Alcohols from allylic silanes

The Pd(MeCN)4(BF4)2-catalysed conversion of allylic alcohols into allylic silanes and boronates has been investigated by product studies, kinetic studies, and H, B, F, and Si NMR (nuclear magnetic resonance) spectroscopy. The two reactions that occur by the same mechanism involve the formation of a palladium allylic alcohol [(a -allyl) palladium] complex after the alcohol is activated by BF3 formed from a BF4 ion of the catalyst. Then, a rate-determining, stereoselective transmetalation with Si(Me)2 and a reductive elimination of the palladium gives the linear silane. B2pin2 replaces Si(Me)2 in the borylation reaction. 

Another recent disclosure examined silicon-to-copper transmetallation as a mild means of synthesizing alkenyl-copper reagents from stable precursors. The method requires activation of the silyl group by an allylic alcohol. Again, the silanes in this work are produced by circuitous means but should be accessible by ruthenium-catalyzed hydrosilylation. Treatment of the silyl alcohol with a stoichiometric amount of copper(l) /rz -butoxide results in the C-to-O migration of the silyl group to produce a vinylcuprate shown to be competent for subsequent allylation to produce 1,4-diene products (Scheme 17). 

The alcohol 10 looks like it might be formed by the addition of a Grignard reagent to an aldehyde. In fact, Patrick Steel of the University of Durham prepared 10 (Tetrahedron Lett. 44 9135,2003) by Diels-Alder addition of the transient silene derived from 7 to the diene 8. More highly substituted dienes lead to more complex arrays of stereogenic centers. The intermediate silacyclohexenes, exemplified by 9, should also engage in the other reactions of allyl silanes. 

Carbamate and amide groups have been found to be stable under these coupling conditions73. In the presence of TiCLt or SnCLt, chiral a-keto amides 36 react with allyl-silane to produce, after hydrolysis, optically active tertiary alcohols 37 with extremely high optical selectivity (equation 23)74. The addition reaction appears to occur from the Si face of the carbonyl group. In a similar manner, a high degree of stereoselectivity is obtained from the reactions of A-Boc-a-amino aldehydes 38 with 2-substituted allylsilanes (equation 24)75. 

The first example was described in the mid-1980s independently by Jeffery, Hallberg, and others in their work on Heck coupling. The former demonstrated the beneficial role of silver salts in the formation of dienols from allylic alcohols and vinyl iodides, as well as the role of counterion or phase transfer conditions (Scheme 10.28).50 The latter found that silver salts accelerated the reaction between aryl iodide and allyl or vinyl silanes, reinforcing regioselectivity and avoiding desilylation (Scheme 10.29).51... 

The radical anions of carbonyl groups can also be generated via PET from activated alkenes, e.g. allylic silanes or stannanes. Triplet excited aromatic ketones, a-dicarbonyls and Michael systems are suitable substrates for oxidizing allylic Group 14 organometallic compounds with subsequent formation of homoallylic alcohols or S-allylated ketones (Scheme 32) [120-122]. 

Allyl silanes react with epoxides, in the presence of Bp3 OEt2 to give 2-allyl alcohols.The reaction of a-bromo lactones and CH2=CHCH2Si(SiMe3)3 and AIBN leads to the a-allyl lactone.On the other hand, silyl epoxides have been prepared from epoxides via reaction with iec-butyllithium and chlorotri-methylsilane. ° a-Silyl-A-Boc-amines were prepared in a similar manner from the A-Boc-amine. " Arylsilanes were prepared by reaction of an aryl-lithium intermediate with TfOSi(OEt)3. In the presence of BEs etherate, allyl silane and a-methoxy A-Cbz amines were coupled. Benzyl silanes coupled with allyl silanes to give ArCHa—R derivatives in the presence of VO(OEt)Cl2 " and allyltin compounds couple with allyl silanes in the presence of SnCl4. Allyl silanes couple to the a-carbon of amines under photolysis conditions. 

Oxazolidine 144 obtained from amino alcohol 143 and ethyl trifluoropyruvate is also a synthetic intermediate for 2-amino-2-trifluoromethylpentanoic acid 145. Lewis acid-catalyzed allylation of 144 with allyl silane occurs in excellent yield with a moderate stereoselectivity. Meanwhile, O-tert-butyldimethylsilyl-protected imine 146 gives better diastereoselectivity although yield is poor (see Scheme 9.31) [57]. 

Two aspects of allyl anion chemistry are combined in the synthesis of bicyclic lactams by Gramain and Remuson.36 Treatment of the unsaturated alcohol 123 with BuLi initially forms the lithium alkoxide 124 and then the allyl lithium 125, possibly better represented as 125a. The proton is removed only from the methyl group rather then the internal CH2 group. Reaction with Me3SiCl and careful acidic workup gives the allyl silane 126. 

More exotic electrophiles include cobalt-stabilised cations derived the alcohol (S)-138 made by a sequence of reactions that shows the stability of allyl silanes to bases. The cuprate from Z-135 adds to a single enantiomer of the epoxide (5)-136 and the tosylate in the product (S) -137 is displaced by a Co(I) anion to give the intermediate (S)-138 as a stable orange solid.38... 

Now coupling to the simpler allyl silane 169 with the same Lewis acid but also with MeOSiMe3 gives the new alcohol as its methyl ether 170. Once again control is from silicon as the aldehyde has no neighbouring chiral centre to exert Felkin control. New chiral centres at C-10 and C-ll are created. 

Table 3. Cyclopropanes from the Simmons-Smith Reaction with Acyclic Chiral Allylic Silanes and Alcohols...

Allylation. y-Selective allylation of aldehydes using chiral reagents formed in situ from tartrate esters and allyldiisopropoxyboranes shows 37-85% ee. On the other hand, the reaction with allylic silanes follows a pathway leading to hydroxyallylation of the double bond, and primary alcohols are obtained from 5-silyl-l,3-dienes. ... 

Scott G. Nelson of the University of Pittsburgh has developed (J. Org. Chem. 2005, 70, 4375) a highly diastereocontrolled route to substituted cyclohexanones using the intramolecular Sakurai reaction. The requisite allyl silane 12 was prepared by Claisen rearrangement of the allylic alcohol 10, followed by homologation. The Ti enolate from the Sakurai addition was trapped with isobutyraldehyde to give 13. Although 32 diastereomers of 13 are possible, the diastereomer illustrated was the dominant product from the cylization. Note that use of the enantiomerically-pure form of the alcohol 10 would have led to enantiomerically-pure 13. 

In photoreactions of di-, tri-, and oligosilanes in the presence of acceptors (A), initiation is by electron transfer from the silanes to A and Si-Si bond cleavage occurs to give silyl radicals and silyl cation components. The silyl radicals display reactivity similar to diat of allyl and aiylmethyl radicals with the radical anions of the electron acceptors. The result is the introduction of silyl groups into the electron-deficient alkenes and arenes. When nucleophiles such as alcohols are... 

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