Boranes reactions with aldehydes

This topological rule readily explained the reaction product 211 (>90% stereoselectivity) of open-chain nitroolefins 209 with open-chain enamines 210. Seebach and Golinski have further pointed out that several condensation reactions can also be rationalized by using this approach (a) cyclopropane formation from olefin and carbene, (b) Wittig reaction with aldehydes yielding cis olefins, (c) trans-dialkyl oxirane from alkylidene triphenylarsane and aldehydes, (d) ketenes and cyclopentadiene 2+2-addition, le) (E)-silyl-nitronate and aldehydes, (f) syn and anti-Li and B-enolates of ketones, esters, amides and aldehydes, (g) Z-allylboranes and aldehydes, (h) E-alkyl-borane or E-allylchromium derivatives and aldehydes, (i) enamine from cyclohexanone and cinnamic aldehyde, (j) E-enamines and E-nitroolefins and finally, (k) enamines from cycloalkanones and styryl sulfone. 

Crotyl silanes offer the possibility of diastereoselectivity in reactions with aldehydes in the same way as the corresponding boranes. The mechanism is completely different because crotyl trialkylsilanes react via an open transition state as the silicon is not Lewis acidic enough to bind the carbonyl oxygen of the electrophile. Instead, the aldehyde has to be activated by an additional Lewis acid or by conversion into a reactive oxonium ion by one of the methods... 

In the same way, reaction of (8) with allenyl- or propargyl-stannanes affords intermediate borane derivatives which, upon reaction with aldehydes, produce the expected adducts with high selectivities (eqs 14 and 15). ... 

Sulfonimidophosphonates Phosphides s. Alkali phosphides Phosphinaminoboranes 16, 374 Phosphine, reactions with aldehydes, aliphatic 16, 696 —, reductions with — 17, 346 Phosphine boranes... 

Enol stannanes of cyclohexanone and propiophenone have been indicated to take part in r/treo-selective aldol reactions with benzaldehyde at low temperatures e.g. —78 °C), but to be erythro-seAsciiwe at higher temperatures ca 45 °C). Two complementary methods have been described for stereoselection in aldol-type reactions. Whilst a-mercurio-ketones show eryr/wo-selection in their reactions with aldehydes in the presence of boron trifluoride diethyl etherate, pre-formed lithium enolates and aldehydes, in the presence of simple trialkyl-boranes, lead to mixtures that are rich in the more stable threo-d do product. Aldol-type products arise from 1,3-alkyl migrations of alk-l-enyl alkyl acetals and ketals, in a reaction that is catalysed by boron trifluoride diethyl etherate (Scheme 52). Diastereoselection is possible, since (.E)-alkenyl acetals give the... 

The allylation reaction has also been extended to enantiomerically pure allylic boranes. For example, the 3-methyl-2-butenyl derivative of bis-(isopinocam-pheyl)borane reacts with aldehydes to give carbinols of >90% enantiomeric excess... 

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