Acetylenic tin reagents

With the exception of photochemical reactions, the chemistry of allyltin compounds is analogous to that of allylsilanes and translates to the use of acetylenic tin reagents. As a final illustration of the utility of tin reagents to the formation of C-glycosides, the use of acetylenic tin reagents, shown in Scheme 7.20, is addressed. In the cited report, Zhai et al. [98] observed the formation of a-C-acetylenic glycosides in good yields. The reactions were catalyzed by zinc chloride and run in carbon tetrachloride. 

Other tin reagents have found use in Pd-catalyzed cross-couplings with halopyridines as well. The Stille coupling of 3-iodopyridine with ethoxy(tributylstannyl)acetylene gave rise to 3-ethoxyethynylpyridine (100), which was then hydrolyzed to the corresponding ethyl 3-pyridylacetate (101) [88], Carbamoylstannane 102 was prepared by sequential treatment of lithiated piperidine with carbon monoxide and trimethyltin chloride. Stille coupling of carbamoylstannane 102 and 3-bromopyridine provided a unique entry to amide 103 [89],... 

Annulation to carbonyl functions has also been achieved with Trost s bifunctional reagents. Whereas the parent silyl acetate (97) yields only simple alkylation products with aldehydes under normal conditions, addition of only a few mole % of trimethyltin acetate to the reaction mixture results in facile formation of methylenetetrahydrofurans Furthermore, excellent diastereoselectivity is observed in the cycloaddition to a galactose-derived aldehyde (125) (equation 136). The tin acetate co-catalyst also promotes addition to relatively unreactive ketone carbonyls, such as in the case of benzofuran (126) and the acetylenic ketone (127) (equations 137, 138). It is remarkable that even the sterically hindered enone (128) reacts preferentially at the ketone function (equation 139). A tributyltin analog (129) of (97) has been used in the stepwise formation of a methylenetetrahydrofuran from aldehydes. Similarly, pyrrolidines can be prepared from the corresponding imines in two steps via a Lewis acid-catalyzed 1,2-addition of the tin reagent, which is then followed by a Pd-catalyzed cyclization (equation 140). Direct formation of pyrrolidine from the imine is possible if one uses a mesylate analog of (97) and a nickel(O) catalyst (equation 141). ... 

Tin Reagents. ipso-Monoiodination of 3,4-bis(trimethylsilyl)thiophene followed by Pd-catalyzed cross-coupling with stannylated acetylene gives the monalkyne 64. Distanny-lated acetylene will give the alkyne substituted at both termini 65 (Scheme 30). 

The overwhelming majoritiy of currently known alkynyliodonium species are prepared by interaction of a terminal, sila- or tin-acetylene with an electrophilic X -iodane, also referred to as a 10-1-3 hypervalent species [14]. Key reagents are iodosobenzene (4), [hydroxyftosyl-oxy)iodo]benzene (5, HTIB) [15] the -oxo-X -iodane 6 [16], and cyano(phenyl)iodonium triflate 7 [17]. 

---