Magnesium unsaturated carbonyl compounds

A simple two-step protocol for the generation of a terminal diene is to add allyl magnesium bromide to an aldehyde or a ketone and subsequent acid or base catalysed dehydration (equation 34)72. Cheng and coworkers used this sequence for the synthesis of some indole natural products (equation 35)72a. Regiospecific dienones can be prepared by 1,2-addition of vinyllithium to a,/l-unsaturated carbonyl compounds and oxidative rearrangement of the resulting dienols with pyridinium dichromate (equation 36)73. 

The reactions of organometallic reagents such as organolithium [696], -zinc [697-700], -magnesium [701], and -aluminum species [702] are facilitated by the presence of TiCU [9] as exemplified in Eq. (308) [703]. Even addition of a titanium compound to aldehydes was promoted in the presence of an extra amount of a titanium salt (Eq. 309) [704,705]. Titanium Lewis acids increase the reactivity of the a-position of a ketone (Eq. 310) [706] and the /3-position of an a,/3-unsaturated carbonyl compound towards nucleophiles (Eq. 311) [608,707-709]. The positive role of TiCU in the photo-hydroxymethylation of ketones and aldimines is ascribed to activation of methanol by the titanium salt (Eq. 312) [710]. 

CycUzations. 8,e-Unsaturated carbonyl compounds undergo cyclization with Mg. The reaction can be carried out electrochemically on a magnesium electrode and its application to a synthesis of muscone has been demonstrated. ... 

A further example is the reaction of the magnesium salt of olivetol with 3-methylbut-2-enal when the yield of the 4-substitited product was 77% and that of the 2- was 3%. By contrast in the reaction of resorcinols with a,p-unsaturated carbonyl compounds in pyridine solution 2-substitution occurs as will be seen in the synthesis of deoxybruceol in section 12.6.5. 

Magnesium chloride promoted Michael additions of these compounds to unsaturated carbonyl compounds were achieved with moderate to good asymmetric induction. Chiral organotin enamines were prepared from... 

The addition of carbonyl compounds towards lithiated 1-siloxy-substituted allenes does not proceed in the manner described above for alkoxyallenes. Tius and co-work-ers found that treatment of 1-siloxy-substituted allene 67 with tert-butyllithium and subsequent addition of aldehydes or ketones led to the formation of ,/i-unsaturated acyl silanes 70 (Scheme 8.19) [66]. This simple and convenient method starts with the usual lithiation of allene 67 at C-l but is followed by a migration of the silyl group from oxygen to C-l, thus forming the lithium enolate 69, which finally adds to the carbonyl species. Transmetalation of the lithiated intermediate 69 to the corresponding zinc enolate provided better access to acylsilanes derived from enolizable aldehydes. For reactions of 69 with ketones, transmetalation to a magnesium species seems to afford optimal results. 

The treatment of vinyl anions with elemental selenium also leads to seleno-carbonyl compounds via eneselenolate anions (Eqs. 4-6). Deprotonation of l,3-selenothio-2-thione with EDA followed by the reaction with elemental selenium proceeds accompanied with the rearrangement of vinylic skeleton to form l,3-dithio-2-selone derivatives (Eq. 4) [38]. The vinyl anions from oxa-zolines and imidazoles were treated with elemental selenium to give the oxazo-line selone and imidazoline selone after the aqueous workup (Eq. 5) [39]. The reaction of vinyl magnesium halides with elemental selenium and allyl bromide gives allyl vinyl selenides that undergo a seleno-Claisen rearrangement to generate y,6-unsaturated selenoaldehydes (Eq. 6) [40]. 

Allyltitanium complexes (22) readily add to carbonyl compounds with high regio- and stereo-selection. They are prepared by reaction of a chlorotitanium complex (21) with an allyl-magnesium or -lithium derivative (equation 13). Some of these unsaturated Ti complexes, like (23)-(25) in Scheme 2, obtained from allylmagnesium halides or allyllithium by reaction with titanium tetraisopropoxide or titanium tetramides, are known as titanium ate complexes . The structure of these ate complexes, at least from a formal point of view, can be written with a pentacoordinate Ti atom. Some ate complexes have synthetic interest, as is the case of (allyl)Ti(OPr )4MgBr which shows sharply enhanced selectivity towards aldehydes in comparison with the simple (allyl)Ti(OPr )3. ... 

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