ca01 only detailed ToC 1..24

2.6.4 Metal–s-Alkyl and –s-Aryl Non-homoleptic Complexes 117
2.6.4.5 Method 5:
Addition of Organochromium(III) Compounds to Carbonyl Compounds
Under certain circumstances, organochromium(III) compounds transfer their alkyl or
aryl groups to aldehydes and, less frequently, to ketones. The particular selectivity and
tolerance of this reaction make it particularly useful in organic synthesis. The use of these
chromium reagents may be advantageous for use with acid-sensitive substrates, because
of their reduced Lewis acidity relative to other transfer reagents [e.g., trichloromethyltitanium(IV)
or alkyltriisopropoxotitanium(IV)].
2.6.4.5.1 Variation 1:
Reaction of Organochromium(III) Compounds Prepared
from Organochromium(III) Chloride by Transmetalation
The organochromium(III) compound may be either isolated before the addition to the carbonyl
compound or prepared in situ, as shown in Scheme 28. Triphenyltris(tetrahydrofuran)chromium(III)
is able to react with ketones, viz. pentan-3-one (67) and cyclohexanone.
[122] On the other hand, chlorodialkyl and dichloroalkyl derivatives are highly
aldehyde selective, while alkylpentaaquachromium(III) is unreactive. The alkyldichlorochromium(III)
reagent is particularly useful as it can be readily generated in situ by transmetalation
from chromium(III) chloride and a Grignard reagent; see the formation of
68. [123] These reagents are able to transfer the alkyl group to the organic substrate even
in an alcoholic medium or in the presence of water. [124] Dichloro[(trimethylsilyl)methyl]chromium(III)
allows an aldehyde-selective alkenation process after acid hydrolysis. [125]
This procedure has been more recently supplanted by those described in the following
variations.
Scheme 28 Reaction of Organochromium(III) Complexes with Carbonyl Compounds [122,123]
( )5
O
Cr(Ph)3(THF) 3
THF, −30 to 20 o C
Ph
67 70%
O
H
R 1 = Me, Pr, Bu, Bn
1. CrR1Cl2(THF) 3, THF, −60 oC 2. H2O 65−85%
OH
+
OH
H
( ) 5
R1 68
Ph OH HO
Reaction of Triphenyltris(tetrahydrofuran)chromium(III) with Pentan-3-one (67): [122]
A briskly stirred suspension of [CrPh 3(THF) 3] [from CrCl 3(THF) 3 (16 g, 43 mmol) suspended
in THF (500 mL) and PhMgBr (129 mmol)] in THF at –30 8C was treated dropwise with freshly
distilled pentan-3-one (67; 20 mL, 190 mmol). The mixture was then allowed to warm
up to rt. After 2 h at 208C the solvent was removed by distillation under reduced pressure
and the product hydrolyzed with H 2O and filtered, both residue and filtrate being washed
with Et 2O. The dried ethereal layer was evaporated and the residue (21.2 g) separated by
distillation. The volatile component, bp 57–62 8C/0.01 Torr (14.9 g, 90 mmol, 70% relative
to PhMgBr) was shown to be 3-phenylpentan-3-ol by a direct comparison of its IR spectrum
with that of an authentic specimen. The semicrystalline residue (5.87 g) was chromatographed
to give traces of oily products and 3-ethyl-4-methyl-5-phenylheptane-3,5diol;
yield: 5.36 g (17% relative to PhMgBr); mp 90–92 8C (hexane).
17%
Et
for references see p 135