Silyl enol ethers Alkynylation

A different approach towards titanium-mediated allene synthesis was used by Hayashi et al. [55], who recently reported rhodium-catalyzed enantioselective 1,6-addition reactions of aryltitanate reagents to 3-alkynyl-2-cycloalkenones 180 (Scheme 2.57). In the presence of chlorotrimethylsilane and (R)-segphos as chiral ligand, alle-nic silyl enol ethers 181 were obtained with good to excellent enantioselectivities and these can be converted further into allenic enol esters or triflates. In contrast to the corresponding copper-mediated 1,6-addition reactions (Section 2.2.2), these transformations probably proceed via alkenylrhodium species (formed by insertion of the C-C triple bond into a rhodium-aryl bond) and subsequent isomerization towards the thermodynamically more stable oxa-jt-allylrhodium intermediates [55],... 

Halogen shifts have been found for tungsten, with assumed formation of iodovinylidenes in reactions of 1-iodo-l-alkynes with W(CO)5(thf) en route to cyclization of 2-(iodoethynyl)styrenes to naphthalenes and of iodo-alkynyl silyl enol ethers [147], while more substantial confirmation is found in Mn =C=C(I)CH (OR)2 (CO)2Cp [R = Me, Et (OR)2 = 0(CH2)30], of which the XRD structure of Mn =C=C(I)CH(OMe)2 (CO)2Cp was determined [148]. 

This reaction in the presence of base was applied to a tandem cyclization. When bis-alkynyl silyl enol ether 93a was irradiated in toluene in the presence of 10 mol % W(CO)6 and DABCO with 1 equiv of H2O, the expected tricyclic ketone 94a was obtained in 80% yield. The five-membered substrate 93b also gave the corresponding tricyclic ketone 94b having the basic carbon skeleton of the cedranes. Thus we can prepare synthetically useful tricyclic compounds utilizing this W (CO)5(L)-catalyzed tandem cyclization in the presence of DABCO [25c] (Scheme 5.29). 

Scheme 5.29 Tandem cyclization of bis-alkynyl silyl enol ethers.

Diastereoselective aldol coupling of alkynyl aldehydes.3 The Co2(CO)6 complexes (2) of alkynyl aldehydes react with silyl enol ethers to form aldols with... 

Belmont s group87 reported a cycloisomerization reaction on quinolines 77 (Scheme 5.34) bearing a silyl enol ether group on position 3 and an alkynyl group on position 2, leading to acridine derivatives 78. 

An interesting reaction between the bis phenyl iodonium triflate of acetylene and the silyl enol ether of acetophenone afforded an allene (PhCOCH=C=C=CHCOPh, 84%) [6], Also, alkynyl iodonium tosylates and carbon monoxide in methanol or ethanol, with palladium catalysis, furnished alkyne carboxylates [53]. 

It has already been noted that the enolates of unactivated monocarbonyl compounds do not undergo alkynylation with alkynyliodonium salts3. It is therefore particularly intriguing that [bis(phenyliodonium)]ethyne ditriflate reacts with the silyl enol ether (SEE) of acetophenone to give an allenic diketone (equation 151)41. Except for the SEE of cyclohexanone, which gives a black tar with the bisiodonium compound41, similar studies of other SEEs have not been reported. 

Very few pericyclic reactions of carbene complexes have been studied that are not in the cycloaddition class. The two examples that are known involve ene reactions and Claisen rearrangements. Both of these reactions have been recently studied and thus future developments in this area are anticipated. Ene reactions have been observed in the the reactions of alkynyl carbene complexes and enol ethers, where a competition can exist with [2 + 2] cycloadditions. Ene products are the major components firom the reaction of silyl enol ethers and [2 + 2] cycloadducts are normally the exclusive products with alkyl enol ethers (Section 9.2.2.1). As indicated in equation (7), methyl cyclohexenyl ether gives the [2 -t- 2] adduct (84a) as the major product along with a minor amount of the ene product (83a). The t-butyldimethylsilyl enol ether of cyclohexanone gives the ene product 9 1 over the [2 + 2] cycloadduct. The reason for this effect of silicon is not known at this time but if the reaction is stepwise, this result is one that would be expected on the basis of the silicon-stabilizing effect on the P-oxonium ion. 

Cyclization. Silyl enol ethers of to-alkynyl ketones undergo cyclization with (THF)W(C0>5. Similarly, 4-alkynols give dihydropyrans. ... 

Alkynyltrichlorotin reactions. The reagents are generated from 1-alkynes by treatment with SnCU-BujN in CH2CI2. Thus alkynylation of aldehydes, acetals, and enones (1,4-mode) is achieved under mild conditions using a one-pot operation. Condensation of silyl enol ethers and ketones with 1-alkynes. Both alkylide-... 

Mukaiyama conditions were used intramolecularly for the successful closure of an 11-membered ring in Smith s synthesis of normethyljatrophone. 64 Conversion of the alkynyl conjugated ketone in 251 to the silyl-enol ether was followed by reaction with TiCU, leading to reaction with the dioxolane moiety and ring closure (sec. 7.3.B.1 for formation of dioxolanes) to give 252 in 47% yield.l " ... 

Matsuda and co-workers found that silyl enol ethers of thiol esters are good nucleophiles in the reaction with propargylic-type esters to form p-alkynyl thiol esters in the presence of a catalytic amount of [Ir(cod)(POPh3)2]OTf 32 [37]. The acetoxy group of 29 was readily substituted at 25 °C to form thiol ester 31 with 93% yield by simply stirring together 29,4 equiv of silyl enol ether 30, and 32 (5 mol%) (Eq. 16). 

The silver(I)-catalysed 5(Q-exo-dig cycloisomerization of alkynyl silyl enol ethers, such as (267), has been reported to produce the spiro-cyclic derivatives (268). Trapping the vinyl silver intermediate with NIS afforded the alkenyl iodides (269). ... 

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