Silylation dienes

Based on bis-silylated dienes another approach to quinoxaline derivatives such as 80 (Scheme 4.10) was found [97]. Fast [4+2] cycloaddition takes place by treatment of Cgo with 2,3-bis(trimethylsilyloxy)butadiene 98, yielding the acyloin-fused fullerene derivative 100 in good yields (Scheme 4.16). The silylated diene is formed in situ by treatment of 98 at 180 °C in o-dichlorobenzene. Controlled bromination of the intermediate 99 leads to the transient diketone 101, which reacts readily in a one-pot reaction with various o-diaminoarenes to yield the quinoxaline-fused fullerenes 102. 

The regioselectivity of alkylations of silyl dienol ethers has been studied87,88. These reactions favor y-alkylation products. In contrast, alkylations of the corresponding lithium enolates mainly occur in the a-position. Substituents on the silyl diene unit, as well as the substituents at the silicon, strongly influence the regioselectivity of the reaction87 91,... 

Yamamoto has proposed a mechanism for the palladium-catalyzed cyclization/hydrosilylation of enynes that accounts for the selective delivery of the silane to the more substituted C=C bond. Initial conversion of [(77 -C3H5)Pd(GOD)] [PF6] to a cationic palladium hydride species followed by complexation of the diyne could form the cationic diynylpalladium hydride intermediate Ib (Scheme 2). Hydrometallation of the less-substituted alkyne would form the palladium alkenyl alkyne complex Ilb that could undergo intramolecular carbometallation to form the palladium dienyl complex Illb. Silylative cleavage of the Pd-G bond, perhaps via cr-bond metathesis, would then release the silylated diene with regeneration of a palladium hydride species (Scheme 2). 

In a process related to GTP, aldehydes initiate the polymerization of silyl vinyl ethers and silyl diene ethers. Here the silyl group is present in the monomer and transfers to the aldehyde ended chains regenerating aldehyde ends [17] (Scheme 8). A Lewis acid catalyst is required. terf-Butyldimethylsilyl works best as a transfer group for vinyl ether while trimethylsilyl is suitable for diene ethers [18]. Even though aldol GTP provides a route to polyvinyl alcohol segments in the subsequent block polymer synthesis, the projected cost of the monomers discouraged further research aimed at commercialization. 

The exocyclic silyl dienes are useful for Diels-Alder reactions. The silyl group can also undergo Pd-catalyzed coupling with aryl iodides as well as oxidation to a hydroxyl group. 

Scheme 3 Silyl diene as substrate in Ni-catalyzed coupling reactions with aryl aldehydes...

In this sub chapter the most important synthetic aspect of the RCM of unsaturated organosilicon compounds will be discussed. For a general review on RCM see Refs. [59,122-124]. For a review on RCM of silyl dienes see Refs. [6,7]. A number of valuable synthetic applications have been proposed over the last few years. As follows from the data available, from the point of view of the application of RCM of vinyl and allylsilanes, the most important are silicon-tethered processes. For a general review on the silicon tethered processes see Refs. [125,126],... 

Silylated dienes (303) (Scheme 65) Resin bearing a ketone (301) placed in a flame-dried flask under Ar was allowed to swell in dry CH2CI2 (5.5 mL). After cooling the resin to 0 °C, EtsN (625 pL, 4.5 mmol) and tert-butyldimethylsilyl trifluorometha-nesuhbnate (775 pL, 3.38 mmol) were added sequentially. The reaction mixture was then gently dried in vacuo. 

The silyl diene (90) photochemically adds benzophenone via a SET mechanism to yield the two oxetanes (91, 18X) and (92, SIX) on irradiation at 436 nm in acetonitrile solution. The oxetanes are accompanied by the (242)-dimer of the diene. The reaction appears to be efficient and can be carried out with a variety of diaryl ketones. 

A good illustration of many aspects of this chemistry comes from Koreeda s synthesis of shikimic acid.22 The silyl diene 123 adds cleanly to methyl acrylate to give essentially one isomer of the adduct 124 (9 1). Dihydroxylation with catalytic 0s04 and the stoichiometric oxidant NMO (A-methylmorpholine-A-oxide) occurs on the opposite face to all three substituents. The alkene 124 is an allylic acetate but this group has no attractive interaction with the reagent. 

The bicyclic lactone 78 reacts with the acyclic dienes 6b-d (heat, 60 °C) to give only endo-adducts 79 with excellent yields (95%, 89% and 74%, respectively). The diastereomeric excess for the silylated diene 6d and 2,3-dimethyl-l,3-butadiene (6c) is d.r. >99.9 0.1, while isoprene (6a) yields, as expected, mixture of regioisomers20-21. 

Racemic 7a in high-pressure mediated [4 + 2] cycloaddition to silylated dienes I3a-c produces adducts 14-c, with excellent regio- and stereoselectivity23. 

Silylated dienes have attracted some interest this year. Trost has described his attempts to prepare 2-(trimethylsilyl)buta-1,3-diene (193) by a Pd(0)-catalysed elimination of acetic acid from (194). 

Our recent study on the activity for metathesis revealed a high reactivity of 1,9-decadiene and cyclooctene in their reactions with triethoxy- and trisiloxy-substituted vinylsilanes [20]. When the mixture containing vinylsilane and 1,9-decadiene was heated in the presence of Grubbs catalyst, the formation of mono- and bis(silyl)dienes, accompanied by a polymeric product, was detected. The replacement of 1,9-diene with cyclooctene in the reaction mixture has resulted in the same products. The reactions can be illustrated by Eqs. 6 and 7. l,10-bis(silyl)-substituted dienes were isolated with 50-70 % yields and characterized spectroscopically. In the case of disubstituted products separated by distillation, no double bond migration was observed. The only process observed in that case was E/Z isomerization, so only Z,Z-disubstituted products could be isolated. 

The two reactions of vinylsilanes with cycloalkenes and alkadienes discussed above compete with ROMP and ADMET polymerization of the organic parent substances. Scheme 1 has been proposed to combine all the competitive/consecutive reactions. The successful formation of silyldiene and bis(silyl)diene occurs exclusively when n = 4 (see Scheme 1). 

Metathesis of 1,9-decadiene and cyclooctene with trialkoxy- and trisiloxy-substituted vinylsilanes in the presence of Grubbs catalyst, carried out in appropriate conditions, leads to the formation of bis(silyl)diene with a high yield. Similar processes performed with divinyl-substituted siloxane lead to the formation of silicon-containing polymers (via ADMET copolymerization and tandem ROM/CD polymerization), thus opening a new convenient route to synthesis of unsaturated organosilicon copolymers. 

This method was employed in the stereospecific construction of a silyl diene in a synthesis of chlorotri-cholide (Scheme 2.80). The boronic acid partner 2.233 was prepared by hydroboration and hydrolysis. The vinyl iodide was 2.235 prepared by hydroalumination-iodination. The silyl group was included in order to boost stereoselectivity in the later Diels-Alder reaction, but also served to facilitate the synthesis of the vinyl iodide coupling partner. The Suzuki coupling yielded the diene 2.236 with retention of the stereochemistry of both alkenes. Thallium hydroxide was employed as the Lewis base. Thallium-containing Lewis bases have been found to be advantageous in a number of cases, but the toxicity of thallium is a serious concern. 

The cross-metathesis of 1,9-octadiene with an excess of trialkoxy- and trisiloxy-substituted vinylsilanes results in the formation of bis(silyl)dienes (Eqs. 7 and 8) [14]. 

Full accounts have been given of the Diels-Alder reactions of 1-acylamino-dienes (156), salient features of which are their wide application (even with reluctant dienophiles such as cyclohexenone), their generally good endo-selec-tivity, and their high regioselectivity with unsymmetrical dienophiles. Full details have also been given of the Diels-Alder reactions of the silyl-dienes (157), which are useful precursors of cyclic allyl silanes. ... 

Hydrosilation Agent. Another application of this difunctional hydrosilane is in the nickel-catalyzed hydrosilation of 1,7-octadiyne, which affords a 1,2-dialkylidenecyclohexane with a (Z)-vinylsilane moiety. This exocyclic silyl diene is treated with an allyl alcohol to give a silicon-tethered triene that undergoes intramolecular Diels-Alder reaction and subsequent oxidative cleavage of the silicon-carbon bond to afford a bicyclic diol (eq 2) ... 

Anionic Additions to Aldehydes. The addition of l,3-bis(silyl)-propenes to aldehydes and ketones to yield the vinyl silyl alcohol was explored. This was done using TBAF and good to excellent yields were achieved (eq 11). A further extension of this work was the addition of the (l,3-bis(silyl)allyl)lithium to ketones and aldehydes (eq 12). In this reaction, the substituted silyl diene was isolated in moderate to good yields. These substrates were then explored as ligands for both iron and manganese complexes. 

Because of the low level of regiocontrol imparted by the silyl group, other substituents on 1-silylated dienes can impart better regiocontrol (eq 5). ... 

More recently, Fiirstner has described the hydrosilylation of internal enyne compound 227 to be catalyzed by platinum carbene complex 228 (Scheme 89) [24]. The corresponding silylated diene 229 is thus efficiently obtained in a regio-and stereocontrolled way, en route to lactimidomycin. 

The synthesis of unusual variants of castanospermine and congeners has been an area of activity this year. The silylated diene 160 (from D-arabinose) undergoes a hetero Diels-Alder reaction to provide the pyrrolo-castanospermine analogue 161 (Scheme 33), and the branched... 

Multi-component reactions open a direct access to poly-functional derivatives/ For instance, reactions of unsaturated substrates and aldehydes in the presence of EtaSiH opened a rapid access to allyllic, homo-allylic or co-unsatu-rated alcohols, and NHC-Ni complexes were recently reported to be efficient catalysts for such oxidative coupling. Mori and co-workers described the first example of such condensation in 2001. They showed that an in situ generated IPr/Ni 1 1 complex led to the stereoselective formation of (Z)-homoal-lylic silyl alcohols [eqn (10.33)], while classical Ni -PPhj catalyst produced only (Ej-coupling products. Of note, IPr/Ni 1 2 and IMes/Ni 1 2 were less efficient in this reaction. These first results were extended to silylated dienes. Interestingly, it was shown that a mixed complex [(IPr)Ni(PPh3)] stereoselectively produced (Z)-allylsilanes while [Ni(PPh3)4] led only to (Ej-isomeric products. 

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