Carbonyl Wittig-type reactions

Thus unsubstituted (R=H) and substituted (R = alkyl) non-stabilized diyiides 1 react with phenylisocyanate and dicyclohexylcarbodiimide (R NCX), leading to the formation of new monoylide type intermediates. These last ones react in situ with carbonyl compounds through a Wittig type reaction leading respectively to a,)8-unsaturated amides 2 and amidines 3, with a high E stereoselectivity, the double bond being di- or tri-substituted [48,49]. By a similar reactional pathway, diyiides also react with carbonic acid derivatives, with the synthesis as final products of -a,/l-unsaturated esters 4 and acids 5 [50]. 

The mixed lithium aminophosphonium azadiylides 7 [47] are known to be involved in reaction with various electrophiles specifically at the carbon center. The reaction with carbonyl known to form betaine adducts was recently used for the first time in Wittig type reactions for the synthesis of di-, tri-, and tetrasubsti-tuted alkenes 8 [52]. 

Different types of the reagents (see Fig. 8-4) have been applied in asymmetric Wittig-type reactions. Because no new sp3 stereocenter is formed in a Witting-type reaction, a substrate containing a stereogenic or pro-stereogenic unit apart from the carbonyl group is usually required to induce an asymmetric process. 

Another disadvantage of Wittig-type reactions is their limitation to aldehydes and ketones as the carbonyl component carboxylic acid derivates... 

Alkynols complexed to cobalt can be oxidized to alkynals without decomplexation. Propargyl aldehydes are protected from polymerization upon complexation with Co2(CO)6. These aldehydes smoothly undergo Wittig-type reactions. Carbonyl-ene reactions have been demonstrated (Scheme 194). Complexation to cobalt protected the enyne in complex (132) from Michael-type reactions (Scheme 195). Alkenyl-substituted complexes undergo [3 + 2]cycloadditions with nitrile A-oxides (Scheme 196). 

In any chain reaction, apart from initiation steps, the termination steps are also important. In metathesis there are many possibilities for termination reactions. Besides the reverse of the initiation step, the reaction between two carbene species is also a possibility (eq. (17)). The observation that, when using the Me4SnAVCl6 system, as well as methane traces of ethylene are also observed [26] is in agreement with this reaction. Further reactions which lead to loss of catalytic activity are (1) the destruction of the metallacyclobutane intermediate resulting in the formation of cyclopropanes or alkenes, and (2) the reaction of the metallacycle or metal carbene with impurities in the system or with the functional group in the case of a functionally substituted alkene (e. g., Wittig-type reactions of the metal carbene with carbonyl groups). 

Dithioacetals react with [Cp3Ti P(OEt)3 2] to produce [Cp2Ti(SR)2] and an alkylidene species [Cp2Ti=C(R )(R )] which is an excellent entry to the Wittig-type reactions from carbonyl, alkene, and alkyne substrates [59]. 

Various carbonyl compounds, such as ( )-3-(benzyloxy)propenal (30), do not react with cyclo-propylidenetriphenylphosphorane. However, the addition of a phase-transfer catalyst can greatly influence the progress of a reaction. Thus, yields obtained from Wittig-type reactions with cyclopropylidenetriphenylphosphoranes are greatly improved by the addition of tris[2-(2-methoxyethoxy)ethyl]amine (TDA-l). The failure of ( )-3-(benzyloxy)propenal to react with cyclopropylidenetriphenylphosphorane was converted into a useful reaction when 10 mol % of TDA-1 was utilized. ... 

Consequently, phospha-l,2-dioxetanes are the only reasonable intermediates that produce the excited carbonyl fragments and simultaneously satisfy the experimental results in the present study. In this study, the substituent effect on the ring closure step in the oxy-Wittig type reaction was revealed by chemiluminescence decay. The promotion of the ring formation results in the... 

Although it is not possible to prepare a,p-unsaturated dihydro-1,3-oxazines (84) by mere dehydration of the adducts (78), these alkenes can be prepared in good overall yields by Wittig-type reactions of carbonyl compounds with the phosphorus-substituted dihydro-1,3-oxazines (80-83 Scheme 30). Generally, better yields of (84) are obtained when aldehydes are employed as the reaction partners for the phosphoranes (80) and (81) and ketones are the reactants for the phosphonates (82) and (83). The a,p-unsaturated dihydrooxazines (84) are highly useful intermediates since they can be converted into a,p-unsaturated aldehydes (Scheme 30) as well as a,p-unsaturated ketones and carboxylic acids (Scheme 31).54... 

In the literature, the term Staudinger reaction is actually associated with any process that involves the conversion of an azide to the iminophosphorane as its initial step. Hydrolysis is the typical fate of this intermediate, but iminophosphoranes (sometimes called phosphazenes) can also undergo a host of synthetically useful tandem reactions, most notably aza-Wittig-type reactions with carbonyl containing compounds.1-4,6-10... 

Na2Mo02 2H2O) and sodium methylamino- or ethylamino-dithiocarbamate. In THF, the Mo compound is treated with various simple diazoalkanes, e.g., phenyl-, diphenyl-, methylphenyl-, and secondary alkyl-diazomethanes. In this reaction (10-21), metalloazines of type 10.54 are formed as yellow crystals in a broad spectrum of yields up to 99%. The ratio of (Z)- to ( )-isomers was determined based on NMR spectroscopy. These metalloazines behave as active carbonyl equivalents in a Wittig-type reaction with phosphoranes (10-22) and form the corresponding ethene derivatives. 

Di-, tri-, and tetra-substituted 2-azadienes (64) carrying electron-withdrawing groups have been synthesised by the aza-Wittig reaction. A one-pot synthesis of olefins from diorganyltelluride, diazo compounds, and carbonyl compounds in the presence of a Cu(I) catalyst is suggested to involve a Wittig-type reaction of an intermediate telluronium ylide. ... 

Methods for the formation of carbon-carbon double bonds in an asymmetric manner through non-Wittig-type reactions [12,13] have also been reported in recent years, including asymmetric induction by reactions with chiral sulfoxides [13], sul-fones (Julia olefination) [14], sulfoximides [12a, 15], or selenides [16] Pd-catalyzed allylic nucleophilic substitutions [17], as well as asymmetric deprotonation [18]. In the context of the topic of asymmetric carbonyl olefination, some of these asymmetric transformations are beyond the scope of this chapter, although a few of the transformations closely related to the Wittig-type reactions will be discussed in a later part of this chapter. 

The most serious problems associated with the Wittig-type reactions stem from sensitivity to steric hindrance and enohzation of the carbonyl substrates under the... 

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