Term
| The majority of reductions of carbonyl compounds and nitriles with nucleophilic reducing agents proceed via. |
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Definition
| nucleophilic transfer of a hydrogen atom with two electrons called a "hydride" from the reducing agent to the carbonyl or cyano carbon |
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Term
| The rate of reduction and the chemoselectivity of a reduc- ing agent toward a given substrate depends on factors such as (3) |
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Definition
nature of the metal cation which serves as a lewis acid to activate the carbonyl or cyano moiety toward hydride transfer
Substitution of the reducing agent hydrogens by alkyl, -OR, or -CN groups
solvent |
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Term
| Reactivity order of substrates for reduction is |
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Definition
| RCHO>R2CO>RCO2R'>RCONR2>RCO2H |
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Term
| LiAlH4 (LAH) is a powerful reducing agent but not |
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Definition
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Term
| The reactivity of LAH may be tempered by the addition of |
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Definition
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Term
| The reactivity and selectivity of LAH can be modified by replacing three of its hydrides with |
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Definition
| t-buxtoxy or ethoxy groups |
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Term
| Li[AlH(OR)3] are _____ reactive but more _______ than LAH |
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Definition
less reactive
more selective |
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Term
| Lithium tri-t-butoxyaluminunm hydride readily reduces |
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Definition
| aldehydes and ketones to the corresponding alcohols and reduces acid chlorides to aldehydes |
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Term
| What makes Lithium tri-t-butoxyaluminunm hydride chemoselective? |
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Definition
| Epoxides, esters, carboxylic acids, tert-amides, and nitriles are not, or only slowly, reduced |
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Term
| A notable difference in Red-A1 selectivity relative to LAH involves the reduction of |
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Definition
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Term
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Definition
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Term
| What cation is a stronger Lewis acid: Li+ or Na+ and what does that mean for reduction |
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Definition
| The Li' cation is a stronger Lewis acid than the Na+cation. Li+ coordination with the carbonyl group enhances the electrophilicity of the carbonyl carbon, thereby facilitating hydride transfer |
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Term
| Lithium borohydride is a more powerful reducing agent than ________ _________: it reduces _______ to primary alcohols but is unreactive towards |
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Definition
1.) sodium borohydride
2.) esters
3.) amides |
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Term
| Zinc borohydride properties |
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Definition
| : it is less basic than NaBH4 and thus it is especially suitable for the reduction of base-sensitive compounds. Also, the zinc cation has a better coordinating ability than either Na+ or Li+, making Zn(BH4)2 often the reagent of choice for chelation-controlled, stereoselective reductions of acyclic ketones |
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Term
| What does zinc borohydride reduce? And what else is it good for? |
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Definition
| aldehydes, ketones and aliphatic esters and carboxylic acids. It is an effective hydroborating agent for alkenes, dienes, and alkynes |
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Term
| The presence of three alkyl groups in lithium trialkylborohydrides imparts |
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Definition
| s increasing nucleophilicity to the hydride, making them more powerful reducing agents than lithium borohydride itself |
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Term
| lithium trialkylborohydrides readily reduces these |
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Definition
| aldehydes, ketones, esters |
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Term
| When using LiEt3BH epoxides are cleaved to give alcohols by |
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Definition
| attach of the hydride at the less substituted carbon (so alcohol is on the more substituted side) |
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Term
| One remarkable feature of L and K selectrides is |
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Definition
| is their unusual ability to introduce major steric control in the reduction of cyclic ketones |
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Term
| What do L and K selectrides reduce |
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Definition
| aldehydes and ketones to corresponding alcohols |
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Term
| Why is oxidative workup essential for using LiEt3BH? |
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Definition
| The trialkyl- boron species formed from these reagents should be oxidized on completion of the reduc- tion, prior to workup, to the correponding alcohols with alkaline hydrogen peroxide |
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Term
| Because of the presence of the electron withdrawing cyano group, NaBH,CN is less ____________ thus more ___________ than NaBH4 |
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Definition
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Term
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Definition
| , it is possible to selectively reduce an aldehyde group in the presence of a keto group or a keto group in the presence of an ester group using NaBH4CN |
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Term
| What reagents can you use for reductive amination? |
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Definition
NaBH3CN is the most popular one
then you can also use Na[BH(OAC)3] and hydrogenation H2 Pd/C
Zn[BH4]2 is particular effective agent for reductive amination |
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Term
| The reactivity of electrophilic reducing agents (R2AlH and BH3) are charcterized by |
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Definition
| their coordination with the carbonyl oxygen prior to hydride transfer, favor reductions of e- rich carbonyl groups. |
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Term
| DIBAL-H is selective reduction of |
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Definition
| substituted esters or nitriles to corresponding aldehydes for the reduction of lactones to lactols |
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Term
| Horner-Wadsworth-Emmons olefination followed by DIBAL-H reduction providesa tandem synthesis of ( )-allylic alcohols |
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Definition
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Term
| An attractive feature of BH3*THF and BH3*SMe2 is the |
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Definition
| the facile reduction of carboxylic acids to primary alcohols. |
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Term
| Reduction of saturated carboxylic acids with the borane derivative, thexylchloro- borane, provides a direct route to ___________ without |
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Definition
| their prior conversion to carboxylic acid derivative^. |
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Term
| NaBH4-CeCl3,-EtOH-HC(OCH3)3 The reason for the selective reduction of the keto group in the presence of the reactive aldehyde is that |
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Definition
| is that the CHO group undergoes preferential acetalization with CeCl3 and the trimethyl orthoforrnate. |
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Term
| Why does BH3*THF or BH3*SMe2 only reduce COOH in the presence of esters or ketones? |
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Definition
| The carbonyl group in triacylboranes resembles the reactivity of an aldehyde or a ketone more than of an ester (ester resonance) due to electron delocalization from the acyl oxygen into the p orbital of boron |
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Term
| There are two possible modes of delivery of the "hydride" to the carbonyl carbon of cyclohexanone |
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Definition
| (1) axial attack with formation of the equatorial alcohol and (2) equatorial attack with formation of the axial alcohol. |
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Term
| Two factors are competing with each other when attacking this carbonyl in cyclohexanone |
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Definition
equatorial is:(1) steric interaction of the incoming "hydride" with the 3,5-diaxial hydrogens in the axial attack and
axial is:(2) torsional strain of the incoming "hydride" with the 2,6-diaxial hydrogens in the equatorial attack. |
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Term
| So what are two factors that can make an equatorial alcohol (aka sterics prevails) |
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Definition
if the carbonyl group is embedded in a rigid system, steric effects prevail, resulting in preferential transfer of the "hydride" from the less hindered side
if there are bulky axial substituents at C(3) or and at C(5) |
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Term
| five-member ring chelates are formed in ________ over six-rnember ring chelates. |
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Definition
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Term
| what two reagents cause 1,3 syn diol from 1-ol and 3-ketone? |
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Definition
MeOBEt2, THF in methanol with NaBH4
AND
i-bu3B with NaBH4 |
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