Propargyl silane

Lithiation of the allenic stannane in the presence of Me3SiCl gave rise to an 86 14mixture of propargylic and allenic silanes (Eq. 9.7). Sequential lithiation followed by addition of Me3SiCl, on the other hand, afforded the propargylic silane to the near exclusion of the allenic isomer. 

Table 9.28 Synthesis of propargylic silanes by SN2 displacement of propargylic sulfmates.

The route has also been applied to TBS-substituted propargylic mesylates (Eq. 9.39) [45]. Interestingly, the isomeric propargylic silanes are not formed despite the more attractive steric environment for a direct SN2 displacement at the primary center. 

A number of additional methods involve the addition of alkynylsilanes to electrophiles with concomitant 1,3-isomerization to afford allenylsilanes geminally substituted with the electrophile moiety. The first of these methods employed a trimethyl-silyl-substituted propargylic silane as the alkynylsilane and various acetals as the electrophile precursors (Table 9.29) [53], The allenylsilanes are formed without contamination by alkynyl isomers. 

Allenyltrichlorosilanes can also be prepared by Sn2 displacement of propargylic chlorides with a Cu or Ni complex of HSiCl3 [56]. The reaction requires an amine base and a donor solvent such as THF or propionitrile (Table 9.32). Conditions can be adjusted to favor the propargylic or allenic silane, which is not isolated, but treated directly with various aldehydes to afford allenylcarbinols (A) or homopropargylic alcohols (B). These reactions presumably proceed by an SE2 pathway, such that the allenyl products arise from the propargylic silane and vice versa. 

Table 9.33 In situ additions of trichloallenic and/or propargylic silane to aldehydes.

Intramolecular Sakurai reaction. Allylic and propargylic silanes can undergo a Lewis acid catalyzed intramolecular Sakurai reaction.1 In cyclization to hydrin-danones, the stereochemical outcome can differ from that obtained by fluoride ion catalysis (presumably kinetically controlled cyclization), equation (I).2... 

Y. Liu, G. Asymmetric synthesis of a-amino allyl, benzyl, and propargyl silanes by metalation and rearrangement. Org. Lett. 2003, 5, 1859-1861. 

Cyclization reactions of a-acyliminium ions with allyl- and propargyl-silanes constitute a useful method for the preparation of various N-bridgehead bicyclic systems. Thus, from a reaction of hydroxylactams 107 and 108, the azaazulenes 109 and 110 were obtained in excellent yields (83TL1407). Similarly, the keto amide 112 was obtained from the imide 111 by reduction to the corresponding hydroxylactam and acid-catalyzed cyclization (84JCS(P1)2477). 

For a review on intramolecular reactions of allylic and propargylic silanes, see D. Schinzer, Synthesis, 1988,4, 263. 

Unfortunately, attempts to perform this substitution reaction on cyclohexenol and geraniol led to the exclusive formation of the corresponding silyl ethers. It thus would seem that one requirement for effective carbon-carbon bond formation is that allylic alcohols be secondary and have possess y,y-disubstitution. Pearson, however, discovered a method with less restriction on the natiue of the substrate he used allylic acetates with y-mono-substitution or primary alcohols [96]. Not only ketene silyl acetals but also a diverse set of nucleophiles including aUyl silane, indoles, MOM vinyl ether, trimethylsilyl azide, trimethylsilyl cyanide, and propargyl silane participate in the substitution of y-aryl allylic alcohol 90 to give allylated 91 (Sch. 45). Further experimental evidence suggests that these reactions proceed via ionization to allylic carboca-tions—alcohols 90 and 92 both afforded the identical product 93. 

Methyl and 9-[(trimethylsilylmethyl)]-9-BBN derivatives are easily synthesized by the reaction of the corresponding lithium reagents with 9-methoxy-9-BBN. Unfortunately, such derivatives are spontaneously flammable in air, making them particularly hazardous to handle for the purpose of isolation. However, selective oxidation with anhydrous triethylamine A -oxide converts them to air-stable borinate esters which are efficient reagents for the methylation [149, 150] of haloalkenes or the syntheses of allylic and propargylic silanes (Scheme 2-52) [151]. 

Conjugated trienes. Alkenylcarhene complexes react with propargylic silanes... 

Another way that allenes can be used to provide an acyl anion equivalent is by the Lewis acid catalysed addition of propargyl silanes to electrophiles 109 followed by oxidative cleavage of the allene. The intermediate vinyl cation 110 is stabilised by the silicon (3-effect (chapter 12) and loss of the Me3Si group gives the allene 111. Oxidative cleavage of the sensitive allene reveals that it has acted as a reagent for the formyl anion. 

The intramolecular addition of propargylic silanes to cyclic enones proceeds smoothly under mild conditions, yielding functionalized exocyclic allenes (Scheme 20). This useful process has been reviewed by Schinzer. ... 

Allenyl- and Propargyl-silanes and -stannanes 2.2.6.4 Vinyl-silanes and -stannanes... 

This reaction type is useful for the synthesis of tropane-like azabicycles (equations 94 and 95). Allyl-and propargyl-silanes are excellent nucleophiles in this process.Enone (125) is probably first transformed into an enol ether (c/. equation 70) before it cyclizes to the homotropane skeleton, which is eventually isolated as a mixture of enone (126) and chloride (127). 

The first example of an epoxide-induced pentacyclization has been describedlos. The process is separated into a tri- and a consecutive bicyclization. Termination of the initial tricyclization leads to a fluorine-stabilized cation, which simultaneously induces the final bicyclization. The whole process is terminated by a 6-exo reaction of a propargyl silane group. Additionally, some mono- and bicyclic byproducts are isolated105. Further investigations with the well-known... 

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