Wittig-type alkenation

Wittig-type alkenation of the carbonyl group is possible with Ti carbene compounds [56], The reaction is explained by the formation of nucleophilic carbene complexes of Ti, although they are not isolated. In the carbonyl alkenation, the oxametallacyclo-butane intermediate 182 is formed by [2+2] cycloaddition of the carbene complex 181 with the carbonyl group. This intermediate is converted to the new alkene 183 and the Ti(IV) oxo species 184, which is a stable compound, and hence the carbonyl alkenation requires a stoichiometric amount of the Ti complex. Also, ester 185 is converted to the enol ether 187 via 186. 

Asymmetric Wittig-type reaction The possibility of carrying out asymmetric version of the Wittig-type reaction is not readily apparent as there is no sp stereocentre formed. However, asymmetric Wittig-type alkenation reactions have been carried out by placing stereogenic centres adjacent to or near the reaction centre. Three types of asymmetric Wittig-type... 

The reduction of a-phosphorus-substituted ketones of the general form 7 has also been quite thoroughly researched (Table 7 84i -84s. The product alcohols arc intermediates in Horner -Wittig type alkene syntheses, and can be induced to undergo stcreospecific syn elimination on conversion to the corresponding sodium or potassium (but not lithium) alkoxides. It is often possible to purify the alcohols by crystallization, allowing the synthesis of stereochemically homogeneous alkencs. 

The (E)/(Z) alkenic ratio in Wittig-type alkenations was shown to be dependent on the amount of Li salt present. ... 

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]. 

Metal alkylidene complexes find application in the metathesis of alkenes, the cvclopropanation of alkenes (Grubbs, Schrock), Wittig type reactions, and the McMurry reaction. In suitable complexes a-elimination can occur twice yielding alkylidync complexes. See Figure 2.21 for an example with tungsten. 

Under very mild conditions, the Wittig-type reaction of dimesitylboron stabilized car-banions with arylketones and aldehydes yields alkenes in good yields (equation 142)530. The stereochemistry of the alkene produced may be controlled, depending upon conditions. 

On the other hand, tertiary phosphanes react with fiuorinated alkenes to form either vinyl-phosphoranes in the case of perfluorinated alk-l-enes, or ylides when perfluorinated alk-2-enes are used as starting materials. The latter lead to useful building blocks for Wittig-type reactions. 

A variety of reagents are used for the construction of carbon-carbon double bonds from carbonyl compounds. For example, ylides such as Wittig-type reagents or carbanions stabilized by an a-heteroatom react with carbonyls to give the corresponding alkenes. 

Methyl 3- /7/-D-daunosaminide 154 has been derived from d-149 via a Wittig-type olefination using (2-thiazolylmethylene)triphenylphosphorane (Scheme 13.53). A 1 1 mixture of (E)- and (Z)-alkenes is obtained, which is isomerized in the presence of iodine into a 9 1 mixture of ( )-152 and (Z)-152. Methylation of the thiazole moiety increases the electrophilicity of the alkene, which then accepts nucleophiles such as benzylamine. The adduct is treated with NaBH4 to give a thiazolidine. Acetylation and mercury-mediated hydrolysis of the thiazolidine ring generates 153, which, on acidic treatment in methanol, yields the A-benzyl 3- /7/-D-daunosaminide 154 [99]. 

The reactions of (39 R = H) with aliphatic aldehydes give alkenes in yields that are strongly temperature dependent. If the reaction is carried out in the presence of TFAA, then excellent yields of the methylene compounds result. However, reaction of iphatic aldehydes with (39 R = (TtHis) in the same conditions proceeds by a most unexpected redox process to yield ketones in good yields after aqueous work-up. The appearance of ketones as products is unique in any Wittig-type of reaction at tlds oxidation level. Alkenyloxyboranes appear to be the intermediates in the reactions, and these react with excess TFAA to give enol trifluoroacetates, which can be isolated and characterized. A possible sequence is shown in Scheme 7. ... 

Disconnection of an internal ( )- or (Z)-double bond or a side chain of an alkene suggests a Wittig-type reaction or an alkylation of a vinylcuprate, respectively. 

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]. 

These two versions of the HWE are close to stereochemical control the formation of either isomer (E or Z) at will from (more or less) the same starting materials. The next two reactions achieve this aim. By purification of Wittig-type intermediates the stereospecific elimination gives a single isomer of the alkene. 

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... 

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