Alkenylating reagents

R = H, Alkyl, R = H, Alkyl, Allyl, c-Alkenyl Reagent = EtgSiH, MegAl, TMS-allyl, 3-TMS-Cyclohexen (-penten) Lewis acid = TiCl4, SnCl4, SnBr4, TfOH Scheme 13 (Table 3)... 

Alkenyl(phenyl)iodonium salts have attracted a significant interest recently as stable and readily available powerful alkenylating reagents. Several convenient, general procedures for the stereoselective synthesis of alkenyliodonium salts from silylated or stannylated alkenes and the appropriate hypervalent iodine reagents are known [5]. The chemistry of alkenyliodonium salts has been extensively covered in several recent reviews [42 - 45]. 

Table 1 summarizes some of the most satisfactory routes to the alkenyl reagents of various classes and the preferred cross-coupling protocols. 

TABLE 1. Some Preferred Alkenyl Reagents for the Synthesis of Arylated Alkenes via Pd- or Ni-Catalyzed Cross-Conpling and Preferred Cross-Conpling Protocols... 

Alkenylcatecholboranes 258, derived from the hydroboration of alkynes, e.g., 256, with catecholborane, are also useful alkenylating reagents Using the original procedure reported by Takaya et the desired products (259) were obtained... 

Starting from the central core with four alkenyl groups, the dendrimer is constructed by repeating sequences of alternating hydrosilylations with chlorosi-lanes as a core, HSiCls as hydrosilylation reagent, and allyl magnesium bromide as ty-alkenylation reagent to obtain dendrimers (Scheme 44). 

These early reports [18, 20] provide support for the proposed role of Pd in stoichiometric synthesis of benzofurans involving reaction of pallada(ll)cyclic Pd(L-C,C)(bpy) (L = [2-(Et02CCH0)C6H4] )with l(C=CR)Ph(BF4) [69, 70], Palladium(IV) intermediates were not detected in this study, but NMR monitoring of reactions of closely related alkenyl reagents I(CH=CHR)Ph(BF4) led to detection of a Pd - intermediate [69, 70],... 

The methods listed above are by no means intended to be exhaustive, even though most, if not all, of the Types III and IV alkenyl reagents may now be prepared by one or more of them. Some other known routes to them that are by and large either not yet weU-developed or not selectively applicable to the syntheses of acyclic unsymmetricaUy substituted alkenes, are not cited here. It should also be mentioned that further explorations and developments in this general area are very desirable. 

Of the four types of alkenyl groups highlighted in gray in Table 3.6, Types V and VI alkenyl reagents may be used for preparing either E- or Z-trisubstituted alkenes... 

I 3 Negishi Coupling and Its Recent Advances Table 3.9 Summary of elementometallation routes to types V-VIII alkenyl reagents. Elementometallation... 

Applications of Types V-VIII Alkenyl Reagents to the Synthesis of Natural Products Containing Conjugated Di- and Oligoenes... 

Allylic alcohols of use in the synthesis of deoxy sugars have been synthesized by reaction of 4-deoxy-L-erythrose derivative (47) (available from the products of cinnamaldehyde in fermenting bakers yeast) with various metal-alkenyl reagents the major isomer (48), formed with vinylmagnesium bromide, was thus converted into the 6-deo3Qr-L-altrofuranosides (49) (Scheme 9). Full details on the synthesis of 2-deojy-L-sugars from l-... 

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