Allylsilanes Preparation

Me3SiCH2CH=CH2i TsOH, CH3CN, 70-80°, 1-2 h, 90-95% yield. This silylating reagent is stable to moisture. Allylsilanes can be used to protect alcohols, phenols, and carboxylic acids there is no reaction with thiophenol except when CF3S03H is used as a catalyst. The method is also applicable to the formation of r-butyldimethylsilyl derivatives the silyl ether of cyclohexanol was prepared in 95% yield from allyl-/-butyldi-methylsilane. Iodine, bromine, trimethylsilyl bromide, and trimethylsilyl iodide have also been used as catalysts. Nafion-H has been shown to be an effective catalyst. 

Oshima and coworkers reported the preparation of allylsilanes such as 148 by ringopening reactions of three-membered rings such as aziridines 147 with silylalu-minium reagents as shown in Scheme 2.38 [58]. 

Lewis acid catalyzed carbonyl addition of allylsilanes (Section D.l.3.3.3.5.) and allylstannanes (Section D.l.3.3.3.6.) usually proceed with clean allylic inversion (Section D.l.3.3.1.2.). Since these compounds are prepared by several routes and are also stable enough to be purified, each regioisomer can be approached. 

A large number of publications appeared on these aspects5, but most of these studies did not address stereochemical questions. In most cases, a given synthetic problem can be better solved by other allylmetals. Grignard reagents have some importance as intermediates for the preparation of allylboronates (Section D.1.3.3.3.3.2.1.), allylsilanes (Section D.1.3.3.3.5.2.L), allyl-stannanes (Section D. 1.3.3.3.6.2.1.1.), or allyltitanium derivatives (Section D.I.3.3.3.8.2.). 

Allylsilanes are widely used in organic synthesis1 4. They are stable with respect to 1,3-migra-tion of the silicon indeed high temperatures or catalysts are required for 1,3-equilibration5. The preparation of allylsilanes has been reviewed recently6-7. 

Allylsilanes have been prepared using Wittig condensations28,29. 

Allylsilanes in which the silyl group is at the more substituted end of the allyl system have been prepared by a reaction sequence involving the conjugate addition of silylcuprates to a, jS-unsat-urated esters followed by reduction and dehydration via selenoxide elimination38. 

Efficient asymmetric transfer is also observed for 1,2-benzenediol induced reactions of a-sub-stituted (Z)- and ( ,)-allyl(trialkoxy)silanes prepared by hydrosilation of conjugated dienes, although in this case the electrophile attacks the C-C double bond of the allylsilane syn with... 

The addition of 2-propenyltris(diethylamino)titanium to 2-(dibenzylamino)alkanals, readily prepared from amino acids, yields the anti-amino alcohols with high stereoselectivity and free of racemization91. Allylsilanes, with Lewis acids under the conditions of chelation control, lead to the iyn-diastereomers91. 

The starting materials for annulative cyclization are cycloalkenones that contain the allylsilane side chain in the 4-position. Such starting materials can easily be prepared from vinylogous esters40. Furthermore, reactions of 3-alkoxy-2-cyclohexenones with functionalized iodides in the presence of lithium diisopropylamide provides an excellent route to such precursors41 34 35. 

Both terminal and internal propargylsilanes are versatile intermediates for the preparation of allylsilanes (Chapter 5). 

A variety of routes are available for the preparation of allylsilanes (/) with the simplest and most direct being the silylation of allyl-metal species. Other routes exemplified in this chapter include Wittig methodology, the use of silyl anions/anionoids in allylic substitution, and hydrometallation of... 

The majority of catalytic enantioselective allylation reactions involve the chiral Lewis-acid-catalysed additions of allylsilanes or allylstannanes to carbonyl compounds. Monothiobinaphthol has been used by Woodward et al. as a chiral promoter in the enantioselective catalytic allylation of aryl ketones with impure Sn(allyl)4, prepared from allyl chloride, air-oxidised magnesium and SnCl4. Therefore, the allylation of arylketones in these conditions was achieved very efficiently, since the corresponding allylic alcohols were formed in... 

A three-component domino process consisting of an ene reaction followed by the addition of an allylsilane to afford polysubstituted tetrahydropyrans in generally good yield was described by Marko and coworkers [115]. However, the nature of the products formed in this process depends heavily on the Lewis acid employed as a catalyst. Thus, reaction of the allylsilane 4-333 with an aldehyde in the presence of BF3-Et20 led to the domino products 4-334, whereas in the presence ofTiCL the diol 4-332, and in the presence of Et2AlCl the alcohol 4-335, were obtained (Scheme 4.74). The latter compound can then be transformed in stepwise manner to 4-334, using the same aldehyde as previously. However, it is also possible to use another aldehyde to prepare tetrahydropyrans of type 4-336. 

One compound, however, which does come within the scope of this section is the furopyridooxazine 513. This has been prepared by the reaction between the allylsilane-linked amino alcohol 512 and glyoxal in TFA <1998T10309> (Scheme 111). 

The reactions of titanium-alkylidenes prepared from thioacetals with unsymmetrical olefins generally produce complex mixtures of olefins. This complexity arises, at least in part, from the concomitant formation of the two isomeric titanacyclobutane intermediates. However, the regiochemistry of the titanacyclobutane formation is controlled when an olefin bearing a specific substituent is employed. Reactions of titanocene-alkylidenes generated from thioacetals with trialkylallylsilanes 30 afford y-substituted allylsilanes 31, along with small amounts of homoallylsilanes 32 (Scheme 14.16) [28]. 

An interesting application of the cydization of alkenyl thioacetals is the stereoselective preparation of olefmic diols. Thus, oxidative cleavage of the silicon—carbon bond [32] in the ring-closed metathesis products, i.e. cyclic allylsilanes such as 35 and 36, affords (Z)-alk-2-ene-1,5-diols 37 and 38 (Scheme 14.18) [33],... 

Allylsilanes from esters.1 A reagent prepared from CeCl, and (CH3)3-SiCH2MgCl is effective for conversion of esters to allylsilanes (equation I). The... 

The most direct route to the 1,4-dicarbonyl equivalent required for the aldol condensation would be to couple the enol ether of an aldehyde with the enol ether of a ketone. However, this sequence proved impractical due to the hydrolytic instability of the ketone enol ether. Even after an extensive effort, the substrate for the electrolysis reaction could not be reproducibly prepared in high yield. These problems were readily avoided with the use of an allylsilane based... 

A norpseudoephedrine auxiliary has been used to achieve >98% ee in the preparation of homoallylic alcohols from aliphatic alcohols and allylsilane. On-line NMR spectroscopy has been used to shed light on the mechanism, including a diversion that occurs if the temperature is not kept low enough. 

The class of 3-silyl-substituted reagents provides, upon addition with aldehydes, allylic silanes that offer many options for further derivatization. Oxidative processes are described in previous sections (see the sections on Preparation of 1,2-Diols and 1,4-Diols). If the appropriate silicon substituents are chosen, formal [3+2] cycloadditions with aldehydes can be promoted under Lewis acid catalysis. For example, the mismatched addition of the Z-3-propyl-3-benzhydryldimethyl allylsilane 183 to an a-benzyloxy aldehyde proceeds with low diastereofacial selectivity in favor of product 184 however, after protection of the secondary alcohol, an efficient [3+2] annulation provides the polysubsubstituted furan 185 in good yield and acceptable stereoselectivity (Scheme 24). ° The latter is brought forward to a tricyclic unit found in the antitumor natural product angelmicin B. 

In a related strategy, the dihydropiperidine skeleton of the macrocyclic alkaloid cannabisativine (208) (Fig. 1.6), isolated from the leaves and roots of the common cannabis plant, was prepared with regio- and stereoselectivity using an intramolecular allylsilane-nitrone cycloaddition reaction as a key step (260). 

The ring opening of an aziridine can also occur in an intramolecular fashion, as observed in the formal [3+2] azitidine-allylsilane cycloaddition reaction of 142. These substrates were used for the preparation of both 5-5 and 6-5 fused ring systems <99T8025>. 

Using catalyst 1, Blechert and co-workers were able to demonstrate the first sterically controlled CM reaction for the exclusive preparation of -olefms (Scheme 6). The best results were obtained using various substituted allylic amines, where the completely stereocontrolled installation of allylsilanes was observed. The mild conditions employed in these examples are noteworthy, as minimal racemization occurred when highly epimerizable chiral allyl amino esters were employed. 

In addition to allylsilanes, CM can also be applied to allylstannanes, which serve as valuable reagents for nucleophilic additions and radical reactions.To date, only eatalyst 1 has been shown to demonstrate CM reactivity in the preparation of 1,2-disubstituted allylstannanes, as ruthenium catalysts were found to be inactive in the presence of this substrate class.Poor stereoselectivities were generally observed, with the exeeption of one instance of >20 1 Z-selectivity in the reaction of allyltributylstannane with an acetyl-protected allyl gluco-side. 

The first total synthesis of the marine dolabellane diterpene (- -)-4,5-deoxyneodolabelline 106 has been accomplished by Williams et alf" The /ra j -disubstituted dihydropyran moiety in key intermediate 69 was efficiently prepared from mixed acetal 103 by RCM with catalyst C and subsequent Lewis acid-catalyzed allylation of ethyl glycosides 104 with allylsilane 107 (Scheme 18). 

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