Trimethylsilylethynyl complexes

The limited C-R bond-length data for R = CN and R = PRj alkynyl complexes have been noted earlier. For trimethylsilylethynyl complexes, the MCC-Si bond lengths span a range of 0.055 A [1.797(7)-1.852(8) A, Table I rf(C-Si) ,ean = 1.819(22) A], which, despite encompassing only nine distances (esd 0.010 A), is broader than that for the MCC-C(Ph) distances of phenylethynyl complexes. Whether this is an indication that greater M-CCR 7r-bonding obtains for trimethylsilylethynyl complexes than for those in which the alkynyl R-group is an alkyl or aryl substituent is uncertain at this juncture, both because there is insufficient detailed... 

The alkynyl reagent 9 was recently introduced for the dia stereoselective synthesis of tertiary propargylic alcohols144. 9 can be prepared as a solid 1 1 complex with tetrahydrofuran by treatment of 9-methoxy-9-borabicyclo[3.3.1]nonane with (trimethylsilylethynyl)lithium, followed by addition of boron trifluoride-diethyl ether complex. The nucleophilic addition of reagent 9 to (R)-2-methoxy-2-methylhexanal (10) afforded a mixture of the diastereomers 11 with a considerable preference to the nonchelation-controlled (3S,4R)-isomer144. 

TT-backbonding, yet its CC-Si bond length [1.812(2) A] is not significantly shorter than that of the mean distance for all trimethylsilylethynyl metal complexes. Despite these uncertainties, the CC-Si bond distance bears close examination in future studies, particularly in conjunction with Si-NMR chemical shifts and MCC-Si nuclear spin-spin coupling constants, which have been reported to correlate with the extent of C-Si rr-bonding (184), because of the preliminary indication that it is shorter for metal-alkynyl complexes than in organic alkynes. 

Single-crystal X-ray structural data have been reported for the following alkynyliodonium compounds the parent ethynyl(phenyl)iodonium triflate (368) [495], phenylethynyl(phenyl)iodonium tosylate (369) [477], cyanoethynyl(phenyl)iodonium triflate (370) [458], propynyl(phenyl)iodonium periodate (371) [502], trimethylsilylethynyl(phenyl)iodonium triflate (372) [503], 3,3,3-trifluoropropynyl(phenyl)iodonium triflate (373) [504], bis(alkynyl)iodonium triflate 374 [505] and complexes of ethynyl(phenyl)iodonium [506] and l-decynyl(phenyl)iodonium [507] tetrafluoroborates with 18-crown-6 (Figure 2.13). 

Alkynyl(phenyl)iodonium salts have found synthetic application for the preparation of various substituted alkynes by the reaction with appropriate nucleophiles, such as enolate anions [980,981], selenide and telluride anions [982-984], dialkylphosphonate anions [985], benzotriazolate anion [986], imidazolate anion [987], N-functionalized amide anions [988-990] and transition metal complexes [991-993]. Scheme 3.291 shows several representative reactions the preparation of Ai-alkynyl carbamates 733 by alkynylation of carbamates 732 using alkynyliodonium triflates 731 [989], synthesis of ynamides 735 by the alkyny-lation/desilylation of tosylanilides 734 using trimethylsilylethynyl(phenyl)iodonium triflate [990] and the preparation of Ir(III) a-acetylide complex 737 by the alkynylation of Vaska s complex 736 [991]. 

The use of cobalt species to catalyze cross-coupling reactions of activated or unactivated alkyl halides with alkynyl Grignard reagents in order to form C(sp)-G(sp ) bonds is also of interest. Cobalt compleres promote the crossbenzyl chlorides and bromides react with 2-trimethylsilylethynyl- and 1-octyn-l-ylmagnesium halides to provide l-aryl-2-alkynes in the presence of a catalytic amount of a cobalt complex [100,101]. 

Dinuclear Pt complexes with bridging ethynediyl and polyynediyl ligands have been prepared by several reactions. The reaction of m-Pt(G6p5)2(CO)(THE) with a bis(trimethylsilylethynyl)platinum complex 805 results in transfer of one alkynyl ligand of 805 to produce a dinuclear complex 806 with an alkynyl ligand bonded to two Pt centers via cr- and TT-bonds (Scheme 107). The reaction of methanol with 806 and a direct reaction in methanol forms the dinuclear complex with a bridging ethynediyl ligand 807. 

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