Stannanes acylation

The stannane-acyl chloride cross-coupling has been directed towards access to 3-acylcyclobutenones and a-heterosubstituted ketones. ... 

A convenient route to highly enantiomerically enriched a-alkoxy tributylslannanes 17 involves the enanlioselective reduction of acyl stannanes 16 with chiral reducing agents10. Thus reaction of acyl stannanes with lithium aluminum hydride, chirally modified by (S)-l,l -bi-naphthalene-2,2 -diol, followed by protection of the hydroxy group, lead to the desired a-alkoxy stannanes 17 in optical purities as high as 98 % ee. 

Scheme 8.13. Synthesis of Ketones from Acyl Chlorides and Stannanes...

An alternative route to nonracemic a-alkoxy stannanes entails the reduction of acyl stannanes with chiral hydrides61 62. Accordingly, conjugated stannyl enones yield (S)-a-alkoxy allylic stannanes by reduction with (J )-(+)-BINAL-H. As expected, (S)-(—)-BINAL-H gives rise to the enantiomeric (7 )-a-alkoxy allylic stannanes (equation 29)61. Upon treatment with Lewis acids, these stannanes undergo a stereospecific anti 1,3-isomerization to the (Z)-y-alkoxy allylic stannanes61. 

Use of di-M-butylstannyl dichloride along with an acyl or silyl halide leads to addition of allylstannanes to the aldehydes.106 Reaction is also promoted by butylstannyl trichloride.107 Both SnCl4 and SnCl2 also catalyze this kind of addition. Reactions of tetraallyl-stannane with aldehydes catalyzed by SnCl4 also appear to involve a halostannane intermediate. It can be demonstrated by NMR that there is a rapid redistribution of the allyl group.108... 

The added advantage of the C (1 )-stannylated glycals is their abUity to participate in palladium-catalyzed coupling reactions with organic halides, a process independently reported by Beau [75] and Friesen [81]. Vinyl stannane 237 can be benzylated, allylated or acylated provided that appropriate catalysts are used [75,77] and representative examples are given in Scheme 59. The C-arylation of... 

Because of the lability of the acyl stannane it is important to have a freshly prepared solution of BINAL-H at -78°C ready for addition of the acyl stannane. This is best achieved by starting the following reduction procedure just prior to the acyl stannane sequence. 

While the a-hydroxy stannane is not as labile as the acyl stannane precursor, it should generally be converted to the ether derivative immediately after isolation. 

The reaction is monitored by TLC (silica gel 1 9 EtOAc-hexane). The TLC plate is developed by dipping into a 0.02 M solution of phosphomolybdic acid in EtOH. The initially formed hydroxy stannane (R = 0.52) will stain blue without heating. The acyl stannane (R = 0.69) is observed by charring the plate. 

Because of the air sensitive nature of the acyl stannane, the flask is vented with argon after removal of solvent. 

Incomplete reduction of the acyl stannane is observed when fewer equivalents of BINAL-H are employed. 

It was discovered by J. C. Saddler and co-workers at Upjohn that for reproducible results it was necessary to reflux the mixture of binaphthol, UiAIH4, and EtOH before performing the reduction. Enantioselective reduction of the acyl stannane can also be effected with Chirald, albeit with slightly diminished enantioselectivity.2... 

Acid chlorides convert l-methylbenzimidazol-2-yl-silanes and -stannanes into 2-acylbenzimida-zoles. The reaction also works with imidazoles, with the stannanes being more reactive than the silanes. The mechanism is believed to involve initial N-acylation, then loss of the silicon or tin substituent to give a zwitterion, and finally N — C-migration of the acyl group (83JHC1011). 

Palladium-mediated addition of silyl stannane reagents to alkynyl ethers has been employed for the synthesis of aliphatic acyl silanes in very good yields via the intermediate a-alkoxy-/J-stannyl vinyl silanes (enol ethers of acyl silanes)82. In a second palladium-catalysed step, the vinyl stannane moiety could be coupled to suitable halides before hydrolysis to the acyl silanes with trifluoroacetic acid (Scheme 11). 

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