Allenyl silyl

Acylsilanes of a variety of substitution patterns have been employed (9) in routes to allenyl silyl enol ethers. 

Allenyl Silyl enol ethers, 86 Allyl alcohol trimethylsilyl ether, 84 Allyl carbonates, 114-15 9 Allyl-ay 2 octalone, 34-5 2-Allyl-2 methylcyclohexanone, 106 (Allyldimethylsilyl)methyl chloride, 58, 59 (AUyldimethylsilyl)methylmagnesium chloride, 59... 

The combination of a silyl-migration from carbon to oxygen and a prototropic isomerization leads to allenyl silyl ethers [246]. 

Whereas deprotonation and halogen-lithium exchange represent the most common methods to access allenic and propargylic lithium intermediates, several less direct routes to more functionalized analogues have also been reported. Additions of various lithium acetylides to acylsilanes followed by Mel or EtI afforded alkylated allenyl silyl ethers (Table 9.4) [10]. The adducts were analyzed after hydrolysis to the related enones. 

Allenyl silyl ethers 40 have also been prepared by the reaction of 2-lithiofurans 38 with acylsilanes 39 via the Brook isomerization (equation 15) . 

Allenylated silyl enolates such as 6-siloxy-l,2,5-trienes and 5-siloxy-l,2,5-trienes undergo W(CO)6-catalyzed 5- and 6-OTr/o-cyclizations to give the corresponding cyclopentenes and cyclohexenes, respectively (Equation (85)).315 These cyclizations proceed via electrophilic activation of the allene moiety by coordination of W(CO)s photochemically generated from W(CO)6-... 

Brook rearrangement. a-Trimethylsilylpropargyl alcohols undergo Brook rearrangement to afford allenyl silyl ethers, which can be used to condense with aldehydes. Silyl group transfer from the ether four bonds apart is preferred after the Sn/Li exchange from silyl ethers of l-tributylstannyl-l,3-alkanediols. °... 

Elimination Tosylhydrazones of a-nitro- and a-benzenesulfonylketones are rapidly converted to the conjugated tosylhydrazones at room temperamre on treatment with DBU. Af,Af-Bistosylhydrazones of the unsubstituted ketones also give the same products. Dehydrochlorination of 2-chloroalkenyl silyl ketones furnishes allenyl silyl ketones (4 examples, 71-86%). ... 

Scheme 13 shows additional examples of constructions of four-carbon chains functionalized appropriately for condensation to pyrroles. Af,Af-bis(trimethylsilyl)propargyl amines can be converted to ketones or esters by lithiation and acylation. These compounds then undergo cuprate addition. The resulting allenyl silyl ethers cyclize to pyrroles on treatment with acid <93T4603). 

Hydrostaimation of arylallenes such as 124 proceeded regio- and stereoselec-tively to provide allylstannane 125 [39]. Reaction of allenyl silyl ethers such as 126 also proceeded regioselectively to produce allylstannanes. A mixture of the stereoisomers 127 and 128 was obtained, but in this case the Z isomer 128 was obtained predominantly. As shown above, allylstannanes are obtained by metallation at the terminal carbon [40]. However, Lautens et al. reported that the vinylstaimane 130 was obtained from alkylallene 129 when ligandless heterogeneous Pd(OH)2 on carbon was used as a catalyst. It was confirmed that the allylstannane 131 was obtained when Pd(PPh3)4 was used. The catalysts changed the selectivity [41]. 

The CuBr2/amine-promoted 2 + 2-cycloaddition reaction of 1,4-bisallenes (17) produced bicyclo[4.2.0]octadiene derivatives (18) in a one-pot procedure (Scheme 7) Phosphoramidite ligands (19) activate the Au(I)-catalysed 2 + 2-cycloaddition reaction of A-allenylsulfonamides with styrenes, at -70 C, to form vinyl cyclobutane derivatives in high yields and enantioselectivity. A new Pt(II) catalyst with a hollow-shaped tri- ethynylphosphine activates the intermolecular 2 + 2-cycloaddition reaction of allenyl silyl ether with vinyl ethers to yield methylenecyclobutanes in good yield. 

Analogously, the silylated / -hydroxyallylsilane 655 condenses with excess dihy-drocinnamaldehyde in the presence of TMSOTf 20 at -78 °C to afford, via 656, the dihydropyran 657 in 82% yield [191] (Scheme 5.58). Condensation of benzaldehyde with methoxytrimethylsilane 13 a and l-trimethylsilyl-2-butyne 658 in the presence of TMSOTf 20 in CH2CI2 affords the allenyl compound 659 in 97% yield and HMDSO 7 [192]. 

Arylative or silylative cyclizations of allenyl aldehydes or ketones have been reported (Equations (101) and (102)).459,459a The intermolecular process, that is, three-component coupling reaction of aldehydes, allenes, and arylboronic acids, is catalyzed by palladium as well (Equation (103)).46O 46Oa These reactions are proposed to proceed through nucleophilic attack of the allylpalladium intermediates to the carbonyl groups. 

The observed /runfavorable interaction between the trimethyl-silyl group and the allenyl group (Scheme 72). 

The Diels-Alder reaction outlined above is a typical example of the utilization of axially chiral allenes, accessible through 1,6-addition or other methods, to generate selectively new stereogenic centers. This transfer of chirality is also possible via in-termolecular Diels-Alder reactions of vinylallenes [57], aldol reactions of allenyl eno-lates [19f] and Ireland-Claisen rearrangements of silyl allenylketene acetals [58]. Furthermore, it has been utilized recently in the diastereoselective oxidation of titanium allenyl enolates (formed by deprotonation of /3-allenecarboxylates of type 65 and transmetalation with titanocene dichloride) with dimethyl dioxirane (DMDO) [25, 59] and in subsequent acid- or gold-catalyzed cycloisomerization reactions of a-hydroxyallenes into 2,5-dihydrofurans (cf. Chapter 15) [25, 59, 60],... 

Gasking and Whitham described the one-pot preparation of 1-silylated 3,3-di-methyl-substituted allenyl sulfides 307 (Scheme 8.82) [170]. Treatment of alkyne 305 with lithium thiolate generates allenyllithium species 306, which is subsequently silylated by trimethylsilyl chloride. Formation of lithiated intermediate 306 is based on a procedure developed by Clinet and Julia [171]. 

Lewis acid-catalyzed ene reactions proceed between allenyl sulfides, e.g. 330, and aldehydes 329 to afford cis-trans mixtures of 1,3-butadienes 331 (Scheme 8.90) [168, 175b], Similar ene reactions observed with imines such as 332 provide the corresponding allylamines [168,177]. It was also found that the ene reaction of 1-silylated allenyl sulfide 333 with various aldehydes (or acetals) furnishes a,/l-unsaturatcd acyl compounds such as 334 and 335 under BF3-etherate catalysis [175b]. 

Scheme 9.7 Mechanistic pathways for reactions of stannane-derived allenyl- and propargyllitihum intermediates with silyl chlorides.

The alkyl allenyl ethers whose chemistry has been discussed are readily prepared and are useful for the synthesis of diverse cyclopentenones, as racemates or in enan-tiomerically enriched form. It is worth noting that silyl allenyl ethers allow entry into a distinct mechanistic manifold [16]. Triisopropylsilyloxyallene 47 can be deprotonat-... 

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