Insertions 1,1,1,2,2,2-hexamethyl

A unique bis-silylation system, in which a bis(silyl)palladium intermediate is generated via recombination of two Si-Si bonds, has been developed.8,97 A bis(disilanyl)dithiane reacts with alkynes in the presence of a palladium/ isocyanide catalyst, giving five-membered ring bis-silylation products in high yield with elimination of hexamethyl-disilane (Scheme 14). The recombination, that is, bond metathesis, is so efficient that no product derived from direct insertion of acetylene into the Si-Si bonds of the bis(silyl)dithiane is formed at all. 

Some aspects of the reactivity of the A-frames formed by Reaction 1 have been explored. Carbon monoxide and sulfur dioxide are readily lost from the respective adducts upon mild heating or exposure to vacuum. The insertions of isocyanides or sulfur have not been reversed. However the oxidation of Pd2(dpm)2(/Lt-S)Cl2 to Pd2(dpm)2-(/x-S02)Cl2 can be effected by using m-chloroperbenzoic acid as oxidant. Acetylene addition is photoreversible photolysis of Pd2(dpm)2-(/Lt-C2 C02CH3 2)C12 forms dimethylacetylene dicarboxylate and Pd2(dpm)2Cl2 (14). Pd2(dpm)2X2 is a catalyst for converting dimethyl-acetylene dicarboxylate into hexamethyl mellitate, and Pd2(dpm)2-(/it-C2 C02CH3 2)X2, which forms during the reaction, is presumed to be an intermediate. 

Recent examples include the insertion of platinum into the Sn—Sn bond of hexamethyl-distannane (equation 116) and of a distannaferrocenophane, 90 (equation 117)273,274. 

Besides olygomerization, one of the typical reactions of silanones is the insertion into Si-O or other polar bonds [1]. So the cathodic generation of diphenylsilanone was performed using hexamethyldisiloxane (HMDS) as a silanone trap. The reduction of O2 dissolved to saturation in a 0.1 M solution of BU4NPF6 in DMF or THF in the presence of Ph2Si(OMe)2 and HMDS resulted in formation of an insertion product — 1,1,1,5,5,5-hexamethyl-3,3-diphenyltrisiloxane (Scheme 2). With permethylcyclosiloxanes D (n = 3, 4), used as trapping molecules instead of HMDS, corresponding cyclic products with extended cycles, D D 2, were obtained. The results of siloxane... 

Chlorotris(triphenylphosphane)rhodium reacts with 3,3-dimethylcyclopropene in a stoichiometric way to form the metallacyclic compound 13 which, on exposure to carbon monoxide, undergoes insertion and reductive elimination to yield the hexamethyl-tris-a-homo-tropone 10 in 73% yield. A minor product of the reaction is the homo-cyclotrimer... 

Metal-catalyzed decomposition of diazomethane in the presence of 1,2,3,4,5,6-hexamethyl-bicyclo[2.2.0]hexa-2,5-diene (11, hexamethyl Dewar benzene) has been shown to give the simple or double cycloaddition products with exo configuration. When the reaction was carried out in the absence of a metal catalyst, the products of C-H insertion to allylic C-H bonds were also observed, in addition to cycloaddition to the double bond to an extent of 25%. A C-C insertion reaction at the unusually long central bond of hexamethyl Dewar benzene was not detected. The reaction conditions were designed to generate methylene in the singlet state, and the observed products were those that would be expected for the reaction of singlet methylene with hexamethyl Dewar benzene. 

Hexafluoroacetone inserts into C—H bonds of the ir-cyclopentadienyl ring of (w-C6H5)Rh(C6Me6) [CsMee = hexamethyl(Dewar)benzene] to give (128). With (7r-C5H5)Rh(CHa CMeCMe CH2) it also forms a link between rhodium and the terminal carbon of the diene to give (129). 

The first radialene to be reported was all-( )-7,8,9,10,ll,12-hexamethyl[6] radialene [1]. The first syntheses of the parent systems [3]radialene 2 [2], [4]radi-alene 3 [3], and [6]radialene 5 [4-6] were reported in 1965,1962 and 1977/1978, respectively, yet unsubstituted [5]radialene 4 (see below, however note added in proof) as well as monocyclic [ ]radialenes with n>6 are still unknown. On the other hand, significant research activities have been devoted recently to the so-called expanded radialenes, which have general structural formulae such as 6 and 7 ( > 1), with ethyne, butadiyne, or benzene spacers being inserted between the corners of the parent [ ]radialenes they are covered in Chapter 9 of this book. 

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