Annulation allenylsilanes

The reaction of allenylsilanes with a,/8-unsaturated acylsilanes presents a new [3 + 3]-cycloaddition approach to a six-membered carbocycle [189]. Lewis acid-promoted ring expansion of the [3 + 2]-annulation product 260 is followed by a second cationic 1,2-silyl migration to produce the cyclohexenone 261 after desilylation. 

The allenylsilanes are excellent nucleophiles and they can react with a variety of electrophilic species in annulation processes that provide access to diverse products. Allenylsilane 112 (Eq. 13.36) reacts with tropylium fluoroborate 111 to provide azu-lene 113 [35]. The reaction is slow and it is necessary to use an acid scavenger so as to inhibit protiodesilylation by the fluoroboric acid that is generated during the course of the annulation. The excess tropylium salt abstracts a hydride from the reaction intermediate leading to the azulene. There are relatively few direct methods for the synthesis of azulenes. 

In summary, the Danheiser reactions are a family of [3 + 2] annulations that take place between allenylsilanes and diverse electrophiles. The products can be carbo-cycles or heterocycles. In all cases, the annulations proceed most efficiently when the allenylsilane has a non-hydrogen substituent on the carbon atom bearing the silicon. This is a consequence of the common mechanistic pathway that proceeds through a vinyl carbocation intermediate. 

Allenylsilanes serve as valuable three-carbon components in a general [3+2] annulation method for the synthesis of five-membered rings. A variety of general synthetic approaches to allenylsilanes have recently been developed and a number of specialized routes to various specific functionalized derivatives are available as well. The present procedure... 

Similar [3 + 2] annulations of allenylsilanes 86 and propargylsilanes 87 with a, /i-unsaturated ketones, ketones, 7V-acyliminium ions, nitrosonium ions and tropylium cations giving the corresponding cyclopentenes (cyclohexenes), dihydrofurans, 3-pyrrolines, isoxazoles and azulene derivatives, respectively, have been reported (equations 60 and 6i)142.t54,i68-t72>... 

Danheiser, R. L., Fink, D. M. The reaction of allenylsilanes with a, 3-unsaturated acylsilanes new annulation approaches to five and six-membered carbocyclic compounds. Tetrahedron Lett. 1985, 26, 2513-2516. 

Allenylsilanes combine with electron-deficient alkenes or alkynes regio- and stereo-selectively to afford hi ly substituted and functionalized cyclopentenes (Scheme 10). The [3 -i- 2] annulation reaction has been used for heteroannulation approaches to five-membered oxygen and nitrogen heterocycles. One particularly useful application of the method is that readily available tropylium salts can function as allenophiles in a general [3 + 2] annulation route to substituted azulenes (equation 3). ... 

Allenylsilanes with a-alkyl substituents undergo annulation reactions, as in equation 107. ... 

Three-Carbon Synthon for [3 + 2] Annulations. Danheiser and co-workers have exploited allenylsilanes as the three-carbon components in a [3 + 2] annulation strategy for the synthesis of a variety of five-membered carbocycles and heterocycles. The pathway by which a typical annulation proceeds is shown in eq 6. Reaction of the 2-carbon component (the allenophile ) at C-3 of the allenylsilane is followed by rapid rearrangement of the silicon-stabilized vinyl cation. Ring closure then affords the five-membered product. 

Allenylsilanes lacking a C-1 alkyl substituent do not function efficiently as three-carbon synthons in the [3 + 2] annulation. This phenomenon is attributable to the relative instability of the terminal vinyl cation intermediate required according to the proposed mechanism for the annulations (eq 6). Fully substituted five-membered rings result from annulations emplo3fing allenylsilanes substituted at both C-1 and C-S. ... 

Synthesis of Furans and Isoxazoles. Electrophilic species of the general form Y=X+ serve as heteroallenophiles, combining with allenylsilanes in a regiocontrolled [3 + 2] annulation method. As illustrated in the mechanism shown in eq 13, addition of the heteroallenophUe at C-3 of the allenylsilane produces a vinyl cation stabilized by hyperconjugative interaction with the adjacent carbon-silicon tr-bond. A 1,2-trialkylsilyl shift then occurs to generate an isomeric vinyl cation, which is intercepted by nucleophilic X. Elimination of H" furnishes the aromatic heterocycle. 

The azulene synthesis proceeds best with 1,3-dialkyl (t-butyldimethylsilyl)allenes. (Trimethylsilyl)allenes desilylate to generate propargyl-substituted cycloheptatrienes as significant byproducts. As observed in the other [3 + 2] annulations discussed already, allenylsilanes lacking C-1 alkyl substituents do not participate in the reaction. 

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