C

H

CH

OSiMe3

Mes

H C cycloadditon Mes

C

Si(SiMe3 )2

ring opening H

C C

C

H

R C

Mes

C OSiMe3

Mes

MesC

CH

OSiMe3

C

H

R

Mes

Si

R

Si C

OSiMe3

radical addition H

Me3 Si

C R

Me3 Si

R ring opening

Mes

(Me 3Si) 2Si

R 115, R = t -Bu 116, R = Ad

Me3 Si

C R

OSiMe 3

C

Mes

OSiMe3

115 and 116

Scheme 33. Rearrangement of silene to silylene [51].

H Si(SiMe3 )2

H

Si Me3 Si

Me 3SiC

H C

Si C

OSiMe 3

Me3 Si

H

CH

C

Scheme 32. Reaction of 51 and 52 with bis(silyl)acetylene [50,51].

51, R = t-Bu 52, R = Ad

(Me3Si) 3SiCOR

MesC

Me3 Si Me 3Si

In marked contrast to the reaction of the acylpolysilanes with various types of acetylenes, in which the 1:1 adducts were produced exclusively, the reaction of acylpolysilanes 51 and 52, and mesitoyltris(trimethylsilyl)silane 80 with mesitylacetylene

MesC

CSiMe3

51, R = t-Bu 52, R = Ad

3.6. Reaction of acylpolysilanes with mesitylacetylene

Me3 Si

SiMe3 C

110, 112, and 114

Scheme 31. Mechanism for the reaction of 51, 52 and 80 with mesitylacetylene [49].

OSiMe3

74

M. Ishikawa et al. / Coordination Chemistry Reviews 335 (2017) 58–75

28.5 TS51-Acetylene 1.265 A' (2.09) B'

Me3 Si

0.0 51-Acetylene

Si

1.226 A' (2.27) B'

Me3 Si

C

5.216 (0.00) A

Si

Me3 SiO

C

SiMe3

5.492 (0.00) B

C

t -Bu

C

2.184 (0.49) A

Me3 SiO

SiMe3

C 2.689 (0.39) B

C

-6.4

t -Bu

115

SiMe 3

SiMe3 θ(CA'SiAC B) = 126.3° TS51-Acetylene

SiMe 3

1.374 A' (1.52) B'

Me3 Si 1.826 (0.97)

Me3 SiO

SiMe3

C A

Si

SiMe3

C

1.827 (0.97) B

C

t -Bu SiMe 3

SiMe3

θ(CA'SiAC B) = 131.4° 51-Acetylene

θ(CA'SiAC B) = 129.5° 115

Fig. 2. Energy profile and optimized intermediates (transition state) for [2+1] cycloaddition of bis(trimethylsilyl)acetylene and silylene. Interatomic distances and Gibbs free energy changes (DG) at 160 °C are presented in Å and kcal/mol, respectively. Mayer bond orders are shown in parentheses [51].

l]-5-(trimethylsiloxy)penta-1,4-diene (109) and (1E,4Z,6Z)-7-tertbutyl-2,4,6-trimesityl-1-[(mesitylethynyl)bis(trimethylsilyl)silyl]7-(trimethylsiloxy)hepta-1,4,6-triene (110) were obtained in 33% and 46% yield, respectively (Scheme 30). Neither 1:1 adduct nor 1:2 adduct was detected in the reaction mixture. The reaction of 52 with mesitylacetylene at 160 °C for 24 h, gave 1:3 adduct (111) and 1:4 adduct (112) in 33% and 48% yield, respectively. The structures of 111 and 112 were verified by X-ray crystallographic analysis. Likewise, the reaction of mesitoylpolysilane 80 with mesitylacetylene produced 1:3 adduct (113) and 1:4 adduct (114) in 32% and 19% yield, respectively. Again, in both reactions, no 1:1 and 1:2 adducts were detected in the reaction mixtures. The formation of the 1:3 adducts 109, 111, and 113, and 1:4 adducts 110, 112, and 114 may be explained by a series of reactions involving the intermediate formation of the silacyclobutadiene derivatives and the ring-opened silene derivatives, as shown in Scheme 31.

3.7. Reaction of acylpolysilanes with bis(trimethylsilyl)acetylene As described above, the cothermolysis of 51 and 52 with silylacetylenes and bis-(silyl)butadiynes cleanly afford the silacyclobutene derivatives. Similar reactions of 51 and 52 with bis (silyl)acetylene afford no silacyclobutenes. Thus, the reaction of 51 with bis(trimethylsilyl)acetylene gave 1-[(tert-butyl)bis(trime thylsilyl)methyl]-1-trimethylsiloxy-2,3-bis(trimethylsilyl)-1-sila-c yclopropene (115) in 87% yield (Scheme 32). Acylpolysilane 52 reacted with the acetylene under the same conditions to give silacyclopropene (116), as the sole product. Similarly, the cothermolysis of 51 and 52 with bis(dimethylphenylsilyl)- and bis (methyldiphenylsilyl)acetylene proceeded cleanly to give the respective silacyclopropenes in high yield. We reported that the formation of the silacyclopropenes would come from isomerization of silacyclobutenes generated from [2+2] cycloaddition of silenes and bis(silyl)acetylenes [50]. However, theoretical studies for these reactions indicate that silylenes formed from isomerization of the silenes derived from the thermolysis of 51 and 52 (silene to silylene rearrangement) react with bis(silyl)acetylenes to give the silacyclopropenes [51] (Scheme 33). The energetics of these reactions are shown in Fig. 2. The activation energy for this reaction was calculated to be 28.5 kcal/mol, and the reaction was slightly exothermic by 6.4 kcal/mol.

4. Conclusion In conclusion, this review describes, in detail, the synthesis and reactions of various types of silacyclobutene derivatives developed mainly in our research group. The silacyclobutenes can readily be synthesized by the nickel-catalyzed reactions of phenylethynylpolysilanes and silacyclopropenes obtained photochemically from the phenylethynylpolysilanes with alkynes, and also by the thermal and photochemical reactions of acylpolysilanes with alkynes. The silacyclobutenes are highly reactive, due to their ring strain, and react with various organic unsaturated compounds to give many types of the products. Many examples for the synthesis and reactions of the silacyclobutenes are shown here. This detailed review on the chemistry of the silacyclobutenes therefore will be of vast help to explore and extend its chemistry. The results obtained from the investigations of the silacyclobutenes are widely applicable to other silicon systems and are thought to contribute to the development of all fields of silicon chemistry. Acknowledgements We express our sincere appreciation to Professor Emeritus Kyoto University T. Yamabe, Nagasaki Institute of Technology, Professor K. Yoshizawa, Institute for Material Chemistry and Engineering, Kyushu University, and Professor J. Ohshita, Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University for their great support. We express our appreciation to Shin-Etsu Chemical Co., Ltd, Toshiba Silicon Co., Ltd, Hokkou Chemical C., Ltd, Nitto Electric Co., Ltd, for financial support. References [1] [2] [3] [4] [5] [6] [7]

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