Silicones migration

The preference for the /3-silyl isomer product complements methods available for hydrostannation of alkynes, for which the a-stannyl regioisomer is formed preferentially.70 7011 70c In addition, the /3-silyl products serve as the platform for a tertiary alcohol synthesis (Scheme 15). Upon treatment of vinylsilanes such as B with tetrabutylam-monium fluoride (TBAF) in DMF at 0 °C, a 1,2 carbon-to-silicon migration occurs, affording the tertiary heterosilane E. Oxidation of the C-Si bond then provides the tertiary alcohol. Good 1,2-diastereocontrol has been demonstrated for y-alkoxy substrates, as in the example shown. The studies suggest that the oxidation of the sterically demanding silane intermediate is facilitated by the intramolecular formation of a silyl hemiketal or silyllactone for ketone or ester substrates, respectively.71... 

Nitronium fluoborate also reacts with silylallenes, as indicated in Eq. 13.40(38], Attack of the electrophile at the allene carbon atom distal to the silicon atom, 1,2-silicon migration and ring closure lead to a silyloxazole. Exposure of this material to bromine in a second step gives 124 in 72% overall yield from 123. This annula-tion is best performed with tert-butyldimethylsilylallenes. With trimethylsilyl compounds, protodesilylation is a competing side reaction. 

Lewis base adducts, 25 63, 65-67 ligand exchange at silicon, 25 60-62 mass spectra, 25 99-100 optical activity, 25 101-102 silicon migration, 25 78 ferrasilacyclopentane, 25 9, 22, 34 fluxionality, 25 93, 97 infrared spectra, 25 91-92... 

Analogues with a siloxy leaving group (58) (R =Ar, R=H, Me) have been generated from A-(silylbenzyl)-benzamides (56) by an intramolecular silicon migration (32). 

The [2+3] cycloaddition of methylidene borane 36 with alkyl azides furnishes the intermediate triazaboroles 37a, which further undergoes silicon migration from carbon to nitrogen resulting in the formation of 37b. The driving force for the reaction is the stabilization of the ring system due to aromatization (6rt electrons) (Scheme 4) <2004ZFA508>. 

Any disiloxane, (SiH3)20, produced as a result of silicon migration from metal to oxygen (Section III,F,2) will rapidly decompose at 450°C... 

Modified Peterson reaction. This reagent was used in an unusual application of the Peterson reaction. The a-trirtiethylsilyloxy aldehyde (1) was converted in 80% yield to the Ws-trimethylsilyl compound (2) by treatment at - 35" with 0.95 equiv. of trimethylsilyllithium in HMPT. Nucleophilic attack at the carbonyl carbon was followed by silicon migration to the secondary alkoxide. The reaction of 2 with 3 equiv. of lithium diisopropylamide in THE containing 5% HMPT at 23" gave the saturated aldehyde 3 in 80% yield. The reaction was used to convert a hindered >C=0 into >CHCH20H. 

Exclusive [2 + 2] cycloaddition reactions of l-seleno-2-silylethenes with the highly electrophilic olefin dimethyl l,l-dicyanoethene-2,2-dicarboxylate occur in the presence of SnCU (Eq. 82) [120a]. In contrast, the reaction of l-seleno-2-silylethenes with methyl vinyl ketone or 2-phosphonoacrylates in the presence of SnCU, involving an unprecedented selenium-mediated 1,2-silicon migration, leads to the [2 + 1] adduct (Eq. 83) [120b,c]. 

When the carbanitm of (61) reacts with aldehydes or kettmes, a 1,4-silicon migration from O to O with subsequent loss of diethyl phosphite is observ (Scheme 31). Hydrolysis of the resulting rearranged product leads to a-hydroxy ketones in good yield. 

The ultimate test would be to have silicon migrate on a simple, unstrained olefin (Eq. 3). With the exception of a few, very strained olefins, such as 1-norbornene [5] (Eq. 4), the thermal rearrangement of an olefin to a carbene is unknown in carbon chemistry. 

Fe-Si bond to the CN bond of the nitrile, followed by a second silicon migration to nitrogen. Migration of the hydrogen atom is then apparently mediated by the metal center230. 

The mechanism initially proposed for the Rubottom oxidation involved epoxidation of the enolsilane to afford intermediate silyloxyoxirane 4. It was suggested that this intermediate undergoes acid-mediated cleavage to afford stabilized carbocation 5, which is transformed to the a-silyloxy ketone 6 via 1,4-silicon migration. Hydrolysis of 6 by aqueous acid in a subsequent step generates the a-hydroxy ketone 7.lb 15 Attempts to provide support for this mechanism via isolation of intermediate silyloxyoxiranes derived from simple ketones proved difficult due to the lability of these compounds. However, Brook demonstrated that the related heterocyclic silyloxyoxirane 8 was isolable and was transformed to ketone 9 upon treatment with /j-TsOH. 

An interesting extention of this methodology using chiral crotyl silane 474 produces tetrahydrofurans directly as a result of 1,2-silicon migration and heterocyclization [152]. Under boron trifluoride etherate catalysis (non-chelation controlled conditions), cis-2,5-disubstituted tetrahydrofuran (475) is produced in 50% yield, whereas tin tetrachloride (chelation control) gives the ra 5-2,5-disubstituted tetrahydrofuran (476) in 75% yield. Stereoselectivities in both cases exceed 40 1. 

Deprotonation of vinylsilane-allylic alcohols using sodium hydride in HMPA is followed by an essentially quantitative C —> O silicon migration (eq 7). ... 

---