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Silicon Based Cross Coupling Reactions Through Intramolecular Activation

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Silicon Based Cross Coupling Reactions Through Intramolecular Activation Silicon ─ Based Cross ─ Coupling Reactions Through Intramolecular Activation Yoshiaki Nakao ＊ and Tamejiro Hiyama ＊＊ ＊ Department of Material Chemistry, Graduate School of Engineering, Kyoto University Kyo＊to＊ University Katsura, Nishikyo ─ ku, Kyoto 615 ─ 8510, Japan Research and Development Initiative, Chuo University 1 ─ 13 ─ 27 Kasuga, Bunkyo ─ ku, Tokyo 112 ─ 8551 (Received July 1, 2011; E ─ mail: [email protected]; [email protected]) Abstract: Tetraorganosilanes having a 2 ─ (hydroxymethyl)phenyl group are found to undergo the silicon ─ based cross ─ coupling reaction. The proximal hydroxy group allows transmetalation of alkyl, alkenyl, and aryl groups on silicon to palladium(II), nickel(II), or copper(I or II) to occur, and hence participation in the cross ─ coupling reaction under mild conditions with excellent chemoselectivity. The high stability of the tetraorganosilicon structure is demonstrated by functionalization under various acidic, basic, and oxidative conditions which leave the silyl group completely intact. Moreover, highly efcient synthesis of functional molecules such as oli- goarenes has been achieved through iterative cross ─ coupling/O ─ deprotection sequences by simply switching reactivity with orthogonal O ─ protection/deprotection. hydroxide to generate pentacoordinate silicates in situ ham- 1． Introduction pered application to the synthesis of highly functionalized tar- Metal ─ catalyzed cross ─ coupling reactions have been devel- get molecules and the manufacture of commodity and spe- oped extensively over the last three decades, and applied widely cialty chemicals. Accordingly, efforts have been made to to synthesis of a number of useful substances ranging from overcome these issues and make the silicon ─ based protocol biologically active compounds such as pharmaceuticals and attractive for synthetic chemists. For example, silicon reagents 5 6 7 agrochemicals to organic materials including liquid crystals having silacyclobutyl, electron ─ poor aryl, benzyl, or allyl and uorescent materials, all on scales varying from the labo- groups, 8 which are converted to highly reactive uorosilanes ratory use to the industrial production. Among the main group and/or silanols in situ upon treatment with uoride, have made organometallic coupling partners, organomagnesium, ─ zinc, possible the use of stable tetraorganosilanes in the cross ─ cou- 1 ─ boron, and ─ tin compounds have been extensively utilized. pling reaction. The use of bench ─ stable silanolate salts, on the However, as the result of wide application of cross ─ coupling other hand, has resulted in smooth transformation under mild chemistry even on varying scales, a number of issues have reaction conditions without the need for external activators. 9 come to light with these conventional cross ─ coupling proto- cols, such as the stability, toxicity, availability and reusability of not only the nucleophilic coupling partners, but also the resultant metal residues. With respect to these issues organo- silicon compounds have been expected to have many advan- tages over other organometallic reagents. Nevertheless, until Hiyama and Hatanaka reported for the rst time in 1988 the cross ─ coupling reaction using tetracoordinate organo(halo)- silanes through formation of pentacoordinate silicate species in situ by the aid of nucleophilic uoride (eq. 1 and Scheme 1), 2 organosilanes had been relatively less employed for cross ─ cou- pling reaction because of low reactivity due to their high sta- bility. 3 This initial protocol has been further developed reveal- ing several inherent issues associated with the original silicon ─ based cross ─ coupling chemistry while at the same time the Scheme 1. A general catalytic cycle for silicon ─ based cross ─ coupling reactions. importance of the silicon ─ based cross ─ coupling reaction has come to be better recognized. 4 In the original protocol, the use of organosilanes having electron ─ withdrawing halogen or oxy- gen bound to silicon was considered to be crucial for making the silicon center Lewis ─ acidic enough to promote and stabi- lize the formation of the requisite pentacoordinate silicates. This reagent design makes the organosilicon reagents suscepti- ble to acid ─ or base ─ mediated hydrolysis and thus less attrac- tive for handling by synthetic chemists irrespective of the aforementioned advantages of organosilicon compounds. In addition, the use of such strong nucleophiles as uoride and Vol.69 No.11 2011 （ 27 ） 1221 有機合成化学69-11＿05論文_Nakao.indd 27 2011/10/20 11:14:34 Ultimately, however, cross ─ coupling reaction using stable transition metals, hypothesizing that the use of 2 ─ (hydroxy- tetraorganosilanes activated by such a mild base as metal car- methyl)phenyl ─ substituted organosilanes (1 ─ 5, Scheme 5) bonate would appear to be ideal, and a general solution would effect transmetalation of an organic group on silicon addressing all the issues has remained elusive until very without the need for a hydroxyl substituent on the silicon atom recently. itself, and so establish a general strategy for enabling tetraor- The robust C ─ Si bonds of tetraorganosilicon compounds ganosilanes to undergo the cross ─ coupling reactions readily. can be cleaved by intramolecular nucleophilic attack as can be Such reagent design of tetraorganosilanes is partly suggested seen in the Brook rearrangement. 10 In particular, rearrange- by the work of Hudrlik and coworkers on allylation and ben- ment is caused by intramolecular attack of anionic oxygen zylation of carbonyls. 13 such as lithium alkoxides (Scheme 2). C ─ Si bond cleavage as in In this account, we describe how tetraorganosilanes 1 ─ 5 the Brook rearrangement has been successfully combined with undergo transmetalation smoothly to copper, nickel, and pal- 11 transmetalation from silicon to copper (Scheme 3) and palla- ladium, and so participate in cross ─ coupling reactions with dium (Scheme 4). 12 We focused our attention on these smooth aryl and alkenyl halides 14 as well as benzyl carbonates. 15 The transmetalations of tetraorganosilicon compounds to late present cross ─ coupling reactions using the newly developed organosilanes proceed smoothly under reaction conditions Scheme 2. The Brook ─ rearrangement. using metal carbonates as a mild activator owing to the pres- ence of the proximal hydroxy group. Moreover, silicon residue 6 can be recovered readily and reused for the synthesis of fur- ther tetraorganosilicon reagents (Scheme 5). 2． Preparation of 2 (Hydroxymethly)phenyl substituted Scheme 3. Transmetalation of [2 ─ (hydroxyalkyl) phenyl](trimethyl) ─ ─ 14,15 ─ silanes to copper. Organosilanes and Related Reagents Typical experimental procedures for the syntheses of com- mon organosilicon compounds are applicable to the prepara- tion of 2 ─ (hydroxymethyl)phenyl ─ substituted tetraorganosili- con compounds. For example, alkenylsilanes 1 are prepared by 16 metal ─ catalyzed hydrosilylation of alkynes using hydrosi- lanes 7a and 7b, which are readily available from 2 ─ bromoben- zyl alcohol, followed by O ─ deprotection. Platinum ─ catalyzed 17 hydrosilylation gives (E) ─ alkenylsilanes stereoselectively 6c,18 (Table 1), whereas ruthenium ─ catalyzed reactions provides an access to (Z) ─ alkenylsilanes (eq. 2). Alkenylsilanes having a Scheme 4. Cross ─ coupling reaction of (Z) ─ β ─ (trimethylsilyl)acrylic range of functional groups can be prepared by these protocols acid. owing to the excellent chemoselectivity achieved with these well ─ established hydrosilylation reactions, as well as the avail- Table 1. Hydrosilylation of alkynes with 7a. Scheme 5. Cross ─ coupling reactions using 2 ─ (hydroxymethyl) ─ phenyl ─ substituted and related organosilanes and their regeneration. 1 222 （ 28 ） J. Synth. Org. Chem., Jpn. 有機合成化学69-11＿05論文_Nakao.indd 28 2011/10/20 11:14:41 Table 2. Ring ─ opening reactions of cyclic silyl ethers 6 with ability of orthogonally O ─ protected hydrosilanes 7a and 7b. organomagnesium or ─ lithium reagents. Alternatively, simple alkenylsilanes such as vinyl ─ and prope- nylsilanes in addition to a number of arylsilanes and hetero- ar ylsilanes, can be prepared by the ring ─ opening reactions of cyclic silyl ethers 6 with highly nucleophilic organolithium or ─ magnesium reagents (Table 2). A cyclohexane variant of cyclic silyl ether 6’, which was readily prepared through O ─ silylation of 2 ─ hydroxyprop ─ 2 ─ yl ─ substituted cyclohexene fol- 19 lowed by platinum ─ catalyzed intramolecular hydrosilylation, also reacts with aryl Grignard reagents to give the correspond- ing arylsilanes 2’ in good yields (Scheme 6). The trans stereo- chemistry of the arylsilyl group and the 2 ─ hydroxylprop ─ 2 ─ yl functionality has been unambiguously conrmed by X ─ ray crystallography of a related 3,5 ─ dinitrobenzoate. Cyclic silyl ether 6a reacts with LiAlH 4, and subsequent one ─ pot O ─ acety- lation gives hydrosilane 7b (eq. 3). These ring ─ opening reac- tions can be used for regeneration of the tetraorganosilicon reagents since the cyclic silyl ethers are silicon residues obtained in high yields upon completion of the cross ─ coupling reaction (vide infra). The tetraorganosilicon compounds thus obtained tolerate a number of reaction conditions convention- ally employed for functional group manipulations as well as standard basic and acidic work ─ up procedures by virtue of the robust tetraorganosilicon moiety (Scheme 7). 3． Cross coupling of 2 (Hydroxymethyl)phenyl substituted Alken─ylsilanes 14a,14c ─ ─
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