Silyl-ketones

Diazaphospholes are known to undergo facile 1,3-dipolar cycloaddditions with a variety of dipoles [2, 4, 7, 98], During recent years, some interesting [2+3] cycloaddition reactions have been reported. 2-Acyl-[l,2,3]diazaphospholes 6 were reported to undergo [2+3] cycloaddition with diazocumulene 92, the minor equilibrium isomer of a-diazo-a-silyl ketones 91, to form a bicyclic cycloadduct 93 (Scheme 29). Thermolysis of the cycloadduct results in the formation of tricyclic phosphorus heterocycle 94, which can be explained due to the possibility of two parallel reactions of cycloadduct. On the one hand, extrusion of molecular nitrogen from 93... 

The reaction of silyl ketone acetals with imines under irradiation has been explored. The versatility of the microwave approach is illustrated in Scheme 8.45. 

RhH(PPh3)4 efficiently isomerized fv-silylallylic alcohols to cr-silyl ketones in excellent yields, but only in the presence of an a-silyl enone as co-catalyst (Equation (13)).48... 

For /3-silylallylie alcohols, [Rh(cod)2]BF4/2PPh3 is a more effective catalyst than RhH(PPh3)4 to afford 3-silyl ketones in good yields (Scheme 1).1 As an interesting extension of this reaction, a tandem hydrosilation-isomerization process, starting from propargylic alcohols, was reported (Scheme 25).49... 

Palladium-catalyzed bis-silylation of methyl vinyl ketone proceeds in a 1,4-fashion, leading to the formation of a silyl enol ether (Equation (47)).121 1,4-Bis-silylation of a wide variety of enones bearing /3-substituents has become possible by the use of unsymmetrical disilanes, such as 1,1-dichloro-l-phenyltrimethyldisilane and 1,1,1-trichloro-trimethyldisilane (Scheme 28).129 The trimethylsilyl enol ethers obtained by the 1,4-bis-silylation are treated with methyllithium, generating lithium enolates, which in turn are reacted with electrophiles. The a-substituted-/3-silyl ketones, thus obtained, are subjected to Tamao oxidation conditions, leading to the formation of /3-hydroxy ketones. This 1,4-bis-silylation reaction has been extended to the asymmetric synthesis of optically active /3-hydroxy ketones (Scheme 29).130 The key to the success of the asymmetric bis-silylation is to use BINAP as the chiral ligand on palladium. Enantiomeric excesses ranging from 74% to 92% have been attained in the 1,4-bis-silylation. 

E)-Enol silyl ethers.1 A new highly stereoselective route to (E)-enol silyl ethers involves addition of CH,Li to silyl ketones substituted at the a -position by a SC6H5 group such as 1. The adduct (a) undergoes a Brook rearrangement and... 

Photolysis of silyl ketones can produce silyl radicals quite clearly. 

But the method suffers from the difficulty of obtaining starting materials and photo induced rearrangement of silyl ketones... 

Conjugated silyl ketones are likewise converted to acylcyclopentenes (Eq. 9.64) [64], In some additions an isomeric cyclohexenone is formed (Eq. 9.65). This latter product is postulated to arise from ring expansion of the initial acylcyclopentene as illustrated in Scheme 9.15. 

Retro-Brook rearrangement of the [l,3]-variant will readily take place in sp and sp carbanion systems. Kuwajima and Takeda and Corey and Rticker have developed the [l,3]-retro-Brook rearrangement of silyl enol ether anions which provide a-silyl ketones (equation 100 and 101). 

ElectrophiHc substitutions with carbon and hetero electrophiles a to the carbonyl group of aldehydes and ketones are among the most important synthetic operations. Such regio-, diastereo-, and enantioselective substitutions can be carried out efficiently with the SAMP/RAMP hydrazone methodology [3]. For cases where virtually complete asymmetric inductions could not be attained, an alternative approach based on a-silylated ketones 2 was developed [4]. They can be prepared easily from ketones 1 in high enantiomeric purity (ee > 98%) by asymmetric carbon silylation employing the SAMP/RAMP hydrazone method (Fig. 1.1.1). After the introduction of various electrophiles via classical enolate chemistry with excellent asymmetric inductions, the desired product ketones 3... 

Although efficient organocatalytic methods for the electrophilic a-fluorination of aldehydes and ketones have recently been developed [7], high enantiomeric excesses can only be reached with aldehydes so far. The asymmetric inductions in the case of ketone fluorinations have remained low ee < 36%) [7a]. Thus, the a-silyl ketone-controlled stoichiometric asymmetric synthesis of a-fluoroketones 10 (Scheme 1.1.1) still constitutes a practical method. 

As depicted in Scheme 1.1.2, the silyl ketones (S)-ll of high enantiomeric purity were converted into the Z-configured silyl enol ethers (S) -12, which were used in the aminomethylation step by treatment with dibenzyl(methoxymethyl)amine in the presence of a Lewis acid. The silylated Mannich bases S,R)-13 were obtained in excellent yields and diastereomeric excesses (de = 92-96%). Finally,... 

Analogues of (751) react with derivatives of phosphorus-containing acids to form 1-phosphoryl-imidazoles (90CHE599). The same compound also silylates ketones to give enol silyl ethers and/or siloxyalkylimidazoles (Scheme 107) (87JC271). 

The 1,4-disilylation of a,/3-unsaturated ketones has been reported by Ito and co-workers.172 Unsymmetrically substituted disilanes, PhCl2SiSiMe3 and Cl3SiSiMe3, undergo reaction in the presence of catalytic amounts of palladium complexes with tertiary alkyl phosphine or bidentate phosphine ligands to yield /3-silyl ketones [Eq. (64)]. 

Silyl enol ethers Trimethylsilyl enol ethers can be obtained from a-silyl ketones by thermal rearrangement or by catalysis with HRh(CO)[P(C6H5)3]3, (CH3)3SiOTf, or ISi(CH3)3. The first two methods are (E)-selective in the case of unsymmetrical ketones, whereas the latter two are (Z)-selective. 

The (diastereoselective) conjugate addition of silylcuprate reagents to a variety of chiral derivatives of a,(3-unsaturated carboxylic acids can be used to prepare optically active p-silyl esters.258 259 Best results are obtained with substrates of type (25). The (related) p-silyl ketones, which also constitute valuable building blocks for (acyclic) stereoselective synthesis, are now accessible in high ee via palladium-catalyzed enantioselective 1,4-disiiylation of a,p-unsaturated ketones (Scheme 76).260... 

Enals vinyl silyl ketones.1 The anion of I reacts smoothly with va rious electrophiles (alkyl halides, epoxides, carbonyl compounds). The products are converted to (E)-enals by oxidation with 30% H202. 

Bromination-desilylbromination of fS-silyl ketones. The preliminary work on protection of oc,/ -enones by this sequence (8, 196-197) has been perfected in several respects1 and shown to be generally applicable to synthesis of oy8-enones.2 Either bromine or phenyltrimethylammonium tribromide (PTAB, 1,855 2,328 4, 386) can be used. In either case, bromine is introduced mainly at the a-position adjacent to the /1-silyl group. Surprisingly, the undesired a -bromo-/(-silyl ketone is converted to the a-bromo-/)-silyl ketone on exposure to hydrogen bromide at 20°. No other acids can clTccl this conversion. Desilylbromination is generally effected with DBU or F". The sequence is applicable to synthesis of a-methylene ketones (equation I) and lactones. 

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