From Alkynylsilanes

A convenient route to both saturated and unsaturated acylsilanes lies in the hydroboration-oxidation of alkynylsilanes (Chapter 7). Recent improvements (6) to this method involve the use of the borane-dimethyl sulphide complex for hydroboration, and of anhydrous trimethylamine IV-oxide for the oxidation of the intermediate vinyl boranes. 

Borane-dimethyl sulphide solution (73 mmol, 2.74 m in THF) was added dropwise, with stirring, to the neat alkynylsilane (200mmol), keeping the reaction temperature below 20°C by use of an ice bath. The mixture was stirred for 2 h at 0-5 °C. At this time, the solution of the trialkenyl borane was diluted with THF (200 ml), and then anhydrous trimethylamine A/-oxide (240 mmol, dried by azeotropic distillation with toluene) was added over 5 min at ambient temperature. The resultant slurry was heated (bath temperature 75 °C) for 4h, and then cooled to ambient temperature. The enol borinate thus formed was hydrolysed by the addition of water (100 ml), followed by stirring for 5 min. The layers were separated, and the aqueous phase was extracted with ether (2x 100ml). The combined organic extracts were washed with brine and dried. Concentration and distillation gave the acylsilane (75-91%). 

This sequence fails with t-BuC CSiMe3, owing to steric hindrance, but succeeds well with the sterically less demanding Me3SiC CSiMe3. General procedure was as above, but with the following amendments. 

(a) The alkylating agent must be reactive, i.e. a methyl, ethyl, allyl or benzyl halide. 

Based on the earlier work of Brook (7), a number of valuable reactions involving Si—C bond cleavage have been described. Both aromatic and 

---

The production of alkynyliodonium tetrafluoroborates from alkynylsilanes with PhIO BF3 may be rationalized in a similar way (equation 7)31. 

Subsequently, a one-pot protocol for the preparation of tri- and tetrasubstituted alkenylsilanes straightforwardly from alkynylsilanes was developed. Within the same carbozirconation protocol, the addition of allyloxytrimethylsilane led to its rapid reaction with the intermediate zirconacyclopropene 288 to expand the... 

The starting materials may easily be obtained from a commercially available ketone, which thus can be transformed in a short sequence into the cyclization substrates41. In addition, dienones containing the alkynylsilane terminator can also easily be transformed to m-fused cyclopcn-tanes with the synthetically useful angular alkenyl side chain41,45. 

A subsidiary approach involves nuclear modification of the arylsilanes so obtained. The requisite organometallics can be prepared from aryl halides, or by deprotonation of a suitably activated (c.g. methoxy-substituted) arene. A more specialized route involves cycloaddition between alkynylsilanes and diynes. 

Previous syntheses of terminal alkynes from aldehydes employed Wittig methodology with phosphonium ylides and phosphonates. 6 7 The DuPont procedure circumvents the use of phosphorus compounds by using lithiated dichloromethane as the source of the terminal carbon. The intermediate lithioalkyne 4 can be quenched with water to provide the terminal alkyne or with various electrophiles, as in the present case, to yield propargylic alcohols, alkynylsilanes, or internal alkynes. Enantioenriched terminal alkynylcarbinols can also be prepared from allylic alcohols by Sharpless epoxidation and subsequent basic elimination of the derived chloro- or bromomethyl epoxide (eq 5). A related method entails Sharpless asymmetric dihydroxylation of an allylic chloride and base treatment of the acetonide derivative.8 In these approaches the product and starting material contain the same number of carbons. 

To see if silacyclobutenes (28) would react in the same manner as the alkynyl-polysilanes (1) and respond similarly to steric differences, compounds 28a and b were heated in the presence of NiCl2(PEt3)2 and PTMSA (Scheme 9). " Compound 28a gave silole 20a in 94% yield,but the only isolable products from the thermolysis of 28b were 26 (51%) and 27b (36%). The products from the thermolysis of silacyclopropene 28b were very similar to that for alkynylsilane lb, but for some reason there were many fewer products for the thermolysis of 28a than for alkynylsilane la. These results suggest that the more sterically hindered lb... 

Brummond reported the silicon-bound products 62 were obtained from the (allenylethyl)alkynylsilane 61 (Equation (31)). Either molybdenum/DMSO or rhodium(i) catalysts affords the desired products, but the regioselec-tivity is dependent on the catalysts used. The trend is in line with what is mentioned previously. ... 

The dominance of an alkoxy group over a silyl group is manifested in the generation of amides from reaction of oc-silyl enol ethers with chlorosulfonyl isocyanate, followed by hydrolysis. Simple vinylsilanes and alkynylsilanes undergo carbamidation [34],... 

Dehydrogenative silylation has also been observed for terminal alkyne substrates. Doyle and co-workers reported in 1991 that a small amount (6%) of alkynylsilane was observed in the product mixture that results from reaction of phenylacetylene and Et3SiH catalyzed by Rh2(pfb)4.41 The remaining components of the product mixture resulted from hydrosilylation. Crabtree and co-workers have found that in reaction of terminal alkynes with various tertiary silanes, dehydrogenative silylation can become the predominant route, depending on reaction conditions [Eq. (7)].42... 

The ethoxycarbocation intermediate (363) produced by the action of acid on the cyclobutenedione monoacetal (362) has been found to react with bis(trimethylsilyl)-acetylene to afford a 2-methylenecyclopent-4-ene-l,3-dione derivative (365). The authors426 proposed that the rearrangement results from an unprecedented cationic 1,2-silyl migration on the alkynylsilane, subsequent ring expansion via a vinyl cation intermediate (364), and re-closure by intramolecular addition of an acyl cation to a silylallene in a 5-exo-trig mode (see Scheme 90). 

Propargylsilanes (alkynylsilanes, silylalkynes) reacted with DIB and several of its ring-substituted analogues in an unusual way the eventual transformation involved the formation of o-iodopropargylarenes. Apart from its mechanistic interest, this constitutes a synthetically useful reaction. 

Other alkynyl phenyliodonium arylsulphonates were prepared by modified procedures, notably from iodosylbenzene and alkynylsilanes, in good yields (62-89%). The decomposition stage proceeded also in satisfactory yields, up to 88% [61]. Bis alkynyl dibenzoates and ditosylates were prepared from the corresponding bis iodonium salts of the general formula PhI+C=C(CH2) C=CI + Ph, where n = 5,6,8 [62]. 

The treatment of alkynylsilanes in acetonitrile with an iodine(III) reagent prepared from iodosylbenzene and two molar equivalents of triflic acid, believed to exhibit structure... 

Alkynylsilanes can be converted into alkynylcoppers by treatment with Cu salts in an aprotic polar solvent such as l,3-dimethyl-2-imidazolidinone (DMI).62 The Cu-promoted coupling reactions via the Si-Cu transmetallation of alkynylsilanes are valuable for the syntheses of alkynyl ketones, 1,3-diynes, 1,3-enynes, and 1-aryl-l-alkynes from acid chlorides,62 alkynyl chlorides,63 alkenyl iodides,64 and aryl iodides,64 respectively (Equation (12)). In addition, symmetric 1,3-diynes can be prepared by the CuCl-mediated homocoupling of alkynylsilanes.65 653... 

A catalytic amount of CuCl was found to activate alkynyl(trimethyl)silanes in the palladium-catalyzed coupling reaction with aryl triflates (Eq. 7) [12]. The catalytic cycle is considered to involve the transfer of an alkynyl group from an alkynylsilane to Cu(I) and then to palladium(II). A sequential palladium-catalyzed reaction of trimethylsilylacetylene gives unsymmetrical diarylacet-ylenes (Eq. 8). 

Allylzinc bromide can undergo regioselectivc addition to l-(trimethylsilyl)-l-alkynes to give adducts in which ZnBr and SiCCH,), are attached to the same carbon atom. In the case of reactive alkynylsilanes, the reaction proceeds by cii-addition. However, if the reaction requires higher temperatures, products resulting from a net trans-addition are obtained (equation II). ... 

---