Base-induced cleavage

The activation of silylene complexes is induced both photochemically or by addition of a base, e.g. pyridine. A similar base-induced cleavage is known from the chemistry of carbene complexes however, in this case the carbenes so formed dimerize to give alkenes. Finally, a silylene cleavage can also be achieved thermally. Melting of the compounds 4-7 in high vacuum yields the dimeric complexes 48-51 with loss of HMPA. The dimers, on the other hand, can be transformed into polysilanes and iron carbonyl clusters above 120 °C. In all cases, the resulting polymers have been identified by spectroscopic methods. 

The addition of any one of several dialkyl chlorophosphates to an arylalkyne-derived vinyl zirconocene in the presence of catalytic amounts of CuBr in THF leads to the corresponding vinyl phosphonate in high yields (78—92% see, for example, Scheme 4.38) [25]. Here, alkyl-substituted acetylenic starting materials do not react beyond the initial hydrozirconation stage. Vinyl phosphonates may be readily converted to acyloins by oxidation to the diol followed by base-induced cleavage. 

Base-induced cleavage of non-enolizable ketones leading to carboxylic amide derivative and a neutral fragment in which the carbonyl group is replaced by a hydrogen. 

Base-induced cleavage of 3-unsubstituted isoxazoles was discussed in Section II,A>5. When the 3-position is substituted and the 5-position is unsubstituted, a different type of base-induced ring-opening takes place, which follows course B in Scheme 2. 3,5-Disubstituted isoxazoles are resistant to base-induced cleavage. 

Base-induced cleavage of 1,2,3-thiadiazoles is a useful method of generating the anions 79. Raap and Micetich first reported the reaction with 4-phenyl-1,2,3-thiadiazole (80) (R = Ph). This compound undergoes hydrogen-lithium exchange at C-5 with butyllithium. The lithio derivative is unstable above — 65°C and the anion (79) (R = Ph) can be intercepted in good... 

Photochemical decomposition of ethyl azidoformate in cyclohexene gives 7-carbethoxy-7-azabicyclo[4.1.0]heptane (108) as main product (56% at 38° and 75% at —75°) together with the three isomeric cyclohexenylurethans.82>83M>8 A nitrene mechanism for the reaction is supported by the fact that the same products in almost the same ratio are obtained in the base-induced cleavage of N-p-nitrobenzenesulfonoxyurethan (107). All evidence indicates that the second reaction yields the carbethoxynitrene by an a-elimination mechanism. The aziridine isomerizes at 140° into 2-ethoxy-4,5-cy clo hexano-A2-oxazoline (109). 

Alcohols can also be prepared from support-bound carbon nucleophiles and carbonyl compounds (Table 7.4). Few examples have been reported of the a-alkylation of resin-bound esters with aldehydes or ketones. This reaction is complicated by the thermal instability of some ester enolates, which can undergo elimination of alkoxide to yield ketenes. Traces of water or alcohols can, furthermore, lead to saponification or transesterification and release of the substrate into solution. Less prone to base-induced cleavage are support-bound imides (Entry 2, Table 7.4 see also Entry 3, Table 13.8 [42]). Alternatively, support-bound thiol esters can be converted into stable silyl ketene acetals, which react with aldehydes under Lewis-acid catalysis (Entries 3 and 4, Table 7.4). 

Figure 10.11. Base-induced cleavage of support-bound carbamates [188,269,272,273],...

Haller-Bauer reaction.1 A study of the base-induced cleavage of the optically active tertiary a-phenyl ketone (l)2 shows that cleavage proceeds with retention with metal amides in benzene and alkoxides in an alcohol or benzene, but with... 

In similar processes, base-induced cleavage of halogenated ketones has been used to prepare cyclopropane derivatives [53] (Figure 6.37). 

Plourde [17] has described the radical addition of alkyl, substituted alkyl and benzyl thiols to polymer-supported cychtol allyl ethers. Treatment of immobilized allyl ether 117 with benzyl thiol in the presence of AIBN provided, after base-induced cleavage from the support and purification, sulfide 119 in high yield and purity (Scheme 25). Similar reactions with hydroxyl- and carboxyl-substituted alkyl thiols also resulted in good yields of products as single regioisomers [17]. 

Reactions of oxime 186, containing an electron-deficient carbon-carbon double bond, were also investigated (Scheme 45) [31]. In the case of stannyl radical addition-cyclization using triethylborane as the radical initiator, the reaction proceeded as efficiently as in previous cases giving the cyclized product 188 in 64% yield, after base-induced cleavage from the resin and hnker. 

In contrast to phosphine oxides, whereby the phosphoryl group is reduced, the reduction of phosphonium salts involves cleavage of a P—C bond. There are several ways this can be achieved, including base-induced cleavage, thermal decomposition, hydride or electrolytic reduction.2... 

Several different methods have been employed for the preparation of diazo(trimethylsilyl)meth-ane (1). At present, the best synthetic method seems to be the diazo transfer reaction between diphenylphosphoryl azide and (trimethylsilyl)methylmagnesium chloride.However, the product often contains (chloromethyl)trimethylsilane and hexamethyldisiloxane as impurities. This method can also be employed for the synthesis of diazo(triethylsilyl)methane and diazo(di-ethylmethylsilyl)methane. The diazo transfer reaction can also be carried out with (trimethyl-silyl)methyllithium and tosyl azide. Other methods are the silylation of diazomethane with (trimethylsilyl)trifluoromethanesulfonate or chloro(trimethyl)silane and the base-induced cleavage of A-nitroso-A-[(trimethylsilyl)methyl]urea or the related carbamate or acet-amide.2° =... 

Here, olfeyl-substituted acetylenic starting materials do not react beyond the initial hydrozirconation stage. Vinyl phosphonates may be readily converted to acyloins by oxidation to the diol followed by base-induced cleavage. 

The structure and absolute stereochemistry of hinesol (282) has now been firmly established by an unequivocal synthesis which involved the tricyclic dienone (329) prepared from 6-methoxy-l-tetralone. Treatment of the dienone with lithium dimethylcopper gave a mixture of syn and anti enones (330). By a series of stereoselective reactions this compound was converted to the diol (331) whose mono-mesylate underwent a base-induced cleavage to give the spiro[4,5]-ketone (332). Elaboration of this ketone to hinesol was accomplished along... 

Carbonate resins have proved valuable for construction of heterocycles by a cyclization-induced cleavage that involves intramolecular attack of an amide nitrogen at the anchor in the presence of base [110,111]. Another example of base-induced cleavage of resin-bound carbamates is provided by the observation that after sulfonylation, treatment with sodium methoxide released sulfonamides [112]. Support 2a has also been found to react with... 

Other important cycloelimination procedures correspond to an elimination of H2O from cyclic ketones. Thus, the a-hydrogen atoms of semicarbazones of cyclic ketones are removed by oxidative cyclization with thionyl chloride or selenium dioxide (Scheme 8-7). The 1,2,3-thiadiazoles (71) or 1,2,3-selenadiazoles (72) which result from these reactions can be cleaved in a second step to yield cyclic alkynes (Scheme 8-8) [28]. Several fragmentation conditions are known, among them thermal decomposition and base-induced cleavage. The mechanism of these reactions has been studied in detaU [29]. It has been noted that the crucial step is the cleavage of the carbon-sulfur or carbon-selenium bond, as in this step the geometrical strain is introduced into the system. Clearly, due to the weakness of the C —Se... 

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