Including Single Quotes In Dynamic Pivot Tables?

I have this pivot table (I only post the static version as the problem only regards the single quotes)

SELECT * from(
select DATEPART(year,DeliverydatePackingSlip) as Year,
CASE WHEN DiffPromiseDateFirst < 0 Then '1 - too early'
WHEN DiffPromiseDateFirst = 0 Then '2 - on time'
ELSE '3 - too late' END as Delivery
from iq4bisprocess.FactOTDCustomer
WHERE OTD_Exclusion = 0)a
PIVOT ( COUNT(Year)
For Year

in ([2012],[2013],[2014],[2015])) as pvtNow, packing everything in a string parameter I always stumble over the single quotes. I tried to replace them with CHAR(39), I tried to define a parameter for each occurrence, but always get a syntax error. What am I doing wrong?declare @sql nvarchar(max)

declare @title1 nvarchar(20)
declare @title2 nvarchar(20)
declare @title3 nvarchar(20)
set @title1 = '1 - too early'
set @title2 = '2 - on time'
set @title3 = '3 - too late'

[Code] .....

exec sp_executesql @sqlThis would throw:Msg 102, Level 15, State 1, Line 3 Incorrect syntax near 'early'.